摘要:

THE ROYAL SOCIETY OF CHEMISTRY Journal of Materials Chemistry Scientific Advisory Editor Editorial Manager Dr. Martin R. Bryce Dr. Robert J. Parker Department of Chemistry The Royal Society of Chemistry University of Durham Thomas Graham House South Road Science Park Durham DH1 3LE, UK Cambridge CB4 4WF, UK Staff Editor: Mrs. Janet M. Leader Senior Assistant Editor: Mrs. S. Shah Assistant Editor: Mrs. S. Youens Editorial Secretary: Miss D. J. Halls Graphics Designer: Ms. C. Taylor-Reid Materials Chemistry Editorial Board Allan E. Underhill (Bangor) (Chairman) Peter G. Bruce (St. Andrews) John W. Goodby (Hull) Martin R. Bryce (Durham) Klaus Praefcke (Berlin) David A. Dunmur (Sheffield) Brian J. Tighe (Aston) Jean Etourneau (Bordeaux) Anthony R.West (Aberdeen) Wendy R. Flavell (UMIST) John D. Wright (Canterbury) Robert J. Parker (Secretary) International Advisory Editorial Board K. Bechgaard (Risa, Denmark) J. S. Miller (Salt Lake City, UT, USA) J. Y. Becker (Beer-Sheva, Israel) K. Mullen (Mainz, Germany) J. D. Birchall (Runcorn, UK) M. Nygren (Stockholm, Sweden) A. J. Bruce (Murray Hill, USA) Y. W. Park (Seoul, Korea) A. K. Cheetham (Santa Barbara, USA) V. Percec (Cleveland, OH, USA) E. Chiellini (Pisa, Italy) N. Plate (Moscow, Russia) D. Coates (Poole, UK) M. Prato (Trieste, Italy) P. Day (London, UK) C. N. R. Rao (Bangalore, India) B. Dunn (Los Angeles, USA) J. Rouxel (Nantes, France) W. J. Feast (Durham, UK) R. Roy (University Park, PA, USA) A. Fukuda (Tokyo, Japan) J. L.Serrano (Zaragoza, Spain) D. Gatteschi (Florence, Italy) J. N. Sherwood (Glasgow, UK) J. B. Goodenough (Austin, TX, USA) J. Simon (Paris, France) A. C. Griffin (Hattiesburg, USA) J. F. Stoddart (Birmingham, UK) S-i. Hirano (Nagoya, Japan) S. Takahashi (Osaka, Japan) P. Hodge (Manchester, UK) J. 0.Thomas (Uppsala, Sweden) H. lnokuchi (Okazaki, Japan) G. J. T. Tiddy (Bebington and Salford, UK) W. Jeitschko (Munster, Germany) Yu. D. Tretyakov (Moscow, Russia) 0. Kahn (Orsay, France] J. W. White (Canberra, Austra4ia) R. McCullough (Pittsburgh, USA) R. Xu (Changchun, China) Y. Yamashita (Okazaki, Japan) Journal of Materials Chemistry (ISSN 0959-9428) is published monthly by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB44WF, UK.All orders accompanied with payment should be sent directly to The Royal Society of Chemistry, Turpin Distribution Services Ltd., Blackhorse Road, Letchworth, Herts SG6 lHN, UK. NB Turpin Distribution Services Ltd., distributors, is wholly owned by The Royal Society of Chemistry. 1994 Annual subscription rate EC (inc. UK) f381.00, USA $718.00, Canada f431.00 (plus GST), Rest of World f410.00. Customers should make payments by cheque in sterling payable on a UK clearing bank or in US dollars payable on a US clearing bank. Air freight and mailing in the USA by Publications Expediting Inc., 200 Meacham Avenue, Elmont, NY 11003. USA POSTMASTER: send address changes to Journal of Materials Chemistry, Publications Expediting Inc., 200 Meacham Avenue, Elmont, NY 11003.Second Class postage paid at Jamaica, NY 11431. All other dispatches outside the UK by Bulk Airmail within Europe, Accelerated Surface Post outside Europe. PRINTED IN THE UK. 0 The Royal Society of Chemistry, 1994. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form, or by any means, electronic, mechanical, photographic, recording, or otherwise, without the prior permission of the publishers. Dr. R. J. Parker, Editorial Manager Tel.: Cambridge (01223) 420066 E-Mail (INTERNET): RSCl@RSC.ORG Fax: (01223) 426017, 420247 or 423623 Advertisement sales: Tel. +44 (0171-287 3091; Fax +44 (0171-494 1134 INFORMATION FOR AUTHORS The Royal Society of Chemistry welcomes submission of manuscripts intended for publication in two forms, Articles and Materials Chemistry Communications.These should describe original work of high quality dealing with the synthesis, structures, properties and applications of materials, particularly those associated with advanced technology. Articles Full papers contain original scientific work that has not been published previously. However, work that has appeared in print in a short form such as a Materials Chemistry Communication is normally acceptable. Four copies of Articles includ- ing a top copy with figures etc. should be sent to The Editor, Journal of Materials chemistry, The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK.Materials Chemistry Communications Materials Chemistry Communications con- tain novel scientific work in short form and of such importance that rapid publication is warranted. The total length is normally restricted to two pages of the double-column A4 format. For a Communication consisting entirely of text and ten refer-ences, with no figures, equations or tables, this corresponds to approximately 1600 words plus an abstract of up to 40 words. However, special consideration will be given to communications with a large amount of essential diagramatic information. Submission of a Materials Chemistry Communication can be made either to The Editor, Journal of Materials Chemistry, The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK, or via a member of the International Advisory Editorial Board. In the latter case, the top copy of the manuscript including any figures etc., together with the name of the person to whom the Communication is being submit- ted, should be sent simultaneously to the Editor at the Cambridge address.Authors may wish to contact the Board member to ensure that he is available to arrange review of the manuscript within reasonable time. In order to avoid delay in publication, proofs of Communications are not sent to authors unless this is specifically requested. Full details of the form of manuscripts for Articles and Materials Chemistry Communications, conditions for accept-ance etc. are given in issue number one of Journal of Materials Chemistry published in January of each year, or may be ob- tained from the Staff Editor. There is no page charge for papers pub- lished in Journal of Materials Chemistry. Fifty reprints are supplied free of charge. Any author who is publishing in Journal of Materials Chemistry is entitled to a free copy of the issue in which the paper appears.

ISSN:0959-9428    

DOI:10.1039/JM99404FX041

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Each Issue with Subject-, Author- and Materials Indexes Additional 10-Volume Indexes 12 volumes per year Annual Subscription Rate: SFr 1320.00 Post agemandling: SFr 120.00 Agency Discount: 70% ISSN 0377-6883 EDITORS: Professor G.E. Murch Department of Mechancal Engineering, The University of Newcastle, NSW 2308, Australia H. Neber-Aesc h bacher Scitec Publications Untermuehleweg 11 CH-6300 Zug, Switzerland Dr. Fred H. Wohlbier Trans Tech Publications Hardstrasse 13 CH-4714 Aedermannsdorf Switzerland ABSTRACT EDITOR: Dr. David J. Fisher Cardiff, United Kingdom Scitec Publications Member of the Trans Tech Group of Publishers Materials Science Solid State Physics Engineering Untermuehleweg11 CH-6300 Zug Switzerland Fax: ++41 -42 32 52 12 E-Mail: ddf@scitec.ch Dimusion The International Journal of the Defect Solid State Pt.A of Diffusion and Defect Data efect and Diffusion Forum is an international journal serving the advanced materials research community as a permanent record of significant developments in the general area of the defect solid state. Started in 1967 as Diffusion Data, the material pre- sented today encompasses the well-known ex-tended abstract section which has made this series a standard in its field, as well as critical reviews, data collections and original contributions. Complete spe- cial issues regularly focus on topics of current inter- est. Particular emphasis is placed on atomic and ionic transport, solid-state defect properties (both structural and electronic), radiation damage and de- fect production. In order to shorten publication time, correspondence can be sent electronically, and manuscripts can be accepted in electronic form. Distributed by: Trans Tech Publications Ltd Trottenstr. 20 / CH-8037 Zurich / Switzerland Fax: (++41) 12 72 10 92 E-Mail: ddf@transtech.ch Write us for: -Detailed scope of coverage -Instructions for authors -Information on further Materials Science titles

ISSN:0959-9428    

DOI:10.1039/JM99404BX043

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

ISSN 0959-9428 JMACEP(11) 1659-1768 (1994) Journal of Materials Chemistry Synthesis, structures, properties and applications of materials, particularly those associated with advanced technology CONTENTS 1659 FEATURE ARTICLE. Chemical routes for preparation of oxide high-temperature superconducting powders and precursors for superconductive ceramics, coatings and composites Yu. G. Metlin and Yu. D. Tretyakov 1667 Pre-tilt angles as a function of polyimide composition for copolyimides H. Yokokura, B. 0.Myrvold, K. Kondo and S. Oh-hara 1673 Synthesis, transition temperatures, some physical properties and the influence of linkages, outboard dipoles and double bonds on smectic C formation in cyclohexylphenylpyrimidines S. M. Kelly and J. Fiinfschilling 1689 8-Fluoro esters incorporating a cyclohexane ring: some new chiral dopants for ferroelectric mixtures S.M. Kelly, R. Buchecker and J. Fiinfschilling 1699 A new type of main-chain liquid-crystal polymer derived from 4-hydroxybiphenyl-4-carboxylic acid and its smectic mesophase behaviour Y. Nakata and J. Watanabe 1705 Effect of spacer length on the thermal properties of side-chain liquid-crystal poly(methacry1ate)s A. A. Craig and C. T. Imrie 1715 Synthesis and mesomorphic properties of 4-[(4-cyanophenyl)acetylenyl]-2,3,5,6-tetrafluorophenyl4-n-alkoxybenzoates J. Wen, H. Yu and Q. Chen 1719 X-Ray crystal structure and solid-state properties of a 1 :1 complex of tetrathiafulvalene (TTF) and 1-Oxo-2,6-dimethyl-4-dicyano-methylenecyclohexa-2,5-diene A.S. Batsanov, M. R. Bryce, S. R. Davies and J. A. K. Howard 1723 X-Ray absorption spectroscopic study of the A1P04-5:ferrocene inclusion compound and its thermally decomposed products A. Lund, D. G. Nicholson, G. Lamble and B. Beagley 1731 Properties of the guest molecules in the 1,lO-dibromodecane/urea inclusion compound: A molecular dynamics simulation study A. R. George and K. D. M. Harris 1737 Ion-exchange properties of lithium aluminium layered double hydroxides I. C. Chisem and W. Jones 1745 Formation and decomposition of L~B~,CU~O,_~ J. M. S. Skakle and A. R. West 1749 170Nuclear magnetic resonance spectroscopy of the structural evolution of vanadium pentaoxide gels A. V. McCormick G. A. Pozarnsky and 1755 Characterisation of silicated anatase powders L.Yi, G. Ramis, G. Busca and V. Lorenzelli MATERIALS CHEMISTRY COMMUNICATIONS 1763 Preliminary crystal structure of mixed-valency Sr,Ni,O,, the actual formula of the so-called Sr,Ni,O,, and C. Renard F. Abraham, S. Minaud 1765 Corrigendum to self-consistent interatomic potentials for the simulation of binary and ternary oxides T. S. Bush, J. D. Gale, C. R. A. Catlow and P. D. Battle 1765 Corrigendum to Examination of the structural features necessary for mesophase formation with aroylhydrazinato-nickel(Ir) and -copper(rr) complexes M. N. Abser, M. Bellwood, C. M. Buckley, M. C. Holmes and R. W. McCabe 1767 Book Reviews G. H. W. Milburn; K. Kawasaki i Cumulative Author Index iv Conference Diary Note: Where an asterisk appears against the name of one or more authors, it is included with the authors’ approval to indicate that correspondence may be addressed to this person. COPIES OF CITED ARTICLES The Royal Society of Chemistry Library can usually supply copies of cited articles.For further details contact: The Library, Royal Society of Chemistry, Burlington House, Piccadilly, London W1V OBN, UK. Tel: +44 (0)71-437 8656, Fax: +44 (0)71-287 9798, Telecom Gold 84: BUR210, Electronic Mailbox (Internet) LIBRARY@RSC.ORG. If the material is not available from the Society’s Library, the staff will be pleased to advise on its availability from other sources. Please note that copies are not available from the RSC at Thomas Graham House, Cambridge. SPECIAL ISSUE ON MOLECULAR CONDUCTORS A special issue is planned for publication in 1995 on the topic of molecular conductors. Anyone interested in submitting a paper for this issue is asked to contact. Dr Martin Bryce Department of Chemistry University of Durham South Road Durham DHI 3LE UK The deadline for submission of manuscripts for this special issue is 31st March 1995. SPECIAL ISSUE ON LIQUID CRYSTALS A special issue is planned for publication in 1995 on the topic of liquid crystals. Anyone interested in submitting a paper for this issue is asked to contact. Professor. John Goodby School of Chemistry University of Hull Hull HU6 7RX UK The deadline for submission of manuscripts for this special issue is 31st May 1995.

ISSN:0959-9428    

DOI:10.1039/JM99404FP099

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Cumulative Author Index 1994 Aarik J., 1239 Abraham F., 1763 Abrahams I.. 185, 775 Abser M. N., 1173, 1765 Afanasiev P., 1653 Agullo J. M., 695 Ahmet M. T., 1201 Ahn S-K., 949 Aidla A,, 1239 Ainslie B. J., 1233 Airoldi C., 1479 Aka G., 907 Akhtar M. J., 1081 Akhtar Z.-u.-N., 1081 Akimoto H., 61 Aksay I. A., 353 Alagna L.. 943 Ah-Adib Z., 1 Aliev A. E., 35 Allan N. L., 817 Al Raihani H., 1331 Alves 0.L., 389, 529 An Y., 985 Ando M., 631 Andreani F., 1035 Angeloni A. S., 429, 437 Angeloni L., 1047 Annila A,. 585 Aoki H., 1497 ap Kendrick D., 399 Ara K., 551 Arai H., 653 Arai K., 275 Aranha N., 529 Armelao L., 407 Armes S. P., 935 Armigliato A., 361 Arnold Jr. F. E.. 105 Aruga Katori H., 915 Asaka N.. 291 Aspin I. P., 385 Attfield J. P., 475, 575 Atwood M. P., 1393 Auld J., 1245, 1249, 1591 Auroux A,, 125 Awaga K., 1377 Azuma K., 139 Baba A,, 51 Babu G.P., 331 Babushkin O., 413 Bach S., 133, 875 Bachir S.. 139 Badwal S. P. S., Badyal J. P. S., 1055 Bae M-K., 991 Baetzold R. C., 299 Baffier N., 133, 875 Bagshaw S. A., 557 Bafios L.. 445 Baram P. S., 817 Barbieri A., 1255 Barbosa L. C., 529 Barker C. P., 1055 Barriga C., 11 17 Barton J. M., 379, 385 Bashall A., 1201 Batsanov A. S., 1719 Battaglin G., 407 Battle P. D., 831, 1457, 1765 Batyuk V. A., 761 Bautista F. M., 311 Bazin D.. 1101 Beagley B.. 1723 Bechgaard K., 675 Bedioui F.. 1215 Bedson J.. 571 Beguin F., 669 Bell R. G., 781 Bellwood M., 1173, 1765 Benzi P., 1067 Bertoncello R., 407 Beveridge M., 119 Bigi S., 361 257, 1437 421, 641, 707, Bignozzi M. C., 429 Billingham N.C., 1508 Bjornholm T., 675 Blasse G., 1349 Bonanos N., 899 Bonardi A., 713 Bond S. E., 23 Booth C., 591, 1507 Booth C. J., 747 Botto L. I., 541, 1641 Bowden K., 1201 Bradley R. H., 487, 1157, Branitsky G. A,, 373 Branton P. J., 1309 Braybrook J. H., 1157, 1357 Brewis D. M., 487, 683 Breysse M., 1653 Brisdon B. J., 1387 Britt S., 161 Brock T., 229 Brodsky C. J., 651 Brown T., 771 Bruce D. W., 479, 1017 Bruce P. G., 167, 1579 Bryant G. C., 209 Bryce M. R., 1719 Buchecker R., 1689 Buckley C. M., 1173, 1765 Buist G. J., 379, 385 Bujanowski V. J., 1181 Bujoli B., 1319 Bulmer G., 1149 Burnell G., 1309 Busca G., 965, 1123, 1755 Bush T. S., 831, 1765 Cabello C., 1641 Cairns J. A., 393 Campelo J. M., 311 Caneschi A,, 319, 1047 Cao X., 417 Capelletti R., 713 Cardwell D. A,, 1393 Carlino S., 99 Carr S.W., 421 Carrazan S. R. G., 47 Carruthers B., 805 Carvalho A,, 515 Casciola M., 1313 Cassagneau T., 189 Castellanos M., 1303 Castiglioni M., 1067 Castillo R., 903 Catlow C. R. A., 1081, 1765 Causa M., 825 Cellucci F., 579 Cervini R., 87 Cesar C. L., 529 Chaair H., 765 Challier T., 367 Chang S-H., 1271 Charlton A., 1233 Chassagneux F., 1331 Cheetham A. K., 641, 707, Chehimi M. M., 305, 741 Chen C., 469 Chen Q., 327, 1715 Chen Z., 1619 Cheng S. Z. D., Chernyaev S. V., 1107 Chevalier B., 463 Chiba K., 551 Chiellini E., 429, 437 Chisem I. C., 1737 Choisnet J., 895 Chu P., 719 Ciacchi F. T., 257 Clegg W., 891 Colbourn E. A., 805 Cole-Hamilton D. J., 657 Coles G. S. V., 23 1189 781, 831, 1457 105, 719 Choy J-H., 1271 Coles H., 869 Colque S., 1343 Connell J. E., 399 Conroy M., 1 Conway L.J., 337 Cook M. J., 209, 1205 Cook S. L., 81 Cooney R. P., 557 Copplestone F. A., 421 Corriu R. J. P., 987 Costa Bizzarri P., 1035 Costa F. M. A., 515 Cotter J. P., 1603 Cox P. A., 805 Craig A. A., 1705 Craig S. R., 977 Crayston J. A., 1093 Crespin M., 895 Critchlow G. W., 1245, 1249, 1591 Cumberbatch T. J., 1393 Dan M., 1195 Daolio S., 1255 Darriet B., 463 David L., 1047 Davidson I. M. T., 13 Davies A., 113 Davies M. J., 813 Davies S. R., 1719 Davis T. P., 1359 Deazle A. S., 385 De Battisti A,, 1255 Dekker J. P., 689 del Arc0 M., 47 del Carmen Prieto M., 1123 Della Casa C., 1035 Delmon B., 903 Dennison S., 41 Depaoli G., 407 Deschenaux R., 679, 1351 De Stefanis A., 959 Devynck J., 1215 Dhas N. A., 491 Diamond D., 145, 217 Diele S., 1547 Dissanayake M.A. K. L., Dong C., 1365 Douglas W. E., 1167 Drabik M., 265, 271 Drennan J., 245 Dunmur D. A., 747 Durand B., 1331 Eda K., 205, 775 Egdell R. G., 1647 Eguchi K., 653 Ekstrand A., 615 Eldred W. K., 305 Ellis A. M., 13 Elsegood M. R. J., 891 Endregard M., 943 Ericsson T., 1101 Erokhin Yu. Yu., 1585 Errington R. J., 891 Etourneau J., 463 Fabretti A., 1047 Facchin B., 1255 Faguy P. W., 771 Fahey J. T., 1533 Fau-Canillac F., 695 Feast W. J., 1159 Feng S., 985 Fernandez J. M., 11 17 Ferraro F., 1047 Fettis G. C., 1157, 1357 Fisher G. A,, 891 Fitzmaurice J. C., 285 Fitzmaurice, J. C., 1603 Fitzpatrick A. D., 1055 Fleming R. J., 87 Fletcher J. G., 1303 Flint S. D., 509 Folkerts H. F., 1349 Forsyth M., 1149 1075, 1307 Foster D. F., 657 Fragala I.L., 1061 Fraoua K., 305 Freakley P. K., 1189 Frechet J. M. J., 1533 Frederiksen P., 675 Friend R. H., 1227 Frialova M., 271 Fuflyigin V. N., 1585 Fuji T., 635 Fujimoto T., 61, 533, 537 Fujita T., 955 Fujiwara Y., 1219 Fukuda A., 237, 997 Fiinfschilling J., 1673, 1689 Gaillon L., 1215 Gale J. D.: 781, 831, 1765 Galikova L., 265, 271 Gallagher M. J., 1359 Gallardo Amores J. M., 965, 1123 Galli G., 429, 437 Ganguli P., 331 Garci O., 1635 Garcia A., 311 Garcia-Martin S., 1307 Garcia-Martinez O., 611 Gatteschi D., 319, 1047 Geantet C., 1653 Gee M. B., 337 Gellman L. J., 1427 George A. R., 1731 Gibb T. C., 1445, 1451 Gibson R. A. G., 393 Gier T. E., 1111 Gil A,, 1491 Gil-Llambias F-J., 47 Gittens G. J., 1508 Glomm B., 55 Godinho M. M., 515 Goodby J. W., 71, 747 Goodenough J.B., 1627 Gopalakrishnan J., 703 Gorbenko 0.Yu., 1585 Gormezano A., 817 Goto T., 915 Gozzi D., 579 Graboy I. E., 1585 Grange P., 1343 Granozzi G., 407 Gravereau P., 463 Greaves C., 931, 1463, 1469 Gregory D. H., 921 Grins J., 445, 1293 Guillon D., 679, 1359 Guo Z., 327 Gutierrez M. P., 1303 Hall P. G., 1309 Hamerton I., 379, 385 Hamstra M. A., 1349 Han Y-S., 1271 Hannington J., 869 Harris F. W., 105 Harris K. D. M., 35, 1731 Harris S. J., 145, 217 Harrison W. T. A,, 11 11 Haslam S. D., 209, 1205 Hastie G. P., 977 Hatayama F., 205, 775 Hayashi A,, 915 Heath K. D., 825 Heath R. J., 487, 683 Hector A. L., 279 Heinrich B., 679 Henshaw G. S., 1427 Hentrich F., 1547 Hermansson L., 413 Herod A. J., 1451 Herrero P., 1433 Hervieu M., 1353 Heughebaert J-C., 765 Heughebaert M., 765 Heywood B.R., 1387 1507 Hickey E., 463 Higuchi A,, 171 Hill C. A. S., 1233 Hinds B. J., 1061 Hirose N., 9 Hitchman M. L., h1 Hix G. B., 189 Hobson R. J., 113 Hochi K., 599 Hodby J. W., 469 Hodge P., 1, 869 Hodson A. G. W., 1387 Holmes M. C., Holmes P. A., 365 Holmgren A., 413 Hong L., 1041 Hopkins J., 1055 Horigome K., 1503 Hosokoshi Y., 1219 Houlton D. J., 1245, 1249, Hourd A. C.. 393 Howard J. A. K., I719 Howlin B. J.. 379, 385 Hu Y., 469 Hubert-Pfalzgraf L. G., 1409 Hudson M. J., 99. 113, 1337 Hudson S. A,, 479 Hughes A. E., 257 Huxham I. M., 253 Ibanez A,, 1101 Ibn-Elhaj M., 1351 Ichimura K., 883 Ikemoto H., 537 Imanishi N., 19 Imayoshi K., 19 Imrie C. T., 1705 Inabe T., 1377 Inada H., 171 Inagaki M., 1475 Indira L., 1487 Inman D., 1331 Inoue T., 1539 Irvine J.T. S., 99i Ishikawa K., 997 Islam M. S., 299 Ismail H., 1189 Isoda S., 291 Isozaki T., 237, 9')7 Itaya A., 1539 Ivanovskaya M. I.. 373 Iyer R. M., 1077 Izaki S., 1581 Jacobson A. J., 1419 Jaek A,, 1239 James M., 575 Janes R., 1071 Jennings R. A., 931 Jimenez R., 5 Jimknez-Lopez A,, 179 Jin-Hua C., 1041 Joachimi D., 1021 Jones A. C., 1245, 1249, Jones D. J.. 189 Jones J. R., 379, 185 Jones P. J. V., 805 Jones W., 1737 Jouanneaux A., I319 Jung K., 161 Jung W-S., 949 Kadokawa J-i., 551 Kaharu T., 859 Kahn-Harari A,, 907 Kakkar A. K., 1:'27 Kamath P. V., 1487 Kang J. S., 747 Kang W-B., 157 Karasu M., 551 Kareiva A., 1267 Karppinen M., 1267 Kassabov S., 15:: 11 "3, 1765 1591 1591 Kall P-O., 1293 i Kato C., 519 Lucas V., 907 Munn R.W., 849 Pettiti I., 541, 1641 Kato R., 915, 1219 Luna D., 311 Munro D. C., 1451 Philippot E., 1101 Katsoulis D. E., 337, 1181 Lund A,, 223, 1723 Murakami, H., 1621 Pic0 C., 547 Kaul A. R., 1585 Ma W., 771 Murray K. S., 87 Picone P. J., 571 Kawamura I., 237 MacFarlane D. R., 1149 Myrvold B. O., 1667 Pigois-Landureau E., 741 Kawara, T., 1571 Machida M., 1621 Nagae S-i., 591 Porta P., 197, 541, 1641 Kawasaki, K., 1768 MacKenzie K. J. D., 1595 Nagase K., 1581 Portier J., 1433 Kelly S. M., 1673, 1689 Macklin W. J., 113 Naito T., 1559 Potter F. H., 1647 Kennedy B. J., 87 Mackrodt W. C., 817, 825 Nakajima H., 1325 Pottgen R., 463 Kerridge D. H., 133 1 Madsen H. G., 675 Nakajima T., 853 Povey I. M., 13 Kershaw S., 1233 Maeda K., 585, 1131 Nakamura T., 1377 Poynter R.H., 1205 Khan M. S., 1227 Maeda S., 935 Nakano H., 171 Pozarnsky G. A., 1749 Kijima T., 1621 Maekawa T., 1259 Nakano Y., 1497 Predieri G., 361 Kim H-B., 883 Mahgoub A. S., 223 Nakata Y., 1699 Pressman H. A,, 501, 1313 King T.. 1 Mai S-M., 591 Nakayama C., 631 Prosperi T., 943 Kinoshita M., 915, 1219 Kiyozumi Y., 585 Maignan A,, 1353 Maireles-Torres P., 179, 189 Nakayama S., 663 Nameta H., 853 Qi F., 1041 Qian Y., 1619 Klein M. L., 793 Malandrino G., 1061 Nanjundaswamy Qiu S., 735 Klippe L., 1585 Malet P., 47 K. S., 1627 Rahmat S., 1201 Klissurski D., 153 Malik M. A,, 1249 Narciso F. J., 1137 Raithby P. R., 1227 Knight K. S., 899 Malins C., 1029 Nazar L.F., 1419 Ramis G., 1755 Knowles J. C., 185, 775 Mani R. S., 623 Neal G. S., 245 Ramsaran A., 605, 1143 KO E. I., 651 Mann S., 1387 Neat R. J., 113 Ramsden J. J., 1263 Kobayashi A., 1559 Manning R. J., 1233 Netoff T. M., 1111 Ranasinghe M. G., 1359 Kobayashi H., 1559 Manthiram A., 1627 Neumayer D. A., 1061 Ranlsv J., 867 Kobayashi T., 291 Marcos M. D., 475 Newton J., 869 Ratcliffe P. J., 1055 Koch B., 903 Kohmoto T., 205, 775 Marder T. B., 1227 Marinas J. M., 3 11 Nguyen P., 1227 Nicholson D. G., 1723 Raveau B., 1353 Raynor J. B., 13 Komatsu T., 533, 537 Marks G., 399 Nicol I., 29 Reau J. M., 1433 Komppa V., 585 Marks T. J., 1061 Nielsen K., 867 Reid M., 1149 Kondo K., 1667 Marsden J. R., 1017 Niinisto L., 1239, 1267, Renard C., 1763 Kossanyi J., 139 Kosztics I., 1351 Kouyate D..139 Martin C., 1353 Martin de Vidales J. L., 1635 Nishiyama I., 449, 983 Niwa S-i., 585, 1131 1409 Rettig W., 1021 Reynolds C. A., 1201 Rhomari M., 189 Kristo J., 1255 Martin T. L., 623 Nix R. M., 1403 Richards B. C., 81 Kriitofik M., 271 Maruyama Y., 1377 Nobutou T., 1539 Richardson R. M., 209, Kubono K., 291 Mather G. C., 1303 Nogdmi T., 1559 1205 Kubranova M., 265 Mathieson I, 1157 Nomura R., 51 Rives V., 47, 1117 Kunitomo M., 205, 775 Matsuba T., 599 Nomura S., 171 Roberts K. J., 977 Kunou I., 955 Matsubayashi G-e., 1325 Norman N. C., 891 Robertson A. D., 457 Kuramoto N., 1195 Matsuda H., 51 Nowinski J. L., 1579 Robertson M.I., 29, 119 Kuroda K., 519 Matsuda T., 955, 1497 Nunes M. R., 515 Rockliffe J. W., 331 Kuwano J., 9, 973 Labajos F. M., 11 17 Lacey D., 1029 Lahti P. M., 161 Matsumoto M., 1377 Matsuzaki I., 853 Maury F., 695 Maza-Rodriguez J., 179 Nygren M., 615, 1275 Nykanen E., 1409 O'Brien N., 1511, 1527 OBrien P., 565, 1249, 1611 Rodriguez-Castellon E., 179 Rodriguez-Reinoso F., 11 37 Rojas R. M., 611, 1433, 1635 Laine-Ylijoki J., 1409 Lamble G., 1723 Landee C., 161 McCabe R. W., 1173, 1765 McCarrick M., 217 McCormick A. V., 1749 Ogawa M., 519 Ogura D., 653 Oh-hara S., 1667 Rojo J. M., 1433 Romanovskaya V. V., 373 Ronfard-Haret J-C., 139 Laus M., 429, 437 McGhee L., 29, 119 Ohlmann A., 1021 Rose R. G., 995 Lavela P., 1413 McKeown N. B., 1153 Ohnishi K., 171 Ross A., 119 Lawrence L.W., 571 Lawrenson B., 393 Lawson A. J., 1511, 1521, McMeekin S. G., 29, 119 McMurdo J., 1205 McMurray H. N., 1283 Ohta K., 61, 533, 537 Ohta S., 1503 Ohtaki M., 653 Rossignol S., 1433 Rothlisberger U., 793 Rourke J. P., 1017 1527 McPartlin M., 1201 Oki K., 635 Rowatt B., 253 Lea M. S., 1017 Le Bideau J., 1319 Meakin P., 1149 Meinhold R. H., 1595 Okuno T., 1377 Oliver S. N., 1233 Rowley A. T., 285 Roziere J., 189 Lee C. K., 525, 1441 Mellen R. S., 421 Olivera-Pastor P., 179 Ruiz P., 903 Lee G. R., 1093 Mendonqa M. H., 515 Osterlund R., 615 Rushworth S. A,, 1245, Lee S., 991 Merkelbach P., 615 Overend A. S., 1167 1249, 1591 Lee S-I., 991 Metcalfe K., 331 Owen J. R., 591 Russell D. K., 13 Leece C.F., 393 Lefebvre F., 125 Le Goff P., 133, 875 Metlin Yu. G., 1659 Michel C., 1353 Milburn, G. H. W., 1767 Pac C., 1571 Pagura C., 1255 Painter J., 1153 Ryan T. G., 209 Sadaoka Y., 663 Saez-Puche R., 1307 le Lirzin A., 319, 1047 Miles D. A,, 1205 Pan W-P., 771 Salatelli E., 1035 Leskela M., 1239, 1409 Letouze F., 1353 Miller J. D., 729 Miller J. R., 1201 Pareti L., 361 Parker M. J., 1071 Sanchez Escribano V., 965, 1123 Le van Mao R., 605, 1143 Lewis A. L., 729 Mills G. P., 13 Minaud S., 1763 Parker S. C., 813 Parkin I. P., 279, 285, 1603 Sano S., 275 Sano T., 1131 Lewis J., 1227 Li J., 413 Li R, 773 Li X., 657 Lightfoot P., 167, 1579 Minelli G., 541, 1641 Min-Hua J., 1041 Mirtcheva E., 611 Mishima S., 853 Miura N., 631, 1259, 1581 Parsonage J.R., 399 Partridge R. D., 1071 Patil K. C., 491 Pauson P. L., 1511, 1521, 1527 Santiago J., 679 Santos M. R. M. C., 1479 Sanz J., 1433 Sastry P. V. P. S. S., 647, 1077 Lim G. S., 1441 Linda11 C. M., 657 Lindback T., 413 Lindgren M., 223 Lindqvist 0.. 1101 Little F. J., 167 Liu C-W., 393 Liu S., 379 Liu-Cai F. X., 125 Lo Jacono M., 197 Long N. J., 1227 Lopez M. L., 547 Lorenzelli V.. 965, 1755 Loubser G., 71 Lowe J. A., 771 Miyamae N., 955 Miyasaka H., 1539 Mizukami F., 585, 1131 Mohanty D. K., 623 Monk P. M. S., 1071 Montes M., 1491 Morales J., 1413 Moreau J. J. E., 987 Moretti G., 541, 1641 Morpurgo S., 197 Mouron P., 895 Mozhaev A. P., 1107 Mueller J., 623 Muller W. F., 895 Mun M-O., 991 Payen C., 1319 Payen E., 1343 Pedrosa de Jesus J.D., 1611 Pelizzi C., 713 Peluau S., 1353 Peng N., 1451 Pennington M., 13 Peraio A., 13 13 Percec V., 719 Pereira-Ramos J-P., 133, Perez G., 959 Perez-Jimenez C., 145 Petrov K., 611 875 Sauer C., 1547 Saunders V. R., 825 Sawa H., 915, 1219 Sawada M., 859 Saydam S., 13 Scholten U., 1351 Schoonman J., 689 Sergeev G. B., 761 Seron A,, 669 Sessoli R., 1047 Shabatina T. I., 761 Shacham-Diamand Y., 1533 Shamlian S. H., 81 Sharma V., 703 Shen D., 105 Shen P., 1289 Sheng E., 487, 683, 1189 Sheridan P., 161 Sherrington D. C.. 229, 253, 1511, 1521, 1527 Sherwood J. N., 977 Shimizu A., 1475 Shimizu Y., 1581 Shimokawatoko T., 51 Shiomi D. 915 Shirota Y 171, 599 Shoji H., 1131 Shukla A. K., 703 Silver J., 1201 Simmons J. M., 1205 Simon M..305 Simpson G. S., 1508 Sinclair D C., 445 Singh N., 509 Skakle J. M. S., 1745 Slade R. c'. T., 265, 367, 501, 509. 1313 Slater P. K.. 1463, 1469 Smart S. P., 35 Smith E. G., 331 Smith J. M., 337 Smith M. E., 245 Snetivy D.. 55 Soininen P., 1409 Solano Revnoso V. C., 529 Solzi M, 361 Song S-W.. 1271 Sotani N., 205, 775 Spagna A,. 793Sprik M., 437 Stainton h.M.. 1159 Stedman Ii.J., 641, 707, 1457 Stern C. L , 1061 Stucky G. D., 1111 Styring P.. 71, 1365 Su Q., 417 Suckut C., 5 Sugiyama K., 1497 Sumathipala H. H.. 1075, 1307 Sundholm F., 499 Sutherland I., 487, 683, I189 Suto S., 631 Suzuki T., 631 Suzuki Y., 237 Svensson G., 1293 Swindell J.. 229 Taga T., 291 Tagaya H.. 551 Tajbakhsh A.R., 1017 Takahashi M., 519 Takahashi S., 859 Takanishi Y., 997 Takano M.. 19 Takatoh K , 1365 Takebe Y., 599 Takeda Y., 19 Takeuchi hf., 955 Takeuchi Y., 1497 Takezoe H.. 237, 997 Tamaki J., 1259 Tamura M.. 915, 1219 Tan M. P., 525 Tanabe K.. 853 Tanaka T., 859 Tanguy B., 1433 Tarasconi P., 713 Tateno A,. 1559 Tetley L., 253 Thanapprapasr K., 591 Thanh Vu N., 1143 Thatcher J. H., 591 Thebault J.. 669 Thepot P., 987 Thery J., 007 Thomas H. J., 541, 1641 Thomas J. O., 839 Thomas M. J. K., 399 Thomson J. B., 167 Thorne A. J., 209 Thorne J. R. G., 1157, 1357 Tian M., 327 Ueno T., 1539 Tirado J. L., 1413 Ulibarri M.-A,, 11 17 Toba M., 585, 1131 Underhill A. E., 1233 Tomellini M., 579 Ungar G., 719 Tomkinson J., 1309 Urbana M.R., 311 Tomlinson A. A. G., 943, Uzunova E., 153 959 Vaillant M., 765 Tondello E., 407 van Aken P. A., 895 Toriumi M., 1539 van der Put P. J. , 689 Torres-Martinez L. M., 5 Van Grieken R., 499 Toyne K. J., 747 Vancso G. J., 55 Tretyakov Yu. D., 1585 Veiga M. L., 547 1659 Veringa H. J., 689 Trigg M. B., 245 Viana B., 907 Trindade T., 161 1 Vidgeon E. A,, 399 Trotter J., 1201 Vila E., 1635 Tschierske C., 1021, 1547 Vivien D., 907 Tseung A. C.C., 1289 Volpe P., 1067 Tsuchida T., 631, 1503 Vuijk J. D., 1365 Udagawa T., 1559 Wahl G., 1585 Ueda M., 883 Wakagi A., 973 Ueda Y., 915, 1219 WangH., 417 Wanklyn B. M., 469 Wong Chi Man M., 987 Yi L., 1755 Watanabe J., 1699 Wong K.K. W., 1387 Yogo T., 353 Watanabe T., 537, 1475 Workman A. D., 13, 1337 Yokokura H., 1667 Watson G. W., 813 Wright S. A., 1365 Yokoyama A,, 983 Watson I. M., 1393 Wu Y. M., 1403 Yonehara H., 1571 Watts J. F., 305 Xiao F-S., 735 Yoon Y., 719 Welford K., 1205 Xiao S., 605, 1143 Yoshizawa A., 449, 983 Weller M. T., 921 Xu R., 735, 985 Young S. M., 1511. 1521. Wen J., 327, 1715 xu w., 735 1527 Wen-Tao Y., 1041 Xu Y., 985 Yu H., 327, 1715 Wessels P. L., 71 Yakhmi J. V., 1077 Yue Y., 985 West A. R., 5, 445, 457, 525, Yamaguchi A., 1377 Zammit M. D., 1359 647, 1075, 1303, 1307, Yamamoto H., 635 Zarbin A. J. G., 389 1441, 1745 Yamamoto I., 61, 533, Zhang M., 1619 West D., 1 537 Zhang W-r., 161 Westin G., 615, 1275 Yamamoto O., 19 Zhao L., 623 Williams D.E., 1427 Yamamoto T., 1539 Zheng Q., 1041 Williams G., 23, 1157 Yamazoe N., 631, 1259, ZhouG., 1619 Williamson C.J., 565 1581 Zhu Y., 1619 Winfield J. M., 29, 119 Yang H., 55 Zhuang Z., 1041 Wittmann F., 1227 Yao J., 605 Ziemelis M. J., 1181 Wolf M., 839 Yarovoy Y. K., 761 Zotov N., 611 ... 111 Conference Diary 1994 November 10 Calorimetry and Thermal Methods Applied to Construction Materials London, UK SCI Conference Secretariat, 14/15 Belgrave Square, London, UK, SWlX 8PS Tel: +44 171 235 3681; Fax: +44 171 823 1698 November 10-12 Second Annual Meeting on Biomaterials of GIB (Interdivisional Group on Biomaterials of the Italian Chemical Society) Italy Ledi Menabue, Executive Committee of GIB, Universith Degli Studi di Modena, Dipartimento di Chimica, via Campi, 183-41100 Modena, Italy.Tel: +39 59 378402; Fax: +39 59 373543 November 14-19 IRaP94: Ionizing Radiation and Polymers Guadeloupe, France Ing. Main LeMoe1, SRSIMLPI, CEA CEN Saclay, Bgtiment 466, F-91191 Gif Sur Yvette Cedex, France Tel: +33 16908 5485; Fax: +33 16908 9600 November 15-18 2nd Color Imaging Conference: Color Science, Systems and Applications Scottsdale, AZ,USA Pam Forness, The Society for Imaging Science and Technology, 7003 Kilworth Lane, Springfield, VA 22151, USA Tel: +1 703 642 9090; Fax: +1 703 642 9094 November 28- 1994 MRS Fall Meeting December 2 Boston, MA, USA Mary Materials Research Society, Meetings Department, 9800 McKnight Road, Pittsburgh, PA 15237-6006,USA Tel: +1412 367 3003; Fax: +1412 367 4373 December 1 Waste Immobilisation London, UK SCI Conference Secretariat, 14/15 Belgrave Square, London, UK, SWlX 8PS Tel: +44 171 235 3681; Fax: +44 171 823 1698 December 8-9 European Workshop on Chemical Sensors for Metallurgical Processes Mol, Belgium F.De Schutter, Energy Division, VITO, Boeretang 200, B-2400 Mol, Belgium Fax: +32 14 32 1185 December 15-16 C-MRS and E-MRS Joint Symposium on Electronic and Optoelectronic Materials Beijing, China Professor Yang Xiongfeng, Secretary General of the Symposium, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China. Tel: +86 01 255 8131 ext.321; Fax:+86 1256 2389 December 19-22 1994 International Conference on Electronic Materials (ICEM'94) & 1994 IUMRS International Conference in Asia (IUMRS-ICA) Hsinchu, Taiwan C/o Materials Research Laboratories, ITRI, Conference Department, IUMRS-ICEIWICA'94, Bldg 77,195 Chung-hsing Rd, Sec. 4, Chutung, Hsinchu, 3105, Taiwan, ROC. Tel: +886 35 820064/916801; Fax: +886 35 820247/820262 1995 January 5-6 Plastics for Portable Electronics Las Vegas, NV, USA Dr. hoop Agrawal, Donnelly Corporation, 4545 E. Fort Lowell Road, Tucson, AZ 85712, USA Tel: +l 602 321 7680; Fax +1602 322 5635 0 January29- Advanced Solid-state Lasers February 1 Memphis, TN, USA Optical Society of America, 2010 Massachusetts Avenue, NW,Washington, DC 20036, USA Tel: +1202 223 8130; Fax: +1202 416 6130 February 16 Radiation Curing and Processing of Materials London, UK SCI Conference Secretariat, 14/15 Belgrave Square, London, UK, SWlX 8PS Tel: +44 171 235 3681; Fax:+44 171 823 1698 March British Liquid Crystal Society, Annual Conference Exeter, UK Professor Roy Sambles, Department of Physics, University of Exeter, Exeter, UK, EX4 4QL March 4-11 IWEPNM 95 International Winterschool on Electronic Properties of Novel Materials, Fderides and Fulleroides Kirchberg, Tyrol, Austria Professor H.Kuzmany, Inst. f. Festkorperphysik der Universitat Wien, Strudlhofg. 4, A-1090 Vienna, Austria March 5-10 ECLC 95 European Conference on Liquid Crystals Bovec, Slovenia Dr. Igor MuieviE, ECLC 95, J.Stefan Institute, Jamova 39, P.O.B. 100, 61111 Ljubljana, Slovenia Fax: +386 61 219385/273677; E-mail: igor.musevic@ijs.si Spring March 13-15 April 2-6 April 3-6 April 24-26 April 24-28 May 7-10 May 9-13 0 May21-26 May28-June 2 0 June26-30 July 17-20 July 24-27 August 7-11 August 19-25 August 27-September 1 September 11-14 COLA '95: The Third International Conference on Laser Ablation Strasbourg, France E. Fogarassy, CNRS, Laboratoire Phase, BP 20, 67037 Strasbourg Cedex 2, France Tel: +33 88 10 62 57; Fax: +33 88 10 62 93 Low- and No-VOC Coating Technologies: 2nd Biennial International Conference Durham, NC, USA Ms. Coleen M. Northeim, Research Triangle Institute, P.O. Box 12194, Research Triangle Park, NC 27709-2194, USA Tel: +1919 541 5816; Fax: +1919 541 7155 Seventh Biennial Workshop on Organometallic Vapor Phase Epitaxy Fort Meyers, FL, USA TMS Meeting Services Department, 420 Commonwealth Drive, Warrendale, PA 15086, USA Tel: +1412 776 9000 ext.241; Fax: +1412 776 3770; E-mail: wilson@tms.org ECIO '95: 7th European Conference on Integrated Optics Delft, The Netherlands EC10'95 Secretariat, P.O. Box 5031, 2600 GA Delft, The Netherlands Tel: +31 15 78 1034; Fax: +31 15 78 4046 19th International Power Sources Symposium 1995 Brighton, Sussex, UK T. Keily (Chairman), International Power Sources Symposium Committee, 1Oakley Drive, Fleet, Hampshire, UK, GU13 9PP ICMCTF' 1995: International Conference on Metallurgical Coatings and Thin Films San Diego, CA, USA Mary S.Gray, ICMCTF 95, Suite 502, 1090 G Smallwood Drive, Waldorf, MD 20603, USA Tel: +1 301 870 8756; Fax: +1 301 645 1426 13th International Conference of Fluidized Bed Combustion Kissimmee, FL, USA Leslie Friedman, Meetings Manager, The American Society of Mechanical Engineers, 345 East 47'b Street, New York, NY 10017-2392, USA Tel: +1212 705 7788; Fax: +1212 705 7856 7th International Conference on Indium Phosphide and Related Materials Sapporo, Hokkaido, Japan IEEE/LEOS, 445 Hoes Lane, P.O. Box 1331, Piscataway, NJ 08855-1331, USA Tel: +1908 562 3893; Fax: +1908 562 8434 CLEO/QELS Conference on Lasers and Electro-optics & Quantum Electronics and Laser Science Conference Baltimore, MD, USA IEEmEOS, 445 Hoes Lane, P.O.Box 1331, Piscataway, NJ 08855-1331, USA Tel: +1908 562 3893; Fax: +1908 562 8434 2nd Mediterranean Workshop and Technical Meeting-Novel Optical Materials and Applications Cetraro, Italy Prof.I.C. Khoo, Electrical Engineering Department, Pennsylvania State University, University Park, PA 16802, USA Tel: +1814 863 2299; Fax: +1814 865 7065 loth International Conference on Integrated Optics and Optical Fiber Communication Hong Kong BDG Communications Mgmt Ltd., IOOC'95 Conference Secretariat, Suite 1104-5, East Town Building, 41 Lockhart bad, Hong Kong Tel: +852 528 6136; Fax: +852 865 1528 MC': 2nd International Conference on Materials Chemistry Canterbury, Kent, UK Dr. J. D. Wright, Chemical Laboratory, University of Kent, Canterbury, Kent, UK, CT2 7NH FLC '95 5th International Conference on Ferroelectric Liquid Crystals Cambridge, UK Prof.W.A. Crossland, Northern Telecom Research Professor of Photophysics, University of Cambridge, Cambridge, UK EnDS XI: Eleventh International Conference on the Electronic Properties of Two-Dimensional Systems Nottingham, UK Prof. L. Eaves (EP2DS XI Chairman), Department of Physics, University of Nottingham, Nottingham, UK, NG72RD Fax: +44 115 9 515180; E-mail: ppzpcm@ppnl.nott.ac.uk Clays and Clay Materiale Science: Euroclay '95 Leuven, Belgium Professor P. Grobet, Secretary Euroclay '95, Centrum voor Oppervlaktechemie en Katalyse, K U Leuven, K Mercierlaan 92, B-3001 Heverlee, Belgium. Tel: +32 16 220931; Fax +32 16 295126 ISCOW95 International Symposium on Crystalline Organic Metals, Superconductors and Fernmagnets Mittelberg, Kleinwalsertal, Austria ISCOM'95,Prof.Heimo J.Keller, Anorganisch Chemisches Institut, Universitat Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany Tel: +49 6221 562 438; Fax: +49 6221 564 197 Euro-Fillers 95: International Conference on Fillers in Polymers Mulhouse, France Dr. E. Papirer, CRPCSS-CNRS, 24 av. Pdt. Kennedy, F-68200 Mulhouse, France Tel: +33 89 42 01 55; Fax: +33 89 32 09 96 V September 11-15 Electrochem '95 Bangor, Wales Dr. Maher Kalaji, Department of Chemistry, University of Wales, Bangor, Gwynedd, Wales, UK, LL57 2UW Tel: +44 1248 351151 ext. 2516; Fax: +44 1248 370528 September 11-16 LB7:The Seventh International Conference on Organized Molecular Films Numana, Ancona, Italy Dr.M. G. 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Hamermesh, SAMPE Technical Director, 1161 Parkview Drive, Covina, CA 91724, USA Tel: +1 818 331 0616 ext. 602; Fax: +1818 332 8929 October 18-20 MOC '95: 5th Microoptics Conference Hiroshima, Japan K. Iga, Representative of the Microoptics Group, Optical Society of Japan (JSAP), Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama, Japan 227. Tel: +8145 922 1111ext.2064; Fax: +8145 921 0698 December loth International Conference on Solid State Ionics Singapore B.V. R. Chowdari, Department of Physics, National University of Singapore, Singapore-0511 Tel: +65 772 2956; Fax: +65 777 6126 June 24-28 ILCC: 16th International Liquid Crystal Conference Kent, OH, USA 16th International Liquid Crystal Conference, Liquid Crystal Institute, Kent State University, P.O. Box 5190, Kent, OH 44242-0001, USA. Tel: +1216 672 2654; Fax: +1216 672 2796; E-mail: ILCCl@alice.kent.edu August 4-9 IUPAC MACRO SEOUL '96: 36th IUPAC International Symposium on Macromolecules Seoul, Korea Dr. Kwang Ung Kim, Secretariat of WAC MACRO SEOUL '96, Division of Polymers, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea Tel: +82 2 957 6104; Fax: +82 2 957 6105; E-mail: iupac@kistmail.kist.re.kr Denotes a new or amended entry this month Entries in the Conference Diary are published free of charge.If you wish to include an announcement please send full details to: Journal of Materials Chemistry Editorial Office, Thomas Graham House, Science Park, Milton Road, Cambridge, UK, CB4 4WF Tel: +44 1223 420066; Fax: +44 1223 426017 vi SCI THE INSTITUTE OF Ibe sodety ofMATERIALS -Iadustry NOMINATIONS ARE INVITED FOR THE Beilby Medal and Prize 1995 The Beilby Medal and Prize(ElOO0) are awarded annually to a scientist or engineer to recognise substantial work of exceptional practical significance in chemical engineering, applied materials science, energy efficiency or a related field.Preference is given to candidates under 40years of age. NOMINATIONS MUST BE MADE USING THE FORM OVERLEAF DEADLINE: 31 DECEMBER 1994 This award is sustained by a Trust Fund commemorating Sir George Beilby FRS (1850-1924)who was President of SCI (1898-99),the Institute of Chemistry (1909- 12) and the Institute of Metals (1 9 16- 18), and founding Chairman of the Fuel Research Board. It is administered in rotation by the Institute of Materials, the Royal Society of Chemistry, and the Society of Chemical Industry. Enquiries about the 1995 Beilby Medal and Prize should be addressed to: Carolyn Shaw, Institute of Materials, 1 Carlton House Terrace, London SWlY 5DB (Tel: +44(0)71 839 4071) (Fax: +44(0)71 839 1702) Closing date for receipt ofapplications:31December 1994 Returnto: Beilby Medal 6r PrizeBEILBY MEDAL AND PRIZE 1995 Carolyn ShawNOMINATION FORM Institute of Materials 1Carlton House Terrace Complete ALL SECTIONSin type or BLOCK CAPITALS Photocopies accepted LondonSWlY 5DB Title Forenames Candidate'sSurname Address for correspondence Date of Birth Telephone No Academic and Professional Qualifications Candidate'scareerhistory (current appointment first): Why candidate should be considered for the Beilby Medal and Prize: Attach a list of the candidate's scientific and technical publications and patents. ~~ Letters of recommendationarerequired from tworeferees. State names: The letters may eitherbe attached to this form or sent separately, provided that they are received by the deadline (31 December 1994).

ISSN:0959-9428    

DOI:10.1039/JM99404BP101

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994,4(11), 1659-1665 FEATURE ARTICLE ~~ Chemical Routes for Preparation of Oxide High-temperature Superconducting Powders and Precursors for Superconductive Ceramics, Coatings and Composites Yuri G. Metlin* and Yuri D. Tretyakov Department of lnorganic Chemistry, Moscow State University, Moscow, 7 7 7899, Russia The importance of chemical routes for the preparation of high-temperature superconducting (HTSC) powders is discussed as well as preparative peculiarities of their synthesis by coprecipitation, sol-gel, freeze-drying and spray pyrolysis techniques. The discovery of ceramic high-temperature superconductors has spurred enormous scientific and practical interest to elucidate the nature of the phenomenon’ as well as to find its practical appli~ation.~.~ Very soon it became evident that success in both of these is determined by the techniques used to obtain HTSC materials and, first of all, of powders as precursors for HTSC ceramics, coatings and composites.At first the so-called ‘ceramic technique’ was used. This involved intimate mechanical mixing of the oxides (for the 123 phase: yttrium and copper oxides with barium carbonate) and repeated ‘beat and heat’ cycles to achieve complete solid- phase interaction of the reagents. In some cases the oxides were replaced by the more readily available carbonates, nitrates or other salts. This technique was well established by its use for obtaining all kinds of constructional and functional ceramics, but it has considerable disadvantages.The greatest disadvantage is the highly crystalline (10 pm) nature of the reagents and the need to achieve heterogeneous mixing, necessitating multiple rep- etitions of prolonged thermal treatment and grinding. In this case uncontrolled crystallite growth takes place and, as a consequence, results in chemical and granulometric heterogen- eity of anisotropic HTSC grains, leading to irreproducibility of electrical and magnetic properties. Between 1987 and the present time many investigations into HTSC technologies have been connected with the devel- opment and application of so-called ‘chemical techniques’ for the preparation of powder^.^,^ Chemical techniques increase the homogeneity of the product by mixing the reagents at the molecular level in solution and maintaining this level (more or less successfully) during the following synthetic stages.The oxide powders prepared in this way have high specific surface area and, consequently, readily undergo solid-state interaction and sintering.6 In Fig. 1 the efficiency of chemical techniques of synthesis for increasing the homogeneity of ceramics is illustrated using values of the standard deviation in an analysis of the composi- tion of Y-and Bi-containing ceramics using sputtered neutral mass spectrometry (SNMS).7Low standard deviations imply greater reproducibility of layer-by-layer analysis of bulk samples. In turn the reproducibility is governed by the chemi- cal homogeneity. From the figure it can be seen that chemical routes are preferable.The use of chemical techniques for the preparation of powders may be useful even for the most popular melting techniques that are used to obtain HTSC powders, notwith- standing the considerable smoothing of the morphological differences of powders with different backgrounds due to freeze spray solid hs! dried dried state -6 P Sr 12 I a I 4 OQ single spray solid crystal dried state Fig. 1 Reproducibility (standard deviation, 6)of SNMS data for (a) YBa,Cu,O, --x and (b) Bi,Sr,CaCu,O, -x ceramics complete or partial melting. In particular, in order to elucidate the nature and influence of various pinning centres on super- conducting properties a considerable role must be played by purity or the controlled content (size and distribution) of impurities, and this can be ensured most easily by chemical techniques of synthesis.We shall examine the following techniques: (1) coprecipi-tation techniques, (2) sol-gel processes, (3) spray drying, (4) plasmochemical techniques and (5) cryochemical techniques. The aim of the present examination is to explain the potential of the different techniques and to highlight specific features of their use in the preparation of HTSC powders. At present some families of oxide HTSC phases are known, including La, -,M,CuO, (M =Ba, Sr), Y,Ba,Cu, + + (n= 1660 I I1 I11 Iv V Fig. 2 Portion of Periodic Table emphasizing the position of HTSC-forming elements 0-2), RE2Ba4CU6+n014+n (n=0-2), Bi2(Ca,Sr)n+ lCUno2n+4 (n= 1-3), TIBa2Can-1Cun02n+3or T1,Ba2Can-lCun02n+4 (n= 1-3), PbSr,ACu,06 or Pb,Sr,ACu,O, (A =Y, RE, RE +Sr, RE +Ca), RE2-,Ce,Cu04, Sr, -,M,CuO, (M =Sr, RE) and HgBa,Ca, -1C~n02n+ ,.They are all complex oxides and include oxides of copper, alkaline-earth elements and oxides of yttrium, rare-earth elements, bismuth, thallium, lead or mercury. Differences in properties of components of HTSC phases conditioned by their position in the Periodic Table (Fig.2) makes it impossible to produce a unified synthesis pattern even for a single synthetic technique. At the same time, the presence of such components as copper or alkaline- earth metals in all HTSC phases allows a prognosis to be made regarding the applicability of different techniques. It is evident that coprecipitation using ammonia can disturb copper stoichiometry, and coprecipitation in the form of carbon-containing salts is always accompanied by the forma- tion of alkaline-earth-metal carbonates and by prolonged times of thermal treatment.Losses of some components of the HTSC phases (thallium, mercury, lead) due to their high volatility can be decreased by treating a complex oxide precursor synthesized by one of chemical techniques in a vapour of the volatile component. This technique was devel- oped for thallium systems' and was shown to be highly efficient for the synthesis of mercury-containing HTSC9 materials. In this review most attention will be paid to the synthesis of Y-and Bi-containing phases as these are the most com- monly used and investigated phases.Note that the attribution of a synthesis technique to one of above-cited classes is purely nominal (especially in case of coprecipitation and sol-gel techniques) and is determined by the author's point of view. Also, we shall consciously limit consideration of powders as precursors because many properties of HTSC ceramics are determined not only by the method of preparing the powders, but also by the conditions of posterior thermal treatment, which are frequently not optimal for a given powder prehistory. Chemical Coprecipitation Techniques Coprecipitation techniques have a wide application in the synthesis of various kinds of ceramics.Therefore it is not surprising that they were among the first chemical techniques to be used to prepare HTSC powders. By using the correct J. MATER. CHEM., 1994, VOL. 4 experimental technique in some cases it is possible to obtain reproducible homogeneous disperse salt mixtures with the desired ratio of cations. In the ideal case (which is not achieved because of the different chemical nature of the components) the optimal conditions are where the precipitation of cations from solution occurs simultaneously and with the same rate. The most common salts for obtaining HTSC precursors are oxalates and carbonates. Coprecipitation of Oxalates To precipitate oxalates the or acetate^""^ are used as one initial reagent and then mixtures of oxalic acid- ammonia, oxalic a~id-triethylarninel~'~ or a saturated solu-tion of ammonium oxalate, with fixed solution acidity (pH), or an aqueous solution of dimethyl oxalate12 are used as precipitants.The precipitation process is complicated by the very different dependence of solubility on pH of yttrium, barium and copper oxalates and the concentrations of initial reagents.1°J8J9 Individual oxalates of yttrium and barium are precipitated completely by pH >2.5 [Fig. 3(a)] and their solubility depends on the initial concentration of nitrate; for yttrium oxalate, e.g.,the solubility is at a minimum at lo-' mol I-' [Fig. 3(b)]. Under the same conditions only 60% pure copper oxalate precipitates, but by simultaneous precipitation with yttrium and barium oxalates in the pH limit 2.38<pH<2.53 it is possible to precipitate up to 90% of the cations from solutions 100 80 60 I I O\ i 5 7 10 20 c/l 0-3 mol dm3 Fig.3 Coprecipitation yield (w) of Y, Ba and Cu oxalates from individual [(a),(b)] and ternary (c) solutions as a function of pH [(a),(c)]and initial concentration of yttrium (b)." J.MATER. CHEM., 1994, VOL. 4 ccutsr 4 Fig.4 Dependence of metal ion residuals in the filtrate on the triethylamine :oxalic acid ratioI7 with concentrations [Y] = lop2mol l-', [Ba] =2 x mol 1-', [Cu]=3 x lop2mol 1-1 [Fig. 3(c)]. A detailed analysis of the factors that affect complete precipitation of oxalates in the synthesis of Bi-containing HTSC in ref. 15 (for a phase with Bi:Sr:Ca:Cu=1:1:1:2) yielded the following recommendation: a mixture of 1moll-' solutions of strontium, calcium and copper nitrates (14, 10 and 20 ml, respectively) and 10ml of a 1mol 1-l solution of bismuth nitrate must be added simultaneously with strong stirring to 102 ml of 0.5 mol 1-' solution of oxalic acid.The suspension obtained must be diluted to 900ml with water and added to 110ml of 1moll-' NaOH (pH 5). The precipi- tate formed must be filtered off, washed with water and then washed three times with acetone and dried at 80°C (12 h). The oxalate mixture obtained has a specific surface area (up to 90 m2 g-l) and consists of agglomerates (1-10 mcm, average size 3.8 pm) of fine (0.25 pm) particles. Coprecipitation of oxalates with a triethylamine-oxalic acid mixture has been used for the preparation of precursors in Yl4*I6and Bi" systems.Precipitation is achieved by adding (dropwise) a mixture of nitrate solutions to the cooled (0°C) precipitant solution. The dependence of the cation concen- tration after precipitation on the molar ratio of triethylamine to oxalic acid is shown in Fig. 4. It is evident that complete precipitation is possible only for a ratio of 2.2: by using lower values the precipitate is depleted in calcium and strontium and by using higher ratios the precipitate is depleted in copper and, partially, in bismuth. Using an aqueous solution of dimethyl oxalate12 as precipi- tant from acidic (pH0.9) solution it was possible (by continu- ous stirring for a week) to precipitate completely a precursor for the synthesis of the 2223 Bi phase.Quantitative precipi- tation of oxalates is also achieved by adding excess (three times) of boiling ethanolic solution of oxalic acid to a hot solution of the corresponding nitrates." Oxalate powders obtained after filtration and drying are characterized by particles of size 0.3-0.5 pm and specific surface area up to 40m2 g-'. Heating the coprecipitated oxalates (dried at 20 "C) at 6 K min-' causes the following processes to occur:l' 65 "C, two-stage dehydration of barium oxalate; 200 "C, decomposition of copper oxalate to oxide; 230 "C, decomposition of barium oxalate to carbonate; 355 "C, decomposition of yttrium oxalate to carbonate; 495 "C, decomposition of yttrium carbonate to oxide.Complete decomposition of barium carbonate occurs when the precipitate is heated to 950°C. As expected, the thermal decomposition of oxalates occurs before the formation of carbonates. From this point of view the process of coprecipi- tation has few advantages over the ceramic technique, but the high degree of homogeneity achieved by coprecipitation and the absence of melting by thermal treatment allow the forma- tion of a single-phase product to be achieved more rapidly. Coprecipitation of Carbonates In principle, coprecipitation of carbonates is analogous to that of oxalates, the difference being that in oxalatc precipi- tation the main attention is paid to complete precipitation of copper, whereas for carbonates the most soluble component is the barium salt.In the coprecipitation of carbonate HTSC precursors an excess of ammonium hydrogen car bonate,21 sodium carbonate22 or trimethylammonium carbonate23 are used as precipitants. Coprecipitation is achieved at pH >8 by the addition of ammonia or sodium hydroxide solution. In the latter case, as well as by precipitation with sodium carbonate, the precipitate must be washed carefullj because alkali-metal impurities impair the properties of HTSC mate- rials (this process may be accompanied by selective dissolution and deviation from stoichiometry). Note that precipitated cations are present in precipitate not only as carbonates, but also as hydroxides. The filtered and dried carbonate-hydroxide mixture consists of 1-3 pm agglomerates of very fine (< 50 nm) particles, and the precur- sors obtained after thermal decomposition have a grain size of ca.0.3 pm.23 Decomposition of barium carbonate by heating the hydroxide-carbonate mixture begins at 600 "Cand a single Y-123-phase is formed after the mixture is annealed at 935 "C for 24 h. Untraditional Coprecipitation Techniques In addition to the techniques of coprecipitation for obtaining HTSC precursors mentioned above some other techniques were of limited usefulness: coprecipitation of hydroxides from acetate solutions with organic amine~,~~ coprecipi tation of hydroxides with alkalis,25 coprecipitation of tartrates with tartric acid26 and coprecipitation of carbonaceous pr-ecursors from nitrate solutions by addition of aqueous sodium hypon- itrite, Na2N202 .27 Sol-Gel Techniques Citrate Technique The variation of the sol-gel technique detailed below was proposed in 1967,28 but it has gained acceptance only for the synthesis of HTSC powders.The technique is based on the ability of hydroxyacrds (citric acid, for example) to chelate metal ions. Heat treatment (100-140 "C) of the chelates with polyfunctional alcohols (e.g. ethylene glycol) yields oligomers by esterification. Further heat treatment ( 180-200 "C) causes further polymerization and a viscous resin (gel) is formed, the decomposition of which yields an oxide powder (Fig. 5). Supposing that com- plexes of citric acid and metal ions are homogeneously distributed in solution and this distribution is conserved after polymerization, the homogeneity should not be a1 tered by decomposition.Therefore sol-gel techniques are recom-mended for work directed at the elucidation of the influence of impurities and/or substitutions on the properties of HTSC materials. Apart from these considerations, the technique is not expensive because it requires no apparatus (absence of centri-fugation, filtration, washing and drying operations), and nitrates are most frequently used as the initial reagtsnts. The possibility of controlling the viscosity of the sol obtained (by varying the ratio of components, the time and temperature of polymerisation, Fig. 6) allows the technique to be considered for preparation not only of powder, but also of thick films, fibres and flat ceramic forms.29 The powders obtained possess a high activity, which allows lower temperatures .ind pro- cessing times to be used for synthesis and sintering.J. MATER. CHEM., 1994, VOL. 4 metakitrate complexes 30 20 &r 10 0 ) 2 4 8 10 12 tlh wt.% Fig. 6 Viscosity of citric acid-ethylene glycol ( 1:4) solution as func- tion of heating time (a) and of weight concentration of salts (for YBa,Cu,O,-, synthesis) (b)29 The sol-gel process is carried out as follows. To a mixture of aqueous nitrates and ethylene glycol (to which ammonia is sometimes added to increase the pH to 3-5,') a solution of citric acid is added in ratio of 1 equivalent of acid to 1 equivalent of metal.Commonly ethylene glycol is in excess because hydroxy groups stabilize metalkitrate complexes in solution and favour the formation of oligomers. A specific problem encountered in the synthesis of Bi- containing HTSC is the stabilization of bismuth cations in soluble form. It is known that Bi3+ cations are readily hydrolysed in aqueous media to form insoluble oxynitrates. To keep bismuth in soluble form it is necessary to add an excess of nitric acid, which in turn suppresses the dissociation of citric acid and disrupts the formation of stable metal- citrate complexes. This problem was resolved for the sol-gel technique in the following way.,' Solutions of bismuth nitrate metaCdiester complexes 120 "C oligomers polymerized complexes Fig.5 Scheme of the chemical processes involved in the citrate sol-gel synthesis of ceramics in ethylene glycol and aqueous citric acid were mixed in the ratio of 3.96 g of citric acid to 10 g of Bi( NO,), * 5H20(+50% excess of ethylene glycol to the water required for dissolution of citric acid). The excess of ethylene glycol, which has two hydroxy groups with strong complexing activity, stabilizes the bismuth cations: despite the low acidity a precipitate does not form. The solution obtained is then mixed with a solution of nitrates of lead, strontium, calcium and copper in an ethylene glycol-citric acid mixture. Dehydration and concentration are achieved by prolonged heating (several hours) at temperatures not exceeding 140 "C.The process is accompanied by segregation due to redox processes in the system. Note that the colloidal solution formed after evolution of NO, is unstable, as shown by local evolutions of NO,, the number of which is dependent on the temperature and time of aging.32 Heating the mixture at ca. 190"C causes polymerization to occur accompanied by an increase in the viscosity and a change in colour from blue-green to brown-black. The size of the colloidal particles in the gel does not exceed 10nm according to electronic microscopic data.33 After the mixture is cooled a light-brown mass is obtained, which is transformed by grinding and heating into a mixture of oxides. The activity that the oxide powders obtain after decompo- sition has been proved3, by synthesis of single-phase Y-123 immediately after annealing at 800 "C (5 h), i.r.considerably lower than the temperatures used in ceramic synthesis, and in ref. 34 where the Y-124 phase was obtained after sintering the sample at 815 "C (120 h). Kakihana et compared the homogeneity of Y-123 ceramics prepared by the sol-gel and ceramic routes by X-ray and Raman spectroscopy. The ceramics were prepared by sintering pressed powders at 940 "C (15 h), cooling to 400 "C, annealing at this temperature (22 h) and further cooling in a J. MATER. CHEM., 1994, VOL. 4 furnace. The ceramics were single phase (X-ray). That the ceramic produced by the sol-gel method was more homo- geneous was proved by the absence of the characteristic bands of BaCuO, (580 and 640cm-') and Y,BaCuO, (390 and 605 cm-') in the Raman spectra.Comparison with standards showed that the impurity content did not exceed 0.5 mass YO. Moreover the authors note that the size of grains is regular (3 pm) in ceramics prepared by the sol-gel route. Conversely the grains produced by the ceramic route were not homo- geneous in size and there was evidence (in photomicrographs) that local melting had occurred during sintering, probably due to the initial chemical heterogeneity of the powder. The sol-gel technique was successfully used for synthesis of precursors for lead-family HTSC.36 Hydrolysis of Metal Alkoxides An alternative technique, frequently classed among 'sol-gel' processes, is the so-called alkoxide technology.It is based on the preparation of powders (or thin films) by slow hydrolysis of a mixture of solutions of metal alkoxides. The method is suitable for the preparation of small quantities of extremely pure and homogeneous powders, as well as fibres, films and ceramics.37A disadvantage of the technique is the inaccessi- bility and expense of the initial reagents. In addition, a specific problem for HTSC production is the difficulty of preparing homogeneous mixtures of alkoxides because there are no readily available copper alkoxides that are soluble in common solvents. A method for increasing the solubility of copper in alcoholic solution has been propo~ed~~,~~ and involves boiling the copper alkoxide in an alcoholic solution of dimethylamino- ethoxide. The compound formed (Cu [OC2H4N(CH3),], 1 is readily soluble in isopropyl alcohol (up to 10.5% of metal by weight ).Hydrolysis of the mixture of alkoxides is carried out at reflux by slow addition of aqueous alcohol (25%). After the solvent has been distilled off at low pressure an amorphous product is obtained, the thermal dEcomposition of which at 700 "C gives a fine (ca. hundreds of A) mixture of yttrium and copper oxides and barium carbonate. Formation of the Y-123 phase was observed at 800 "C and a single-phase product was formed at 850°C (air, 3 h). Despite the fact that the alkoxide technique has no advantage from the point of view of tempera-ture lowering and decreasing the time of synthesis, it may be useful for the preparation of coatings by the sedimentation technique.Thermal Decomposition of Solutions Pyrolysisof Aerosols None of the chemical techniques for obtaining HTSC powders has such wide application as the pyrolysis of aerosols. This is a widespread technique for the preparation of fine disperse active powders for the production of HTSC materials. The essence of the technique consists of the following: using an ultrasonic pulveriser a mixture of salt solutions is transformed into an aerosol with a particle size of 0.5-0.8 pm and then carried by a flow of carrier gas to a hot chamber where instant (complete or partial) decomposition of the particles takes place.The oxide product is collected on a filter. Mixing the components at the atomic level in solution and the practically instant dehydration and decomposition of microdrops of the aerosol allow a homogeneous product to be obtained avoiding the multiple grinding and annealing that characterise the ceramic technique and make the product impure and lead to uncontrolled growth of the grains. Nevertheless, the powders obtained may be contaminated by hdbw dried 123 crystallites t nitrate particles mixed oxides t nitrates secondary densritlon I grain growth % sold singlecrystal solid 123 particle particle (polycrystalline) Fig. 7 Mechanism for formation of solid reacted particles fi om aero- sol droplets4' material from the pulverising chamber (high temperature, presence of free acid); in addition, to avoid formation of barium carbonate, it is necessary to purify large volumes of carrier gas (oxygen) from impurities (CO,).The parameters that determine the quality and properties of the powders are, first, the concentration of the l;olution, the temperature of the reactor and the time the particles spend in the hot zone of the reactor. This time is determined by the aerodynamic conditions of the reactor (geometry, gas For example, relatively low temperatures (700-800 "C) and short heating times lead to the formation of porous particles (25-50 nm) and agglomerates of such particles that make their condensation difficult by following sintering, whereas at 1000 "C and times of ca.15 s it is possible to obtain monocrystalline (not agglomerated, 100- 250 nm) and practically single-phase particles (Fig. 7).40 By using solutions of concentration 0.03 mol 1-l the ceramic obtained has an average grain size of 0.75 pm and from solutions of concentration 0.45 mol 1-' (with the same thermal history) average grains sizes of 2 pm are ~btained.~, The most widely used salts for this technique are nitrates. Their use allows the Y-124 phase to be obtained by annealing at 750 "C for 24 h (Po, =1atm)40 and a high content of the Bi-2223 phase to be obtained even after 16 h of annealing at 750°C.41 The Y-123 phase was successfully preparecl by the pyrolysis of aerosols of citrate complexes with ethylene f~rrnates,~~glycinate (alanide)-nitrate45 and acetate- nitrate46 mixtures.In the last case, by the use of an optimal component ratio (oxidant :reducer) it is possible to perform the decomposition in one step (a peculiar version of self- propagation synthesis) at a substantially lower temperature (Fig. 8). Plasmochemical Technology Plasmochemical technology of preparation of powder precur- sors for HTSC ceramics is, in general, analogous to that of pyrolysis of aerosols. The technique is based on the introduc- tion of an initial solution (concentration up to 40g 1-') by excessive pressure into a flow of low-temperature plasma formed by several plasmotrons (the output of a single plasmo- tron can achieve tens ~fkW).~~ There the dispersed solution decomposes into solid (powder) and gaseous (water vapour, nitrogen and oxides of carbon) phases, which are captured and absorbed by corresponding filters.Argon or air are commonly used as the gas-forming plasma. The powders prepared have a size of 1-3pm and specific surface areas of 5 m2 g-' and they are a mixture of copper and yttrium oxides and barium nitrate. In addition to the disadvantages of the aerosol pyrolysis technique, plasmochem- J. MATER. CHEM., 1994, VOL. 4 = O 200 400 600 I I I 400 600 TI'C Fig. 8 DSC curves for the decomposition of all acetate precursors of the Y-123 phase (a) and the acetate-nitrate (6: 7) mixture (b)45 ical technology requires a great expense of energy and of carrier gas (tens of litres per min) and does not compensate for this in terms of product quality.Cryochemical Technology The disadvantages inherent in the majority of chemical tech- niques for the synthesis of HTSC powders (in both Y and Bi systems) can be eliminated by using cryochemical technology (spray-free~ing/freeze-drying).~~These are based on prep-aration of a finely dispersed and highly homogeneous salt (and then oxide) precursor using fast freezing of a finely dispersed solution of salts (obtaining a cryogranulate) and subsequent sublimation of water. It is necessary to carry out the experiment under conditions which avoid physicochemical processes leading to disturbance of the chemical and granulo- metric homogeneity of product. Such processes include: (1) stratification of dispersed microdrops into regions rich and poor in solvent due to insufficiently high cooling rates [this can be avoided if the solution is dispersed not into liquid nitrogen (or in cooled hexane) but onto a massive metal plate cooled to liquid-nitrogen temperature]. (2) Melting of the cryogranulate in the course of freeze-drying due to formation of low-melting eutectics (in the case of nitrate solutions there is a eutectic of H,O-HNO, with T,= -43 "C).To remove this problem it is necessary to try to exchange nitrate solutions for acetates4' (but in this case the problem of formation of carbonates and their decomposition arises), or for nitrate- nitrite solutions,50 or to use dilute solutions of nitrates with low acidity (0.1 mol 1-1 Bi at pH 0.7) by synthesis of the Bi( Pb)-2223 phase.'l (3) Segregation of components in the freeze-dried product (containing up to 3 mass YOof water) during its thermal treatment due to the melting of hydrates, Ca(NO,), .4H,O (42 "C) and Cu( NO,), 3H,O ( 112"C).In this case the drying must be carried out in a thin layer at slow (5 K h-I) heating to 125 "C in an argon flow.51 (4) Undesirable formation of carbonates during annealing due to the presence of CO, in the atmosphere. It is not sufficient in this case to use air that is cleansed of carbon dioxide; it is more efficient to achieve the necessary partial pressure of oxygen by mixing oxygen and argon in the required ratio. (5) The occurrence of solid-state interactions under conditions that exclude the formation of liquid phases.This is especially vital in the synthesis of Bi-containing HTSC because the 60 80 100 120 140 160 TIK Fig. 9 Temperature dependence of the normalized resistivity for the Bi(Pb)-2223 phase ceramics with different prehistory: 1, solid state; 2, sol-gel; 3, free~e-drying~~ temperature of formation of liquid phases in the process depends very strongly on partial pressure of oxygen (836 "C at po,)= 1 atm, 808 "C at 0.2 atm, 772 "C at 2 x lo-* atm). In addition, it is rec~mmended~~?~~ that the salt product is introduced into a furnace that has been pre-heated to the necessary temperature. Use of finely dispersed systems (hundreds of A) and highly homogeneous precursors prepared by cryochemical tech-niques allowed HTSC phases which are not easily prepared by other techniques to be obtained.Thus the Bi-2223 phase was synthesized from a 'cryochemical' precursor after 12 h of annealing" (cf. 200-300 h annealing using traditional technol- ogies), and the Y-124 phase was prepared without the use of a high pressure (up to 100atm) of ~xygen.'~,~~ which is normally necessary. A comparison of the kinetics of phase formation of the Bi- 2223 phase from different precursors allowed the following series of precursor activity to be e~tablished:'~ cryochemical technique (nitrates) > thermolysis of solutions (nitrates) -> citrate technique > ceramic technique As expected, precur- sors without carbonates have an advantage, and for salts having the same chemical nature the advantage lies with the technique that allows a greater degree of homogeneous mixing of components (Fig.9).54 Conclusions In this article we have tried to discuss the advantages (and disadvantages) of an array of chemical routes for HTSC powder preparation. Summarizing, one can conclude that the advantages of chemical routes, compared with the conven- tional ceramic route, are most often exhibited by: (a) lower temperatures and times of heat treatment for preparation of single-phase samples; (b)the possibility of synthesis of phases which cannot be prepared directly by the ceramic route; (c) more chemical and granulometric homogeneity of the samples; and (d) better physical (electric and magnetic) properties of HTSC ceramics.Of the chemical routes one can also list preferences. In particular, the comparison of kinetics of phase formation of the Bi-2223 phase from different precursors allowed a series of precursor activity to be established. References 1 V. L. Ginsburg, Superconductivity: Physics, Chemistry und Technique (Russia), 1992,5, 1. 2 D. Larbalestier, IEEE Trans. Magn., 1988,24, 71 1. 3 R. Simon, Solid State Technol., 1989,32, 141. 4 H. S. Horowitz, R. K. Bordia, C. C. Torardi, K. J. Morrissey, M. J. MATER. 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Kodas, K Ott and D. Kroeger, J. Aerosol Sci., 1991,22,601. T. L. Ward, S. Lyons, T. T. Kodas, J. Brynestad, D. MI. Kroeger and H. Hsu, Physica C., 1992,200,31. N. Tohge, M. Tatsumisago, T. Minami, M. Adachi, Y Kowaka and K. Okuyama, J. Am. Ceram. SOC., 1991,74,2117. S. C. Zhang, G.L. Messing and W. Huebner, J. Aerosol Sci., 1991, 22, 585. J. Block and L. E. Dolhert, Muter. Lett., 1991,11,334. K. Kourtakis, M. Robbins and P. K. Gallagher, J. Solid State Chem., 1990,84,88. K. Kourtakis, M. Robbins and P. K. Gallagher, J. Solid State Chem., 1989,82,290. H. Zhu, Y. C. Lau and E. Pfender, J. Supercond., 1990,3,171. Yu. D. Tretyakov, N. N. Oleynikov and A. P. Mojaev, Usnovy Kriochimicheskoy Technologii (The Basics of Crj ochemical Technology), High School, Moscow, 1987 (Russian). M. Matsuda, Y. Ogawa, Y. Aihara, K. Yamashita and T. Umegaki, J. Am. Ceram. SOC., 1993,76,1618. A. P. Mojaev, V. I. Pershin and V. P. Shabatin. J. All-Union Mendeleev Chem. Soc., 1989,34,504. 25 26 27 28 Lett. B, 1989,3, 301. G. N. Novitzkaya, V. S. Flis, V. M. Pan, S. V. Polyanetskaya and K. P. Daniltchenko, Ukranian Chem. J. 1991,57, 586 (Russian). Q. Xu, L. Bi, D. Peng, G. Memg, G. Zhou, Z. Mao, C. Fan and Y. Zhang, Supercond. Sci. Technol., 1990,3, 564. S. Hirano and T. Hayashi, Thermochim. Acta., 1991,174,169. M. Pechini, US.Pat. 3 330 697, July 11, 1967. 51 52 53 54 P. Krishnaraj, M. Lelovic, N. G. Eror and U. Balazhandran, Physica C, 1993,215,305. K. Takahashi, T. Ito, H. Yoshikawa and A. Hiraki, Jpl. J. Appl. Phys., 1993,32, L1211. S. X. Dou, H. K. Liu and C. C. Sorrell, Muter. Forum, 1990,14,92. K. Song, H. Liu, S. Dou and C. Sorrell, J. Am. Ceram. Soc., 1990, 73, 1771. 29 S. C. Zhang, G. L. Messing, W. Huebner and M. Coleman, J. Muter. Res., 1990, 5, 1806. Paper 4/02098G; Received 8th April, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401659

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(11), 1667-1671 Pre-tilt Angles as a Function of Polyimide Composition for Copolyimides Hisao Yokokura, Bernt 0. Mywold,*+ Katsumi Kondo and Shuichi Oh-hara Hitachi Research Laboratory, I-I Oh-mika-cho, 7-chome, Hitachi-shi, 319-12 Japan Six series of copolyimides have been synthesized. The pre-tilt angle has been measured as a function of the coinposition of these copolyimides. A linear relationship was found between the pre-tilt angle and the composition when both the simple polyimides are amorphous. When one or more of the simple polyimides is semi-crystalline there is normally a strong deviation from linearity. The copolyimides will then usually give pre-tilt angles lower than expected from an average of these for the two simple polyimides.There have been a few investigations relating the pre-tilt angle of the nematic director to the properties of the polyimide alignment layers,'-" but most of them do not give complete structural Most studies relating the structure of the polyimide or polymer to the pre-tilt angle use very simple polyimides, usually one diamine and one tetracarboxylic acid anhydride are combined. For a polyimide to be useful for display manufacturers it is not enough that it gives the required pre-tilt angle. The adhesion to IT0 and glass should be good, the relative permittivity should be low and the curing temperature should also be low for easy fabrication. For safe handling it should be soluble in non-aggressive solvents, for easy production the solvents should also be non-hygroscopic.To fulfil all (or as many as possible) of these requirements, polyimides for real applications are normally copolyimides with a mixture of several diamines and tetracarboxylic acid anhydrides. The purpose of this investigation is to ascertain whether the alignment properties of copolyimides can be found as a simple (linear) combination of the alignment properties of the individual simple polyimides. In this study simple copolyimides are used, either with one tetracarboxylic acid dianhydride and a mixture of two diamines, or with one diamine and a mixture of tetracarboxylic acid dianhydrides. Experimenta1 The polyimides in this study were all synthesized by mixing the two diamines and dissolving them in N,N-dimethylaceta- mide (DMAc) or N-methylpyrrolidone (NMP).A slurry of the acid dianhydride (either pure or a 1: 1 mixture) in the same solvent was added to the diamines. We used a 1 :1 ratio of the total diamine content to the total acid dianhydride. The resulting polyamic acids were spin coated on to clean TTO-covered glasses. The final cures were at 250 "Cfor 30 min. The different diamines and acid dianhydrides used in this study are shown in Fig. 1 together with their abbreviations. All the polyimides were buffed under standard conditions and thick cells with anti-parallel rubbing directions were made. Details of the cell preparation have been given previously.l1 The pre-tilt angles for the director of these cells were determined by the crystal rotation method.$ The nematic mixture ZLI-1132 based on cyanophenylcyclohexyls was used in all cases, and the measurement performed at room tempera- ture.The refractive indices for ZLI-1132 are n,= 1.635 and no=1.493. The standard deviations for the pre-tilt angles of the samples used are 0.1-0.3" (calculated from two identically t Present address: University of Oslo, Department of Chemistry, P.O. Box 1033 Blindern, N-0315 Oslo, Norway. dianh ydrides diamines 0 H2N-Q-NH2 0 0 PPDPMDA 0 DDE 0m o 0 0 H2N-NH2 BTDA MOD 0 O@ \/ \/ 0 0 S-BPDA AH2 ADA F-BAPP Fig. 1 Compounds used in this study together with their standard abbreviations. PMDA =benzene- 1,2,4,5-tetracarboxylic acid dian-hydride (pyromellitic acid dianhydride), BTDA =4,4'-carbonylbis (benzene-1,2- dicarboxylic acid anhydride), s-BPDA =3,3',4,4'-biphe-nyltetracarboxylic acid dianhydride, PPD =p-phenylenediamine, DDE =4,4'-diaminodiphenyl ether, MOD =1,s-diaminooctane, HMD =1,6-diaminohexane, ADA =4,4-diamino-Y-rnethyltetraphenyl-methane, F-BAPP =2,2-bis[4-(4-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane. treated cells with three measuring points for each cell).This is also the typical standard deviation found from a large number of ~amples.~ Theory We have recently pointed out that the pre-tilt angle determined by the crystal rotation method is really the pre-tilt angle of the nematic director, i.e. the symmetry axis, of the nematic pha~e.'.'~,~~It is, in general, not possible to derive the orien- tation of the molecules at the surface from this measurement, particularly as the molecules at the surface show a wide azimuthal distrib~tion,'~-~~ and probably also some polar distribution. The complete derivation of the theory is published else- where.13 We have pr~posed~~~'~ that the pre-tilt angle of the symmetry axis can be related to the orientational distribution at the surface as follows: where bobsis the observed pre-tilt angle of the symmetry axis in the bulk, a, is the in-plane order giving the difference in tendency for the molecules to lay parallel as opposed to anti- parallel to the rubbing dire~tion,'~*'~ Po is the (polar) angle between the optical axis of individual molecules and the surface. a is the angle between the optical and mechanical axis of individual molecules, d is the cell gap, d* the thickness of a surface layer where the averaging from the surface distribution to the bulk orientation takes place and zis any net deformation of the ~urface.'~*'~ For mild rubbing con-ditions the last term can be neglected.The surface layer is less than 1 pm thickI7 as determined by optical methods. For the 120 ym thick cells used in this study the surface layer can be neglected and eqn. (1) simplified to: a has been estimated for a number of single compounds18 and is usually between 1 and 10". In this study the fluid used for the measurements is kept fixed, cc is then a constant and the differences seen in observed pre-tilt angles will be a result of changing in-plane order (a,) or polar angle between the first monolayer of mesogens and the surface (Po).The linear relationship between a,,measured by second-harmonic gener- ation (SHG), and bobs, measured by the crystal rotation method, has already been des~ribed.'~,~' We will show that by using eqn. (2) together with commonly accepted relation- ships between the structure and crystallinity of polymers we can rationalise our experimental results for several series of pol yimides. Semi-crystalline polymers have a higher degree of order than amorphous polymers. This will also translate to a higher degree of order in the surface layer. Hence a higher a, is found for semi-crystalline polyimides than for amorphous ones. According to eqn.(1) and (2) this will give a higher pre- tilt angle. For the series of dialkylene pyromellitimides it was found that the amorphous propylene and pentylene PMDA gave in-plane orders of 0.20 and 0.18, respectively, while the crystalline butylene and hexylene PMDA gave a, values of 0.23 and 0.35 for the liquid crystal 8CB.19 8CB and related liquid crystals usually form dimeric pairs in the bulk. Also at the surface there will be a monomer- dimer equilibrium. This equilibrium is sensitive to the nature of the surface, as evidenced by the different strengths of the SHG signal with different alignment layers. The dimeric pair will, by definition, give a,=0, because the two members of the pair are pointing in opposite directions.The SHG signal originates from non-centrosymmetric species, e.g. the mon- omers. The in-plane order derived from SHG measurements will thus be an upper limit, as the contribution from the dimeric pairs is not observed. Taking this into account a much stronger odd-even effect for the in-plane order has been found." For the crystalline butylene, hexylene and octylene J. MATER. CHEM., 1994, VOL. 4 derivatives of PMDA a, was found to be 0.15, 0.29 and 0.22, respectively. The amorphous pentylene and heptylene deriva- tives gave lower values (0.05 and 0.1, respectively). Although the exact values for a,depend on the liquid crj.stal used, the relative differences between different substrates have been the same for all liquid crystals or liquid-crystalline mixtures used so far, as long as the liquid-crystalline compounds have a cyano group at one end.'4,'5,'8,20 The angle Po is difficult to predict.It will depend on a balance between the dipolar forces and the dispersive forces. The dipolar forces will favour a high Po, with any polar end groups as close to the surface as possible. The dispersive forces on the other hand will favour a low Po with as close contact with the whole of the molecule as possible. Two crystalline surfaces investigated with a wide variety of nematic liquid crystals showed the same trends and a good correlation between the pre-tilt angles found for a given fluid on the two different surface^.^ It thus seems that the very specific steric interactions, at least to a first approximation, can be neglected.The same was found for three different amorphous polyim- ides.3 Although, there was no correlation between the amorph- ous and crystalline polyimides. Results and Discussion Amorphous/Amorphous Polyimides Linear, slim and symmetrical groups along the polymer chain favour crystallinity, while branched, bulky and unsymmetrical groups will make packing more difficult and lead to amorph- ous polymers.21 Statistical copolymers are expected to be amorphous as the packing is difficult for these polymers, too." Fig. 2 and 3 show two systems with the long and strongly bent F-BAPP as one of the components. In Fig. 2, the bulky ADA is the other component, and the pre-tilt angle is nearly constant.Neither of these components facilitates crystallis- ation, and both the 'pure' polyimides (0 or 100% of one diamine) and the copolyimides are expected to be amorphous. There is thus no drastic change in the in-plane order across this system. a, in eqn. (2) will be low and fairly constant across this series and there are no great changes in the observed pre-tilt angle. Fig. 3 shows several mixtures with F-BAPP as one of the components and PPD as the other. The acid dianhydride is a 1:1 mixture of s-BPDA and PMDA (but the same ratio for all the polyimides). With this mixed acid dianhydride the crystallinity is probably low and independent of the concen- trations of the two diamines.Although PPD itself is rigid, linear and symmetrical, and thus greatly promotes crystallis- 5 14 I 41 '1 0.0 0.2 0.4 0.6 0.8 1.0 mole fraction ADA 1.0 0.8 0.6 0.4 0.2 0.0 mole fraction F-BAPP Fig. 2 Pre-tilt angles (BOB)found for mixtures of F-BAPP and ADA, with the acid dianhydride s-BPDA J. MATER. CHEM., 1994, VOL. 4 1669 -1 I 0.0 0.2 0.4 0.6 0.8 1.0 mole fraction F-BAPP 1.0 0.8 0.6 0.4 0.2 0.0 mole fraction PPD Fig. 3 Pre-tilt angles found for mixtures of F-BAPP and PPD, with a 1 :1 mixture of the acid dianhydrides s-BPDA and PMDA ation, the mixed acid anhydride will offset most of this effect. We thus expect the polyimides to be amorphous and indepen- dent of diamine composition in this system.The increase in observed pre-tilt angle with increased F-BAPP concentration is thus not due to changes in the in-plane order (a,), but rather to changes in the surface pre-tilt angle (Po). We see that the changes in the pre-tilt angle in this case are nearly linear with changes in the composition. This is in sharp contrast to the behaviour of the amorphous/semi-crystalline systems, and also to at least some of the semi-crystalline/semi- crystalline systems discussed below. Amorphous/ Semi-crystalline Polyimides Fig. 4 shows the results with s-BPDA as the acid dianhydride and a mixture of MOD and ADA. For the pure diaminooctane the polyimide is crystalline' and the pre-tilt angle high. The pre-tilt angle for s-BPDA-MOD is the same as reported previous1y.l Addition of a small amount of the very bulky and unsymmetrical ADA gives a drastic reduction of the pre- tilt angle.This bulky component will of course effectively destroy any crystallinity and thus reduce the in-plane order (al).With more than 50% of ADA the pre-tilt angle saturates at an intermediate level, which is the same as for the ADA- F-BAPP mixtures in Fig. 2. In Fig. 5 the results for copolyimides between DDE and MOD are shown. DDE is much less bulky than ADA and has about the same molecular length as MOD. In this case about a 10-15% addition to the semi-crystalline s-BPDA-MOD is 0 0 0!!li2 0 OlO 0.2 0.4 0.6 0.8 1.0 mole fraction MOD 1.0 0.8 0.6 0.4 0.2 0.0 mole fraction ADA Fig. 4 Pre-tilt angles found for mixtures of MOD and ADA, with the acid dianhydride s-BPDA 5-v)-a,L 4-0,-a, -0 3--2 cz 2: 'I1-0 0 I * I' 1 Fig.5 Pre-tilt angles found for mixtures of MOD and DDE, with the acid dianhydride s-BPDA tolerated before there is a drastic drop in the pre-tilt angle. At low concentrations DDE can be regarded as a contaminant in the s-BPDA-MOD matrix. This contaminant mill prefer- ably be incorporated into the amorphous regions of' the semi- crystalline matrix. It seems that DDE can be incorporated to a larger degree than ADA before the crystallinity is disrupted appreciably. When the crystallinity is disrupted there is also a rapid decrease in the in-plane order (al), and according to eqn. (2) there will also be a drop in the observed pre-tilt angle.The value for s-BPDA-MOD given here is tht: same as that given in Fig. 4,and also for another series of experiments described previously.' Semi-crystalline/ Semi-crystalline Polyimides S-BPDA-MOD,' PMDA-MOD22 and BTDA-MOD23 are all crystalline polyimides. Fig. 6 shows what happens when mixtures of the different acid dianhydrides are used. In all cases the pre-tilt angles of the copolyimides are far from the average of the two simple polyimides. For both polyimides containing s-BPDA as one component the pre-tilt angle is far below the average. This is to be expected if the random mixing disrupts the crystallinity. In this case the order is reduced and a, is lowered. This will result in a reduced pre-tilt angle according to eqn. (1) and (2).For the mixing of BTDA and PMDA the pre-tilt angle is actually higher for rhe mixed polyimide; this is more difficult to explain. It could mean that Fig. 6 Pre-tilt angles found for mixtures of the acid dianhydrides BTDA s-BPDA and PMDA; the diamine is always MOT) the crystal structures of the two pure polyimides are so similar that mixed crystals can also exist; in other words the crystal- linity is not much affected. The PMDA moiety is linear, whereas the BTDA moiety can exist both as a linear group (depicted in Fig. 1) or in two different bent conformation^.^^ The flexibility of the BTDA group is probably also the reason that alkylene chains, with either an odd or an even number of methylene groups, give crystalline polyimides with this whereas the PMDA22 or s-BPDA1 groups only give crystalline polyimides for the chains with an even number of methylene groups.For PMDA-MOD,20 a, =0.22 using 5CB as the liquid crystal, whereas for BTDA-MOD, a1=0.31. Po was 10 & 4"for BTDA-MOD and 13 f.3" for PMDA-MOD.20 The pre-tilt angle values given here for the pure PMDA-MOD and BTDA-MOD are 1.5-1.6" lower than reported previou~ly.~,~ The value for s-BPDA-MOD is the same as reported previously.' This is most likely because the polyimides in this series of experiments are cured at 250°C for 30 min. Higher values were obtained for BTDA-MOD and PMDA-MOD, cured at 300°C for 2 h. The effect of different curing conditions has been described in detail for the polyimides based on PMDA and butylenediamine and pen- tylenediamine." We have previously found a strong odd-even effect for the pre-tilt angle when polyimides based on s-BPDA and alkylene- diamines were used for the alignment layer.' The polyimides with an even alkylene chain length are crystalline.Fig. 7 gives the pre-tilt angle as a function of composition for mixtures of HMD and MOD. In this case the pre-tilt angle is nearly linear in composition. In homologous series it is normal that all members have very similar crystal structures.22 It thus seems likely the crystal structures of s-BPDA-HMD and s-BPDA-MOD are similar enough for mixed crystals to exist, in the same way as mixed crystals have been found in series of p~lyamides.~~ As for the amorphous/amorphous case in Fig.3 a linear change is found in the pre-tilt angle when a, stays (nearly) constant. For s-BPDA-HMD the in-plane order (al) has been determined" to be 0.29 with 5CB as the liquid crystal, while the surface pre-tilt angle (Po) was 16". We see that, in general, copolyimides where one, or both, of the pure polyimides are crystalline give non-linear changes in the pre-tilt angles for the copolyimides. If both pure polyimides are amorphous the changes are more nearly linear. As crystallinity is critically dependent on the packing of the polymer chains it is very difficult to obtain good crystallinity with two, or more, units randomly alternating along the polymer chain.The disruption of the crystallinity will lower 0.0 0.2 0.4 0.6 018 1:O mole fraction HMD 1.0 0.8 0.6 0.4 0.2 0.0 mole fraction MOD Fig. 7 Pre-tilt angles found for mixtures of MOD and HMD, with the acid dianhydride s-BPDA J. MATER. CHEM.. 1994, VOL. 4 the in-plane order drastically and thus lower the pre-tilt angles observed. During the course of this investigation changes were made to our rubbing machine, the pre-tilt angles found for this set of experiments is thus not directly comparable with previous values for s-BPDA-MOD or S-BPDA-HMD.' Neither are the values for s-BPDA-MOD in Fig. 7 directlq- comparable with the values in Fig. 4, 5 and 6. A complete discussion of the effect of processing parameters on the pre-tilt angle is beyond the scope of this work.We have found the pre-tilt angle depends on the curing temperature and time for two simple polyimides based on PMDA." The pre-tilt angle also depends on any heat treatment after rubbing.'' Similar find- ings have been reported on commercially available pol yimides.' The rubbing will strongly influence the pre-tilt angle. Both the rotating speed of the rubbing roll, the translational speed of the substrate and the number of times the substrate is rubbed will be important, these factors have been combined into the concept of rubbing ~ork.',~~,~' In addition, the pressure the roll exhorts on the substrate,27 the penetration depth of the fibres in the rubbing ~10th~~ or the contact length27 and the type of fibres" or rubbing material26 used will be important; unfortunately none of these factors are easily quantified.We have also found that the diameter of the rubbing roll is of importance. In our analysis of the problem, changes in the rubbing parameters will influence both a, and Po in eqn. (2). The relationship between rubbing and these two parameters has been studied elsewhere.20 We would also expect different polyimides to show different sensitivity to changes in the processing parameters, it is thus probably not feasible to obtain any universal relationship between the processing parameters and the pre-tilt angles. This is illus- trated by the 25% changes in pre-tilt angles of s-BPDA-MOD (from 5.7" shown in Fig.6 to 4.2" shown in Fig. 7) when we changed the rubbing roll, whereas the structurally very similar s-BPDA-HMD only changed insignificantly from 3.0" (given previously') to 2.8". Conclusions Copolyimides where one, or both, of the pure polyimides are crystalline usually give non-linear changes in the pre-tilt angles for the copolyimides. If both pure polyimides are amorphous the changes are more nearly linear. The results are within our recently proposed population distribution model. The in-plane order (a, ) is strongly depen- dent on the crystallinity of the polyimides and thus varies non-linearly with composition. This will also give rise to a non-linear change in the pre-tilt angles observed. References 1 H. Yokokura, M. Oh-e, K.Kondo and S. Oh-hara, Mol. Cryst. Liq. Cryst., 1993,225, 253. 2 B. 0. Myrvold, K. Kondo and S. Oh-hara, Liy. Cryst., 1993, 15,429. 3 B. 0.Myrvold, K. Kondo and S. Oh-hara, Mol. Cryst. Liq. Cryst., 1994,239,211. 4 B. 0. Myrvold, H. Yokokura, Y. Iwakabe, K. Kondo and S. Oh-hara, Proc. Jpn. Display '92, Society for Information Display, Evanston, 1992, p. 827. 5 C. Nozaki, N. Imamura and Y. Sano, Jpn. J. Appl. Phys., 1993, 32,4352. 6 M. Nishikawa, T. Miyamoto, S. Kawamura, Y. Tsuda and N. Bessho, Proc. Jpn. Display '92, Society for Information Display, Evanston, 1992, p.819. 7 H. Fukuro and S. Kobayashi, Mol. Cryst. Liq. Crjist., 1988, 163, 1 CI1J I. J. MATER. CHEM., 1994, VOL. 4 1671 8 P. A. Gass, A. Mosley, B. M. Nicholas, J.T. Brown, C. P. 19 D. Johannsmann, H. Zhou, P. Sonderkaer, H. Wierenga, Edwards, and D. G. McDonnell, Proc. SID, 1987,28,381. B. 0.Myrvold and Y. R. Shen, Phys. Rev., 1993,48,1889. 9 D-S. Seo, K. Muroi and S. Kobayashi, Mol. Cryst. Liq. Cryst., 20 B. 0.Myrvold and K. Kondo, Mol. Cryst. Liq. Cryst.. 1994 to be 1992,213,223. published, LC-GH-5161. 10 Y. S. Negi, Y. Suzuki, T. Hagiwara, I. Kawamura, N. Yamamoto, 21 For example, L. H. Sperling, Introduction to Physical Polymer K. Mori, Y. Yamada, M. Kakimoto and Y. Imai, Liq. Cryst., 1993, Science, John Wiley, New York, NY, 1986. 13, 153. 22 V. V. Korshak, T. M. Babchinitser, L. G. Kazaryan, V. A. 11 B. 0. Myrvold, Y. Iwakabe, K. Kondo and S. Oh-hara, Jpn. Vasilyev, Ya. V. Genin, A. Ye. Azriel, Ya.S.Vygodsky, N. A. J. Appl. Phys., 1993,32, 5052. Churokina, S. V. Vinogradova and D. Ya. Tsvankin, J. Polym. Sci. 12 B. 0. Myrvold, 1993, Am. Chem. SOC. 205th Annual Meeting, Polym. Phys. Ed., 1980,18,247.Denver. 23 B. 0.Myrvold, S. Oh-hara and K. Kondo, Mol. Cryst. Liq. Cryst., 13 B. 0. Myrvold and K. Kondo, Liq. Cryst., 1994, in the press 1994 to be published. 3.10 875. 24 S. Numata, K. Fujisaki and N. Kinjo, Polymer, 1987,28,2282.14 M. Barmentlo, R. W. J. Hollering and N. A. J. M. van Aerle, Phys. 25 0.B. Edgar and R. Hill, J. Polym. Sci., 1952,8, 1. Rev. A, 1992,46, R4490. 26 M. E. Becker, R. A. Kilian, B. B. Kosmowski and D. A. Mlynski,15 M. Barmentlo, R. W. J. Hollering and N. A. J. M. van Aerle, Liq. Mol. Cryst. Liq. Cryst., 1986, 130,167. Cryst., 1993,13,475. 27 Y. Sato, K. Sat0 and T. Uchida, Jpn. J. Appl. Phys., 1992,31, L579. 16 W. Chen, M. B. Feller and Y. R. Shen, Phys. Rev. Lett., 1989, 28 H. Matsuda, D-S. Seo, T. Isogami, Y. Yabe and S. Kobayashi,63,2665. Proc. 17th Jpn. LC Conf., 1991, p.142. 17 B. 0.Myrvold, K. Kondo and S. Oh-hara, J.SID, 1994,93-22, in the press. 18 B. 0.Myrvold and K. Kondo, Liq. Cryst., 1994, in the press. Paper 4/02712D; Received 6th May, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401667

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( ll), 1673-1688 Synthesis, Transition Temperatures, some Physical Properties and the Influence of Linkages, Outboard Dipoles and Double Bonds on Smectic C Formation in Cyclohexylphenylpyrimidines Stephen M. Kelly" and Jurg Funfschilling F. Hoffmann-La Roche Ltd., Dept. RLCR, CH-4002 Basle, Switzerland A trans-l,4-disubstituted cyclohexane ring has been introduced into known two-ring phenylpyrimidines to produce a wide variety of new three-ring cyclohexylphenylpyrimidines. The length and type of the terminal chains and linking units have been varied systematically. The effect of introducing a carbon-carbon double bond of defined configuration into various positions of both terminal chains has also been investigated. The influence of lateral dipoles (i.e oxygen and carboxyl groups) in different positions (central and terminal) in the molecular core of a model system on the smectic C (S,) transition temperature has been studied and related in a simple empirical way to standard theories for S, phase formation.Isolated, non-conjugated outboard dipoles (i.e. in cyclohexyl ethers and esters) have been found to destabilise the Sc and nematic (N) phases. Conjugated outboard dipoles (i.e. in phenyl ethers and esters) lead to substantial increases in the Sc transition temperature and usually to a widening of the Sc temperature range. Most of the new cyclohexylphenylpyrimidines exhibit a variety of smectic phases as well as Sc and N phases. Several homologous series of the most interesting cyclohexylphenylpyrimidines incorporating oxygen atoms or carboxy groups and/or a carbon-carbon double bond were synthesized and found to exhibit a relatively wide-range Sc phase at elevated temperatures.In admixture with a chiral smectic C (S,. base mixture, some of the new three-ring cyclohexyl- phenylpyrimidines can induce a substantial increase in the Sc-and N transition temperatures without increasing the viscosity (and thus response times) excessively. Electro-optic display devices utilising ferroelectric liquid crys- tals (FLCDs) are being developed for commercial applications with a high information Content.'-'' These display devices are characterised by exceptionally fast response times (ps), high contrast and good viewing angle dependence. The successful commercialisation of these displays depends not only the resolution of problems concerned with display device con- struction, stability and addressing, but also on the optimis- ation of the ferroelectric mixtures used, which must exhibit a broad spectrum of narrowly defined physical A chiral smectic C (S,*) phase in conjunction with a narrow smectic A (S,) and nematic (N) phases are usually regarded as in order to obtain good orientation for sur- face-stabilised ferroelectric display devices ( SSFLCDS).',~A smectic A phase is not required for those effects based on orientation by electric fields (e.g.short pitch bistable and dis- torted helix ferroelectric display devices SBFLCDs and DHFLCDs).'-'* Although SSFLCDs require only a moder- ately high value of spontaneous polarisation (P,) and long pitch values (p) and SBFLCDs and DHFLCDs require the opposite (i.e.very high P, values and a short pitch), the non- optically active base mixture for each display type can be devised using different amounts of the same components and then doped with the appropriate optically active (chiral) dopants to achieve the desired values for the pitch and spontaneous polari~ation.l~-~~ Each base mixture must be chemically, thermally, photo- and electro-chemically stable, exhibit a small birefringence (An), a low rotational viscosity (y), low melting point (Tm),large tilt angle (0) and high smectic C (S,) transition temperatures (TSc*).Thus, there is a techno- logical requirement for compounds with high S, transition temperatures that can be used to increase the S,* transition temperature (Ts,*) without being seriously detrimental to the other essential physical properties [e.g.melting point, response times (T)~tilt angle].Three-ring systems offer the possibility of introducing the trans-1,4-disubstituted cyclohexane ring into the core in order to reduce the viscosity and birefringence. Almost no two-ring compounds incorporating the trans-1,4-disubstituted cyclo- hexane ring have been found to exhibit an S, phase. There are numerous reports concerning alkyl/alkoxy-su bstituted two- and three-ring phenylpyrimidine~,l~-~~ some ()f which incorporated a trans-1,4-disubstituted cyclohexane ring.29-34 However, preliminary investigations of a small number of 5-(trans-4-alkylcyclohexyl)-2-(4-alkylphenyl)pyrimidinr~s34with various linking units (e.g.CH,CH2, CH,O, CO,) between the cyclohexane and benzene rings revealed that these materials could be used to increase the S, transition temperature of a host mixture without increasing the viscosity (and .bus, the response time) excessively, while also decreasing the birefrin- gence. These investigations have now been extended to include variations in the length and type of the terminal chains and linking units. As it has recently been sho~n~~-~~ that the introduction of a carbon-carbon double bond in certain positions and configurations into various positions of the terminal chains of two-ring phenylpyrimidine~~~-~~ can lead to improvements in the transition temperatures, response times, tilt angles etc.of the base mixtures containing them, this effect has also been investigated for the new 5-(trans-4- alkylcyclohexyl)-2-(4-alkylphenyl)pyrimidines with various linking units [ie. -(direct bond), CH2CH2, CH20, COz] between the cyclohexane and benzene rings under study. Only substances containing a trans-l,4-disubstituted cyclo- hexane ring have been prepared as it is most probiible that the corresponding materials incorporating a cis-1,4-disubsti- tuted cyclohexane ring would not be liquid crystalline. Even small amounts of the latter lead to substantial reductions in the liquid-crystal transition temperatures of the former and must be removed during synthesis. This is due to the angled, non-collinear configuration of the cis-1,4-disubstitutcd cyclo- hexane This is well documented and in order to form calamitic liquid crystals a semi-rigid, lath-like structure is required (i.e.the ring-substituent bonds should be co-axial or at least parallel to each ~ther).l~-~l J. MATER. CHEM., 1994, VOL. 4 Most statistical theories for the Sc phase postulate that lateral dipole moments at an angle to the molecular axis promote S, mesophase behavio~r.~’~~~ However, it has been shown that such dipole moments are not essential for Sc formation, but often still lead to higher Sc transition tempera- ture~.~~,~’However, only fully aromatic systems were investi- gated.44,45 This is equally valid for models based on packing forces, where only steric forces are taken into account.Therefore, it was decided to investigate the influence of different lateral dipoles in various positions in the molecular core of a three-ring cyclohexylphenylpyrimidine model system on the S, transition temperature in order to determine the validity of these theories for the case where the outboard terminal dipoles are attached alternatively to aromatic and/or aliphatic rings. c5Hll~--Z+CN -HCVEtOWtoluene1 NHfitOHl 1 ‘N NaOCH,JCH30H CQ-’ “-/+ Experimental Synthesis The three-ring cyclohexylphenylpyrimidines 1-6 and 13-18 either directly linked or with an ethyl linkage [Z = -(direct bond) or C2H4] were synthesized as shown in Scheme 1 from the known two-ring cyclohexyl ben~onitriles.~~.~~ The nitrile function was converted in the usual way with ethanolic hydrochloric acid into a benzimidoethyl ether hydrochloride and then into an benzimidamide hydrochloride using ethanolic Condensation with various straight chain (2-methoxymethylidene) aldehydes prepared in situ from the corresponding tetraacetals as ~s~a1~~,~~ yielded the desired three-ring cyclohexylphenylpyrimidines 1-6 and 13-18.A Williamson alkylation of the required 4-(5-n-alkylpyrimidin-1-6 and 13-18 59 and 69-73 \ 44,54and 65-68 Scheme 1 J. MATER. CHEM., 1994, VOL. 4 2-yl)phenols3' with the appropriate (trans-4-n-alkylcyclohex-y1)methyl bromides4' yielded ethers 7-9 and 19-21, see Scheme 2.Esters 10-12 and 22-32 were prepared in the usual way by esterification of the required 4-( 5-n-alkylpyrimidin-2- yl)phenols3' with the necessary trans-4-alkyl- and trans-4- alkenyl-cyclohexane-1-carboxylic a~ids~~,~'using N,N-dicyclohexylcarbodimide (DCC),sO see Scheme 2. Ether 33 was prepared by alkylation of 2-[4-trans-4-hydroxycyclohexyl)phenyl]-5-decylpyrimidine" using butyl tolulene-p-sulfonate,52 see Scheme 3. Ether 34 was prepared by alkylation of trans-4-propylcyclohexan-1-01~~with 4-( 5- decylpyrimidin-2-yl) benzyl toluene-p-sulfonate prepared from 4-( 5-decylpyrimidin-2-y1) benzyl alcohol after reduction of 4-( 5-decylpyrimidin-2-y1) benzoic acid3' with lithium aluminium hydride. see Scheme 4.Alkoxy-substituted ether 35 was synthesized from 2- [4-(trans-pentylcyclo hexyl) phenyl ]pyrmi-din-5-olS3 as shown in Scheme 1. The 4-(trans-4-pentylcyclo- hexy1)benzimidamide hydr~chloride~~.~~ was reacted with (2-ethoxy -3-dimethylaminopropenylidene) dimethylammon -ium perchlorateS4 to yield 2-[ 4-trans-pentylcyclohexyl)phenyl] 5-ethoxypyrimidine. Removal of the ethoxy group by base at high temperature^^^ resulted in 2-[ 4-(trans-pentylcyclo- hexyl)phenyl]pyrimidin-5-01,~~which was alkylated with bromononane in a Williamson ether synthesis to produce nonyloxy substituted ether 35. Ether 36 was prepared by alkylation of 2-[ 4-(trans-4-hydroxycyclohexyl)phenyl]-5-non-yloxypyrimidinesl using butyl toluene-p-sulfonate,54 see Scheme 3.Ether 37 was prepared by alkylation of 4-(5-de~ylpyrimidin-2-yl)phenol~~with (trans-hydroxycylohexy1)-methyl toluene-p-sulfonate,33~55and subsequent alkylation of the hydroxy group and ethyl toluene-p-sulfonate, see Scheme 2. Ether 38 was prepared by alkylation of 4-(5-benzyloxypyrimidin-2-yl)phenolS2with (trans-4-propylcyclo- 10-1 2 and 22-24 I 7-9 and 19-21 hexy1)methanol in a Mitsunobu reaction, followed by depro- tection with hydrogen and palladium on charcoal to produce 4-{ 5-[( trans-4-prop ylcyclohexyl ) methoxy] pyrimidin-2- y1)phenol and subsequent alkylation with bromononane, see Scheme 5. Ether 39 was synthesized by alkylation of 4-(5- benzyloxypyrimidin-2-yl)phenols2with (trans-hydroxycyclo- hexy1)methyl toluene-p-sulfonate,33.55 alkylation of the hydroxy group with ethyl toluene-p-sulfonate, removal of the benzyl group and subsequent alkylation with bromononane, see Scheme 5.Ester 40 was synthesized by esterification of 2-[4-(trans-4-hydroxycyclohexyl ) phenyl 1-5-de~ylpyrimidine~~with butanoic acid as usual using DCC," see Scheme 3. Ester 41 was prepared as usual by esterification of trans-4-propylcyclohexan-1-01~~with 4-( 5-decylpy rimidin-2-y1)benzoic acid using DCC, see Scheme 4.The 2-[ 4-trans- pent ylcyclohexyl )phenyl ]pyrimidine- 5-oIS3 was es terif ied with nonanoic acid to produce ester 42, see Scheme 1. Ester 43 was synthesized by esterification of 4-{5-[(trans-4-propylcyclo-hexyl)methoxy] pyrimidin-2-yl} phenol with nonanoic acid, see Scheme 5.Esterification of 2-[ 4-(trans-pentylcyclohexyl) phenyl] pyrimidin-5-olS3 with nonanoic acid yielded ester 45, see Scheme 1. The 2-[4-(trans-pentylcyclohexyl)phenyl]pyri-midin-5-olS3 was alkylated in a Mitsunobu reaction with the appropriate (E)-alk-2-en-l-ols and alken-1-01s with a terminal double bond to yield alkenyloxy-substituted ethers 44, 54 and 65-68 and 59 and 69-73, respectively, or alkylated in a Williamson ether synthesis with bromononane to produce nonyloxy-substituted ether 53 or esterified with alkanoic acids using DCC to produce esters 60-64, see Scheme 1. The methods of synthesis and structural analysis of the new three-ring cyclohexyl phenylpyrimidines are described in detail below. The configuration of the carbonsarbon double bond in the alkenyl chain of new ester 45 and ethers 44, 54 and n 37 Scheme 2 J.MATER. CHEM., 1994, VOL. 4 33 \ 1 40 -00 --OCQHIQ 36 Scheme 3 65-68 was confirmed by 'H nuclear magnetic resonance (NMR) spectroscopy (the trans-olefinic coupling constants, z12-18 Hz, are larger than those of the corresponding cis- olefinic coupling constants, ~7-11 Hz) and by infrared (IR) spectroscopy (the trans-absorption bands are narrow and exact, M 970-960 cm-', while the cis-absorption bands are observed at distinctly different wavelengths, M 730-675 cm- I). The structural and isomeric purity was determined by differential thermal analysis (DTA) and capillary gas chroma- tography (GC) as usual and, where necessary, on liquid-crystal-packed columns.56 The transition temperatures of the compounds prepared were determined by optical microscopy using a Leitz Ortholux I1 POL BK microscope in conjunction with a Mettler FP 82 heating stage and FP 80 control unit.All the monotropic liquid crystal phases could be observed using a microscope and no virtual values (extrapolated) were deter- mined. The transition temperatures were also determined using a Mettler DTA TA 2000. The purity of the compounds was determined by thin layer chromatography (TLC), GC and DTA. A Perkin-Elmer 83 10 capillary gas chromatograph and GP-100 graphics printer were used. Precoated TLC plates (4 cm x 8 cm), SiO, SIL G/IV2s4, layer thickness 0.25 mm (Machery-Nagel, Diiren, Germany) were utilised. Column chromatography was carried out using silica gel 60 (230-400 mesh ASTM).Reaction solvents and liquid reagents were purified by distillation or drying shortly before use. Reactions were carried out under N, unless water was present as a reagent or solvent. All temperatures were meas- ured externally unless otherwise states. The 'H NMR spectra were recorded at 60 MHz (Varian T-60), 80 MHz (Bruker QP-80) or 250 MHz (Bruker HX-270). Tetramethylsilane was used as the internal standard. Mass spectrometry (MS) was carried out by using an MS9 (AEZ Manchester) spectrometer. The S,* mixture SCO 1014 consists of 4-(trans-4-{[(R)- 2-fluorohexanoyl] oxy) cyclohexy1)phenyl 2,3-difluoro-4-(octy- 1oxy)benzoate( 16 wt.%), 2-[4-( hexyloxy)phenyl]-5-nonylpyr-imidine (24 wt.%), 2-[4-(nonyloxy)pheny1]-5-nonylpyri-midine (24 wt.%), 2-[ 4-(nonyloxy)phenyl]-5-heptylpyri-midine ( 12 wt.%), 2-[4-( hexyloxy)phenyl]-5-octylpyrimidine ( 12 wt.%) and 2-[ 4-(decyloxy)phenyl]-5-octylpyrimidine (12 wt.%).The determination of the physical properties of the chiral mixtures containing the new esters was carried out as pre- viously de~cribed.~~.~~ Synthesis of Ethoxy [4-(tvans-4-pentylcycIohexyl)phen ylme t hyl- amine Hydrochloride A solution of 4-(trans-4-pentylcyclohexyl)ben~onitrile~~ (51.1 g, 0.2 mol) in ethanol (35 cm3) and toluene (200 cm3) was saturated with hydrogen chloride at 0 "C and then stirred at room temperature for 2 days. The reaction mixture was evaporated down under reduced pressure, shaken with ether (500 cm3), filtered, washed with portions of ether and finally J.MATER. CHEM., 1994, VOL. 4 o~;>cloH21HO a\LIAIH,/ether H&12 HO/434>ClOH2' 0 41 34 Scheme 4 dried under vacuum to yield 59.6 g (88%) of the hydrochloride. IR (KBr) vmax/cm-': 2994,2922,2850, 1648, 1612,1443,1053 849. MS m/z: 301 (M'), 273, (C,,H,,NO+). 'H NMR aH (CDCI,; TMS standard; 250 MHz): 0.84-0.89 (3 H, t), 1.05-1.80 (16 H, overlapping peaks), 2.50-2.60 (1 H, overlap- ping peaks), 4.58-4.67 (2 H, q), 7.48-7.51 (2 H, d), 8.05-8.08 (2 H, d), 8.44 (2 H, s). Synthesis of 4-(tvans-4-Pentylcyclohexyl )benzimidamide Hydrochloride A saturated ethanolic ammonia solution (350 cm3) was added to a solution of 4-(trans-4-pentylcyclohexyl)phenylimido-ethyl ether hydrochloride (59.6 g, 176 mmol) and ethanol (350 cm3).The reaction mixture was stirred at room tempera- ture for 2 days and then evaporated down. The solid residue was shaken with ether (500cm3), filtered, washed with por- tions of ether and finally dried under vacuum to yield 53.6 g (98%) of the benzamidine. IR (KBr) vma,/cm-l: 3252, 3073, 2952, 2923, 2850, 1666, 1610, 1539, 1498, 1448, 849. MS m/z: (M'), 256, (C18H26N'). 'H NMR JH(CDCl,; TMS standard; 250 MHz): 0.84-1.50 (16 H, overlapping peaks), 1.81 (4 H, t), 2.50-2.58 (1 H, overlapping peaks), 2.50-2.58 (1 H, s), 7.45-7.48 (2 H, d), 7.76-7.80 (2 H, d), 9.19 (4 H, s). Synthesis of 2-[ 4-(trans-4-Propylcyclohexyl)phenyll-5-heptylpyrimidine, 1 A 30mol% solution of sodium methoxide in methanol (10cm3) was added dropwise to a mixture of 2-(methoxy-methylidene)n~nenal~~*~~(4.6 mmol), 4-(trans-4-pentylcyclo- hexyl) benzimidamide hydrochloride ( 1.O g, 3.6 mm ol), and methanol (15 cm3) at room temperature and stirred overnight.Concentrated hydrochloric acid was added (pH 3-41 and the inorganic material filtered off. The filtrate was conc;entrated under reduced pressure, dichloromethane (50cm,) was added and the resultant solution washed with water (2 x 100 cm3), then dried (MgSO,), filtered and evaporated down. The residue was purified by column chromatography on silica gel using a 9: 1 hexane-ethyl acetate mixture as eluent and recrystallised from tert-butyl methyl ether to yield 6.8 g (33%) of the desired product (1).IR (KBr) vma,/cm-': 2855, 2824, 2852, 1610, 1584, 1539, 1429, 797. MS m/z: 378 (hil'). The transition temperatures of ether 1 and of the similar ethers 2-6 and 13-18, prepared using this general method, are collated in Tables 1 and 2. Synthesis of 2-(4-[(trans-4-Propylcyclohexyl) methoxj 1-phenyl)-5-heptylpyrimidine, 7 A mixture of (trans-4-propylcyclohexyl)methyl bromide4' (0.24 g, 1.1 mmol), 4-( 5-heptylpyrimidin-2-yl)phen01.'~(0.30 g, 1.1mmol), potassium carbonate (0.43 g, 1.4 mmol) and butan- 2-one (50cm3) was heated under reflux overnight, filtered to remove any inorganic material, diluted with water ( 500 cm3) and then extracted into diethyl ether (3 x 100 cm3).'The com- bined organic extracts were washed with water (2 x 500 cm3), dried (MgSO,), filtered and then evaporated dciwn. The residue was purified by column chromatography on silica gel using a 9: 1 hexanHthy1 acetate mixture as eluent and recrystallised from ethanol to yield 0.35 g (77%) of the pure J. MATER. CHEM., 1994, VOL. 4 43 39 Scheme 5 Table 1 Transition temperatures for 1-12" ether. IR (KBr) v,,,/cm-l: 2922,2859,1585, 1547, 1519, 1427, 1255, 1167, 1028, 844, 796. MS m/z: 408 (M'), 270 (Cl7Hz2N2O+),185, (CllH9N20+). The transition tempera- tures of ether 7 and of similar ethers 8,9 and 19-21, prepared using this general method, are collated in Tables 1 and 2. Synthesis of 4-(5-Heptylpyrimidin-2-yl )phenyl trans-4-1 3 106 2 5 75 Propylcyclohexane-1-carboxylate,10 3 7 64 A solution of DCC (0.23 g, 1.1 mmol) in dichloromethane 4 3 101 (10 cm3) was added slowly to a solution of 4-(5-heptylpyrimi-5 5 92 6 7 86 din-2-yl)~henol~~(0.25 g, 0.9 mmol), trans-4-propylcyclohex- 7 3 113 ane-1-carboxylic acid48 (0.13 g, 0.9 mmol, 4-(dimethylamino)- 8 5 104 pyridine (0.04 g) and dichloromethane (25 cm3) at 0 "C.The 9 7 102 mixture was stirred at room temperature overnight, filtered 10 3 99 to remove precipitated material and then the filtrate was11 5 108 12 7 91 evaporated down under reduced pressure. The residue was purified by column chromatography on silica gel using a 9 : 1 a Values given in parentheses represent a monotropic transition hexane+thyl acetate mixture as eluent and then recrystallised temperature.from ethanol. The transition temperatures of ester 10 and J. MATER. CHEM., 1994, VOL. 4 Table 2 Transition temperatures for 13-24 compound Z n (C-S)/OC 13 3 72 77 116 151 14 5 60 83 93 131 152 -15 7 50 101 136 149 C2H4 3 88 92 105 131 C2H4 5 84 98 118 132 C2H4 7 70 101 124 130 19 CH20 3 93 101 -139 20 CH20 5 87 118 -142 21 CH20 7 86 120 -136 22 co2 3 70 89 -161 23 co2 5 64 85 104 -161 24 co2 7 78 89 116 -158 Table 3 Transition temperatures for esters 22-29 25 1 76 77 124 26 2 82 82 142 22 3 76 89 161 27 4 40 82 99 160 23 5 64 85 I04 161 6 24 7 78 89 116 158 8 28 9 77 85 123 153 29 10 77 87 125 150 Table 4 Transition temperatures for esters 23, 27 and 30-32 ~~ compound R (C-S,/S,/Sc)/°C (S,-S,)/OC (S4-S3)/”C (S,-S,)/”C (S,-S,-)/OC (S,-N)/”C (N-I)/”C 27 40 ---82 99 160W W30 71 ----92 162 W23 64 ---85 104 161 31 v 71 ----97 150 32 --If--52 56 64 69 -92 170 esters 11, 12, 22-32, prepared using this general procedure, (0.25 g, 0.8 mmol), potassium tert-butoxide (0.43 g, 3.1 mmol) are given in Tables 1-4.‘H NMR S, (CDC1,; TMS standard; and 1,2-dimethoxyethane (50 cm3) was stirred at room tem- 250 MHz): 0.88-0.90 (6 H, overlapping peaks), 1.27-1.32 perature overnight. It was then filtered to remove inorganic (10 H, overlapping peaks), 1.52-1.66 (4 H, overlapping peaks), material, diluted with water (500 cm3) and then extracted into 2.54-2.61 (4 H, overlapping peaks), 7.16-7.26 (2 H, overlap-diethyl ether (3 x 100cm3).The combined organic extracts ping peaks), 8.42-8.46 (2 H, d), 8.61 (2 H, s). IR (KBr) were washed with water (2 x 500 cm3), dried (MgSO,), filtered v,a,/cm-l: 2956,2925, 2853, 1731, 1598, 1476, 1205, 843. MS and then evaporated down. The residue was purified by 125 (C,H130). column chromatography on silica gel using a 9: 1 hexane-m/z: 396 (M’), 272 (C17H24N20), ethyl acetate mixture as eluent and recrystallised from ethanol to yield 0.25 g (66%) of the desired pyrimidine. IR (KBr)Synthesis of 2-[ 4-(truns-Butoxycyclohexyl)phenyl]-5-v,,,/cm-’: 2923, 2851, 1582, 1543, 1513, 1426, 1254, 1025, decylpyrimidine, 33 847, 790. MS m/z: 490 (M’), 270 (CI7Hz2N20+),185, A mixture of butyl toluene-4-sulfonate (TCI; 0.19 g, 0.9 mmol), (Cl,H,N20+).NMR 6, (CDCI,; TMS standard; 2SO MHz): 2-[4-( trans-4- h ydr ox yc yclo hex yl )p henyl ]-5 -de~ylpyrimidine~~ 0.88-0.89 (6 H, overlapping peaks), 1.29 (18 H, overlapping peaks), 1.62 (2 H, overlapping peaks), 2.60 (2 H, t), 4.02 (2 H, t) 6.96-7.00 (2 H, d), 8.32-8.36 (2 H, d), 8.57 (2 H, s). The transition temperatures of ether 33 are collated in Table 5. 4-(5-Decylpyrimidin-2-y1) benzyl Alcohol A solution of 4-( 5-decylpyrimidin-2-y1) benzoic acid3' (1.1 g, 3.9 mmol) in diethyl ether (25 cm3) was added dropwise to a solution of lithium aluminium hydride (0.2 g, 5.1 mmol) and diethyl ether (50cm3), which was cooled in an ice-bath. The reaction mixture was heated under gentle reflux for a further 2 h and then cooled to 0 "C in an ice-bath.Water (25 cm3) and 25% hydrochloric acid (50cm3) were added dropwise to the cooled reaction mixture. The organic layer was separated off and the aqueous layer extracted with ether (3 x 50 cm'). The combined organic layers were washed with water (500cm3) and saturated potassium carbonate (3 x 50 cm3), then dried (MgSO,), filtered and evaporated down to yield 1.Og (95%) of the desired pyrimidine. IR (KBr) v,,,/cm-': 3404, 3108, 3064, 2918, 2850, 1702, 1634, 1591, 1565, 1510, 1426,1299,1054,830. MS m/z:326 (M+),297 (C19H25N20'), 213 (CI3Hl3N2O+). 4-( 5-Decylpyrimidin-2-y1) benzyl Toluene-p-sulfonate A solution of toluene-4-sulfonyl chloride (0.60 g, 3.2 mmol) in dichloromethane (10 cm3) was added slowly to a solution of 44 5-decylpyrimidin-2-y1)benzyl alcohol ( 1.0 g, 3.1 mmol), pyridine (1.2 g, 15.3 mmol) and dichloromethane (50 cm3) at 0 "C.The reaction mixture was stirred at 0 "C for 6 h, washed with dilute hydrochloric acid (2 x 50 cm3), water (2 x 50 cm3) and dilute sodium carbonate solution (2 x 50 cm3), then dried (MgSO,), filtered and evaporated down to yield 0.9 g (62%) of the desired toluene-p-sulfonate. Synthesis of trans-l-[4-( 5-Decylpyrimidin-2-y1) benzyloxyll-4- propylcyclohexane, 34 A mixture of 4-( 5-decylpyrimidin-2-y1) benzyl toluene-p-sul- fonate (0.9 g, 0.6 mmol), trans-4-propylcyclohexan-1-0148 (0.3 g, 0.6 mmol), potassium tert-butoxide (0.23 g, 2.1 mmol) and 1,2-dimethoxyethane (50cm3) was stirred at room tem- perature overnight, then worked up and purified as above for 33 to yield 0.25 g (66%) of the desired pyrimidine.IR (KBr) vm,,/cm-': 2955, 2921, 2852, 1767, 1606, 1582, 1430, 1234, 1114, 1027, 851, 794. MS m/z: 466 (M'), 326 (C21H30N20f), J. MATER. CHEM., 1994, VOL. 4 188 (C, ,Hl2N20+). The liquid crystal transition temperatures of ether 34 are collated in Table 5. Synthesis of 2-[ 4-(tvans-4-Pentylcyclohexyl)phenyll-5-nonyloxypyrimidine, 35 A mixture of l-bromononane (Fluka; 0.19 g, 0.9 mmol), 2-[4- trans-4-pentylcyclohexyl) phenyl] pyrimidin-5-01 53 (0.25 g, 0.8 mmol), potassium carbonate (0.43 g, 3.1 mmol) and butan-2- one (50cm3) was heated under reflux overnight and then worked up and purified as described above for 7 to yield 0.25 g (66%) of the desired ether. IR (KBr) vmax/cm-': 2921, 2850, 1582, 1548, 1436, 1278, 1015, 851, 796. MS m/z: 450 (M').'H NMR bH (CDC1,; TMS standard; 00 MHz): 0.89-1.60 (34 H, overlapping peaks), 1.85-1.90 (6 H, overlap-ping peaks), 2.50 (1 H, t), 4.05-4.11 (2 H, t), 7.23-7.32 (2 H, t), 8.23-8.26 (2 H, d),8.43 (2 H, s).The transition temperatures of ether 35 and ether 53, prepared using this general method, are collated in Tables 5 and 6. Synthesis of 2-[ 4-(trans-4-Butyloxycyclohexyl)phenyll-5-nonyloxypyrimidine, 36 A mixture of butyl toluene-4-sulfonate (TCI; 0.17 g, 0.76 mmol), 2-[ 4-(trans-4-hydroxycyclohexyl)phenyl]-5-non-yl~xypyrimidine~' (0.10 g, 0.25 mmol), potassium tert-butox- ide (0.09 g, 0.83 mmol) and 1,2-dimethoxymethane (20 cm3) was heated under reflux overnight and then worked up and purified as described as above for 7 to yield 0.05 g (44%) of the desired ether.IR (KBr) v,,,/cm-': 3436, 2928, 2854, 1610, 1576, 1544, 1513, 1437, 1279, 1107, 847, 794. MS m/z: 452 (M'), 378 (C25H34N20+). 'H NMR 6, (CDCI,; TMS stan- dard; 250 MHz): 0.88-0.96 (6 H, overlapping peaks), 1.28-1.58 (22 H, overlapping peaks), 1.62 (2 H, overlapping peaks), 2.00 (2 H, d), 2.18 (2 H, d), 2.60 (1 H, t), 3.25 (1 H, overlapping peaks), 3.47-3.52 (1 H, t), 4.06-4.11 (1 H, t), 7.26-7.32 (2 H, t), 8.23-8.27 (2 H, d), 8.43 (2 H, s). The transition temperatures of ether 36 are collated in Table 5. Synthesis of 2-( [4-(trans-4-E thox yc yclohex y 1 )met hox y ]-phenyl}-5-decylypyrimidine,37 A mixture of ethyl toluene-4-sulfonate (TCI; 0.3 I g, 1.6 mmol), 2- (4- [(trans-4-hydroxycyclohexyl)methoxyJ phenyl- 5-decyl- pyrimidine33 (0.20 g, 0.5 mmol), potassium tert-butoxide (0.15 g, 1.4 mmol) and 1,2-dimethoxyethane (50 cm3) was Table 5 Transition temperatures for 14, 19, 22 and 33-43" Rl-0 R2.*I> 14 - 60 33 - 59 34 OCH2 79 19 CH,O 93 35 - 73 36 - 88 37 CH,O 95 38 CH,O 100 39 CH20 88 40 - 82 41 02c 90 22 COZ 76 42 - 122 43 CH,O 125 a Values given in parentheses represent a monotropic transition temperature.131 152 137 143 ~~ 91 139 139 176 153 167 111 130 -158 -154 150 158 96 142 -161 -178 -169 J. MATER.CHEM., 1994, VOL. 4 168 1 Table 6 Transition temperatures for two-ring ethers 46-52 and three-ring ethers 53-59" compound X R (C-S,/S,/N/I)/"C (S,-S,/N)/"C (S,-S,/N)/"C (SA-N/I)/"C (N-I)/"C ref. 46 36 - 53 85 - 39 47 56 - 65 82 - 39 48 52 (44) - 39 49 47 82 - 39 50 38 58 - 39 51 51 86 - 39 52 34 - 38 77 - 39 53 65 83 119 - 181 - 54 97 - 115 - 176 - 55 93 - 161 - 56 87 - 184 - 57 86 - 168 - 58 81 130 185 - 59 67 121 176 - a Values given in parentheses represent a monotropic transition temperature. heated under reflux overnight and then worked up and purified as described above for ether 7 to yield 0.03 g (14%) of the desired ether. IR (KBr) v,,,/cm-': 2923, 2851, 1607, 1583, 1541, 1513, 1430, 1252, 1165, 1110,1035, 847, 800.MS m/~:452 (M'), 312 (C19H24N202).'H NMR 6, (CDCl,; TMS standard; 250 MHz): 0.87 (3 H, d), 1.18-1.26 (21 H, overlapping peaks), 1.59 (2 H, overlapping peaks), 1.80 (2H, overlapping peaks), 2.00 (2 H, d), 2.18 (2 H, d), 2.60 (2 H, t), 3.25 (1 H, overlapping peaks), 3.52-3.56 (2 H, q), 3.81-3.84 (2H, d), 6.94-6.98(2 H, d), 7.26 (2 H, s), 8.32-8.36(2H,d), 8.56(2 H, s). The transition temperatures of this ether are collated in Table 5. Synthesis of 2-(4-[(trans-4-Propylcyclohexyl)methoxy]-phenyl)-5-benzylox ypyrimidine A mixture of (trans-4-propylcyclohexyl)methyliodide47(4.0g, 15.0mmol), 4-(5-benzyloxypyrimidin-2-yl)pheno158( 1.O g, 3.6mmol), potassium carbonate (2.0g, 14.3mmol) and butan- 2-one (50 cm3) was heated under reflux overnight.The reac- tion mixture was worked up and purified as described above for 7 to yield 0.6 g (40%) of the desired ether. IR (KBr) ~,,~/cm-~:2956, 2920, 2852, 1606, 1582, 1550, 1515, 1445, 1248, 1179, 1101, 846, 789, 747, 711. MS m/z: 416 (M'). Synthesis of 2-{4-[(trans-4-Propylcyclohexyl )methoxy]-phenyl)pyrimidin-5-01 A solution of 2-(4-[(trans-4-propylcyclohexyl)methoxy]-phenyl) -5-benzyloxypyrimidine (0.6g, 1.4mmol), ethyl acet- ate (50cm3), ethanol (50cm3), acetic acid (2cm3) and 10% palladium on active charcoal (0.2g) were hydrogenated until no more hydrogen was taken up. The catalyst was filtered off and the filtrate evaporated down and purified by recrystallis- ation from ethyl acetate to yield 1.Og (68%) of thr desired pyrimidine; mp 209-210 "C.IR (KBr) v,,,/cm-l: 3431, 2920, 2847, 1611, 1581, 1558, 1431, 1288, 844, 790. MS m/z: 326 (M + ), 188 (C,,H*N,O,). Synthesis of 2-{4-[(trans-4-Propylcyclohexyl) methoxyJ-pheny1)-5-nonyloxypyrimidine,38 A mixture of 1-bromononane (Fluka; 0.23g, 1.1 mmol), 2-(4-[(trans-4-propylcyclohexyl)methoxy]phenyl} pyrimi din-5-01 (0.25 g, 0.7mmol), potassium carbonate (0.43g, 2 9 mmol) and butan-2-one (50 cm3) was heated under reflux overnight. The reaction mixture was worked up and purified as described above for 7 to yield 0.10 g (32%) of the desired tbther. IR (KBr) vmax/cm-l: 2922, 2852, 1605, 1544, 1515, 1411, 1278, 1250, 1171, 1003, 845, 786.MS m/z: 452 (M'), 314 (CI9H2,N2O2),188 (C1,H8N202). 'H NMR dH (CD<'13; TMS standard; 250 MHz): 0.89-1.60 (29 H, overlapping peaks), 3.80-3.83 (2 H, d), 4.05-4.11 (2 H, t), 6.94-6.98 (2 H, d), 8.24-8.28 (2 H, d), 8.41 (2 H, s). The transition temperatures of this ether are collated in Table 5. Synthesis of 2-{[4-(trans-4-Hydroxycyclohexyl) methouy 1-phenyl)-5-benzyloxypyrimidine A mixture of (trans-4-hydroxycyclohexyl)methyltr duene-4-s~lfonate~~(2.2g, 7.7mmol), 4-(5-benzyloxypynmidin-2- J. MATER. CHEM., 1994, VOL. 4 (2.0 g, 7.0 mmol), potassium carbonate (3.9 g, Synthesisof 2-(4-[ trans-4-(Butanoyloxy)cyclohexyl] pheny1)- yl)phen01~~ 28.0 mmol) and butan-2-one (50cm3) was heated under reflux 5-decylpyrimidine, 40 overnight and then worked up and purified as described above for ether 7 to yield 0.5 g (18%) of the desired ether.IR (KBR) vmax/cm-': 3433, 2927, 2858, 1740, 1607, 1547, 1515, 1440, 1278, 1249, 1171, 844, 790. MS m/z: 390 (M'). Synthesis of 2-{[4-(trans-4-Ethoxycyclohexyl)methoxyl-phenyl)-5-benzyloxypyrimidine Amixture of ethyl toluene-4-sulfonate (TCI; 0.85 g, 4.2 mmol), 2-{4-[(trans-4-hydroxycyclohexyl)methoxy]phen yl} -5- benzyl- oxypyrimidine (0.50 g, 1.3 mmol), potassium tert-butoxide (0.43 g, 3.8 mmol) and 1,2-dimethoxyethane (50 cm3) was heated under reflux overnight and then worked up and purified as described above for ether 7 to yield 0.2 g (37%) of the desired ether. MS m/z: 418 (M+), 327 (Cl9H2,N2O3'), 278 (C17H16N202+ 1. Synthesis of 2-{4-[(trans-4-Ethoxycyclohexyl) methoxy 1- phenyl }pyrimidin-5-01 A solution of 2-(4-[( trans-4-ethoxycyclohexyl)methoxy]-phenyl} -5-benzyloxypyrimidine (0.15 g), ethyl acetate (50 cm3), ethanol (50cm3), acetic acid (1cm3) and 10% palladium on active charcoal (0.2 g) were hydrogenated until no more hydrogen was taken up and then worked up and purified as described above to yield 0.1 g (88%) of the desired pyrimidine. MS m/z: 328 (M').Synthesis of 2-{4[(~rans-4-Ethoxycylohexyl)methoxyl-phenyl}-5-nonyloxypyrimidine,39 A mixture of 1-bromononane (Fluka; 0.06 g, 0.3 mmol), 2-(4- [(trans-4-ethoxycyclohexyl)methoxy]phenyl) pyrimidin-5-01 (0.08 g, 0.25 mmol), potassium carbonate (0.14 g, 1.0 mmol) and butan-2-one (25 cm3) was heated under reflux overnight and then worked up and purified as described above for 7 to yield 0.06 g (53%) of the desired ether. IR (KBr) v,,,/cm-': 2926, 2854, 1606, 1544, 1514, 1434, 1273, 1250, 1168, 1109, 1031, 850, 791.MS m/z: 454 (M'), 314 (Cl9HZ6N2o2), 188 (CloH8N2O2). 'H NMR dH (CDC1,; TMS standard; 250 MHz): 0.88 (2 H, t), 1.18-1.28 (19 H, overlapping peaks), 1.58 (3 H, overlapping peaks), 1.82 (2 H, d), 2.18 (2 H, d), 3.25 (1 H, overlapping peaks), 3.52-3.55 (2 H, q), 3.81-3.83 (2 H, d), 4.05-4.10 (2 H, t), 6.93-6.97 (2 H, d), 8.24-8.28 (2 H, d), 8.41 (2 H, s). The transition temperatures of this ether are collated in Table 5. A solution of DCC (0.40g, 0.9 mmol) in dichloromethane (10cm3) was added slowly to a solution of 2-[4-(trans-4-hydroxycyclohexyl)phenyl]-5-decylpyrimidine (0.50 g, 0.8 mmol), butanoic acid (Fluka; 0.17 g, 0.8 mmol), 4-(di- methy1amino)pyridine (0.04 g) and dichloromet hane (50 cm3) at 0°C and then worked up and purified as described above for ester 10 to yield 0.32 g (84%) of the desired ester.'H NMR 6, (CDCl,; TMS standard; 250 MHz): 0.88-0.99 (6 H, overlap- ping peaks), 1.26-1.68 (24 H, overlapping peaks), 2.00 (2 H, d), 2.18 (2 H, d), 2.60 (2 h, t), 2.65 (3 H, t), 3.47-3.52 (1 H, t), 4.06-4.11 (1 H, overlapping peaks), 7.26-7.33 (2 H, t), 8.31-8.34 (2 H, d), 8.60 (2 H, s). v,,,/cm-': 2924, 2859, 1729, 1611, 1586, 1546, 1431, 1181, 1013, 801. MS m/z: 464 (M+), 376 (C25H32N20+). The transition temperatures of ester 40 are collated in Table 5.Synthesis of tvans-4-Propylcyclohexyl4-(5-Decylpyrimidin-2-yl)benzoate, 41 A solution of DCC (0.43 g, 2.1 mmol) in dichloromethane (10cm3) was added slowly to a solution of rrans-4-propyl-cyclohexan-1-01 (0.25 g, 1.8 mmol), 4-( 5-decylpyrimidin-2- yl) benzoic acid (0.6 g, 1.8 mmol), 4-(dimethy1amino)pyridine (0.04 g) and dichloromethane (25 cm3) at 0 "C and then worked up and purified as described above for ester 10 to yield 0.32 g (84%) of the desired ester. 'H NMR 6, (CDCl,; TMS standard; 250 MHz): 0.87-0.90 (6 H, overlapping peaks), 1.26-1.56 (27 H, overlapping peaks), 1.91 (2 H, d), 2.22-2.35 (2 H, d), 2.60-2.66 (2 H, t), 4.88-5.02 (1 H, overlapping peaks), 8.12-8.15 (2 H, d), 8.46-8.49 (2 H, d), 8.65 (2 H, s). IR (KBr) v,,,/cm-': 2925,2852,1711, 1545, 1432, 1274, 1133, 762.MS m/z: 464 (M'), 323 (CZ1Hz7N20). The transition temperatures of benzoate 41 are collated in Table 5. Synthesis of 2-[ 4-(trans-4-Pentylcyclohexyl)phenyl] pyrimidin- 5-yl Nonanoate, 42 A solution of DCC (0.19g7 0.9mmol) in dichloromethane (10 cm3) was added slowly to a solution of 2-[4-(trans-4-pentylcyclohexyl)phenyl]pyrimidin-5-oI (0.25 g, 0.8 mmol), nonanoic acid (Fluka; 0.14 g, 0.8 mmol), 4-(dimethylamino)- pyridine (0.04 g) and dichloromethane (25 cm3) at 0 "C and then worked up and purified as described above for ester 10 to yield 0.32 g (84%) of the desired ester. 'H NMR 6, (CDCl,; TMS standard; 250 MHz): 0.89-0.90 (6 H, overlapping peaks), Table 7 Transition temperatures for 35,42,44and 45 c5Hl,---~~~z-c6H13 ~ _____~ 35 -O< 73 128 139 176 44 - O u 94 125 - 170 42 122 113 -_ 178 -OF0 110 --21645 -*y0 J.MATER. CHEM., 1994, VOL. 4 1.28 (28 H, overlapping peaks), 1.77 (2 H, q), 1.91 (4 H, t), 2.60-2.66 (3 H, t), 7.26-7.34 (2 H, t), 8.29-8.32 (2 H, d), 8.61 (2 H, s). TR (KBr) vmax/cm-l: 2920, 2850, 1768, 1546, 1429, 1232, 1133, 857. MS m/z:464 (M'). The transition tempera- tures of nonanoate 42, (E)-non-2-enoate 45 and esters 60-64 prepared using this general method are collated in Tables 5, 7 and 8. Synthesis of 2-{4-[(tvans-4-Propylcyclohexyl) methoxy1-phenyl )pyrimidind-yl Nonanoate, 43 A solution of DCC (0.13 g, 0.7 mmol) in dichloromethane (10cm3) was added slowly to a solution of 2-(4-[(trans-4-propylcyclohexyl)methoxy] phenyl} pyrimidin-5-01 (0.20 g, 0.6 mmol), nonanoic acid (Fluka; 0.10 g, 0.6 mmol), 4-(dimethy1amino)pyridine (0.04 g) and dichloromethane (25 cm3) at 0 "C and then worked up and purified as described above for ester 10 to yield 0.32 g (84%) of the desired ester.IR (KBr) v,,,/cm-': 2921,2852,1767,1606, 1508,1430,1234, 1027, 851. MS m/z: 466 (M'), 326 (C2,H2,N202), 188 (C,,H,N202). The transition temperatures of nonanoate 43 are listed in Table 5. Synthesis of 2-[ 4-(trans-4-Pentylcyclohexyl)phenyl]-5-{[(E)-non-2-en-l-yl] oxy) pyrimidine, 44 A solution of (E)-non-2-en-l-o1 (Johnson Matthey; 0.10 g, 8 mmol ), 2-[4-(trans-4-pentylcyclohexyl)phenyl]pyrimidin-5-01~~(0.25 g, 8 mmol), diethyl azodicarboxylate (0.13 g, 8 mmol), triphenylphosphine (0.20 g, 8 mmol) and tetrahydro- furan (25 cm3) was stirred at room temperature overnight and then evaporated down.The solid residue was taken up in warm hexane (25 cm3), filtered to remove the precipitate ( PPh30) and evaporated down once more. Purification of the residue by column chromatography on silica gel using a 9 :1 hexane-ethyl acetate mixture as eluent and then recrystallis- ation from ethanol yielded 0.16 g (48%) of the desired ether. IR (KBr) v,,,/cm-': 2956,2922,2850, 1574, 1543,1439,1390, 1277, 1000, 971, 789. MS m/z: 448 (M'), 324 (C21H2,N20). 'H NMR BH (CDC1,; TMS standard; 250MHz): 0.87-0.90 (6 H, q), 1.27 (21 H, overlapping peaks), 1.90 (4 H, t), 2.08-2.12 (2 H, q), 2.45-2.56 (1 H, t), 4.59-4.62 (2 H, d), 7.26-7.32 (2 H, t), 8.23-8.26 (2 H, d), 8.45 (2 H, s).The transition temperatures of ether 44 and similar ethers 53-59 and 65-73 prepared using this general method are recorded in Tables 6, 7, 9 and 10. Results and Discussion The transition temperatures of all the compounds synthesized were determined as single components. This allows several Table 8 Transition temperatures for esters 42 and 60-64" CmHzm+1 compound m (C-S,-/N)/"C ( S,-N)/"C (N-I)/"C 60 6 112 -188 61 7 118 -185 42 8 122 (113) 178 62 9 120 125 177 63 10 118 133 172 64 11 120 138 169 Values given in parentheses represent a monotropic transition temperature. 1683 Table 9 Transition temperatures for ethers 44, 54 and 65-68 compound m (C-S,-/N)/OC (S,-N)/"C ( N-I)/"C 65 3 99 -187 66 4 99 -177 54 5 97 115 176 44 6 94 125 170 67 7 86 135 167 68 8 93 140 163 Table 10 Transition temperatures for ethers 59 and 69 -73 C5H11---m:>O-(cHz)mk compound m (C-S,/N)/"C (S,-SA)/"C (S,N)/"C ~~~ (N-I)/"C 69 4 82 - - 184 70 5 55 65 112 185 59 6 67 96 121 176 71 7 59 91 142 176 72 8 55 103 145 169 73 9 57 97 151 168 important qualitative properties of the pure compounds to be determined, such as the tendency to form liquid-crystal phases, as reflected in the absolute value of the clearing point.Similarly, the tendency to form (tilted) smectic phases is shown by the upper temperature limit of the highest smectic (tilted) phase.However, the high melting point of a three-ring compound may obscure the existence of monotropic trans- itions. Additional information can be gained from mixtures of selected compounds in a base mixture, which exhibits all the phases (smectic and nematic) of interest. The addition of a fixed (relatively small) amount of the compound to be investigated results in relatively limited changes in the trans- ition temperatures of the base mixture. This allows compo- nents with greatly differing absolute transition temperatures and phase types to be compared. Furthermore, propcrties of interest of S,* mixtures designed for use in FLCDs such as the spontaneous polarization and switching time are only measurable in mixtures.Therefore, a fixed amount (1 5 wt.%) of a selection of the new compounds was added to a standard S,* base mixture SCO 1014, which exhibits the fcdlowing phases: C/Sx-Sc* = -7.6 "C, Sc*-SA =60.6 "C, S \-N* = 67.7 "C and N*-I =74.6 "C). The liquid-crystal transition tem- peratures (C-Sc*, SrSc*, Sc*-SA, SA-N* and N*-I) the spontaneous polarisation (P,) and the observed switching time (z) of the resulting mixtures were determined under standard conditions (z: 1OVpp p-' square wave, time to maximum current, at 25 "C; P,: 10 Hz, 10 Vpp p-' triangle). Influence of Linkages The transition temperatures of the heptyl homologues of the three-ring cyclohexylphenylpyrimidines (1-12) either directly linked or with ethyl, methoxy or ester linkages (denoted as -, CH2CH2, CH20 and C02, respectively) are collated in Table 1.The melting points (T',) of the directly linked com- pounds (1-3), the ethanes (4-6), the ethers (7-9) and the esters (10-12) are moderate for three-ring compounds (82, 93, 106 and 100"C, on average, respectively). The corresponding values for the clearing point (TNI)are relatively low (163, 140, 149 and 177"C, on average, respectively). Only three com- pounds (3, 6 and 9) exhibit an S, mesophase (monotropic). J. MATER. CHEM., 1994, VOL. 4 Compound 3 also possesses an enantiotropic SA phase. The transition temperatures for the corresponding decyl homol- ogues (13-24) are collated in Table 2. The extension of the terminal alkyl chain by three methylene units (CH,) results in lower melting and clearing points (ca.-10 to 26 "C) for all of the compounds studied. However, 11 of the 12 homol-ogues prepared now exhibit an S, phase at relatively elevated temperatures. In addition, the directly linked compounds (13-15) and the ethanes (16-18) possess and enantiotropic SA phase above the Sc phase. An ordered smectic phase (S3, not yet identified, and S,) is observed for the longest chain lengths for three of the four series studied. This is the usual behaviour observed for most mesogens of this type.17-22 Influence of Chain Length and Double Bonds The data collated in Table 3 for a homologous series of three- ring esters (22-29) reveal that even compounds with the shortest chains (n= 1)exhibit S, and N phases.This is unusual for cyclohexane compounds, where the plots of the Sc and N phases usually rise very sharply from the very low values of the short chains, they reach a maximum and then decrease s10wly.'~-~~Ordered smectic phases (S, and S,) are observed for most homologues. The effect of a carbon<arbon double bond on the transition temperatures of a number of esters (23, 27 and 30-32) is shown in Table 4. It is seen that T,, is lower for the alkenyl- substituted esters (30-32), while the ordered phase (S,) is completely suppressed. The TNIis sometimes higher but in one case it is lower. Although T, is higher for two alkenyl- substituted esters (30 and 31), these changes still result in an increase in the S, transition temperature range by the lowering of the ordered smectic (S, and S,) transition temperatures.The elimination or lowering of the ordered smectic phase is important for the low temperature behaviour of mixtures of these compounds (the double bond in a terminal position can also contribute to a low value for the birefringence). These observations are not completely consistent with previous results for related systems with an alkyl chain attached to a cyclohexane ring. Influence of Dipoles The effect of introducing an additional lateral dipole in the form of either an oxygen atom (0)or a carboxy group (0,C or CO,) into the core of an almost apolar dialkyl pyrimidine (14) is demonstrated by reference to Table 3. The model substance (14) exhibits three smectic modifications (S,, Sc and S,) as well as an N phase.The total lengths of the dialkyl pyrimidine (14), the ethers (19 and 33-39), the esters (22, 40-42) and the combined ester/ether (43) are kept constant (five units attached to cyclohexane ring and 10 units attached to the pyrimidine ring when all the rings are bonded directly and three units when separated by a two-unit linking group). The replacement of the apolar CH, unit between the cyclohexane ring and the alkyl chain of dialkyl pyrimidine (14) by an oxygen atom to yield the cyclohexyl ether (33) leads to the disappearance of the Sc phase. The transition temperatures of the orthogonal smectic phases (A and B) are increased (+9 and +8 "C, respectively), while TN, is lower (-9 "C).The introduction of a epoxymethano group (OCH,) between the cyclohexane ring and the pyrimidine ring with the oxygen atom attached to the cyclohexane ring to produce the ether (34) leads to the total elimination of the smectic phases. TNIis much lower (-61 "C) than that of the model substance (14).Thus, a single isolated (non-conjugated) dipole, either as an outboard or central dipole appears to destabilise the Sc and the N phases. A central dipole in the form of an oxygen atom attached to the benzene ring (CH,O) in the core of the aromatic ether (19) leads to an increase (+8 "C) in the S, transition temperature compared with the reference sub- stance (14). The orthogonal SA and SB phases are totally suppressed; TNIis also lower (-13"C).However, an oxygen atom attached to the pyrimidine ring in a terminal position in the aromatic ether (35) results in an even larger increase (+35 "C) in T,, and TNIand the broadest Sc temperature range (55 "C) of all the compounds collated in Table 5. Thus, an outboard or central dipole in conjugation with an aromatic ring stabilises the Sc and N phases. The presence of two outboard dipoles in the shape of oxygen atoms in terminal positions (one isolated next to the cyclohexane ring and one conjugated with the pyrimidine ring) in the ether (36) causes a decrease C-8°C) in Ts, compared with the reference substance (14). TNIis increased (+15"C), but not by as much as for the monoether (35). These transition temperatures could be regarded as the prod- uct of the two competing tendencies described above.The SA transition temperature is increased significantly (+ 22 "C).No ordered phase could be observed. The diether (37) with one isolated outboard dipole and one central conjugated oxygen atom possesses values for T, and T,, that do not differ greatly from those of the corresponding monoether (19). TNIis lower than that of either monoether (19 and 33) with oxygen atoms in the same positions. Thus, the transition temperatures are, in this case, clearly non-additive. Diether 38, with two conju- gated dipoles (one central, one outboard), exhibits an Sc transition temperature and a TNIat almost exactly intermedi- ate values between those of the corresponding monoethers (19 and 35) with the oxygen atoms in the same positions.However, the melting point for diether 38 is much higher than that of either monoether and thus, the Sc range is narrow (10 "C). Ether 39, with an oxygen atom in the three possible positions under consideration, exhibits a moderate T,, a relatively high Ts, and a relatively broad Sc temperature range (30°C). TNIis relatively high. The introduction of a third oxygen atom as an isolated, non-conjugated dipole next to the cyclohexane ring of diether 38 to produce triether 39 results in an increase in the S, and a decrease in TNI. The introduction of the larger dipole moment associated with the carboxy group (CO, and 0,C) of esters 22 and 40-42 results in similar trends in the temperatures as observed for the corresponding monoethers 19 and 33- 35, although at higher absolute temperatures, except for T,,.The replacement of the apolar CH, unit next to the cyclohexane ring of dialkyl pyrimidine 14 by a carboxy group to yield ester 40 leads to the disappearance of the S, phase. The transition temperatures of the orthogonal smectic phases (A and B) are increased substantially (+ 33 and +19 "C, respectively), while TNIis somethwat higher (+6 "C). This is also the case for ester 41 with a carboxy group (0,C) between the cyclohexane and the benzene rings with the oxygen atom attached to the cyclohex- ane ring. Only an SA and an N phase could be observed. Thus, an isolated (non-conjugated) outboard dipole in the form of a carboxy (ester) function also destabilises the Sc phase.The presence of the carboxy group between the cyclo- hexane and the benzene rings in ester 22 with the oxygen atom of the carboxy group attached to the benzene ring results in an increase in T, and TNI(+ 14 and +9 "C, respect-ively). The SAand SBphases of the model dialkyl pyrimidine (14) have been totally suppressed and Ts, is a little lower (-4°C). The carboxy group with the oxygen atom bonded to the pyrimidine ring in ester 42 increases T, and TNIsubstan-tially (+62 and +26"C, respectively). T,, is increased to a lesser extent (+ 20 "C) and is, as a consequence, monotropic. SA or SB phases could not be observed. J. MATER. CHEM., 1994, VOL. 4 The combination of an expoxymethano group (CH,O) and an ester group (0,C) in 43 lead to the highest melting point observed for this series and, thus, a monotropic S, transition temperature.The clearing point (TN,)is high, but lower than that of the corresponding ester (13) with a direct linkage instead of the epoxymethano linkage. This is consistent with the other results collated in Table 5. The S,* transition temperatures (Tsc*)for mixtures 1-14 containing compounds 14, 19 and 33-43 are arranged in Table 11 in order of ascending value. It is clear from the data in the table that an outboard dipole [either in the form of an oxygen atom or a carboxy (ester)] group in position 1 or 2, when the dipole is attached to the cyclohexane ring, leads to a low Tsc,.The highest values are for compounds with a dipole attached to the polarisable pyrimidine ring. Intermediate values are obtained for compounds with a dipole in the middle of the molecule, when the dipole is bonded to the polarisable benzene ring. For compounds with two, or more conjugated dipoles the effects on the Ts,* are more or less additive. This is shown clearly in Fig. 1, where the transition temperatures of mixtures 1-14 are plotted in order of increasing Ts,*. The clearing point (N*-I) also increases generally in the same order. However, the SA-N* transition temperature shows no such dependency and is missing alto- gether for three mixtures. This indicates that the position and nature of the dipoles often has the same effect on the N and Sc phases, but not on the SA phase. This infers that previous the~ries~'-~' for the S, phase are primarily valid only for fully aromatic compounds without any isolated (i.e.non-conjugated) dipoles. Thus, thc expla- nation~~',~'proposed in order to explain the low nematic transition temperatures (or complete absence of an N phase) of mesogens incorporating isolated dipoles59p63 (e g. non-conjugated oxygen or nitrogen atoms) would also appear to be valid for the S, phase. These theories invoke intermolecular repulsive interactions between dipoles in adjacent molecules assuming certain packing arrangement^.^^,^' The z and P, values for the mixtures containing conipounds 14, 19 and 33-43 are shown in Fig. 2.There is a general dependence of the switching times and spontaneous polaris- ation on Ts,* suggesting that these values are primarily a result of the temperature dependence of the tilt angle. This infers surprisingly similar viscosity values. The major excep- tions are for mixtures 12 and 14 containing ether 35 and ester 42, respectively, where a high spontaneous polarization value and a relatively short switching time infer a low viscosity Table 11 Chiral smectic C transition temperatures (T,J for mixtures 1-14 consisting of 15 wt.% of selected members of compounds 14, 19, 22 and 34-43 and 85 wt.% of the mixture SCO 1014 1 2 3 mixture T,,,/"C position 1" position 2" position 3" compound 1 24.6 OCH, 34 2 53.7 OK 41 3 57.6 -33 4 61.0 -40 5 63.3 -36 6 63.9 -14 7 63.9 CH20 37 8 65.8 co2 22 9 67.0 CH,O 43 10 67.1 11 67.4 12 67.8 13 69.3 14 69.7 " 0=oxygen; CO, and 0,C =ester; CH,O and OCH, =epoxymethano; Fig.1 Chiral nematic-isotropic ( W, N*-I) smectic A-chiral nematic (0,SA-N*) and the chiral smectic C-smectic A (A,Sc*-SA) transition temperatures for mixtures 1-4 containing compounds 14, 19, 22 and 33-34 CH,O 19 CH,O 39 -35 CH,O 38 -42 -=direct bond. 20 200 180 18 cu 160 0516 140Q 120 14 100 135791113 n Fig. 2 Spontaneous polarisation (W, P,) and switching times for mixtures 1-14 containing compounds 14, 19, 22 and 33-43 J. MATER. CHEM., 1994, VOL. 4 associated with a high Ts,* value. Thus, ether 35 and ester lOOr 42 are the most interesting compounds of the 14 screened.Mixture 13 incorporating diether 38 exhibits an almost equal 90 ITsc*,but a lower spontaneous polarisation and a much higher switching time. A surprising characteristic of the data in Table6 and Fig. 1 and 2 is the fact that the presence or absence of an SAphase for the mixture seem to have no effect on the spontaneous polarisation or the switching time. Therefore, derivatives of monoether 35 and ester 42 were chosen for further study. Influence of Double Bonds and Dipoles It has recently been shown that the incorporation of carbon- 2 3 4 5 6 7 carbon double bonds of defined configuration in certain n positions of alkoxy and alkanoyloxy chains can beneficially Fig. 3 Chiral nematic-isotropic (m, N*-I), smectic '4-chiral nematic influence both the transition temperatures and other physical (m, SA-N*) and the chiral smectic C-smectic A( A,S,.*-SA)transition properties of direct relevance for commercial FLCDS.~~-~~temperatures of the mixtures, containing 15 wt.% of the two-ring Therefore, an additional trans-carbon-carbon double bond was introduced into nonyloxy ether 35 to yield nonenyloxy ether 44.The result of this manipulation is shown in Table 7. T, is increased (+21 "C) while both Tsc, and TNIare both marginally lower (-3 and -6 "C, respectively). The SA phase has been suppressed. The effect of a similar manipulation on nonyloxy ester 42 to yield (E)-non-2-enoyloxy ester 45 is to suppress the S, phase completely, while decreasing the T, (-10 "C) and increasing the clearing point (+39 "C) signifi-cantly. This kind of behaviour has also been observed for analogous two-ring phenylpyrirnidine~.~~ The data compiled in Table 6 allow a comparison of the transition temperatures of two-ring phenylpyrimidines (46-52)39with a nonyl chain attached to the pyrimidine ring (X =C,H,) and an octyloxy/octenyloxy chain on the benzene ring with those of the analogous three-ring ethers (53-59) with an additional trans-l,4-disubstituted cyclohexane ring (X =C,H,,).The most striking aspects of the results collated in Table 6 are the substitution of an N phase for the SA phase of two-ring ethers 46-52 for all but two isomers 58 and 59 of three-ring ethers 53-59.TNIof three-ring ethers 53-59 is high (176 "C,on average). The melting point is higher (+37 "C,on average) than that of two-ring ethers 46-52.58 However, Tsc is increased more (+54 "C, on average, comparing only those homologues with an S, phase for both series), which results in an enantiotropic S, phase (22 "C, on average). Thus, the increased rigidity of the trans-1,4-cyclohexane ring results, as expected, in higher transition temperatures. As the nonyl chain of the two-ring phenylpyrimidines can adopt many more non-linear conformations than the trans-4-pentylcyclo- hexyl moiety, the effective length: breadth ratio of the latter will be greater, thus leading to the higher transition tempera- tures observed. The transition temperatures of the mixtures containing 15 wt.% of two-ring phenylpyrimidines 46-5239 or three-ring ethers 53-59 and 85 wt.% of the base mixture SCO 1014 are shown in Fig.3. The spontaneous polarisation and switching times of the same mxtures are shown in Fig. 4. Larger differences in the values determined for the spontaneous polarisation are observed than can be explained by differences in Ts,*. Therefore, it can be assumed that two-ring ethers 46-52 possess a smaller tilt angle. However, the switching times are lower than can be explained by the even lower tilt angle. Therefore, it must be concluded that the viscosity of the two-ring ethers (46-52) is lower (as could have been expected) than that of the three-ring ethers (53-59). However, the difference is not that large, thus, inferring that the viscosity of the three-ring ethers is low.Hence, the trans-1,4-cyclohexane ring has been shown to induce a low viscosity in the Sc phase as also observed for nematogens. phenylpyrimidines (46-52; solid lines) and the corresponding three- ring ethers (53-59; broken lines) and 85 wt.% of the chiral smectic C mixture SCO 1014 uersus the number of carbon atoms (n) from the core, where the carbon-carbon double bond in the alkenyloxy chain starts for each of the two series 2o '2oo 18 I 6 22 16 14 n Fig. 4 Spontaneous polarisation (m, P,) and switching times (e,z) of the mixtures containing 15 wt.% of same ethers as in Fig. 3, i.e. the two-ring phenylpyrimidines (46-52; solid lines) and the corre- sponding three-ring ethers (53-59 broken lines) and 85 wt.% of the chiral smectic C mixture SCO 1014 uersus the number of carbon atoms from the core, where the carbon-carbon double bond in the alkenyloxy chain starts for each of the two series The transition temperatures of a homologous series of esters (42 and 60-64) are collated in Table 8.Only S,-and N phases could be observed. T, is high (118 "C, on average) and, as a consequence, the range of the S, phase is narrow (13 "C, on average). TNIdecreases with increasing chain length. This leads to a significant narrowing of the nematic temperature range. The normal pattern of alternation for TNIis observed. Attempts to determine the relative merits of these esters in mixtures were not successful owing to solubility problems at room temperature for several of the homologues.The thermal data of a short homologous series of alkeny- loxy-substituted three-ring ethers (44, 54 and 65-68) are recorded in Table 9. The melting points are remarkably con- stant (95"C, on average). The S, phase is injected for octeny-loxy ether 54 and rises steeply as the chain lengthens. Thus, the temperature range of the S, phase increases strongly for longer chain lengths. Only an S, phase and an N phase could be determined. TNIdecreses steadily with increasing chain length and exhibits the normal alternation pattern. The transition temperatures of a short homologous series of alkenyloxy-substituted three-ring ethers (59 and 69-73) are J.MATER. CHEM., 1994, VOL. 4 listed in Table 10. S, and SAphases as well as an N phase are observed. T, and Tscare relatively low for three-ring systems (63 and 90 "C, on average, respectively), while the SA and N transition temperatures are high (134 and 176 "C, on average, respectively). As a consequence the temperature range of the S, and N phases is large (44 and 86 "C, on average, respect- ively). Both the s, and SA transition temperatures increase with increasing chain length, whereas TNI decreases pro- portionately. This is normal behaviour for such systems. The normal pattern of alternation for the clearing point is observed. The transition temperatures of the mixtures containing 15 wt.% of alkenyloxy-substituted three-ring ethers (44, 54 and 65-68; 59 and 69-73) and 85 wt.% of the base mixture SCO 1014 are shown in Fig.5. The clearing point (N*-I) is similar for both series. However, compounds 44,54 and 65-68 with a trans-carbonxarbon double bond exhibit a higher Tsc* and a broader N phase. Thus, the temperature range of the SA phase is much narrower than that observed for the corresponding series of alkenyloxy substituted three-ring ethers (59 and 69-73) with a carbon-carbon double bond in a terminal position. Fig. 6 shows the values of the spontaneous polarisation and switching ties for the same mixtures. They go together, i.e. their variation can be explained by differences 90 r Fig. 5 Chiral nematic-isotropic (N*-I), smectic A-chiral nematic (SA-N*) and the chiral smectic C-smectic A (Sc*-SA) transition temperatures of the mixtures containing 15 wt.% of ethers 44, 54 and 65-68 (solid lines), and 59 and 69-73 (broken lines) and 85 wt.% of the chiral smectic C mixture SCO 1014 versus the number of carbon atoms (n) in the alkenyloxy chain 6 7 8 9 10 11 n Fig.6 Spontaneous polarisation (m, P,) and switching times (0,z) of the mixtures containing 15 wt.% of the same ethers as in Fig. 5 [44,54 and 65-68 (solid lines) and 59 and 69-73 (broken lines)] and 85 wt.% of the chiral smectic C mixture SCO 1014 versus the number of carbon atoms (n) in the alkenyloxy chain [d =2, solid lines; d = (n -1)broken lines] 251 20-N'5 15-$?2 10-5-Ol I I10 20 30 40 50 60 70 TI'C Fig.7 Spontaneous polarisation (P,) and switching times (7) of mixture 11 versus temperature in the tilt angle, which is governed by the distance from Tsc,. This is shown clearly in Fig. 7, where the dependence of the spontaneous polarisation on the temperature is depicted. At about room temperature the increase in the spontaneous polarisation is ca. 0.15 nC cm-2 "C-'. Therefore, an average difference of 8°C in Tsc,corresponds to a difference in the spontaneous polarisation of 1.2 nC cmP2, which is equal to the differences observed. Conclusions Polar linkages between the cyclohexane and benzene rings in a cyclohexylphenylpyrimidine model system lead to higher S, transition temperatures than those observed for the analogous compounds with apolar linkages.Isolated dipoles such as oxygen atoms or carboxy groups attached to a non-polarisable ring (e.g. cyclohexane) leads to a lowering of Tsc and the clearing point (TNI)or to the total disappearance of the S, phase. The same dipoles attached to polarisable rings (e.g. benzene or pyrimidine rings) result in high Ts, and T,, values. Conjugated dipoles in the centre of the molecule usually exhibit intermediate effects. Combinations of dipole positions normally give rise to intermediate (additive or subtractive) effects. Conjugated outboard (terminal) dipoles have d greater effect on Ts, than central (conjugated) dipoles. 'The (E)-carbon-carbon double bond induces the highest T,,, and spontaneous polarisation of Sc* mixtures. The presence of the trans-l,4-disubstituted-cyclohexanering in the model system leads to compounds with moderately high Ts, and low viscosity values (i.e.short switching times) and are of interest for commercial mixture development for liquid-crj stal dis- plays based on ferroelectric effects. The authors express their gratitude to Mr. C. Haby and Mr. W. Janz for technical assistance in the preparation of the compounds and the determination of their physical data. Dr. W. Arnold (NMR), Mr. W. Meister (MS), Dr. M. Grosjean (IR), Mr. F. Wild and Mr. B. Halm (DTA) are thanked for the measurement and interpretation of the required spectra. Dr. R. Buchecker and Mr. T. Lukac are thanked for very constructive discussions and the generous donation of import-ant reaction intermediates.References 1 N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett., 1980 36,899. 2 N. A. Clark, M. A. Hanschy and S. T. Lagerwall, Mol. c:ryst. Liq. Cryst., 1983,94,213. 3 L. A. Beresnev, V. G. Chigrinov, D. I. Dergachev, E. P. Poshidaev, J. Funfschilling and M. Schadt, Liq. Cryst., 1989,5, 117 i . 1688 J. MATER. CHEM., 1994, VOL. 4 4 J. Funfschilling and M. Schadt, J. Appl. Phys., 1989,66 3877. 33 R. Buchecker, S. M. Kelly and J. Funfschilling, Liq. Cryst., 1990, 5 J. Funfschilling and M. Schadt, Proc. SID, 1990,31, 119. 8, 217. 6 J. Funfschilling and M. Schadt, SID 90 Digest, 1990, 106. 34 S. M. Kelly, J. Funfschilling and F. Leenhouts, Liq. Cryst., 1991, 7 J. Funfschilling and M.Schadt, Jpn. J. Appl. Phys., 1991,30, 741. 10,243. 8 J. Fiinfschilling, Proc. 21 st. Freiburger Arbeitstagung Flussig- 35 S. M. Kelly, Liq. Cryst., 1993, 14 675. kristalle, Freiburg, 1992. 36 J. Fiinfschilling, S. M. Kelly and A. Villiger, L7q. Cryst., 1993, 9 M. Schadt, Liq. Cryst., 1993, 14,73. 14, 713. 10 J. Funfschilling and M. Schadt, Proc. Euro Display '93, 37 S. M. Kelly, J. Fiinfschilling and A. Villiger, Liq. Cryst., 1993, Strasbourg, 1993, p. 63. 14, 1169. 11 K. Skarp and N. A. Clark, Mol. Cryst. Liq. Cryst., 1988,59,69. 38 S. M. Kelly and J. Funfschilling, J. Mater. Chem., 1993,3, 953. 12 C. Escher, Kontakte, 1986,2, 3. 39 S. M. Kelly, J. Funfschilling and A. Villiger, Liq. Cryst., 1994, 13 M. Brunet, J. Phjx Colloq., 1975,36, C1-321.16, 813. 14 R. Eidenschink, R. Hopf, B. S. Scheuble and A. E. F. Waechtler, 40 W. W. McMillan, Phys. Rev. A, 1973,8, 1921. Proc. 16th Freiburger Arbeitstagung Flussigkristalle, Freiburg, 41 A. Wulf, Phys. Rev. A, 1975, 11, 365. 1986. 42 R. G. Priest, J. Chem. Phys., 1976,65,408. 15 T. Geelhaar, C. Escher and E. Bohm, Proc. 17th Freiburger 43 A. Wiegeleben and D. Demus, Liq. Cryst., 19Y2, 11, 111, and Arbeitstagung Flussigkristalle, Freiburg, 1987. references cited therein. 16 J. Fiinfschilling and S. M. Kelly, Proc. 20th Freiburger 44 J. W. Goodby, G. W. Gray and D. G. McDonnell, Mol. Cryst. Arbeitstagung Fliissigkristalle, Freiburg, 1991. Liq. Cryst. Lett., 1977,34, 183. 17 D. Demus, H. Demus and H. Zaschke, Flussige Kristalle in 45 J. W.Goodby, and G. W. Gray, Ann. Phys., 1978,3, 123. Tabellen, Deutscher Verlag fur Grundstoff Industrie, Leipzig, 46 R. Eidenschink, D. Erdmann, J. Krause and L. Pohl, Angew.1974. Chem., Int. Ed. Engl., 1977, 16, 100. 18 D. Demus and H. Zaschke, Flussige Kristalle in Tabellen, 47 N. Carr, D. G. McDonnell and G. W. Gray, Mol. Cryst. Liq. Deutscher Verlag fur Grundstoff Industrie, Leipzig, 1984, vol. 11. Cryst., 1983,97, 13. 19 G. W. Gray, in Advances in Liquid Crystals, ed. G. H. Brown, 48 H.-J. Deutscher, B. Laaser, W. Dolling and H. Schubert, J. Prakt. Academic Press, New York, 1976, vol. 2, p. 16, and references Chem., 1978,320,191. cited therein. 49 M. Petrzilka, R. Buchecker, S. Lee-Schiederer, M. Schadt and 20 K. Toyne, in Thermotropic Liquid Crystals, ed.G. W. Gray, Wiley, A. Germann, Mol. Cryst. Liq. Cryst., 1987, 148, 173. New York, 1987, and references cited therein. 50 B. Neises and W. Steglich, Angew. Chem., 1978,90,556.21 D. Coates, in Liquid Crystal Applications and Uses, ed. 51 S. M. Kelly, R. Buchecker and J. Funfschilling, J. Mater. Chem., T. Bahudar, World Scientific Press, New Jersey, 1990, vol. 1, p. 91, 1994,4, 1689. and references cited therein. 52 M. Ettinger, R. Nardin, S. Ray Mahasay and L. M. Stock, J. Org.22 J. W. Goodby, in Ferroelectric Liquid Crystals, Principals, Chem., 1986,51,2840.Properties and Applications, Gordon and Breach, Philadelphia, 53 G. Illian, I. Muller and R. Wingen, 1992, EPA 0477901. PA, 1991. 54 Z. Arnold, Collect Czech. Chem. Commun., 1973,38, 1168. 23 H. Zaschke, Z. Chem., 1975,15,441. 55 L. N...Owen and D. A. Robins, J. Chem. SOC., 1949,326.24 H. Zaschke, J. Prakt. Chem., 1975,317,617. 56 G. Osterheld, P. Marrug, R. Riihrer and A. Germann,25 S. Saito, K. Kitano, K. Ohno, H. Inoue and M. Ushiodo, 1988, J. Chromatogr., 1982,234,99.EPA 0 293 764. 57 F. Leenhouts, S. M. Kelly and A. Villiger, Displaw, 1990,41.26 H. Zaschke and H. Schubert, J. Pract. Chem., 1970,312,494; 1973, 58 E. P. Janulis, C. G. Johnson, P. M. Savu and D. T. Spawn, 1993, 315,1113. WO93229396.27 H. Zaschke, A. Isenberg and H. Schubert, J. Pract. Chem., 1979, 59 N. Carr, G. W. Gray and S. M. Kelly, MoZ. Cryst. Liq. Cryst., 321,619. 1981,66, 267. 28 A. Biering, D. Demus, L. Richter, H. Sackman, H. Wiegeleben 60 G. W. Gray, Mol. Cryst. Liq. Cryst., 1981,63, 3. and A. Zaschke, Mol. Cryst. Liq. Cryst., 1980,62, 1. 61 H. J. Dietrich and E. L. Steiger, Mol. Cryst. Liq. Cryst., 1972,29 G. Kraus and H. Zaschke, J. Pract. Chem., 1981,323,199. 16,263.30 S. M. Kelly and A. Villiger, Liq. Cryst., 1988,3, 1173. 31 W. Hemmerling, I. Muller and R. Wingen, Ferroelectrics, 1988, 62 E. L. Steiger and H. J. Dietrich, Mol. Cryst. Liq. Cryst., 1972, 85,393. 16,279. 32 C. Escher, W. Hemmerling, G. Illian, I. Muller, P. Wegener and 63 M. A. Osman and L. Revesz, Mol. Cryst. Liq. Cryst., 1980,56, 133. R. Wingen, presented at the 13th International Liquid Crystal Conference, Vancouver, BC, Canada, 1990. Paper 4/01811G:Received 25th March, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401673

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994,4(11), 1689-1697 a-Fluoro Esters Incorporating a Cyclohexane Ring: Some New Chiral Dopants for Ferroelectric Mixtures Stephen M. Kelly,* Richard Buchecker and Jurg Funfschilling F. Hoffmann-La Roche Ltd., Dept. RLCR, CH-4002 Bade, Switzerland Several new homologous series of optically active a-fluoro esters incorporating a cyclohexane ring and a number of different cores have been synthesized. Several of these chiral dopants possess an enantiotropic chiral smectic C (Sc*) phase at elevated temperatures. They are characterised by a relatively high spontaneous polarisation, long pitch and low rotational viscosity. In addition they exhibit exceptional chemical, thermal and photo- and electro-chemical stability. It is shown that an ester group in a terminal position of the molecular core of these new chiral dopants leads to an increase in the spontaneous polarisation (compared with the values for the analogous ethers) with only a small increase in the rotational viscosity, as determined in a standard Sc base mixture.An ester group in a central position of the molecular core has the opposite effect on these parameters. It is shown that at least two aromatic rings are normally necessary for S, formation. An optically active a-chloro ester incorporating a cyclohexane ring was also prepared and compared with the analogous a-fluoro ester. A higher spontaneous viscosity were determined far the a-fluoro ester. These new a-fluoro esters are excellent chiral dopants for chiral smectic C mixtures for use in the surface- stabilised ferroelectric liquid-crystal display.Surface-stabilised ferroelectric liquid-crystal devices (SSFLCDs)'-' are being developed for commercial appli- cations with a high information content and/or fast response times (e.g. computer screens, printer heads, spatial light modu- lators). The display devices are characterised by exceptionally fast response times (ps), high contrast, good viewing angle dependence and memory (bistability). Most commercially available chiral smectic C (S,*) mixtures designed for SSFLCDs consist of a non-optically active base S, mixture doped with at least one optically active (chiral) dopant.6-8 The chiral dopant should induce the desired value of spon- taneous polarisation (Ps)and a long helical pitch in the base S, mixture (in order to avoid the necessity of pitch compen- sation) without depressing the S, transition temperature (S,*-S,/N) or increasing the rotational viscosity (y), or the birefringence (An) of the mixture excessively.The dopant can also effect the tilt angle (O), which influences the switching times and the contrast (see Mixture Properties). All the mixture components must be chemically, thermally and photo- and electro-chemically stable. For most applications it is essential to utilise chiral dopants, which induce a large spontaneous polarisation without increasing the rotational viscosity excessively. The resultant induced spontaneous polarisation of the mixture must also not be too large (to avoid charge effects), but a large intrinsic value of P, for the chiral dopant allows smaller amounts to be The induced spontaneous polarisation depends, amongst other factors, on the dipole moment at the optically active centre of the chiral dopant.Hence, at least one of the substituents attached to the optically active centre is usually strongly electronegative (e.g. flu~rine,'~-~~ nitrile26,27) in ~hlorine,~~,~' order to polarise the carbon-substituent bond. Other polar groups in the vicinity of the chiral centre (e.g.oxygen," ester,26 oxirane16-'8) also serve to increase the polarisation. The optically active centre should be as close as possible to the core of the molecule (commensurate with phase stability) in order to optimise dipole and steric interactions and thus minimise the rotational freedom of the dipole moment. Similar effects can also be obtained by using a variety of optically active five- and six-membered rings containing polar groups (e.g.dioxolane,28 la~tone,,~-~~ proline rings36).o~azolidine,~~ We have recently reported a wide variety of new, optically active a-fluoro esters incorporating the aliphatic cyclohexane ring and a number of different cores.15 Several of these chiral dopants possess an enantiotropic Sc* phase at elevated tem- peratures, characterised by high spontaneous polarisation, long pitch and low rotational viscosity. In addition they exhibit exceptional chemical, thermal and photo- and electro- chemical stability. Although the spontaneous polarisation is reduced by about 40% compared with that of fully aromatic analogue^,'^.'^ a significantly lower rotational viscosity leads to much shorter response times.Additionally the substitution of the aliphatic trans-l,4-disubstituted cyclohexane ring for an aromatic (benzene) ring was found to result in lower values for the birefringence and in a substantially longer pitch.I5 We now report on attempts to improve further the physical properties of these chiral dopants as determined in a standard Sc base mixture (e.g. reduced response times, higher S,* transition temperatures) by modification of core structures and terminal chains. It has recently been shown that in contrast to the situation observed for nematics, the introduction of an ester group (CO,) between the terminal chain and the core of the rmiolecule of base S, components can lead to lower viscosity values and thus, response Therefore, it was decided to investi- gate the effects of an ester group in this position of chiral dopants in order to determine whether lower induced viscosit- ies for the Sc* mixture could be obtained.It has also been demonstrated recently that introduction of a carbon carbon double bond of a defined configuration in certain positions of the terminal alkanoyloxy chain of a series of 4454-alkylpyrimidin-2-y1)phenylalkanoates can lead to improve- ments in the transition temperatures and other physical properties of relevance to display device appli~:itions.~~ Therefore, it was decided to extend this system to the chiral dopants as described above.Only (E)-alk-2-enoyloxy esters were synthesized as this has been shown to be the most advantageous position and configuration of the carbon-carbon double bond.39 A preliminary screening of one homologue each of the trans-4-(2',3'-difluoro-4'-n-alkoxybiphenyl-4-yl)cyclohexyl(R)-2-fluorohexanoates and trans-4-[4-(5-alkylpyrimid1n-2-y1)-phenyl] cyclohexyl (R)-2-fluorohexanoates had indicated that such chiral dopants were promising candidates for commercial SSFLCD mixt~res.'~ Therefore, it was decided to synthesize a homologous series of each type, in order to be able to identify the esters with the most advantageous comhination of physical properties.Large variations are often observed within a homologous series (e.g. a factor of two in switching times) and trends are often impossible to predict despite the large body of information already available for such sys- tem~.~~-~~It was also hoped that by substituting an alkoxy chain for the alkyl chain of the trans-4-[ 4-(5-alkylpyrimidin-2-yl)phenyl]cyclohexyl (R)-2-fluorohexanoates Sc* phases could be induced. One optically active (R)-2-chlorohexanoate analogue of J. MATER. CHEM., 1994, VOL. 4 (i.e. no racemisation occurred during esterificat~on). The struc- tural and isomeric purity was determined by differential thermal analysis and capillary gas chromatography as usual and, where necessary, on liquid-crystal-packed columns.48 The transition temperatures of the esters prepared, recorded in Tables 1-5, were determined by optical microscopy using a Leitz Otholux I1 POL BK microscope in conjunction with a Mettler FP 82 heating stage and FP 80 control unit.All the monotropic liquid-crystal phases could be observed using the tra~zs-4-(2’,3’-difluoro-4’-n-alkoxybiphenyl-4-yl)cyclohexyl (R)-2-fluorohexanoates was also prepared in order to deter- mine the relative merits of these two widely used polar groups in otherwise identical chiral dopant^.^^,^^ A small series of biphenylyl- and phenylpyrimidinyl-cyclo-hexyl (R)-2-fluorohexanoates with alkyl and alkoxy chains were prepared in order to determine the necessity of hetero- atoms (e.g. nitrogen or oxygen) for the formation of Sc phases in such systems.46 Experimental Synthesis The trans-4-(4-n-alkanoyloxyphenyl)cyclohexyl(R)-2-fluoro-hexanoates (l-ll) and trans-4-{4-[(E)-alk-2-enoyloxy]phen-yl} cyclohexyl (R)-2-fluorohexanoates ( 12-18) were prepared by selective esterification of 4-(trans-4-hydroxycyclohexyl)-phen01’~ at the phenolic hydroxy group with either alkanoic or (E)-a1 k-2-enoic acids to produce the corresponding trans-4-(alkanoyloxy-or (E)-alk-2-enoyloxy-phenyl)cyclohexan-l-01s.These cyclohexanols were then esterified a second time with (R)-2-fluorohexanoic a~id’~,~~ to yield the desired diesters (l-ll) and (12-18). The general method of synthesis of the trans-4-(2’,3’-difluoro-4’-n-alkoxybiphenyl-4-yl)cyclohexyl (R)-2-fluoro-hexanoates ( 19-30) and trans-4-[ 4-(5-alkylpyrimidin-2-y1) phenyl] cyclohexyl (R)-2-fluorohexanoates (32-36) has already been described for one homologue of each series.” The (R)-2-chlorohexanoate (31) was prepared as described for a microscope and no virtual values (extrapolated) had to be determined.When necessary the Mettler stage could be cooled (-50°C) by allowing N2 gas, cooled by liquid N2, to pass through the stage at a controlled rate. The liquid-crystal transition temperatures were also determined using a Mettler DTA TA 2000. The purity of the compounds was determined by a thin- layer chromatography (TLC), gas chromatography and differential thermal analysis (DTA). A Perkin-Elmer 8310 capillary gas chromatograph and GP-100 graphics printer were used.Precoated TLC plates, 4cm x 8 cm, SiOz SIL G/UV254, layer thickness 0.25 mm (Machery -Nagel, Duren, Germany), were utilised. Column chromatography was carried out using silica gel 60 (230-400 mesh ASTM). Reaction solvents and liquid reagents were purified by either distillation or drying shortly before use. Reactions were carried out under N, unless water was present as a reagent or solvent. All temperatures were Table 1 Transition temperatures for the trans-4-( 4-n-alkanoyloxy- pheny1)cyclohexyl (R)-2-fluorohexanoates (1-1 1 ) ~ the trans-4-(2’,3’-difluoro-4’-n-alkoxybiphenyl-4-yl)cyclohexyl 1 1 48 (R)-2-fluorohexanoates (19-30) using (R)-2-chlorohexanoic acid14 instead of (R)-2-fluorohexanoic acid. The trans-4-[4-(5-alkoxypyrimidin-2-yl) phenyl] cyclohexyl (R)-2-fluorohexanoates (37-41) were synthesized by condensation of 4-(trans-4-h y droxycyclohexyl)benzamidine hydrochloridelS with the aldehyde prepared in situ from benzyloxyacetaldehydediethylacetal to yield trans-4-[ 4-( 5-benzyloxypyrimidin-2-yl)phenyl]cyclohexanol.Removal of the benzyl protection group by catalytic hydrogenation resulted in the corresponding phenol, which could be selec- tively alkylated in a Williamson ether synthesis to produce trans-4-[ 4-(5-alkoxypyrimidin-2-yl)phenyl]cyclohexanol. Ester- ification with (R)-2-fluorohexanoic acid as above yielded the desired esters (37-41).The methods of synthesis and structural analysis of the new esters (1-42) are described in detail below. The configuration of the carbon-carbon double bond in the alkenyl chain of the new esters (12-18) was confirmed by ‘H nuclear magnetic resonance (NMR) spectroscopy (the trans-olefinic coupling constants, ca.12-18 Hz, are larger than those of the corre- sponding cis-olefinic coupling constants, ca. 7-11 Hz) and by infrared (IR) spectroscopy (the trans-absorption bands are narrow and exact ca. 970-960 cm -I, whereas the cis-absorp- tion bands are observed at distinctly different wavelengths ca. 730-675 cm-’). The optical purity (90.6% ee) of the optically active (R)-2-fluorohexanoic acid (prepared and purified as described in the 1iterat~re.I~~~~) was determined according to literature rnethod~.”~~~ The optical purity of the (R)-2-fluoro- hexanoates was determined similarly and found to be identical 2 2 51 3 3 38 - 4 4 25 51 5 5 42 59 6 6 52 59 7 7 42 64 8 8 41 67 9 9 45 68 10 10 52 71 11 11 50 72 Table 2 Transition temperatures for the tran.s-4-{4-[(E)-alk-2-enoyloxy] pheny1)cyclohexyl (R)-2-fluorohexanoates (12-18)” 12 1 13 2 14 3 15 4 16 5 17 6 18 7 ‘Values given in temperature.81 (611 -76 78-92 -(46)< -80 74-66 77 57 -77 75 -78 parentheses represent a monotropic transition J. MATER. CHEM., 1994, VOL. 4 Table 3 Transition temperatures and enthalpies of fusion for the trans-4-(2’,3’-difluoro-4‘-n-alkoxybiphenyl-4-yl)cyclohexyl(R)-Zfluoro-hexanoates ( 19-30) and the trans-4-( 2’,3’-difluoro-4‘-decyloxybiphenyl-4-y1)cyclohexyl (R)-2-chlorohexanoate (31) EF ester n X (C-SA/N*)/OC (SA-N*/I)/”C (N*-I)/”C AH/kJ mol-’ 19 1 F 70 -116 22.0 20 2 F 73 108 138 23.5 21 3 F 59 122 127 25.2 22 4 F 44 133 -26.5 23 5 F 33 131 -16.6 24 6 F 38 133 -22.7 25 7 F 37 129 -22.4 26 8F 33 134 -18.1 27 9F 41 127 -23.6 28 10 F 35 128 -22.3 29 11 F 42 125 30 12 F 38 134 -29.8 -31 10 c1 34 112 -measured externally unless otherwise stated.The ‘H NMR spectra were recorded at 60 MHz (Varian T-60), 80 MHz (Bruker WP-80) or 250 MHz (Bruker HX-270). Tetra-methylsilane was used as the internal standard. Mass spectra were recorded on a MS9 (AEZ Manchester) spectrometer. The S,* mixture SC9-1219 consists of 5-(5-heptyl-1,3-di-oxan-2-y1)-2-(4-octyloxypheny1)pyridine(7.0 wt.%), 5-( 5-octyl-1,3-dioxan -2 -yl) -2 -(4-octyloxyphenyl) pyridine (7.0 wt.%), 5-(5-decyl- 1,3 -dioxan-2-yl)-2-(4-octyloxyphenyl)pyridine (6.0 wt.%), 4-[ 2-(trans-4-pentylcyclohexyl)ethyl]phenyl 4-decyloxybenzoate ( 15.9 wt.%), 4-[ 2-(trans-4-pentylcyclohex-yl)ethyl] phenyl 4-dodecyloxybenzoate (7.1 wt.%), 4-[ 2-(trans-4-pentylcyclohexyl)ethyl]phenyl 2,3-difluoro-4-(und~cyloxy)-benzoate (7.0 wt.%), 2-(4-hexyloxyphenyl)-5-non!lpyrimi-dine ( 14.9 wt.Yo), 5-nonyl-2-( 4-nonyloxyphenyl )pyrimi- dine ( 19.9 wt.%),5-heptyl-2-( 4-octyloxypheny1)py rimidine (5.0 wt.%) and 2-(4-hexyloxyphenyl)-5-ocytlpyrimidine (10.1 wt.%). The determination of the physical properties of the chiral mixtures containing the new esters was carried out as pre- viously des~ribed.~’,~’ Synthesis of trans-4-( CAcetoxyphen yl )cyclohexanol A solution of N,N-dicyclohexylcarbodiimide (0.78 g, 0.0038 mol) in dichloromethane (50 cm3) was added slowly to a solution of 4-(trans-4-hydroxycyclohexyl)pheno11~(0.60 g, 0.0031mol), acetic acid (Fluka) (0.18 g, 0.0031mol), 4- (dimethy1amino)pyridine (0.04 g) and dichloromethane (25 cm3) at 0 “C, stirred at room temperature overnight, filtered and the filtrate evaporated down under reduced pressure.The residue was purified by column chromatography on silica gel using a 1: 1 hexane:ethyl acetate mixture as eluent followed by recrystallisation from ethanol to yield 0.65 g (90%) of the pure ester.v,,,/cm-’: 3409, 3329, 2927, 2852, 1756, 1626, 1575, 1223, 835. Mass spectrometry (MS) m/z: 234 (M+), 192 (C12H1602), 174 (C12H14O). Table 4 Transition temperatures for the trans-4-[ 4-( 5-n-alkylpyrimidin-2-yl)phenyl]cyclohexyl (R)-2-fluorohexanoates (32-36) and tr6zns-4-14-(5-n-alkoxypyrimidin-2-yl) phenyl]cyclohexyl (R)-2-fluorohexanoates (37-41) -.32 34 98 -137 -.33 53 101 -143 -.34 41 109 -143 -.35 49 113 -145 -.36 48 116 -145 37 58 -81 161 105 38 44 78 95 162 103 -.39 53 88 102 162 -.40 60 -92 106 163 -.41 35 70 98 108 165 Table 5 Comparison of the transition temperatures for the trans-4-(4’-decylbiphenyl-4-yl)cyclohexyl(R)-2-fluorohexanoate (42), trans-4-[4-( 5- nonylpyrimidin-2-yl) phenyl]cyclohexyl (R)-2-fluorohexanoates (36) and trans-4-[ 4-( 5-nonyloxypyrimidin-2-yl)phenyl]cyclohexyl (R)-Xuoro-hexanoates (40) --42 145 36 48 116 -145 40 60 90 108 165 Synthesisof trans-4( 4-Acetoxypheny1)cyclohexyl(R)-2-Fluorohexanoate, 1 A solution of N,N-dicyclohexylcarbodiimide (0.68 g, 0.0033 mol) in dichloromethane (50 cm3) was added slowly to a solution of trans-4-(4-acetoxyphenyl)cyclohexanol(O.65g, 0.0028 mol), (R)-2-fluorohexanoic acid15 (0.37 g, 0.0028 mol), 4-(dimethy1amino)pyridine (0.04 g) and dichloromethane (25 cm3) at 0 "C and then stirred at room temperature over- night.The reaction mixture was worked up and purified as described above to yield 0.85 g (42%) of the pure ester. 'H NMR 8, (CDCI,; standard TMS; 250MHz): 0.90-0.95 (3 H, t), 1.25-2.14 (16 H, overlapping peaks), 2.29 (3 H, s), 2.62 (1 H, overlapping peaks), 4.77-4.81 (1 H, t), 4.96-5.01 ( 1 H, overlapping peaks), 6.99-7.02 (2 H, d), 7.18-7.26 (2 H, d).v,,,/cm-': 2942, 2862, 1751, 1509, 1373, 1200, 840. MS m/z: 350 (M'), 308 (C18H2503).Microanalysis found (expected): C 68.4 (68.5), H 7.8 (7.7), F 5.5 (5.4)%. [a],,=+8.3 (c. 0.0060 g cm-,; CHC1,). The transition temperatures of this ester (1) and similar esters (2-11 and 12-18) prepared using this general method are collated in Tables 1 and 2. Synthesis of trans-4-[ 4-( 5-Benzyloxypyrimidin-2-yl )phenyl] cyclohexanol N,N-Dimethylformamide (8.9 cm3, 115 mmol) was added dropwise to phosporyl chloride (8.6 cm3, 94 mmol) at 0 "C and then stirred for 15 min.A solution of benzyloxyacetal- dehyde diethyl acetal (14.0 g, 62 mmol) in N,N-dimethylfor- mamide (30 cm3) was added dropwise to the reaction mixture, which was heated at 50°C for 18 h. A solution of 4-(trans- 4-hydroxycyclohexyl) benzamidine hydrochloride ( 15.9 g, 62 mmol) in N,N-dimethylformamide (60 cm3) was added dropwise to the cooled reaction mixture (room temperature) and then stirred for 30 min. Triethylamine (69 cm3) was added dropwise and the reaction mixture heated at 50°C for 2 h, poured onto water (500 cm3), cooled to 0 "C, acidified with 36% hydrochloric acid (pH 3-4), stirred for 20 min at this temperature and then extracted into ethyl acetate (3 x 300 cm'). The combined organic layers were washed with water (3 x 300 cm3), dried (Na,SO,), filtered and evaporated down.The residue was purified by column chromatography on silica gel using a 1:1 ethyl acetate: toluene mixture as eluent and recrystallised from tert-butyl methyl ether to yield 6.3 g (33%) of the desired alcohol; mp 220-222 "C. 'H NMR BH (CDCl,; standard TMS; 250 MHz): 1.50 (5 H, overlapping peaks), 1.94 (4 H, overlapping peaks), 2.62 (1 H, overlapping peaks), 3.60, (1 H, overlapping peaks), 4.14-4.21 (2 H, q), 7.26-7.29 (2 H, overlapping peaks), 8.23-8.27 (2 H, d), 8.51 (2 H, s). v,,,/cm-': 3430,2927,2854, 1604, 1542, 1436, 1269, 1063, 993, 781 cm-'. MS m/z: 360 (M'), 342 (C23H2zN20). Synthesis of trans-4-[ 4-( 5-Hydroxypyrimidin-2-yl )phenyl] cyclohexanol A mixture of trans-4-[ 4-( 5-benzyloxypyrimidin-2-yljphenyll-cyclohexanol (1.0 g, 2.7 mmol), ethyl acetate (20 cm3), and 10% palladium on active charcoal (0.3 g) were hydrogenated until no more hydrogen was taken up.The catalyst was filtered off and the filtrate evaporated down. The residue was purified by column chromatography on silica gel using a 20 :1 dichloromet hane-met hanol mixture as eluent and recrys tal- lised from ethanol to yield 0.5 g (60%) of the desired alcohol. 'H NMR 6, (CDCI,; standard TMS; 250MHz): 1.89 (8 H, overlapping peaks), 2.49-2.51 (1 H, s), 3.34 (1 H, s), 7.29-7.33 (2 H, d), 8.14-8.18 (2 H, d), 8.40 (2 H, s). vrnax/cm-': 3424, 3257,2930,2855,2725,1611,1555,1429,1284,1050,793 cm-'. MS m/Z: 270 (M'), 252 (C16H16N20). J. MATER.CHEM., 1994, VOL. 4 Synthesis of trans-4-[ 44 5-Decyloxypyrimidin-2-yl )phenyl] cyclohexanol A mixture of 1-bromodecane (Fluka; 0.7 g, 0.0031 mol), trans-4-[ 4-( 5-hydroxypyrimidin-2-yl)phenyl]cyclohexanol (0.5 g, 0.0026 mol), potassium carbonate (0.14 g, 0.0104 mol) and butan-2-one (50 cm3) was heated under gentle reflux over- night, then filtered to remove inorganic material. The filtrate was diluted with water (1000 cm3) and then extracted into diethyl ether (3 x 100 cm3). The combined organic extracts were washed with water (2 x 500 cm3), dried (MgSO,), filtered and then evaporated down. The residue was purified by column chromatography on silica gel using a 9: 1 hexane- ethyl acetate mixture as eluent and recrystallised from ethanol to yield 0.5 g (47%) of the desired alcohol; mp, 160-162°C.v,,,/cm-': 3421, 2925, 2853, 1609, 1541, 1438, 1273, 1064, 840, 782 Cm-'. MS m/Z: 410 (M'), 392 (C26H36N20). Synthesis of trans-4-[ 44 5-Decyloxypyrimidin-2-yl )phenyl] cyclohexyl (R)-2-fluorohexanoate, 41 A solution of N,N-dicyclohexylcarbodiimide (0.3 g, 1.2 mmol) in dichloromethane (10 cm3) was added slowly to a solution of trans-4-[ 4-( 5-decyloxypryrimidin-2-yl )phenyl ]cyclohex-anol (0.5 g, 1.0mmol), (R)-2-fluorohexanoic acid15 (0.2 g, 1.0 mmol), 4-(dimethy1amino)pyridine (0.04g t and dichloro- methane (25 cm3) at 0 "Cand then stirred at room temperature overnight. The reaction mixture was worked up and purified, as described above, to yield 0.4 g (62%) of the desired ester.'H NMR 8, (CDC1,; standard TMS; 250 MHz): 0.88-0.96 (6 H, overlapping peaks), 1.28-2.00 (34 H, overlapping peaks), 2.62 (1 H, overlapping peaks), 4.06-4.11 (2 H, t), 4.76-5.11 (2 H, t), 7.26-7.29 (2 H, overlapping peaks), 8.25-8.28 (2 H, d), 8.44 (2 H, s). vmax/cm-': 2925, 2855, 1733, 1608, 1540, 1436, 1278, 1082, 854, 778. MS m/z: 526 (M'), 392 (C&35N20'). Microanalysis found (expected): C 75.2 (75.31, H 9.2 (9.3), N 5.5 (5.5), F 3.6 (3.7)%. +4.3 (C 0.0080 g cm-,; CHCl,). The transition temperatures of ester 41 and similar esters 37-40, prepared using this general method, are collated in Table 4. Mesomorphic Properties The transition temperatures of an homologous series of trans-4-(4-n-alkanoyloxyphenyl)cyclohexyl(R)-2-fluorohexanoates (1-11) are recorded in Table 1.The first three homologues (n= 1-3) do not exhibit mesomorphic behaviour. The other members of the series only exhibit an SB mesophase above the crystalline state. The plots of the S, transition temperature against the number of carbon atoms (n) in the alkanoyloxy chain rise with increasing chain length and show the normal pattern of alternation. No other mesophases could be observed. Table 2 contains the transition temperatures of the corre- sponding trans-4- (4-[(E)-alk-2-enoyloxy] phenyl} cyclohexyl (R)-2-fluorohexanoates (12-18) with an additional carbon- carbon double bond. The introduction of the trans double bond into diesters 1-11 to yield diesters 12-18 results in a small increase in the SB transition temperature (+3"C, on average, comparing only homologues of equal chain length); this is unusual. In all previous investigations of this effect in esters39,45$46and ordered mesophases were partially or totally suppressed, Sc or N phases were induced and the width of the Sc phase was extended.A melting point for several homologues could not be determined due to the low tendency for crystallisation of the ordered SB phase. The (E)-but-2-enoyloxy substituted ester (12) exhibits an N phase instead of the SB phase observed for the other homologues. J. MATER. CHEM., 1994, VOL. 4 This unusual behaviour underlies the strong nematic tendenc- ies of the (E)-but-2-enoyloxy function recently described for aromatic esters39 and aliphatic cyclohexyl ester^.^^,^^ The transition temperatures of an homologous series of the trans-4-(2',3'-difluoro-4'-n-alkoxybiphenyl-4-yl)cyclohexyl(R)-2-fluorohexanoates (19-30) are listed in Table 3.The first three members of the series exhibit an N* phase at relatively elevated temperatures (127 "C, on average). Eleven homol- ogues possess an enantiotropic SA phase at similar tempera- tures (126"C, on average). The plots of the s, transition temperature uersus the number of carbon atoms in the alk- anoyloxy chain show an increase with short chain lengths and then remain relatively flat for longer chain lengths. The plots show the normal pattern of alternation. The melting point is higher for short chain lengths than for longer chains.However, the melting point does not vary greatly with chain length (45 "C, on average). No other mesophases could be determined. This is unusual considering that the esters contain three-rings and a combined chain length of up to 18 carbon atoms. This must be in part due to the two lateral fluorine atoms, which have been shown to decrease the tendency for ordered phase A comparison between the thermal data of esters 19-30 with a direct linkage between the two phenyl rings and the corresponding die~ters'~ with an additional carboxy group (CO,) between the same two rings reveal that the diesters are superior with respect to the liquid-crystal transition temperatures. The diesters exhibit the desired order of phases for SSFLCDs (i.e.Sc*, SA and N* phases). However, the monoesters (19-30) do not depress the Sc* phase transition temperature of the base mixture excess- ively (see Mixture Properties). The clearing point (S,-I) of the chloro-substituted ester (31)is significantly lower (-14 "C) than that of the corresponding r-fluoro-substituted ester (28) with the same chain length (n= 10). This is almost certainly due to the larger van der Waals radius of the chlorine atom. Table 4 contains the thermal data for the two homologous series of trans-4-[ 4-( 5-alkylpyrimidin-2-y1)phenyllcyclo-hexyl (R)-2-fluorohexanoates (32-36) and trans-4-[ 4-( Salk- oxypyrimidin-2-yl) phenyl] cyclohexyl (R)-2-fluorohexanoates (37-41). The alkyl substituted series (32-36) only possesses orthogonal smectic phases (B and A) at elevated tempera- tures (107 "C and 143 "C, respectively).The melting point is relatively low and unusually uniform (45 "C, on average). The alkoxy-substituted series also exhibits SB and SA phases. However, the transition temperatures for the SB phase are lower (-1SoC, on average), whereas those of the SA phase are higher (+2O"C, on average) to an almost equal extent. An N* phase is observed for two homologues with short chains (37 and 38). All the homologues prepared possess an Sc* phase (98 "C, on average). An ordered (as yet unidentified) smectic phase is observed for homologue 41 with the longest chain studied. The melting point of the alkyl and alkoxy- substituted series are very similar (45 and 50"C, on average, respectively).This is unusual and is probably due to the presence of the S, phase. The thermal data collated in Table 5 show clearly that, even for three-ring systems, compounds containing the biphenyl moiety (e.g. 42) do not exhibit an S, phase. Indeed the alkyl- substituted phenylpyrimidine (36) also only exhibits ortho- gonal phases. Only the combination of the phenylpyrimidine core and an alkoxy chain in the ester (40) gives rise to an Sc* phase. The dependence of the Sc+ phase on the nature and position of dipoles in the cyclohexyl-phenyl-pyrimidine mesogenic system will be discussed in detail elsewhere.47 The liquid-crystal transition temperatures of the three-ring pyrimidines (37-41) show clearly that substances containing the trans- 1,4-disubstituted cyclohexane ring can exhibit an enantiotropic Sc* phase.This is in accord with previous results for a variety of three-ring phenyl ben~oates~"'~ with various linking units (e.g. single bond, epoxymethano, ethyl, carboxy, four-unit-linking group) and phenylpyrim-idines.52,55,56The results in Tables 1-5 show clearly that two- ring systems incorporating a 1,4-disubstituted cyclohexane ring are not sufficient for S, formation (exceptions containing a strong lateral dipole are known) and the results also show that a minimum of two aromatic rings per aliphatic; ring is required for S, formation (phenyl benzoates with 1,4-&substi- tuted bicyclo C2.2.21 octane ring in place of the 1,4-disubsti- tuted cyclohexane ring of the diesters also exhibit an Sc pha~e.~~,~~This is in contrast to statistical theories of the Sc phase; the theories normally assume a fully arom;ttic core str~cture.~'+~~ Mixture Properties Since most of the new chiral dopants (1-42) do not themselves possess an Sc* phase, parameters such as the spontaneous polarisation or switching time have to be determined in mixtures.Therefore, a small amount (7 wt.%) of the dopant is added to a standard non-chiral base mixture (S('9-1219) with the phase sequence Sc-SA =76 "C, SA-N =81 "C and N-I =103 "C. The transition temperatures, the spontaneous polarisation and the switching time of the resultant mixtures are then determined under standard conditions (T: 15 Vpp/p square wave, time to maximum current; P,: 10 Vpp/p triangu- lar wave form).In order to discuss the differences in response tinies, it is necessary to define more exactly the parameters involved. The spontaneous polarisation, the effective viscosity ;md the switching time, all depend on the Sc tilt angle 8. However, it is difficult to measure 6' reliably as details of the surface alignment influence the result. Therefore, attempts were made to estimate and then eliminate the influence of variations of 0 without actually measuring it. The spontaneous polxisation can be related to the tilt angle by62 P,= Po sin 8 (1) where Po is a constant characteristic for the dopant. An effective viscosity yeff can be defined63 via yeff dyl/dt +P, sin ylE =0 (2) where q is the angle of rotation on the Sc* cone and E the applied electric field.This equation allows the definition of a characteristic time, z (3) The experimental switching time is proportional to :. Based on geometrical considerations it follows63 that yeff is related to 6' by yeff=yo sin2 8 (4) where yo is independent of 8 and represents the rotational viscosity of a hypothetical nematic-like Sc structure with H = 90°C. Combining eqn. (l), (2)and (4) finally leads to zE =(yo/Po)sin f3 From these equations it follows: (i) In contrast to the expec- tation that a higher P, necessarily means a shorter switching time, an increase of P,, which is due to an increase in 8, leads to longer switching times.(ii) If both z and P, increase (or decrease) upon changing the side chain within a homologous series of dopants, then this is probably a change of 8 of the mixture induced by the dopant. (iii) If T decreases and P, increases, this is probably due to a change of Po, especially for low dopant concentrations, where changes of viscosity are small. The transition temperatures (Sc*-SA, SA-N*and N*-I) of a series of mixtures of the trans-4-(4-n-alkanoyloxypheny1)cy-clohexyl (R)-2-fluorohexanoates (l-ll)are plotted versus the numbers of carbon atoms in the terminal chain of the esters in Fig. 1. The Sc* and S, transition temperatures both increase with increasing chain length, whereas the clearing point remains basically constant. This is unusual as the pure esters only exhibit an SBmesophase (see Table 1).The spontaneous polarisation and the observed switching time of the same mixtures are plotted uersus the number of carbon atoms in the terminal chain of the esters in Fig. 2. There are significant variations from one homologue to another, but no systematic dependence on the chain length is observed. The high value of the spontaneous polarisation and short response time of the mixture containing ester 10 are particularly interesting. Similar trends for the transition temperatures, spontaneous polarisation and response times of the trans-4-{4-[(E)-alk-2-enoyloxy] phenyl} cyclohexyl (R)-2-fluorohexanoates (12-18) as those of the trans-4-(4-n-alkanoyloxyphenyl)cyclohexyl(R)-2-fluorohexanoates (1-11) are shown in Fig. 3 and 4.The presence of the trans-carbon-carbon double bond does not seem to lead to any significant improvement. The transition temperatures of a series of mixtures of the trans-4-(2’,3’-difluoro-4’-n-alkoxybiphenyl-4-yl)cyclohexyl(R)-2-fluorohexanoates (19-30) are plotted uersus the numbers of carbon atoms in the terminal chain of the esters in Fig. 5. The Sc* and S, transition temperatures both rise with increas- t N* 1 3 5 7 9 11 n Fig. 1 Chiral nematic-isotropic (N*-I), smectic A-chiral nematic (SA-N*)and chiral smectic C-smectic A (Sc*-SA) transition tempera- tures versus the number of carbon atoms (n)in the alkanoyloxy chain of the trans-4-(4-n-alkanoyloxyphenyl)cyclohexyl(R)-2-fluorohexa-noates (l-llj mixtures Fig.2 Spontaneous polarisation (W, P,) and switching time (0,T) uersus the number of carbon atoms (n)in the alkanoyloxy chain of the trans-4-(4-n-alkanoyloxyphenyl)cyclohexyl (R)-2-fluorohexa-noates (l-ll)mixtures J.MATER. CHEM., 1994, VOL. 4 9 \t-4 5 6 7 8 9 10 n Fig. 3 Chiral nematic-isotropic (N*-I), smectic A-chiral nematic (SA-N*)and chiral smectic C-smectic A (Sc*-SA)transition tempera- tures versus the number of carbon atoms (nj in the alkanoyloxy chain of the trans-4-{ 4-[(E)-alk-2-enoyloxy] phenyl) cyclohexyl (R)-2-fluoro- hexanoate (12-18) mixtures 7.0 ’500 t 4.0I 13001 I I 4 6 8 10 n Fig. 4 Spontaneous polarisation (W, P,) and switching time (0,z) versus the number of carbon atoms (n) in the alkanoyloxy chain of the trans-4-{4-[(E)-alk-2-enoyloxy] pheny1)cyclohexyl (R)-2-fluoro-hexanoate (12-18) mixtures 6ot 501 , J 2 4 6 8 10 12 n Fig.5 Chiral nematic-isotropic (N*-I), smectic A--chiral nematic (SA-N*)and chiral smectic C-smectic A (Sc*-SA)transition tempera- tures versus the number of carbon atoms (n)in the alkoxy chain of the trans-4-( 2‘,3’-diflouro-4’-n-alkoxybiphenyl-4-yljcyclohexyl (R)-2-fluorohexanoate (19-30) mixtures ing chain length, they appear to reach a maximum for intermediate chain lengths, and then appear to stabilise. The clearing point (N*-I) is remarkably independent of chain length. The absolute values for each of the three transitions are higher than those observed for the corresponding mixtures incorporating an equal amount of any of the two-ring esters J.MATER. CHEM., 1994, VOL. 4 9.0 I 1600 n Fig. 6 Spontaneous polarisation (a,P,) and switching time (0,5) versus the number of carbon atoms (n) in the alkoxy chain of the trans-4-(2',3'-diflouro-4'-n-alkoxybiphenyl-4-yl)cyclohexyl (R)-2-fluoro- hexanoate (19-30) mixtures (l-ll and 12-18). The spontaneous polarisation and response times of the same mixtures are plotted versus the numbers of carbon atoms in the terminal chain of the esters in Fig. 6. Although the plots exhibit considerable scatter, the switching times increase with increasing chain length. The switching times are in general longer than those of the two-ring esters (l-ll).Hence, the two-ring esters (l-ll and 12-18) and the three-ring difluoro esters (19-30) offer the possibility of choos-ing between higher transition temperatures and shorter response times according to application specifications. The difluoro esters (19-30) are of especial interest owing to the negative value of the dielectric anisotropy attributable to the two fluorine atoms in a lateral position.This facilitates a good orientation by electric field effects. The transition temperatures of mixtures incorporating the trans-4-[ 4-( 5-alkylpyrimidin-2-yl)phenylIcyclohexyl (R)-2-fluorohexanoates (32-36) and trans-4-[4-(5-alkoxypyrimidin-2-yl )phenyl]cyclohexyl (R)-2-fluorohexanoates (37-41) differing only in the presence of an additional oxygen atom attached to the pyrimidine ring of the esters (37-41) are plotted versus the number of (methylene and oxygen) units in the terminal alkyl and alkoxy chains in Fig.7. The data were plotted in this way in order to reveal possible odd-even effects for esters with alkyl and alkoxy chains of the same total 50 6 7 a 9 10 11 12 chain length Fig. 7 Chiral nematic-isotropic (N*-I), smectic A-chiral nematic (SA-N*)and chiral smectic C-smectic A (Sc*-SA) transition tempera- tures uersus the total length (oxygen and methylene units) of the alkyl/alkoxy chain of the trans-4-[4-( 5-alkylpyrimidin-2-y1)phenyll-cyclohexyl (R)-2-fluorohexanoate (32-36) and trans-4-[4-( 5-alkoxy- p yrimidin-2-yl ) phenyl] cyclohex yl (R)-2-fluorohexanoate (37-41 ) mixtures. (Note that the x-axis is the total number of atoms in the side chains including oxygen.) 1695 length.The corresponding values for the spontaneous polaris- ation and switching time are plotted in Fig. 8. In contrast to the alkoxy-substituted esters (37-41) the alkyl-substituted esters (32-36) do not possess an S,* phase as single compo- nents. Thus, the mixtures containing them exhibit lower Sc* transition temperatures. The mixtures containing the alkyl- substituted esters possess significantly higher values for spon-taneous polarisation and shorter switching times. As discussed above this indicates a significantly larger value for Po. This suggests that the dipole moment of the oxygen atom in the alkoxy chain compensates the dipole moments attached to the chiral centre.This is quite remarkable and implies a rather strong correlation of the orientation of the dipoles over several freely rotatable bonds. In the alkoxy-substituted esters a distinct odd-even effect for both the spontaneous po1,irisation and switching time is observed. This is attributed to the odd-even effect of Po. This is another indication that the dipole moment resulting from the oxygen atom in the alkoxy chain is correlated with the dipole moment of the chir;il centre. In Fig. 1-8 clear odd-even effects for the phase tiansition temperatures or the spontaneous polarisation and tht: switch- ing times are only observed for the alkoxy-substituted esters. Odd-even effects are most pronounced if the bond:, for the even (all trans) positions are closely aligned to the effective molecular axis.This seems to be the case for the alkoxy chains, but not for the alkyl chains or the esters. The data collated in Table 6 allow the effect of an ester group in a terminal position in the core of a chiral dopant to be determined. The ether15 and diester 9 differ only in the 61 1aoo chain length Fig. 8 Spontaneous polarisation (a,P,) and switching tiine (0,5) versus the total length (oxygen and methylene units) of [he alkyl/ alkoxy chain of the trans-4-[ 4-( 5-alkylpyrimidin-2-y I )phenyl] cyclohexyl (R)-2-fluorohexanoates (32-36) and trans-4-[ 4-( 5-alkoxy-pyrimidin-2-yl)phenyl] cyclohexyl (R)-2-fluorohexanoate (37-41 ) mixtures. (Note that the x-axis is the total number of atoms in the side chains including oxygen.) Table 6 Comparison of the transition temperatures, spc mtaneous polarisation and response times for two mixtures consisting of 7 wt.% the reference ether trans-4-( 4-decyloxyphenyl)cyclohexyl (R)-2-fluor~hexanoate'~and trans-4-(4-decanoyloxyphenyl)cyclohexyl(R)-2-fluorohexanoate (9) and 93 wt.% of the base Sc mixture 929-1219 X (S,,-S,)/OC (S,N*)/"C (N*-I)/"C PJnC cm -2 ~/ps CH, 64.5 82.6 97.8 3.8 460 co 68.0 81.5 97.1 5.7 400 presence of a carbonyl group (CO) instead of a methylene unit (CH,), i.e.the chain lengths are the same. The S,* transition temperatures for the mixture containing 7 wt.% of the ester (X =CO) is significantly higher (+3.5 “C) than that incorporating an equal amount of the ether (X =CH,).The spontaneous polarisation is significantly higher (+1.9 nC cmP2) for the ester than for the ether, indicating a greater value for Po, as shown by the significantly shorter response time. Shorter response times for ester mixtures compared with related ether mixtures have already been observed for non-optically active esters exhibiting an Sc phase, where the carbonyl function was also in a terminal instead of a central position in the core of the m~lecule.~’-~~ However, this was probably owing to a lower tilt angle rather than to a larger value for Po as the chiral dopant is the same. The effect of an ester group in a central position of a chiral dopant can be elucidated from the data in Table 7.The Sc* transition temperature for the mixture containing the reference a-fluoro (di-)ester15 with a second ester group in a central position is higher (+1.2 “C) than that incorporating an equal amount of the a-fluoro (mono-)ester (28).This is not surprising as the reference a-fluoro (di-)ester15 exhibits an enantiotropic Sc phase at elevated temperatures in the pure state. The SA transition temperature for the mixture containing ester 28 is higher (+3.8 “C) reflecting the high SAtransition temperature of the pure material (see Table 3). The clearing points are almost equal, which is surprising considering the absence of an N* phase for ester 28 and the high N* phase transition temperature for the diester. Spontaneous polarisation is sig- nificantly higher (+ 1.9 nC cmP2) for ester 28 than for the diester.This is probably owing to a change of Po as well as of y, because of presence of the second carboxy (ester) group. Even if Po were constant and the tilt angle were fully respon- sible for the increase of P,, the rotational viscosity of mono-ester 28 would still be substantially lower. The data collated in Table 8 allow a valid comparison of the relative effects of either a fluorine or a chlorine atom attached directly to the optically active centre of the chiral dopant. The transition temperatures of the mixture containing 7 wt.% of a-fluoro ester 28 are all higher than those observed for the corresponding mixture containing an equal amount of the otherwise identical a-chloro ester 31.The spontaneous polarisation for 3-fluoro ester 28 is greater (88.5 nC cmP2, extrapolated to 100%) than that of the analogous a-chloro ester 31 (64 nC cmP2, extrapolated to 100%).This could be because of the stronger electronegativity of the fluorine atom. In addition, the rotational viscosity of the a-fluoro ester must also be lower than that of the a-chloro ester as shown by the significantly lower response time, which is only partially due Table 7 Comparison of the transition temperatures, spontaneous polarisation and response times for two mixtures consisting of 7 wt.% reference ester trans-4-[ 4-(2’,3’-difluoro-4-decyloxybenzoyloxy)phen-yl]cyclohexyl (R)-2-fl~orohexanoate’~and trans-4-(2’,3’-difluoro-4-decyloxybiphenyl-4-y1)cyclohexyl (R)-2-fluorohexanoate (28) and 93 wt.% of the base Sc mixture SC9-1219 (S,*-SA)/”C (SA-N*)/3C (N*-I)/”C PJnC cm-’ Z/~S CO, 76.7 83.5 103.9 5.4 650 ~ 75.5 88.3 104.3 6.2 415 J.MATER. CHEILI., 1994, VOL. 4 Table 8 Comparison of the transition temperatures, spontaneous polarisation and response times for two mixtures consisting of 7 wt.% trans-4-(2’,3’-difluoro-4-decyloxybiphenyl-4-yl)cyclohexyl (R)-2-fluorohexanoate (28) and trans-4-(2’,3’-difluoro-4‘-decyloxybiphenyl-4-y1)cyclohexyl (R)-2-~hlorohexanoate (31) and 93 R t.% of the base S, mixture SC9-1219 F.F ..-c10H210 o* ox X (SC*-SA)/T (SA-N*)/”C (N*-I)/”C PJnC cmp2 Z/~S F 75.5 88.3 104.3 6.2 41 5 c1 74.5 86.3 102.3 4.5 630 to the higher value of Po.This is a reflection of the smaller size of the fluorine atom and the shorter carbon-fluorine bond compared with that of chlorine. These results highlight the attractiveness of fluorine as a substituent attached to the optically active centre of chiral dopants. The authors express their gratitude to Mr. C. Haby, Mr. W. Janz and Mr. J. Reichardt for technical assistance in the preparation and evaluation of the compounds. Dr. W. Arnold (NMR), Mr. W. Meister (MS), Dr. M. Grosjean (IR), Mr. F. Wild and Mr. B. Halm (DTA) are thanked for the measure- ment and interpretation of the required spectra. References 1 N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett., 1989,36, 899. 2 N. A. Clark, M. A. Hanschy and S. T. Lagerwall. Mol.Cryst. Liq. Cryst., 1983,94,213. 3 K. Skarp and M. A. Hanschy, Mol. Cryst. Liq. Cryst., 1988, 165, 439. 4 M. A. Hanschy and N. A. Clark, Ferroelectrics, 1984,59, 69. 5 S. T. Lagerwall, N. A. Clark, J. Dijon and J. F. Clerck, Ferroelectrics, 1989,94, 3. 6 M. Brunet, J. Phys. Colloq., 1975,36, C1-321. 7 R. Eidenschink, R. Hopf, B. S. Scheuble and A. E. F. Wachtler, Proc. 16th Freiburger Arbeitstagung Fliissigkristalle, Freiburg, 1986. 8 T. Geelhaar, T. Escher and E. Bohm, Proc. 17th Freihurger Arbeitstagung Flussigkristalle, Freiburg, 1987. 9 J. W. Goodby and T. Leslie, in Liquid Crystals and Ordered Fluids, ed. A. C. Griffin and J. F. Johnson, Plenum, New York, 1984, vol. 4, p. 1. 10 C. Escher, Kontakte, 1986,2,3. 11 D.M. Walba, S. C. Slater, W. N. Thurme5, N. A. Clark, M. A. Hanschy and F. Supon, J. Am. Chem. Soc., 1986,108,521 1. 12 J. P. le Pesant, B. Mourney, M. Hareng, G. Decobert and J. C.Dubois, Paris Display ’84,1984, 217. 13 J. W. Goodby, Science, 1986,231, 350. 14 J. Bomelburg, G. Heppke and A. Ranft, Z. Nuturforsch. B: Chem. Sci., 1989,44, 1127. 15 R. Buchecker, S. M. Kelly and J. Funfschilling, Iiq. Crpst., 1990, 8,217, and references cited therein. 16 D. M. Walba, R. Vohra, N. A. Clark, M. A. Hanschy, J. Xue, D. S. Parma, S. T. Lagerwall and K. Skarp, J. 4m. Chrrn. Soc., 1986,108,7424. 17 D. M. Walba and N. A. Clark, Ferroelectrics, 1988,84, 65. 18 D. M. Walba, H. A. Razawi, N. A. Clark and D. S. Parma, J. Am. Chern. SOC., 1988,110,8686.19 D. M. Walba, K. F. Eidman and R. C. Haltiwanger, J. Org. Chem., 1989,54,4943. 20 M. D. Wand, R. Vohra, D. M. Walba, N. A. Clark and R. Shao, Mol. Cryst. Liq. Cryst., 1991,202, 183. 21 S. Nakamura and H. Nohira, Mol. Crpst. Liq. Cryst., 1990, 185, 199. J. MATER. CHEM., 1994, VOL. 4 1697 22 H. Nohira, S. Nakamura and M. Kamei, Mol. Cryst. Liq. Cryst., 42 S. M. Kelly, Liq. Cryst., 1993, 14, 675. 1990,180,379. 43 S. M. Kelly, J. Funfschilling and A. Villiger, Liq. Crvst., 1993, 23 S. Arakawa, K. Nit0 and J. Seto, Mol. Cryst. Liq. Cryst., 1991, 14, 813. 204, 15. 44 S. M. Kelly, Liq. Cryst., 1993, 16, 67. 24 C. Bahr and G. Heppke, Mof. Cryst. Liq. Cryst., 1987, 148, 29. 45 S. M. Kelly, A. Germann and M. Schadt, Liq. Cryst., 1994,16,491.25 T. Sakurai, N. Mikami, R. Higurchi, M. Honma, M. Ozaki and 46 S. M. Kelly and J. Funfschilling, J. Muter. Chem., 1994, 4, 1673. K. Yoshino, J. Chem. SOC.,Chem. Commun., 1986,978. 47 A. Focella, F. Bizzarro and C. Exon, Synth. Commun.. 1991, 21, 26 T. Geelhaar, H. A. Kurmeier and A. E. F. Wachtler, Liq. Cryst., 2165. 1989,5, 1269. 48 G. Osterheld, P. Marrug, R. Ruhrer and A. Germann, 27 L. K. M. Chan, G. W. Gray, D. Lacey, R. M. Scrowston, I. G. J. Chromatogr., 1982,234,99. Shenouda and K. J. Toyne, Mol. Cryst. Liq. Cryst., 1989, 172, 125. 49 S. M. Kelly, R. Buchecker, J. Fromm and M Schadt, 28 G. Scherowsky and M. Sefkow, Mol. Cryst. Liq. Cryst., 1991, Ferroelectrics, 1988,85, 385. 202,207. 50 S. M. Kelly and R. Buchecker, Helv. Chim. Acta, 1983, 71, 451 29 G. Scherowsky and J.Gay, Liq. Cryst., 1989,5,1253. and 461. 30 G. Scherowsky, J. Gay and M. Gunararte, Liq. Cryst., 1992, 11, 51 S. M. Kelly, R. Buchecker and M. Schadt, Liq. Cryst, 1988, 3, 745. 1115 and 1125. 31 G. Scherowsky and M. Sefkow, Liq. Cryst., 1992,12,355. 52 S. M. Kelly and A. Villiger, Liq. Cryst., 1988,3, 1173. 32 T. Kusumoto, A. Nakayama, Chem. Lett., 1992,2047. 53 S. M. Kelly, Liq. Cryst., 1989,5, 171. 33 K. Sakaguchi and T. Kitamura, Ferroelectrics, 1991, 114,265. 54 S. M. Kelly, Helv. Chim. Acta, 1989,72, 594. 34 K. Sakaguchi, T. Kitamura, Y. Shiomi, M. Koden and 55 S. M. Kelly, J. Funfschilling and F. Leenhouts, Liq. Crvst., 1991, T. Kuratate, Chem. Lett., 1991, 1383. 10,243. 35 T. Kusumoto, K. Sato, T. Hiyama, S. Takehara, M. Osawa, 56 S. M. Kelly, Mol. Cryst. Liq. Cryst., 1991,204,27. A. Nakayama and T. Fujisawa, Chem. Lett., 1991, 1623. 57 R. Dabrowski, J. Dziaduszek, B. Sosnowska and J. Przedmojski, 36 H. R. Dubal, C. Escher, D. Gunter, W. Hemmerling, Y.Inoguchi, Ferroelectrics, 1991, 114, 229. I. Muller, M. Murakami, D. Ohlendorf and R. Wingen, Jpn. 58 R. Dabrowski, J. Dziaduszek, J. Szulc, K. Czuprjnski and J. Appl. Phys., 1988,27, L2241. B. Sosnowska, Mol. Cryst. Liq. Cryst., 1991,209,201. 37 J. Funfschilling and S. M. Kelly, Proc. 20th Freiburger 59 W. W. McMillan, Phys. Rev. A: Gen. Phys., 1973,8, 1921. Arheitstugung Fliissigkristalle, Freiburg, 199 1. 60 A. Wulf, Phys. Rev. A: Gen. Phys., 1975, 11, 365. 38 S. M. Kelly, J. Funfschilling and A. Villiger, Liq. Cryst., 1993, 61 R. G. Priest, J. Chem. Phys., 1976,65,408. 14,699. 62 K. Siemensmeier and H. Stegemeier, Chem. Phys. Lett., 1988, 148, 39 S. M. Kelly and J. Funfschilling, J. Muter. Chem., 1993,3,953. 409. 40 J. Funfschilling, S. M. Kelly and A. Villiger, Liq. Cryst., 1993, 63 C. Escher, T. Geelhaar and E. Bohm, Liq. Cryst., 1988,3,469. 14, 713. 41 F. Leenhouts, S. M. Kelly and A. Villiger, Displays, 1990,41. Paper 4/01809E; Received 25th March, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401689

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( ll), 1699-1703 1699 A New Type of Main-chain Liquid-crystal Polymer derived from 4'-Hydroxybiphenyl-4-carboxylic Acid and its Smectic Mesophase Behaviour Yasukazu Nakatat and Junji Watanabe" Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 7 52, Japan A homologous series of polymers (PHBC-n) has been prepared from 4'-hydroxybiphenyl-4-carboxylic acid and o-bromoalkan-1-01s containing five to nine methylene units which can serve as mesogen and flexible spacer groups, respectively. The PHBC-n polymers are characterized by having two different linkage groups, ether and ester groups, to connect the biphenyl mesogen to the methylene spacer, and are classified on a new type of main-chain liquid- crystal polymer.All polymers exhibit the enantiotropic smectic phase. The smectic behaviour and structure were examined by differential scanning calorimetry, optical microscopic and X-ray diffraction methods, and are discussed in comparison with the corresponding date of the BB-n polyesters, which include only the ester linkage group. In recent papers,lP3 Watanabe et a!. have reported the meso- can be observed only for the c-director but not for the n-phase properties and structure of a homologous series of director.2 Such an odd-even effect on the transition param- main-chain BB-n polyesters which were prepared from p,p'-eters and mesophase structure shows how the nature of the dibenzoic acid and alkanediols. flexible spacer is important for understanding the meso-morphic behaviour in this type of polyester.This effect is believed to result from a coupling of polymeric and mesogenic effects, in which the mesogenic groups are confined in .;pa~e.~*~ In this study, we prepared a new class of main-chain liquid- crystal PHBC-n polymers with the following formula These BB-n polyesters invariably form smectic mesophases and their isotropization temperature, ?I, and entropy, ASi, exhibit an odd-even oscillation in which the larger values are observed for polyesters with even n. The odd-even nature of the alkylene spacer also reflects the type of smectic liquid crystal.' In BB-n where 11 is even a normal SA phase is formed Here n is the number of methylene units in the flexible spacer.with both axes of the polymer chain and the biphenyl mesogen The PHBC-n polymers can be differentiated from the BB-n lying perpendicular to the layers [see Fig. l(a)]. In contrast, polyesters, since they possess two different linkage groups, an the smectic structure of BB-n with n odd was identified as a ester group on one side and an ether group on the other new type of smectic phase, S,,,$ in which the tilt direction of which connect the mesogen group to the alkylene spacer. The the mesogenic groups is the same in every second layer but mesophase properties and structures were examined by opposite between neighbouring layers [see Fig. 1(b)].The SCA differential scanning calorimetry (DSC), optical microscopy phase is quite novel and interesting since uniaxial ordering and X-ray measurements.t Present address: LINTEC Co., Nishikicho, Warabishi, Saitama Experimental335, Japan. In previous reports, this phase has been termed S,,, but Sc2 is a Syntheses confusing notation in the context of low molar mass systems. For this reason, we will hereafter adopt the notation SCA,which has been PHBC-n polymers were synthesized in two steps according to coined for the same type of smectic phase in low molar mass systems Scheme 1. by Fukuda and co-~orkers.~ EHW-n IAt Fig. 1 Schematic illustration of (a) the SA phase formed in even-membered BB-n and (b)the s,, phase in odd-membered BB-n. In the SCA phase, the mesogenic groups in each layer are tilted by about 25" to the layer normal but with opposite tilt directions in neighbouring layers.Scheme 1 1700 J. MATER. CHEILI., 1994, VOL. 4 Preparation of Ethyl 4'-(o-Hydroxyalkoxy)biphenyl-4-carboxylate (EHBC-n) Monomers 1 r In a dry, three-neck, 100ml round-bottom flask, equipped 0U with a magnetic spinner, condenser, additional funnel and Q) nitrogen inlet tube, was placed 3.0 g (1.24 mmol) of ethyl 4'-hydroxybiphenyl-4-carboxylate in 30.0 ml of N,N'-dimethyl- formamide and 1.3 g of anhydrous potassium carbonate. The additional funnel was charged with 15.0 mmol of w-bromo- Y alkan-1-01. After heating to 120 "C, 0-bromoalkan-1-01 was added dropwise. Stirring was continued for 4h after the 9050 130 170 210 addition was complete and the contents were then extracted TI'C with 500 ml of 1 mol 1-l HC1.Purification was achieved by column chromatography using silica gel and chloroform as Fig.2 DSC thermograms of (a) EHBC-8 monomer a nd (h) PHBC- eluent, followed by successive crystallizations from isopropyl 8 polymer alcohol-hexane. Preparation of PHBC-n Polymers The lowest transition at crystal transition, the hig hest one at T, correspon ?; to the isotropization ds to the crystal-liquid PHBC-n polymers were prepared by melt transesterification of the liquid crystal. The intermediat e transition at is due of the EHBC-n monomers, which was performed by placing to the transition between two liquid crystals. T he transition 1 g of EHBC-n with a small amount of tetraisopropyl orthotit- temperatures, as collected in Table 1 and plotted ag ainst n in anate as catalyst in a polymerization tube.The polymerization Fig. 3, decrease monotoni cally with in creasing YZ. Furthermore, tube was heated to 230 "C in a mantle heater, and a continuous it is found that the mono mers of EHB C-6 to EH BC-9 exhibit stream of nitrogen bubbles was passed through the melt by two mesophases while EH BC-5 show s only one me sophase. means of a capillary tube. The ethanol was distilled off for From optical microsco py observati ons, all of t he monomer 2 h, after which the temperature was raised to 250-300°C mesophases appear to be composed of thin rod s on cooling and the pressure was reduced to 0.2 mmHg; these conditions from the isotropic melt. After the t hin rods c oalesce upon were maintained for 1 h.The inherent viscosities of the further cooling, a mesop hase develops with ho meotropic polymers were measured at 30°C using a Ubbelohde vis- alignment which may resu lt from the t endency of t he molecules cometer for 0.5 g dl-' solutions in a 60:40 mixture of phenol to attach their polar hyd roxy end gro up to the glass surface. and tetrachloroethane. homeotropic mesophase, In this no birefringence can be Characterization of EHBC-n Monomers and PHBC-n observed. This feature rethe lower-temperature me sophase, indmains unalt icating thaered upon t bo tra nsition to th phases Polymers are optically uniaxial sme ctic phases. The calorimetric behaviour was studied using a Perkin-Elmer - DSC-I1 calorimeter.10 mg samples were heated and cooled 320 at a rate of 10°C min-' under a flow of dry nitrogen. In an (b) effort to provide a common thermal prehistory, the data were collected during the first cooling from an isotropic melt and - during the second heating from room temperature. Optical 240 microscopic observations were performed using an Olympus BH-2 polarizing microscope equipped with a Mettler FP-82 P hot stage and a Mettler FP-80 temperature controller. Wide- k angle X-ray diffractograms were recorded with a flat-plate 160 - camera using Ni-filtered Cu-Ka radiation. The distance from (a ) sample to film was calibrated using silicon powder. The sample temperature was regulated using the Mettler hot stage and controller.80 - Results 4 6 8 10 The EHBC-n monomers exhibit mesophases whose meso- n morphic behaviour and structure are briefly described here. Fig. 3 Variation of transitio n temperatur e with num ber of carbon A typical DSC thermogram is shown in Fig. 2(a) for EHBC- atoms in the methylene spa cer, n, for (a) EHBC-n m on omers and 8. All the EHBC-n monomers exhibit two or three transitions. (b)PHBC-n polymers; (0)T,, (a)?; and (0)?; Table 1 Characteristics of the EHBC-n monomers calorimetric data' monomer T,/"C T/"C 7,'/"C A,S/J mol-' K-' AJ/J K-l AiS/J mol-'mol-' Kpl d(&Jb;A d(S,)b/A EHBC-5 124 146 14.2 25.9 20.7 EHBC-6 107 111 140 7.1 4.6 22.2 22.7 22.0 EHBC-7 96 109 134 7.0 4.2 27.2 23.7 23.0 EHBC-8 93 105 129 16.7 5.0 31.8 25.0 24.2 EHBC-9 91 102 126 61.3 4.6 30.5 26.3 25.5 'Calorimetric data are collected on heating.'d is the layer spacing. J. MATER. CHEM., 1994, VOL. 4 4 6 8 10 n Fig. 4 Variation of d-spacings with n for (a)EHBC-n monomers and (b)PHBC-n polymers: (@) S,; (0)SA The higher-temperature mesophase can be assigned to the smectic A (S,) phase from its characteristic X-ray pattern which includes a sharp inner reflection (the layer reflection), and a broad outer reflect+. The spacing of the layer reflection varies from 20.7 to 25.5 A with a variation of n from 5 to 9, while the sp!cing of the outer broad reflection is constant at around 4.5A. On cooling to the lower-temperature meso- pha!e, the outer reflection becomes sharp with a spacing of 4.4 A.This, together with the optical microscopic observations, suggests that the lower-temperature mesophase is smectic B (SB).The layer-reflection spacings are plotted against n in Fig.4. Although !he spacing in the SB phase is somewhat larger by 0.7-0.8 A than that in S,, the spacing in each phase increases linearly with incre?sing n. The average increment of spacing per unit of n is 1.2 A for both phases. This increment, as well as the value of the layer spacing, is reasonable for the uniaxial smectic phases in which the molecules, in nearly extended form, lie perpendicular to the layers. Fig. 2(b) shows the DSC thermograms of the PHBC-8 polymer. Two peaks can be observed upon heating and cooling, All other polymers also exhibit two enantiotropic peaks.The peak at the lower temperature (T,) corresponds to the crystal-liquid crystal transition and the higher-tempera- ture peak at to the isotropization of the liquid crystal. The thermodynamic data obtained from the heating curves are listed in Table 2. In Fig. 3, the transition temperatures are plotted against the number of methylene units, n, in the flexible spacer. As n increases, the vertical spacing of the two transition tempera- tures, which represents the temperature span of the mesophase, becomes narrow, showing a marked odd-even oscillation. The isotropization entropy, AJ, also exhibits odd-even oscil-lation with the larger value found for PHBC-n with n odd as illustrated in Fig.5. A fan-like texture was observed for all of the polymer mesophases by optical microscopy showing the smectic nature of the mesophase. X-Ray diffraction also verifies the smectic layered mesophase; the diffraction pattern consists of sharp inner and diffuse outer reflections. The spacings for the inner- layer reflections are listed in Table 2 and plotted against n in n Fig. 5 Variation of smectic-phase isotropization entropy with n in the polymeric PHBC-n system Fig. 4. One can also find here a marked odd-even oscillation, with the larger value occurring for polymers with odd n. To determine the structure of the smectic phase in detail, X-ray diffraction of the oriented fibre specimens, which can be prepared by drawing the isotropic melt, was undertaken.Typical oriented X-ray patterns are shown for PHBC-7 and PHBC-8 in Fig. 6(u)and (b),respectively, where the jibre axis corresponding to the chain axis is placed in the vertical direction. From Fig. 6(u), it can be seen that in PHWC-7 the layer reflection is placed at the meridian and the outer broad reflections appear on the equator. A similar pattern is observed in two other odd-n homologues, PHBC-5 and PHBC-9. Therefore, we conclude that the polymers with odd n form an SA phase, such as that depicted in Fig. l(u). In the polymers with even n (PHBC-6 and PHBC-8), in contrast, the outer broad reflections are split into two portions, lying above and below the equator, the layer reflection remaining on the meridian [Fig.6(b)].This suggests the formation of SCA,as illustrated in Fig. l(b), the structural details of which have been described in previous paper^.^.^ Discussion Compared with the monomeric EHBC-n, PHBC-n polymers show two distinct features of mesophase behaviour arid struc- ture: (i) the PHBC-n polymer smectic mesophases appear in a higher-temperature region than those of EHBC-n monomers, as shown in Fig. 3; (ii) the marked odd-even alternation appears in the thermodynamic parameters of the transition as well as in the smectic structure (see Fig. 3-5). These can be explained by the polymeric effect, which results from the linkage of mesogenic groups into the polymer backbone through the flexible spacer.6 The odd-even alternation of the smectic structure is interes-ting.As stated in Introduction, this was first observed in the BB-n polyesters and was believed to result from the different conformational constraints on the angular displacement of the mesogenic groups for the odd- and even-n Supplementary evidence for this conformational constraint has been derived from the computational analyses.' Abes Table 2 Characteristics of the PHBC-n polymers calorimetric data' polymer qinhb/dlg ~ T,/"C TJT A,S/J mol-' K-' AiS/J mol - K - &JA PHBC-5 0.43 270 307 8.8 19.2 16.2 PHBC-6 0.37 178 215 3.8 14.2 16.1 PHBC-7 0.33 238 261 10.9 23.0 18.5 PHBC-8 0.41 178 204 5.9 19.6 18.3 PHBC-9 0.35 210 22 1 10.9 25.1 20.7 'Calorimetric data are collected on heating.vinh is the inherent viscosity. 'Layer spacing. J. MATER. CHEM., 1994. VOL. 4 (a) Fig. 6 X-Ray diffraction patterns for the oriented smectic phases of (a) PHBC-7 and (b)PHBC-8 fibres. The fibre was prepared by drawing the isotropic melt. The axis is placed in a vertical direction. performed a conformational analysis, within the framework of the rotational isomeric state model, and evaluated the angle, 0, defined by unit vectors attached to two successive mesogenic groups, for all possible conformations of the alky- lene spacer. The results indicate that the angular distribution of main-chain polyesters such as BB-n is different, depending on the odd-even parity of n. When n is even, 0 values are distributed in the two regions 0-30" and 85-130".For odd n most of the angles are located in the region 50-90", and to some degree orientations are also permitted in the region above 150". In each system, the conformers with smaller angular displacements of the successive mesogens are in the more extended form, whereas those with larger angular dis- placements are in the folded form. Comparing the calculations with the observations for the BB-n polyesters,132 we have arrived at the following conclusions as to the conformational constraint imposed by the smectic field. (1) Where n is even, parallel orientation of the successive mesogenic groups is allowed, conforming more or less to the concept of an ordinary uniaxial ordering of the n-director, but where n is odd, uniaxial orientation of successive mesogenic groups is not expected.(2) The conformers with smaller angular displacements in both the odd and even systems are those which participate in the experimentally observed smectic layer structures. These points help to explain how the different types of smectic phases appear to be dependent on the odd-even parity of n and, at the same time, why the new type of smectic phase, SCA,is formed from the BB-n polyesters with odd n. The computational analyses by Abe have also shown that similar angular distributions can be attained in main-chain polyethers in which the mesogen and alkylene spacer are connected by an ether linkage group.' This suggests that the polyethers may form two types of smectic structures, in the same way as the polyesters.So far, the main-chain polyethers have been synthesized with a variety of mesogenic groups. However, these tend to form nematic liquid cry~tals.~-'~ Only polyethers based on the biphenyl mesogen have been reported to form smectic me so phase^,'^,^^ but structural analyses have not been made in relation to the odd-even character of the flexible spacer. Therefore, the question of how the odd-even character of the flexible spacer in the polyethers affects the smectic structure remains unsolved. On this point, it is interes- ting that the present PHBC-n polymers showed a similar odd-even alternation to the BB-n polyesters. The PHBC-n polymers have an ester linkage on one side of the biphenyl mesogen, but an ether linkage on the other, and hence it is suggested that the ether linkage may serve to produce similar odd-even effects to the ester linkage.Note that the odd-even alternation of the smectic structures appears with ah opposite trend for the PHBC-n and BB-n polymers. The SA phase is observed here for PHBC-n with odd n, while it is formed from even-n polymers in the BB-n series. Also, the SCAphase is observed in even-u PHBC-n and odd-n BB-n. The layer thickness, as well as the thermodynamic parameters of the transition, also shows opposite oscillations with n. This may be due to the fact that the number of atoms in the spacer directly connecting two successive biphenyl mesogens is fewer by a unit in PHBC-n than in BB-n, because of the lack of a carbonyl group in one of the two linkage groups.Therefore, for the angular displacements of the biphe- nyl groups in PHBC-n and BB-(n-1) are similar. This can be understood from Fig. 7, where the molecules in a fully extended form are illustrated for PHBC-7 and BB-6. This simple argument is also supported by the observation that the absolute value of the layer thickness, as well its odd-even oscillation, is comparable for the PHBC-n and BB-(n-1) systems (see Fig. 8). Fig. 7 Comparison of the extended-chain structures of (a) PHBC-7 and (b)BB-6 J. MATER. CHEM., 1994, VOL. 4 1703 22 r 3 J. Watanabe, M. Hayashi, A. Morita and T. Niori, hlol. Cryst. Liq. Cryst., in the press.4 A. D. L. Chandani, E. Gorecka, Y. Ouchi, H. Takezoe and 20-'5 18--0 5 6 A. Fukuda, Jpn. J. Appl. Phys., 1989,28, L1265. Y. Takanishi, H. Takezoe, A. Fukuda and J. Watanabe, Phys. Ret.. B, 1992,45,7684. A. Ciferri, in Polymer Liquid Crystals, ed. A. Ciferri, 16 /+---d- W. R. Krigbaum and R. B. Meyer, Academic Press. London, 1982, pp. 63-83. 7 J. Watanabe, H. Komura and T. Niori, Liq. Cryst., 199?, 13,455. 8 A. Abe, Macromolecules, 1984,17,2280. 4 6 8 10 9 S. Antoun, R. W. Lenz and J-I. Jin, J. Polym. Sci., Polvm. Chem. n Ed., 1981, 19, 1901. Fig. 8 Comparison of the smectic layer spacings of the PHBC-n (0) and BB-(n -1) (0)polymers 10 11 A. C. Griffin and S. J. Havens, J. Polym. Sci., Polym. Phys. Ed., 1981, 19, 951. J-I.Jin, E-J. Choi, S-C. Ryu and R. W. Lenz, Polym. J., 1988, 18,63. Finally, note that the main-chain PHBC-n polymers have non-centrosymmetric arrangements of the repeating unit within a polymer chain. In other words, the polymer has a head-to-tail character and a non-centrosymmetric alignment of the dipole moments. In addition to having two different 12 13 14 15 A. Abe and H. Furuya, Macromolecules, 1989,22,2982. J-I. Jin and J-H. Park, Eur. Polym. J., 1987,23,973. T. Shaffer and V. Percec, J. Polym. Sci., Polym. Lett. Ed., 1985, 23, 185. T. Shaffer, M. Jamaludin and V. Percec, J. Polym. Sli, Polym. Chem. Ed., 1986, 24, 15. linkage groups, this structural characteristic places the PHBC-n polymers as a new class of main-chain liquid-crystal poly- mer.l6 In this new type of polymeric system, ferroelectricity (one of the most predominant properties in a liquid-crystalline field) can be achieved if the polymers are packed into smectic 16 17 18 A. M. Ahmed, W. J. Feast and J. Tsibouklis, Polymer. 1993, 34, 1297. R. G. Petscheck and K. M. Wiefling, Phys. Ret.. Lett. 1987, 59, 343. J. Watanabe, Y. Nakata and K. Shimizu, J. Phys. I1 Fr., 1994, 4, 581. layers with a non-centro-symmetric alignment.17*'s This matter is currently under investigation. Paper 4/02999B; Received 20th hfuy, 1994 References 1 J. Watanabe and M. Hayashi, Macromolecules, 1989,22,4083. 2 J. Watanabe and S. Kinoshita, J. Phys. I1 Fr., 1992,2, 1237.

ISSN:0959-9428    

DOI:10.1039/JM9940401699

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( ll), 1705-1714 Effect of Spacer Length on the Thermal Properties of Side-chain Liquid-crystal Poly(methacry1ate)s Aileen A. Craig and Corrie T. Imrie* Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen, UK AB9 2UE A series of side-chain liquid crystal polymers, the poly[w-(4’-methoxybiphenyl-4-yloxy)alkylmethacrylatels, have been synthesized in which the spacer length is varied from 3 to 12 methylene units. This is the first example of a poly(methacry1ate)-based side-chain liquid crystal polymer series for which as many as 10 homologues have been prepared. The thermal properties have been characterised using differential scanning calorimetry and polarised light microscopy. All 10 homologues exhibit smectic behaviour; in addition, the butyl homologue is nematogenic.The clearing temperatures and associated entropy changes exhibit a distinct odd-even effect as the length and parity of the spacer is varied with the odd members exhibiting the higher values. This behaviour is rationalised in terms of the change in the average shape of the side chain on varying the parity of the spacer. The properties of the polymers are compared to those containing the same mesogenic group but different polymer backbones in order to consider the effect of flexibility on thermal behaviour. In general, these comparisons support the view that increasing backbone flexibility enhances the clearing transition while tending to decrease the entropy change associated with the transition. Exceptions to this rule are rationalised either in terms of molecular weight dependent effects or in the case of poly(norbornene)-based materials, are thought to arise from the complex microstructure of the polymer chains.Replacing the alkyl spacer by an oligo(ethy1ene oxide) chain reduces the clearing transition of the polymer. This is rationalised in terms of the flexibility and preferred conformations of an oligo(ethy1ene oxide) spacer. Side-chain liquid-crystal polymers are currently attracting considerable interest not only for their application potential in a wide range of electro-optic devices but also because they provide a demanding challenge to our understanding of the molecular factors that promote self-organisation in condensed phases.’ A side-chain liquid-crystal polymer comprises three distinct structural units: a polymer backbone, a flexible alkyl spacer and a mesogenic gro~p.~,~ The liquid-crystal unit is attached as a pendant group to the polymer backbone via the flexible spacer.The effect on the thermal properties of the polymer of varying the chemical nature of the mesogenic group is extensively documented and well By contrast, the dependence of the transitional behaviour on the chemical nature of the polymer backbone and the length and parity of the spacer has still to be fully established and understood. In particular; conclusions have been drawn con- cerning the role played by the spacer in determining the transitional behaviour of the polymer from incomplete homo- logous series and such extrapolations may have overlooked subtle but important effects.This situation has arisen mainly as a result of synthetic difficulties and availability of key intermediates. An example of this can be found in polysiloxane chemistry in which w-bromo-a-alkenes are key intermediates.’ Only selected members of this series are readily available and this has limited the range of spacer lengths that have been attached to polysiloxane backbones. In recent years, however, synthetic methodologies have been developed to allow the synthesis of mesogenic poly(viny1 ether) poly(norborn-ene)s15,16 and polystyrene^'^^^' for which as many as 10 homologues can be readily obtained. In this contribution we report the thermal properties of a poly(methacry1ate)-based system, the poly [co-( 4’-methoxybiphenyl-4-yloxy)alkylmeth--1-L--t-Jx 1 acrylatels, 1, comprising 10 homologues.We believe lhat this is the first example of a poly(methacry1ate)-based scries for which so many homologues have been prepared and characterised. The notation used to describe these polymers is n-OMe in which n denotes the number of carbon atoms in the flexible alkyl spacer. This particular series was chosen for investigation for three main reasons. First, the mesogenic group has been attached to a range of polymer backbones and this allows for the effects on the thermal behaviour of the polymer of the nature of the backbone to be determined. Secondly, the most complete studies of such effects2’ have been made using polymers containing the 4’-cyanobiphenyl-4-yloxy furiction, a group which normally gives rise to partially intertligitated smectic pha~es.~In contrast, 4‘-methoxybiphenyl-4-yloxy-based polymers will exhibit different smectic structures.Finally, members of this series, 1, were recently prepared by anionic polymerization, resulting in highly stereoregular poly- mer~.~’,~~By comparison, the polymers reported here have been prepared by free-radical polymerization and 1his will allow for the effects of stereoregularity to be considered. Experimental The polymers were prepared using the synthetic routc shown in Scheme 1. Identical experimental procedures were used to prepare all members of series 3 and 4 (see Scheme l), and hence only a representative description is given for the monomer synthesis. Monomer Synthesis 4-Hydroxy-4’-methoxybiphenyl (2) was prepared according to the method described by Rodriguez and Per~ec.-’~ Thus, 4,4’-dihydroxybiphenyl (62.6 g, 0.34 mol) was dissolved in 5% aqueous sodium hydroxide (700ml) and cooled in an ice- salt-water bath.Dimethyl sulfate (18.18 g, 0.14 mol) was slowly added dropwise and the solution subsequently allowed to warm to room temperature. The resulting precipitate was collected, added to boiling water (300ml) and filtered hot. J. MATER. CHEM., 1994, VOL. 4-dlmethyl sulfate H 3 C O w O H 0"C - - 2 2 Br(CHd),Br, n=3-12 K2C03,acetone, A t 3 3 methacryllc acid KHC03, DMF, A b 0 H3COwO(CH2)nOCCCH3II II-- 4 CH2 4 AIBN, A benzene b CO I O(C H 2)n0 - - OCH3 Scheme 1 Dilute hydrochloric acid was added to the filtrate and the resulting precipitate was collected, recrystallised twice from ethanol and dried under vacuum.The yield was consistently much lower than that reported by other Yield: 13.5 g, 20%. mp: 184.8-186.3 "C (literature mp 182-183 T).23-26IR (KBr): vmax 3399 cm-' (OH), 'H NMR (CDCl,) 6: 6.9, 7.5 (m, 8 H, aromatic), 4.8 (s, 1 H, OH), 3.8 (s, 3 H, OCH,). l-Bromo-9-(4'-methoxybiphenyl-4-yloxy)nonane(3) was prepared using a modification of the procedure described by Crivello et 2, (2.56 g, 12.8 mmol), 1,9-dibromononane (34.8 g, 128 mmol) and potassium carbonate (13.2 g, 96 mmol) were refluxed with stirring in acetone (200 ml) overnight.The reaction mixture was filtered hot, the residue washed with acetone, and the acetone removed under reduced pressure. Light petroleum (40-60 "C) was added to the concentrated organic extracts and the resulting precipitate collected and dried. The crude product was recrystallised twice from ethanol with hot filtration to ensure the complete removal of the dimeric side-product, 1,9-(4'-methoxybiphenyl-4-yloxy)non-ane. Yield: 3.3 g, 65%. mp: 88.4 "C, purity (DSC): >99.5%. 'H NMR (CDCl,) 6: 6.9, 7.5 (m, 8 H, aromatic), 4.0 (t, 2 H, J 6.5, OCH,), 3.8 (s, 3 H, OCH,), 3.4 (t, 2 H, J 6.8, CH2Br), 1.8-2.0 (m, 4 H, OCH,CH,, CH,CH,Br), 1.3-1.5 [m, 10 H, 0(CH2 )2(CH,)5(CHd,Brl.9-[ 4-( 4-Methoxypheny1)phenoxyl nonyl methacrylate (4 n=9) was prepared using a modification of the procedure described by Nakano et al.,, Methacrylic acid (0.72 g, 7.0 mmol) was stirred with potassium hydrogen carbonate (0.80 g, 8.0 mmol) at room temperature for 5 min to form potassium methacrylate. This salt was added to 3 (2.19 g, 5.4 mmol) and hydroquinone (0.016 g, 0.55 mmol) in N,N'-dimethylformamide (66 ml) and the resulting mixture was stirred at 100°C for 24 h. The reaction mixture was allowed to cool and was poured into water (ca. 300 ml). The resulting precipitate was filtered and dissolved in dichloromethane. The organic solution was washed with 5% aqueous sodium hydroxide and then water. The organic layer was dried over MgSO,, filtered and the solvent removed.The crude product was recrystallised twice from ethanol. Yield: 1.52 g, 69%. mp: 89.5"C, purity (DSC): ~99.5%. IR (KBr) v/cm-': 1715 (vs C=O), 1638 (C=C). 'H NMR (CDC1,) 6: 6.9, 7.5 (m, 8 H, aromatic), 5.6, 6.1 (s, 2 H, CH,=C), 4.2, [t, 2 H, J 6.7, H2COC(0)], 4.0 (t, 2 H, J 6.5, OCH,), 3.8 (5, 3 H, OCH,), 2.0 (s, 3 H, CH,), 1.5-1.8 [m, 4 H, OCH,CH,, CH,CH,OC(O), 1.2-1.5 [m, 10 H, O(CH,),(CH,),(CH,),OC~O)]. Polymerisa tion Monomer 4 (1g) was dissolved in benzene (10 ml) and 1mol% AlBN added as initiator. The reaction mixture was flushed with argon for 20min and then heated in a water bath at 60°C to initiate the polymerisation. After 48 h the reaction was terminated by adding THF (15 ml) and the polymer precipitated into a large amount of methanol. The product was redissolved in THF and reprecipitated into methanol.The removal of the monomer was monitored spectroscopically. The alkene stretch at 1638 cm-' in the IR spectra of the monomers and the peaks associated with the alkene protons at 5.6 and 6.1 ppm in the 'H NMR spectra were not present in those of the corresponding polymer. 9- OMe: Yield: 0.84 g, 71%. IR (KBr): vmax 1728 cm-' (vs C=O). 'H NMR (CDC1,) 6: 6.9, 7.4 (m, 8 H, aromatic), 3.8-4.0 (m, 4 H, OCH,), 3.7 (s, 3 H, OCH,), 1.3, 1.6, 1.8 (m, 16 H, CH,), 0.9, 1.1 (m, 3 H, CH,). General The proposed structures of all the compounds were verified using 'H NMR and IR spectroscopy. 'H NMR spectra were measured in CDC1, on a Bruker AC-F 250MHz NMR spectrometer.The tacticities of the polymers were determined from spectra measured at 60 "C in CDCl,. IR spectra were recorded using a Philips Analytical PU9800 FTIR spec-trometer. The purities of all the intermediates were verified using thin layer chromatography (TLC) and differential scan- ning calorimetry (DSC). The molecular weights of the poly- mers were measured by gel permeation chromatography (GPC) using a Knauer Instruments chromatograph equipped with two PL gel 10pm mixed columns and controlled by Polymer Laboratories GPC SEC V5.1 Software. Chloroform was used as the eluent. A calibration curve was obtained using polystyrene standards. The thermal properties of the polymers were characterised J.MATER. CHEM., 1994,VOL. 4 1707 by DSC using a Polymer Laboratories PL-DSC equipped made of their properties both across the series and ~ith other with an autocool accessory and calibrated using an indium polymers reported in the literature. standard. Two samples were used for each polymer and the The data listed in Table 1 were extracted from the second results averaged. The time-temperature profile was identical heating cycle of the DSC time-temperature profile and these for each polymer. Thus, each sample was heated from 25 to traces are collected in Fig. 1. For the propyl homologue, i.e. 230'C, maintained at 230 "C for 3 min, cooled to -5O"C, 3-OMe, a single peak is evident in the DSC trace, dthough maintained at -50 "C for 3 min and finally reheated to 230 "C.this has a reproducible shoulder at ca. 172°C (Fig. 1). No The heating and cooling rate in all cases was 10°C min-'. The glass transition was observed for this polymer. U'hen the identification of the liquid-crystalline phases was performed by polarised light microscopy using an Olympus BH-2 optical microscope equipped with a Linkam THMS 600 heating stage n and TMS 91 control unit. Clear. characteristic optical textures 12from which phase assignments were possible were obtained by cooling the polymer at either 0.2" or 0.1 "C min-l from 11 ca. 1O'C above the clearing temperature to below the glass- transition temperature or, in the absence of a Tp, to room 10 temperature. 9 8 Results and Discussion 7 The thermal properties of the poly [w-( 4-methoxybiphenyl-4'-y1oxy)alkyl methacrylate] s, 1, are listed in Table 1 while 6 Table 2 contains the data reported for members of the series 5reported elsewhere in the literature.The number-average molecular weights of the polymers were all in excess of V 4 29 000 g mol-' (average degree of polymerization > 90) with associated polydispersities in the range 2.2-4.0. These high 3 molecular weights ensured that the thermal properties of the polymers lay outside the molecular weight dependent regime.27 ==7rThe hexyl and octyl homologues exhibited the broadest peaks and contained a high molecular weight fraction. However, I I I I IKomiya et have shown that polydispersity does not effect 0 I 50 100 1 50 200 250either the transition temperatures or the width of the biphasic TPCregion provided that the component polymers are in the molecular weight independent regime.This is the case for the Fig. 1 The normalised DSC traces obtained on the second heating materials reported here, allowing valid comparisons to be of the n-OMe series Table 1 Thermal properties of the polymers 1 --179 -12.41 -3.30 4 -(125) (139) (5.22) (0.78) (1.57) (0.23) 5 92 155 169 6.31 4.89 1.77 1.33 6 88 117 134 5.56 2.18 1.72 0.64 7 55 114 145 3.62 4.22 1.12 1.22 8 -89 129 2.13 3.04 0.71 0.91 9 48 91 148 0.81 4.80 0.27 1.37 10 51 78 131 0.93 4.20 0.32 1.25 11 45 83 140 1.02 5.36 0.34 1.56 12 57 82 136 1.15 4.58 0.39 1.35 Table 2 Thermal properties of the poly [o-(4'-methoxybiphenyl-4'-yloxy)alkylmethacrylate] s reported in the literature 2 120 -(152) (0.87) -(0.25) 32 2 140 2.18 -0.64 23 3 175 -9.13 -2.45 23 4 (145) C3.911 (0.51) Cl.151 (0.15) 23 5 177 4.25 3.54 1.17 0.95 23 6 136 (4.64) 2.61 (1.42) 0.77 32 6 131" (11.78) 2.72" (3.63) 0.81" 30 6' 132 --22 6 132' 5.15' 2.36' 1.59' 0.70' 23 11 142 1.09 6.78 0.36 1.97 31 X denotes either a crystal or a highly ordered smectic phase."A combined S-N-I transition. 'Authors did not assign the phxse types. 'Combined transitions. isotropic phase was cooled, bgtonnets developed which exhib- ited arcs crossing their backs (Plate 1). These coalesced on further cooling to give the arced focal conic fan texture shown in Plate 2.This optical texture is indicative of an E phase.28 The relatively large entropy change associated with the clear- ing temperature, see Table 1, is consistent with this view. The arced focal conic fan texture is, however, a paramorphotic texture,28 suggesting the existence of a narrow smectic A phase, but this was not observed using the polarising micro- scope. A change in the optical texture could not be correlated with the shoulder associated with the peak in the DSC trace. The thermal data for 3-OMe show good agreement with the data for 3-OMe prepared by a free-radical method,23 see Table 2, although no shoulder was observed in the latter DSC trace. However, the DSC trace of a predominantly syndiotactic sample prepared by the same authors did exhibit this shoulder, suggesting that the polymers reported here also possess a significant syndiotactic fraction.In order to investigate this possibility 'H NMR spectroscopy was used to characterise the tacticities of the n-OMe series. Typical spectra are shown in Fig. 2 while Table 3 lists the relative amounts of each configuration. All the polymers possess a significant syndiotac- tic fraction, presumably resulting from steric considerations. However, the tacticity of 3-OMe is similar to that of the polymer prepared by free-radical p~lymerization~~ so it would appear that the shoulder is not an effect of tacticity. A more plausible explanation, therefore, notes that the polymer reported by Nakano et al. actually exhibits a slightly lower clearing temperature than 3-OMe.This shift of the main DSC peak, albeit by a small amount, may obscure the shoulder Plate 1 Bitonnets separating from the isotropic phase for 3-OMe (177 C) J. MATER. CHEM., 1994, VOL. 4 1.2 1.o 0.8 6 Fig.2 Region of the 'H NMR spectrum for (a) 7-OMe and (h) 4-OMe showing the peaks associated with the methyl protons Table 3 Tacticities of polymers 1 n ~~ I S H 3 1 4 5 4 1 5 9 5 1 4 7 6 1 5 8 7 0 2 3 8 0 5 8 9 0 1 2 10 0 1 2 11 0 2 3 12 0 1 2 observed for 3-OMe (Fig. 1). Nakano et al. failed to obtain any clear, characteristic optical textures for this polymer and so it is not possible to compare phase assignments.The DSC trace exhibited by 4-OMe contains two peaks (Fig. 1). No glass transition was observed for 4-OMe. When the isotropic phase was cooled, a nematic schlicrerz texture developed (Plate 3). This assignment is supported by the small entropy change associated with the clearing transition which is comparable to that exhibited by other side-chain liquid- crystal polymers2' as well as by low molar mass liquid Plate 2 Arced focal conic fan texture exhibited by 3-OMe (100"C) Plate 3 Nematic schlieren texture exhibited by 4-OMe (135 'C) J. MATER. CHEM., 1994, VOL. 4 crystals.29 When the sample is cooled further the schlieren texture becomes somewhat sanded (Plate 4), indicating a smectic C-nematic transition. The entropy change associated with a smectic C-nematic transition is dependent on the length of the nematic phase.For 4-OMe the nematic phase is relatively short, 14T, and hence a relatively large entropy value is observed. The thermal data for this polymer are in reasonable agreement with those reported by Nakano et al., Table 2. although the authors offered no phase assignments because clear, characteristic optical textures were not obtained. The DSC trace of 5-OMe contained two peaks and in addition, a weak second-order transition. The latter transition is too weak to be observed on the scale used in Fig. 1 and has been assigned as a glass transition. When the isotropic phase was cooled, bitonnets developed which coalesced giving rise to a well defined focal-conic fan texture (Plate 5).As a consequence, this is assigned as a smectic A phase and the magnitude of the entropy change associated with the clearing temperature supports this view. When the sample was cooled further. continuous bands developed across the backs of the fans which persisted to room temperature (Plate 6). These changes are indicative of an E-smectic A phase transition.28 The thermal data for 5-OMe are in reasonable agreement with those reported by Nakano et ~l.,*~but again no phase assignments were offered by the authors. The DSC trace for 6-OMe comprises a weak second-order transition assigned as a glass transition and two endothermic peaks. The peak associated with the clearing transition has a small shoulder associated with it.When viewed through the polarising microscope, a rather poorly developed focal-conic Plate 4 Smectic C schlieren texture exhibited by 4-OMe obtained on cooling the preparation in Plate 3 (120 "C) Plate 5 Smectic A focal conic fan texture exhibited by 5-OMe (165 "C) Plate6 Arced focal conic fan texture exhibited by 5-OMe obtained on cooling the preparation in Plate 5 (78"C) fan texture was obtained when the isotropic phase wah cooled. When the sample was cooled further, faint bands appeared across the backs of the fans which persisted to room tempera- ture. By analogy with 3-OMe and 5-OMe, the higher-tsmpera- ture phase is assigned as a smectic A and the lower-temperature phase as an E phase. No change in thc. optical texture could be correlated with the shoulder associated with the higher-temperature peak in the DSC trace.The ti ansition temperatures reported for 6-OMe are in excellent agreement with those reported elsewhere (Table 2). A similarlj shaped clearing peak was reported by Hahn et ~1.;~'the authors rationalised this in terms of a combined smectic A-nematic- isotropic transition. However, no schlieren texture was observed for 6-OMe. The DSC trace for 7-OMe contains a second-order glass transition and two endothermic peaks. When the isotropic phase was cooled, batonnets developed then coalesced, giving rise to a well defined focal conic fan texture. When the sample was cooled further, continuous bands developed across the backs of the fans, which persisted to room temperature.This behaviour is similar to that described for 5-OMe and thus, by analogy, the higher-temperature phase is assigned as a smectic A phase and the lower-temperature phase as an E phase. The DSC traces for 8-OMe and 10-OMe consisted of a weak second-order glass transition and two end( Pthermic peaks. No clear characteristic optical textures were obtained for these polymers. When the sample was cooled from the isotropic phase a poorly defined focal conic fan texture was formed (Plate 7), so the phase is tentatively assigned as a smectic A phase. When the lower-temperature transition was passed, no detectable change in the optical texture was observed. This absence of change may indicate a smectic B or crystal B-smectic A transition, although given the poorly defined texture this is a rather speculative assignment.The DSC trace exhibited by 9-OMe contains a second-order glass transition and two endothermic peaks. A focal conic fan texture was obtained when the sample was cooled from the isotropic phase. As the bdtonnets developed and coalesced, continuous bands crossed the backs of the fans but these did not persist into the phase, which is thus tentatively assigned as a smectic A phase. This texture subsequently remained unchanged when the sample was cooled to room temperature, suggesting that the lower-temperature phase was either a crystal or smectic B phase. The DSC trace of 11-OMe showed a glass transition and two endothermic peaks.When the isotropic phase was cooled, biitonnets formed and coalesced to give a focal conic fan 1710 J. MATER. CHEhl., 1994, VOL. 4 ture phase which was obtained by annealing the sample for 15 h at 137 "C. The DSC trace of 12-OMe exhibited a glass transition and two endothermic peaks. When the sample was cooled, similar textures were obtained to those described for 11-OMe. However, the focal conic fans were somewhat less well defined, Plate7 Poorly defined focal conic fan texture exhibited OMe (120 C) texture in coexistence with regions of homeotropic (Plate 8). This can be assigned as a smectic A phathe sample was cooled further, bitonnets continued tfrom the homeotropic regions while the existing fansmoother (Plate 9). In certain regions of the preparabands formed across the backs of the fans, suggincrease in the in-plane ordering of the molecules. As consequence, the lower-temperature phase is tassigned as either a crystal or smectic B phase.Hreported a similar focal conic texture for the higher alignse. When o develop s became esting an enta-tem by 8-ment tion, faint a tively su et aL31 pera- but by analogy to 11-OMe the higher-temperature phase is assigned as a smectic A phase and the lower temperature phase as either a crystal or smectic B phase. Fig. 3 shows the dependence of the transition temperatures on the number of carbon atoms in the alkyl spacer for the ri-OMe series, including 2-OMe.24332 The glass-t ransition tem- peratures initially decreased with increasing spacing length before reaching a limiting value of ca.50 "C. Similar behaviour was observed for mesogenic polymethacrylates containing the 4'-cyanobiphenyl-4-yloxy group, although the limiting value was in the region of 20"c.20 The higher glass-transition temperatures exhibited by the n-OMe series probably reflects the higher packing density present in the highly ordered lower-temperature smectic phase exhibited by these polymers over that found in the interdigitated smectic phases exhibited by the 4'-cyanobiphenyl-4-yloxy-basedmaterials. Thus, the higher glass-transition temperatures result from a reduction in the specific free volume in the smectic phase. There is no clear evidence that the glass-transition temperatures alternate with the parity of n, as has been found for mesogenic polystyrenes.18p20 The clearing temperatures exhibit a distinct odd-even effect with the number of carbon atoms in the spacer, 11, with the odd members of the series exhibiting the higher values (Fig.3). Note that although different clearing transitions are being compared, it is generally found that within a given series in which the nature of the clearing transition varies, the alter- nation in smectic-isotropic transition temperatures parallels that for the nematic-isotropic transition temperature^.^^ Hence, the shape of the curve is not dependent on the types of transition being considered. For the n-OMe series the alternation in the clearing temperatures is initially pronounced but attenuates on increasing n. The established rationalisation for such behaviour considers the alternation in the average shape of the side chain and its effect on the relative orientation of the mesogenic groups on varying the parit! of the spacer (Fig.4).2 Thus for an odd-membered spacer the mesogenic unit is orthogonal to the backbone whereas for an even-membered spacer the mesogenic groups are constrained to lie - Plate8 Focal conic fan and homeotropic textures of the phase exhibited by 11-OMe (133 'C) smectic A 190 150- 02 110- 70- 30! I , . . , , , I I I 2 4 6 81012 n Plate 9 Focal conic fan and homeotropic textures of temperature smectic phase exhibited by 1I-OMe (27 'C) the lower Fig. 3 Dependence of the glass-transition temperatures (A), E-isotropic (A),smectic A-isotropic (O), nematic isotropic (O), smectic C-nematic (+), E-smectic A (m) and smectic B/B-smectic A (0).Transition temperatures for 2-OMe have been taken from the literature.24332I, isotropic; N, nematic; g, glass; E, crystal E; S,, amectic A; S,, smectic C; B, crystal B; SB,smectic B.J. MATER. CHEM., 1994, VOL. 4 1711 00 00>LooO Y6O-O 6 6;t 3 g0L70 Fig. 4Diagrammatic representation of the effects of the introduction of a single gauche defect into the spacer of a side-chain liquid-crystal polymer containing (a)an odd-membered and (b)an even-membered spacer at some angle with respect to the backbone. This model assumes that the backbone lies in a plane orthogonal to the director and this view is supported, in part, by experimental studies using techniques including X-ray diffra~tion,~~ neutron scattering3’ and ’H NMR.36 However, the flexible spacer is unlikely to exist exclusively in an all-trans conformation and so this rationalisation must be extended to consider a confor- mational distribution for the spacer rather than just a single conformation.18 The effects of introducing a single gauche linkage into the spacer and moving it sequentially along the chain are also shown in Fig.4. For the odd-membered spacer there are more conformations for which the mesogenic groups are coparallel [Fig. 4(u)]. In such arrangements the aniso- tropic interactions between the mesogenic units are maximised and hence higher clearing temperatures result. The attenuation in the alternation exhibited by the clearing temperatures may then be explained by the dilution of this shape change resulting from the greater number of conformations available to the spacer as its length is increased.This interpretation of the dependence of the clearing temperatures on n strictly accounts only for clearing temperatures that increase with n. This is not the case for the n-OMe series, and to allow for this the explanation must be further extended to consider chain flexi- bility. This acts to reduce the clearing temperature and thus the spacer has a dual role in determining the clearing tempera- ture. Increasing the spacer length enhances the shape ani- sotropy of the side chain, so promoting liquid crystallinity, but the increase in molecular flexibility serves to decrease the clearing temperature.In this respect the role of the spacer in determining transition temperatures may be considered to be analogous to the role played by terminal alkyl chains in determining the liquid-crystal properties of low molar mass mesogens.*’ The more elongated conformers invoked to account for the dependence of the clearing temperatures on n will be favoured in the anisotropic liquid-crystalline environment. Thus a consequence of this explanation is that a greater change in the conformational component of the entropy associated with the clearing transition would be anticipated for an odd member rather than an even member. Fig. 5 shows the depen- dence of the clearing entropy, expressed as the dimensionless quantity ASIR, on the number of carbon atoms in the flexible spacer, n, for the n-OMe series including 2-OMe.32 The 4T -1 I 1 1 , I Io! ,,,,I 2 4 6 81012 n Fig.5 Dependence of the entropies associated with the thermal transitions on the length of the alkyl spacer. The symbols dcnote the same transitions as in Fig. 3. The nematic-isotropic clearing entropy for 2-OMe was taken from ref. 32. entropy change exhibited by 3-OMe is significantlj higher than that shown by the remaining members of the series and is consistent with the assignment of an E-I phase transition. By contrast, that the clearing entropies exhibited by 2-OMe and 4-OMe are considerably smaller than that shown by the other homologues reflects that these members are nemato- genic. The clearing entropies of the remaining members may be compared as they are all associated with smectic A-isotropic transitions.A distinct odd-even effect is evident in which the odd members exhibit the higher values. This supports the suggestion that the conformational component of the entropy change is larger for the odd members. However, it must be remembered that there are three main contributions to the smectic A-isotropic entropy: conformational, orien- tational and translational. It is the subtle interplay of these three contributions that determines the overall entropy value. Thus while it is tempting to account for the alternation in the clearing entropies solely in terms of an alternation in the conformational component such a view must be treated with some caution.Indeed, further speculation on this matrer must await the results of model calculations. We now turn our attention to the effects of the flexibility 1712 of the backbone on the liquid-crystalline behaviour of the polymer. The analogous poly (acry1ate)-based material to 11-0~e,31 I 1 942 exhibits a smectic-isotropic transition at 147 "C, 7 "C higher than that exhibited by 11-OMe. The smectic phase was not identified unambiguously but the entropy change associated with the clearing transition, expressed as the dimensionless quantity ASIR, was 1.22. This suggests that the phase was a smectic A and the entropy change is lower than that reported here for 11-OMe. This observation supports the view that increasing backbone flexibility increases the clearing transition temperature while reducing the associated entropy change.20 Three polysiloxane-based materials have been reported J.MATER. CHEhl., 1994, VOL. 4 would have been anticipated over that of 10-OMe, the ana- logous poly(methacry1ate) having a spacer of the same length. However, the poly(viny1 ether) was of relatively low molecular weight and its thermal properties may still lie in the molecular weight dependent regime. The properties of the hexyl member of a poly(propeny1 ether)-based series have been reported,24 4-Me-CH and as with the poly(viny1 ether), the clearing temperature, 110"C, is considerably lower than would be anticipated on the basis of the increase in backbone flexibility over the n-OMe series.Again this may be attributed to a molecular weight effect. The most complete set of data with which to compare the 4-0 IMe-Si-(CH,),O M O M e I n = 3,4,11 The propyl and butyl homologues of this series were reported to be crystalline in nature, exhibiting crystalkisotropic trans- ition temperatures of 125 and 123 "C, re~pectively.~~This is a surprising result because the highly flexible polysiloxane chain would be anticipated to increase the clearing temperature over the analogous members of the n-OMe series. By contrast, the undecyl member of the series38 exhibits a smectic-isotropic transition at 154°C. In this instance the behaviour of the polysiloxane should not be compared to that of 11-OMe but rather 9-OMe as it is the total number of atoms contributing to the length of the spacer that must be considered.The clearing temperature of the polysiloxane-based material is 6°C higher than that exhibited by 9-OMe, in agreement with the observation that increasing backbone flexibility enhances the clearing transition temperature.20 However, this modest increase is not as large as anticipated on the basis of 4'- cyanobiphenyl-4-yloxy containing polymers.20 The entropy associated with the clearing transition exhibited by the nonyl member of the polysiloxane-based series, AS/R =1.66, is higher than that reported here for 9-OMe. However, it is difficult to comment on this as the smectic phase exhibited by the polysiloxane was not unambiguously identified. The undecyl member of a poly(viny1 ether)-based series containing 4'-methoxybiphenylyl has been reported,39 -rx and exhibited a crystal-isotropic transition at 131 "C as well as a monotropic smectic A phase.This is also a somewhat surprising result as an increase in the clearing temperature thermal properties of the n-OMe series has been reported for containing the methoxybiphenylyl mesogenic gro~p:~~,~~ a series of poly(norbornene)-based materials,15*16 The homologues with n =2-10 exhibit nematic behaviour, while the undecyl and dodecyl homologues are smectogens. The clearing temperatures of the complete series lie in the range 84-96 "C, considerably lower than the range of tempera- tures reported for the n-OMe series.Komiya et di5considered these polymers to be derivatives of poly (norbornene), which exhibits a glass-transition temperature of ca. 40°C. Thus on the basis of backbone flexibility the poly(norbornene)-based materials would be anticipated to exhibit clearing tem-peratures somewhat higher than those of the n-OMe series. This is not the case, however, and more realistically the poly(norbornene)-based materials should be considered as derivatives of phwButI' CO,Me rather than poly(norbornene) itself. The Tp for this polymer is not available, although using the empirical relationships relating molecular structure to Tgit would be predicted that its Tgshould be considerably higher than that of poly(norbor- nene).This accounts, therefore, for the lower clearing tempera- tures. However, the Tgs exhibited by these polymers are also .lower than those shown by the n-OMe series. In addition, the clearing entropies exhibited by the two smectogenic members of the poly(norbornene)-based series are lower than those of the analogous n-OMe homologues. The behaviour of the poly(norbornene)-based materials does not appear to support the view that decreasing backbone flexibility increases Tg and AS/Rbut decreases the clearing temperature.20 In this instance, however, the anomalous behaviour may be attributed to the complex microstructure of the poly(norbornene) chains involving cis and trans double bonds as well as head-to-head, head-to-tail and tail-to-tail arrangements of the repeat units.15 J.MATER. CHEM., 1994, VOL. 4 The synthesis and properties of a closely related set of polymers have been reported by Duran et a1.25340-42and by Rodriguez and Per~ec~~ in which the flexible alkyl spacer is replaced by an oligo(ethy1ene oxide) chain, y2I Me-C-C-[0(CH2),lnO1:: n = 2,3 The members of the n-OMe series analogous to n=2 and 3 are 5-OMe and 8-OMe, respectively. For both polymers the clearing temperatures of the n-OMe members are ca. 18"C higher while the Tg of 5-OMe is lower. Both the flexibility and the ground-state conformations of a poly(ethy1ene oxide) chain are thought to differ from those of an alkyl chain.43 These differences may be expected to influence the liquid- crystal behaviour of side-chain liquid-crystal polymers pos- sessing such a spacer.This observation now requires further study. In general, therefore, a comparison of the thermal behaviour of the n-OMe series with that of other series differing only in the chemical nature of the backbone, supports the view that increasing backbone flexibility (i) decreases q,(ii) enhances the clearing temperature and (iii) reduces the entropy change associated with the clearing transition. A new model, the virtual trimer, was recently proposed to account for these effects.20 This considers that the mesogenic group comprises two side chains linked or correlated by a backbone segment. In a rigid polymer, the formation of such species is restricted and hence lower clearing temperatures would be anticipated.Also, the formation of such trimers in a flexible polymer actually increases the rigidity of the backbone, enhancing Tg relative to the unsubstituted polymer, and this is observed experimentally. Within the framework of the virtual trimer model, the trend in the clearing entropies may be interpreted in terms of the relative contributions of the conformational, orientational and translational components to the overall entropy. This is somewhat speculative, however, because of the uncertainty in correlating entropy change and molecular An alternative explanation to account for the relationship between the clearing entropy and backbone flexibility has been proposed by Percec and Tomazos and this considers the backbone is varied.Its ability successfully to account for the subtle trends in the clearing entropy within a given series, such as the n-OMe series (Fig. 5), has still to be investigated and it is possible that the conformational statistics of the spacer will be of greater importance than those of the back- bone in determining the small differences in AS/R as n is varied. Conclusions The synthesis and thermal properties for 10 members of a poly(methacry1ate)-based side-chain liquid-crystal polymer series have been reported. To our knowledge this is the most complete series of such polymers and allows us to examine in detail the role of the spacer in determining transitional behaviour. The glass-transition temperatures decrease initially with increasing n before reaching a limiting value.By contrast the clearing temperatures show a regular odd-even effect as the length and parity of the spacer is varied, with the odd members exhibiting the higher values. The entropy change associated with the clearing transition exhibits a similar dependence on n. This behaviour may be rationalised by considering the change in the average shape of the side chain as the parity of the spacer is varied. This may account also for the difference in behaviour of polymers containing oligo (ethylene oxide) spacers. The thermal behaviour of the n-OMe series has also been compared to other polymers containing the same mesogenic unit but different main chains.This revealed some wpport for the view that increasing the flexibility of the polymer chain enhances the clearing transition temperature but reduces the associated entropy change. Exceptions to this rule are r,itional- ised in terms of either molecular weight or in the case of poly (norbornene)-based materials complex microstructures. However, the significance of the anomalous behaviour of members of a polysiloxane-based series was not clear. It should not be overlooked that Emerson and Luckh~rst~~ have shown, using a molecular field theory for flexible molecules, that changes in transition temperatures and associated entrop- ies for low molar mass materials can be accounted for solely in terms of the molecular geometry of the link that connects the terminal chain to the mesogenic core.This may also be an important consideration in understanding the transitional behaviour of side-chain liquid-crystal polymers for which the geometry of the link connecting the side chain to the polymer backbone may be an important factor. This may account for differences, for example, between poly(methacry1atc)s and polysiloxanes. conformation of the polymer chain in the mesopha~e.~?~~,~~ The University of Aberdeen Research Committee are grate- For a flexible polymer such as a polysiloxane, the backbone is considered to be confined by the smectic field to lie between the smectic layers, forming essentially a microphase-separated morphology. In contrast, for a rigid backbone, e.g.a poly (methacrylate), the backbone is no longer confined by the smectic field but instead crosses the smectic layers. In doing so, the backbone is forced to adopt more extended confor- mations and hence at the clearing transition there will be a greater change in the conformational distribution of a rigid backbone than for a flexible backbone. As a consequence, a higher clearing entropy would be anticipated for the system based on the rigid backbone. It is difficult to comment upon the validity of this model for this particular set of materials because we do not know the molecular organisation within the mesophase. Indeed, the relationship between the chemical structure of the polymer backbone and the conformation it fully acknowledged for the award of a studentship tcr A.A.C.and for a grant to purchase the PL-DSC. References 1 Side Chain Liquid Crystal Polymers, ed. C. B. McArdle Blackie and Sons, Glasgow, 1989. 2 V. Percec and C. Pugh, Side Chain Liquid Crystal Poljmers, ed. C. B. McArdle, Blackie and Sons, Glasgow, 1989, ch. 3. 3 V. Percec and D. Tomazos, Comprehensive Polymer Science, First Suppl., ed. S. L. Aggarwal and S. Russo, Pergamon Press. Oxford, 1992, ch. 14. 4 C. T. Imrie, T. Schleeh, F. E. Karasz and G. S. Attard, Macromolecules, 1993,26, 539. 5 G. W. Gray, Side Chain Liquid Crystal Polymers, ed. C. B. McArdle, Blackie and Sons, Glasgow, 1989,ch. 4. 6 H. Jonsson, P.-E. Sundell, V. Percec, U. W. Gedde and A. Hult, Polym. Bull., 1991,25, 649. adopts in a mesophase has still to be fully e~tablished.~~,~* V.Heroguez, M. Schappacher, E. Papon and A. Deffieuu, Polym.7 However, this is a plausible model with which to account for Bull., 1991,25, 307. the trends in the clearing entropy as the flexibility of the 8 V. Percec and M. Lee, Macromolecules, 1991,24,4963. 1714 J. MATER. CHEM., 1994, VOL. 4 9 V. Percec, M. Lee and H. Jonsson, J. Polym. Sci., Part A: Chem. 30 B. Hahn, J. H. Wendorff, M. Portugal1 and H. Ringsdorf, Colloid Ed., 1991, 29, 327. Polym. Sci., 1981,259, 875. 10 V. Percec and M. Lee, Macromolecules, 1991,24, 1017. 31 C. S. Hsu, J. M. Rodriguez-Parada and V. Percec, Mukromol. 11 V. Percec and M. Lee, Polym. Bull., 1991, 25, 131. Chem., 1987,188,1017. 12 V. Percec and M.Lee, Macromolecules, 1991,24,2780. 32 H. Finkelmann, M. Happ, M. Portugal and H. Ringsdorf, 13 V. Percec and M. Lee, Polymer, 1991,32,2862. Makromol. Chem., 1978,179,2541. 14 V. Percec, M. Lee and C. 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Allcock and C. Kim, Macromolecules, 1990,23,3881. 46 V. Percec and D. Tomazos, Adu. Muter., 1992,4. 548. 27 C. T. Imrie, F. E. Karasz and G. S. Attard, J. Macromol. Sci-47 P. Davidson and A. M. Levelut, Liq. Cryst., 1992, 11,469. Pure Appl. Chem., 1994, A31,1221. 48 L. Noirez, P. Davidson, W. Schwarz and G. Pepy, Liq. Cryst., 28 G. W. Gray and J. W. Goodby, Smectic Liquid Crystals- 1994,16,1081. Textures and Structures, Leonard-Hill, Glasgow, UK, 1984. 29 C. T. Imrie and L. Taylor, Liq. Cryst., 1989,6, 1. Paper 4102793K; Received 1lth May, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401705

出版商:RSC    

年代:1994

数据来源: RSC