摘要:

THE ROYAL SOCIETY OF CHEMISTRY Journal of Materials Chemistry Scientific Editor Staff Editor Professor Anthony R. West Mrs. Janet M. Leader Department of Chemistry The Royal Society of Chemistry University of Aberdeen Thomas Graham House Meston Walk Science Park Aberdeen AB9 2UE, UK Cambridge CB4 4WF, UK Editorial Secretary: Miss J. E. Chapman Materials Chemistry Editorial Board Anthony R. West (Aberdeen) (Chairman) C. Richard A. Catlow (London) David A. Rice (Reading) David A. Dunmur (Sheffield) Rodney P. Townsend (Bebington) H. Monty Frey (Reading) Allan E. Underhill (Bangor) John W. Goodby (Hull) Graham Williams (Swansea) John D. Wright (Canterbury) ~ ~~~ International Advisory Editorial Board M. A. Alario- Franco (Madrid) D.Kohl (Aachen) K. Bechgaard (Copenhagen) M. Lahav (Rehovot) J. D. Birchall (Runcorn) A. J. Leadbetter (Daresbi ry) D. Bloor (Durham) P. M. Maitlis (Sheffield) A. K. Cheetham (Oxford) J. S. Miller (Wilmington) E. Chiellini (Pisa) P. S. Nicholson (Hamilton) M. G. Clark (Wembley) M. Nygren (Stockholm) P. Day (Grenoble) V. Percec (Cleveland) D. Demus (Halle) C. N. R. Rao (Bangalore) B. Dunn (Los Angeles) M. Ratner (Evanston) W. J. Feast (Durham) J. Rouxel (Nantes) A. Fukuda (Tokyo) R. Roy (University Park, PA) D. Gatteschi (Florence) J. L. Serrano (Zaragoza) A. M. Glass (Murray Hill) J. N. Sherwood (Glasgow) J. B. Goodenough (Austin) J. Simon (Paris) G. W. Gray (Poole) J. F. Stoddart (Sheffield) A. C. Griffin (Cambridge) S.Takahashi (Osaka) S-i. Hirano (Nagoya) G. J. T. Tiddy (Bebington and Salford) P. Hodge (Manchester) B. J. Tighe (Birmingham) H. lnokuchi (Okazaki) Yu. D. Tretyakov (Moscow) W. Jeitschko (Miinster) R. J. P. Williams (Oxford) 0. Kahn (Orsay) R. Xu (Changchun) Journal of Materials Chemistry (ISSN 0959-9428) is published six times a year by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK. All orders accompanied with payment should be sent directly to The Royal Society of Chemistry, Turpin Transactions Ltd., Blackhorse Road, Letchworth, Herts SG6 1 HN, UK. NB Turpin Transactions Ltd., distributors, is wholly owned by The Royal Society of Chemistry. 1991 Annual subscription rate EC (inc. UK) f 175.00, USA $395.00, Rest of World f195.00.Customers should make payments by cheque in stirling 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 11 003. USA Postmaster: send address changes to Journal of Materials Chemistry, Publications Expediting Inc., 200 Meacham Avenue, Elmont, NY 11 003. 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. @ The Royal Society of Chemistry, 1991. 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.Professor A. R. West, Scientific Editor Mrs. J. M. Leader, Staff Editor Tel.: Aberdeen (0224) 27291 8 Tel.: Cambridge (0223) 420066 Fax: (0224) 272938 E-Mail (JANET): Telex: 73458 UNIABN G RSCl @UK.AC.RL.GB Fax: (0223) 420247 or 423623 Telex: 818293 ROYAL G INFORMATION FOR AUTHORS The Royal Society of Chemistry welcomes submission of manuscripts intended for pub- lication 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.Art icles Full papers contain original scientific work that has not been published previously. How- ever, work that has appeared in print in a short form such as a Materials Chemistry Com- munication is normally acceptable. Four copies of Articles including 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 CB44WF. UK. Materials Chemistry Communications Materials Chemistry Communications contain novel scientific work in short form and of such importance that rapid publication is war-ranted. The total length is rigorously restric- ted to two pages of the double-column A4 format.The manuscript will be returned for reduction if this length is exceeded. For a Communication consisting entirely of text and ten references, with no figures, equations or tables, this corresponds to approximately 1600 words plus an abstract of up to 40 words. Submission of a Materials Chemistry Com- munication 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 viaa member of the Interna- tional Advisory Editorial Board. In the latter case, the top copy of the manuscript includ- ing any figures etc., together with the name of the person to whom the Communication is being submitted, should be sent simultan- eously to the Editor at the Cambridge address. Authors may wish to contact the Board mem- ber 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 Communi- cations, conditions for acceptance etc. are given in issue number one of Journal of Materials Chemistry published in January of each year, or may be obtained from the Staff Editor. There is no page charge for papers published in Journal of Materials Chemistry Fifty reprints are supplied free of charge. Any author who is publishing in Journal of Materials Chemistry for the first time is entitled to a free copy of the issue in which the paper appears.

ISSN:0959-9428    

DOI:10.1039/JM99101FX009

出版商:RSC    

年代:1991

数据来源: RSC

ISSN:0959-9428    

DOI:10.1039/JM99101BX011

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ISSN 0959-9428 JMACEP(3) 307-488 (1991) Journal of Materials Chemistry Synthesis, structures, properties and applications of materials, particularly those associated with advanced technology CONTENTS 307 FEATURE ARTICLE. Chirality in liquid crystals J. W. Goodby 3 19 Porous cross-linked materials formed by oligomeric aluminium hydroxides and a-tin phosphate P. Maireles-Torres, P. Olivera-Pastor, E. Rodriguez-Castellon, A. Jimenez-Lopez, L. Alagna and A. A. G. Tomlinson 327 6Li Magic angle spinning nuclear magnetic resonance spectroscopy: A powerful probe for the study of lithium-containing materials S. P. Bond, A. Gelder, J. Homer, W. R. McWhinnie and M. C. Perry 331 Dielectric relaxation spectroscopy and molecular dynamics of a liquid-crystalline polyacrylate containing spiropyran groups E.J. C. Kellar, G. Williams, V. Krongauz and S. Yitzchaik --X.339 Room-temperature electrochemical reduction of YBa,Cu,O, Solid-state and solution chemical results M. Schwartz, Y. Scolnik, M. Rappaport, G. Hodes and D. Cahen 347 Vitrification in low-molecular-weight mesogenic compounds W. Wedler, D. Demus, H. Zaschke, K. Mohr, W. Schafer and W. Weissflog 357 Characterization of epitaxially grown ZnS:Mn films on a GaAs( 100)substrate prepared by the hot-wall epitaxy technique T. Nakamura, H. Muramatsu, Y. Takeuchi, H. Fujiyasu and Y. Nakanishi 361 Evolution of structural changes during flash calcination of kaolinite. A 29Si and 27Alnuclear magnetic resonance spectroscopy study R. C.T. Slade and T. W. Davies 365 Preparation of ternary composite hydrogels of agarose, concanavalin A and a glycolipid monolayer, and their permeation properties N.Higashi, M.Takematsu and M. Niwa 37 1 High-surface-area resins derived from 2,3-epoxypropyl methacrylate cross-linked with trimethylolpropane trimethacrylate P. D. Verweij and D. C. Sherrington 375 High-resolution solid-state 31Pand "'Sn magic-angle spinning nuclear magnetic resonance studies of amorphous and microcrystalline layered metal(1v) hydrogenphosphates M. J. Hudson and A. D. Workman 38 1 Electrochromic Nb205 and Nb,O,/silicone composite thin films prepared by sol-gel processing G. R. Lee and J. A. Crayston 387 Nature of dangling-bond sites in native plasma-polymerized films of unsaturated hydrocarbons, and electron paramagnetic resonance kinetics on heat treatment of the films M.Kuzuya, M. Ishikawa, A. Noguchi, H. Ito, K. Kamiya and T. Kawaguchi 393 Dispersion of silicon carbide whiskers and powders in aqueous and non-aqueous media T. P. O'Sullivan and S. E. Taylor 401 Radiation chemical yields and lithographic performance of electron-beam resists based on poly(methy1styrene-co-chlorostyrene) R. G. Jones, P. C. Miller Tate and D. R. Brambley 409 Electron microscopic study of the morphology of lead sulphide and silver sulphide crystals obtained by the silica gel crystal growth technique P. Aragbn-Santamaria, M. J. Santos-Delgado, A. Maceira-Vidan and L. M. Polo-Diez 415 Computer-simulation study of alkali-metal insertion into a-U308 R. G. J. Ball and P.G. Dickens 423 Progress towards preparation of high-surface-area rare-earth oxides L.A. Bruce, S. Hardin, M. Hoang and T. W. Turney 429 Reorientation of Mo-co-ordinated water molecules in high-hydrogen-content oxide bronzes HI,,Moo3 and H2,,Mo03.A neutron scattering study R. C. T. Slade, P. R. Hirst and H. A. Pressman 437 Luminescence properties of A2ReC1, crystals M. Bettinelli, C. D. Flint and G. Ingletto 441 Generation of charge carriers and an H/D isotope effect in proton-conducting doped barium cerate ceramics R. C. T. Slade' and N. Singh 447 Matrix surface modification by plasma polymerization for enzyme immobilization M. Mutlu, S. Mutlu, M. F. Rosenberg, J. Kane, M. N.Jones and P. Vadgama 45 1 Infrared reflectance spectra of Sb-doped Sn02 ceramics C.S. Rastomjee, P. A. Cox, R. G. Egdell, J. P. Kemp and W. R. Flavell 457 Formation of ordered multilayers by stepwise oligomerisation J. Y.Jin and R. A. W. Johnstone 46 1 Thermal decomposition of cobalt(I1) acetate tetrahydrate studied with time-resolved neutron diffraction and thermogravimetric analysis R. W. Grimes and A. N.Fitch 469 Surface structure of a glassy carbon. Scanning tunnelling microscopy study N. M. D. Brown and H. X. You ~~ MATERIALS CHEMISTRY COMMUNICATIONS 473 Crystallization of sparingly soluble salts on functionalized polymers E. Dalas 475 Tetrathiafulvalene-FePS, layered intercalation compound: A new type of organic-inorganic metal L. Lomas, P. Lacroix, J. P. Audi6re and R. Clement 477 Madelung energy and hole location in YBa2Cu,08 S.K. Ramasesha and K. T. Jacob 479 Novel co-operative magnetic properties of decamethylmanganocenium 2,3-dichloro-5,6-dicyanobenzoquinoneide, 3[Mn(C,Me,),]"[DDQ].-J. S. Miller, R. S. McLean, C. Vazquez, G. T. Yee, K. S. Narayan and A. J. Epstein 481 Photoinduced reversible refractive-index changes in tailored siloxane-based polymers S. H. Barley, A. Gilbert and G. R. Mitchell 483 Simple and convenient method for preparing functionalised network organopolysilanes H. Watanabe, M. Abe, K. Sonoda, M. Uchida, Y.Ishikawa and M. Inomiya 485 Synthesis and optical spectroscopy of platinum-metal-containing di- and tri-acetylenic polymers B. F. G. Johnson, A. K. Kakkar, M. S. Khan, J. Lewis, A. E. Dray, R. H. Friend and F. Wittmann 487 Book Reviews: D. M. Adams and A. G. Christy; J. H. R. Clarke; D. R. Rosseinsky 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.

ISSN:0959-9428    

DOI:10.1039/JM99101FP031

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

Cumulative Author Index 1991 Abe M., 483 Desiraju G. R., 201 Irvine J. T. S., 147, 289 Adams D. M., 487 Dickens P. G.., 105., 137,415 Ishikawa M., 387 , Adams J. M., 43 Dodd S.M., 11 Ishikawa Y., 483 Adams P. N., 141 Doi T., 169 Ito H., 387 Alagna L., 319 Dray A. E., 485 Iwasawa N., 37 Allen Sir Geoffrey, 1 Dunmur D. A., 251, 255 Jacob K. T., 477 Allen G. C., 69, 73 Dunn B., 265 Jarman R. H., 113,297 Alonso P. J., 197 Dyer A., 43 Jiang M. R. M., 11 Annen M. J., 79 Eddy M. M., 223 Jimenez-Lopez A., 3 19 Apblett A. W., 143 Edge S., 103 Jin J. Y., 457 Aragon-Santamaria P., 409 Edwards D. J., 223 Johnson B. F. G., 485 Arhancet J. P., 79 Egdell R. G., 63,451 Johnson O., 223 Audiere J. P., 475 Epstein A. J., 479 Johnstone R. A. W., 457 Ball R.G. J., 105, 415 Esteruelas M. A., 251 Jones A. C., 139 Barbieri A., 191 Faber J., 175 Jones M.N., 447 Barley S. H., 481 Fernando Q., 2 13 Jones R. G., 401 Barrer R. M., 305 Fitch A. N., 461 Jorgensen J. D., 175 Barron A. R., 143 FitzGerald E. T., 51 Jutson J.A., 73 Battaglin G., 191 Flavell W. R., 63, 451 Kaduk J. A., 113,297 Bettinelli M., 437 Flint C. D., 437 Kaharu T., 145 Bicelli L. P., 259 Formstone C. A., 51, 205 Kakkar A. K., 485 Binks J. H., 289 Friend R. H., 485 KHll P-O., 239 Blau W. J., 245 Fujiyasu H., 357 Kamiya K., 387 Bond S. P., 327 Gelder A., 327 Kane J., 447 Boscolo Boscoletto A., 191 Giatti A., 191 Kathirgamanathan P., 103, Brambley D. R., 401 Gibb T.C., 23 141 Brock T., 151 Gier T. E., 153 Kawaguchi T., 387 Brown I.T., 69 Gilbert A., 303, 481 Kellar E. J. C., 331 Brown N. M. D., 469 Glasser F. P., 305 Kemp J. P., 451 Bruce D. W., 251,255 Golden M. S., 63 Khan M. S., 485 Bruce L.A., 423 Golden S. J., 63 Kimura K., 293 Byrne H. J., 245 Goodby J. W., 5, 307 Kordas G., 97, 175, 181 Cahen D., 339 Gourier D., 265 Korgul P., 239 Campillos E., 197 Greaves C., 17 Krongauz V., 331 Cardin D. J., 245 Green M. L. H., 205 Kurmoo M., 51 Catlow C. R. A., 233 Grey C. P., 113 Kusabayashi S., 169 Centonze D., 259 Grimes R. W., 461 Kuzuya M., 387 Chambers R. D., 59 Grins J., 239 Lacroix P., 475 Chatakondu K., 205 Grossel M. C., 223 Lee C.K., 149 Cheatham L. K., 143 Hardin S., 423 Lee G. R., 381 Cheetham A. K., 113, 223, Harrison K.J., 121 Legge C. H., 303 Harrison W. T. A., 153 Le Lagadec R., 251 Chen C-Y., 79 Hayter J. B., 181 Lewis J., 485 Cho C. G., 217 Higashi N., 365 Li H-X., 79 Christy A. G., 487 Hirao M., 293 Liddell K., 239 Chvatal Z., 59 Hirst P. R., 281, 429 Lo Jacono M., 129 Clarke J. H. R., 487 Hitterman R. L., 175 Lomas L., 475 Clement R., 475 Hoang M., 423 Lukes P., 29 Cogle T. J., 289 Hodes G., 339 Maceira-Vidan A., 409 Cook M. J., 121 Hoffman D., 87 Maffi S., 259 Cox P. A., 51,451 Homer J., 327 Maireles-Torres P., 3 19 Crayston J. A., 381 Horner P. J., 271 Maitlis P. M., 251, 255 Crennell S. J., 113, 297 Howe S.D., 29 Male S. E., 69 Currie D. B., 295 Howlin B., 29 Malitesta C., 259 Dalas E., 473 Hudson M. J., 375 Mandal K. C., 301 Daniel M.F., 121 Hunt S. E., 251 Manterfield M. M., 255 Daolio S., 191 Hursthouse M. B., 139 Marcos M., 197 Davey A. P., 245 Imaeda K., 37 Marsden J. R., 251 Davies T. W., 361 Ingletto G., 437 Maruyama Y., 37 Davis M. E., 79 Inokuchi H., 37 Mateus C. A. S., 289 De Battisti A., 191 Inomiya M., 483 MatijeviC E., 87 Demus D., 347 Inoue M., 213 Matsubara H., 145 Dent A. J., 103 Inoue M. B., 213 McKeown N. B., 121 McLean R. S., 479 McWhinnie W. R., 327 Milburn G. H. W., 155 Miller J. S., 479 Miller Tate P. C., 401 Mitchell G. R., 303, 481 Mohr K., 347 Moon B.M., 97 Moore G. A., 175, 181 Moretti G., 129 Mori T., 37 Morris M., 43 Motevalli M., 139 Muramatsu H., 357 Musicanti M., 129 Mutlu M., 447 Mutlu S., 447 Nakamura T., 357 Nakanishi Y., 357 Nakano C., 37 Nalini V., 201 Narayan K.S., 479 Nardella A., 129 Nebesny K. W., 213 Nishihata Y., 169 Niwa M., 365 Noguchi A., 387 O'Brien P., 139 Ocaiia M., 87 OConnor P., 103 OHare D., 51, 205 Olivera-Pastor P., 3 19 Olsson P-O., 239 Orr R., 255 OSullivan T. P., 393 Owen J. J., 113 Percec V., 217 Perry M. C., 327 Polo-Diez L. M., 409 Porta P., 129 Postle S. R., 223 Powell A. V., 137 Pressman H. A,, 429 Pugh C., 217 Quill K.; 141 Ramasesha S. K., 477 Rao C. N. R., 299 Rappaport M., 339 Rastomjee C. S., 451 Rayment T., 299 Richardson R. M., 121 Rodriguez-Castellon E., 3 19 Rosenberg M. F., 447 Roser S.J., 121 Rosseinsky D. R., 487 Rotella F., 175 Sabbatini L., 259 Saito G., 37 Sakurai Y., 169 Salamon M. B., 181 Salmon L., 265 Sankar G., 299 Santos-Delgado M. J., 409 Savadogo O., 301 Schafer W., 347 Schwartz M., 339 Scolnik Y., 339 Serrano J. L., 197 Sherrington D. C., 151, 371 Silver J., 29 Simmons J. M., 121 Sinclair D. C., 147 Singh N., 441 Slade R. C. T., 281, 361, 429, 441 Slaney A. J., 5 Slater P. R., 17 Smith R. I., 91 Sola E., 251 Sonoda K., 483 Stacey J. M., 251 Stucky G. D., 153 Takahashi S., 145 Takematsu M., 365 Takenaka S., 169 Takeuchi Y., 357 Tamatani A,, 169 Taylor S. E., 393 Templeton-Knight R., 59 Terauchi H., 169 Thompson D. P., 239 Thompson W.C., 305 Thomson A. J., 121 Tilley R. J. D., 155 Tomlinson A. A. G., 319 Turney T. W., 423 Twyman J. M., 205 Uchida M., 483 Underhill A. E., 103, 141 Vadgama P., 447 Vazquez C., 479 Verweij P. D., 371 Vivien D., 265 Volin K. J., 175 Walsh J. R., 139 Watanabe H., 483 Wedler W., 347 Weissflog W., 347 Weller M. T., 11, 295 West A. R., 91, 147, 149, 157, 163 West B. C., 281 Whitcombe M. J., 303 Whiteley R. H., 271 Williams G., 331 Wiseman P. J., 205 Wittmann F., 485 Workman A. D., 375 Yee G. T., 479 Yitzchaik S., 331 Yokoyama M., 293 You H.X., 469 Zambonin P. G., 259 Zaschke H., 347 Zhang X., 233 Zheng C., 163 1 Conference Diary 1991 May 1-5 Society for Biomaterials: 17th Annual Meeting in Conjunction with the 23rd International Biomaterials Symposium Scottsdale, USA Society for Biomaterials, 1991 Annual Meeting, Post Office Box 717, Algonquin, Illinois 60102-0717, USA May 5-9 Rolduc Polymer Meeting 6 Rolduc, The Netherlands P.J.Lemstra, Eindhoven University of Technology, PO Box 513,5600 MB Eindhoven, The Netherlands May 5-10 Sensors: 179th Meeting of the Electrochemical Society. Sensors Based on Organic Electroactive Materials Washington DC,USA DrP. Kathir, Cookson Technology Centre, Sandy Lane, Yamton, Oxford OX5 lPF, UK May 5-11 ECCG-3: 3rd European Conference on Crystal Growth Budapest, Hungary A. Lorinczy, Conference Secretary, Res. Inst. for Technical Physics, Budapest, Ujpest 1.Pf. 76, Hungary-1325 May 14-16 Sensor 91 Nuremberg, Germany ACS Organisations GmbH, Von-Miinchhausen-Strasse29, D-3050 Wunstorf 2, Germany May 19-29 International Workshop on Modern Magnetic Materials and their Technological Impact La Habana, Cuba Professor C. Rodriguez Castellanos, Dean Physics Faculty, La Habana University, Vedado-Collina Universitaria, La Habana, Cuba May 20-24 New and Alternative Materials for the Automotive Industries Florence,Italy ISATA Secretariat, 42 Lloyd Park Avenue, Croydon CRO 5SB. UK May 21 -25 Tenth International Conference on Solid Compounds of 'Itansition Elements Miinster, Germany SCTE-10, Anorganisch-Chemisches Institut der Universibt, Wilhelm-Klemm-Str. 8, D-4400Miinster, Germany Tel.: (+ 49 251) 83 3121; FAX: (+ 49 251) 83 3169 May 22-23 Second International Symposium on Metal-Containing Liquid Crystals St.Pierre de Chartreuse, France DrA-M. Giroud-Godquin, DRFLCH, Centre d'Etudes Nucleaires de Grenoble, 85X,38041 Grenoble Cedex, France May 22-24 Thirteenth International Conference on Advances in Stabilization and Controlled Degradation of Polymers Lucerne, Switzerland In Europe: DrN. C. Billingham, MOB, University of Sussex, Brighton BNl 9QJ. UK In the USA: Professor A. V. Patsis, Materials Laboratory, CSB 209, State University of New York, New Paltz, New York 12561, USA May 26-30 MatTech '91-The Second European East-West Symposium on Materials and Processes Helsinki, Finland Professor Kaj Lilius, PL 121, SF-02101 Espoo, Finland.Tel.: + 358-0-451 2769. FAX: + 358-0-4512799; + 358-0-4552250; + 358-0-4512 660 May 26-June 2 National Conference -Work Shop: "Progress and Problems in Liquid Crystals" Leningrad, USSR S. I. Vavilov State optlcal Institute, 199034 Universitetskaja nab., 5, Leningrad, USSR FAX: 2184172 May 27-3 1 International Conference on Advanced Materials ICAM -91 (E-MRSSpring Meeting) Strasbourg, France E.h4RS/P. Siffert, C. R. N., B. P. 20, F-67037 Strasbourg-Cedex, France May 27-3 1 6th International Symposium on Intercalation Compounds Orleans, France Secretariat ISIC 6, CRSOCI-CNRS,45071 Orleans Cedex 02,France May 27-3 1 The Second International Conference on Rare Earth Development and Applications (ICRE'91) Beijing, China Senior Engineer Liu Aisheng, Senior Engineer Jin Jinghong, The Chinese Society of Rare Earths, 76 Xueyuan Nan Lu,Beijing 1ooO81, China.Tel.: 8312541 or 891666. FAX: 8312 14. May 28-30 12th International Conference and Exhibition of the Society for the Advancement of Materials and ProcessEngineering (SAMPE) Maastricht, Netherlands Maastricht Exhibition and Congress Centre, Congfess Organizing Dept./SAMPE '91, PO Box 1630, NL-6201 BP Maastricht, The Netherlands. Tel.: +31-(0) 43-838383. FAX: +31-(0) 43-838300. June 10-14 8th Bratislava International Conference on Modified Polymers High Tatras, Czechoslovakia DrD. Lath, Polymer Institute, Slovak Academy of Sciences, 842-36 Bratislava, Dubravska cesta, Czechoslovakia June 17-21 Cordon Conference on Liquid Crystals Brewster Academy, Wolfeboro, New Hampshire, USA Professor N.A. Clark, Department of Physics, Condensed Matter Laboratory, Campus Box 390,Boulder, CO 90309-0390,USA 11 June 17-21 3rd International Symposium on Polymer Electrolytes Annecy, France A. Gandini, EFPG BP 65,38402 St Martin d’Heres, France June 19-21 1991 EIectronic Materials Conference Boulder, Colorado, USA Barbara J. Kampeman, Meetings Manager, The Mineral Metals and Materials Society, 420 Commonwealth Drive, Warrendale, PA 15086, USA. Tel.: (412) 776-9050. FAX: (412) 776-3770 June 19-21 Conference on Fracture Processesin Brittle Disordered Materials Noordwijk, The Netherlands Congress Office ASD, PO Box 54,2640 AB Pijnacker, The Netherlands. Tel.: 31-1736 95356. FAX: 31-1736 92242 June 24-26 Fifth International Symposium on Catalyst Deactivation Northwestern University, Illinois, USA DrK.K. Robinson, Amm Research Center,Research and Development Department, PO Box 301 1, Naperville, IL60566,USA. Tel.:(708)420-4964. FAX: (708)-420-5303 June 24-28 Summer School on Organic Materials for Phdonics ~=ggen,Professor G. Zerbi, Dipartimento chimicaIndusuiale, Politecnico, Piazza L.Da Vinci 32,20133 Milano, Italy Tel.: (+39) 2 2399 3235/3230. FAX: (+39) 2 236 2589. E-Mail: Darstellung@imicl64. Bitnet: RCHIINlO IMIPOLI. June 24-28 Third International Conference on Ferroelectric Liquid Crystals Boulder, Colorado, USA Professor N. A. Clark, FLC91, Mice of Conference Services, Campus Box 454, University of Colorado, Boulder, Colorado, CO 80309,USA June 24-28 Transducers ’91 (6th International Conference on Sensors and Actuators) San Francisco, California, USA Mrs Linda Reid, University Extension, University of California, Berkeley, CA 94720, USA.Tel: (415) 642-4151 June 25 Macro Group UK: Transition Metal Mediated Pdymerlsations hdon, UK Professor W. J. Feast, IRC in Polymer Science and Technology, University of Duham,South Road, DURHAM DH13LE, UK. July 1-4 ICIM 91 (Second International Conference on Inorganic Membranes) Montpellier, France Professor L Cot, c/o Icu12-91, ENSCM, 8 rue de 1’Ecole Normale, 34053 Montpellier Cedex 1, France Tel.: 33 67540085. FAX: 33 67635970 July 2-5 International Symposium: Supported Reagent Chemistry Yo&, UK DrJohn F. Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN, UK July 3-5 Understanding Self-Assembly and Organisation in Liquid Crystals (Joint British Liquid Crystal Society and Statistical Mechanics and Thermodynamics Group of the RSC) Leeds, UK DrJ.R Henderson, School of Chemistry, University of Leeds, Leeds LS29JT July 7-12 10th International Conference on the Chemistry dthe Organic Mid State University of British Columbia, Vancouver, Canada Conference Secretariat, ICCOSS X,c/o Venue West Ltd,MS-375 Water Street, Vancouver, B.C. Canada V6B 5C6. Tel.: (604)681-5226. FAX: (604) 681-2503 July 7- 12 7th International Conference on Surface and Colloid Science Compiigne, France Secretariat of the 7th ICSCS, c/o Wagons-Liu Tourisme, BP 244, F-92307 Levallois-Pem Cedex, France July 9-10 Polymers in Extreme Environments Nottingham, UK Conference Department (7129, The Plastics and Rubber Institute, 11Hobart Place, London SW 1 W OHL,UK July 9- 13 The VIth International Conference on the Chemistry dSelenium and Tellurium Osaka, Japan Professor Nobo~ Sonoda,Osaka University, Dept of Applied Chemistry, Faculty of Engineering, Suita, Osaka 565, Japan.Tel.: (81) 6-877-5111 Ext. 4276. FAX: (81) 6-876-4754 July 15-18 Rheology of Polymer Melts Rague, Cnxhoslovakia PMM Secretariat, c/oInstitute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 16206 Prague, Czechoslovakia July 15-17 Advanced Inorganic Materials Ambleside, UK Dr Julia Wates, Akm Chemicals Ltd, HollingworthRoad, Littleborough, Lancs.FAX: 070673628. July 17-19 DCEM 11, Deposition and Characterisation of Electronic Materials (ASSCG/RSC Dalton) Manchester, UK Dr M. E. Pemble, Department of Chemistry, UMIST, PO Box 88, Sackville Street, Manchester M60 lQD, UK or Mn E. S. Wellingham, Field End House, Bude Close, Nailsea, Bristol BS19 2FQ, UK July 21 -26 Polymer Surfaces and Interfaces I1 Dumam, UK Professor W. J. Feast, Department of Chemistry, Univenity of Durham, South Road, Durham DH 1 3LE, UK ... 111 July 22-24 August 17-22 August 19-22 August 20-24 August 25-30 August 26-30 August 26-30 September 1-6 September 2-4 September 4-6 September 4-6 September8-13 September 9-1 1 September 9- 14 September 18-20 September 22-25 September 23-27 September 30-October 2 EUROMAT 91: The 2nd European Conference on Advanced Materials and Processes Cambridge, UK Emat 91, Conference Department, The Institute of Metals, 1 Carlton House Terrace, London SWlY 5DB, UK 33rd IUPAC Congress Budapes4 Hungary E.Pugor, c/o Hungarian Academy of Sciences, H-111 1 Budapest, Gellert ter 4, Hungary International Topical Conference on Optical Probes ofConjugated Polymers Snowbird, Utah, USA Department of canferences and Institutes, Division ofccdnuing Education, 2174 Annex, University of Utah, Salt Lake City, UT 84112, USA. Tel.: (801) 581-5809. FAX: (801) 581-3165 International Conference on Polytypes, Modulated Structures and QuasicrystaIs(M0SpQQ '91) Balatonszqlak, Hungary Secretary of MOSPOQ '91, Roland WOSPhysical Society, PO Box 433, H-1371 Budapest, Hungary ACS Autumn Meeting New Yo&, USA ACS, International Activities office,155 16th Street NW,Washington DC 20036, USA F'ifth International Conference on Langmuir-Blodgett Films Paris, France Annie Ruaudel-Teixier, Chairman, Service de Chimie Molc4culaire, Bit.125, C.E.N. Saclay, 91 191, Gif-sur-Yvette Cedex, France. Tel: 69-08-54-55. FAX: 69-08-79-63 Summer European Liquid Crystal Conference Vilnius University, Lithuania Dr P. Adomenas, 1991SELCC,Department of Chemistry, Vilnius University, Naugarduko 24,232006 Vilnius, Lithuania. Tel.: Vilnius 661553. Telex: 261212 VUSU. FAX: (007-012-2) 613473 XVI International Symposium on Maaocyclic Chemistry (ISMC 1991) Sheffield, UK Dr Nom A.Stoddart,Department of Chemistry, The University, Sheffield S37HF, UK. Tel.: (0742) 768555 Ex. 4522. FAX: (0742) 739826 Crystalline Structures and Defects in Ceramics Ljubljana, Yugoslavia M. Dmfenik, Institute Jozef Stefan, Yu-61001 Ljubljana, PO Box 100, Yugoslavia IWASES-I1 (International Workshop on Auger Spectroscopy and Electronic Structure) Lund, Sweden Dr C-O. Almbladh, Department of Theoretical Physics, Siilvegatan 14 A, S-22362 Lund, Sweden E-mail: COA@SELDC!52.BITNET. FAX: 46-(0)46-104710 PolymerStabilization: Mechanisms and Applications Birmingham, UK ProfessorN. S.Allen, Depmrtment of Chemistry, Manchester Polytechnic, John Dalton Building, Chester Street, Manchester M15GD International Symposium: zedite Chemistry and Catalysis Prague, Czchoslovakia Dr B.Wichterlod (Zeolite Chemistry and Catalysis), The J. Heymvsky Institute of Physical Chemistry and Electrochemistry, Dolejskova 3,182 23 Prague 8, Cz;echoslovakia. Tel.:(422) 858 4120, (422) 815 35%. Telex: 122018 ATOM C. FAX: (422) 858 4569. 6th Biennial Polymer Conference Leeds, UK Meetings Wice,'he Institute of Physics, 47 Belgrave Square, Won SWlX 8QX, UK Physics of PolymerNetworks Ballenstedt, Germany Organizing commi#ee,Physics of Polymer Networks, Technical University of Merseburg, Dept of Physics, Merseburg, DDR-420, Germany Recyding of Polymers Marbella, Spain W. Heitz, Philipps-Universit Marbuqj FB14 Physikalische Chemie, Polymere, Hans-Meenveinstrasse, D-3550 Marburg, Germany 5th Sensors and their Applications Conference Edinburgh, UK Meetings Office, The Institute of Physics, 47 Belgrave Square, London SWlX SQX, UK 7th International Symposium on Electrets Potsdam, Germany Professor Detlev Geiss, Academy of Sciences, Instituteof Polymer Chemistry 'En& Correns', Kantsmsse 55, 1530 TeltowSeehof, Germany Eurosensors V Rome, Italy ProfessorA. D'Amico, Universita' di Rama "roR VERGATA, Dipartimentodi Ingegneria El&ca, Via Orazio Raimondo, 140173 Roma, Italy iv September 30- Speciality Polymers '91 October 2 Mainz, Germany Mereace Organkr, SP'91, Butterworth Scientific Ltd,PO Box 63, Westbury House.Bury Street, Guildford, Surrey GU25BH,UK September30-Macromdecule-Metal Complexes IV October 5 Siena, Italy ProfessorRoland0 Barbud, Dputunento di Chimica, Universita di Siena, Piano dei Mantellini 44,53 100 Siena, Italy October6-11 Sixth International Workshop on Glasses and Ceramics from Gels seville, SpainDr Luis EsquiviasFedriani(WorkshopChairman), Depanamento de Esmaura y Pqnedades de los Materiales, Aptado, 40-11510 hemReal (Wiz),Spain October 13-17 Unmvemtional Photoactive aids (UPS) Symposium on Molecular Systems Okazaki, JapanTadaoki Mitani, Institutefor MolecularScience, Myodaiji, Okazaki 444,Japan October 14-18 ECASIA 91(4thEuropean Conference on Applications dSurface and Interface Analysis) Budapest, HwwY ECASIA 91, MTA ATOMMI, Pf.51, H-4001 Debrecen, Hungary. Tel.:(36)-52-16181.Telex: 72210 (atom h). FAX: (36)-52-16181 October 14-18 Olefin and Vinyl Polymerization and Functionalhation Hangzhou,chinaProfessorKun-Yuan Qiu, Secretary-General of SymposiUm 91 Hangzhu, Chemistry Department, Peking University, Beijing 100871, China October20-26 Mid State Ionics 8th International Conference Lake Louise, Canada Dr P. S.Nicholson, McMaster University, JHE-249, Hamilton, Ontario, Canada L8S 4L7. Tel.: (416) 529-7575. FAX: (416) 529-5994 October 23 -25 Polymersin a Marine Environment HI London,UK Rhian Bufton, conference Organiser, The Institute of Marine Engineen, The Manorial Building, 76 Mark Lane, London EQR 7JN, UK October 28- Eumphysics Conference on Macromolecular Physics: PdymerNetworks November 2 Wemigerode, Germany S.Wartewig, 'Carl Schorlemmer' Technical University Le~ma-Merseburg, Department of Physics, DDR-420 Merseburg, Germany November 4-8 15th International Symposium on the Sdentific Bash for Nudear Waste Management Strashxlrg, France I? Siffert,EMRS 1991 Fall Meeting, C.T. T.,B. I? 20,67037 Strasbourg, Cedex, France November 1 1 -15 5th International Kyoto Conference on New A@ of Organic Chemistry (IKCOC-5) (Session 2: Organic Synthesis for Materials Sdence) Kyoto, Japan Professor YoshikiOhshim, Chairman ofIKCOC-5,Dept of Applied Chemistry, Osaka University, Yamadaoka 2-1, Suita, Osaka 565, Japan November 30- International Symposium on New Polymers December 1 Kyoto, Japan ProfessorToshinobu Higashiura, Department of PolymerChemistry, Kyoto University, Yoshida, Sakyoku, Kyoto 606,Japan December 1-6 Electrical, Optical, and Magnetic Ropertfesof Organic Wid State Materials(MRS Symposium) Boston, USA D.J. !Sandman, GTE Laboratories Incorporated,40Sylvan Road, Waltham,MA02254, USA. TeL: (617) 466-4216 December 2-6 Cellulose'91 New Orleans, USA Dr Noelie R. Bemmiere, USDA ARS Southern Regional Research Center, 1100 Robert E. Lee Boulevard,PoBox 1%87, New Orleans, LA 70179-0687, USA December 2-7 Materials Research Society Fall Meeting Prague, Czechoslovakia J. Heymvsky, Instituteof PhysicalChemistry and Electrochemistry,Dolejskova 3,182 23 Prague 8, Czechoslovakia December 16-17 Protein Engineering: Structure Prediction and Ugand Interactions London, UK Dr Necra Brokakoti, c/o Roche Prcducts hi,Broadwater Road, Welwyn Garden Cky, Hem AL7 3AY, UK Conference Diary 1992 January 6-8 Fluompolymers92 Manckstcr, UK DrE.smithar DrS.hnm,Departmentof Chemistry, UMIST, h4anckster M60 1QD.UK February 3-5 March 30-31 April 5-10 May 17-21 July 13-18 July 14-16 August 23-28 August 24-28 August 3 1 -&ptember 4 19Ch Australian Polymer Symposium Fremantle, Australia Dr Graeme George, Secretary RACI Polymer Division,Ckanistry Department, University of Queensland, St. Lucia QLD 4067, Australia Kautschukelnstische Pdymersysteme Bad Nauheirn, Germany GDCh-Geschiiftsstelle, Abteilung Tagungen, Postfach 90 0440,DdOOO Frankfurt am Main, Germany ACS Spring Meeting San Francisco,USA ACS, International Activities Office, 155 16thStreet NW,Washington DC20036, USA 7th International Conference on Relation Between Homogeneous and Heterogeneous Catalysis Tokyo,Japan Professor Yasuhiro Iwasawa, Chairman of W SHHC, Department of Chemistry, The University of Tokyo, Hongo, Tokyo 113, Japan 34th IUPAC International Symposium on Macromolecules Prague, Czechoslovakia IUPAC Macro '92Secretariat, Instituteof Macromolecular Chemistry, Czechoslovak Academy of Sciences, Heyrovkeho nm.1888D.162 06 Prague 6, Czechoslovakia Speciality Polymers '92 London, UK Conference Organizer, Spiality Polymers '92, Burnorth Scientific Limited, Po Box 63, Westbuxy House, Bury Street, Guildford, Surrey GU2 5BH, UK ACS Autumn Meeting Washington, USA ACS,InternationalActivities Office, 155 16th Street NW,Washington DC20036,USA European Conferenceon Molecular Electronics Padua, Italy ProfessorR Bozio, ECME 92, Department of Physical Chemistry, 2 Via Lotedan, 1-35131 Padua, Italy Macromolecules '92 Cantehury, UK Dr A Am&, Department of Chemical Engineering and Applied Chemistry, University of Aston, Aston Triangle, Birmingham B4 7ET, UK vi

ISSN:0959-9428    

DOI:10.1039/JM99101BP033

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(3), 307-318 FEATURE ARTICLE Chirality in Liquid Crystals John W. Goodby The School of Chemistry, The University, Hull HU6 7RX, UK Chirality has become arguably the most important and complex topic of research in liquid crystals today. The reduced symmetry in these organized phases leads to a variety of novel phase structures, properties, and applications. Molecular asymmetry imparts form chirality to liquid-crystalline phases, which is manifested in the formation of helical ordering of the constituent molecules of the phase. Similarly, molecular asymmetry imposes a reduction in the space symmetry, which leads to some phases having unusual non-linear properties, such as ferroelectricity and pyroelectricity. In the following article the various effects of chirality in liquid- crystalline systems are described in general terms.Keywords: Liquid crys ta I; Chira lity ; Helicity; Fe rro electricity; Feature article 1. Introduction Over 100 years ago, in 1888, the first thermotropic liquid crystal mesophases were discovered in two derivatives of cholesterol, cholesteryl benzoate and cholesteryl acetate.' RCO, w 1 These compounds have asymmetric molecular structures, and are therefore optically active and chiral. This discovery made by Lehmann and Reinitzer had a twofold effect in that it initiated research in the new field of liquid crystals2 and heralded an era of fascination with optical activity and chirality in organized fluids. When heated these particular derivatives of cholesterol melt in stages from organized crystalline solids to disordered amorphous liquids via intermediary orientationally ordered fluid liquid-crystalline phases.The intermediary phase was found to be identical for both compounds of 1; because it was discovered in derivatives of cholesterol it was given the name cholesteric mesophase. Today, however, it is sometimes referred to as the chiral nemutic phase. This extraordinary mesophase was found to have unusual optical properties in that, under certain circumstances, it can selectively reflect light.3 This physical property is a consequence of Bragg-like reflections from the helical structure of the mesophase. On average, the rod-like molecules in this liquid-like phase are locally packed parallel to one another so that they are orientationally ordered; however, the molecules have no pos- itional or translational order.If we consider an object mol- ecule; on moving away from the side of this rod-like molecule there is a twist or rotation of the orientational ordering of the molecules, giving rise to a helical macrostructure, as shown in Fig. 1. This helical formation is commonly known as a single-twist structure. This simple example of the first liquid crystal to be dis- covered demonstrates two important aspects of symmetry in complex fluids. First, the molecular structures of the constitu- ent molecules of the phase have no mirror symmetry, and Fig. 1 Schematic representation of the structure of the cholesteric phase. The molecules are shown as elliptical rods which stack in a single-twist structure therefore have the property of being handed.Thus, the mol- ecules are optically active and chiral. Secondly, the organiz- ation of these molecules in the fluid, cholesteric phase leads to a helical liquid crystal being formed that is also optically active and chiral. In essence, molecular chirality imparts form chirajity to the mesophase. These two particular features of symmetry in liquid crystals are uniquely and inextricably linked. Molecular chirality, for example, can be generated systematically through the design and synthesis of materials, and in turn, the molecular chirality created can be used in the control of the chirality of the liquid-crystalline mesophase.For example, the pitch and handedness of the helical structure of the cholesteric phase can be determined and finely controlled by the molecular geometry (or geometries) of the constituent molecules in the phase. These rules of thumb developed for material design, construction, and mixing of cholestrogens have been used very successfully in recent times to generate encapsulated cholesteric mixtures for the ubiquitous low-cost liquid-crystal- line thermometers and related thermochromic paints and coatings used in surface therm~graphy.~.~ 308 After 100 years of research in liquid crystals, the study of chiral systems still proves to be one of the most fascinating, fertile and exciting areas for investigations and applications of both low-molar-mass and polymeric materials.The study of chiral systems has, in fact, been further boosted recently by the invention of new methods of synthesizing novel chiral organic compounds via asymmetric induction of prochiral substrates. In the following sections the various forms of symmetry in liquid crystals will be discussed, and some of the more recent and unusual discoveries concerning symmetry and phase structure in organized complex fluids will be described. 2. Molecular Structure and Liquid-crystalline Systems Organic, metal-containing organic, and polymeric materials that have dichotomous molecular structures are often found to exhibit liquid-crystalline phases. In this respect, a dichot- omous structure is used to describe a material that has a molecular structure composed of at least two different por- tions.The two portions may have different structural proper- ties (for example, one part might be flexible and the other portion might be rigid) or different chemical properties, as in hydrophobic-hydrophilic, aromatic-aliphatic, or hydro-carbon-fluorocarbon mixed systems. It is believed that liquid- crystalline phases are formed through a ‘molecular phase- separation process’ in materials of this type. Like-parts of the molecules pack together, thereby causing a kind of phase separation, but on a molecular scale. Some molecules that exhibit liquid-crystalline phases are shown in Fig. 2. The macroscopic and microscopic ordering of the molecules in these systems can be predicted to some extent from the steric shape and dipolar character of the molecular structure of the material.These guidelines allow the design and engineering nematic liquid crystal discotic liquid crystal H O a o , smectic liquid crystal Ho HO C8H17 F\ /F smectic and nematic liquid crystal 4-side-chain polymer CH,-Si-(CH,),O e c o 2 e c N liquid crystal I0 Fig. 2 Mesogenic compounds J. MATER. CHEM., 1991, VOL. I of new molecules that may be expected to self-assemble in predicted ways in order to produce materials that have desirable properties. In the following sections the liquid-crystalline properties of molecules that have elongated molecular structures will be discussed.The shapes of the molecules are referred to as lath- or rod-like, and the phases that they exhibit are sometimes called calamitic liquid-crystalline mesophases. 3. Phase Structures in Calamitic Liquid Crystals First consider what happens when a simple rod-like, non- chiral (achiral) liquid-crystalline compound melts to the iso- tropic liquid.’ The compound will not simply melt with the structure of the solid breaking down to give a disordered liquid; the material will in fact melt in stages. The first possibility for the breakdown in the structure of the organized solid is for the molecules to start to rotate about their long axes. This produces a transition from the crystal to a soft- crystal phase. Sometimes these phases are referred to as anisotropic plastic crystals.There are a number of these quasi- crystalline phases, which are classified by the packing arrange- ments and layer ordering of the molecules. Within the layers, the molecules have long-range positional order; similarly the out-of-plane ordering is also long range. The long axes of the molecules can be either orthogonal or tilted with respect to the layer planes. These quasi-smectic phases are coded by the letters E, J, G,H, K and B (cryst).6 Further heating of the material can induce another phase transition where the periodic ordering of the molecules breaks down. This change in order can produce a number of novel phases where the molecules are still organized in layers but there is only short-range positional order of the molecules in the plane of the layers.There is no out-of-plane ordering of the molecules; however, the molecules still maintain the same orientational order from layer to layer. Even under these constraints the molecules in these phases are able to rotate rapidly about their long axes, pass from layer to layer, and tumble, albeit in a restricted way, about their long axes. Phases with this form of gross structure are called smectic (lamellar) phases. Again this state is subdivided into a number of groups depending on the local packing of the molecules. The molecules may be tilted with respect to the layer planes, as in smectic C,and may also have local hexagonal packing, as in smectics I and F. Alternatively, the molecules can have their long axes orthogonal to the layer planes, as in the smectic A phase, or local hexagonal packing, as in smectic B.7 The structures of some of these phases are depicted schematically in Fig. 3.Further heating of the material can induce a subsequent phase transition to the nematic phase, where the molecules do not have any positional or layer ordering. The only structural feature that remains, which distinguishes the phase from the isotropic liquid, is the orientational order of the molecules. The long axes of the molecules tend to lie approxi- mately parallel to one another, even though the molecules are free to tumble and slide past one another. A liquid-crystalline compound does not necessarily have to melt through all of these stages to the isotropic liquid; for example, some materials may melt directly to the nematic phase whereas others might melt through the smectic phases to the nematic and then to the amorphous liquid. When chiral molecules are introduced into these liquid-crystalline phases a variety of new phases are formed that have very unusual properties.These compounds and the phases that they form are discussed in the following sections. J. MATER. CHEM., 1991, VOL. 1 glass slide )-f=.-....-.g direction Disordered layer structure of the rmectic A phase Hexagonally ordered structure of the smectic B phase Disordered layer structure of the smectic C phase, the molecules are tilted Fig. 3 Structures of smectic phases 4.Classification of Liquid-crystalline Phases Liquid-crystalline phases are commonly classified by thermal microscopic investigations and by compatibility studies in binary mixtures. Investigations of the textures formed by liquid-crystalline phases are used extensively to identify mesophase types. The classification of the defects is made when the sample is cooled from the isotropic liquid into its mesophase(s). The defects obtained in this way are characteristic (paramorphotic) of the mesophase and not the crystal, and therefore can be used for the preliminary identification of the mesophase. However, a discussion of the structures of defect textures in liquid crystals will not be given in detail here because of the complexity of the Miscibility experiments are another useful way of classifying liquid-crystalline phases.Liquid crystals of the same type tend to be completely miscible across the full composition range of the phase diagram when compounds that possess the same phases are mixed. Again, thermal optical microscopy is used to identify phase transitions in binary mixtures. The transition temperatures obtained are plotted against composition in order to produce a miscibility diagram. Complete identifi- cation and classification, however, is better achieved by X-ray structural analysis. Thermal optical microscopic investigation of the textures of liquid-crystalline phases can be done quite simply by sandwiching a thin sample of the material between glass plates and observing the textures formed when the sample is heated or cooled between crossed polars.In the case of smectic liquid crystals, an alternative approach is to spread a thin film of the mesophase across a small hole (2 mm diameter) in a metal plate,g see Fig. 4. A free-standing film can be prepared by heating the material into its smectic state and then pulling a thin film across the hole by using the edge of a microscope coverslip as a spreader. This suspended film provides an oriented sample for study where the molecular layers are arranged normal to the direction of observation. Plate 1 shows the texture of a free-standing film of a smectic liquid crystal, suspended across such a small hole. When the long metal plate liquid ciystal in its smectic phase Fig.4 Preparation of free-standing films axes of the molecules are normal to the layer planes the field of view appears black (homeotropic), but the fact that the defect texture formed in Plate 1 is birefringent indicates that the molecules are tilted with respect to the layer planes. This simple experiment indicates one of the ways in which the structure of a liquid crystal can be probed. 5. Molecular Symmetry Symmetry in molecular structure is well understood and has been carefully defined in recent years. Cahn, Ingold and Prelog produced an invaluable and universally used method of defining and labelling chiral centres in molecular struc- tures."*" The R and S absolute spatial configuration system defines the local molecular structure about a chiral atom in terms of the atomic number@) of the substituents attached to the asymmetric atom.In liquid crystals, however, other aspects apart from the labelling by atomic number are also important. An archetypical structure of a chiral, calamitic, rod-like liquid-crystalline compound which has two flexible aliphatic chains attached to a rigid aromatic or quasi-rigid alicyclic core is shown below: I* C,,H2n+lA~B-(CH2),,,-C-{CH2),,-CH3I Y where n, m,and p are integers and (B + m)is the parity; A and B are polar groups; X and Y are different substituents; *denotes a chiral centre. The chirality in this compound, with respect to the rules of Cahn et al.,'O*llis determined by the atomic numbers of the substituents X and Y.However, in liquid crystals other aspects of molecular chirality are import- ant. It is well known, for example, that the steric size and shape of the substituents X and Y and their location with respect to the rigid core are important in determining phase structure type and phase characteristics. The absolute spatial configuration of the chiral atom and its location with respect to the core are often found to determine the handedness of the helix of any helical phase exhibited by the material. For example, the number of atoms (B+m) that the chiral centre is removed from the core determines the handedness of the helix. As the parity count (B+m) switches from odd to even so the handedness switches from left to right or vice uersa.Similarly, if the absolute spatial configuration of the chiral centre is inverted, say from R to S so the handedness of the helix also reverses.12 For example, we might expect for the ester, which has an absolute configuration S, that as the value of n alternates from odd to even, the handedness of the helical structure of the phase will also invert. Thus, the sign of the helix is dependent on the structure of the material. Similarly the handedness of the helix will alter if the spatial configur- ation is changed for any particular value of the parity (n+l in this case). Gray and McDonnell suggested12 that the spatial configuration of the chiral centre of the molecular structure is related to the screw direction of helical structures in the following way: Sol Sed Rod Re1 where S or R is the absolute spatial configuration of the chiral centre, o or e is the odd or even parity for the atom count that the chiral centre is removed from the core, and d or l refers to the handedness of the helical structure.A left-hand helix will be d and a right-hand helix will be 1. This relationship works very well for simple molecules that exhibit cholesteric phases, but for more complex systems the relationship is not as useful. In helical phases, it is generally found that when the chiral centre is brought closer to the core the pitch of the helix becomes shorter, and therefore the chirality increases. This is thought to be caused by the increased steric hindrance to the rotation of the asymmetric centre about the long axis of the molecule.Hence the degree of chirality of the mesophase, determined by the pitch of the helix, can be predicted to some degree from the molecular structure of the material in ques- tion. Similarly, the dipolar nature of the chiral centre with respect to its local molecular environment is also very import- ant in determining the nature of the properties of the liquid- crystalline phase. For example, if the substituents at the chiral centre are very polar they can often suppress liquid-crystalline properties, but conversely strong polarity can result in strong ferroelectric or pyroelectric behaviour of the material (see later). Again the absolute spatial configuration of the chiral centre determines the direction of the associated dipole, and hence the direction of any resulting spontaneous polarization of the material.13.14 6.Helical Structures in Liquid-crystalline Phases Liquid-crystalline phases that are composed of chiral mol- ecules usually have helical macrostructures. Typically, the orientational ordering of the molecules becomes twisted and forms a helical structure. There are two basic helical arrange- ments of the molecules in liquid-crystalline phases. In one, the orientational ordering of the molecules occurs in one preferential direction, thereby creating a single-twist structure. In the other case, the helix forms in more than one preferential direction, thereby creating a three-dimensional helical net- work.This is called a double-twist structure. 6.1 Double-twist and Blue-phase Helical Structures Consider a uniaxial phase that is composed of rod-like molecules. In the simplest situation, the helix can form in a direction perpendicular to the long axis of an object molecule, as shown in Fig. 5(a). This example is analogous to the structure of the cholesteric phase, shown in Fig. 1. In the direction parallel to the long axis of the molecule no twist can be effected. Now consider a similar situation, but this time the twist in the orientational order can occur in more than one direction in the plane perpendicular to the long axis of an object molecule, as shown in Fig. 5(b). In the simplest picture, two helices are formed with their axes perpendicular J.MATER. CHEM., 1991, VOL. 1 (a) (b) (c) Fig. 5 The helical arrangement of molecules in liquid-crystalline phases. Single-twist (a)or double-twist (b)structures can be generated. A two-dimensional double-twist structure forms a lattice of defects (4 to one another in the plane at right angles to the direction of the long axis of the molecule. Expanding this structure in two-dimensions the two helices can intersect to form a two- dimensional lattice. However, the helices cannot fill space uniformly and completely, and hence defects are formed, see Fig. 5(c). As helices are periodic structures, the locations of the defects created by their inability to fill space uniformly are also periodic.Thus a two-dimensional lattice of defects is created. This inability to pack molecules uniformly can be extended to three dimensions to give various cubic arrays of defects. The different lattices of defects provide the structural network required for the formation of a range of novel liquid- crystalline phases, which are called blue phases. ‘s In principle, these phases are frustrated structures where the molecules would like to fill space with a double twist structure but are prevented from doing so and therefore form defects. Thus, the formation of defects stabilizes the structure of the phase. One possible structure for a blue phase is shown below in Fig. 6. These phases were called blue phases because when they were first observed by Coates and Gray16 they appeared blue in the microscope.Their strong blue colour is due to the selective reflection of light. Other materials were later disco- vered which exhibited blue phases where the selective reflec- tion was in the red or green region of the spectrum. Experimentally it was found, however, that the helical pitch length must be of a similar length to the wavelength of visible light for a material to exhibit a blue phase, so the lattice period is of the order of 5000 A. 6.2 Smectic A* ‘Abrikosov’ Phases Blue phases are not the only frustrated structures that can be formed in liquid crystals. Recently, a new ‘frustrated phase’ has been discovered in smectic liquid crystals. This new phase was theoretically predicted by Renn and Luben~ky’~ to occur in the vicinity of the transition from the cholesteric to the smectic A phase.At this transition the helical ordering of the cholesteric phase gives way to the formation of the layered Fig. 6 Cubic blue phase 0’symmetry J. MATER. CHEM., 1991, VOL. I structure of the smectic A phase. Under certain circumstances it is possible that this transition can be mediated by the formation of an intermediary phase. They called this new phase the twist grain boundary phase. In this intermediary phase the molecules try to form a helical structure, where the axis of the helix is perpendicular to the long axes of the molecules (as in the cholesteric phase). However, the molecules also wish to form the lamellar structure of the smectic A phase [see Fig.7(a)]. These two structures are incompatible and cannot coexist and fill space uniformly without forming defects. The matter is resolved by the formation of a lattice of screw dislocations which enables a quasi-helical structure to coexist with a layered structure. This is achieved by having small blocks of molecules that have a smectic A structure, being rotated with respect to one another, thereby forming a helical structure. The blocks are separated from one another by screw dislocations. This type of defect allows the blocks to be rotated through a small angle with respect to one another. As a helix is formed with the aid of screw dislocations, the dislocations themselves are periodic.It is predicted that the screw dislocations will form grain boundaries in the phase and hence the new phase was referred to as the twist grain boundary (TGB) phase. The structure of the twisted smectic A* (TGB) phase is shown in Fig. 7(a). Examples of this theoretically predicted phase were found in some propiolate esters of 'high liquid-crystalline chirality'. Certain members of this family of materials exhibit exotic phase behaviour in that they form the twist grain boundary phase directly on cooling the isotropic liquid. '*ql' Further cooling produces transitions to ferroelectric, ferrielectric and antiferroelectric smectic C* phases (see later): CH3 The helical structure in the smectic A* phase appears from experiments to have a pitch length in the range of 0.38-0.63 pm; therefore the phase, like the cholesteric phase, selec- tively reflects light.Studies appear to indicate that the block size is ca. 185 A, which is quite small considering that the phase is still smectic. Thus, these remarkable phases and phase transitions are all products of the competition between chirality and conventional phase structure. Earlier, de Gennes had predicted that phase transitions of this type would occur in mechanically distorted liquid-crystal- line phases and drew an analogy between the physical nature of these phase transitions and that of transitions in supercon- ductors.*' de Gennes suggested that twist and bend distortions can be incorporated into a layered smectic A structure via the presence of an array of screw or edge dislocations. The screw dislocations permeate the normal phase to produce a lattice; this is similar to how the conducting phase permeates the superconducting phase to form a lattice of vortices in Type I1 superconductors.It is thought that the defects in the phase might be stabilized by impurities in the material. In the case of chemically pure compounds, the impurity could be due to the presence of the minority enantiomer in materials where the enantiomeric excess is <loo%. Thus, the enantiomer present in the lesser amount could migrate to the cores of the defects, thereby stabilizing them. This suggests that these frustrated phases (blue phases and Abrikosov phases) are inhomogeneous in the distribution of their enantiomeric components.This hypothesis is supported by indications from experimental studies which suggest that the greater the optical purity of the compound, the narrower the temperature range of the frustrated phase. Abrikosov phases have also been detected in optically active polymeric side-chain liquid-crystalline polymers. For example, the chiral polymer of the structure shown below was found to exhibit a twisted smectic A* phase.21 The side group is a derivative of cholesterol, and is therefore optically active. When this polymer forms a liquid-crystalline phase it is thought that the main chain of the polymer prevents a cholesteric phase from forming and instead induces the forma- tion of a smectic A phase.However, the strong chirality of the side group forces the layers to twist to give an Abrikosov phase, as depicted in Fig. 7(b). 6.3 Helical Smectic Phases Smectic phases where the long axes of the molecules are tilted with respect to the layer planes, as in the smectic C* phase, can form different helical structures to that of the cholesteric phase. This is because the tilt of the molecules can precess about an axis normal to the layers on passing from layer to layer. The rotation of the tilt is always in the same direction for a given material and hence a macroscopic helix is formed, as shown in Fig. 8. For the structure of the C* phase shown in Fig. 8, the molecules are tilted at an angle 8 with respect to the layer planes. The angle between the vertical planes that contain the long axis of an object molecule and the normal to the layer plane in subsequent layers is called the azimuthal angle.If both of these angles are large then the pitch of the helix formed will be relatively short in length. Usually the pitch in the smectic C* phase is longer than that in the cholesteric phase if both phases are exhibited by the same material. The other tilted smectic phases, smectics I* and F*, can also exhibit helical structures, although the pitch length is again Fig. 7 Twist grain boundary phases in low molar mass and polymeric liquid crystals. (a) Twisted smectic A* mesophase with spiral layer ordering; (b)twisted A* polymer P spiralling polarization D molecules 180” twist in the tilt direction Fig.8 Structure of the chiral smectic C* phase longer than in the smectic C* phase (when these phases are exhibited by the same material). The tilt angle (0) in smectic C* phases usually increases as the temperature decreases. To a first approximation, after a second-order transition to the smectic C* phase from a smectic A phase, the tilt angle varies in the following manner with respect to temperature where T, is the transition temperature, T is the temperature, O0 is a constant, and CI is an exponent (which has a theoretical value of 0.5). Thus, the tilt angle increases as the temperature is reduced, and as a consequence the pitch in a helical smectic C* is shortened concomitantly in length. For smectic C* phases, where the pitch length of the helix is of a similar magnitude to the wavelength of light, light will be selectively reflected. The iridescent colour change that takes place passes through red to blue when the sample is cooled because the tilt angle increases with falling temperature. This is the opposite effect to that observed for the cholesteric phase. Thus a material that exhibits both cholesteric and smectic C* phases can reflect light through its spectral range twice on heating or cooling, e.g. on cooling the reflected colours pass from blue to red in the cholesteric phase and from red to blue in the smectic C* phase.The pitch of the helix in the smectic C* phase is usually determined spectroscopically when the pitch is of a similar length to that of the wavelength of light; however, when the pitch length is of the order of a few micrometres it can be determined optically from the defect textures exhibited by the phase.The defects are formed when the helical structure interacts with the surface of the preparation. When the molecules at the surface are oriented differently from those in the bulk then a line defect is formed. The line defects can be seen in the polarizing microscope,22 where the distance between the lines is directly related to the pitch of the phase. Plate 2 shows the focal-conic defect texture of the smectic C* phase where the pitch bands shown are parallel to the molecular layers. The distance between the lines can be J.MATER. CHEM., 1991, VOL. I determined easily by using a Filar eyepiece or a graticule, and when the specimen is placed in an oven the pitch can be determined as a function of temperature. The helical twist sense of a material can be determined to be either right-handed or left-handed by mixture studies. For example, a contact preparation can be made between two liquid crystals when they are in their liquid-crystalline phases. At the contact the liquid crystals mix slightly. If the materials have the same helical twist direction then the pitch will not diverge across the contact region of the sample. However, if the materials have the opposite handedness then the helical structures will compensate for each other and the pitch will diverge in the contact region. Typically, this experiment is carried out with the specimens contained between a micro- scope slide and a cover slip; however, a similar experiment can be carried out using a free-standing film that is composed of two materials that have been spread across the film hole at the same time.The way that this contact film is prepared is shown schematically in Fig. 9. Plate 3 shows a contact preparation in a free-standing film for the R and S enantiomers of I-methylpentyl 4’-(4”-n-decyloxybenzoyloxy)biphenyl-4-carboxylate.This plate shows that pitch diverges and becomes infinite in the centre of the field of view and therefore the two enantiomers have the opposite twist directions. At the edges of the preparation some colouration is seen; these colours are due to the selective reflection of light.Thus, the pitch varies from infinity (white areas) to ca. 0.5 pm across the field of view. This method has been used extensively to classify the handedness of the helical phases of various liquid crystals. Once a material has been classified as either right- or left-handed it can then be used as a standard compound in the classification of other materials. Problems can sometimes be encountered with this method of classification when the two materials form a strong complex or association in mixtures. For example, two right- handed materials can form a pairing that might be left- handed, thereby producing two regions in the contact preparation where the pitch diverges.If these regions are not well separated it can appear that the two compounds have the opposite twist sense. Therefore, some care needs to be taken in using this method of defining twist directions of materials. 6.4 Optical Properties of Helical Structures So far the helical arrangement of the molecules in various liquid-crystalline phases has been described, and in each case the selective reflection of light has been inferred but not fully discussed. Therefore in this section the general optical proper- ties of helical structures are presented. When the helical structure of the phase is uniformly aligned with the axis of the helix normal to the substrate surface, then the phase will selectively reflect light, (see Plate 4).The selective reflection of visible light occurs when the pitch of glass slide pull direction /?-liquid crystal S-liquid crystal Fig. 9 Preparation of a contact free-standing film P J. MATER. CHEM.,1991, VOL. 1 Plate 1 A free-standing film of a smectic C liquid crystal (magnification x 100) Plate 2 A focal-conic defect texture of the smectic C* phase. The defect lines are related to the pitch of the helix in the phase; they are sometimes called pitch bands. (Magnification x 100) J. W. Goodby (Facingp. 3 12) Plate 3 A contact preparation in a free-standing film of two materials that have opposite helical twist directions in their smectic C* phases. The whitish band across the centre is where the pitch approaches infinity.The coloured bands on the top and bottom edges are where the two respective materials selectively reflect light. (Magnification x 100) Plate 4 The Grande-Jean plane texture of the cholesteric phase. The material selectively reflects red light. (Magnification x 100) J. MATER. CHEM., 1991, VOL. 1 the phase is of a similar magnitude to the wavelength of light. However, the brilliant colours that are exhibited by helical phases are only observed under certain circumstances. For example, for a narrow wavelength range about the wavelength A. of an incident light beam parallel to the helical axis, the light beam will be split into its two circularly polarized components by the helical structure. One of the circularly polarized components will pass through the specimen, whereas the other component will be reflected.The component that is reflected has the same rotation sense as that of the screw direction of the helix. Incident linearly polarized light that has a wavelength outside that of the reflection band will be transmitted through and rotated by the helical structure. The optical rotary power of helical liquid crystals can be very large, i.e. of the order of thousands of degrees, thereby demonstrating that the form optical activity is much stronger than the molecular optical activity. The sign of the rotary power is different above and below the reflection band. For pitch lengths greater than the wavelength of the incident light, the plane of incident polarized light tends to be rotated in the same direction as the helix, whereas for pitches shorter than the wavelength of light the reverse is true.The maximum reflection of incident light occurs when A. =np where p is the pitch and n is the mean refractive index within a plane normal to the helical axis: The width of the reflection band, AA, is given by AA =Anp where the anisotropy of the perpendicular and parallel refrac- tive indices is given by An=(nll -nl) For obliquely incident light, the reflection band is shifted towards shorter wavelengths, and sometimes higher-order reflection bands are formed which are not present for normal incident light. All helical liquid-crystalline phases will selec- tively reflect light; however, the cholesteric phase is typically used for surface thermography because this phase can usually be obtained with a pitch length in the required regime for selective reflection to occur.Often the pitch length in smectic phases is too long for the reflection to occur in the visible range. 7. Reduced Symmetry in Liquid Crystals 7.1 Ferroelectricity in Smectic Phases Symmetry Arguments We have seen how form chirality is expressed in helical structures in various liquid-crystalline phases; now attention is turned towards space symmetry in these phases. Essentially, this section deals with smectic phases where the molecules are tilted with respect to the layer planes, for example as in the smectic C, I and F phases, or the crystal J, G, H and K phases. When the smectic C phase is constituted of achiral molecules then the environmental or space symmetry consists of a centre of inversion, a mirror plane normal to the layers, and a C2 axis parallel to the layers and normal to the tilt direction.Thus the symmetry is described as C2,,. When the molecules of the phase are chiral the local symmetry in the layers is reduced to a polar C2 axis. As the molecules are themselves polar, there is an inequivalence with respect to the dipoles along the C2 axis.23 This inequivalence occurs even though the molecules are undergoing rapid reorientational motion about their long axes. The time-dependent alignment of the dipoles along the C2 axis causes a spontaneous polarization to develop along this direction, parallel to the layer planes, and therefore each individual layer has a spontaneous polariz- ation.The arguments used so far only apply to individual layers, but when the layers are stacked one on top of another the tilt direction is rotated about the normal to the layer planes; consequently the layer polarizations are avaraged out to zero and the phase can be described as heliele~tric.~~The structure of the helielectric phase is shown in Fig. 8. If the helix is unwound the ‘layer polarizations’ point in the same direction and hence the phase becomes ferroelectric and therefore pyroelectric. The arguments presented for this reduction in symmetry are depicted pictorially in Fig. 10. We assume that in the layers there are as many molecules pointing up as there are down; this situation is represented in the figure by two asymmetric molecules that are shown as fish” (after de Gennes).If we assume that each molecule has a dipole pointing out of the eye of each of the fish then we see that the dipoles are pointing in the same direction, and the drawing has a twofold axis pointing out of the page. The shape of the molecules shows that the layer structure has no centre of inversion or mirror planes. Hence a degenerate structure with the dipoles pointing back into the page cannot be produced simply by rotating the molecules about their long axes. The equivalent layer structure can only be produced if the mol- ecules are tilted in the opposite direction in the plane of the page with their dipoles pointing downwards.This degeneracy is used to produce an electro-optic effect that is implemented in surface-stabilized ferroelectric liquid-crystalline displays (SSFLCDS).26 In these displays, when the polarity of the applied electric field is reversed, the molecules are reoriented by rotating around the surface of a cone, thereby moving through an angle equal to twice that of the tilt angle, see Fig. 11. Fig. 10 Reduced symmetry in the smectic C* phase. The dipoles represented by p are parallel to the C2axis and are perpendicular to the plane of the page Kmolecule P Q 4 * change in polarity of applied field Fig. 11 The effect produced in SSFLCDS when the polarity of the applied electric field is reversed.The molecules are reoriented through an angle equal to twice the tilt angle. The polarization directions are opposite to one another and perpendicular to the plane of the page Polarization Direction The polarization is a vector quantity and can point in one of two directions. Consequently, a material will also have one of two possible spontaneous polarization directions27 associ- ated with it, as shown in Fig. 12. The polarization direction is dependent to some extent on the absolute spatial configur- ation of the molecule; for example, the R and S isomers will have opposite polarization directions. Some atempts have been made to relate the direction of the spontaneous polarization to the spatial configuration of the molecule in a similar way to that used by Gray and McDonnell for helical structures.The following relationships were found to have some use in smectic C* liquid crystals. SedP(-) +I, Sol P(+)+I, Sod P(-) -I, Sel P(+) -I Red P(-) -I, Rol P(+) -I, Rod P(-) +I, Re1 P(+) +I where S or R is the absolute spatial configuration, o or e is the parity, 1 or d is the screw direction of the helix, P(+) or P(-) is the polarization direction, and -I or +I is the inductive effect of the off-axis substituent at the chiral centre. The inductive effect essentially determines the direction of the dipole at the chiral centre. As with the relationship between spatial configuration and helical twist direction, this relation- ship works very well for simple molecules, but for more J.MATER. CHEM., 1991, VOL. 1 molecule. These properties are interlinked and describe some- what how the dipole at the chiral centre is coupled to the lateral molecular dipole. A simple example of how the polariz- ation is effected by rotational freedom is illustrated by the following homologous series of corn pound^^^: As the terminal alkyl chain length (n) is increased, so the motion of the chiral centre about the long axis of the molecule becomes increasingly damped. Thus, for the simplest chiral system with n=2 the maximum value for the spontaneous polarization is ca. 150 pC m-'; however, when the value n= 6 the polarization increases sevenfold. Similarly it can be shown that when the dipole associated with the chiral centre is increased so too does the polarization. For example, con- sider the following materials: complicated structures it is not particularly ac~urate.l~*'~ However, the fact that such simple correlations exist shows that the microscopic and macroscopic properties of these materials are closely related.K Fig. 12 Polarization direction in smectic C* phases. This convention produces the same sign of the polarization as that used by Clark- Lagerwall even though the opposite spatial relationship is employed. This arises owing to the differences in symbols used by chemists and physicists to describe dipole directions Magnitude of the Spontaneous Polarization The magnitude of the spontaneous polarization in ferroelectric liquid crystals is relatively low with respect to inorganic systems. Low values are usually obtained because the mol- ecules are undergoing rapid reorientational motion about their long axes, and therefore it is only the time-dependent alignment of the dipoles which contributes to the polarization. Moreover, the types of molecule that form ferroelectric smectic phases are not appreciably polar in comparison to inorganic materials.As the phases are fluid, the molecules relax easily after being poled in applied electric fields. This means that the phases are subject to loss with respect to their pyroelectric properties. Closer examination of ferroelectricity in liquid crystals shows that the magnitude of the spontaneous polarization is dependent on the tilt angle (6)of the phase,28 the size of the dipole at the chiral centre, and the amount of freedom that the chiral centre has to rotate about the long axis of the CH3 50 C1OHPI0~CH=N~CH=CHC02CH2~HC2H51 CH3 450 CloH210~CH31~CH=CHC02$HC0,CHC,H.I CI 400 C,H130~CH=N-(&CH=CHC02CH,~HCH3 When the polarity of the lateral substituent at the asymmetric carbon atom increases so too does the p~larization.~~-~~ For instance, when a methyl substituent is replaced with a chlorine atom at the chiral centre the polarization increases by ca.7-10-fold. Furthermore, when the motion of the chiral centre is also trapped by steric hindrance of other substituents that are further down the terminal aliphatic chain away from the aromatic core, the polarization increases substantially.These studies point to the fact that the nature and location of the chiral centre are important factors in determining the magnitude of the spontaneous polarization. However, it can also be shown that increasing the overall polarity of the molecule does not necessarily improve the polarization. There- fore, inserting more polar groups, such as esters, into the structure of a ferroelectric liquid crystal does not mean that the polarization will be increased automatically. The reason for this is that the dipoles of the extra groups are not necessarily coupled to the local dipole at the chiral centre. The list of materials shown on the next page demonstrate this effect quite clearly. J.MATER. CHEM., 1991,VOL. 1 CH3 50 c10H21c02~c02cH2~Hc2HSI CH3 50 c10H21~~c02~c02cH2~Hc2H5I 25 c10H210~c02~c02cH2$Hc2H5I 50 C70H21 0 ~ c 0 2 ~ c 0 2 c H 2 F H c 2IH5 CH3 50 CloH210~CH=N~C02CH2$HC2H5I CH3 50 CloH210~CH=N~CH=CHC02CH2$HC2H5I Until recently there used to be considerable interest in the magnitude of the spontaneous polarization because the switch- ing time of a ferroelectric device is inversely proportional to the p~larization.~~This relationship is expressed in the equation zzq1P-E where zis the reorientational time, q is the viscosity, P is the polarization, and E is the applied electric field. It was proposed that the polarization should be increased in order to reduce the switching speed in electro-optic devices. Unfortunately, it was found that this practice creates relatively strong internal fields that have to be overcome before the molecules will latch and switch.Recently, however, it was found that it is necessary to reduce both the reorientational viscosity and the polariz- ation in order to obtain switching times in the region of 1 -10 p,which are necessary for video rate displays. This is achieved by making mixtures of achiral materials that exhibit smectic C phases and chiral dopants that have large polariza- tions but are not necessarily liquid crystals.35 The dopants are only present in the mixture in low concentrations, i.e. of the order of 10%. Temperature Dependence of the Spontaneous Polarization The spontaneous polarization has also been shown to be temperature dependent.A 'Landau' type of expression has been derived to predict the behaviour of the polarization, as follows P=Po( T,-T)' where P is the magnitude of the polarization at a given temperature T, Po is a constant for the material, T, is the 315 transition temperature for the second-order smectic A to smectic C* phase change, T is the temperature, and is an exponent which, theoretically, should equal 0.5.36This form of temperature dependence of the polarization is similar to that for the temperature dependence of the tilt angle. To a first approximation, the two equations can be used to fit experimental data. The values for the exponents can be determined from these fits, but the results obtained for the exponents are not usually in agreement with their theoretical values of 0.5, see for example Fig.13. Some research has shown that higher terms of the free-energy expansion need to be included before the data can be fitted to the theory.37 Unusual results for the dependence of the polarization on temperature can sometimes be obtained. For instance, the polarization can be found to fall with decreasing temperature so that the sign of the polarization inverts.38 One particular material, (S)-Zmethylbutyl 4'-n-nonoyloxybiphenyl-4-car-boxylate, has a negative polarization at high temperatures, but as the temperature is reduced the polarization falls and its direction inverts and becomes positive at low temperatures (Fig.14).This material is not the only example of a substance that shows this behaviour; some liquid-crystalline polymers also show polarization inversions when the temperature is changed e.g. the polymeric epoxide3' 1 13 N1 €1000%. \ C 8001.-0 CI .-2 600 I g 400 SAtoSvtransition.I200 Curie point 65 70 75 80 85 90 95 100 105 110 TI "C Fig. 13 The spontaneous polarization (pCm-2) of (S)-1-methylheptyl 4-(4-n-decyloxybenzoyloxy)biphenyl-4-carboxylatemeasured as a function of temperature ("C) 30 40 .-2 20 Curie point Q 60 TI "C Fig. 14 The spontaneous polarization (pCm-2) of (S)-Zmethylbutyl 4'-n-nonoyloxybiphenyl-4-carboxylatemeasured as a function of tem-perature. The material has the following transition temperatures and properties: Iso. liq.59.7"CSA41.8"CSc*;P(-) to P(+)17.5"C It is thought that the polarization sign inversion is caused by the competition between various conformational species which happen to have different polarization directions.40 A theoreti- cal model was proposed where the conformational isomers are interconvertible, and therefore the relative concentrations of the various species vary with temperature. At a certain point the concentrations of the species with respect to their polarities balance and the spontaneous polarization falls to zero and changes sign. From this approach the energy barrier to interconversion can be determined by iterative fits to the experimental data. The activation energy is found to be of a similar magnitude to the torsional energy encountered for the rotation of carbon-carbon bonds in normal alkanes. This suggests that the various species are conformational isomers where the chiral centre is in either a trans or a gauche relationship with the overall structure of the molecule, as shown in Fig.15. Experimental studies on the materials depicted in Fig. 15 show that the polarization does in fact fall to zero at the inversion point. This result is evident from the measurements of the helical unwinding voltage as a function of temperat~re.~' At the crossover temperature the unwinding voltage is very high because the helix is unwound via the coupling of the dielectric anisotropy to the applied field, whereas for tempera- tures away from the crossover point the unwinding voltage is very small because the helix is unwound by the coupling of the polarization to the applied field.Pyroelectric studies on these materials suggest that the phase is composed of clusters of molecules that are undergoing dynamic intercon- version.42 The cluster size was found to be of similar magni- tude to that of the in-plane positional correlation length for the smectic C* phase determined by X-ray diffraction stud- ie~.~~These results suggest that ferroelectric liquid crystals are not too dissimilar from ferroelectric crystals in that they are inhomogeneous and are composed of clusters of differently poled regions. The major difference arises from the fact that liquid-crystalline phases are liquid-like and are therefore in dynamic equilibrium.This leads to the unusual results reported above, results that are not usually seen in solid-state systems. 7.2 Antiferroelectric and Ferrielectric Phases The novel phenomena of antiferroelectricity and ferrielectricity have been observed recently in smectic C*pha~es.~-~~ This behaviour has so far been found in materials where the degree of chirality is relatively high, for example in materials where J. MATER. CHEM., 1991, VOL. 1 the chiral centre is near to the rigid aromatic core of the molecule or where the peripheral aliphatic chain is relatively long. In these materials the motion of the chiral centre about the long axis of the molecule is damped, and therefore the degree of chirality is increased.The increase in chirality over that in closely related systems is manifested in a shortening of the pitch length of the helix in chiral phases and a substantial increase in the spontaneous polarization. This progressive change in chirality can be seen in homologous series, such as the (S)-or (R)-1-methylalkyl 4-(4-decyloxybenzoyloxy)biphenyl-4-carboxylates,when the peripheral alkyl chain is extended. When the alkyl chain is short in length, e.g. propyl, normal phase behaviour is observed. The materials cool from the isotropic liquid into smectic A phase and then undergo a transition into the smectic C* phase, where the pitch of the helix is ca. 2-3 pm. However, for the longer chain homologues, e.g.heptyl, the pitch in the C* phase drops to a value of ~0.5 pm. Similarly, the magnitude of the polarization increases six- to seven-fold as the alkyl chain is lengthened from propyl to heptyl. For the longer chain lengths, e.g. heptyl, other phases are observed on cooling the smectic C* phase. The phase changes are clearly defined by large changes in the optical textures of the materials observed by thermal polarized light microscopy. These changes are characterized by a sudden jump in the pitch length from ca. 0.5 to ca. 10 prn as the temperature is red~ced.~'This effect is clearly seen in free-standing films where the phase changes from being iridescent in its smectic C* phase (Plate 5) to being non-iridescent but still helical in the lower-temperature phase (Plate 6).Further cooling pro- duces a transition to another phase (Plate 7) where the pitch again decreases to ca. 0.5 pm (Plate 8). It is clear from optical rotation studies that the pitch does not diverge at these phase transitions, and so the effect observed is not due to a twist inversion, e.g. the helix screw direction changing from left to right to left. Differential scanning calorimetry of these mater- ials shows that there is no measurable enthalpy associated with these phase changes. Surprisingly, the magnitude of the polarization for these materials falls at the C* to the first phase transition, and in the lowest-temperature phase approaches zero. This fall in polarization is matched by similar changes seen for the tilt angle when it is determined as a function of the applied electric field, as shown in Fig.16. At the Curie point the spontaneous polarization and the tilt angle initially rise, reach a maximum value and then fall rapidly at the transition to the intermediary phase and reach very low values in the low-temperature phase, see Fig. 16(a). " 0 20 AT= <-T 40 60 Fig. 16 The optical tilt angle (0) measured as a function of reduced temperature from the Curie point for (S)-1-methylpentyl 4-(4-n-The tilt angle was meas- Fig. 15 Conformational isomers of (S)-Zrnethylbutyl4-n-nonoyloxy-decyloxybenzoyloxy)biphenyl-4-carboxylate. biphenyl-4-carboxylate. The two conformers have opposite polariz- ured in cells with a 10 pm spacing.The applied field was 67.5 V (A), ation directions: top, P(-); bottom, P(+) and 135 V (0) J. MATER. CHEM., 1991, VOL. 1 Plate 5 A free-standing film of a chiral smectic C* liquid crystal. In this preparation we are looking directly down the helical axis of the phase which selectively reflects light in the blue region of the spectrum. (Magnification x 100) Plate 6 The transition from the smectic C* phase to the ferrielectric phase in a free-standing film (same area as Plate 5). At the transition the pitch changes dramatically and becomes longer and the iridescent colour disappears. (Magnification x 100) J. W. Goodby (Facingp. 3 16) Plate 7 The transition to the antiferroelectric C* phase from the ferrielectric phase on cooling (same area as Plate 5-6). In the new phase the pitch length shortens and once again the specimen selectively reflects light.(Magnification x 100) Plate 8 The texture of a free-standing film (same area as Plate 5-7) of an antiferroelectric smectic C* phase. The helix unwinds as the temperature is lowered and so the phase now selectively reflects red light. (Magnification x 100) There is no sign inversion accompanying the changes in the polarization, thereby indicating that the effect is not being caused by the competition of various conformers. If the applied field is increased then the temperature dependences of the polarization and tilt angle are restored to their typical forms, as shown in Fig. 16(b) when the applied field is doubled.Fukuda and co-w~rkers~~ have rationalized this behaviour in various materials by suggesting that the compounds undergo a transition from a ferroelectric phase into a ferrielectric phase where the polarization falls. Subsequent cooling produces a transition to an antiferroelectric phase where the polarization essentially becomes zero. The proposed structure of the antiferroelectric is shown first (Fig. 17) because it is the easier of the two new structures to rationalize. It is proposed that the layers are stacked in such a way that the polarization vectors in subsequent layers point in the opposite directions, thereby cancelling each other. This results in the spontaneous polarization falling to zero. On the application of a strong electric field this layer ordering is broken up and the phase returns to the normal ferroelectric structure; hence the polarization-temperature curve returns to its usual form.A schematic representation of the switching process is shown in Fig. 18. In the unpoled antiferroelectric state, subsequent layers are tilted in opposite directions to one another. If the antiferroelectric phase is subjected to a d.c. voltage the layer ordering will break up, and the molecules will be poled in one direction depending on the polarity of the applied electric field in order to give a ferroelectric phase. Switching from the antiferroelectric to the ferroelectric phase occurs at a defined applied field; therefore, there is a sharp threshold for switching.This behaviour is different from switching in a conventional ferroelectric liquid-crystalline phase where the process appears to be continuous, i.e. switch-layers Fig. 17 Antiferroelectric smectic C* phase \ ing occurs without a threshold field in cases where the switched states are not surface stabilized. The presence of a threshold field to switching is useful in display applications that rely on the multiplexing of a large number of pixels in order to operate. The switching process can be investigated by measuring the tilt angle as a function of the applied voltage. No change in tilt angle occurs for low voltages, and at a critical field the structure of the phase changes from being antiferroelectric to ferroelectric. At this point the tilt angle increases substantially, and reaches the full tilt angle of the smectic C*phase.There is a degree of hysteresis when the voltage is cycled up and down or the polarity is reversed, as shown in Fig. 19 (after ref. 48). Unlike the structure of the normal smectic C*phase which is repeated for each 360" rotation of the helix, the helical structure of the antiferroelectric phase is repeated for every 180" rotation. Therefore, the pitch appears to change quite dramatically as the temperature is altered. A similar structure is proposed for the ferrielectric phase except that the layers are stacked in such a way that there is a net polarization, i.e. the number of layers of opposite polarization direction are not equal. There might be for instance twice as many layers where the polarization is opposite to that of the other layer, as shown in Fig.20. However, it is also suggested that the stacking of the layers is regular so that the phase has two interpenetrating sub- lattices. For this stacking arrangement the ferrielectric phase will have a measurable polarization. This value will be lower I n-I -301 I 1 0 40 Fig. 19 Switching from an antiferroelectric state in smectic C* liquid crystals P polarization up unwound state polarization down Fig. 18 Antiferroelectric-ferroelectric switching Fig. 20 Ferrielectric smectic C* phase 318 J. MATER. CHEM., 1991, VOL. 1 than that for the ferroelectric phase, but it will not approach zero as in the antiferroelectric phase.Moreover, the relative proportions of the two opposing layer structures can vary with temperature. Therefore, it is to be expected that the polarization will drop in an almost stepwise fashion with falling temperature as the ratio of the opposing layers changes 16 17 18 19 20 D. Coates and G. W. Gray, Phys. Lett., 1973, 45A,115. S. R. Renn and T. C. Lubensky, Phys. Rev. A, 1988,38, 2132. J. W. Goodby, M. A. Waugh, S. M. Stein, E. Chin, R. Pindak and J. S. Patel, J. Am. Chem. SOC., 1989, 111, 8119. G. Srajer, R. Pindak, M. A. Waugh, J. W. Goodby and J. S. Patel, Phys. Rev. Lett., 1990, 64, 1545. P. G. de Gennes, Solid State Commun., 1972, 10, 753, from 2: 1 to 3:4 to 5: 7 etc. In fact, there is some evidence from electrical-switching studies to support this hypothesis where the ratio of the opposing layers should also be depen- dent on the strength of the applied field.This effect is just one of the many interesting and new phenomena that are being unearthed in chiral liquid crystals. 21 22 23 24 Ya. S. Freidzon, Ye. G. Tropsha, V. V. Tsukruk, V. V. Shilov, V. P. Shibaev and Yu. S. Lipatov, J. Polym.Chem. (USSR), 1987, 29, 1371. M. Glogarova, L. Lejek, J. Pavel, U. Janovec and F. Fousek. Mol. Cryst. Liq. Cryst., 1983, 91, 309. R. B. Meyer, Mol. Cryst. Liq. Cryst., 1976, 40,74. H. R. Brand, P. E. Cladis and P. L. Finn, Phys. Rev. A, 1985, The properties of these systems have yet to be fully investigated and exploited in applications. Other effects such as electroclin- ism and flexoelectricity are not discussed in detail here, but they are also of interest in relation to chirality and fast switching effects in liquid crystals.25 26 31, 361. P. G. de Gennes, The Physics of Liquid Crystals, Oxford Univer- sity Press, Oxford, 1974, p. 321. N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett., 1980,36, 899; M. Handschy and N. A. Clark, Ferroelectrics, 1984, 59, 69; N. A. Clark and S. T. Lagerwall, Ferroelectrics, 1984, 59, 25. 27 J. S. Patel and J. W. Goodby, Opt. Eng., 1987, 26, 373. 8. Summary 28 J. S. Patel and J. W. Goodby, Mol. Cryst. Liq. Cryst., 1987, 144, 117. I have sought in this article to give only a flavour of the complex but exciting nature of current research activities in liquid crystals. However, it should be noted that the topic of chirality is one of the fastest growing areas of research in liquid crystals today. The discoveries of new methods of making optically pure materials from prochiral substrates have led the way in producing new phases that have unusual physical properties.These properties can be utilized in a variety of applications, for example, in displays and sensors. 29 30 31 32 33 34 35 J. W. Goodby, J. S. Patel, and E. Chin, J. Phys. Chem., 1987, 91, 5151. R. B. Meyer, L. Liebert, L. Strezlecki and P. Keller, J. Phys. Lett. (Paris), 1975, 36, L69. Ch. Bahr and G. Heppke, Mol. Cryst. Liq. Cryst., 1987, 148, 29. L. A. Beresnev, E. P. Pozhidaev, L. M. Blinov, A. Paulguchen and N. B. Etingen, JETP Lett., 1982, 35, 531. K. Yoshino, M. Ozaki, T. Sakurai, K. Sakamoto and M.Honma, Jpn. J. Appl. Phys., 1984, 23, L175. N. A. Clark and S. T. Lagerwall, in Liquid Crystals of One- and Two-Dimensional Order, ed. W. Helfrich and G. Heppke, Springer-Verlag, Berlin, 1980. G. W. Gray, M. Hird, D. Lacey and K. J. Toyne, Mol. Cryst. References 1 2 3 4 F. Reinitzer, Monatsh. Chem., 1888,9,421; 0.Z. Lehmann, Phys. Chem. (Leipzig), 1989, 4, 462. H. Kelker and P. M. Knoll, Liq. Cryst., 1989, 5, 19. J. G. Grabmaier, in Applications of Liquid Crystals, ed. G. Meier, E. Sackmann and J. G. Grabmaier, Springer-Verlag, Berlin-New York, 1975, pp. 83-160. See for example, H. Kelker and R. Hatz, in Handbook of Liquid 36 37 38 39 Liq. Cryst., 1990, 191, 1. Ph. Martin6t-Lagarde, R. Duke and G. Durand, Mol. Cryst. Liq. Cryst., 1981,75,249; K. Kondo, S. Eva, M.Isagai and A. Mukok, Jpn. J. Appl. Phys., 1985, 24, 1389. C. C. Huang and J. Viner, Phys. Rev. A, 1982,25, 3385. J. W. Goodby, E. Chin, J. M. Geary, J. S. Patel and P. L. Finn, J. Chem. SOC.,Faraday Trans. I, 1987,83, 3429. G. Scherowsky, A. Beer, J. Gay, U. Muller, A. Schliwa, E. Rhode, K. Kuhnpast, J. Springer and P. Harnischfeger, Proc. 13th Znt. 5 6 7 Crystals, Verlag Chemie, Basel, 1980, and references therein. J. W. Goodby, Chemalog Hi-lites, 1987, 11, 3. J. W. Goodby, Mol. Cryst. Liq. Cryst. Lett., 1983, 92, 171. See for example G. W. Gray and J. W. Goodby, Smectic Liquid Crystals: Textures and Structures, Blackie Press, Glasgow-Lon- 40 41 42 Liq. Cryst. Con$, Vancouver, July, 1990, Book 1, p. 105. J. S. Patel and J. W. Goodby, Philos. Mag. Lett., 1987, 55, 283.J. S. Patel, personal communication. A. M. Glass, J. W. Goodby, D. H. Olson and J. S. Patel, Phys. Rev. A, 1988, 38, 1673. 8 9 don, 1984. D. Demus and L. Ritchter, Textures of Liquid Crystals, Verlag Chemie, New York, 1978. R. Pindak and D. Moncton, Phys. Today, 1982, 35, 57; C. Y. 43 44 J. D. Brock, A. Aharony, R. J. Birgeneau, K. W. Evans-Lutterodt, J. D. Lister, P. M. Horn, G. B. Stephenson and A. R. Tajbakhsh, Phys. Rev. Lett., 1986, 57, 98. A. D. L. Chandari, Y. Ouchi, H. Takezoe, A. Fukuda, K. Young, R. Pindak, N. A. Clark and R. B. Meyer, Phys. Rev. Terashima, K. Furukawa and A. Kishi, Jpn. J. Appl. Phys., 1989, Lett., 1978, 40, 773; R. Pindak, C. Y. Young, R. B. Meyer and 28, L1261. N.A. Clark, Phys. Rev. Lett., 1980, 45, 1193. 45 E. Gorecka, A. D. L. Chandari, Y. Ouchi, M. Takezoe and A. 10 R. S. Cahn, C. K. Ingold and V. Prelog, Angew. Chem. Znt. Ed., 1966, 5, 385. 46 Fukuda, Jpn. J. Appl. Phys., 1990, 29, 131. S. Inui, S. Kawano, M. Saito, H. Iwane, Y. Takanishi, K. Hiraoka, 11 12 13 R. S. Cahn and C. K. Ingold, J. Chem. SOC., 1951, 612. G. W. Gray and D.G. McDonnell, Mol. Cryst. Liq. Cryst. Lett., 1977, 34, 211. J. W. Goodby, E. Chin, T. M. Leslie, J. M. Geary and J. S. Patel, 47 48 Y. Ouchi, H. Takezoe and A. Fukuda, Jpn. J. Appl. Phys., 1990, 29, L987. J. W. Goodby and E. Chin, Liq. Cryst., 1988, 3, 1245. M. Johno, A. D. L. Chandari, J. Lee, Y. Ouchi, H. Takezoe, A. 14 J. Am. Chem. Soc., 1986, 108,4729. J. W. Goodby and E. Chin, J. Am. Chem. SOC., 1986, 108,4736. Fukuda, K. Itoh, and T. Kitazume, Proc. Jpn. Display, 1989, 22. 15 P. P. Crooker, Liq. Cryst., 1989, 5, 751. Paper 1/00981H; Received 4th March, 199 1

ISSN:0959-9428    

DOI:10.1039/JM9910100307

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(3), 319-326 Porous Cross-linked Materials formed by Oligomeric Aluminium Hydroxides and a-Tin Phosphate Pedro Maireles-Torres," Pascual Olivera-Pastor,' Enrique Rodriguez-Castellon," Antonio Jimenez- Lopez,*' Lucilla Alagnab and Anthony A. G. Tomlinson*' a Departamento de Quimica Inorganica, Cristalogra fia y Mineralogia, Universidad de Malaga, Apartado 59, Malaga, Spain I. T.S.E.,Area di Ricerca del C.N.R., C.P.10 Monterotondo Staz., 00016 Rome, Italy The intercalation of the tridecameric polyhydroxyaluminium Keggin-type cation, formally +[AI0,A I ,( 0H)24(0H,) ,] , in t o a-Sn(H PO,) ,H,O via the co IIoid a I tetra met h y Iam m oniu m -exc h ang ed in t e rm ed iate x-Sn[NMe,]o,,~,,oHl,l~l~o(P04)2~4H,0and the alumina-pillared materials obtained after calcination are described.Two different intercalated precursor materials are obtained, depending on whether the Keggin ion inserted derives from the commercial product ('Chlorhydrol') or from the polyhydroxyaluminium cation generated in situ. Calcination leads to materials differing in free heights and in alumina contents. Their surface areas (B.E.T., N,, 77 K) are quite high: 190 m2 g-' [chlor-SnP (400 "C)] and 228 m2 g-' [All,-SnP (400 "C)]. Pore-size calculations show them to be mainly mesoporous, but with some micropore contribution (>50% of pores in width range 15-40 A). The higher microporosity of the former with respect to the latter is ascribed to lateral-order differences between the alumina pillars.High cation-exchange capacities (for Co2+, Ni2+ and Cu2+) confirm that both solids are porous and have more accessible interlayer sites than does the parent material. Optical spectra of the transition-metal ion- exchanged materials indicate that the sites available in the two solids differ, and that both differ from those present in the starting a-tin phosphate. Site geometries are suggested. Keywords: Pillaring; x-Tin phosphate; Aluminium hydroxide The last 10 years have seen an explosive growth in attempts to prepare reproducibly two-dimensional porous materials from smectite clays by inserting large cationic species.' The original motive was to find catalytically active large-pore materials robust enough to withstand reforming after use as heavy-fraction catalysts in the petroleum industry.' However, those clay-based materials which most closely fulfill the three criteria required of such a catalyst, i.e.(i) a high surface area in pores large enough to allow fast diffusion of small- to medium-sized molecules (8-12 A), (ii) acidic, ion-exchange properties allowing access to metal ions as promoters, and (iii) high thermal stability (>400 "C), tend to collapse under mild hydrothermal condition^.^ Although layered Group 14 phosphates are not as readily swellable as smectites,, it has recently been shown that they can be swelled partially to allow insertion of organic ligands with subsequent development of a complex-pillar chemistry.' We now describe a general method for inserting the Keggin- like cation [A104A112(0H)24(0H2)12]7+as both the commer- cial product ('Chlorhydrol') and as prepared in situ, into a-tin phosphate.Calcination of these products gives rise to two series of phase-pure stable pillared materials which are porous, with interlayer sites more accessible than those present in the parent materiak6 Similar techniques have been reported for x-zirconium and y-titanium phosphates7 and layered titanates8 Experimental a-Tin phosphate [a-Sn(HP04)2 H20, ('a-SnP')] was prepared as described previouslyg and had dOo2=7.8 A (interlayer dis- tance) and a specific surface area (B.E.T., N2, 77 K) of 11 m' g-'. It was conserved at 70% relative humidity. 'Chlorh ydrol', [A104A1 2( OH),,( OHz) '1 Cl,, was a commer- cial product (Reheis, NJ, USA; supplied as a powder).Chemi- cal analysis for A1 and thermogravimetry (TG) confirmed the formulation, and the product was used as received. (All products derived from this material are referred to as chlor- SnP.) The All, oligomer was prepared essentially as described by Tokarz and Shabtai.'O An aqueous solution of 0.2 mol dm-3 NaOH was added slowly to a 0.2 mol dm-, solution of AlCl, (commercial pure product, Merck) to a final pH of 4.35. The resulting solution was left to stand for 20 days at 25 "C and then used immediately. (All products derived from this precur- sor are referred to as All, oligomer-SnP.) [NMe,]Cl and n-propylamine (nPr) were commercial products and were used as received.Formation of a-Tin Phosphate/n-Propylamine Colloidal Suspension a-SnP (5 g) was suspended in a 0.1 mol dm- aqueous solution of n-propylamine [corresponding to 60% of the theoretical cation exchange capacity (c.e.c.)] for 2 h at 25 "C. The half- exchanged form 'a-Sn * HnPrA * HP, was obtained [as demon- strated by TG, X-ray diffraction (XRD) and chemical analysis on the filtered, washed solid). The colloidal suspension could be preserved as such for >1 month without flocculation and readily formed films on casting, similar to those of or-zirconium phosphate previously described." Insertion of Aluminium Polyhydroxy Cations into the mealf fa-SnP Colloidal Intermediate When freshly prepared a-Sn HnPrA -HP colloid was con- tacted with either Chlorhydrol or All, oligomer solutions, at various mole ratios and temperatures, the solids separated were amorphous or showed high collapse at low calcination temperatures (<200 "C).They also contained organic frag- ments. It was first necessary to sweep out interlayer protons partially. To the colloidal suspension of a-Sn-HnPrA-HP was added an aqueous solution (50 cm3) of 16.72 g of [NMe,]Cl equiva-lent to 10 times the theoretical c.e.c., based on starting a-SnP. After 1 day of stirring at 25 "C, the fine suspen- sion was centrifuged and the solid was washed with water and air-dried. It was analysed (chemical and TG) for a-Sn[NMe4]o~9-l.oHl .'-.-,(PO& *4H20 (referred to as NMe,-SnP). It formed stable, colloidal suspensions, could be cast as a film, and XRD and IR spectra showed that neither a-SnP nor starting n-PrA intercalate was present.A1 Oligorner-SnP Aliquots of Al13 oligomer solutions were contacted with colloidal NMe,-SnP suspensions at various molar ratios, each being stirred for 1 day at 25 "C. The suspension was centri- fuged, and the solid washed well with water and air-dried. The A1 content of the remaining solution was estimated colorimetrically (alizarin method). Each solid was also treated with 0.2mol dm-3 HCl, which extracted A1 (to leave a-SnP) and again analysed for Al. The two methods gave identical results. Chlor-SnP Chlorhydrol(l.28 g) in 50 cm3 of water was added to colloidal NMe,-SnP formed from 1 g of NMe,-SnP and the suspension stirred for 1 day.The solid was separated from the equilibrium solution by centrifugation (15 000 rpm, 10 min), washed well with water and air-dried. In a second series of preparations, after contacting, the suspension was dialysed (Visking 9-36/32" tube) against water with four daily changes to a conductance of remaining water of 20 pS. The dialysate was then filtered on a milli- pore and dried in air. This material is described in detail elsewhere.12 Calcination to Alumina-containing Products The two final materials were calcined at various temperatures and basal-spacing changes were monitored by XRD (Fig. 3, later). The products obtained after calcination at 400 "C were those used in conventional surface-area determinations (together with the initial a-SnP, and the intercalated precursor material).Cation exchange on these same products was carried out with conventional batch methods5 with metal@) acetate solutions. Physical Measurements XRD patterns were registered on a Siemens D-501 diffractometer both on powders and on cast films. Surface areas were measured with a Carlo Erba Sorptomatic 1800 instrument. Samples were first degassed at 150 "C, and adsorp- tion-desorption of N2 followed at 77.4 K. Electronic spectra, as reflectance, were mezwred on Beckmann DK 2A and Shimadzu MPC-3 100 spectrophotometers against BaSO, or MgO as reference. TG and differential thermal analysis (DTA) data were recorded on a Rigaku Thermoflex, using calcined alumina as a standard.Results and Discussion When either aged All, oligomer solution or Chlorhydrol solution is contacted with colloidal a-Sn HnPrA *HP there is only low, disordered, intercalation of the polyhydroxy cation. Conversely, preliminary exchange with "Me,] provides a + simple preparative route for intercalated precursors, with complete removal of the "Me,] after contacting. + J. MATER. CHEM., 1991, VOL. 1 B 17.4 3) A 11.1 281" 8.6 c)-11.1 d, 10.5A6 10 14 281" Fig. 1 Cu-KaXRD patterns (A) starting a-Sn-HnPrA-HP (B) a-Sn[NMe4]o~,-,~oH,,~-l~o(P04)2.4H@ precursor, showing phase changes on dehydration. T/ "C:(a)25, (b) 100, (c) 200, (d)300 As shown by the XRD (Fig. 1) the solid has an interlayer distance that is much larger (17.4A) than expected on the basis of the diameter of "Me,]' (5.3 A) alone.The assump- tion that a-SnP has a layer thickness roughly the same as that of a-Zr(HP04)2H20, i.e. 6.5 A: leads to an expected interlayer distance of ca. 11.8 A. The material has a large amount of interlayer water, most of which is removed at 200 "C with collapse from 17.4 to 11.1 A. This may be rationalised by assuming that there is ordering of water +molecules between the "Me,] and layer -P-OH groups, as demonstrated for a-VO(P04)*5H20.i3 As expected, at 300 "C there is a final collapse to mixed phases of a-SnP and a 10.4A phase in which the "Me,]' resides in the cavity formed by groups of interlayer -P-OH., The uptake of the two polyhydroxy cations into a-SnP is similar at lower polyhydroxy cation :a-SnP ratios, but diverges at higher ratios (Fig.2). Two well known phenomena may be in operation: (i) A change in nuclearity caused by pH changes. An increase in pH is expected to increase the degree of hydrolysis:' J. MATER. CHEM., 1991, VOL. 1 321 -c E mol Al moiety added/g Me,NSnP Fig. 2 Uptake of [A104Al,,(OH),4(OH,),217 + by NMe4-SnP from AlC1,-NaOH mixed solutions (-) and from Chlorhydrol solutions (---) (assuming that the sole species present is the Keggin ion) +[A1 1304(0H)24(0H2)121 + OH -[A1 04(OH),,( OH ,) 1] + +H 0fo" -CAI 13°4(0H)26(H20) 101 + 2O which, in turn will increase the number of A1 species exchanged. (ii) A change in nuclearity; NMR measurements show that species of higher nuclearity, e.g.[A116(0H)36(H20)24]1Z+,and other unidentified one^,^.^^ exist. Exchange of such species is expected to give rise to a decrease in A1 species uptake. Equilibrium pH values were in both cases comparable (4.1-4.5) so the uptake differences are more readily imputed to the presence of counter-ions. Thus, the Na+ present in the All, oligomer solution will partially exchange for NMel on the open phosphate surfaces and is subsequently less easily exchanged. The lower surface charge leaves fewer sites available for exchanging A1 species, as observed. Other effects are also operating, as is clear from the XRD of Fig. 3 and the associated TG/DTA analyses of Fig. 4. In the clay-pillaring studies reported to date, the interlayer spacing of the smectite increases from ca.9.3 to 18-19 A.This has been taken naively to mean that the ion intercalated is indeed [A10A112(0H)24(0H2)12]7+,1~a prolatewhich has ellipsoidal shape with a long axis of ca. 9.5 A.16Al13-SnP more closely fits this picture than does chlor-SnP, the expan- sion being 17.7-7.8 A =9.9 A. However, insertion of Chlorhy- A 12.3 ,j '-' 13.1----------._ _-___. (c) 14.4 :: II ; .>, .I ' IIIIIIII ,,,,,,,1116 12 8 4 16 12 8 4 2eto Fig. 3 Variation of XRD (Cu-Kcr) of (A) All,-SnP and (B) chlor-SnP with temperature: (a) 25, (b) 200, (c)300, (d) 400 and (e) 500 "C I I IIII I II 7°C Fig.4 TG and DTA curves of: (a) Chlorhydrol; (b) All,-SnP (c) chlor-SnP.(-) Weight loss; (---) DTA drol leads to an expansion of 16.4 A,suggesting that either a bilayer is formed or the pH conditions on the phosphate surface (about which little is known) are different from those in the Na+-buffered surface of the Al13 oligomer case. This can give rise to more than one cluster species via hydrolysis (Scheme 1). The evidence points to both effects being present. Thus, after calcination, apart from the characteristic dOo2reflection, the chlor-SnP materials give broad do04 reflections, indicating that a second phase is present. Secondly, in both materials, the water molecules are removed smoothly up to ca. 400 "C (in itself unusual, there is no clear evidence for a distinction between intercalate water and zeolitic-type water).However, chlor-SnP collapses by ca. 8.7 A at 200 "C, whereas Al,,-SnP collapses by only 3.3 A. More water is lost with much smaller changes in interlayer distance, and these are then followed by a broad, composite exotherm in chlor-SnP (yet further evi- Scheme 1 (a) Possible bilayer formulation, showing coalescence of precursor species after calcination. (b) Hydration sphere formulation, showing possible hydrogen-bonding to layers. (Cross-hatched, hydroxide; hatched, oxide) dence for the presence of more than one intercalate precursor) which is much less evident in Al13-SnP. Note that aluminium hydroxides and oxyhydroxides give endotherms in DTA; that for Chlorhydrol is shown for comparison in Fig. 4 (see also ref.17). Furthermore, the TG/ DTA curves show no evidence for a PO:- >P20$- conden- sation (expected to lie at 450-500 "C). Moreover, treatment with 0.2 mol dmP3 HCI solutions did not lead to changes in the basal spacings of the materials calcined at 400 "C. Table 1 provides an estimate of the differing pillar densities in both calcined materials on the assumption that [A10,A1,2(0H)24(0H2)12]7+ is the entering species. Surface Area and Pore Distribution N2 adsorption-desorption isotherms for the intercalates and the pillared materials [shown by the temperature of calci- nation, e.g. Al,,-SnP (400 "C)] are shown in Fig. 5. a-SnP itself is non-porous, with a B.E.T. surface area of 11 m2 g-', whereas the intercalate precursors and pillared materials are porous, the former having B.E.T.surface areas of 80-90 m2 g-' and the latter 190-230 m2 g-'. The many hazards in partitioning porosity effects are well documented, as exemplified by the extensive literature on carbons.18 The pore volumes obtained are considered appar- ent pore volumes because their shapes are not known.lg The curves were analysed using both Pierce's method (closed cylindrical pore model)20 and Cranston and Inkley's method (open cylindrical pore model).'l Both the 't'22 and the a, method23 were used for assessing the presence of microporos- ity. For both, a-SnP itself was used for the reference isotherm, and application of the a,-method, as shown in Fig. 6, demon-Table 1 Analyses of Keggin-type intercalatesn ~ _______~ A1 A120, no.of oligomer (mmol All, H20' (mmol g-' units per unit per g SnP) (YO) (SnP) celld A1 ,-SnP 1.97 30.14 2.95 0.4 chlor-SnP 3.05 30.05 4.49 0.6 In all cases, organic matter <1%; using alizarin S; 'from the TG of Fig. 4; assuming the radius of A1,,04(0H)24(0H2)12]7+=ca. 5 A, and using a= 5.02 A, b=8.61 A for u-S~(HPO,)~.H,O, the oligomer intercalate occupies ca. 32/86.4 =40% of the area ab. J. MATER. CHEM., 1991, VOL. 1 0.2 0.4 0.6 0.8 1.0 PIP, Fig. 5 Adsorption-desorption isotherms of N2 on (a) Al,,-SnP (400 "C); (b) chlor-SnP (400 "C) 200 160 7 I m "E 120 0.iT I-5 Y)D 80 1.o 2.0 US Fig.6 Application of the us-method to the N2 adsorption curves. (a) u-SnP (reference); (b) chlor-SnP (400 "C); (c) All,-SnP (400 "C) strates that this choice was valid.The adsorption data reveal that although all the materials are predominantly mesoporous, there is some contribution by micropores in the pillared (i.e. calcined) materials. Table 2 lists the main textural parameters obtained. For both calcined materials, most pores (>50% of B.E.T. surface area) have radii in the range 15-40 A, as seen in Fig. 8 (later). Note that: (i) the radii of Fig.8 are Kelvin radii, and application of the Kelvin equation breaks down at fp<ca. 10 i.e. above the range of micropores expected in pillared materials; (ii) the range which constitutes micro- and meso-porosity is controversial. Mesopores have been defined J.MATER. CHEM., 1991, VOL. 1 323 Table 2 Surface parameters of a-tin phosphate, aluminium oligomer exchanged tin phosphate and alumina pillared tin phosphate material S,.,.,./m2 g-' S,/m2 g-' * S,/m2 g-' K/cm3 g-' Vmic/cm3 g-' a rplAb a-SnP 11.0 -12.0 0.052 -86.7 A1 ,-SnP 89.5 -80.9 0.320 -79.1 All,-SnP (400 "C) 228.1 197.0 193.7 0.460 0.022 47.5 chlor-SnP (400 "C) 190.0 158.2 141.5 0.295 0.019 41.7 a Sing's method: Vmic=micropore volume (a,-plot); S, =specific surface area. Pierce's method: S, =accumulated surface area; V,=accumulated volume; rp =average pore radius. as having a width 20-5OOA (the 20-30A range being am- biguousZ3) and the pores of interest in pillared clays have been defined as those with diameter <30 A.24Mesopores are believed to be induced by platelet stacking or end-end interactions, as shown in Scheme 2(~).'~ Given the probable more disordered lateral pillaring, coupled with the suscepti- bility of a-SnP to hydrolysis, Scheme 2(b) seems the more realistic.26 In other words, pillaring may induce higher porosity at the lower end of the mesopore range, which is not rationalisable from pore sizes deduced from XRD and (idealised) pillar density. From Fig.7, it appears that more such sites have been induced in chlor-SnP (400 "C) than in AlI3 oligomer-SnP (400 "C). Cation Exchange and Site Accessibility The sites in the interlayer are expected to be modified by pillaring. Fig. 8 shows the uptake curves for Co2+,Ni2+, and I 0 10 30 50 70 90 r,/A I 0 10 30 50 70 90 r,/A Fig.7 Pore size distribution curves. (a) All,-SnP (400 "C); (b) chlor-SnP (400 "C);(-) Pierce's method;" (------) Cranston and Inkley's met hod2 Scheme 2 End-side particle interactions giving rise to mesopores. (a) Ideal regular pillaring; (b) with 'cut' layers due to hydrolysis (or other long-range disorder) c2u02 6 10 M2+lg(ad.) 0.2 '2 6 10 M2+/g(ad.) M2+/g(ad.) Fig. 8 Uptake curves of Co2+ (M), Ni2+ (A)and Cu2+ (0)by Al,,-SnP (400 "C)and chlor-SnP (400 "C), (a) by chlor-SnP (400 "C) containing two different concentrations of the precursor: (0)point (i); (0)point (ii); (b) by All,-SnP (400 "C); (c) by All,-SnP (400 "C), first treated with NH3 gas Cu2+ (in no case was there evidence for pillar elution).A comparison of Cu2+ uptake by calcined materials at points (i) and (ii) of the Chlorhydrol uptake curve (see Fig.2) underlines the dramatic difference caused by in-layer pillar ordering: uptake of Cu2+ is almost an order of magnitude lower by material (ii) than by (i). The uptake of Cu2+ by Al,,-SnP (400°C) is very similar in magnitude to that of chlor-SnP (400 "C) [see Fig. 8(c)]. However, the exchange is faster than in the Chlorhydrol analogue, which is in agreement with the (naive) calculation of A1203 pillar density shown for the former in Table 1, and against changes in ion exchange being due to differences in inter-stacks such as those in Scheme2(a). The influence of specific hydrogen-bonding in the interlayer also needs to be taken into account.According to Va~ghan~~ calcining All,- exchanged clay in an NH, atmosphere leads to maximum cation-exchange capacity for the PILC (pillared layered clay) produced. Presumably, interlayer ammonium ions are pro- duced that are no longer available for migration into vacancies in the octahedral part of the framework, thus enhancing the c.e.c. of the interlayer. Although the single interlayer of the phosphate is very different than in a PILC,26 Fig. 7(c) shows that All,-SnP c.e.c.s are enhanced by almost an order of magnitude on prior ammoniation. They are much higher than those in smectite clays (0.5-1.5 mmol g-', ref. 27). Ammoniation also causes an inversion of uptake rates between Cu2+ and Co2+.In studies of pore accessibility in complex-pillared zirconium phosphates carried out to date, the order has been found to be: Cu2+ >Coz+ >Ni2+.28 The reverse order obtained here suggests that the species exchang- ing into the All,-SnP material are the hydrates, M(OH,);+ (M =Co2+, Ni2+) and Cu(OH2)i+, which is further evidence for easy accessibility on pillaring. Particularly clear evidence that different sites are present, as well as different accessibilities, comes from Ni2 +-exchanged AI A J. MATER. CHEM., 1991, VOL. 1 NH,-treated chlor-SnP (400 "C) and AlI3-SnP (400 "C) (Fig. 9). Two sites are present, one pseudo-octahedral, as shown by weak d-d bands at 9600 and 13 500 cm-' ascribable to ,T1 t,A2 transitions of a cis rather than a trans geometry,29 and application of the classical average environment rule leads to the conclusion that this species contains an [NiN303] chr~mophore.~'The major Ni2 + site is, however, represented by that giving a d-d band at 16 700 cm-', an energy too low to be due to a square-planar [NiN4] chromophore (e.g. 'AZg+-'Alg, 22 500 cm-I for [Ni(NH3)4]2+)31 but applicable in terms of an [Ni04] chromophore deriving from weak (in ligand-field terms) PO:- ligands. On removal of all water and NH,, almost all are no longer present and there is only a distinct shoulder at high energy: 24 400 cm-'.Such a high energy can be due only to a square-planar [Ni04] unit in which the oxygen donors are provided by the A1203 pillar.32 In All,-SnP (400 "C), Ni2+ is clearly in a very different site [Fig.9(b)] with a low-energy d-d band, at 7700 cm-' [com-pare with Ni(OH2);+ having v1 at 8500 cm-', ref. 301. When taken together with the low energies of the other d-d bands characteristic of a pseudo-octahedral environment, this clearly indicates that the Ni2+ is in a constrained environment of six oxygen atoms provided by phosphate groups. On complete dehydration and deammoniation, the site geometry changes drastically, the spectrum being very reminiscent of the pseudo- trigonal bipyramidal [Ni05] site present in low-Ni2 +-loaded a-zirconium ph~sphate.~, These sites are completely different from those available in a-SnP it~elf.~ WJIIIII'1IIIII1IIII11I m LIIJnm 006 11 IJnmFig.9 Electronic spectra of Ni2+-exchanged pillared materials (first treated with NH, gas). (a)Chlor-SnP (400 "C);(b)All,-SnP (400 "C); Fig. 10 Electronic spectra of Co2+-exchanged Al,,-SnP (400 "C), (a) (-) as prepared; (---) after heating at 350 "C (d-d band positions without and (b)with prior adsorption of NH, gas: (-) as prepared; shown in cm-') (---) after heating at 350 "C (d-d band positions shown in cm-') J. MATER. CHEM., 1991, VOL. 1 The spectra of Co2+-exchanged species also show that the sites accessible are different from those in the non-pillared material (Fig. 10). Co2+-exchanged Al13-SnP gives d-d bands characteristic of a pseudo-octahedral en~ironment,~~ which do not change to those of a pseudo-tetrahedral moiety on dehydration.Conversely, the first-ammoniated material [sep- arated at the plateau of Fig. 7(c)] also has a pseudo-octahedral Co2 site, which on dehydration and deammoniation changes + to a pseudo-tetrahedral site, [Coo,]; the low energy of the d-d bands suggesting that the Co2+ is bonded to the layer phosphate groups. Turning to the Cu2 +-exchanged materials, there seems little obvious difference between the chlor-SnP and Al,,-SnP mater- ials. Both give broad d-d bands at 13 000-14 000 cm-', apparently deriving from a single site. The relatively low energy is as expected for a tetragonal-octahedra1 geometry. Little change occurs on either dehydration or calcination, the d-d bands shift to lower energy. We note that Cu2+/a-A1203 gives very-low-energy d-d bands due to the presence of both tetragonal-octahedral and pseudo-tetrahedral sites.34 We infer that the Cu2+ loads at the layer sites.Conversely, at higher loading levels d-d bands broaden on calcination [Fig. ll(c)], suggesting that Cu2+ co-ordinates to more than one site. Finally, Fig. 12 shows the reflectance spectra of several low- 1, 13100 \(b) 12500 I -. 'L.-/\--11111111111111111 1 1 1 1000 2000 Ilnrn Fig. 11 Electronic absorption spectra of Cu2+-exchanged pillared materials at different temperatures: (-) 25 "C; (---) 120 "C; (. -.-.-) 350 "C. (a) Low-loaded Cu'+-exchanged chlor-SnP (400 "C); (b) Low-loaded Cu2+-exchanged Al,,-SnP (400 "C); (c) High-loaded Cu2+-exchanged AI,,-SnP (400"C) (d-d band positions shown in cm-') 16 300 17 100 Ilrllllll,llllll,l 1000 2000 Ilnrn Fig.12 Electronic absorption spectra of Cu2+-exchanged pillared materials, before (-) and after (-----) amine loading. (a) 150% c.e.c. after absorption of NH, (3 h, 25 "C); (b) 150% c.e.c. after absorption of en (3 h, 50 "C); (c) as for (b), 10% c.e.c. (d-d band positions shown in cm- ') and high-loaded Cu2+ samples before and after contact with NH3 or ethane-1,2-diamine (en). 10% c.e.c. Cu2 +-chlor-SnP has a low-intensity band at 13 100cm-', enhanced in the 150% c.e.c. Cu2 +-exchanged analogue. After absorbing en, the former gives rise to a new band at 17 100 cm-' and the latter one at 16 300 cm-' with a clear shoulder at 10 500 cm-'. (Compare with a d-d band at 14 300 cm-' in or-Sn[Cu(en),~,](P04)2 *2.2H20 itself, for which a very restric- ted site accessibility was suggested.'} Instead, the d-d energies indicate that two sites are present and both allow access to bi- dentate en.Similarly, after the absorbance of NH3, the d-d band position in Cu2+-exchanged chlor-SnP, 15 650 cm-' (with unclear shoulder at ca. 11 000 cm-'), is very different from that in the [Cu(NH3),J2'-SnP analogue.' Again, we suggest that the interlayer Cu2+ is more accessible in the former and the electronic spectrum is consistent with a five- co-ordinate moiety (although probably one containing four rather than five NH3 molecules). Conclusions The tridecameric polyhydroxyaluminium cation [A104A112( OH)24(OHZ)12] can be intercalated into or-tin + phosphate, both from Chlorhydrol solutions and as the in situ prepared moiety.Two different alumina pillared materials are obtained on calcination, and surface measurements show that both have high specific surface area (190-228m2 g-'). They are predominantly mesoporous but some microporosity J. MATER. CHEM., 1991, VOL. 1 They are predominantly mesoporous but some microporosity induced by the pillaring is present. Further work is necessary to unravel how lowered layer charge influences pillar density and effects due to hydrolysis. The contribution to porosity of amorphous components also requires investigation. Cation exchange demonstrates that accessibility of acid sites in the interlayer is quite high, especially when enhanced by exchange with a better leaving group, such as NHZ. A possible explanation is that the cleaning effect of ion exchange may remove decomposition products, which block narrower pores, that pillar-pillar gaps are extremely critical and that access to large areas of the surface is forbidden for nitrogen.13 14 15 16 17 A. Jimenez-Lopez, L. Alagna and A. A. G. Tomlinson, Eur. Pat., applied for. L. Alagna, D. Attanasio, T. Prosperi and A.A.G. Tomlinson, J. Chem. SOC., Faraday Trans., 1989,85, 689. J. H. Patterson and S. Y. Tyree Jr., J. Colloid Interface Sci., 1973, 43,389; J. D. Hem and C. E. Robertson, U.S. Geol. Survey Water Supply paper., 1967, 1827-A, p. 55; J. W. Akitt and A. Farthing, J. Chem. SOC.,Dalton Trans., 1981, 1617; 1981, 1624; N.Lahav, V. Shani and J. Shabtai, Clays Clay Mineral, 1978, 26, 107. G. Johansson, Acta Chem. Scand., 1960, 14, 769. T. J. Pinnavaia, M-S. Tzou, S. D. Laudau and R. H. Raythatha, J. Mol. Catal., 1984, 27, 194. Minerals in Soil Environments, ed. J. B. Dixon and S. B. Weed, Soil Science Society of America, Madison, Wisconsin, USA, 1977. Further investigations of the pore properties of these, and other alumina-pillared materials are underway. 18 F. Rodriguez-Reinoso and A. Linares-Solano, in Chemistry and Physics of Carbon, ed. P. A. Thrower, Marcel Dekker, New York, 1988, vol. 21, p. 1. We are extremely grateful to the E.E.C. EURAM Programme (Contract No. MA 1E/0027/I) and C.A.I.C.Y.T. (Project No. PB86/244) for their financial support. 19 20 21 22 P.J. M. Carrott and K. S. W. Sing in Characterisation of Porous Solids, ed. K. Unger, J. Rouquerol, K. S. W. Sing and H. Kral, Elsevier, Amsterdam, 1988, p. 77. C. Pierce, J. Phys. Chem., 1953, 57, 149. R. W. Cranston and F. A. Inkley, Adu. Catal., 1957, 9, 143. A. Lecloux and J. P. Pirard, J. Colloid Interface Sci., 1978, 70, 265. References 23 S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and 1 R. M. Barrer, J. Inclusion Phenom., 1986, 456; Catalysis Today. 24 Porosity, Academic Press, London, 1982, p. 98. D. E. W. Vaughan, in Catalysis Today, Pillared Clays, ed. 2 Pillared Clays, ed. R. Burch, Elsevier, Amsterdam, 1988. M. L. Occelli, R. A. Innes, F. S. S. Hure and J. W. Hightower, 25 R. Burch, Elsevier, Amsterdam, 1988, p. 190.T. J. Pinnavaia, in Heterogeneous Catalysis, ed. B. Shapiro, Texas J. Appl. Catal., 1985,14, 69; D. E. W. Vaughan, R. J. Lussier and A & M University Press, College Station, TX, 1985, p. 85. J. S. Magee Jr., US. Pat., 4 271 043, 1981, to W. R. Grace and 26 A. A. G. Tomlinson, in Pillared Layered Structures. Current Co., D. E. W. Vaughan and R. J. Lussier, 5th Int. Con$ Zeolites, Trends and Applications, ed. I. V. Mitchell, Elsevier Applied 3 Naples, Italy, Heyden, London, 1980. A. Clearfield, in Surface Organometallic Chemistry: Molecular 27 Science, Amsterdam, 1990, p. 91. Crystal Structures of Clay Minerals and Their X-Ray Identifi- 4 Approaches to Surface Catalysis, Kluwer, Dordrecht, 1988,p. 271. G. Alberti, in Intercalation Chemsitry, ed.M. S. Whittingham, cation, ed. G. W. Brindley and G. Brown, Mineral. SOC., London, 1980. 5 Academic Press, New York, 1982. C. Ferragina, M. A. Massucci and A. A. G. Tomlinson, J. Chem. 28 C. Ferragina, A. La Ginestra, M. A. Massucci, P. Patron0 and A. A. G. Tomlinson, J. Phys. Chem., 1985,89,4762. SOC., Dalton Trans., 1990, 1191 and refs. therein; L. Alagna, 29 A. A. G. Tomlinson, M. Bonamico, G. Dessy, V. Fares and A. A. G. Tomlinson, E. Rodriguez-Castellon, P. Olivera-Pastor L. Scaramuzza, J. Chem. SOC., Dalton Trans., 1972, 1672. 6 and S. Bruque, J. Chem. SOC., Dalton Trans., 1990, 1183. L. Alagna, A. A. G. Tomlinson, P. Maireles-Torres, P. Olivera- Pastor, E. Rodriguez-Castellon and A. Jimenez-Lopez, J. Chem. 30 31 C. K. Jorgensen, Absorption Spectra and Chemical Bonding in Complexes, Pergamon, Oxford, 1962. R. Stomberg, S. Svensson and A. A. G. Tomlinson, Acta Chem. SOC., Chem. Commun., 1989, 751. Scand., 1973, 1672. 7 A. Clearfield and B. D. Roberts, Inorg. Chem., 1988, 27, 3237. 32 A. B. Lever, Inorganic Electronic Spectroscopy, Elsevier, Amster- 8 9 S. Cheng and T. C. Wang, Znorg. Chem., 1989,28, 1283. E. Rodriguez-Castellon, A. Rodriguez-Garcia and S. Bruque, 33 dam, 2nd edn., 1984, p. 535. L. Alagna, A. A. G. Tomlinson, C. Ferragina and A. La Ginestra, Mater. Res. Bull., 1985, 20, 115. J. Chem. SOC., Dalton Trans., 1981, 2376. 10 M. Tokarz and J. Shabtai, Clays Clay Mineral, 1985, 33, 89. 34 R. A. Friedman and B. Freeman, J. Chem. SOC., Faraday Trans., 11 G. Alberti, M. Casciola and U. Costantino, J. Colloid Interface 1972, 568. Sci., 1985, 107, 256, 12 P. Maireles-Torres, P. Olivera-Pastor, E. Rodriguez-Castellon, Paper 0/03600E;Received 6th August, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100319

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(3), 327-330 6Li Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy: A Powerful Probe for the Study of Lithium-containing Materials Stephen P. Bond, Andrew Gelder, John Homer, William R. McWhinnie* and Michael C. Perry Department of Chemical Engineering and Applied Chemis trx As ton Universitx Aston Triangle, Birmingham 84 7ET, UK A narrow chemical-shift range has been established for a variety of lithium compounds via study of their 6Li magic angle spinning nuclear magnetic resonance (MAS NMR) spectra. Comparative studies of 6Li and 7Li spectra (MAS, and 'off-angle' spinning) establish that, for solid-state (and even solution) analytical purposes, 6Li is the preferred nucleus, since the advantage of narrow absorption lines outweighs the poorer sensitivity of 6Li relative to 7Li.6Li MAS NMR spectra have been obtained for laponite clay, which had been thermally treated at 200,400, 600, 800 and 1300 "C; at the lower temperatures (<800"C) both dehydrated and rehydrated specimens were considered. The data are consistent with mobility of lithium ions from the trioctahedral clay sites at 600 "C. Both conventional and microwave methods were used to prepare lithium-exchanged laponite. The superior resolution achievable in 6Li MAS NMR in comparison with 7Li MAS NMR is demonstrated with the microwave specimen where use of 6Li spectroscopy revealed two lithium sites. On storage of the sample for 3 months, the two sites give way to a single lithium environment. Possible causes are discussed.Keywords: Magic angle spinning nuclear magnetic resonance spectroscopy; Laponite; Mobility in clays; Lithium6 ; Ion exchange; Microwave heating The use of 7Li NMR spectroscopy is well established in solution studies. The nucleus has a high natural abundance (92.5%) and a favourable receptivity (1 500, 13C =1.OO); how-ever, the quadrupole moment (-4.5 x e m2) can give rise to broad lines in non-cubic environments. It is, therefore, sometimes profitable in solution studies to examine 6Li spectra despite the lower abundance (7.5%) and the less favourable receptivity (3.58, 13C =1.00) as the quadrupole moment is also significantly lower (-8 x e m'), and narrower spectral lines can The use of 6Li rather than 7Li for solid-state MAS NMR studies has been less widely explored.In this paper information is presented which supports the view that, for solid-state work, 6Li is the preferred nucleus for study because the narrower lines may enable resonances of similar frequency to be resolved using 6Li MAS NMR when an apparent singlet may be seen in the 'Li MAS NMR spectrum. Experimental Lithium compounds were obtained from commercial sources: e.g. LiCl, LiBr, LiI (Aldrich), lithium metasilicate (Pfalz and Bauer). Lithium (12-crown-4) bromide was prepared by a literature method' (Found: C, 36.5; H, 6.03%. CsH16BrLi04 requires C, 36.5; H, 6.08%). Materials 8-Lithoxoquinoline sesquihydrate. 8-Hydroxyquinoline (15 g, 0.103 mol) was treated with an ethanolic solution of lithium ethoxide (0.103 mol).A precipitate was formed which was filtered from the yellow solution and Soxhlet extracted with ethanol. The resulting yellow solution was evaporated to afford yellow crystals, which were washed with cold ethanol and dried in vacuo; yield 60%; m.p. >300 "C (Found: C, 67.4; H, 4.54; N, 8.62; 0, 15.2%. C9H7LiN01.5 requires C, 67.5; H, 4.38; N, 8.76; 0, 15.0%). Laponite RD (laponite in this paper) was obtained from Laporte Industries Ltd. The thermal treatment, and rehy- dration (200, 400, 600, 800, 1300 "C) was carried out exactly as described in an earlier paper,6 using a Carbolite furnace. The clay was ion exchanged with lithium both using a conventional method7 and by treating 1 g laponite with an aqueous solution (10 cm3) of 1 mol dmP3 LiCl in a screw- capped Teflon container (Savillex Corporation, Minnetonka, Minnesota, USA) and heating for 5 min in a Sharp Carousel domestic microwave oven (650 W, med.-high setting in five 1 min bursts). Physical Measurements 6Li and 7Li MAS NMR spectra for powdered specimens were measured using a Bruker AC(E) 300 MHz spectrometer.The 'magic angle' was set with KBr and samples were packed into Delrin rotors and spun at ca. 5 kHz. The observation frequen- cies were 44.168 MHz (6Li) and 116.644 MHz (7Li). Apparent chemical shifts for both nuclei were measured with respect to a saturated aqueous solution of LiC1. Some spectra were obtained by spinning off the 'magic angle', and for 8-lithox- oquinoline sesquihydrate comparisons of line width for MAS NMR spectra were carried out for 6Li and 7Li. Results and Discussion Table 1 contains comparative data for the 6Li and 7Li nuclei, Table 2 reports 6Li chemical-shift data for some lithium compounds, Table 3 records data obtained from laponite heated to the indicated temperatures, and Table 4 gives data for Li-exchanged laponite.Following heat treatment and cooling to room temperature, half of each sample was treated with distilled water for 1 h, after which, following decantation of excess water, it was air dried for 1 week. Chemical-shift data for both dehydrated and rehydrated laponite are given in Table 3. The purpose for considering 6Li MAS NMR as an analytical tool arose from current work with lithium-containing minerals Table 1 Measurements of linewidth for 6Li and 7Li resonance lines under a variety of experimental conditions compound/measurement conditions G(ppm)” FWHM/Hzb 84ithoxoquinoline sesquihydrate 6Li MAS (dipolar decoupling, ‘H) 6Li MAS (no dipolar decoupling) 1.397 128 135 %i ‘off angle’ spinning 7Li MAS -1.881 -0.437 750 926 7Li static 5150 lithium (12-crown-4) bromide in D20 6Li solution (linewidth measurements made with high digital resolution) 0.504 0.39 7Li solution (linewidth measurements -0.169 0.46 made with high digital resolution) ‘‘Apparent’ chemical shift, i.e.at experimental peak maximum; FWHM =full width at half maximum. Table2 Some 6Li chemical-shift data obtained from MAS NMR studies of lithium compounds compound G(ppm us.satd. aq. LiCl) LiI -2.312 --2.152LiBr -0.359 laponi te -0.735 Li,Si03 0.200 Li( 12-crown-4)Br 0.659 Li(C,H6NO) * )H20 1.397 Table3 6Li MAS NMR data for thermally treated laponite and for Li +-exchanged laponite G(ppm us. satd. aq. LiCl) voc heated specimen rehydrated -room temp. -0.735 200 -0.63 1 -0.8 1 400 -0.395 -0.52 600 -0.227 -0.12 800 -0.145 0.05 1 1300 -0.077 e.g. laponite. It was considered prudent to survey some solid-state 6Li data for a variety of ‘simple’ compounds. Accordingly, the range of materials listed in Table 2 was considered. Before the experimental data in Tables 2 and 3 can be addressed it is necessary to point out that comparisons When the nucleus is located in a ligand field of less :.spin of J.MATER. CHEM., 1991, VOL. 1 between 6Li and 7Li spectra are not simple. 7Li has a nuclear than cubic symmetry [i.e. when the electric field gradient (EFG) is >O], the maximum of the -+++ transition observed in the MAS spectrum does not correspond to the isotropic chemical shift since spinning at the ‘magic angle’ does not average second-order quadrupole effects to zero. The situation for 6Li is different. The nuclear spin is integral (I= 1) and the quadrupole effect would be expected to produce a doublet for an orientated 6Li sample. The effect of magic angle spinning on such spectra is not well documented and this paper attempts to address this problem.In Table 1 some comparisons are made between 6Li and 7Li resonances in the same compounds. For MAS NMR spectra the dipolar broadening should be eliminated so that remaining effects should be dominated by quadrupole interac- tions, with 6Li and 7Li subject to the same EFG. Spinning off the ‘magic angle’ will introduce dipolar broadening but because the 6Li has a lower gyromagnetic ratio than 7Li the dipolar effect should be less on 6Li than on 7Li. 8-Lithoxo- quinoline sesquihydrate was used to investigate these points. Although the structure of this compound is not known, the symmetry of the lithium environment must be low. The 6Li MAS NMR spectrum of the compound has a FWHM= 128 Hz which increases to 135 Hz in the absence of proton dipolar coupling (a result not inconsistent with the co-ordi- nation of water).Spinning slightly off the ‘magic angle’ does introduce dipolar broadening. The 7Li MAS NMR linewidth is greater (some 6.86 times), as expected, than that for 6Li in the identical chemical environment (the ratio 7Li :6Li of the quadrupole moments is 56.25). For 7Li, the MAS experiment produces a 5.56-fold narrowing of the static linewidth. Attempts were made to obtain solution spectra for lithium in a low-symmetry environment. Solubility considerations narrowed the choice to lithium (12-crown-4) bromide; how- ever, it seems from the very narrow lines observed that the D20 solutions contain Li(D,O); species, thus the data are not comparable to those obtained from solid materials reported in Table 2.Despite this, measurement of linewidths under conditions of high digital resolution indicate that, even in the liquid phase, 6Li shows lines sharper than the corre- sponding ones from 7Li, and in the example cited the 7Li line is 18% broader than the 6Li line. Thus the data of Table 1 establish that, under all experimental conditions considered, 6Li gives narrower lines. Since neither series of spectra will give isotropic chemical shifts directly from the observed resonance maximum, the direct comparison of 6Li and ’Li ‘chemical shifts’ is not valid. However, comparisons of 6Li ‘chemical shifts’ measured under the same conditions should provide chemical information. Prior to using 6Li for this purpose information was sought regarding the range of 6Li chemical shifts. Table2 presents Table4 Some 6Li and 7Li NMR data on Li+-exchanged laponite material (experimental conditions) 6Li MAS (microwave method; fresh specimen) 6Li ‘off anglelc (microwave method; fresh specimen; small departure from magic angle) 6Li ‘off angle’ (microwave method; fresh specimen; large departure from magic angle) 6Li MAS (microwave method; aged specimen, 3 months) 6Li MAS (conventional method7) 7Li MAS (microwave method)d ’Li ‘off angle’ (microwave method;d small departure from magic angle) ‘Chemical shift, i.e.position of resonance maximum; FWHM =full width at half maximum; was used for both 6Li and 7Li spectra; freshly prepared specimens. S”(PPm) FWHMb/Hz -0.192 25.0 -0.438 (dv= 11.3 Hz) Av=11.9Hz 37.1 broad singlet 86.8 -0.179 37.7 0.232 -0.316 86.5 broad singlet 267 the same small departure from the ‘magic angle’ J.MATER. CHEM., 1991, VOL. I some data obtained for this purpose. The data of Table 2 do imply that the lithium is most shielded when it is more covalent (LiI); interestingly, the most 'ionic' of the samples considered was Li(CgH6ON) *3H2) which must imply that co- ordination of the nitrogen lone pair is, at best, extremely weak. Table 2 establishes a narrow chemical-shift range for Ti. The data for LiBr are of some interest. In the initial stages of the accumulation of the spectrum, a single resonance was observed. However, as the experiment proceeded a split resonance was seen with the parameters reported in Table 2.The new, weaker, component was the more deshielded and is attributed to LiBr (as) reflecting the hygroscopic nature of the salt; the more shielded resonance arises from LiBr. Laponite, a synthetic smectite clay (the closest natural counterpart is hectorite) contains ca. 0.62% lithium when obtained in the normal sodium-exchanged form: Nao.67 (Lio.67Mg5,33)Si8020(oH)4.The thermolysis of this mineral was recently the subject of a combined NMR (29Si, 23Na) and X-ray diffraction study, and it was thus of interest to examine the 6Li NMR spectra of specimens subjected to the same heat treatment. Despite the low lithium content, the natural- abundance MAS NMR spectra were of excellent quality and were obtained without excessive demands on instrument time; the FWHM of the spectral lines was of the order of 62 Hz.The 29Si MAS NMR study of laponite thermolysis sug- gested that even at 400 "C some loss of crystallinity and even breakdown of the silicate structure could occur.6 However, the effects seemed almost reversible on rehydration. The 6Li data in Table 3 reflect this quite well. Thus, on heating to 400 "C there is a deshielding of the lithium, possibly mainly caused by dehydration of the interlamellar sodium ions, since, on rehydration, the lithium undergoes an upfield shift to values closer to the -0.735 ppm characteristic shift of untreated laponite. The earlier study6 suggested that, on heating laponite to 600 "C, the lithium ions migrated from the trioctahedral sites to edge sites.In this instance, rehy- dration leads to a downfield shift consistent with the formation of aquated lithium ions on edge or interlamellar sites. By 800 "C the silicate network has essentially broken down and crystalline enstatite (MgSiO,) is the dominant phase. It was suggested that the lithium formed a silicate phase also; 6Li data are not inconsistent with this view but, at this juncture, are insufficient to identify the phase (no distinctive XRD lines were seen, the 800 "C and 1300 "C XRD traces were dominated by the enstatite polymorphs6). It was decided to ion exchange laponite with lithium to evaluate the possibility of distinguishing the structural and interlamellar ions.One method of ion exchange used was the classic but lengthy method of Posner and Q~irk.~This produced a material giving a single 6Li resonance, FWHM ~80Hz, centred on 6 =0.232 ppm. However, use of a new microwave method (see Experimental) gave a completely exchanged clay which was examined by both 6Li and 7Li MAS NMR spectroscopy (Table 4). Excellent spectra were obtained in both cases (Fig. l), but whereas a single resonance was obtained with 7Li (Table 4), clear resolution into a doublet is seen in the 6Li spectrum. However, re-examination of the spectrum after a lapse of 3 months revealed a single 6Li resonance (Table 4). A fresh preparation involving the micro- wave procedure again produced material which showed a doublet spectrum.For both preparations, the lines have a 1 : 1 intensity ratio. The spectrum observed may be that of a single lithium site giving rise to a quadrupole split line. However, we believe two pieces of evidence establish that the components of the doublet are chemically shifted. First, spinning 'off angle' by a small amount causes dipolar broaden- ing of the lines, but the separation of the components remains L I I I I I I I 1 I I 150 100 50 0 -50 150 100 50 0 -50 6 (PPW 6 (PPm Fig. 1 Comparison of the 'Li MAS NMR (a) and 6Li MAS NMR spectra (b) of lithium-exchanged laponite (microwave method) constant (within the error of the measurement, f0.5 Hz) at 11.3 Hz (MAS) and 11.9 Hz ('off angle').Secondly, there is no obvious reason why a quadrupole split resonance should change with time. It should be noted that spinning at large deviations from the 'magic angle' introduced sufficient dipolar broadening for resolution to be lost (Table 4). The 1 : 1 intensity ratio is consistent with expectation if the signals arise from interlamellar (exchanged) lithium ions and from structural lithium ions. The more deshielded resonance is assigned to interlamellar lithium ions (aquated) and the peak at 6= -0.438 ppm is assigned to the structural trioc- tahedral lithium ions; the slight deshielding relative to sodium laponite reflects the sensitivity of the lithium chemical shift to the identity of the interlamellar cations. It was of interest to note that investigation of an aged specimen failed to reveal two sites.Also, if the lithium was exchanged by a conventional method7 (i.e. washing several times with 1 mol dm-3 LiCl, stirring for 36 h at pH 4, followed by extensive washing and dialysis of the clay, a procedure of several weeks) a single resonance was noted (Table4). The data are consistent with a slow migration of lithium from trioctahedral sites into a common environment; the acceleration of the ion-exchange process in the microwave method enables the distinct lithium sites to be detected, but only if 6Li MAS NMR spectroscopy is used, the 7Li spectrum is a singlet. During the preparation of this manuscript Eckert et al. published their investigation of Li2S-P2S5 glasses by multi- nuclear NMR methods.8 Included were some 6Li MAS NMR data.The potential value of the superior resolution achievable in the 6Li spectra was noted in that work also. A. G. thanks the Royal Aircraft Establishment, Farnborough for support. S. P. B. thanks SERC and James River Graphics Corporation for a CASE award. This work has been carried out with the support of Procurement Executive MOD. References 1 S. Harder, J. Boersma, L. Brandsma, J. A. Kantas, W. Bauer and P. von R. Schleyer, Organometallics, 1989, 8, 1696. 2 D. R. Armstrong, D. Barr, W. Clegg, S. M. Hodgson, R. E. Mulvey, D. Reed, R. Snaith and D. S. Wright, J. Am. Chem. SOC., 1989, 111, 4719. 3 C. Brevard and P. Granger, Handbook of High Resolution Multi- nuclear NMR, Wiley, New York, 1981. 330 J. MATER. CHEM., 1991, VOL. 1 4 J. H. Gilchrist, A. T. Harrison, D. J. Fuller and D. B. Collum, 7 A. M. Posner and J. P. Quirk, Proc. R. SOC. London, Ser. A, J. Am. Chem. SOC., 1990,112,4069. 1964, 278, 35. 5 N. Poonia, J. Am. Chem. SOC., 1974,96, 1012. 8 H. Eckert, Z. Zhang and J. H. Kennedy, Chem. Muter., 1990, 2, 6 A-P. S. Mandair, P. J. Michael and W. R. McWhinnie, Poly-273. hedron, 1990,9, 517. Paper 0/04383D; Received 28th September, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100327

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(3), 331-337 33 1 Dielectric Relaxation Spectroscopy and Molecular Dynamics of a Liquid-crystalline Polyacrylate containing Spiropyran Groups Ewen J. C. Kellar," Graham Williams,*" Valeri Krongauzb and Shlomo Yitzchaikb a Department of Chemistry, University College of Swansea, Singleton Park, Swansea SA2 8PP, UK Department of Structural Chemistry, The Weizmann Institute of Science, Rehovot 761000, Israel The dielectric properties of a liquid-crystalline (LC) copolymer containing photochromic spiropyran groups have been determined over wide ranges of frequency and temperature. The observed loss curves contain a component from the dipole reorientations of the mesogenic groups and a further component from a conductivity-related process. An electrical cleaning method was used to reduce the contribution from the latter process and a subtraction method was devised to separate the overall loss curves into the two components.Using electrical/ thermal treatments it was found possible to align the copolymer homeotropically but not planarly. The dielectric loss spectra for unaligned and homeotropically aligned samples were very different. The data were analysed to yield two relaxation parameters for unaligned and aligned samples and were interpreted in terms of the anisotropic motions of dipolar mesogenic groups as have been discussed previously. In addition, the temporal stability of alignment of homeotropic samples at temperatures close to the clearing temperature was determined using the dielectric method.Keywords: Liquid crystal; Dielectric relaxation; Photochromism; A.C. alignment Since the late 1970s when liquid-crystalline (LC) side-chain polymers were first synthesized, much work has been done in assessing and varying their properties with a view toward their potential applications in the field of molecular elec- tronics, e.g. as media for optical information storage, optical waveguides and non-linear optical devices. '-'One such way in which the optical and spectroscopic properties of an LC polymer can be modified is by the incorporation of dye molecules alongside the mesogenic units either physically, as in mixtures, or chemically, by including the dye within the chemical structure to form a copolymer. In the particular case studied here, the LC polymer is a random copolymer in which 20% of the mesogenic groups have been substituted by a spiropyran derivative (see Scheme 1 and Experimental later).Note that the representation of the dimer below is schematic spiropyran yellow red blue merocyanine -* merocyanine dime rised isolated 0-0-0-Scheme 1 only. A proper description would take into account its sand- wich structure in which the rings ~verlap.~ Spiropyran copolymers have been the centre of much attention for several years owing to their photochromic and thermochromic properties."-" The polymer material con- taining the spiropyran group is yellow but irradiation by UV light at low temperatures, below the polymer glass-transition temperature (Q, causes the colour of the polymer to change to deep blue.The change is indicative of a photochemical conversion of the spiropyran to merocyanine, see Scheme 1. The reaction can be reversed by irradiation with visible light. If the copolymer film containing the spiropyran group is irradiated with UV radiation at higher temperatures the compound changes colour to red, as a result of complete or partial merocyanine dimerisation. This reaction is reversible thermally or in visible light.l4-'' Merocyanine groups will also be formed owing to the thermochromic reaction and dimerise if a sample is melted, and so a colour change from yellow to red is also possible. From previous work by Wis- montski-Knittel and Krongauz" and Goldburt and Kron- gauz,13 it has been demonstrated that the merocyanine tends to aggregate further to form higher aggregates and even a semicrystalline phase.It was possible to incorporate up to 40mol% of the photochrome units and still observe the liquid-crystalline properties of the material, though the clear- ing temperature (T,) declined with increasing photochrome concentration. Using UV spectrophotometry," the local order parameter, S, for the dye was found to be ca. 0.1 even when the polymer was aligned in an electric field. Yitzchaik et al." have proposed therefore that the photochromic side groups are in some way separate from the rest of the liquid-crystalline mesophase. Apart from the work of Yitzchaik et al.,18 who demonstrated alignment of the copolymer by virtue of the sample becoming optically transparent in the presence of a d.c. electric field between interdigitated electrodes mounted on a glass plate, no work has been done to date on the precise nature and extent of alignment within the sample and the effect, if any, of the addition of the spiropyran into the system.Dielectric relaxation spectroscopy is an ideal technique for such a and is used extensively in this paper to explore further the properties of this photochromic copolymer. CH, CH, CH,=CHCONH(CH,)SCONH -No2 1 2 Experimental The copolymer studied was produced by the free-radical copolymerisation of the two monomers 1 and 2 and was synthesized by The content of the photochromic como- nomer in the polymer is ca.20%. The polymer was purified by reprecipitation from a tetrahydrofuran solution with meth- anol and drying in a vacuum.17 In order to study both the electrical properties and the optical transparency variations occurring within the copoly- mer, the LC sample was melted and sandwiched between two conducting indium tin oxide glass electrodes.A PTFE spacer was used to maintain constant thickness (80 pm) and to insulate one electrode from the other. The sample area was ca.0.5 cm2. The glass cell was placed in an insulated mount inside a watertight metal jacket and each end was connected to a metal contact so that electrical measurements across the cell could be made. Also contained within the metal housing was an optical-fibre arrangement which enabled the optical transmission characteristics of the sample to be monitored via photo diode^.^^ The output was recorded on an XY chart recorder.The whole apparatus was immersed in a water bath so that the temperature of the sample could be controlled accurately, provided sufficient time (10 min) was left for the system to equilibrate after each temperature change. The electrical connections to the cell allowed the sample to be subjected to various d.c. and a.c. fields. All dielectric experiments were made using a Genrad 1693 RLC Digibridge over the range 12-10' Hz coupled to a Hewlett-Packard computer system, to collect, store and manipulate the data thus acquired. The equivalent parallel capacitance, C,, and conductance, G,, of the sample were taken at 20 spot frequencies by a substitution method involv- ing first zeroing the Digibridge without the sample and then measuring with the sample present.A scan across the fre- quency range required ca.8 min. The values of C, and G, are related to the relative permittivity E' and loss factor E" according to the equations,28 c, =ElC, +c1 G,/w =E"C, where C, is the geometrical capacitance of the interelectrode space, C1is the fringing capacitance and w =2nfis the angular frequency of the measurement. Thus plots of C, and G,/o against frequency are sufficient to indicate the dielectric relaxation behaviour of the liquid-crystalline polymer. The use of an Olympus BHSP polarizing microscope coupled to a Linkam hot stage (THM 600) enabled accurate measurements (within 0.1 "C) to be made of the clearing point for the LC copolymer, which was found to lie between 95.7 and 95.9 "C.Results and Discussion The copolymer was studied primarily using dielectric relax- ation spectroscopy which gave useful information about the J. MATER. CHEM., 1991, VOL. I dynamic molecular motion of the molecules, especially the mesogenic side groups. It is therefore possible to assess the degree of alignment of the liquid crystal from the size and shape of the loss peak. Further information concerning opti- mum conditions for sample alignment, alignment stability and T, can also be obtained. Initial Studies and D.C. Cleaning Fig. l(a) shows the loss spectra obtained for an unaligned sample at different temperatures.Consider first the data at the lower temperatures. As the frequency decreases, the loss increases with no evidence of a maximum. As the temperature is increased, the loss values increase markedly and there is clear evidence that a loss peak is emerging from the low- frequency conductivity 'tail', until at the highest temperatures studied, the two processes can be resolved. An alternative, and informative, representation of these data is shown in Fig. 2(a), where again, the emergence of a dielectric relaxation process from the dominant conductivity tail is observed as the sample temperature is increased. The sample was subjected to repeated pumping at high temperatures in the LC state in an attempt to remove impurities, which would contribute to the low-frequency conductivity process, but this process was unaffected by such treatment.In order to reduce the conductivity process, and hence enhance the dielectric loss process at higher frequencies, we used the technique of 'electrical cleaning' as described by Osaki et ~21.~' and Cebe and Gr~bb.~' This required the sample in the melt (T>T,) to be subjected to a large d.c. voltage (ca.300 V) for a period of several hours. During this time, extraneous ions which led to the conductivity process seen in Fig. l(a) are swept to the electrodes. As time progressed the effective d.c. conductivity fell, so it was possible to raise the applied voltage to assist the cleaning process.On cooling the sample back into the LC state, with the voltage still maintained, the ions are trapped at the electrodes and their greatly reduced mobility in the LC state, compared with that in the isotropic state, means that their contribution to the low-frequency conductivity tail is reduced. In practice, as the current traversing the sample, during the cleaning process, approached very small values irrespective of the value of the applied d.c. voltage, the electrical cleaning process was deemed to be complete, so the sample was cooled to just below < (ca.45 "C) and subsequently its dielectric properties were measured at successively higher temperatures. Fig. l(b) shows data for a sample that was treated in the way just described and the change in behaviour is evident.The loss peak is now seen over a wide range of sample temperatures since the magnitude of the low-frequency con- ductivity process has been reduced significantly. Fig. 2(b) shows the three-dimensional representation of these data and comparison with Fig. 2(a) shows how the low-frequency con- ductivity process has been reduced and the loss peak enhanced. Clearly, the cleaning process is successful if the sample tem- perature is below ca. 80 "C. However, the effect becomes short- lived at higher temperatures as T, is approached, because the ions which had been swept to the electrodes diffuse back into the bulk of the material and restore the original levels of low-frequency conductivity. The other effect of the d.c.voltage is to cause partial alignment of the polymer film as the LC phase reforms on cooling from the melt. This gives rise to an increase in the height of the emerging ion peak [compare Fig. l(a), l(b) and also 2(a), 2(b)]. J. MATER. CHEM., 1991, VOL. 1 4050/ LLa.30: I3 L-I . . ..'* 1-. . .__ .~-.. 1 2 3 4 5 2 3 4 5 log ( f/Hz) log ( f/Hz) log ( f/Hz) 50 (f t 2 3 4 5 log ( f/Hz) log ( f/Hz) log ( f/Hz) Fig. 1 G/w us. log f over a range of temperatures (50-99.5 "C) for (a)uncleaned and unaligned sample, (b)d.c.-cleaned sample (300 V, 3 h, 110 "C),(c) a.c.-aligned sample (200 V, 300 Hz, 2 h, 85 "C), (d) uncleaned and unaligned sample with conductivity tail subtracted, (e)d.c.-cleaned sample (300 V, 3 h, 110 "C) with conductivity tail subtracted, (f) a.c.-aligned sample (200 V, 300 Hz, 2 h, 85 "C) with conductivity tail subtracted A.C.Alignment The sample was subjected to a field of 300 Hz, 200 V for 2 h at 85 "Cfollowed by cooling (with the field removed) to room temperature. The resulting loss spectra are shown in Fig. l(c) and 2(c). The almost two-fold increase in the height of the loss peak compared with that for the unaligned sample indicates that the sample is almost totally homeotropically (h-) aligned i.e. the long axes of the liquid crystal side groups are aligned normal to the electrode surfaces. Conductivity is still dominant but the intensity of the loss peak is such that it is not so fully obscured at low temperatures as was the case in the unaligned sample.Conductivity Subtraction A third technique to remove the low-frequency conductivity feature was employed and in many respects was the most successful for all the observed loss curves. It took advantage of the fact that at high temperatures the conductivity process dominates at low frequencies, so its functional behaviour can be estimated over the extra frequency range. Once calculated, the theoretical curve can be subtracted from the original data to reveal a residual (or 'cleaned') loss feature. Such a method required a plot of log(G/w) us. log f to be made. For a true d.c. conductivity process, the loss factor should be proportional to f-'. However, it is usually found for organic solids that an f-" law applies, and this is also the case for the present work. For the low-frequency conductivity regime log(G/u)=A -nlog f (3) We have determined values (A, n) for each sample at each temperature from the best fits to the limiting low-frequency data where the conductivity process was dominant.An esti- mate of the accuracy of fit was made by examining the residuals in the low-frequency range where the dipole loss contribution was negligible. A good fit was assumed if the first low-frequency points gave differences of <1% of the averaged value. This method allowed the conductivity tail to be subtracted from ca.40% of the loss spectra, but for the remainder it was necessary to extrapolate (A, n) values with respect to the temperature, and to see if the dielectric relax- ation peak obtained subsequently was well behaved before accepting the (A, n) values in a particular case. Fig.3 and 4 show the values of n(T)and A(T)thus determined. The values of n range from ca. 0.5 at 50 "C to ca. 1.0 at 100 "C,whereas A changes by two orders of magnitude in the same temperature range. The plot of log A us. T-' curved upwards, showing that conductivity increases more strongly with increasing temperature than expected by a simple Arrhenius relation. At higher temperatures n4 1 (a simple conductivity process), whereas n<l implies dispersive transport of ions. The loss curves for dipole relaxation were obtained by subtracting the calculated loss due to a conductivity process from the observed loss curves; the resulting data are shown in Fig.l(d)-(f) and 2(d)-( f). Comparing Fig. l(d)-( f),the preferred alignment (homeotropic) leads to an increase in the height of the loss peak. For the range 95-97.5 "C we see a marked decrease in peak height, showing that the material is transformed from h-aligned liquid crystal to isotropic liquid over a narrow temperature range. Discussion of Dielectric Relaxation Spectroscopy Results The data of Fig. l(a)-l(c) show that the dipole relaxation process is partly obscured by the low-frequency conductivity process, but they may be separated with the aid of an electrical cleaning operation and numerical subtraction of the conduc- tivity process using eqn.(3). The subtraction process is well illustrated in the comparison of the curves shown in Fig. 5 J. MATER. CHEM., 1991, VOL. 1 10 Fig. 2 Three-dimensional plot of G/o us. log f us. T for (a) uncleaned and unaligned sample, (b) d.c.-cleaned sample (300 V, 3 h, 110 "C), (c) a.c.-aligned sample (200 V, 300 Hz,2 h, 85 "C),(d) uncleaned and unaligned sample with conductivity tail subtracted, (e) d.c.-cleaned sample (300 V, 3 h, 110 "C)with conductivity tail subtracted, (f)a.c.-aligned sample (200 V, 300 Hz, 2 h, 85 "C)with conductivity tail subtracted and 6 for different samples at 85 "C. Note the d.c.-cleaned and unaligned loss curves reflect different weighted sums of sample gives a loss peak greater than the untreated unaligned the underlying dipole relaxation modes23 (see also Fig.8 later). sample, since the cleaning process induces some h-alignment. Note that an isosbestic point occurs at log f=3.9, at which The shift of the frequency of maximum loss to lower values point the loss curves all have the same value. This is predicted on partial alignment (for d.c.-cleaned sample) and fuller align- from a semi-macroscopic theory for the complex permittivity ment (for a.c.-aligned sample) is expected since the h-aligned E(o), where c0=2n$ for a uniaxial sample whose macroscopic J. MATER. CHEM., 1991, VOL. 1 c3ao 0.7 40.01" 0.61 g 0 O "p 30.0 0.5 0.4 0.0 1°.OL0.01 2 3 4 log( f1Hz) Fig. 6 Plot of G/w us. log f for uncleaned and unaligned (U),d.c.-cleaned (A)and a.c.-aligned (0)sample at 85 "C with conductivity tail removed specimens, respectively.E(O) =&'(a)-id'(a), where &''(a)is the dielectric loss factor (= G/oC,, where C, is the inter-electrode 00 8 capacitance). Thus, for the isosbestic frequency for loss factor, E",,(O)=E"~(O) and hence &"(a)at this frequency is the same for any value of sd. A similar condition applies to &'(a)but 0 the value of the isosbestic frequency at which ~',,(a)=E'&) O O is different from that for the loss factor. Fig. 7 shows the plots 0 of C, against logf, and the isosbestic frequency (cross-over -0.5 1 0 0 0 0 0 frequency) is seen to occur at ca. 700 Hz and is, coincidentally, close to the frequency of maximum loss factor f, (Fig.6). Thus, the frequency of 300Hz, chosen to align the sample homeotropically at 85 "C,was conveniently placed away from the dielectric loss maximum, which causes dielectric heating, -1 .oIand from the conductivity tail, which also causes dielectric 2.6 2.7 2.8 2.9 3.0 3.1 103~1~ Fig. 4 log A us. 1/Tfor uncleaned and unaligned 0)and a.c.-aligned sample (0) 40.0 -30.0-Q.h -20.0 10.0 -i0.0 L AL-12-Lld 1 2 3 4 5 log ( f/W Fig. 5 Plot of G/ous. log f for uncleaned and unaligned (0),d.c.-cleaned (A)and a.c.-aligned (0)sample at 85 "C heating. According to Fig. 7, it should be possible to align the sample using a directing a.c. field the frequency of which exceeds the cross-over frequency, but we did not succeed in doing so in our experiments.The plots of logf, us. T-' for the unaligned and h-aligned samples are shown in Fig. 8, and are seen to be approximately linear giving apparent activation energies of 184 kJ mol-' and 209 kJ mol-' for unaligned and h-aligned samples, respectively. In contrast, the apparent activation energy for the conductivity process, as obtained from the slope of log A 170 I 160 --150 -140 130 -120 -U ,n 110 -O" 100 -90 -80 -70 --60 alignment is intermediate between the h- and planarly (p-) 501 I "1111" I "111"1 I'I""" ' iI1'llllaligned conditions. We obtained23 log ( f/Hz) E(W)=(1 +2Sd)&li(o)/3+2(1 -Sd)&L(a)/3 (4) Fig. 7 C, us. logffor uncleaned and unaligned (U),d.c.-cleaned (A)where Sd is equal to 1.0 and -0.5 for fully h- and p-aligned and a.c.-aligned (e)sample at 85 "C 336 5 0.4 0 0 h 0' \2 3 0 .-E cn- 0 2 1 103~1T Fig. 8 logfus. 1/T for uncleaned and unaligned (0)and a.c.-aligned (0)sample us. T-' in Fig. 4, is only ca. 0.33 of that for the dipole relaxation process (Fig. 8) and this differenceenables the latter process to emerge from the conductivity tail as is shown in Fig. 1 and 2. In amorphous solid polymers the apparent activation ener-gies for the principal (a)relaxation process (segmentalmotion) and conductivity process are often found to be similar, implying that the diffusion of ions is effected via segmental motions. For the LC polymer, the dielectric relaxation process observed in Fig.1 and 2 is to be assigned to a weighted sum of relaxation modes, 00, 01, 10 and 11 (as has been described earlier in ref. 22-25, 31) in which the longitudinal dipole moment pl and transverse dipole moment pt associated with both the mesogenic groups and the photochromic groups are relaxed,z3and it is evident that these anisotropic motions and the motions of extraneous ions through the material have very different mechanisms, and hence different activation energies. We note that the half-width of the loss curve for the h-aligned sample (Fig. 6) is ca. 1.5, which is a little broader than that (1.14) for a single relaxation time process. Also the curve has a long high-frequency tail, which is indicative of the 01 process.The loss curve for the unaligned sample is very broad and asymmetrical. If we take values for ~",,(f)and &""naljgned(f) from Fig. 6, then using eqn. (4) it is possible to calculate the loss curve for a p-aligned sample. The calculated curve shows a rising loss with frequency, and passes through the isosbestic frequencyz3at ca. 9 kHz. The molecular origin of the losses for a p-aligned sample is the 10 and 11 relaxation modes as we have discussed earlier (ref. 22-25, 31). The unaligned sample gives a dielectric loss spectrum which is a weighted sum of all four relaxation modes [see eqn. (4), for sd=o]. Stability of Homeotropic Alignment Dielectric relaxation spectroscopy provides a simple and direct method for monitoring the stability of the field-induced align-ment of the polymer.The h-aligned material was prepared using 200 V at 300 Hz applied for 2 h at 85 "C, and samples were studied at different temperatures by cooling the sample quickly to the desired temperature, removing the voltage and monitoring the dielectric loss factor at a frequency close to fm at that temperature at 1 min intervals over a period of 3 h. In order to standardise the experiments, a delay of 3 min from field removal was imposed at the start of each run to take into account the time required to achieve a constant J. MATER. CHEM., 1991, VOL. 1 43 42 .. 41 40 39 38 LL a 37 \236 30 -'...... '...... ...... -..... ......... 29 -.. ........ ".'.. ........,, ...... . ......... .... ....... ......... 0 1800 3600 5400 7200 9000 10 800 tls Fig.9 Plot of G/o peak decay at different temperatures us. t: top (log f=2), 75 "C;middle (log f=2.25), 80 "C;bottom (log f= 2.Q 85 "C temperature in the cell. Fig. 9 shows our data for 85, 80 and 75 "C. As the temperature is decreased the rate of disalignment decreases. The observation that disalignment occurs in the LC phase is in contrast with our earlier observation for siloxane-chain homopolymers where no disalignment occurred below z.22-z5We have also observed disalignment in siloxane-chain LC copolymers where the longitudinally attached mesogenic groups are diluted by the presence of transversely attached mesogenic groups.32 It is evident from Fig.9 that disalignment occurs even at temperatures 20 "C below T,. Optical Studies Some optical studies were made with a sample contained between conducting glass electrodes as the temperature was varied and the directing voltage was applied. Optical trans-mission of the whole sample area was detected by a fibre-optic bundle linked to a photodetector. The optical trans-mission of the sample increased steadily for T> with a final rapid increase to transparency in the range 95-95.7 "C. On application of a directing a.c. voltage, the optical transmission increased with time but with this method it was not possible to estimate the degree of homeotropic alignment achieved as time increased.Conclusions Dielectric loss spectra for unaligned, partially h-aligned and h-aligned samples of a photochromic polymer have been obtained over a range of frequencies for different sample temperatures. The spectra have been resolved into conduc-tivity and relaxation component processes and their phenom-enological parameters have been determined over a range of temperatures and sample conditions. The resolved dipole-relaxation process is mainly due to the 6 (or 00) motional process of the mesogenic groups, but clear evidence is obtained for the presence of 01, 10 and 11 relaxation modes, which result from the anisotropic motions of the dipolar mesogenic groups. A feature of the work is that it was found possible to 'electrically clean' the samples, allowing the conductivity process to be suppressed for T< T, and the dipole relaxation process to be made more prominent.Disalignment of h-aligned samples was observed at temperatures well below T, after removing the directing voltage and monitoring the dielectric loss factor at a frequency chosen to be close to J. MATER. CHEM., 1991, VOL. 1 337 logf, at that temperature. The rate of decay increases with increasing temperature. 14 15 I. Cabrera, V. Krongauz and H. Ringsdorf, Mol. Cryst. Liq. Cryst., 1988, 155, 221. I. Cabrera and V. Krongauz, Macromolecules, 1987, 20, 2713. 16 I. Cabrera and V. Krongauz, Nature (London), 1987, 326, 582. The authors would like to thank SERC for the postdoctoral award to E.J.C.K.,which enabled much of this work to take place.Thanks are also made to Dr. M. Garley for his help in developing the program required for the three-dimensional 17 18 19 I. Cabrera, V. Krongauz and H. Ringsdorf, Angew. Chem. Znt. Ed. Engl., 1987, 26, 1178. S. Yitzchaik, I. Cabrera, F. Buchholtz and V. Krongauz, Macro-molecules, 1990, 23, 707. W. Haase and H. Pranoto, Polymeric Liquid Crystals, ed. A. plots in Fig. 2. 20 Blumstein, Plenum, New York, 1985, p. 313. W. Haase, H. Pranoto and F. J. Bormuth, Ber. Bunsenges. Phys. Chem., 1985,89, 1229. 21 F. J. Bormuth and W. Haase, Liq. Cryst., 1988, 3, 881. References 22 23 G. S. Attard and G. Williams, Liq. Cryst., 1986, 1, 253. G. S. Attard, K. Araki and G. Williams, Br. Polym. J., 1987, 19, ~1 2 H.Finkelmann and G. Rehage, Ado. Polym. Sci., 1984, 60/61, 99. V. P. Shibaev and N. A. Plate, Adv. Polym. Sci., 1984, 60/61, 24 25 119. G. S. Attard, Mol. Phys., 1986, 58, 1087. G. S. Attard, J. J. Moura-Ramos and G. Williams, J. Polym. 3 173. G. S. Attard and G. Williams, Chem. Br., 1986, 22, 919. 26 Sci., Polym. Phys. Ed., 1987, 25, 1099. G. S. Attard, K. Araki, J. J. Moura-Ramos and G. Williams, 4 5 G. S. Attard and G. Williams, Nature (London), 1987, 326, 544. M. Engel, B. Hisgen, G. Keller, W. Kreuder, B. Reck, H. Ringsdorf, H. W. Schmidt and P. Tschirner, Pure Appl. Chem., 1987, 57, 1009. 27 28 Liq. Cryst., 1988, 3, 861. K. Araki, A. Kozak, G. Williams, G. W. Gray, D. Lacey and G. Nestor, J. Chem. Soc., Faraday Trans. 2, 1988,84, 1067.N. G. McCrum, B. E. Read and G. Williams, Anelastic and 6 H. J. Coles, in Developments in Crystalline Polymers, ed. D. C. Dielectric Efects in Polymeric Solids, Wiley, London-New York, 7 Bassett, Elsevier Applied Science, Barking, 1988, vol. 2, p. 297. Side Chain Liquid Crystal Polymers, ed. C. B. McArdle, Blackie, 29 1967. S. Osaki, S. Uemura and Y. Ishida, J. Polym. Sci., Polym. Phys. Glasgow, 1989. Ed., 1971, 9, 585. 8 I. Cabrera, M. Engel and H. Ringsdorf, Extended abstract of a 30 P. Cebe and D. T. Grubb, Macromolecules, 1984, 17, 1374. paper presented at the 3rd EEC Workshop on Photochemical 31 K. Araki, G. S. Attard, A. Kozak, G. Williams, G. W. Gray, D. and Photobiological Processes for the Production of Energy Lacey and G. Nestor, J. Chem. Soc., Faraday Trans. 2, 1988,84, 9 Rich Compounds, London, April 18-2 1, 1989. The Theory of the Photographic Process, ed. H. James, McMillan, 32 1067. G. Williams, A. Nazemi, F. E. Karasz, J. S. Hill, D. Lacey and New York, 1977, ch. 7, 8. G. W. Gray, Macromolecules, in the press. 10 Y. Kalisky and D. J. Williams, Macromolecules, 1984, 17, 292. 33 G. S. Attard, K. Araki, J. J. Moura-Ramos, G. Williams, A. C. 11 F. Shvartsman and V. Krongauz, Nature (London), 1984, 309, Griffin, K. Bhatti and R. S. L. Hung, in Polymer Association 12 609. T. Wismontski-Knittel and V. Krongauz, Macromolecules, 1984, Structures, Microemulsions and Liquid Crystals, ed. M. A. El Nokaly, ACS Symp. Ser. 1989, 3484,255. 18, 2124. 13 E. Goldburt and V. Krongauz, Macromolecules, 1986, 19, 247. Paper 0/04402D; Received 1st October, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100331

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(3), 339-346 Room-temperature Electrochemical Reduction of YBa,Cu,O, -Solid-state and Solution Chemical Results Michael Schwartz,ta Yosef Scolnik," Michael Rappaport,b Gary HodesC and David Cahen*a a Departments of Structural Chemistry, Nuclear Physics and Materials Research, The Weizmann Institute of Science, Rehovot 76100, Israel The oxygen content of polycrystalline samples of YBa,Cu,O,-, can be reduced quantitatively, in a controlled fashion, by electrochemical techniques at room temperature in propylene carbonate. Upon reduction, the propylene carbonate undergoes an unusual reaction at the YBa,Cu,O,-, cathode to produce propanal, apparently because of the production of an active oxygen species on the surface. The reduced materials have been characterized by X-ray powder diffraction, electrical resistivity and magnetic susceptibility.The large-grained, reduced pellets are found to be inhomogeneous with respect to x. The reduced materials exhibit a broadened transition to the superconducting state. This effect is ascribed to the formation of metastable phases formed during reduction. After a low-temperature anneal, 80, 60 and 20 K transition temperatures are observed. These results indicate that T, is a continuous function of oxygen content, but a discontinuous function of oxygen ordering. Keywords: Electrochemical reduction; Y-Ba-C u-0 system; Superconductivity Among the superconducting cuprates, YBazCu307 --x (123) is the one most readily amenable to controlled solid-state chemi- cal studies owing to the relative ease of its (de)oxygenation at moderate-to-high temperatures.This property leads to a continuous change in the oxygen content from seven (x=O) to six (x = l).',' Results from a variety of techniques, including re~xygenation,~*~high-temperature solid-state electro~hemistry,~*~inelastic relaxation measurements,'o and polarized light microscopy (to follow the position of the orthorhombic/tetragonal phase boundary)," $ show that, not- withstanding appreciable differences between values arrived at by different methods, the chemical diffusion coefficient for oxygen in this material is (very) high. For example, values of 2.2 XIO-'~cm2 s-' (at ca. 200 "CI,~ 5 xIOb8cm2 s-' (at ca. 550"C),87.5~10-~cm's-' (at ca.300°C)" and even 6~10-~cm's-' (at ca. room temperature)," have been measured or deduced. This oxygen lability is the more interesting because it has been shown that there is a marked non-linear dependence of T, and of the crystallographic parameters on oxygen con- These results have been obtained on samples that are prepared at temperatures >350 "C by annealing with Zr as an oxygen getter" or at even higher temperatures by vacuum annealingI3 and q~enching.'~ Careful preparation of oxygen-deficient 123 by these methods has led to the obser- vation of a 'plateau' behaviour of T, with decreasing oxygen content, in which two T,s of ca. 90 and 60K are observed for samples with x between 0 and 0.6.129'3 Electron diffraction and single-crystal X-ray diffraction data have been used to suggest the occurrence of long-range ordering of oxygen vacancies in the oxygen sublattice to form two The occurrence of ordering is supported by results from thermodynamic ~alculations.'~*'~ However, the existence of a distinct, microscopically homogeneous 60 K phase has been questioned on the basis of high-resolution electron microscopy t Present address: Xsirius Superconductivity Materials Ltd., studies." A study of reduced samples prepared by solid-state electrochemical techniques at 500 "C showed the presence of many phases, each with only slightly different oxygen contents." The behaviour of oxygen in this material can be rational- ized, on the basis of its crystal structure,21 which is derived from the ideal perovskite structure, AB03, with Cu occupying the B sites, and Ba and Y the A sites.The combination of Y and Ba ordering and oxygen deficiency leads to a tripled unit cell relative to the basic perovskite unit cell. In the fully oxygenated compound the oxygen vacancies (two per unit cell) are ordered with oxygens missing from the Y plane[(f,0, f) and (0, f,3)sites] and from the basal plane [(f,0 0) site]. These ordered vacancies lead to planes of square-pyramidal CuO, units perpendicular to c and to chains of square planar CuO, units parallel to b (Fig. 1). Powder ,Cu-0 chain 0 cu 0BaII 0 ll Oy 0 Jerusalem, Israel. C $ In ref. 11, use is made of the fact that the oxygen out-diffusion, at sufficiently high temperatures, leads to a phase transformation kafrom an orthorhombic to a tetragonal phase of 123 and that the boundary between these phases can be visualized clearly, when a sample is observed by polarized light microscopy.Fig. 1 Structure of YBa,Cu,O,-, (after ref. 21) neutron diffraction studies on oxygen-deficient material have shown that the most labile oxygen comes from the (O,+,O) ~ite.'~*~~Loss of this oxygen breaks up the chains, and in the end-member x= 1, no chains are present. Changes in oxygen content also play a role in the electronic J. MATER. CHEM., 1991, VOL. I of the total oxidative power of the samples [as expressed in (Cu-0)' units and assuming three Cu per formula unit and an essentially 1OO% pure material].32 Lao~,Sr0.,CoO3 -x was prepared according to ref.29 and was a single phase as determined by X-ray powder diffraction. The parameter x structure. Above T,, 123 has metallic hole cond~ctivity.~~ was deduced to be 0.01 from iodometric titration. Reduction of the oxygen content to below ca. 6.4 leads to a new semiconducting, tetragonal phase.24 In the fully oxygen- ated material (x=O), the average, formal valence of Cu is +2.33 (assuming Y3+, Ba2+ and 02-)and in the semicond- ucting phase (x = I), the average formal Cu valence is +1.67. Formal valencies in such a complex, metallic oxide should be used with caution and it is probably more realistic to describe the holes in the conduction band as belonging to (Cu-0)' units rather than to discrete Cu3+-02- or Cu2+-O- units.Regardless of the formal description, it is clear that the number of charge carriers is governed by the oxygen content. We report here solid-state and solution chemical results of room-temperature electrochemical reduction of this material The bulk electrochemical reductions were performed in a single-compartment cell using propylene carbonate-tetra-butylammonium perchlorate (0.1 mol dm- 3, as the electrolyte. The samples were in the form of pressed pellets, ca. 100 mg, 8 mm diameter by 0.5 mm thick, with a density of 4.6- 5.0 gcm-3. The pellets were pressed onto an undersize Pt disc which served as the current collector. The Pt disc and wire were encased in a glass tube to prevent direct contact with the electrolyte.Two Pt wires were used as counter and quasi-reference electrodes. Ar was bubbled through the solu- tion during the reduction. Standard electrochemical equip- ment, including a digital coulometer, was used. The cathodic limit for propylene carbonate-tetrabutylammonium perchlor-(preliminary accounts of this work have been pre~ented~~-~~). ate decomposition on Pt was more negative than -1 V and We thought that such a reduction could be possible because of the relatively high oxygen diffusivities near room tempera- ture that have been observed in other oxygen-deficient perov- ~kites.~~,~'Thus, oxygen-deficient 123 could be prepared from fully oxygenated 123 at room temperature using electrochemi- cal reduction according to YBa2Cu307-x +nye- +YBa2Cu307 -x-y +y[O"-] (1) The square brackets around 0"-and the use of the parameter n indicate that we do not know the exact form in which oxygen diffuses through or in which it leaves the solid.By working at room temperature we hoped to restrict large-scale structural rearrangements, which can occur by higher tem- perature preparation and to allow for the formation of phases that are not stable at higher temperatures. In addition, if n is known, electrochemical reduction permits precise control of the degree of reduction via coulometry, which is not always possible with higher-temperature techniques. Because of the impracticality of using gas/solid reactions to exploit room-temperature oxygen mobility, we explored liquid/solid electrochemical reactions, as was done for the 'normal' oxygen-deficient perovskites Nd0.50Sr0.-x2850C~03 and Lao,50Sro.50C003 -x29 in aqueous solvents. The known reactivity of 123 with H20 required the use of non-aqueous electrolyte^.^^ We chose propylene carbonate because of its wide working potential window and because we found that it is chemically inert towards 123. Some experiments were done also in acetonitrile. We note that electrochemical tech- niques at room temperature have been used also to intercalate H and Li into 123.31 Experimental 123 was prepared by heating a mixture of the precipitated nitrates. The mixture was heated to dryness three times to remove any excess acid and then heated overnight at 120 "C.The powder was ground thoroughly, pressed into pellets and fired twice at 950°C for 12h in O2 and then cooled at 50 "C h-' to room temperature with one grinding in between. After these firings, the powder was reground, passed through a 400 mesh (37 pm) sieve, repressed and refired as above except that a 12 h anneal at 500 "C was added. Typically an elongated grain size resulted, with a long dimension of 5-10 pm, as determined by scanning electron microscopy. X-Ray powder diffraction showed the orthorhombic pattern of 123.l Some batches contained a small amount of other phases. The starting material showed, reproducibly, an oxygen content of 6.97f0.02 as determined by iodometric titration was not approached during reductions or during the separate measurements of the diffusion coefficient. The reductions were carried out potentiostatically.The voltage was set at -1 V relative to the Pt quasi-reference electrode. The potential of the Pt quasi-reference electrode was found to be ca. 250 mV more positive than the saturated calomel electrode and stable over the course of a few hours. Initial current densities were of the order of 1 mA cm-2 and the current decayed during the course of the experiment. Initially, there was a fast decay which may be due to adsorbed oxygen. This decay accounted for no more than 30mC of charge passed, i.e. less than 2% of the total charge needed to reduce the oxygen content by 0.05, equivalent to x =0.001. The initial voltage at the counter electrode was > +2 V relative to the reference electrode. The reduction was allowed to continue until the desired amount of charge had been passed as measured by coulometry.Typical times were 4-10 h. Reduced samples were washed with acetone and stored in uacuo. Iodometric titration was used to analyse for oxygen content.32 Simply soaking the pellets in the electrolyte had no effect on their electrical, magnetic and structural properties. These experiments were also repeated with samples of 5Sr0.5c003* Some samples were annealed after reduction by sealing them in fused silica ampoules in a vacuum of better than 1 x lop2 Pa. These samples were heated at 150 or 300 "C for 24 h and allowed to cool in the furnace.Experiments to study the reaction products in solution were performed in a two-compartment cell in which the counter electrode was separated from the working electrode by a sintered glass frit. The working electrode was prepared by attaching a wire to the 123 (or Lao~5Sro~,Co03~x) pellet with Ag paste, covering this area with insulating epoxy to prevent contact with the electrolyte. After an initial Ar purge, the electrolyte was blanketed by Ar so as to minimize loss of any gaseous species dissolved in the electrolyte. Reduction conditions were the same as above. After a certain degree of reduction (x=0.10-0.19, cyclic voltammetry of the solution in the working electrode compartment was performed using a Pt wire in place of the 123 electrode.Both propylene carbonate and acetonitrile solutions were used. The solutions in the working electrode compartment were analysed by gas chromatography and mass spectrometry using a Finnegan model #4500 GC/MS equipped with a Supelco SBP5 30m capillary column. Semi-quantitative gas chromatography analyses were performed with a Tracor 560 GC with an OV1 1-101 column of 1.5 m. An effective value for the chemical diffusion coefficient of J. MATER. CHEM., 1991, VOL. 1 oxygen in polycrystalline 1237, at room temperature, was determined from the time decay of the current under potentio-static condition^,^' using acetonitrile in addition to propylene carbonate, both with tetrabutylammonium perchlorate as the electrolyte. Electrical resistivity measurements were performed using the van der Pauw four-probe method with measuring currents of 1-50mA, and contact was made using Ag paint [ca.(1-2) x cm2 area]. Minimum detectable resistance was R. Magnetic susceptibility measurements were done using the a.c. mutual inductance technique at a frequency of 75 Hz and a field of ca. 0.33 G. The samples were cooled in the field. D.c. magnetic susceptibility measurements were also performed on a vibrating sample magnetometer and found to be similar to the a.c. susceptibility results. X-Ray powder diffraction patterns were obtained with Cu-Kcc radiation. Results Reduced Samples Structural Evidence for Electrochemical Reduction of 123 The X-ray diffraction patterns in the region of the (200) and (020)+(006) reflections for an unreduced and highly reduced sample are shown in Fig.2. These reflections are unambiguous in that each peak is directly related to a lattice constant. Comparison of the two patterns shows that the (200)reflection is broadened while the (020)+(006) reflection is not broadened further upon reduction. In addition, the (200) reflection is shifted to lower Bragg angles, indicating that the lattice parameter a has increased. Because the X-ray powder diffrac-tion was performed on pieces of pellets that had been thor-oughly ground, the bulk of the sample, rather than only those regions near the surface exposed to the electrolyte, was probed. We note that it was possible, via heavy reduction, to obtain samples that showed no more splitting of the (200) and the (020)+(006) peaks (x FZ 6.2).Because of the above-mentioned broadening of the peaks upon reduction (of bulk samples), this is only suggestive of a transition to a tetragonal phase. 48.0 47.5 47.0 46.5 46.0 281" Fig. 2 X-Ray powder diffraction patterns of bulk (pellet) YBazCu30,-, in the 20 region 48.0-46.0", Cu-Kcr radiation and a 20 scan rate of 0.25 " min-'. (a) Starting material, x=O.O5; (b)x= 0.29, before annealing; (c)x =0.29, after annealing. Dashed lines are at the centre of full-width at half-maximum for the x=O.O5 sample. The arrows represent the expected shifts in the (200) and (006) reflections for a sample with x=O.29 based on data from ref. 12 and 14 341 Stoichiometric Evidence for Electrochemical Reduction of 123 A second result that supports oxygen removal from the bulk is the exact agreement between y in eqn.(1) as determined by coulometry (using n=2) and the final oxygen content of the sample as derived form iodometric titration. Finally, it was observed that the density of the pellet was important. Typical densities were 4.6-5.0 g cmP3(ca. 75% of the theoretical density). For pellets with much higher densities, ca. 5.8 g cmP3 (91% of the theoretical density), the current decayed very rapidly and reduction was limited to 1-3% of the labile oxygen (x =0.01-0.03). Samples with lower density have greater porosity and, therefore, a larger effective surface area for contact with the liquid electrolyte.The rapid current decay that was observed with dense pellets can then be understood by realizing that in a wet electrochemical set-up actual electron transfer occurs at all the grain surfaces that are contacted by the electrolyte, rather than only at the surface of the pellet. This situation is quite different from that in (high-temperature) solid-state electrochemical experiments, where as dense a pellet as possible is needed to obtain the highest possible area of direct contact between solid electrode and electrolyte [cf. ref. 9(b)]. Magnetic Susceptibility and Electrical Resistivity of Reduced Samples Plots of the temperature dependence of the magnetic suscepti-bility and electrical resistivity have been published in prelimi-nary communication^.^^-^^ Representative plots are shown in Fig.3 and 4 and the data are summarized in Table 1. The data show that the transition to the superconducting state is broadened relative to the starting material [cf.also ref. 12(b)]. The degree of broadening increases for larger amounts of Omom 0.04 TIK Fig. 3 Plots of electrical resistivity us. temperature for two samples of reduced YBazCu30,-, x, x=0.14, before annealing; +, x =0.10, after annealing at 300 "C 2.5, ~0.04 G hl.04 : Rc 0.02 Q-0.01 G 0.0 0.00OS5-0 2.0 40 60 80 100 TIK Fig. 4 Plots of electrical resistivity us. temperature for a sample of reduced YBa2C~30,-x.x , x=0.20 before annealing; +, x=0.29 after annealing at 300 "C J.MATER. CHEM., 1991, VOL. 1 Table 1 Effect of room-temperature reduction and subsequent low-temperature anneal on the superconducting transition temperature (from electrical resistivity measurements) and u lattice parameter of YBa2Cu30, -x onset temperature/K before after before after anneal anneal anneal anneal 0.05 0.05 83 84 0.10 0.10 85 81, 59 0.15 0.23 80 79, 57 0.20 0.29 75 61, 20 0.22 78 0.29 70 'From ref. 12; 'from ref. 14. reduction. Except for the unreduced material, the electrically measured transition to the superconducting state is broadened as the current is increased (10-50 mA). There is also a slight decrease in the onset temperature for superconductivity (Table 1). In addition, increased reduction leads to thermally activated normal-state electrical behaviour. The effective acti- vation energy in the normal state increases with increasing reduction (from In p us.l/Tplots). Reduced and Annealed Samples After reduction, some samples were annealed at 150 and 300 "C under conditions described in the Experimental. For samples with x20.10, additional oxygen loss occurred upon annealing. This suggests that oxygen lability increases with increasing reduction, possibly owing to lattice destabilization as a result of second (and higher) nearest-neighbour effects (i.e.decreasing distance between oxygen vacancies). Initial and final oxygen contents are given in Table 1. The X-ray diffrac- tion patterns were also sharper than those obtained for the unannealed samples [Fig.2(c)]. The a values remain larger with respect to unreduced samples and are summarized in Table 1. They agree well with those obtained for samples reduced by high-temperature methods with similar oxygen content.l2-I4 Plots of magnetic susceptibility (x) and of electrical resis- tivity (p) as a function of temperature (T)for the annealed samples have been presented Representative p-T plots are shown in Fig. 3 and 4 and the results are summarized in Table 1. After the 150 "C anneal, a break in the transition to the superconducting state is observed at ca. 70 K. After the 300 "C anneal, the X-T measurements show distinct transitions to the superconducting state. For samples with x<O.25, two transitions at T,zt:O and 80 K are found.For x=O.29, one T, at ca. 60 K is observed. The electrical resistivity measurements agree with the magnetic measure- ments except that a T,z20 K is also osberved for the x =0.29 sample (cf: also the recent results of Jorgensen et al. on the effects of (sub)room temperature anneals of quench-reduced samples33). The fact that this T, is observed by the electrical resistivity measurements and not by the magnetic suscepti- bility measurements indicates that only a small amount (but above the percolation limit) of the sample has this T,. The presence of the distinct 60 and 80 K transitions in the reduced, annealed samples is similar to the 'plateau' behaviour observed in oxygen-deficient samples prepared by other method^.'^,'^,^^ After the low-temperature annealing treatment, the normal- state electrical resistivity of the reduced samples is thermally activated.The degree of activation decreases as T, is approached. Similar effective activation energies for samples with different final oxygen contents are found over similar temperature ranges. transition width/K before anneal 5 22 25 37 Solution Reactions a lattice parameter/pm after anneal 38 1.7(4) 382.4(8) 382.6(8) 383.2(4) 382.7(2)" 383.52' To complement the results obtained from the measurements on the electrode, experiments were performed to elucidate the oxygen chemistry occurring at the surface during reduction, i.e. to identify the product(s) in the reaction y[02-1 +electrolyte-+products (2) After a certain degree of reduction (x =0.10-0.19, cyclic voltammograms were performed on the solution.As shown in Fig. 5, a species is formed which is not present initially. This species is clearly not 02,which has a very distinct cyclic voltammogram on Pt in propylene carbonate [Fig. 5(c)]. GC/ MS analyses were then performed on the solution from the working electrode compartment and a reaction product, ident- ified as propanal, was observed. The amount of propanal formed increased with the amount of charge passed. Semiquantitative analyses indicated that the amount of pro- panal was consistent with the amount of oxygen produced as determined by coulometry, i.e. it is a major product. However, exact figures cannot be obtained since the stoichiometry of eqn.(2) is not known. The electroactive species observed in the cyclic voltammogram is not propanal as the cyclic voltam- mograms of the unknown were different from cyclic voltam- mograms of propanal. The unknown may be due to Cog-(see Discussion later). Attempts to precipitate Cog-with Li+ $. -lo@ +10 pA f ' ~ ' I ' " l ' ~ ' 1 -1 .o 0 1.o potential/V vs. Pt Fig. 5 Cyclic voltammograms of propylene carbonate-tetrabutyl- ammonium perchlorate (0.1 mol dm- 3, electrolyte in working elec- trode compartment using Pt wires as the working and quasi-reference electrodes: (a) before reduction of 123; (b) after reduction of 123 (x= 0.15); (c) saturated with O2 J.MATER. CHEM., 1991, VOL. 1 from the reaction solution were not successful. It was also observed that neither trace amounts of HzO nor saturation of the electrolyte with oxygen had any effect upon the reduction. In addition, the presence of an oxidizing species in the counter-electrode compartment was detected using I -. In order to determine whether the formation of the propanal was due to reaction with the oxygen from 123 or due to some catalytic effect of the metal oxide and the electrical potential difference, a sample of YBazCU306 (1236) was used in place of 123. The 1236 was held at the same potential as that used for the reduction of 123 for the same period of time. Only very small amounts of current could be passed and the total charge accumulated was equivalent to x=O.Ol. This can be understood by realizing that if 123 does not contain labile oxygen, it is no longer a mixed ionic/electronic conductor but just an electronic one, which then becomes a blocking elec- trode to the liquid electrolyte. Under these conditions reaction (1) cannot take place.This lack of reactivity is not due to a density effect since the density of the 1236 pellets was 4.5 g cm-3 (72% of the theoretical density). Cyclic voltammo- grams were unchanged from the initial background scans and subsequent GC/MS analysis of this solution showed no traces of propanal or any other reaction products. This experiment was also repeated using Lao.5Sro.5C003 -x as the working electrode. Current could be passed as in the case of 1237 and after reduction equivalent to x=0.07, GC/ MS analyses of the solution in the working electrode compart- ment were made.There were three reaction products: the two major products were identified as propanal and tributylamine from their mass spectra; the other product could not be identified. Diffusion-coefficient Measurements The effective chemical diffusion coefficient for oxygen in polycrystalline 123, B, measured at room temperature by the method of ref. 29, was found to be 4+_2 x cmz s-' irrespective of grain size, pellet density, porosity, surface treatment, organic solvent or applied voltage, as described in detail elsewhere. 34 a Discussion Effects of Reduction on 123 All experimental observations on material that has been reduced electrochemically at room temperature indicate bulk oxygen loss.While the density dependence of the reduction process and the agreement between y in eqn. (1) as determined by coulometry and by iodometric titration could be explained by either a near surface or a bulk electrochemical reduction of the material (but not by a chemical decomposition), these results, taken together with the X-ray diffraction data, show that a bulk reduction occurs. If only near-surface reduction were to take place, we would expect to see X-ray diffraction from the non-reduced core. Using accepted estimated sensi- tivity limits for X-ray powder diffraction, and the 10 pm average grain diameter found from scanning electron microscopy on our samples, we calculate that unreduced cores with diameters no less than 3 pm should have been detected in the X-ray diffraction patterns.Smaller unreduced cores can constitute d 3% of the sample. In addition, the persistence of significant amounts of unreduced cores in the grain can be excluded since they would be detected in the magnetic suscep- tibility and electrical resistivity measurements of the unan- nealed samples as a sharp drop followed by a In our samples, only a continuous drop is observed. (Recent results on fine-grained, polycrystalline thin films, obtained from Tel Aviv University, show near-uniform changes in the XRD patterns upon r.t. reduction, again supporting the occurrence of bulk reduction.34b Because the reduction is carried out at room temperature, the issue of oxygen mobility at this temperature is a crucial one.Straight extrapolation of high-temperature values obtained from solid-state electrochemical measurements, sug- gests a value for 0" of ca. 10-zocmz s-'.~This is similar to what can be estimated by extrapolating the 600-300 "C self- diffusion data, obtained from tracer studies on single crystals and oriented polycrystalline samples,36 to room temperature. Such a small diffusion coefficient would limit oxygen loss to a surface region only and therefore cannot explain our results. Extrapolation of the values obtained between 900 and 300 "C from the earlier men_tioned polarized light microscropy experi- mects" suggests a D of 7 x lo-'' cmz s-' at 300 K.Estimates of D based on the amount of oxygen removed over time from the average grain size of our samples give values on the order of 10-'o-lO-'z cm_' s-'. These estimates agree with our measurements of D (according to the method of ref. 29) at room temperature. These results should be compared with the values of 1.4x10-" and 5 x1O-l5 cm2 s-' at 25 "C for in other perovskites, Ndo~50Sro~50C00328and 50C~03Lao.50Sro, -x,z9 respectively. All these estimates assume that diffusion of the oxygen species within the grain to the surface is the rate-limiting step, rather than subsequent chemi- cal reactions at the surface with species in the electrolyte. This assumption is based, inter ah, on our measurements of the effective diffusion coefficient (see Results section)34 and fits with the shell model that will be described below.Clearly, the room-temperature reduction process leads to materials that are not homogeneous, as shown by the (h 0 0), (h k 0) and (h k 1) peaks in X-ray powder diffraction patterns and by broadened transitions to the superconducting state. However, careful analysis of the experimental results can yield information on the electronic and structural properties, which are important for understanding superconductivity in these materials. Both the shift observed in the a parameter and the broadening indicate that a number of domains/phases with different a parameters form. Such inhomogeneity is to be expected because of the low temperature of the reduction process.This shift in the a parameter is consistent with oxygen removal, as shown by diffraction studies on oxygen-deficient material prepared by other high-temperature When we compare the lattice parameters reported for such samples with our samples of similar average oxygen content, we find that the shifts for our reduced, unannealed samples show regions with (and an averaged value of) a larger and a c value that is significantly smaller than found in the former (assuming minimal shifts in b, cf. Fig. 2). This then suggests that room-temperature reduction produces samples contain- ing regions with a structure different from those obtained in more homogeneous samples prepared by high-temperature reduction.The effect of these metastable structures is seen in the occurrence of the 20 K transition after annealing. Indeed, the observation of a distinct 20 K transition alongside the well known 60 K one may indicate the existence of another 'plateau' of stable phases. This idea is supported by the recent results of Jorgensen et which confirm our report and support our deductionz7 for the existence of a stable, ordered 20 K phase. The 20 K transition is not observed (initially) in samples prepared by higher-temperature technique^.'^-'^ This may be an indication of the lower stability of this phase as compared to those with higher T,. Evidence for metastable regions of disordered oxygen within a crystallite has also been obtained from electron micros~opy.'~ Models for Room-temperature Reduction of 123 The broadening in the superconducting transitions can have several explanations.One possibility is physical degradation 344 of the grain boundaries leading to weak links. This can provide one simple explanation for the observed normal-state thermal activation of the electrical resistivity in strongly reduced samples. However, this can be discarded as the sole cause for several reasons. First, a grain-boundary effect cannot explain the results from the X-ray powder diffraction and magnetic susceptibility experiments, which indicate a bulk effect. In addition, samples in which the grain boundaries have been damaged by ion-beam irradiation3’ show electrical behaviour at T, which is substantially different from the behaviour observed in our samples.The ion-beam irradiated samples show a sharp drop in electrical resistivity at T,and then a tail. In some severely damaged samples, only a dip at the original T, is observed and the resistivity increases with increasing temperature. In such samples, ion-beam irradiation has no effect on the crystallinity. In comparison, X-ray powder diffraction showed that the bulk crystallinity of our samples had been affected (Fig. 2). Also, the annealed samples in their normal states showed thermally activated behaviour with effective activation energies similar to those found for the as- reduced samples over comparable temperature intervals, Finally, we tested to see if the conditions used for post- reduction annealing are sufficient to promote sintering.This was checked by annealing at 300 “Cfor 24 h an unsintered pellet of 123. Although the transition to the superconducting state was observed by magnetic susceptibility on this sample, neither a full transition nor a sharp drop in resistivity was observed in the electrical-resistivity measurements, indicating that the grain boundaries are not reconstructed under these annealing conditions. These results confirm that grain-bound- ary degradation cannot be the sole cause for the broadening. The most likely major cause for the broadened transitions upon reduction is the presence of a residual gradient of oxygen content within each grain, which will disrupt long-range order.This is consistent with the low temperature of preparation and the X-ray powder diffraction data. This gradient can arise from the fact that oxygen mobility at room temperature is still insufficient for complete equilibration of the oxygen concentration in our relatively large-grained, room-tempera- ture reduced samples. (It is worthwhile here to point out that room- temperature reduction of ca. 1 pm grain-size polycrys- talline films does not lead to broadening of the X-ray diffrac- tion peaks.34 During reduction oxygen loss will occur near the surface of the grains where most of the electrical potential drop (A4) occurs. As a result of initial oxygen loss, a chemical potential gradient (Ap) is set up which extends deeper into the grain.The resulting gradient in electrochemical potential (AD) will decrease with increasing depth into the grains. After reduction (A$=O), a finite Ap will remain. From the exper- imentally observed change in rest potential (ca. lo2 mV) during tens of minutes after reduction, we deduce that this Ap can lead to some further migration of oxygen, thus decreasing the final gradient, although this may be predominantly a surface effect. The possibility of such room-temperature migration of oxygen, under the influence of a chemical potential gradient alone, is underscored by the recent results of Jorgensen et .~~~1 and by the transmission electron microscopy obser- vations of Miiller et aL3’ As Ap decreases, this process will become slower until it finally becomes negligible on our timescale, leaving a grain with gradually decreasing oxygen content towards the surface (Fig.6). The changes in the physical properties upon reduction can be understood by comparison with results obtained for more homogeneously reduced samples. T, is expected to be higher (and the normal-state resistivity is expected to be more metallic) further inside each grain, where less reduction has occurred. This leads to a shell model, shown in idealized fashion in Fig. 7. A somewhat related model, to explain the J. MATER. CHEM., 1991, VOL. 1 *t I b--VO gt 0 distance into particle ‘ ‘ distance into particle Fig. 6 Effect of electrochemical reduction on oxygen content x and chemical potential Ap as a function of time of reduction and distance into an individual particle.V, is an oxygen vacancy in the 123 lattice, 0, is an oxygen atom on one of the possible oxygen sites in the basal plane, t, and t2 are successively increasing times of reduction and p, is the chemical potential of oxygen Fig. 7 Schematic illustration of the idealized shell model for 123, reduced at room temperature. The change in density of shading represents changes in oxygen content in a particle experimentally found lack of dependence on [O], i.e. x in YB~,CU~O~-~of the out-diffusion rate of oxygen, has been suggested by Tu et aL3*In the shell model more shells undergo the transition to the superconducting state, as the temperature is gradually lowered, and the resistivity of each grain gradually decreases.Full superconductivity is reached when the surface (most reduced) of each grain becomes superconducting (although tunnelling and proximity effects could lead to full superconductivity while the immediate surface region is still not superconducting). The observed effect of changes in the current density on the electrically measured transition to the superconducting state can be construed as support for this model. Another model that could explain the gradual decrease in resistivity of the reduced samples with decreasing temperature involves differences in overall oxygen content from one grain to another. According to this model, one grain might be completely superconducting, while another would not yet be superconducting, again leading to a gradual decrease in resistivity with decreasing temperature.However, there appears to be no obvious physical reason for such an abrupt change of properties between grains. The first model suggests a range of T,,i.e. not only at 60 and 90 K (similar behaviour was noted also by Namgung et QZ.,~’ using samples quenched in Hg). We attribute the occur- rence of such intermediate T, values to the lack of full relaxation of the lattice (cf: discussion of Fig. 2, above) and the absence of long-range order in the a direction with respect to oxygen vacancies [cf: also ref. 12(b) for a related idea]. Again, this is an effect of the low temperature of preparation. We note that results from high-resolution electron microscopy studies on oxygen-deficient 123 have been interpreted to suggest that superconductivity can occur even in samples that show very-short-range ordering of oxygen and vacancies.” J.MATER. CHEM., 1991, VOL. 1 Subsequent annealing of the room-temperature reduced samples allows for long-range order as deduced from the observation of definite 60 and 90 K transitions. Structural Implications of the Model Neutron powder diffraction data of oxygen deficient samples have shown that the most labile oxygen is located on the (O,+, 0) site.14 From this, together with the observation that for our reduced samples it is mainly the u lattice parameter that is affected, we deduce that the gradient in oxygen content leads to increasingly larger sections of uninterrupted -Cu(l)-0-Cu(1)-0-chains and longer distances between interrupted chain fragments deeper into the core of the grains.Long-range order of the type observed by X-ray diffraction in reduced samples'' or even the (probably shorter- range) order observed by electron diffraction in higher-tem- perature quench-reduced samplesI6 is not expected to be observed here owing to the low temperature of preparation. Near(est)-neighbour effects may lead to some degree of local ordering of oxygen vacancies but this would not be detected in the X-ray powder diffraction. The low temperature of preparation will also limit any structural rearrangements to local relaxation of the atoms. The broadening in the X-ray powder diffraction (the scattering is primarily due to the heavy atoms) can then be explained by the different degrees of relaxation of the heavy atoms due to the oxygen gradient.40 There is the additional consideration that not all grains have to respond in exactly the same manner to reduction.(Strain and small particle size can also cause broadening in X-ray powder diffraction peaks but cannot account for the shifts in a). Further structural studies are necessary to clarify these points. Upon annealing the sample, oxygen lability is increased owing to the higher temperatures, and structural rearrange- ment to stable phases occurs on a very short timescale, as reflected in the observation of distinct transitions to the superconducting state at 60 and at 20 K.This type of struc- tural rearrangement is probably an oxygen vacancy ordering, similar to that observed in samples reduced by other method^.".^^ The recent results of Jorgensen et confirm that such rearrangement results from the metastability of the disordered 0 sublattice. The differences in behaviour between unannealed and annealed samples with the same oxygen content can then be attributed primarily to the ordering of the oxygens. Reduction Products in the Electrolyte The oxygen that is extracted electrochemically from 123 reacts further with propylene carbonate to form propanal. The reaction can be described by the following equation zOCOCH2CH(CH3)0+YBa2Cu307--x +2ze--+zCH3CH2CH0+zC0; -+YBa2Cu307--x -(3) The cathodic decomposition of propylene carbonate has been studied previously and it was found that on metal electrodes (Pt, Ni, Li etc.)41*42 and propylene carbonate decomposes to propene and C0:- according to the following equation: OCOCH2CH(CH3)O+2e-+CH3CH=CH2+COi-(4) It is conceivable, although not likely, that propylene carbonate decomposes chemically, OCOCH2CH(CH3)O+CH3CH2CH0+C02 (5) However, this decomposition does not seem to be catalysed by 1236. The observed absence of measurable amounts of propanal using 1236 could simply be due to the very small amounts of current that we succeeded in passing, giving concentrations of propanal below the detection limit.How- ever, the main purpose of this control experiment was to show that the reaction is not a chemical reaction catalysed by an oxide biased with an electrical potential. There are also no reports of this type of decomposition, except by heating at temperatures >150 "C, in which case other products are formed besides propanal and carbon At room temperature, propylene carbonate undergoes an alkaline hydrolysis to yield propylene glycol, not pr~panal.~' Based on this discussion, we propose the following reactions [02-] +Cog- (6) +OCOCH2CH(CH3)b-+CH3CH2CH0 or [02-]+H20+20H- (74 OH-+OCOCH2CH(CH3)O- CH3CH2CH0+HCO, (74 The observation of the formation of tributylamine along with propanal during the reduction of Lao.5Sr,.5C003 -x is indica- tive of the presence of a reactive oxygen species on the surface of the oxides, since it is known that tetraalkylammonium salts react with strong bases to form trialkylamine~.~~ Under the conditions of the present experiments, [02-1 may be reacting directly or it may react first with trace amounts of water in the solvent to form OH-, which then reacts with the propylene carbonate and the tetrabutylammonium ion.The agreement between the values of y in eqn. (1) as calculated by coulometry and by iodometric titration, using oxygen di-anions [n=2, in eqn. (l)], shows that, when the oxygen leaves 1237 during reduction, it is not in a peroxide or superoxide form. The fact that only propanal was observed with 1237 may be due to differences in catalytic activity between Lao.5Sro~5Co03--x and 1237, and in particular the presence of La and Co instead of Y and Cu.The reaction of a 123 cathode with propylene carbonate to produce propanal and C0:-is analogous to the high- temperature loss of oxygen in 123. This process can be written in terms of the defect chemistry of oxygen47 (at least in the region above the metal-insulator transition) as [2zhib, YBa2Cu3O7]7+zO2 +[zV& YBa2Cu307 -,] (8) where hib is a hole in the valence band and V, is a dipositive oxygen vacancy. The square brackets enclose electrically neutral entities. The equivalent reaction for the electrochemi- cal reduction of 123 and subsequent reaction with propylene carbonate can be written as I I2ze-+[2zh;,, YBa2Cu307]+zOCOCH2CH(CH3)0 -+[zV& YBa2Cu307-,I +zCH3CH2CH0+zC0:-(9) where the electrons are introduced electrochemically.In both instances YBa2C~307-x is in the same final electronic state. As noted above, the formation of propanal is not an electro- chemical reaction, but is due to the presence of a reactive oxygen species on the surface of the 1237 electrode. Comparison between eqn. (8) and (9) (high-temperature DS. room-temperature methods of reduction of 1237) shows both the similarity of the two methods in that the stoichiometry of 1237 is identical (although there is a structural difference) and the difference in the fate of the oxygen. This difference, taken together with the difference between the products of propylene carbonate decomposition on 123 on the one hand and those obtained on metals or on 1236 on the other hand is significant and confirms earlier observations of 123 as an 346 active catal~st~~-~' and suggests its use in other catalytic and electrocatalytic systems.Conclusion We have found that the oxygen content of 123 can be reduced at room temperature in an electrochemical cell with a propyl- ene carbonate-tetrabutylammonium perchlorate electrolyte. This reduction is accompanied by an unusual reaction of the propylene carbonate on the 123 electrode which can be attributed to the relatively high oxygen mobility in this material and different surface chemistry as compared to a metal electrode. The as-reduced samples are inhomogeneous (probably with respect to an oxygen gradient), and appear to contain regions of new metastable structures, distinct from those obtained by high-temperature reduction.Comparison of the physical properties of the samples, as-reduced, with samples after annealing show that T, is a continuous function of oxygen content. The formation of samples with a T, of 60 K (and 20K) can be ascribed then to the formation of structures at temperatures that are high enough to allow for ordering of the oxygen vacancies. The distinct 20 K transition appears to be accessible only uia initial room-temperature reduction or by low-to-room-temperature rearrangement of quench-reduced samples.33 This work was supported by the United States-Israel Binational Science Foundation, Jerusalem.We wish to thank N. Fleischer for electrochemical advice, S. Reich for use of electrical resistivity equipment. A. Tishbee for help with the GC/MS analysis, and I. 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ISSN:0959-9428    

DOI:10.1039/JM9910100339

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

347J. MATER. CHEM., 1991, 1(3), 347-356 Vitrification in Low-molecular-weight Mesogenic Compounds Wolfgang Wedler,” Dietrich Dernus/ Horst Zaschke,a Kristina Mohr,” Wolfgang Schaferb and Wolfgang Weissf Iog “Martin-Luther-Universitat HaIle- Wittenberg, Sektion Chemie, 0-4020 HaIle (Saale), Weinbergweg 16, Germany Werk fur Fernsehelektronik, 0-1160 Berlin, Ostendstr. 1-5, Germany “Spezia Ich em ie Leipzig, 0- 7030 Leipzig, Elstera ue 9, Germany Systematic investigations have allowed us to formulate a structural model representing the relationship between vitrification and mesogenity in low-molecular-weight liquid-crystalline compounds. The tendency to transform the highly viscous mesophases into glass phases is caused mainly by the bulkiness of the molecules.The freezing of such mesophases, oriented by external influences at high temperatures, can be used for information storage. Keywords: Liquid crystal; Vitrification; Differential scanning calorimetry; Viscosity According to the well known Tammann rule,’ all substances length :breath ratios. The common feature of all these mol- in nature can be transformed into the glassy state below their ecules is their unconventional structure. l l melting temperatures. However in most cases, the glassy state In the following, we present a survey of some important cannot be reached, because a strong tendency for crystalliz- unconventionally constructed mesogenic compounds with ation at temperatures between the melting point and the glass- strong glass-forming tendencies.transition temperature, Tg, exists. The glassy state in nematic, smectic and cholesteric liquid- One-string Compounds crystalline phases can be useful for the construction of infor- (1) Lateral branches: mation-storing materials. Information can be transferred to ref. 12 the mesophase at temperatures above Tg and can be stored in the glassy state for a long Though glass-transition phenomena seemed to be a privilege of polymeric liquid crystals, they have also been observed in some low-molecular- ref. 13weight mesophases.6-8 In most of these mesophases the glass-transition temperature was much lower than room temperature. ‘c0,-x 0..Kuhrmann’ and Baentsch’ synthesized and described liquid-crystalline low-molecular-weight substances, which ref.3showed glass-transformation behaviour. Based on their exper- imental results and on assumptions of the free-volume model of glass-forming liquids, we have developed a generalized structural model. This model allows us to find a compromise between the mesogenity of the substances on the one hand, and the glass-forming tendency on the other. The aim is to create enantiotropic, mostly calamitic, liquid-crystalline sub- (2) Terminal branches: stances with glass transitions above room temperature. ref. 14 The calorimetric and viscosimetric measurements reported here enable the systematic investigation of the relationship ‘swallow-tailed’ compounds between the glass-forming tendency and molecular structure. (b) Ro~C;c~~~~2C~co~~c~=c,c~*~,C02R’ ref.14 RO,C Materials ‘bi-swallow-tailed’ compounds Molecular Structure (c) Laterally and terminally branched, non-symmetric com- The substances of ref. 9 and 10 show a significant deviation pounds: from the rod-like shape of mesogenic molecules. They are branched in the centre of the molecular unit by condensed ring systems and are ‘bulky’ in shape, e.g. Twin Structure (1) ‘Siamese twin’ molecules: lS ref. 10 cr 142.2 N 207.0 IS (see Tables 1-9, later) “;px (x=O, 1, 2) In the recent years other types of mesogenic compounds have been synthesized, which have molecules with low 348 J. MATER. CHEM., 1991, VOL. 1 (2) Twin molecules with flexible bridge groups:' (C) Synthesis of compound /C02-CH2-@02 'CO2-R'Q /CO2-iy The elongated shape of the molecules, which guarantees the existence of the niesophases, can be disturbed by several structural fragments.In almost all cases the mesophase stab- ility is weakened, whereas the tendency of vitrification is strengthened. Synthesis The synthesis of one-string compounds type l(a), l(b), 2(a) and 2(b) as well as of the twin-structured mesogens has been discussed in detail in the references given. To elucidate the synthesis routes of the cases l(c), 3 and of some special compounds of type l(b) (see Table 2 later) we will give examples. (A) Synthesis of compound C3Hf19C029c02a Br CH,O 4-Propyloxy-3-bromobenzoic acid was esterified with 4-hydroxy-3-methoxybenzaldehyde(vanillin) according to the method of Einhorn.The obtained 4-(4-propyloxy-3-bromo) benzoyloxy-3-methoxybenzaldehydewas oxidized by Cr03 and concentrated acetic acid. This yielded 4-(4-propyloxy-3- bromo)benzoyloxy-3-methoxybenzoic acid, which was esterified again with 2-naphthole according to the Einhorn method. Repeated purification in mixtures of ethanol and toluene gave a compound with the following phase sequence (temperatures in "C): cr 151 (N 87) IS; TfN=31 "C (symbols and abbreviations see at the beginning of Tables 1-9). (B) Synthesis of compound The first step of the synthesis was the (4:2) cycloaddition (Diels-Alder reaction) of 2,3-dimethylbuta- lY3-diene with p-benzoquinone. The Diels-Alder adduct was isolated in high yield.The same good result was obtained by using other dienes, such as cyclopentadiene, cyclohexa- 1,3-diene or buta- 1,3-diene. In the following reaction the quinone adduct was transformed by isomerization with hydrochloric acid in etha- nol solution to 5,8-dihydro-6,7-dimethyl-1,4-dihydroxynaph-thalene. This hydroquinone derivative was acylated after the method of Einhorn with 4-ethyloxybenzoyl chloride at room temperature. For purification the prepared ester was recrys- tallized from ethanol to yield light crystals with the following phase sequences: cr 215 N 241.6 IS; T:N=52 "C (see also Table 3 later). The substance was prepared in two reaction steps. The esterification of 1,4-dihydroxy-2-naphthoicacid with 4-nitrob- enzyl bromide was performed in dried acetone.The formed hydrogen bromide can be eliminated by triethylamine without saponification of the halogenide. The 4-nitrobenzyl 1,4-dihydroxy-2-naphthoate was recrystallized from dimethylfor- mamide; melting range 218-220 "C. This phenolic intermedi- ate was acylated with 4-n-propyloxybenzoyl chloride in pyridine following the known procedure described by Einhorn. The crude product was recrystallized from pentan-1-01. The melting behaviour is given in Table 2: cr 204 (N 100) IS; Ty =49 "C. Experimental Calorimetric Measurements Calorimetric measurements were carried out on a Perkin-Elmer DSC-2 device which was cooled constantly by a COz-ethanol mixture. Heating and cooling experiments were made in amounts of 5-8 mg, placed in aluminium capsules.We performed the calorimetric observation in three steps: (1) heating the crystalline material into the isotropic liquid phase (heating rate 20 K min-'; observation of melting and other phase-transition processes); (2) quenching the sample (cooling rate, 40 K min-';observation of phase-transition processes and of the glass-transition step); (3) heating the liquid-crystal- line glassy material again into the isotropic liquid phase (heating rate 20 K min-';observation of the transformation interval, of the crystallization and of subsequent phase tran- sitions). Sometimes the supercooled liquid crystallized during quenching in the calorimetric device. In these cases the samples were heated and quenched outside the calorimeter using a heating plate and a cooling medium (e.g.dry ice). Step 3 of the procedure was then continued. Viscosimetric Measurements We made measurements on a Rheotest-2 viscosimeter to gain information about the dynamic viscosity of pure and mixed glass-forming liquid-crystalline phases. The substance was placed between a fixed metallic plate and a rotating metallic cone, The plate could be thermostatted. The cone had a diameter of 36 mm, was driven by a motor and had rotation velocities of 81 and 243 rpm. This enabled shearing tensions between 7540 and 45 200 Pa to be reached and a velocity gradient between 1620 and 4860 s-'. An external magnetic or electric field to orient the director of the sample could not be used.So we assume that, owing to mechanical forces, the molecules were oriented along the shearing direction. As Schneider and Kneppe' demonstrated, the effective shear viscosity under the condition of stream orientation is close to the shear viscosity q1 defined by Mieso~icz'~ (director parallel to the vector of shearing velocity). Provided that the Leslie- Ericksen coefficients a2 and a3are both negative, then accord- ing to ref. 16 one can suppose that under the influence of the shearing tension the director is oriented in the shearing plane. Moreover, the equilibrium angle of stream orientation, O,, between the director and the vector of shearing velocity has a small value (5-1 5" for MBBA [N-(4-methoxybenzylidene)- J. MATER. CHEM., 1991,VOL.1 20 K min-'-/Icr Tm IS -40 TNI -----\ K min-' glassy nematic- N \I II " iliIS W 50 100 150 ' ' 200-91°C Fig. 1 DSC plot of 2-naphthyl-4-(4-cyanobenzoyloxy)-3-methoxy benzoate: showing a glassy phase with nematic structure I I -SC -NV IS glassy SC Fig. 2. DSC plot of 4,4-dimethyl-(4-n-octyloxybenzoyloxy)benzoyl-oxybenzylidene rnal~nate,'~ showing a glassy S, phase 4-butylaniline]). From this point of view, we suppose that our measured viscosities are near to the appropriate ql Miesowicz viscosity, in the best case. On the other hand, this assumption cannot be true if the coefficients a2 and a3 have opposite signs or if instabilities above a critical shearing tension appear. l6 Results Calorimetric Measurements The calorimetric plots of glass-forming mesogenic materials will be explained for a nematic and a smectic compound.Fig. 1 shows a typical three-step DSC-plot of a compound4 that creates a nematic glassy liquid-crystalline phase. The crystalline substance was heated with a heating rate of 20 K min-'. The melting point could be detected at a temperature of 189.8 "C, the corresponding phase-transition enthalpy had a value of 33.8 kJ mol-' (see first run). The second run was a cooling process of the first isotropic liquid phase. Using a cooling rate of 40K min-', an isotropic-nematic phase transition could be registered at 175.8 "C, having an enthalpy of 1.24 kJ mol -'. With further cooling no crystallization occurred, and the highly viscous, nematic substance was frozen into the glassy state at ca.40 "C. The glass-transition interval had a width of ca. 10 K. After this cooling process a new heating period again showed the transformation from the Table 1 Calorimetric investigation of the homologous series no. n cr N IS Tp/ "C 138.8(-11 1.1)141.1(. 136.4) 112.6(*106.2)99.6 * 109.0 93.5(-87.4) 81.0 92.0 27 22 13 5 -5 -8 78.3 82.5 -15 84.0 84.4 -16 no. n A,H/kJmol-' ANiHlkJ mol-' ACplJmol-' K-' 32.8' 1.36 181 39.3b 2.52 206 35.1' 2.07 224 35.2' 2.72 200 51.9" 2.22 197 38.2" 2.66 268 38.6" 2.43 220 45.5" 2.66 274 " First heating; 'second heating. cr =crystalline phase; Sc =smectic C phase; S,=smectic A phase; Sx=smectic phase, type unclear; N= nematic phase; IS =isotropic phase; TF =lower limit of the glass- transition interval (onset of glass-transition temperature); AmH= transition enthalpy of the melting process; ANiH =transition enthalpy of the transition nematic phase/isotropic phase; $NH =transition enthalpy of the transition smectic/nematic; ACp=jump of heat capacity caused by glass transition. nematic glassy phase to the supercooled, highly viscous nematic phase. This transformation can be recognised by a step in the curve, which corresponds to a C, variation of 150 J K-' mol-'.Further heating yields a slow, steady crystallization, which is characterized by an exothermic peak with a maximum at ca. 125 "C. The now crystalline material begins to melt at 181 "C.In Fig. 2 the three-step DSC plot for a smectic compo~nd'~ is depicted. All conditions are the same as in Fig. 1. This 'swallow-tailed' compound has a phase transition from the crystalline to the SA phase at ca. 104.4"C with a transition enthalpy of 37.4 kJ mol-'. The smectic phase transforms at 124 "C into a nematic phase (phase-transition enthalpy: 0.59 kJ mol-') and at 161 "C from the nematic into the isotropic phase (phase-transition enthalpy: 1.16 kJ mol-'). During the cooling process the phase sequence is encountered in reverse and at a temperature of 66°C the SA phase transforms into an Sc phase. The compound does not crys- tallize, if cooling is continued. Consequently, the highly viscous Sc phase transforms into the glassy state at CQ.0°C. Sub-sequent heating induces a step in the curve, beginning at -5 "C. The step has a height, corresponding to ca. 225 J K-' mol -'. Soon after forming the highly viscous, supercooled Sc phase from the glassy Sc phase, there is an intensive exother- mic effect at 20 "C indicating crystallization. The crystalline material melts at CQ. 94 "C,and then the phase sequence SA, N, isotropic liquid can be observed again. Tables 1-9 give a representative review of the phase behav- iour and the calorimetric data of some important classes of liquid-crys talline glass-forming substances (all temperatures in "C). With the aid of Table 7 the relationship between the glass- transition temperature and the clearing temperature can be 3 50 J. MATER.CHEM., 1991, VOL. 1 Table 2Calorimetric investigation of some laterally aromatically branched compounds no. compound cr N IS -204.0 100.0) c3H709c02~02c0m3H7 Co2-CH2+o2 213 'BHI 70~ c 0 2 ~ 0 2 c ~ 0 c 8 H7 I * 90.0 (-61.0) 91.0 C02~CH2)2~C(CH313 214 -113.0 96.5)- compound no. Ty/"C A,H/kJ mol-' AsXNH/kJmol-' ANiH/kJmol-' ACJJ mol-' K-' 14 34.0 49 47.6 3 44.9 -7 53.4 demonstrated. The elongation of the alkyl chain in this homologous series of initially isotropic 1 -naphthyl esters causes the creation of an anisotropic, probably nematic, phase. The clearing temperature rises with chain length, whereas the glass-transition temperature decreases. In this interpretation, growing alkyl chains stabilize the liquid-crystalline state but effect a decrease of the glass-transition temperature.Another phenomenon was observed for compound 8/1 (Table 8). This compound with a glass-transition temperature of 58 "C forms an isotropic glassy phase immediately after quenching. Observations by polarization microscopy over ca. 1.85 1.35 250 - 1.60 284 4.56 1.60 I46 - 1.56 234 3 days indicate molecular mobility in the glassy phase. Even at room temperature, ca. 25 K below the glass-transition interval, after 21 h beginnings of an anisotropic phase are visible (see Fig. 3), which grow and after ca. 2 days form a final picture that is compatible with a nematic phase (see Fig. 4).X-Ray investigations gave no indication of a crystalline phase.The isotropic-nematic phase transition which should occur at temperatures below the glass-transition temperature, in this compound is strongly supercooled. Kresse et d.'* have already demonstrated by dielectric relaxation measurements that ca. 30 K below the calorimetric glass-transition temperature the motions of the whole mol- Fig.3 Observation of the isotropic glassy phase of compound 8/1, 21 h after freezing. Small domains of an anisotropic phase are visible. Fig. 4.Final state of sample from Fig. 3 after 3 days. No covering No covering plates are used; 80-fold magnification, crossed polarizers plates; 80-fold magnification, crossed polarizers J. MATER. CHEM., 1991, VOL. 1 351 Table 3 Calorimetric investigation of mesogenic compounds with condensed ring systems at the centre of the molecules (4 no.n X cr N IS Ty/"C 3/la 2 215.0 -241.6 528H3C CH3 3l2a 3 * 172.2 -212.5 42-fJH3C CH3 3/3a 2 160.6 -172.8 23 no. ACJJ K-' mol-' A,H/kJ mol-' 3/la 121 41.6 4.20 3/2a 112 39.7 (was not measured) 3/3a 170 39.1 2.65 ref. 3, 10 X-no. X cr N IS Tr/"C 03/lb 192.4 * 200.4 38 3/2b * 150.0 -167.6 17 H3CeCH=N-3/3b 102.2 -146.6 .14 3/4b * 130.0 87.5) 23(a no. AC&J K-' mol-' A,H/kJ mol-' ANiH/kJmol-' 3/lb 155 41.4 1.34 3/2b 207 31.2 0.46 3/3b 180 44.1 0.58 3/4b 188 37.4 0.29 ecules are slowed down. This takes place according to a Viscosimetric Measurements dramatic drop in the free volume, available for mostly rotational motions around the long and the short molecular In addition to the calorimetric investigations we made viscosi- axes.The temperature behaviour of this relaxation mechanism metric measurements on pure compounds l/S-l/S (Table 1) follows the Vogel-Fulcher-Tammann-equation [see eqn. (1) and on mixtures. We were able to measure dynamic viscosities later]. Contrary to this, the motion of individual molecular only from ca. 150 mPa s (near to the clearing point) to ca. fragments, showing an Arrhenius-like behaviour, is unabatedly 1000 mPa s. Then, as a consequence of the mechanical forces, active. So we interprete the occurrence of such supercooling the emerging crystallization prevented further observation. effects in the glassy state as a microscopically visible expression In Fig.5 the log q us. T-' dependence is depicted. Clearing of molecular relaxation processes, which are still active in the temperatures can be recognized by a characteristic decrease glassy state. in the dynamic viscosity when the substances are cooled from J. MATER. CHEM., 1991, VOL. 1 Table 4 Calorimetric investigation of the homologous series no. n cr 411 1 412 2 413 3 414 4 no. ACJJ K-' mol-' 228 279 193 190 ref. 14 SC SA N IS Ty/"C 100.8( -69.0) 126.0 * 162.0 -5 104.0(*58.5) 110.0 144.0 -16 65.0( * 53.5) 99.0 130.0 -23 70.0(* 5 1 .O) -92.0 -115.0 -28 A,H/kJ mol-' AsANH/kJ mol-' ANiH/kJ moi- ' 37.4 0.59 1.16 48.6 0.31 0.82 45.9 0.33 0.88 38.3 0.41 0.88 Table 5 Calorimetric investigation of the homologous series no.n cr1 -176.0 * 195.0 * 102.3 * 80.0 * 81.4 -141.2 80.0 83.0 ACJJ K-' mol-' 208 252 251 26 1 274 239 222 218 cr2 SC N IS T:N/ "C -297.0 36 m232.0 2 a201.0 -5 112.5 94.5 (-60.0) (-60.0) ( * 74.2) ( -80.0) * 165.0-150.5 * 120.4 * 120.4 * 109.2 -15 -17 -16 -16 -16 A,H/kJ mol-' A,,,H/kJ mol-' ANiH/kJ mol-' 39.7 - 0.90" ? - 0.45' 45.0 - 0.35' 36.3 - 0.40b 19.4119.4 (2 peaks) 1.23 0.32 38.3 1.80 0.27 39.6 1.87 0.39 66.6 2.0 I 0.39 a Own measurement, decomposition probably in progress; 'values given in ref. 14. the isotropic into the nematic state. Furthermore, below the clearing temperatures in this Arrhenius plot a non-linear dependence occurs.This is characteristic behaviour of glass- forming liquids. In ref. 18 and 19 it was shown, that the Fig. 5. Arrhenius plot for temperature dependence of dynamic-vis- cosity for higher homologues in the series of 2-tert-butylhydroquinonebis(4-n-alkyloxy benzoates). Values of n: +, 5; 0,6; x ,7; 0,8 temperature dependence of relaxation processes is governed by an activation part and a free-volume part. At high tempera- tures the activation part dominates and at low temperatures the free-volume part dominates. We assume that according to the 'bulky' molecular shape the influence of the free volume dominates. Other influences are most effective in the vicinity of the clearing point.Consequently, we used the empirical Vogel-Fulcher-Tammann (VFT)equation,20, to fit our data. As Fig. 6 shows, this VFT model satisfactorily reflects the temperature dependence of the dynamic viscosity. Table 10 gives a s.ummary of all fitted and measured data of the four pure substances 1/5-1/8. Also in this table the temperature for a dynamic viscosity 9 = 10'' mPa s is given. According to an often cited rule2' this temperature should coincide with the calorimetric glass transition. Within the limits of error, the values of Table 10 are in good agreement, which also confirms the validity of the VFT model for our glass-forming nematic compounds. J. MATER. CHEM., 1991, VOL. 1 Table 6 Calorimetric investigation of several 2-naphthyl esters4 R R" R"' cr N ISno.R (aH CH30 -161.2 135.2) * 134.4 -218.8CH3 H H H 134.6 -250.0 204.6H H -177.2 H CH30 * 155.2 (* 89.4) H CH30 * 143.4 H Br * 136.8 * 185.2 H Br 102.6 124.0 H H 174.4 -308.0 H CH30 * 189.8 ( * 176.0) Ty/ "C ACp/J K-' mol-' A,H/kJ mol-ANiH/kJmol-' no. 25 137 34.5 0.90 2 196 28.6 0.78 15 219 27.1 not measured 12 218 47.5 0.68 26 179 43.6 0.79 7 ? not measured -0 141 35.0 1.12 -5 249 39.4 0.69 17 126 32.8 not measured 40 150 33.8 1.24 Table 7 Calorimetric observation of a homologous series of 1-naphthyl Table 8 Calorimetric observation of the homologous series esters4 / ref. 15so anisotropic no. n cr phase IS T:N/oC (Siamese-twin mesogens) 4 116.8 10 * 66.6 (~12.4) -12 no.m n cr N IS TY/"C * 87.8 ( * 30.0) -15 8/l 1 * 189.2 58 no. AC,/J K-' mol-' A,H/kJ mol-' A,,H/kJ mol-' 812 2 ~211.4 (. 76) 49 813 3 * 194.0 80.4) 46(a 71 1 156 32.3 -814 4 ~171.8 115.2) 41 0.60 815 5 -142.8 (a 109.7) 33(a712 I06 30.4 713 138 53.2 1.06 816 6 -148.0 ( 119.0) 31 817 7 -119.8 (* 1 1 3.6) 24 A,,H =enthalpy of phase transition from anisotropic, probably 818 8 132.2 (* 1 1 5.0) 24 nematic, to isotropic phase. 819 9 * 107.2 (* 87.0) 10 no. ACp/J K-' mol-' A,H/kJ mol-" ANiH/kJmol-' -1o4 273 40.1 a310 65.0 286 58.5 1.75 210 63.8 2.70 297 50.4 2.40:lo3 352 62.5 2.59 0 21 1 72.0 2.8 1 1 E 284 57.0 2.97e 1 40 44.6 2.14 1o2 a Could not be measured.Mixtures 15 10 5 The prevention of crystallization in liquid-crystalline phases (T-~,)-1/10-3 K-1 of pure low-molecular-weight compounds is one of the most Fig. 6. Vogel-Fulcher-Tammann plot of the same compounds as serious problems with respect to applications. It is mostly plotted in Fig. 5. Values of n: +, 5; 0,6; x ,7; a,8 pure substances with high glass-transition temperatures that J. MATER. CHEM., 1991, VOL. 1 Table 9 Calorimetric observation of some twin mesogens with aromatic bridges: C8H 17O~ c 0 2 ~ 0 2 c ~ o c 817 H X ref. 13 no. X cr N IS T:N/oC 911 -CO,fCH& 0 120.8 142.2 12 vOlcMirOpC- 912 125.6 12* 147.6 0 99.8 -179.6 21 914 * 140.6 ( * 127.4) 17 -Cop -CH2 -t$C:p -0pC - CH30 no.AC,/J K-' mol-' 371 554 475 296 A,H/kJ mol-' 45.8 33.4135.8 (two peaks) 50.7 75.7 ANiH/kJmol-' 4.67 3.38 4.25 3.54 Table 10 Viscosimetric data of the pure compounds l/5-1/8 0.099 885 241 (-32) 265 (-8) -5 0.461 607 253 (-20) 270 (-3) -8 0.020 1112 225 (-48) 254 (-19) -15 0.179 777 234 (-39) 256 (-17) -16 crystallize easily. Consequently, we prepared mixtures in order to decrease the formation rate of crystallites and to improve the conditions for freezing liquid-crystalline structures. Already in ref. 18 and 19 we have reported measurements on two glass-forming liquid-crystalline mixtures, consisting of four components. The principles for the creation of such glass- forming mixtures could be: (1) selection of not more than four components; (2) components should have preferably enanti- otropic liquid-crystalline phases; (3) creation of eutectic mix- tures.Composition and melting temperatures can be estimated by the Schroeder-Van Laar equation.22 The prerequisite is that the components should have different molecular struc- tures. (4) The beginning of the glass-transition interval changes approximately to a linear equation between the appropriate temperatures of the pure components (Ty):23 N Ty(mixture)= C xiT$N (2)i= 1 (5) The pure components should have low crystallization rates, but simultaneously, high glass-transition temperatures. For that reason we used compounds with alkyl chains contain- ing two to four methylene groups.As an illustration, we have chosen two mixtures (see Tables 11 and 12). Fig. 7 gives the calorimetric curves of both mixtures. The four-component mixture seems to be really eutectic. It does not crystallize, whereas the heating curve of the three-component mixture indicates a slight crystallization: at a heating rate of 20 K min-', ca. 9% of the substance crystallizes (exothermic effect at ca. 135 "C). Computed and measured melting temperatures also deviate from each other. This is a hint for the non-eutectic character which surely has its origin in the similar chemical structure of the 2-naphthyles- ter compounds. Evidently, eqn. (2) satisfactorily predicts the measured glass-transition temperatures. Discussion All the above experimental results allow us to formulate a structural model for the causes of glass-transition behaviour under moderate conditions at high temperatures in low- molecular-weight liquid-crystalline phases.(1) The main ingredient is a 'bulky' structural element at a central or terminal position of the rod-like molecule, resulting in a decreased 1ength:breadth ratio. Because of this, J. MATER. CHEM., 1991, VOL. 1 Table 11 Composition and calorimetric data of a four-component mixture (all temperatures in "C) ~~ component 6i9 1 /4 TF [according eq. (2)]: TfN(measured): T, (eutectic, computed and measured: TNi: 13 "C 10 "C 88 "C 127.5-131.5 "C X TY cr N IS 0.100 36 -188.2 263.0 0.124 17 174.4 a308.0 0.086 0.690 40 5 * 189.8 * 99.6 (* 176.0)-109.0 AC,= 189 J K-' mol-' A,H =38.6 kJ mol-' see Fig.7(a) ANjH= 1.45 kJ mol-' Table 12 Composition and calorimetric data of a three-component mixture (all temperatures in "C) component X TYI "C cr N IS c 3 H 7 0 ~ c 0 2 p c 0 2 ~ 0.365 31 -151.0 (-87.0) 615 Br CH30 0.352 26 -155.2 (-89.4) 5i 1 0.283 36 * 176.0 -297.0 T:N [according eqn. (2)] 31 "C T:N (measured) 30 "C AC,= 196 J K-' mol-' see Fig. 7(b) T, (eutectic, computed) T, (measured) 123.4 "C 146.3 "C second heating: AmH=3.9 kJ mol- ' TNi(wide clearing interval) 143-169 "C ANiH=0.55kJ mol-' x =molar fraction of the component monotropic phases are observed in most cases. The situation can be optimized, if the bulky element is situated in the centre of the molecule (e.g.1,4-disubstituted naphthalene derivatives show more enantiotropic phases, whereas 2-naphthylesters show monotropic phases and 1 -naphthylesters mostly give isotropic glassy phases; see Tables 3, 6 and 7). (2) Investigations into several homologous series has proved that the glass-transition temperature decreases if the number of methylene groups in the chains increases. The alkyl chains as well as alicyclic ring systems in the molecules decrease the glass-transition temperature (see Tables 1, 4, 5, 7 and 8). The systematic decay of the glass-transition tempera- ture for the substances from Table 1 is demonstrated in Fig. 8. nematic nematic isotropicglass T~~~= 10 oc no crystallization I I ....t... --4w1 100 1500 91°C --40 K min-' appearing of the nematic phase- 20 K min-'-L nematic glass nematic-w isotropiccrystallization____.._ = 30 "' (ca.9% of the mixture) 1-50 100 150 91°C The influence of alicyclic ring systems may be illustrated by the following example:I3 R T:N/oC cr N is GC5HIl 9 -142.8( 130.8) ---@Hl1 4 * 139.0(a 94.4) -(3) Additional lateral substituents increase the glass-tran- sition temperature because the bulkiness also increases. The length :breadth ratio becomes lower. For this reason the clearing temperature decreases or the liquid-crystalline phase generally vanishes. This may be demonstrated by the following 2-naphthyle~ters:~ X TfN/"C cr N is H 27 176.6 -217.8 176.8(0 92.0)CH30 46 (4) Polar substituents in the molecules increase both the clearing temperature and the glass-transition temperature.In our interpretation, the polar parts increase the density at constant temperature^.^^ This causes a comparatively lower free volume and higher viscosity and, in terms of the free- Fig. 7. DSC plots for two mixtures. See Tables 11 and 12 for volume a higher glass-transition temperature. We compositions also give an example to illustrate this4 Fig.8. Melting and clearing temperatures as well as lower limit of glass-transition intervals as a function of the chain length in the homologous series of 2-tert-butylhydroquinone bis(4-n-alkyloxy benzoates) T;"/'C cr N is CH30 25 161.2 (435.2) NC 40 * 189.8 (-176.0) The compound with the shortest alkyl chain in a homologous series has the highest glass-transition temperature. As a matter of experience, long alkyl chains cause the appearance of smectic phases.It consequently follows that the glass-tran- sition in smectic phases should predominantly be observed at low temperatures (e.g. Tables 4 and 5). In general, the above results indicate that vitrification at high temperatures and mesogenity are in contradiction to each other in low-molecular-weight substances. Using the concept of the free-volume theory25 for forming glass phases, the above rules may be interpreted by the reduction of the accessible free volume by polar groups, stiff molecules with strong attractive forces, bulky substituents in lateral or ter- minal position.26 Molecules following this structural arrange- ment have a low mesogenity so that compromises in the molecular structure are necessary.On the other hand, in order to observe glass transitions it is necessary to avoid crystalliz- ation. This may be reached by a non-symmetric molecular shape, elongation of alkyl chains (optimum C5-C7) and creation of mixtures. Obviously, this demands further compro- mises. Therefore, obtaining low-molecular-weight liquid crys- tals with high glass-transition temperatures is a difficult, although not impossible, task. J. MATER. CHEM., 1991, VOL. 1 The authors are indebted to Professor K. H. Dehne and Dr. A.Roger, Giistrow, for the syntheses of some of the substances mentioned in this work, as well as for fruitful discussions. References 1 G. Tammann, Aggregatzustande, Leipzig, 1923. 2 (a)D. Demus, G. Pelzl, W. Wedler, Anisotropes Festes Optisches Medium, DD WP C09 K/282 705/8, 1985; (b)D. Demus, G. Pelzl, Polarisatoren, DD WP 242 625 A 1, 1985; (c) D. Demus, G. Pelzl, Thermo-elektrooptisches Speicherdisplay, DD WP 242 624 A 1, 1985; (d) D. Demus, G. Pelzl and W. Wedler, Proc. Eurodisplay '87, London, September 1987, p. 71. 3 D. Demus, W. Wedler, K. Mohr, W. Weissflog, W. Schafer and R. Schmidt, Nematische Flussigkristalle mit Glasphasen, DD WP C09 K/307 180/2, 1987. 4 D. Demus, W. Wedler, W. Schafer, G. Uhlig and W. Weissflog, Anisotrope Optische Medien, DD WP C09 K/316 239/7, 1988.5 D. Demus, W. Weissflog, G. Pelzl, W. Wedler and A. Humke, Niedermolekulare Glasbildende Gemische mit Smektischen Phasen, DD WP C09 K/318 571/5, 1988. 6 J. Grebovicz and B. Wunderlich, Mol. Cryst. Liq. Cryst., 1981, 76, 287. 7 H. Yosioka, M. Sorai and H. Suka, Mol. Cryst. Liq. Cryst., 1983, 95, 11. 8 K. S. Kunihisa and Y. Satomi, Mol. Cryst. Liq. Cryst., 1986, 141, 1. 9 Ch. Kuhrmann, Dissertation, Halle, 1926. 10 S. Baentsch, Dissertation, Halle, 1931. 11 D. Demus, Liq. Cryst., 1989, 5, 75. 12 W. Weissflog, R. Schlick and D. Demus, Z. Chem., 1981,21,452. 13 W. Weissflog, D. Demus, S. Diele, P. Nitschke and W. Wedler, Liq. Cryst., 1989, 5, I 11. 14 W. Weissflog, A. Wiegeleben, S. Diele and D.Demus, Cryst. Res. Technol., 1984, 19, 583. 15 H. Dehne, A. Roger, D. Demus, S. Diele, H. Kresse, G. Pelzl, W. Wedler and W. Weissflog, Liq. Cryst., 1989, 6, 47. 16 F. Schneider and H. Kneppe, Fliejverhalten von Stoflen und Stoflgemischen, ed. W-M. Kulicke, Hutling und Wepf, 1986, ch. 8, pp. 318-368. 17 M. Miesowicz, Mol. Cryst. Liq. Cryst., 1983, 97, 1. 18 H. Kresse, S. Ernst, W. Wedler, D. Demus and F. Kremer, Ber. Bunsenges. Phys. Chem., 1990, 94, 1478. 19 R. Stannarius, W. Gunther, M. Grigutsch, A. Scharkowski, W. Wedler and D. Demus, Liq. Cryst., in the press. 20 (a) H. Vogel, Phys. Z., 1921, 22, 645; (b) G. S. Fulcher, J. Am. Chem. SOC., 1925,8,789; (c)G. Tammann and G. Hesse, Z. Anorg. Allg. Chem., 1926, 156, 245. 21 A. Feltz, Amorphe und Glasartige Anorganische Festkorper, Akad-emie-Verlag, Berlin, 1983. 22 (a) I. Z. Schroeder, 2. Phys. Chem., 1893, 11, 449; (b) J. J. van Laar, 2. Phys. Chem., 1908, 63, 216. 23 H. Schad and H. L. Zeller, Phys. Rev. A, 1982, 26, 2940. 24 D. Demus, 2.Chem., 1986, 26, 6. 25 (a) A. K. Doolittle and D. B. Doolittle, J. Appl. Phys., 1957, 28, 901; (b)W. Brostow, Polymer, 1980, 21, 1410. 26 (a) D. Demus, W. Wedler, W. Schafer, H. Zaschke, G. Uhlig, H. Kresse, S. Diele, G. Pelzl and W. Weissflog, Proc. 8th Liq. Cryst. Conf. Socialist Countries, Krakow, August-September, 1989, abstracts A-D; (b) W. Schafer, G. Uhlig, H. Zaschke, D. Demus, S. Diele, H. Kresse, S. Ernst and W. Wedler, Mol. Cryst. Liq. Cryst., 1990, 191, 269. Paper 0/04716C; Received 19th October, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100347

出版商:RSC    

年代:1991

数据来源: RSC