摘要:

Journal of Materials Chemistry Scientific Advisory Editor Profcssor Martin R. Bryce Departmcnt of Chemistry University of Durham South Road Durham DHl 3LE. UK Associate Editor Professor Jean Etourneau ICMCB Avcnue du Docteur Schweitzcr 33600 Pcssac Francc Editorial board Allan E. Underhill (Chairman) Bangor Peter G. Bruce St. Andrews Martin R. Bryce Durham Jcan Etourneau Bordeirux Managing Editor Janet L. Dean Deputy Editor Zoe G. Lewin Assistant Editor Graham F. McCann Editorial Secretary Miss D. J. Halls Wendy R. Flavell UMIST John W. Goodby Hull Klaus Praefcke Berlin Brian J. Tighe Aston International advisory editorial board K. Bechgaard Riso. Denmark J. Y. Becker Beer-Sheuu, Israel A. J. Bruce Murray Hill, USA E.Chiellini Pisrr, Iraly D. Coates PO&. UK P. Day London, UK D. A. Dunmur Sheffield. UK B. Dunn Los Angeles, USA W. J. Feast Durham, UK R. H. Friend Ctrmhridp, UK A. Fukuda Tokyo, Japan D. Gatteschi Florence, Ital-v P. Hodge Manclwster, UK Information for authors The Royal Society of Chemistry welcomes submission of manuscripts intended for publication in two forms, Articles and Materials Chemistry Communications. These should describe original work of high quality dealing with the synthesis, structures, properties and applications of materials, particularly those associated with advanced technology. Full papers contain original scientific work that has not been published previously. However, work that has appeared in print in a short form such as a Materials Chemistry Communication is normally acceptable.Four copies of Articles including a top copy with figures etc. should be sent to the Managing Editor at the Cambridge address. Journal of Muterials Chemistry (ISSN 0959-9428) is published monthly by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK. All ordcrs accompanied with payment should be sent directly to The Royal Society of Chemistry, Turpin Distribution Services Ltd.. Blackhorse Road, Letchworth. Herts SG6 IHN, UK. NB Turpin Distribution Services Ltd., distributors, is wholly owned by The Royal Society of Chemistry. 1996 Annual subscription rate EEA (incl. UK) E519.00, USA $934.00. Rest of World E532.00.Customers ~ A. B. Holmes Cambridge, UK H. Inokuchi Okuzuki, Japan W. Jcitschko Miinstrr, Germany 0.Kahn Bordeuu?r, France J. Livage Paris, France R. McCullough Pittsburgh, USA J. S. Miller Suit Lake City, USA K. Mullen Muinz, Germany L. Niinisto Espoo, Finland M. Nygren Stockholm, Sweden Y. W. Park Seoul, Korea N. Plate Moscow, Russia Materials Chemistry Communications contain novel scientific work in short form and of such importance that rapid publication is warranted. The total length is normally restricted to two printed A4 pages. However, special consideration will be given to communications with a large amount of essential diagramatic information.Submission of a Materials Chemistry Communication can be made either to the Managing Editor at the Cambridge addrcss, or via a member of the International Advisory Editorial Board. In the latter case, the top copy of the manuscript including any figures etc., together with the name of the person to whom the Communication is being submitted, should be sent simultaneously to the Managing Editor at the Cambridge address. All authors submitting work for publication are should make payments by cheque in sterling payable on a UK clearing bank or in US dollars payable on a US clearing bank. Air freight and mailing in the USA by Publications Expediting Inc., 200 Meacham Avenue, Elmont, NY 11003. USA POSTMASTER: send address changes to Journal qf Materials Chemistry, Publications Expediting Inc., 200 Meacham Avenue, Elmont, NY 11003.Second Class postage paid at Jamaica, NY 11431. All other dispatches outside the UK by Bulk Airmail within Europe, Accelerated Surface Post outside Europe. PRINTED IN THE UK. ~ ~~~~ Production Editor Stephanie Shah Technical Editors Carole J. Nerney Alan J. Holder Graphics Designer Ms C. Taylor-Reid Anthony R. West Aherdeen John D. Wright Canterhury Janet L. Dean (Secretary) M. Prato Trirste, Italy C. N. R. Rao Bangalore, India B. Raveau Caen, Frcrnce T. Rojo Bilhao, Spain J. Rouxel Nantes, France A. Simon Stuttgart. Germuny M. A. Subrdmanian Wilmington, USA S. Takahashi Osaka, Japrrn J. 0.Thomas Uppsulu, Sweden M.Vallet-Regi Madrid. Spain D. E. W. Vaughan Annundale, USA Y. Yamashita Okazaki, Japan required to sign an exclusive copyright licence. All submissions should be accompanied by a completed form (a blank for photocopying is reproduced at the end of the Information for Authors in Issue l), without which publication cannot proceed. A completed graphical abstract template should also accompany each submission. Full details of the form of manuscripts for Articles and Materials Chemistry Communications, conditions for acceptance etc. are given in Issue 1 of Journal of Muterials Chemistry published in January of each year, on the world wide web (htpp://chemistry.rsc.org/rsc/) or may be obtained from the Managing Editor. There is no page charge for papers published in Journal of Muterials Chemistry. Fifty reprints are supplied free of charge. Janet L. Dean, Managing Editor Tel.: Cambridge (01223) 420066 E-Mail (INTERNET): DEANJ@,RSC.ORG Fax: (01223) 420247 Advertisement sales: Tel. +44 (0)171-287 3091; Fax +44 (0)171-494 1134 ~~___ 0The Royal Society of Chemistry, 1996. All rights reserved. No part of this publication may be reproduced, storcd 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.

ISSN:0959-9428    

DOI:10.1039/JM99606FX001

出版商:RSC    

年代:1996

数据来源: RSC

ISSN:0959-9428    

DOI:10.1039/JM99606BX003

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Cumulative Author Index 1995 Alcantara-Rodriguez M., 247 de Souza D. P. F., de Souza M. F., 233 233 Iyoda M., 501 Jackson P. D., 137 Moine B., 381 Monk P. M. S., 183 Serrano J. L., 349 Shirota Y., 117 Ali F., 261 Dirken P. J., 337 Jacobson A. J., 81 Moon J. H., 365 Shitara Y., 11 Ali-Adib Z., 15 Allen J. L., 165 Domingues-Rodrigues A,, 207 James M., 57 Janes R., 183 Morales J., 37, 41 MoriT., 501 Slade R. C. T., 73 Smart L. E., 221 Al-Raihani H., Arai H., 455 495 Dragone R., 403 Durand B., 495 Jansson K., Jefferies G., 97, 213 131, 137 Moriga T., 459 Morineau R., 505 Smith J. R., 295 Smith M. E., 261, 337 Arai K., 11 Arriortua M. I., 421 Ashwell G. J., Attfield J. P., 57 Bahra G. S., 23 23, 131, 137 Dussack L. L., 81 Eadon D., 221 Eguchi K., 455 Ellert 0.G., 207 Enoki T., 119 Jimtnez-L6pez E.R- C. A., 247 Jones J. R., 305 Jumas J-C., 41 Kaczorowski D., 429 Naito H., 33 Nakano H., 117 Neat R. J., 49 Newport R. J., 337, 449 Nieminen M., 27 Southern J. C., 73 Steuernagel S., 261 Stoev M., 377 Su Q., 265 Suarez M., 415 Barberis G. E., Bardosova M., 421 375 Enomoto M., 119 Etter the late M. C., 123 Kagawa S., 97 Kanamura K., 33 Nii H., 97 Niinisto L., 27 Subrt J., 155 Sudholter E. J. R., 357 Barrans Y., 5 Barrel1 K. J., 323 Barton J. M., 305 Bassoul P., 5 Bast1 Z., 155 Battle P. D., 201, 395 Evans P., 289, 295 Ewen R. J., 289 Farcy J., 37 Farr I. V., 103 Foster D. F., 507 Franklin K. R., 109 Kanniainen T., 161 Katerski A., 377 Kawaguchi K., 117 Kennard C. H. L., Kikuchi K., 501 Kim J.H., 365 23, 137 Noma N., 117 Nygren M., 97 O’Brien P., 343 Ogawa K., 143 Ohyama T., 11 Olivera-Pastor P., 247 Sugahara Y., 69 Sugiyama S., 459 Suzuki H., 501 Takahashi M., 119 Takeda M., 119 Takehara Z-I., 33 Baur W. H., 271 Bay B. H., 331 Berry F. J., 221 Beteille F., 505 Bieniok A., 271 Ganguli M., 391 Gao Y., 369 Garcia J. R., 415 Garcia-Granda S., Gazzoli D., 403 415 Kim S. B., 365 Kitazawa T., 119 Knowles J. A., 89 Kochubey D. I., 207 Koivusaari K. J., 449 Olivier-Fourcade J., 41 Omenat A., 349 Oriakhi C. O., 103 Otterstedt J-E., 213 Pac C., 143 Tanaka M., 459 Tatam R. P., 131 Taylor R., 155 Teare G. C., 301 Teraoka Y., 97 Blin J. L., 385 Gentle I. R., 137 Koto K., 459 Parent C., 381 Teunis C. J., 357 Bomben A,, 15 Bornholdt K., 271 Boutinaud P., 381 George C.D., 131 Glenis S., 1 Gomi Y., 119 Kuroda K., 69 Labes M. M., 1 Lai S-W., 469 Park J. W., 365 Partridge R. D., 183 Pedrini C., 381 Tirado J. L., 37, 41 Tomkinson J., 449 Torncrona A., 213 Boyle D. S., 227 Bravic G., 5 Breen C., 253 Brendel U., 271 Goiii A., 421 Gore J. G., 201 Green D. A., 449 Grobet P., 239 Lavela P., 41 Lee C. K., 331 Lee E., 109 Lee G. R., 187 Peeters K., 239 Pelloquin D., 175 Pereira-Ramos J-P., Pertierra P., 415 37 Tran V. H., 429 Treacher K. E., 3 15 Tredgold R. H., 375 Trindade T., 343 Britton D., 123 GUOL-H., 369 Le Flem G., 381 Petrunenko I. A., 207 Troc R., 429 Brown C. R., 23 Bukhtenko 0.V., 207 Guzman G., Hahn J. H., 505 365 Lequan M., 5 Lequan R. M., 5 Pickett N.L., 507 Pizarro J. L., 421 Tsodikov M. V., Tundo P., 15 207 Bush T. S., 395 Bushnell-Wye G., Byrn S. R., 123 Cabrera S., 175 337,449 Hall S. B., 183 Hamada D., 69 Hamerton I., 305, 311 Hamet J-F., 165 Lerner M. M., 103 Leskela M., 161 Lezama L. M., 421 Lin C. L., 1 Pola J., 155 Pottgen R., 63, 429 Prellier W., 165 Qian M., 435 Vaidhyanathan B., 391 Valigi M., 403 Valli L., 15 van de Velde G. M. H., 357 CaldCs M. T., 175 Campbell S. A., 295 Campillos E., 349 Chane-Ching K., 5 Charters R. B., 131 Chassagneux F., 495 Chasseau D., 5 Chen J., 465 Hamilton D. G., 23 Harkema S., 357 Harrison W. T. A., 81 Hayashi H., 459 Heald C. R., 311 Hernan L., 37 Hersans R., 149 Hervieu M., 165, 175 Lin J., 265 Lindroos S., 161 Liu S., 305 Livage J., 505 Llavona R., 415 Lorriaux-Rubbens A., Lowendahl L., 213 Lynch D.E., 23 385 Ranjan R., 131 Rao K. J., 391 Rasheed R. K., 277 Rasika Abeysinghe J., 155 Ratcliffe N. M., 289, 295, Rauhala E., 27 Raveau B., 165, 175 30 1 van Dijk M., 357 Vansant E. F., 239 Vaughey J. T., 81 Vente J. F., 395 Vogt T., 81 Wallart F., 385 Wang S., 265 Warman J. M., 357 Chen X., 1 Hitch T. J. A. R., 285 Machida M., 69, 455 Rawson J. O., 253 Watts J. F., 479 Choi J. U., 365 Hobson R. J., 49 Macklin W. J., 49 Razafitrimo H., 369 West A. R., 331 Chung D. D. L., 469 Chung H., 365 Clarkson G. J., 315 Cole-Hamilton D. J., 507 Cook M. J., 149 Cooke S., 1 Crayston J. A., 187 Davies A., 49 Hodge P., 15, 375 Hoffmann R-D., 429 Holloway J., 221 Honeybourne C. L., 277, 285, 289, 323 Howlin B.J., 305, 311 Huang K-S., 123 Hudson M. J., 49, 89 Maksimov Y. V., 207 Mann B. E., 253 Marcos M., 349 Marucci A., 403 MatijeviC E., 443 Matsuyama H., 501 Mattei G., 403 McKeown N. B., 315 Rigden J. S., 337, 449 Rodriguez J., 415 Rodriguez M. L., 415 Rodriguez-Castell6n E., Rojo T., 421 Russell D. A,, 149 Saadoune I., 193 Saito K., 501 247 Whitfield H. J., 261 Wignacourt J. P., 385 Williams D. E., 409 Winfield J. M., 227 Woolley M., 375 Xu R., 465 Yao J., 143 Yao T., 33 Davies T. W., 73 Humberstone P., 315 McLendon G., 369 Salvad6 M. A., 415 Yonehara H., 143 Davis F., 15 Ihanus J., 161 McMurdo J., 149 Salvador S., 73 Yoshino H., 501 Davis S. J., 479 Dawson D. H., 409 Ikemoto I., 501 Imae I., 117 Mercey B., Michel C., 165 175 Sanchez L., 37 Sanders G.M., 357 Yu J., 465 Yue Y., 465 de Lacy Costello B. P. J., 289 Delmas C., 193 Ingram-Jones V. J., Inman D., 495 Inoue H., 455 73 Minami T., 459 Miyazaki A., 119 Moffat J. B., 459 Sasaki S., 501 Schouten P. G., SegalN., 395 357 Zeng H. C., 435 Zhang H., 265 Zhong Q., 443 i Conference Diary April 22-26 International Conference on Metallurgical Coatings and Thin Films California, USA Mary S Gray, Conference Secretary, ICMCTF 96, Suite 502, 1090 G Smallwood Drive, Waldorf, MD 20603 USA. May 2-7 Metal Clusters in Chemistry Strasbourg, France Dr Josip Hendekovic, European Science Foundation, 1quai Lezay-Marnesia, 67080 Strasbourg Cedex, France. E-mail: euresco@esf.c-strasbourg.fr;Tel: +33 88 76 71 35; Fax: +33 88 36 69 87.May 6-10 Second European Solid Oxide Fuel Cell Forum Oslo, Norway European SOFC Secretariat, PO Box 1929, CH-5400 Baden, Switzerland. Fax: +41 56 218466. May 12-15 Fifth European Symposium on Polymer Blends Maastricht, The Netherlands E. Meuris/M. Keulen, P.O. Box 5511, 6202 XA Maastricht, The Netherlands. Tel: +31 46 767252/761596; Fax: +31 46 767605. May 18-24 IS&T 49th Annual Conference Minneapolis, MN, USA Conference Manager, IS&T, 7003 Kilworth Lane, Springfield, VA 22151, USA. E-mail: imagesoc@us.net; Tel: +1703 642 9090; Fax: +1 703 642 9094. May 27-31 The Polymer Processing Society 12th Annual Meeting Sorrento, Italy Professor Jose M. Kenny, 12th PPS Meeting, Department of Materials and Production Engineering, University of Naples, P.Tecchio, 80125 Naples, Italy May 29/31 International Symposium on Nitrides Saint-Malo, France Professor P Verdier, ISN'T Secretary, U.R.A. "Verres et Ceramiques" Universite de Rennes I. Av. Genkral Leclerc. 35042 -Rennes Cedex, France. June 1996 International Conference on Intelligent Materials Lyon, France Mrs Claude Bernavon ICIM 96, Group #Etudes de Metallurgie Physique et de Physique de Materiaux, B&t. 502 -ler etage, Institut National des Sciences Appliques de Lyon, 20 avenue Albert Einstein, F 69621 Villeurbanne Cedex, France. E-mail: bernavon@insa.insa-1yon.fr;Tel: +33 72 43 83 85; Fax: +33 72 43 88 30. June 3-6 Dedicated Conference on Materials for Energy-Efficient Vehicles Florence, Italy The ISATA Secretariat, 42 Lloyd Park Avenue, Croydon CRO 5SB, UK.E-mail: 100270.1263@COMPUSERVE.COM;Tel: +44 181 681 3069; Fax: +44 181 686 1490 June 3-7 4th World Surfactants Congress Barcelona, Spain J. Sanchez Leal, General Secretary, CESIO AEPSAT, Comite Espanol de la Detergencia (CED). -Jordi Girona, 18-26. 08034 Barcelona, Spain. E-mail: cesio96@cid.csic.es; Tel: (343) 204 02 12 -400 61 00; Fax: (343) 280 53 00 -204 59 04. June 8-13 Fundamental Aspects of Surface Science: Semiconductor Surfaces Blankenberge, Belgium Dr Josip Hendekovic, European Science Foundation, 1quai Lezay-Marnesia, 67080 Strasbourg Cedex, France. E-mail: euresco@esf.c-strasbourg.fr; Tel: +33 88 76 71 35; Fax: +33 88 36 69 87. June 10-13 Science and Technology of Pigment Dispersion Luzern, Switzerland Dr A.V. Patsis, Director, Institute for Materials Science, State University of New York, New Paltz, NY 12561, USA. Tel: +914 255 0757; Fax: +914 255 0978. June 17-19 18th International Conference in Stabilization and Controlled Degradation of Polymers Luzern, Switzerland Dr A.V. Patsis, Director, Institute for Materials Science, State University of New York, New Paltz, NY 12561, USA. Tel: +914 255 0757; Fax: +914 255 0978. June 24-28 ILCC: 16th International Liquid Crystal Conference Kent, OH, USA 16th International Liquid Crystal Conference, Liquid Crystal Institute, Kent State University, P.O. Box 5190, Kent, OH 44242-0001, USA. E-mail: ILCClG@alice.kent.edu; Tel: +1216 672 2654; Fax: +1 216 672 2796.June 24-28 11th Bratislava IUPAC International Conference on Polymers High Tatras Bratislava, Slovakia Dr Lyda Rychla, Polymer Institute, Slovak Academy of Sciences, Dubravska cesta, CS 842-36 Bratislava, Slovakia. E-mail: upolrych@savba.sk; Tel: 0042 7 37 34 48; Fax: 0042 7 37 59 23. June 26-28 TMS, 1996 38th Electronic Materials Conference The Minerals, Metals & Materials Society (TMS), 420 Commonwealth Drive, Warrendale, PA 15086, USA E-mail: csc@tms.org; Tel: 412-776-9000; Fax: 412-776-3770 July 1-5 22nd International Conference in Organic Coatings -Waterborne, High Solids, Powder Coatings Athens, Greece Dr A.V. Patsis, Director, Institute for Materials Science, State University of New York, New Paltz, NY 12561, USA. Tel: +914 155 0757; Fax: +914 255 0978.July 6-12 Solid State Chemistry '96 Bratislava, Slovak Republic SSCH '96, Institute of Inorganic Chemistry, Slovak Academy of Sciences, SK-842 36 Bratislava, Slovak Republic July 7-12 XVIIth International Conference Organometallic Chemistry Brisbane, Australia XVIIth ICOMC Secretariat, Faculty of Science and Technology, Griffith University, Brisbane 41 11, Australia. E-mail: ICOMC@sct.gu.edu.au; Tel: +61 (0)7 3875 7217; Fax: +G1 (017 3875 7656. July 8-10 ESOPS 12 -"12th European Symposium on Polymer Spectroscopy" Lyon, France Dr G. Lachenal, Universite Lyon 1,Laboratoire des Materiaux Plastiques, Bld du 11Novembre 69622 Villeurbanne, Cedex France. E-mail: lachenal@matplast.univ-lyon1.fr;Tel: +33 72 43 12 11; Fax: +33 72 43 12 49.July 29-Recent Advances in Polymer Synthesis August 2 York, UK Professor P. Hodge, Department of Chemistry, University of Manchester, Oxford Road, Manchester, UK M13 9PL. E-mail: Philip.Hodge@man.ac.uk; Tel: +44 (0)161 275 4706; Fax: +44 (0)161 275 4598. August 4-9 IUPAC MACRO SEOUL '96: 36th IUPAC International Symposium on Macromolecules Seoul, Korea Dr. Kwang Ung Kim, Secretariat of IUPAC MACRO SEOUL '96, Division of Polymers, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea. E-mail: iupac@kistmail.kist.re.kr; Tel: +82 2 957 6104; Fax: +82 2 957 6105. August 4-9 The Tenth American Conference on Crystal Growth Colorado, USA Anthony L. Gentile, American Association for Crystal Growth, PO Box 3233, Thousand Oaks, CA 91359-0233 USA Fax: 805 492 4062 August 28-31 10th Conference of the European Society of Biomechanics Leuven, Belgium Dr. J.Vander Sloten, Executive Secretary, 10th Conference of the European Society of Biomechanics, Katholieke Universiteit Leuven, Division of Biomechanics and Engineering Design, Celestijnenlaan 200A, B-3001 Heverlee, Belgium. E-mail: jos.vandersloten@mech.kuleuven.ac.be;Tel: +32 16 32 70 96; Fax: +32 16 29 27 16. September 1-6 XIth International Symposium on Organosilicon Chemistry Montpellier, France Professor R.J.P. Corriu, Laboratoire des Precurseurs Organometalliques de Materiaux, UMR CNRS 44, Universite de Montpellier 11, Place E. Bataillon, CC 007, F34095 Montpellier Cedex 5, France Fax: +67 14 38 88.September 1-6 ECME 96, Third European Conference on Molecular Electronics Leuven, Belgium Professor F.C. De Schryver, Department of Chemistry, K.U. Leuven, Celestijnenlaan 200 F, B-3001 Heverlee, Belgium. October 9-14 Physical Metallurgy: Interfacial Engineering in Materials Castelvecchio Pascoli, Italy Dr Josip Hendekovic, European Science Foundation, 1quai Lezay-Marnesia, 67080 Strasbourg Cedex, France. E-mail: eurescoG3esf.c-strasbourg.fr;Tel: +33 88 76 71 35; Fax: +33 88 36 69 87. October 13-18 ISLC '96 15th International Semiconductor Laser Conference Haifa, Can Carniel, Israel IEEE/LEOS, 445 Hoes Lane, P.O. Box 1331, Piscataway, NJ 08855-1331, USA. Tel: +1 908 562 3898; Fax: +1 908 562 8434. October 27-12th International Congress on Advances in Non-Impact Printing Technologies November 1 San Antonio, TX, USA Conference Manager, IS&T, 7003 Kilworth Lane, Springfield, VA 22151, USA. E-mail: imagesoc@us.net; Tel: +1 703 642 9090; Fax: +1 703 642 9094.November 16-23 IS&T/SID Fourth Color Imaging Conference: Color Science, Systems & Applications Scottsdale, AZ,USA Conference Manager, IS&T, 7003 Kilworth Lane, Springfield, VA 22 151, USA. E-mail: imagesoc&s.net; Tel: +1 703 642 9090; Fax: +1 703 642 9094. August 24-27 ZMPC '97 International Symposium on Zeolites and Microporous Crystals Tokyo, Japan Dr Takahashi Tatsumi, Secretary, ZMPC '97, Engineering Research Institute, Faculty of Engineering, The University of Tokyo, Yayoi, Tokyo 113, Japan.Denotes a new or amended entry this month. Entries in the Conference Diary are published free of charge. If you wish to include an announcement please send full details to: Journal of Materials Chemzstry Editorial Office, Thomas Graham House, Science Park, Milton Road, Cambridge, UK, CB4 4WF. Tel: +44 1223 420066; Fax: +44 1223 426017. ... 111 MacroGroup UK International Conference on RECENT ADVANCESIN POLYMER SYNTHESIS Invited Speakers so fa accepted J M G Cowie Heriot-Watt E W Meijer Eindhoven W JFeast Durham A H E Muller Maim M K Georges Xerox, Canada S Nakahama Tokyo E Goethals Gent M Sawamoto Kyoto R H Grubbs Caltech A D Schluter Berlin R J Jer6me LiGge D A Tirrell UM4SS York University, UK 29 July -2 August 1996 PROGRAMME COMMITTEE Chairman: Professor Phil Hodge Manchester Secretary: Mr Ron Feasey EPSRC Dr Allan Amass Aston Professor Jim Feast Durham Dr Terry McGrail ICI Dr David Bott Courtaulds Professor Bruce Gilbert YorR Dr Neil McKeown Mmchester Dr John Ebdon Luncaster Dr Andy Holmes Cambridge Professor David SherringtonStrathcZydk INTERNATIONAL COMMITTEEADVISORY Dr D C Bott Courtaukds Professor W J Feast Durham Professor R P Qurk Akron Professor F Ciardelli Pzsa Professor E Goethals Gent Professor F Schue Monpellier Professor J M G Cowie Heriot-Watt Professor S Nakaharna Tokyo Professor D A Tirrell UMSS MEETING ANNOUNCEMENT Polymer Degradation Discussion Group (a sub-group of MACRO Group UK) PREDICTING AND EXTENDING POLYMER LIFETIMES The twenty-first annual meeting of the Polymer Degradation Discussion Group will be held at the University of Warwick, Warwick, UK.September 11th -13th 1996 with the title: PREDICTING AND EXTENDING POLYMER LIFETIMES Short communications and posters are invited. An initial abstract should be submitted to Dr S. Al-Malaika at the address below, for consideration by the scientific committee. Full details can be obtained from: Dr Sahar Al-Malaika Speciality Materials Research Group Chemical Engineering & Applied Chemistry Aston University Aston Triangle BIRMINGHAM B4 7ET e-mail: S.Al-Malaika@ Aston.ac.uk COURSE ANNOUNCEMENT Sixth Post-Doctoral Course on Degradation and Stabilization of Polymeric Materials The sixth Post-Doctoral Course on Degradation and Stabilization of Polymeric Materials will be held in Brighton, England, October 14 -18 1996.This course is aimed at scientists from academic or industrial laboratories wishing to obtain state-of-the-art understanding of the field of polymer degradation and stabilization. All of the lecturers are acknowledged experts in the field and delegates receive a course manual covering all of the lecture material. Full details can be obtained from: Dr N C Billingham School of Chemistry and Molecular Sciences University of Sussex BRIGHTON BNl9QJ England Tel: (44) 1273 678313 Fax: (44) 1213 677196 e-mail: N.Billingham@sussex.ac.uk J MATERIALS CHEMISTRY FORUM The Materials Chemistry Forum of the Royal Society of Chemistry was established in 1993 to promote the emerging discipline of Materials Chemistry, particularly through conferences and cross-subject-group activities.It organises the International Conference Series on Materials Chemistry, MCn as well as a range of more specialised meetings; regional meetings on Materials Chemistry are planned, with the first Scottish meeting scheduled for 1996. The Forum operates in parallel with the Journal of Materials Chemistry so that, together, they give the RSC a broad involvement in developments occurring at the interface between Materials and Chemistry. A key feature of the Forum is the representation on it from seven materials subject groups, not all of which are otherwise affiliated to the RSC.These subject groups are Macrogroup, The British Liquid Crystal Society, The Applied Solid State Chemistry Group, The Polar Solids Group, The Molecular Crystals Discussion Group, the RSC 'Materials Chemistry Sector' and the Society of Chemical Industry's 'Materials Chemistry Group'. For the first time, these various subject groups have a common venue, at the Forum, to discuss and collaborate on topics of common interest. TERMS OF REFERENCE FOR FORUM MEMBERSHIP The Forum membership, with terms of office, is as follows (Nov. 1995): 1. CHAIRMAN: Period of Service: 3 years, renewable 1996 Responsibility of Appointment: SAB 2. CHAIRMAN, JOURNAL OF MATERIALS CHEMISTRY EDITORIAL BOARD ex officio 3. SAB MEMBER: Period of service not set down 4-9. GROUP MEMBERS: Period of Service: 3 years, renewable as follows: MACROGROUP: 1998 BRITISH LIQUID CRYSTAL SOCIETY: 1997 POLAR SOLIDS GROUP: 1996 APPLIED SOLID STATE CHEMISTRY GROUP: 1997 MOLECULAR CRYSTALS GROUP: 1995 MATERIALS CHEMISTRY SECTOR: 1998 10-12. ORGANISERS OF MC" MEETINGS Service commences after MCn-2 and terminates after Forum Committee meeting following MCn. 13- 14. CO-OPTED MEMBERS optional Current Forum members are: 1. A.R. West 2. A.E. Underhill 3. H.M. Frey 4. S. Armes 5 and 10. D.W. Bruce 6. A.V Chadwick 7. P. O'Brien 8. M. Willis 9. M. Hawkins 11. J.M. Kelly

ISSN:0959-9428    

DOI:10.1039/JM99606BP009

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Nitrogen dioxide and volatile sulfide sensing properties of copper, zinc and nickel chromite? Colin L. Honeybourne and Raymond K. Rasheed Faculty of Applied Sciences, University of the West of England, Bristol, UK BSl6 1QY Polycrystalline powders of CuCr20,, ZnCr204 and NiCr204 were prepared from the decomposition of metal nitrates with glycine, and their structures confirmed by X-ray diffraction. The three spinels were mixed with ethyl cellulose dissolved in a suitable solvent (80 :20 toluene-ethanol) to form pastes which were applied to substrates and dried. The gas sensing properties of the resulting thick films were investigated with ppm levels of NO2, CH3SH and C,H5SH at 220 "C. It was found that the spinels can be used to detect the presence of these gases.Further experiments showed that by substituting some of the A or B cations with Li and Mg, respectively, the gas sensing properties of the spinels can be altered significantly. The monitoring of volatile sulfides and nitrogen dioxide is increasing in importance in industrial and domestic areas, because even in dilute concentrations they can be harmful, and in the case of sulfides their smell can be offensive. The majority of metal oxide sensors for volatile sulfides and nitrogen dioxide B (0.04 moles) were dissolved in 100ml of distilled water. Using valency values of -2, + 1 and 0 for 0, H and N, respectively, the oxidising valency for one mole of each metal nitrate was calculated. These values were then corrected for each nitrate to correspond to the number of moles used in the are based on tin dioxide containing additives or d0~ants.l~~ reaction and then summed to give a The aim of the research described here is to demonstrate the potential of chromite spinels as sensitive gas sensors.To this end it was decided to focus on metal oxides containing transition-metal ions, and target gases that may bring about a change in their oxidation state. The resulting changes in oxidation state would then alter the conductivity of the oxide, and give a measurable response to the target gas. The oxides chosen for this study were CuCr,O,, ZnCr,O, and NiCr20,. These oxides belong to a class of inorganic compounds known as spinels which have the general formula AB20,., The oxide ions form a close-packed array and the A and B cations occupy tetrahedral and octahedral sites, respectively.Spinels are ther- mally stable and because of their high activity they are used in a variety of important industrial reactions, such as the dehydrogenation of hydrocarbons.' Chromites are p-type semi- conductors and conduction proceeds via hopping of charge carriers between Cr3+ and Cr4+.6,7 The catalytic and gas sensing properties of CuCr,O,, ZnCr20, and NiCr,O,, among other chromites, have been studied.6*8 In most catalytic reactions involving spinels the reaction follows the Mars- van Krevelen mechani~m.~ Lattice oxygen is used and appears in the oxidation products. The resulting oxygen vacancies in the spinel are then replenished by gaseous oxygen from the atmosphere.It therefore seemed logical to investigate the gas sensing properties of CuCr204, ZnCr20, and NiCr,O,. The effects of substituting some of the A or B cations with other cations of different valencies were also studied. Experimental Preparation of spinels The spinels (AB204) were synthesized from nitrates using a wet chemical method." The basis of the reaction is the exothermic redox decomposition of metal nitrates and a suit- able organic compound. The starting materials used were Cu(NO,), (99.999%), Cr(NO,), 9H20 (99.99%), LiNO, (99.99%), Mg(N0,)- 6H20 (99.995%), Zn(N0,) (99.999%), Ni( 6H20 (99.999%) and glycine (99 +YO),all obtained from Aldrich. Metal nitrate A (0.02 moles) and metal nitrate i Presented at the Second International Conference on Materials Chemistry, MC', University of Kent at Canterbury, 17-21 July 1995.combined oxidising valency. Using the same elemental valency values for 0,H, N and +4 for C, the reducing valency for one mole of glycine was calculated. This value was used to calculate the number of moles of glycine required to balance the combined oxidising valency of the nitrates in solution. The glycine was then added to the solution and dissolved. The solution was heated at 100°C to remove most of the water. During this stage of the reaction a fine powder appeared. As the solution became more concentrated the viscosity increased rapidly and a clear resin was obtained. The resin was then heated at 300°C on a hotplate, causing it to ignite and yield a fine precursor powder.In the final step of the preparation, the precursor powder was calcined at 800 "C for 24 h in air to produce the spinel product. The spinels obtained were identified by X-ray diffraction (XRD) with Cr-Ka radiation, and the particles were observed under the transmission electron microscope (TEM). Electron spectroscopy for chemical analysis (ESCA) was used to deter- mine the cationic species present and their relative amounts. The analyses were carried out using a VG ESCALAB Mark I1 instrument with in-house software. Fabrication and testing of sensor elements Silver electrodes, approximately 1 mm apart, were painted onto glass substrates (25 mmx 8 mm) and dried at room temperature for 1 h.The powdered spinel was mixed into a paste with ethyl cellulose dissolved in 80 :20 toluene-ethanol. The ethyl cellulose was used as a binder and its loading was fixed at 5% by mass. The paste was then applied to the substrate to produce a thick film that partially covered the electrodes. In all preparations the thick film was dried for 3 h at 200 "C. The prepared sensors were mounted on heaters and tested at 220 "C to ppm levels of methanethiol, ethanethiol and NO2. A gas-flow rig was constructed so that the responses could be measured accurately. Together with a gas chamber, the major components of the flow rig are a Keithley 617 programmable electrometer, a Keithley 705 scanner, a Keithley 7056 voltage card and 7158 current card, thermal mass flow controllers, and a Viglen IV/33 personal computer.Electrical measurements were made using the Keithley 617 programm- able electrometer coupled with the Keithley 705 scanner. The electrical responses of the gas sensors to different atmospheres were recorded as current changes. This was achieved by using J. Muter. Chem., 1996, 6(3), 277-283 277 the electrometer to apply a potential difference across the sensors and measure the resulting current In expenments using NO2 and CH,SH, model 5850TR thermal mass flow controllers were used to administer a specified gas together with a diluting gas (air) from separate gas cylinders to the gas chamber Measurements of sensor responses were made under continuous flow conditions In experments with C2H5SH the chamber was sealed with rubber septums and C2H5SH vapour was injected into the chamber through a septum with a gas- tight syringe Results and Discussion XRD The XRD patterns for all the samples prepared confirm the formation of a spinel phase Many of the peaks are very broad owing to the fact that the average crystal size in the powder samples is rather small In XRD broadening of diffracted X-ray beams may occur if the average particle size is below 200 nm l1 Gas sensing properties of CuCr,04, ZnCr204 and NiCr204 Fig 1 and Table 1show that thick films of CuCr204 can detect NOz and that the sensitivity can be altered by substituting some of the Cu or Cr for Li and Mg respectively (see Fig 2 and 3 and Table 1) Exposure to NO2 causes the conductivity of CuCr204 to increase steadily, and in some cases the time taken to reach a steady-state response may be as long as 1 h Upon the removal of NO2 the sensors recovered quickly The 51 f.0 tlmin 80 Fig. 1 Response of a CuCr,O, sensor at 220°C to 20 ppm NO, (sensitivity 6 65) a. 0 t/min 90 Fig. 2 Response of a Cuo 8Lio ZCrzO4 sensor at 220 "C to 20 ppm NO, (sensitivity 41 76) 0 tlmin Fig. 3 Response of a CuCr, 9Mgo sensor at 220 "C to 20 ppm NO, (sensitivity 1102) rise in conductivity produced by NO2 is the result of processes that occur on the surface whereby the density and/or mobility of charge carriers is increased It is probable that NO2 adsorbed on the surface behaves in the same way as oxygen adsorbed on tin dioxide, in that electrons in the conduction band or acceptor states become trapped at the surface12 To see how this would lead to an increase in conductivity it is necessary to examine bnefly the processes involved in charge-carrier formation As previously stated, the charge carriers in chro- mites are mobile holes that hop from one Cr3+ site to another Their formation in CuCr204 is balanced by the reduction of Cu2+ to Cu+, and by the adsorption of oxygen to a lesser extent Note that Cr3+ and Cu2+ lose a significant amount of crystal-field stabilisation energy through oxidation and reduction, respectively This may limit the number of charge carriers produced in that Cu+ may revert to Cu2+ and Cr4+ to Cr3+, z e the holes are annihilated If it is assumed that charge-carrier formation is in equilibrium with opposing fac- tors, then the trapping of electrons at the surface would be expected to shift the equilibnum in favour of charge-carrier formation Since NO2is an electron-seeking gas, its adsorption in the immediate vicinity of Cu+ would be expected to trap electrons at that locale, and thus prevent the annihilation of holes (charge carriers) An additional factor leading to the observed increase in conductivity may be the existence of oxygen vacancies These may react with oxygen from the decomposition of NO2 to produce charge carriers, this is shown in eqn (1) using Kroger-Vink notation Vo ++O+OOx+2h (1) Introduction of Li for Cu and Mg for Cr both enhance the sensitivity of CuCr204 to NO2 (see Fig 2 and 3) The com- pounds that showed the most significant improvements in sensitivity were Cu, *Lie 2Cr204 and CuCr, ,Mgo XRD studies of these compounds revealed a spinel phase only, and TEM images (Fig 4) show that there are very minor differences in their microstructures It is proposed that sensitivity is enhanced by the formation of oxygen vacancies in the spinel lattice The presence of Li+ for Cu2+, and Mg2+ for Cr3+, would be expected to produce ionised oxygen vacancies as Table 1 Synopsis of observed sensitivity values for copper chromites after activation of the sensor surface by an initial influx of the gas to be detected (values for some repeated observations are given) ~~~~ PPm ~ sensitivity of gas used C u C r 0, cuO gL1O ZCrZo4 cuO gL10 ICrZ04 CuCrl 9Mg0 lo, NO, 20 6 65 36 65 - 13 09 50 5 25 37 10 - 13 79 50 4 99 42 61 - 14 78 CH,SH 10 0 90 124 6 33 - CzH,SH 67 6 71 7 11 12 05 - HZS 10 - - 6 21 - 278 J Muter Chem , 1996, 6(3), 277-283 Fig.4 TEM images of (a)CuCrz04,(b)CuCr, 9Mg0,104 and (c) Cuo 9Lio ,CrZO4. The particles of each sample are very similar. shown in eqn. (2): Li,O 2cuo 42Li,,' +0,"+V,' . (2) and likewise for Mg2+ [eqn. (3)]: 2Mg0 92Mg,,' +20," +V,' * (3) Note that in SnO, oxygen vacancies act as chemisorption sites, and this would also be true for spinels. The extra oxygen vacancies created by the presence of Li' and Mg2+ in the compounds Ch.8Li, 2Cr204 and CUC~~.~M~,-,~O~ would there- fore lead to a greater interaction with NO, and hence promote sensitivty. Exposure toppm levels of volatile sulfides causes a sharp conductivity decrease in CuCr,04 (see Fig.5). Some typical sensitivity values are reported in Table 1. It is not known exactly how volatile sulfides interact with CuCr,O,, but two a C 0 t Imin 30 Fig. 5 Response of a CuCrz04 sensor at 220 "C to 67 ppm CH,CH,SH (sensitivity 4.36) processes are believed to occur. In the first, volatile sulfides incident on the surface of CuCr204 react with chemisorbed oxygen to give C02, H20 and SO2. The loss of oxygen ions from the surface releases electrons back into the lattice and these may combine with Cr4+, thus annihilating the charge J.Mater. Chem., 1996, 6(3), 277-283 279 carriers and lowering the conductivity. It is also possible that some of the electrons could combine with Cu2+ ions. This would have no effect on the conductivity because the charge carriers would remain intact. In the second process it is assumed that Cu+ exists on the surface, and that lattice oxygen is used in the decomposition of volatile sulfides. Again, oxygen vacancies act as chemisorption sites. It is then proposed that the adsorbed sulfides react with lattice oxygen in accordance with the Mars-van Krevelen mechanism, leave the surface as C02 and HzO, and deposit sulfur as S2-ions. The S2-ions would then be expected to interact with Cu+ cations since Cu+ is a soft metal and forms strong complexes with ligands such as alkyl sulfides.The interaction between S2-and Cu+ to form Cu-S bonds requires that Cu+ loses an electron. The free electrons combine with Cr4+and thus lower conductivity. The long recovery times of the sensors due to the difficulty in breaking these bonds supports this idea. It was found that introduction of Li for Cu to give Cuo.9Lio.1Cr204improved sensitivity towards volatile sulfides (see Fig. 6 and Table 1). As well as containing Cr4+and Cu+,Cuo.9Lio.lCrz04will contain additional oxygen vacancies [see eqn. (2)] and will thus allow greater interaction between volatile sulfides and the spinel nature of the surface of spinels has not been determined, there is evidence to suggest that the cations in tetrahedral sites are not exposed to the atm~sphere.~If this were the case, then only through oxygen vacancies would the Cu+ ions become exposed and accessible to chemisorbed species.Furthermore, the increased sensitivity of Cuo~9Lio~lCr,0,would then be due to the greater number of exposed Cu' ions. It was found that the introduction of Mg had no significant effect on the sensing properties of CuCr204towards volatile sulfides. This could be due to the fact that oxygen vacancies produced by the presence of Mg2+ are not ideally situated in that they do not expose Cu+ ions to the atmosphere. The results of ESCA studies on the compounds CuCr204, CUC~~.~M~.,O, are shown in Fig.7. Forand Cuo.9Lio.1Cr204 illustrative purposes the three spectra are placed on the same set of axes. The region between 930 and 935 eV is of particular interest. Close examination of this region reveals that there are two peaks for each sample. The larger peak at ca. 935 eV can be attributed to tetrahedrally coordinated Cu2+ions, and the smaller peak at ca. 932eV to Cu' in the same environ-ment.' For CuCr20, the two peaks are very close together, the Cu' peak appearing as a shoulder on the Cu2+ peak. The lattice. Further substitution of Cu to give CU,,~L~~.~C~~O,shoulder becomes more distinguished for CuCrl~9Mgo~104,had and a less significant effect on sensitivity; sensitivity values of this for Cb.9Lio.lCr204the two peaks are almost completely compound are only slightly greater than those of CuCr,O,.resolved. In this sequence it can be seen that the binding Before proposing the second mechanism, it was assumed that energy corresponding to Cu+ decreases slightly. This is due to Cu+ ions exist on the surface. CuCr204 is a normal spinel the reduced influence of oxygen ions on the Cu+ cation and with Cu2+located in tetrahedral sites, and although the exact indicates the presence of oxygen vacancies. The fact that these shifts can be detected by ESCA proves that the process of oxygen vacancy formation is not limited to the bulk. From the results shown in Fig. 8 and Table 2 it can be seen that greater sensitivity to volatile sulfides is achieved with263m-1521a1 I ZnCr,O,.Like CuCr,O,, ZnCr,O, is a p-type semiconductor and the charge carriers are expected to be formed in the same way.' In CuCr,O, most of the valence electrons released by Cr3+ are accepted by Cu2+ to give Cu+. A similar process in ZnCr20, is unlikely because in spinels Zn2+ is generally con- 0 t/min . 40 sidered to be inert. The valence electrons can then only be accepted by gaseous oxygen through chemisorption, and Fig. 6 Response of a Cu0,,Li,,,Cr2O4 sensor at 220°C to CH,SH (sensitivity 6.33) 10ppm acceptor states lying just above the valence band. It is then reasonable to assume that at the surface of ZnCr20, oxygen al"bY 04 I I I I1 I I I I I 1 925 930 935 940 945 950 955 960 965 970 975 binding energylev Fig.7 Results of ESCA studies of (a) CuCr,O,, (b) CuCr,~,Mgo,,04and (c) Cu,.,Lio.,Cr,04 280 J. Mater. Chew., 1996, 6(3), 277-283 Table 2 Synopsis of observed sensitivity values for zinc chromites after activation of the sensor surface by an initial influx of the gas to be detected (values for some repeated observations are gven) Termmatron of 691 06 GR CH-,SFI~trcaxn~ 12 3 0 t/min 50 Fig. 8 Responses of ZnCr,O, to (a) 10 ppm CH3SH (sensitivity 299 23) and (b)1 ppm CH3SH (sensitivity 3.20) has the same function as Cu2+ in CuCr,O,. As with CuCr,O,, it is proposed that two processes occur on the surface of ZnCr,O, when volatile sulfides are adsorbed. The first process is a simple reaction between volatile sulfides and chemisorbed oxygen to give CO,, H,O and SO,.All the electrons released by the loss of chemisorbed oxygen would then recombine with Cr4+ and hence cause a fall in conductivity. This may be one reason why ZnCr,O, is more sensitive than CuCr,O,. In CuCr,O, some of the released electrons may be accepted by Cu2+ instead of Cr4+ and thus have no affect on conductivity. The second process thought to occur follows the Mars-van Krevelen mechanism in which lattice oxygen reacts with gases NO2 20 299 287 20 290 285 CH3SH 1 3 20 2 88 10 297 292 10 299 29 1 C,H,SH 15 278 262 from the air and the oxidised products leave the surface. This produces oxygen vacancies in the ZnCr,O, lattice which, when ionised, releases electrons that could annihilate charge carriers and reduce conductivity. Substituting some of the Cr for Mg to give ZnCr, ,Mg, had little effect on the microstructure (see Fig.9) or the sensitivity; the sensitivity values remained fairly high (see Table 2). It is therefore assumed that the interactions between volatile sulfides and the two samples are the same. The two samples gave similar responses to NO, and also showed greater sensitivity than CuCr,O, (see Fig. 10 and Table 2). The rise in conductivity would be due to NO, abstracting electrons from the valence band during chemisorption and thus produc- ing hole charge carriers. Again, the presence of Cu+ may explain why CuCr,O, is less sensitive than ZnCr,04 to NO,. As well as the valence band, the Cu+ ions may act as a source of electrons for the chemisorption of NO, by being oxidised to Cu2+.As a result the conductivity of CuCr,O, changes by a smaller amount than that of ZnCr,O,.Fig. 9 TEM images of (a)ZnCr,O, and (b)ZnCr, ,Mg, The particles of each sample are very similar and cuboid in shape J. Mater. Chem., 1996,6(3), 277-283 281 Fig. 10 Response of a ZnCr204 sensor at 220 "C to 20 ppm NO,(sensitivity 290 36) From the results shown in Fig 11 and Table 3 it can be seen that reasonable responses to CH3CH2SHcan be achieved with NiCr204 Although the NiCr,04 sensors are less sensitive than the ZnCr,04 sensors, they are more sensitive than the CuCr204 sensors In companng ZnCr204and CuCr204sensors, it was proposed that the ease with which copper ions change oxi-dation state is detrimental to sensor performance This means that processes occurnng on the surface of CuCr204involving the exchange of electrons are not limited to the valence band where charge are formed Or destroyed The Of copper ions to change oxidation state means that they can interact with adsorbed species and as a result the electrons involved do not affect conductivity In ZnCr,04 sensors, electronic processes are restricted to the valence band, which explains their greater sensitivity Since NiCr204shows greater sensitivity to than CuCr204, It Is --279MQ Q: 13 1 -< 400 tlmn Fig.ll Response of an NiCr204 sensor at 220°C to 67ppm CH3CH2SH(sensitivity 26 33) 8000 -.I h 7000 5000 a u) g!r 4000 3000 2000 Table 3 Synopsis of observed sensitivity values for nickel chromites after activation of the sensor surface by an initial influx of the gas to be detected (values for some repeated observations are given) PPm sensitivities of gas gas used NiCr,O, NIO SLIO 2cr204 NO2 20 0 43 188 50 0 59 170 CH3SH 10 3 33 C2H5SH 67 16 64 67 26 33 For the detection of NO,, sensitivity S is defined as S=(R,,, -RNo~)/RNo~~ are the resistances in air and NO2,where Rair and RNO~ respectlvel~For the detection of volatile sulfides (VS) sensitivity is defined as S =(Rvs -R,,,)/R,,,, where Rvs and R,,, are the resistances in and to assume that nickel ions do not change oxidation state as readily as copper ions With respect to gas sensing, this means that more of the electrons released by the reactions betweenadsorbed oxygen and are accepted by Cr4+ ions Since fewer electrons are accepted by Ni2+ to give Ni', conductivity in NiCr204 changes to a greater extent ESCA studies on this compound (Fig 12) revealed two peaks for Ni at binding energy values of 856 50 and 861 95 eV It is likely that these peaks correspond to Ni2+,the presence of Ni+ could not be confirmed The ESCA spectra for the compounds NiCr, ,Mg, ,04and Ni, &lo 2Cr2O4 (Fig 12) also had Ni peaks at the same binding energy values Thus it was concluded that the introduction of Mg for Cr and Li for Ni had no effect on the environment of the Ni2+ions However, the behaviours of the two compounds were quite different NiCr, ,Mg, gave reasonable responses to CH3CH2SH with slightly lower sensitivity values than NiCr204 Ni, &io 2Cr2O4, on the other hand, showed very little sensitivity to volatile sulfides Based on the assumption that Li has no affect on the environment of Ni2+,it is proposed that its presence in NiOsLio2Cr2O4 is balanced by the oxidation of 25% of the remaining Ni2+ions or 10% of the Cr3+ ions TEM images also show that the microstructures of the compounds are very similar (Fig 13) o! I1 I II I1 I I 1 I 1 I I1 i 845 e50 855 860 865 a70 875 880 885 890 895 binding energy/eV Fig.12 Results of ESCA studies of (a) Nio *Lie &r204, (b) NiCr2O4 and (c)NiCr, ,Mg, 282 J Muter Chem , 1996,6(3), 277-283 Fig.13 TEM images of (a)NiCr,O,, (b) NiCr, ,Mg, The NO, sensing properties of NiCr,O, were found to be relatively poor in comparison with ZnCr204. Table 3 shows the sensitivity values obtained for NiCr,04 to various concen- trations of NO2. NiCrl.,Mgo.lO, was found to be even less sensitive, but the sensitivity of Ni0.8Li0.2Cr204 was surprisingly high (Table 3). TEM images of the compounds NiCr204 and Nio.8Lio.2Cr204showed no difference in their microstructures; therefore, it is reasonable to assume that the enhancement in sensitivity is due to the presence of Li. We propose that the electropositive nature of Li modifies the surface of Nio~,Li0.,Cr,O4 and makes it more reactive to oxidising species such as NOz.Conclusion We have shown that the chromites of copper, zinc and nickel have potential use as solid-state sensors of both oxidising and reducing species. We have observed that the sensing character- istics of chromites can be modified by replacing some of the A or B cations with cations of different valencies. and (c)Ni, *Lie ,Cr,O,. The particles of each sample are very similar. References 1 G. Sberveglieri and S. Groppelli, Sensors Actuators B, 1991,4,457. 2 J. Mizsei and V. Lantto, Sensors Actuators B, 1991,521. 3 J. Tamaki, Chem. Lett., 1991, 575. 4 A. F. Wells, Structural Inorganic Chemistry, Oxford University Press, London, 1975, p. 489. 5 N. J. Jebarathinam and M. Eswaramoorthy, Bull. Chem. SOC.Jpn., 1994,67,3334. 6 D. Basak and J. Ghose, J.Solid State Chem., 1994,112,222. 7 K. S. R. C. Murthy and J. Ghose, J. Catal., 1994,147,171. 8 Y. Shimizu and S. Kusano, J.Am. Ceram. SOC., 1990,73,818. 9 J. Jacobs and A. Maltha, J. Catal., 1994,147,294. 10 J. J. Kingsley and L. A. Chick, Better Ceramics Through Chemistry V,Mater. Res. SOC.Symp. Proc., 1992,271, 113. 11 A. R. West, Solid State Chemistry and its Applications, Wiley, Chichester, 1984,p. 173. 12 P. T. Mosely, A. M. Stoneham and D. E. Williams, Techniques and Mechanisms in Gas Sensing, Adam Hilger, Bristol, 1991, pp. 108-138. Paper 5/04826E;Received 21st July, 1995 J. Mater. Chem., 1996, 6(3), 277-283 283

ISSN:0959-9428    

DOI:10.1039/JM9960600277

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Vapour sensing properties of a cadmium oxide-antimony oxide system ceramic, Cd,Sb*O6.2 Toby J. A. R.Hitch and Colin L. Honeybourne University of the West of England, Coldharbour Lane, Bristol, UK BS16 1 QY The vapour sensing properties of a tetragonal pseudo-pyrochlore, Cd,Sb206.8, are reported. Cd2Sb206.8, prepared by a high- temperature solid-state synthesis, is an n-type semiconductor. Thick-film sensors incorporating Cd2Sb,06 8 as the active material were fabricated utilising screen printing technology. Sensors heated to 250-500 "C were exposed to low concentrations of vapours associated with food spoilage (ethanol, ethyl acetate, limonene, pinene and toluene). The sensors were monitored through the current across the electrodes as a result of a small applied voltage.Tin dioxide sensors were also fabricated to act as a comparison. The results highlight considerable selectivity differences between the Cd2Sb206.8 and SnO, sensors, particularly with regard to humidity responses where the lack of response for Cd2Sb206.8 is crucial for an operational gas sensor. An unusual feature is 'chiral' responses exhibited by Cd2Sb2O6.8 to limonene (orange oil) and pinene (pine oil) which may have practical applications in the stereochemistry field. The concept that adsorption of a gas or vapour on the surface of a semiconducting oxide may facilitate a change in the electrical properties of the oxide has been well The so-called 'gas sensor' has been extensively researched with the main body of work concentrating on tin oxide.This is unsurprising as it is cheap and responds well to small alter- ations in the surrounding atmosphere. The inherent problem with Sn02 as a gas sensor is the fact that it is largely non- selective, i.e. it yields a response to almost any impurity in the atmosphere, including water vapour. To a certain extent this problem has been overcome by the addition of catalysts6 and molecular filter^.^ More recently, other oxides have been investigated as possible gas-sensitive elements. For example, Arakawa and co-worker~~~~have investigated a range of perovskite oxides as methane sensors. In fact, sensitivity of the conductance of oxides to traces of reactive gas in air is a widespread phenom- enon. However, there is as yet no a priori system which can predict the gas sensitivity of oxides and can determine which, if any, might show advantageous properties and, in particular, selectivity of response.This paper reports and discusses the vapour sensing proper- ties of a cadmium antimony semiconducting oxide with a pseudo-pyrochlore" structure, CdzSb206.8, towards atmos- pheres containing low concentrations of a range of organic vapours. Electrical Conduction Behaviour The electrical behaviour of Cd2Sb206.8 has been studied by Li and Zhang," who found that two different mechanisms operate at high and low temperatures. At high temperature (> 560 "C), the conduction is mainly related to the defect structure. Cd2Sb20,8 has a tetragonal pseudo-pyrochlore structure.Within this structure six of every seven oxygen atoms are locked in a distorted octahedral network. The remaining oxygen is situated at the centre of the octahedron from where it can migrate easily. Therefore, oxygen vacancies can be described thus: oot+vo++02 (1) Applying the law of mass action, the following equation is t Presented at the Second International Conference on Materials Chemistry, MC2, University of Kent at Canterbury, 17-21 July 1995. reached, in which K, is the quotient of the forward and reverse rate constants implicit in eqn. (1): [VOlP8, =Kl (2) For simplicity, the concentration of occupied octahedral sites COO] can be considered a constant, which gives the following expression for the concentration of oxygen vacancies: [Val==ti, (3) where K is a constant.Because the carrier concentration is proportional to the oxygen vacancy concentration the compound displays a higher electrical conductivity under a reducing atmosphere, which is consistent with the material being classed as an n-type semicon- ducting oxide. In the low-temperature region the dominant mechanism is one of mixed valence (3:5)exhibited by the central metal ion. To meet the electroneutrality condition within the structure, some Sb5+ must be converted to Sb3+, thereby generating a mixed-valence non-stoichiometric oxide. Experimental Procedure Cd2Sb206.8 was prepared using a solid-state oxide synthesis route.12 Ground CdO and Sb205 (Aldrich, both 99.9%) were mixed in a 2 :1 ratio in methanol for 12 h, dried and pelletized.The pellets were calcined at 800 "C for 5 h (20 "C min-' ramp), reground and pelletized and then sintered at 1200°C for 5 h. An X-ray diffractometer was used to confirm the crystal structure.12 Sensors manufactured utilising Cd2Sb@6.s as the active material are illustrated in Fig. 1. A screen-printable paste was prepared containing Cd2Sb@6.8 (>75%), glass frit, surfactant and an organic medium. This was printed onto 1 in2$ (5 per square) alumina substrates over Pt/Au interdigitated electrodes and fired at 800 "C for 3 h (25"C min-' ramp). This gave film thicknesses of approximately 70 pm. The sensors were heated by means of a screen-printed Pt/Au circuit on the reverse face of the substrate.The surface of the sensor was studied by means of scanning electron microscopy (SEM) in conjunction with an energy dispersive analysis by X-rays (EDAX) probe. Sensors were also produced utilizing SnO, (99.999 YO,Aldrich) as the active material in the same way, to act as a contrast to the Cd,Sb,O,., sensors. Sensors were tested using the set-up ~~ ~ $ 1 in=2.54 cm. J. Mater. Chem., 1996, 6(3),285-288 285 -0.2 in FRONT Fig. 1 Screen-printed sensor design (+electrode bar separation = 250 pm) Fig. 2 Experimental apparatus used in vapour sensing investigations. Sensors are inserted into PCB edge connectors. illustrated in Fig. 2 in the temperature range 250-500°C in 50 "C increments. A voltage of 0.25 V was applied across the electrodes and the current was monitored.Sensors were initially run in the ambient atmosphere for 48h to acts as a 'run-in' period. During this time it was observed that the current across the electrodes increased slowly to a steady equilibrium level. Injections of known concentrations of vapour were made to the main chamber via a gas syringe. All materials used were silanated to minimize the sequestration of vapour on the walls of the chamber. The chamber was purged after each injection with 5 dm3 min-l air. When the current had recovered to its equilibrium level another injection could be performed. Results and Discussion A typical experimental run is shown in Fig. 3. Upon injection of EtOH the current across the electrodes increases sharply, i.e.the conductance increases. After the peak current is reached the response decays owing to the decreasing EtOH concen- tration in the chamber. Upon purging a large decrease in current is observed owing to a corresponding decrease in the Fig. 3 Experimental run for Cd,Sb,0,.8 sensor at 400 "Cexposed to EtOH. A, 5ppm EtOH injection; B, chamber purged with 5dm3 min-' air; C, equilibrium time; D, 10 pprn EtOH injection; E, 25 ppm EtOH injection. sensor temperature Responses are displayed in terms of I/Io, where I is the peak response current and I, is the current at ambient atmosphere. When interpreting the results, some factors must be borne in mind, although the conclusions that can be drawn from the results are relatively unaffected by them.Firstly, although the chamber atmosphere was dried, the by-products of many surface-catalysed reactions may include water vapour and this may cause changes in the patterns of response. Secondly, whilst X-ray diffraction indicated that the Cd2Sb206.8 synthesised was in a single phase, the possibility that the gas response effects might be due to minor impurities cannot be ruled out. Furthermore, there will undoubtedly be heterogeneities on a fine scale at free surfaces and grain boundaries. However, we consider that the results are not greatly affected by these complicating factors. Indeed, the results were found to be repeatable to a high accuracy (<5%). Comparison of Cd2Sb20,8 and SnO, sensors Table 1 shows a comparison between Cd2Sb20,~, and SnO, tested at 350°C to 10 ppm of a range of organic vapours.The responses are of the same order but significant differences can be seen. Cd,Sb,O,., shows a much higher response to ethyl acetate than SnO,. This is also the case for other temperatures and the difference is magnified at higher vapour concentrations. In the case of toluene the reverse situation operates, where Cd2Sb206.8 displays a smaller response than for SnO,. Again the differences in response are magnified at higher concen- trations. The responses to the terpenes are very closely related. This emphasises the fact that n-type semiconductors utilised as gas sensors do indeed show fundamental differences in response between each other, as observed by Moseley et It would be expected to be especially true for semiconductors of widely differing structures, as is the case here.The most significant differences in response are those for humidity changes. Cd2Sb206.8 yields very small responses to large humidity changes, which is also the case at higher temperatures (up to 5OOOC). This is important because one of the main by-products of many surface-catalysed reactions is water vapour, which would distort the response of moisture- sensitive materials. The sensitivity of SnO, to moisture is seen as one of the main drawbacks to the operational use of gas sensors based on SnO,. Responses to optically active terpenes Codistillation of many plant materials with steam produces a fragrant mixture of liquids called plant essential oils.These oils consist largely of cyclic hydrocarbons called terpenes. Damage/disease in plant material can cause emissions of these terpenes. In this investigation the optically active terpenes limonene (orange oil) and pinene (pine oil) are investigated. Table 1 Comparison of response characteristics for Cd,Sb2o6 and SnO, sensors at 350 "C upon exposure to 10 ppm of various organic vapours organic vapour Cd2Sb2068 Sn02 ethanol 2.11 2.49 ethyl acetate toluene 3.18 1.09 2.21 1.65 (S)-(-)-limonene (R)-(+)-limonene(S)-(-)-a-pinene 1.54 1.41 1.46 1.54 1.52 1.72 (R)-(+)-a-pinene (S)-(-)-fl-pinene 0-10096 R.H." 1.43 1.37 1.09 1.7 1.3 1.Y2 a R.H. =relative humidity.286 J. Mater. Chem., 1996,6(3), 285-288 The racemic forms of limonene and pinene are illustrated in Fig. 4.The responses of Cd2Sbz06,8at 450 "Cto these vapours are illustrated in Fig. 5 and 6. Fig, 5 shows that the sensors show significant variations in response to the (R)and (S)forms of lhonene, the (S) response being the greater. This is an unusual phenomenon as the two forms have almost identical volatilities and are approximately equivalent in size. One possible explanation for this is that the porosity of the films may exclude the (R)form from penetrating as deeply into the film as the (S)form. An SEM image of the Cd,Sb20,.8 sensor surface is shown in Fig. 7. The high porosity of the films can be seen clearly. However, the Sn02 films show an equivalent porosity but the chiral differences in response are not observed.It should also be noted that (S)-limonene is very slightly more volatile than (R)-limonene and this variation may be magnified on the sensor surface. These response variations become more pronounced with increasing temperature up to the maximum response temperature of 450 "C. Above this temperature the responses and the (R)and (S) differences decrease rapidly. In the case of pinene (Fig. 6), the response characteristics for the (R)-a and (S)-a forms are almost identical for the temperature range used. However, it was observed that the (S1-p form Yields significantly dlanced responses. This is Fig. 4 Racemic isomers of limonene (orange oil) and pinene (pine oil) 3.0-2.5-4O -z4 2.0-1.5-i.oJ 0 5 10 15 20 limonene (ppm) Fig.5 Limonene response data for Cd2Sb206,8 sensor at 350 "C. (a)(S)-Limonene;(b)(R)-limonene. 5 4-I/___ I0-0 5 10 15 20 pinene (ppm) Fig. 7 SEM image of Cd2Sbz06.g sensor surface (magnification x 200000) rather less surprising than with the case with limonene, because structural differences between the o! and p forms could easily account for the variation in response. This is despite the fact that, as with limonene, the volatilities of all three forms are almost identical. Again, as with limonene, the responses reach their peak at 450°C,with a subsequent sharp decline at elevated temperatures. Table 2 illustrates the variations in response for limonene and pinene (20ppm) at the peak response temperature.All the responses discussed were repeat- able to a high degree. Note that for limonene and pinene the responses vary linearly with vapour concentration. The cause of this will be discussed. Responses to other organic vapours The response data for ethanol, ethyl acetate and toluene are shown in Fig. 8. The prime feature of note is that, in contrast to the terpenes, ethanol and ethyl acetate display square-root- dependent response characteristics. Therefore it would appear that a different response and/or adsorption mechanism is in operation for the two sets of compounds. Toluene also exhibits a square-roo t response dependence, although data for higher concentrations is lacking.The maximum responses for both ethanol and ethyl acetate are seen at 400°C. Mechanistic discussion It is well known from empirical gas-sensing studiesI4 that the conductance change, Ag, induced by the introduction of a particular gas of partial pressure PRis given by the relationship: Ag =aPvR (4) Table 2 Peak response data for CdzSb206.8 sensor exposed to 20 ppm of various aromatic terpenes aromatic terpene maximum 1/1, Ta/T (S)-(-)-limonene(R)-(+)-limonene (S)-(-)-a-pinene (R)-(+)-a-pinene (S)-(-)-/?-pinene 3.64 3.18 2.78 2.8 1 5.64 450 450 450 450 450 Fig. 6 Pinene response data for Cd2Sb@,,, sensor at 350°C. (a)(S)-8-pinene; (b) 0, a(S)-a-pinene; M, (R)-a-pinene. Temperature of maximum response.J. Mater. Chem., 1996, 6(3), 285-288 287 in which a is normally a constant for a given type of substrate of order of magnitude and v is said to characterise the order of the surface reaction that dominates induced changes in surface charge-carner density This index is frequently fractional for semiconducting oxides Many workers have recently reported a square-root dependence of the electrical response upon the partial pressure of a reducing gas in dry air and moist air utilising materials such as SnO, (and metal- doped Sn0,),16 chromium titanium oxide (a p-type binary oxide),17 ZnO and ZnO/CuO contact ceramics In this work, the dependence of the electrical response upon the partial pressure of the reducing gas differs between ethanol and ethyl acetate (dependence on the square-root of the partial pressure) and the larger hydrocarbons limonene and pinene (linearly dependent upon the partial pressure) Note that the large hydrocarbons can undergo catalytic dehydrogenation (as a first stage, say, in their decomposition) whereas ethanol and ethyl acetate cannot readily undergo dehydrogenation and are more likely to proceed uzaelimination of water C2HSOH+C2H, + H2O (5) or other small organic molecules Kohl and co-workers have noted the range of product^'^ that, in the case of n-type oxides,20 can be observed from the surface interactions of even small adsorbed molecules Decomposition reactions do not necessarily have a direct bearing on changes in charge-carrier density, however, oxidation of decomposition products by surface-bound oxygen anions can be of key importance 2o We can illustrate the process of direct combustion by using the model of McAleer et al k2RO,,, + n' -Re, + 0, (7) with the requirements that the rate of release of the surface oxygen monoanion (Os-) is very much less than its rate of reaction with the reducing gas, R, and that the rate of formation of Os-is very much larger than its rate of reaction with R Species other than Os-(eg O,s-and 02s2-)can also be utilised in similar simple schemes In our work R(g) could be a species in the air that we are aiming to detect However, we suggest that R(g) is more likely to be one of the species produced by catalytic decomposition (such as H2 or C2H4) that has desorbed subsequent to the decomposition process and can react readily with surface- bound oxygen ions, thereby generating 'free' electron charge carners (n') at the surface The large hydrocarbons limonene 10 91 / 8-7-6-5-41 /I Fig.8 Response data for CdzSbzOss sensor at 400°C exposed to (a)ethyl acetate, (h)ethanol and (c)toluene 288 J Muter Chem , 1996,6(3), 285-288 and pinene can dehydrogenate to release hydrogen and a hydrocarbon with more double bonds This IS especially appro- priate for limonene, for by such a process a planar aromatic molecule is formed We suggest that the observed difference in behaviour between these hydrocarbons and the small, oxygen- containing species arises from the combustion of hydrogen released by catalytic dehydrogenation of the hydrocarbons Experiments using thermal desorption mass spectrometry must be performed before any further comments can be made Conclusions Screen-printed thick-film sensors were produced utilising an n-type semiconducting ceramic, Cd,Sb,O, 8, as the active material The responses of these sensors were compared with those of SnO, Significant differences in response were obtained for ethyl acetate and toluene An important difference was the lack of sensitivity of Cd2Sb2068 to humidity changes This could prove useful in a commercial gas sensing device The responses of Cd,Sb,O, to optically active terpenes were unusual in the fact that in the case of limonene the (S) form gave larger responses than the (R)form In the case of pinene the B form gave the larger responses This behaviour was not displayed by SnO, This property could have potential practi- cal usage in distinguishing optical isomers, where polansed light is currently employed To gain a clearer picture other optically active compounds will be investigated The response characteristics of the vapours tested for were found to differ with respect to vapour concentration The terpene responses were linear with respect to concentration and the smaller hydrocarbons displayed a square-root depen- dence It is suggested that dehydrogenation could be the key to these differences This will be actively investigated using mass spectrometric methods References C Wagner, J Chem Phys , 1950,18,69 K Hauffe and H J Engell, Z Electrochem , 1952,56,366 N Taguchi, US Pat, 3 695 848,1972 H Windischmann and P Mark, J Electrochem SOC, 1979, 126, 627 5 J Watson, Sensors Actuators, 1984,5,29 6 J F McAleer, P T Moseley, J 0 W Norris, D E Williams and B C Tofield, J Chem SOC Faraday Trans 1,1988,84441 7 K Nakashima and S Suzuki, Anal Chim Acta, 1984,162,153 8 T Arakawa, Chem Ind (Dekker), 1993,50,361 9 T Arakawa, N Ohara, H Kurachi and J Shiokawa, Anal Chem Symp Ser , 1983,17,159 10 R A McCauley, J Appl Phys, 1980,52,290 11 B Li and J Zhang, J Am Ceram SOC ,1989,72,2377 12 B Li, J Am Ceram SOC , 1988,71, C78 13 P T Moseley, A M Stoneham and D E Williams, in Techniques and Mechanisms in Gas Sensing, ed P T Moseley, J 0 W Norris and D E Williams, Adam Hilger, Bristol, 1991 14 P Van Geloven, J Moons, M Honore and J Roggen, Sensors Actuators B, 1989,17, 361 15 J Vetrone, Y W Cheung, R Covicchi and S Semancik, J Appl Phys, 1993,73 8371 16 P T Moseley, Sensors Actuators B, 1991,3,167 17 D H Dawson, G S Henshaw and D E Williams, Sensors Actuators B, 1995,26,76 18 S T Jun and G M Choi, Sensors Actuators B, 1994,17, 175 19 D Kohl, W Thoren, U Schnakenberg, G Schuill and G Heiland, J Chem SOC Faraday Trans, 1991,87,2647 20 D Kohl, Sensors Actuators B 1990,1,158 21 J F McAleer, P T Moseley, J 0 W Norm and D E Williams, J Chem SOC Faraday Trans 1 1987,83,1323 Paper 5/04833H, Received 21st July, 1995

ISSN:0959-9428    

DOI:10.1039/JM9960600285

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Novel composite organic-inorganic semiconductor sensors for the quantitative detection of target organic vapourd Benjamin P. J. de Lacy Costello, Phillip Evans, Richard J. Ewen, Colin L. Honeybourne and Norman M. Ratcliffe" Faculty of Applied Sciences, University of the West of England, Coldharbour Lane, Frenchay, Bristol, UK BS16 1QY Composites of tin dioxide (an n-type semiconductor) and derivatives of the conducting polymer polypyrrole (a p-type semiconductor) gave reversible changes in electrical resistance at room temperature when exposed to a range of organic vapours. The optimum amount of polymer giving highest sensitivity was found by experiment to be 2.5% by mass for the polypyrrole chloride-tin dioxide composite. Composites containing 2.5% polymer by mass, but differing in polymer derivative, were fabricated and exposed to low concentrations of ethanol, methanol, acetone, methyl acetate and ethyl acetate.All were found to give significant and reversible decreases in electrical resistance. Direct comparison with sensors constructed solely of tin dioxide or polypyrrole at room temperature showed the composites to be more sensitive. The gas sensitivity of the composite materials depended on the type of polymer derivative incorporated and the dopant anion associated with the polymer. The composites were simple to fabricate and gave differing response profiles to a range of organic vapours. Organic semiconductors, particularly polypyrrole and its derivatives, have been reported to change their conductivities when exposed to a variety of organic and inorganic vapours.' Recently, polypyrroles with different anions and/or substituents have been incorporated into sensor arrays for artificial nose applications.Inorganic semiconductors such as tin dioxide, either undoped3 or doped4 with catalytic metals, have been exten- sively reported to act as chemical sensors when operated at elevated temperature^,^ typically 350 "C. There are numerous examples of applications of inorganic semiconducting mixtures, e.g. as catalysts and promoters, and in electronic devices (for instance, the electroluminescence exhibited by zinc sulfide and copper6). In contrast, there are few reports describing the applications of organic semiconduct- ing mixt~res.~" It should be observed that films of organic semiconductors layered with inorganic semiconductor films are currently receiving some attention.For instance, poly( p-phenylene vinyl- ene) has been found to emit light when sandwiched between materials having high (e.g. indium oxide) and low (e.g. alu-minium) work functions.' Polypyrrole films on n-type cadmium sulfide (prepared by electrochemical polymerisation of pyrrole on the cadmium sulfide) have markedly improved the stability of cadmium sulfide against photocorrosion, while permitting a light-assisted water cleavage reaction." Films of inorganic and organic semiconductors have also been shown to possess diode characteristics," and photovoltaic properties at very low efficiencies.12 However, there have been no reports of the use of composites of organic and inorganic semiconductors for vapour sensing of organic compounds.Reported worki3 on the analysis of soft rot infections (caused by the bacterium Erwinia carotouora) of Russet Burbank cultivar and Kennebec cultivar potato tubers by GC-MS cites an increase in the concentration of organic vapours above the infected tubers. The volatiles emitted in the highest concen- trations were ethanol, acetone and butan-2-one. Additional volatiles detected in the headspace of the diseased tubers included acetaldehyde, methyl acetate, ethyl acetate, pro- pane thiol, hydrogen sulfide, dimethyl disulfide, n-propanol and isobutyl alcohol. Current methods for the detection of soft rot in stored t Presented at the Second International Conference on Materials Chemistry, MC', University of Kent at Canterbury, 17-21 July 1995.potato tubers are expensive and bulky, as in the case of GC-MS, or largely ineffective, as in the case of thermographic monitoring. A system is required which is inexpensive whilst being sensitive to low levels (typically <10ppm) of the vapours. A device of this type would enable early remedial action to be taken, thus reducing financial losses. The requirements for useful gas sensing devices in a variety of applications are high sensitivity, low cost and portability. Whilst devices based on heated inorganic semiconductors are sensitive to a variety of organic species at low levels, including ethan01,~ methan01,'~ ethyl acetatei5 and acetone,16 the need for elevated temperatures for operation increases the cost of the device and limits the portability.Multi-sensor arrays based on electropolymerised conducting polymers have found a wide variety of applications, especially in the assessment of and beverage2 quality. Devices of this type, whilst operating at room temperature, are expensive to produce and require complex signal processing. There is scope for the development of new materials displaying different response profiles to those currently available. We have assessed the potential of com- posites of p-type polypyrrole derivatives and n-type tin dioxide as possible sensors for detecting key analytes of value for the detection of soft rot in stored potato tubers. Experimenta1 Tin(1v) oxide powder (99.9999% purity) was obtained from Janssen Chimica (Geel, Belgium).Pyrrole (98% purity) and N-methyl pyrrole (99% purity) were obtained from the Aldrich Chemical Company (Gillingham, Dorset, UK). The tin dioxide was used as received. Pyrrole and N-methyl pyrrole were distilled prior to use and stored over 4A molecular sieves at 4 "C. Polymerisation of pyrrole and N-methyl pyrrole was based upon standard methodologies.18 Preparation of polypyrrole chloride (PPCl ) Copper(r1) chloride (1.6 g, 11.9 mmol dm-3) was dissolved in acetonitrile (35ml). Pyrrole (0.19 g, 0.2 ml, 2.9 mmol dm-3) was added rapidly and the mixture was stirred for 3 h. The black polymer produced was then filtered and washed with acetonitrile (300 ml) before drying in vucuo (0.95 mmHg, 72 "C, 2 h).Yield: 0.14 g [63.6% based on a 3: 1 monomer: dopant ratio (see Fig. l)]. Elemental analysis: C, 50.70; H, 2.72; N, 14.76; C1, 20.11; ash, 0.37%. J. Muter. Chem., 1996, 6(3), 289-294 289 L Jn where X-= CI-NO3-R=H CH, Fig. 1 Idealised polymer structure based on a 3 1 monomer dopant anion ratio Preparation of polypyrrole nitrate (PPN03) Iron(1rr) nitrate nonahydrate (4 54 g, 11 2 mmol dm-3) was dissolved in water (33 ml) Pyrrole (0 19 g, 0 2 ml, 2 9 mmol dm-3) was rapidly added and stirred for 90 min The polymer was then filtered off and washed with distilled water (260ml) before drying in uucuo (0 95 mmHg, 72 "C, 2 h) Yield 0 21 g (858% of the theoretical based on a 3 1 monomer dopant ratio) Elemental analysis C, 5403, H, 2 87, N, 18 19, ash, 0 16% Preparation of poly-N-met hyl pyrrole chloride (PNMPCl ) Iron(m) chloride (1 88 g, 11 mmol dmV3) was dissolved in water (38 5 ml) N-Methyl pyrrole (0 23 g, 0 25 ml, 2 9 mmol dm-3) was dispersed in water (1 5 ml) and added rapidly to the iron(m) chloride solution with vigorous stirnng, which was maintained for 3 h, prior to filtration The polymer was then washed with water (320 ml) and dned zn D~CUU(0 95 mmHg, 72 "C,2 h) Yield 0 08 g (30% of the theoretical based on a 3 1 monomer dopant ratio) Elemental analysis C, 5924, H, 3 97, N, 13 67, Cl, 11 64, ash, 6 21% Preparation of poly-N-methyl pyrrole nitrate (PNMPNO,) The preparation was the same as for PNMPCl, with iron(m) nitrate nonahydrate (4 44 g, 0 011 mol) used as the oxidant replacing iron(1n) chlonde Yield 0 14 g (50% of the theoretical based on a 3 1 monomer dopant ratio) Elemental analysis C, 61 01, H, 3 87, N, 15 50, ash, 047% Each polymer was prepared twice (batch 1 and batch 2 polymers) using the methods descnbed, and elemental analysis was carried out on each polymer to ascertain the reproduc- ibility of the syntheses The results displayed in Tables 1 and 2 are for composites comprising batch 1 polymers Scanning electron microscopy study of materials incorporated into the composites SEM images of the polymers showed that the particle size distribution was in the range 10-60 pm The micrographs of the polypyrrole further showed that the large particles were formed of agglomerated polydispersed spheres SEM images of the tin dioxide showed the particle size distnbution to be in the range 10-30 pm Fabrication of substrates for gas sensor testing Interdigtated electrodes, possessing three pairs of interpen-etrating bars with an electrode gap of 0 5 mm, were prepared on alumina substrates The design was such that four separate electrodes with a common voltage connection and separate measurement connections fitted onto one alumina square The alumina substrates were cleaned in deionised water pnor to sputter coating with gold (Emscope gold sputterer coating current 15 mA for 3 min) The electrode pattern was then created using photolithographic techniques 290 J Mater Chem, 1996,6(3), 289-294 Table 1 Relative responses of composite sensors with differing percent ages of PPCl incorporated in their structure when exposed to 100 ppm of ethanol vapour polymer (mass%) in the composite relative response to ethanol 0 04 0 68 0 81 125 0 98 25 1 5 0 48 10 0 34 25 0 16 50 0 75 -0 09 100 -0 08 Production and optimisation of composite gas sensitive materials Composite materials were produced by grinding various pro- portions of polypyrrole chloride with tin dioxide in an aqueous slurry Composites containing 0 675, 1 25, 2 5, 5, 10, 25, 50 and 75% PPCl by mass were produced These were assessed for gas sensitivity using the method described in the following section Sensors comprising polypyrrole alone and tin dioxide alone were also tested The results (displayed in Table 1) showed the composite containing 2 5% PPCl by mass to give the greatest sensitivity and this was considered to be the optimum polymer weighting Composite sensors containing 25% polymer by mass were then produced using the other polymer derivatives Each polymer (0025 g) was added to tin dioxide (0 0975 g) and ground in water (2 ml) to produce a coherent paste A known volume was then deposited across the interdigitated gold electrode and dned for 15 min at 60 "C All sensors were then left for 48 h in clean air to equilibrate Duplicate sensors of each type were produced from batch 1 polymers (A and B sensors) and tested under identical con- ditions in order to assess consistency within a batch Similarly, composite sensors comprising batch 2 polymers were produced to assess the reproducibility of fabrication For comparison, batches of sensors comprising PPCl and tin dioxide alone were produced and tested All sensors were tested for their electncal responses to duplicate vapour injections of three diffenng concentrations (1, 10, 100 ppm) of the test analytes methanol, ethanol, ethyl acetate, methyl acetate and acetone All sensors were tested for their response to the organic vapours at two different temperatures 20 and 40 "C The results are displayed in Table 2 All sensors were retained and retested after six weeks to assess any ageing affects Sensor testing apparatus The sensors were tested in an in-house designed test chamber This consisted of a multi-port sphencal glass vessel (total volume, 3 375 dm3) maintained in a heated (variable up to ca 40 "C) chamber Three ports possessed electncal feed-throughs, two for connection to the sensors and one for connection to a semiconductor temperature sensor Two ports (one inlet, one outlet) were connected uza electrically operated valves to provide a purging air supply, where the source was a blended synthetic air cylinder (MG Gas Products Ltd , Reigate, UK) One port contained a silicone rubber septum through which test vapours could be injected via a gas-tight syringe The sensors were connected through a Keithley 705 scanner to a Keithley 617 electrometer (Keithley Instruments Ltd ,Reading, UK) Both Keithley instruments were connected uza an IEEE488 interface to an IBM-compatible computer The software to gather and display the data was written in-house The voltage applied across the electrodes was 1 0 V, supplied Table 2 Percentage change in current (applied voltage 1,0V), upon exposure to organic vapours, of tin dioxide, PPC1, PPCl composite, PPNO, composite, PNMPCl composite and PNMPNO, composite at 20 and 40 "C test vapour conc.(ppm) T/"C SnO, PPCl ~~ ethanol 1 20 9.8 -2.5 1 40 12.6 -2.1 10 20 15.3 -4.4 10 40 20.7 -4.1 100 20 30.7 -6.1 100 40 41.2 -5.7 methanol 1 20 8.1 -2.1 1 40 9.5 -2.2 10 20 13.2 -2.4 10 40 14.3 -2.1 100 20 17.8 -3.6 100 40 21.3 -2.9 ethyl acetate 1 20 6.1 -1.6 1 40 7.3 -1.7 10 20 11.5 -2 10 40 12.6 -1.9 100 20 14.2 -2.6 100 40 14.9 -2.2 methyl acetate 1 20 5.7 -1.4 1 40 6.4 -1.6 10 20 10.1 -2.1 10 40 11.3 -2.3 100 20 15.1 -2.8 100 40 16.4 -2.7 acetone 1 20 8.1 -2.1 1 40 7.9 -2.2 10 20 13.4 -2.8 10 40 14.1 -2.6 100 20 20.7 -4.6 100 40 20.4 -3.3 from a programmable voltage source built into the Keithley 617 electrometer.All sensitivity results were measured in terms of percentage change in current. Results and Discussion Comparison of C :N, N :H, C :H and Cl :N/C :C1 (where appli- cable) ratios, ascertained from elemental analysis results per- formed on batch 1 and batch 2 polymers, showed that the reproducibility of synthesis was within 2% for each polymer type.The percentage yields of polymers were relatively low owing to the short reaction times used. Better yields might have been obtained if the reaction had been left for 24 h, although this could have resulted in over-oxidation of the polymer. Losses can also be explained by the formation of soluble oligomers which would have been removed during the work-up. The optimum polymer weighting was found to be 2.5% by mass for the composite sensors incorporating PPCl. The effect on the response to 100 ppm of ethanol vapour of varying the mix ratio is shown in Table 1, where the results are quoted relative to the response of the most sensitive composite. On testing with the other vapours of interest, the 2.5% mix ratio also gave the highest sensitivity.The composite sensors, where the polymer is present at a low level, exhibited a resistance decrease implying overall n-type conduction, as was observed for tin dioxide alone. The negative results indicate a resistance increase upon exposure to the vapours, implying a reversion from n-type to p-type conduction. This was observed for composites which contained more than 50% polypyrrole by mass. These results suggest that two competitive mechanisms occur within the composite sensors. Sensitivity results displayed in Table 2 were all calculated using the following relationship: lOO(1, -Io)/Io, where I, is the current two minutes after the injection of the test vapour, and I, is the stable baseline current prior to injection. The results YOchange in current PPCl PPNO, PNMPCl PNMPN03 composite composite composite composite ~~ 21.7 9.8 11.3 9.5 30.7 15.1 17.1 16.3 32.3 17.6 22.1 20.1 65.4 26.1 27.2 24.1 76.4 34.6 33.7 37.9 103.1 37.2 45.5 38.9 11.3 4.6 7.2 8.1 18.5 9.1 9.5 10.1 21.7 9.3 14.1 13.4 32.5 15.3 16.1 15.2 41.9 16.5 23.4 20.6 49.9 21.2 21.6 21.3 10.2 5.7 7.1 6.1 11.1 6.2 9.1 8.3 17.1 11.4 11.6 11.1 20.2 10.1 14.6 14.3 32.3 20.3 21.4 21.7 29.7 17.8 22.3 22.6 9.1 5.2 6.3 6.5 15.1 7.2 10.1 9.1 18.3 9.1 11.2 12.3 26.6 11.3 15.3 14.6 26.1 15.2 16.3 18.1 36.1 16.8 20.8 21.3 14.2 7.1 8.3 8.3 16.7 9.2 10.3 10.1 23.6 11.1 11.5 13.2 27.3 12.1 15.3 15.7 35.7 18.2 19.1 21.4 35.3 16.9 20.3 21.3 show that at room temperature the composite sensors generally gave enhanced responses to the organic vapours when com- pared with the responses of sensors constructed of tin dioxide or polymer alone.As discussed earlier, the electronic properties of the com- posites appear to be governed by tin dioxide, and therefore when exposed to electron-donating vapours or gases they exhibit a decrease in resistance, whereas sensors composed solely of polypyrrole exhibit an increase in resistance. It is postulated that the observed increased sensitivity for composite materials may be due to the creation of a positively charged depletion layer on the surface of the tin dioxide which could be formed owing to inter-particle electron migration from tin dioxide to polypyrrole at the heterojunctions.This would cause a lowering of the activation energy and enthalpy of physisorp- tion for vapours with good electron-donating characteristics, whilst still permitting reversibility. The results in Table 2 show little correlation between gas sensitivity and temperature for the composite sensors. Although slight enhancements in sensitivity are observed with some composite sensors, the difference is not considered great enough to warrant external heating of the sensors in a practical device. The results displayed in Table 2 are for group A sensors made up with batch 1 polymers.Testing of group B sensors and sensors made up with batch 2 polymers elucidated similar response profiles with similar sensitivity values obtained (within 10%). Therefore sensors which are reproducible in terms of sensitivity both intra-and inter-batch can be produced. Variations in the initial baseline current of up to 40% may occur between sensors of the same type. The composite sensors tended to drift towards lower resistance, with a figure of 5% per hour typical when under test. A number of solutions to improve the reproducibility of fabrication are currently under investigation, such as screen printing or pre-coating of the tin dioxide particles with polymer prior to sensor production. J. Mater. Chem., 1996, 6(3), 289-294 291 The reproducibility of response for the sensors containing only tin dioxide was poor, typically 30% difference between batches The baselines of these tin dioxide sensors tended to drift towards lower resistance (up to 15% per hour when under test) resulting in poor response data The sensors containing only polypyrrole derivatives gave highly reproducible responses, typically within 5% The baseline stability was high with no appreciable drift during a week of testing These sensors gave very low sensitivities when compared to the tin dioxide or the composite sensors A typical run profile carried out in dry air (Fig 2) is included which shows the response of the PPCl composite sensor to three consecutive vapour injections of ethanol in the range 10-1000 ppm The responses reached 90% of the maximum within 3 min and were 90% reversible within 5min for the case of a 500 ppm injection of ethanol It should be noted that the adsorption kinetics of composite sensors at lower concen- trations such as 10 ppm are slower, with 90% of the maximum response reached within 4 min The adsorption and desorption kinetics of the polymer are marginally faster than those of the composites with 90% of the maximurn response reached within 2min and 90% recovery within 3min Sensors constructed from tin dioxide alone exhibit similar adsorption kinetics to those of the composite sensors However, the desorption kinetics are slow with only 50% recovery after 5 min For all composite sensors and individual component sensors the adsorption kinetics were found to be dependent on analyte concentration, but independent of analyte type (for the vapours tested) In contrast, the desorption kinetics of the sensors were found to be independent of analyte concentration, but depen- dent upon analyte type The desorption kinetics were slower when the sensors had been exposed to the esters For example, a PPCl composite was 90% reversible after 6min when exposed to a 500 ppm injection of ethyl acetate Fig 3 displays current us time data for the PPCl composite when exposed to 500 ppm ethanol Fig 4 shows that saturation of composite and individual Fig.2 Change in current of a PPC1-tin dioxide composite upon exposure to ethanol vapour in the range 10-1000 ppm (10 V applied) 8 7 : a 0 -20 I I I I I I I I I I 0 200 400 600 800 1000 vapour concentration (ppm) Fig.4 Percentage change in current us ethanol concentration for the PPCl composite (W), tin dioxide (C) and PPCl (+ ) sensors component sensors occurred at ethanol vapour concentrations >500 ppm The saturation effect at this level is evident for all the analytes tested, and this is shown in Fig 5 Baseline stability studies under ambient conditions over two weeks showed that after an initial settling period of four days the composite sensors exhibited good stability with fluctuations in the initial resistance of <7% per week This compares favourably with literature values reported for an electropolym- erised polymer film of a 10% change in baseline resistance over two weeks" As reported, the fluctuation was greater when the sensors were tested continuously However, when testing was complete, the composite sensors returned to within 2% of their original baseline value The baseline stability of the tin dioxide sensors is poor, with fluctuations of 20% per week under ambient conditions In contrast, the baseline fluctuations of the polymer sensors were typically less than 5% per week Studies of the responses of the sensors to different relative humidities over the range 20-98% (Fig 6-9) showed that the composite sensors, as well as polypyrrole and tin dioxide alone, were humidity dependent Composites incorporating poly-N- methyl pyrroles were found to be particularly sensitive to rn Y-140[1 0 0 0 200 400 600 800 1000vapour concentration (ppm) Fig.5 Percentage change in current of the PPCl composite sensor us analyte concentration B,ethanol, 0,methanol, +, ethylacetate, 0, methyl acetate, A,acetone o 025r m= 0 024, E '6 0023, 5 I I I 1 I I I I 1 I 1 I I0 022i ' 1 2 3 4 20 40 60 80 100 tmin relative humidity (%) Fig. 3 Current us time data for the PPCl composite when exposed to Fig. 6 Resistance change for the PPCl sensor on exposure to a range three 500 ppm injections (El,W, +) of ethanol vapour of relative humidities 292 J Muter Chem , 1996, 6(3), 289-294 35 I30 25 I G $20 15 8 10 "0 20 40 60 80 100 relative humidity (%) Fig.7 Resistance change for the tin dioxide sensor on exposure to a range of relative humidities 0 6 0 0 On urn I 1 1 I I I I ? =, 0 20 40 60 80 100 relative hum idity (%) Fig.8 Resistance change for the PPCl (B)and PPNO, (0)composite sensors on exposure to a range of relative humidities 500 0 4001 33001 8 200 ,001 0 0 on " 0 20 40 60 80 100 relative humidity (%) Fig. 9 Resistance change for the PNMPCl (m)and PNMPNO, (0) composite sensors on exposure to a range of relative humidities humidity changes. Composites incorporating PPCl were rela- tively unaffected by humidity changes, whilst being most sensitive to organic vapours. These observations may be partly explained by the differential interaction of water vapour with the solvation sheaths surrounding the dopant anions of the polymers.However, the results show the polymer type to be the major contributing factor to the humidity dependence. Distinct response patterns to humidity were observed, which would permit simple compensation in a practical device, assuming baseline drift can be reduced. Testing of the composites and individual components towards ethanol (500 ppm) under 50% humidity conditions gave sensitivity values typically 65% of those obtained in dry conditions, suggesting that competitive rather than additive effects are observed with humidity. Under ambient humidity conditions all composite sensors exhibited slower adsorption and desorption kinetics.The composite sensors reached 90% of the maximum response within 4min and were 90% revers- ible within 6min. Sensors constructed solely of tin dioxide gave sensitivity values typically 50% of those observed when tested under dry conditions. Adsorption kinetics were similar to the composites, but desorption kinetics were slower with sensors only 50% reversible within 8 min, Sensors constructed solely of polypyrrole derivatives gave sensitivity values typically 75% of those observed when tested under dry conditions. Adsorption and desorption kinetics were 50% slower than when tested under dry conditions. All sensors were retested after six weeks and the results showed that despite a small decrease in sensitivity (typically 5-10%), the response profiles and baseline stabilities were similar.The behaviour of the PPCl sensors was erratic with a substantial loss of gas sensitivity in some sensors. This can be explained by the low mechanical strength of sensors based on chemically prepared polypyrroles which are highly intractable materials. The sensors based on tin dioxide alone had drifted to much lower resistances over this time, but still gave sensi- tivity values comparable to those obtained when they were first tested (within 20% of the original values). Gas sensitivity was found to be dependent upon the polymer derivative, the associated anion, and the synthesis conditions of the polymers incorporated in the composites. This is in agreement with literature on the responses of pyrrole- based systems for gas sensing.The sensitivity of the composite sensors compares favourably at lower vapour concentrations with heated thin-film systems cited in the literat~re,~ whilst power consumption is minimal in comparison. Comparison with reported" sensitivities of electropolymerised polymers to organic vapours at room tem- perature shows composite sensors generally to be more sensi- tive, especially to alcohols. Sensitivities of 0.5% are quoted on exposure to 1600 ppm of methanol vapour for a single polymer incorporated in an array-type device." Quoted linear sensi- tivity ranges for both thin-film sensors and electropolymerised films are superior to composite sensors where saturation occurs above the 500 ppm level.However, for applications in the early detection of soft rot in stored potato tubers the concentrations of interest will be in the low ppm range. Conclusion Novel vapour sensors for the detection of organic vapours have been fabricated from composites of polypyrrole deriva- tives and tin dioxide. The sensors, which operate at room temperature, respond reproducibly and reversibly to low con- centrations of a range of organic species, such as esters, alcohols and ketones. Sensitivities at low concentrations compare favourably with common existing sensor types based on ceramic materials and electropolymerised polymer films. Importantly, some of the sensors tested possess different rela- tive responses to the vapours, thus permitting their use as part of an array of sensors for vapour discrimination.All these factors suggest possible applications in the quality control of foodstuffs, specifically in the early detection of soft rot in potato tubers. Future work will be involved in optimising the fabrication conditions to improve the reproducibility of manu-facture and addressing problems such as humidity dependence and baseline stability. Further optimisation must be carried out before the composites could be incorporated into a practical device for vapour sensing. References 1 J. J. Miasik, A. Hooper and B. C. Tofield, J. Chem. Soc., Faraday Trans. 1, 1986,82, 117. 2 T. C. Pearce, J. W. Gardner, S. Friel, P. N. Bartlett and N. Blair, Analyst, 1993,118, 371.3 Y. K. Fang and J. J. Lee, Thin Solid Films, 1989, 169, 51. 4 T. Maekawa, J. Tamaki, N. Miura and N. Yamazoe, Sensors Actuators, 1992,9, 63. 5 J. Watson and K. Ihokura, The Stannic Oxide Gas Sensor; Principles and Applications, CRC Press, Boca Raton, FL, 1994. 6 W. E. Howard, J. Luminescence, 1981,24125,835. J. Mater. Chem., 1996, 6(3), 289-294 293 7 8 9 10 I Rodriguez and J Gonzalez Velasco, J Muter Sci Lett, 1987, 6,1319 S Maeda and S Armes, Chew Mater , 1995,7,171 J H Burroughes, D. D Bradley, A R Brown, R N Marks, K Mackay, R H Friend, P L Burns and A B Holmes Nature, 1990,347 539 A J Frank and K Honda, J Phys Chem, 1982,815,1933 16 17 18 19 T Nenov and S Yordanov Sensors Actuators 8,1992,8,117 J V Hatfield,P J Hicks P Neaves,K C PersaudandP Travers, Sensors (Vl ) Technology Systems and Applicutions, ed K T V Grattan and A T Augostini, Institute of Physlcs, Bristol, 1993 R E Myers, J Electron Muter 1986, 15, 61 H V Shurmer, P Corcoran and J W Gardner, Sensors Actuurors 11 12 T Woda, A Takeno, M Iwaki and H Sasabe, Synth Met 1987, 18,585 H Sasabe, T Furuno and T Wada, Mol Cryst Liq Cryst, 1988, 160,281 20 21 B, 1991,4,29 T Hanawa, S Kuwabata and H Yoneyama, J Chew SOC, Faraday Trans I, 1988,84,1587 J M Slater and E J Watt, Analyst, 1992, 117, 1265 13 14 J L Varns and M T Glynn, Am Pot Jn, 1979,56,185 J W Gardner, E L Hines and H C Tang, Sensors Actuators B, 22 J V Hatfield, P Neaves, P J Hicks, K Persaud and P Travers, Sensors Actuators B, 1994,lS-19,221 1992,9,9 15 K Persaud and G Dodd, Nature, 1982,299,352 Paper 5/04829J, Received 21st July, 1995 294 J Mater Chem , 1996,6(3), 289-294

ISSN:0959-9428    

DOI:10.1039/JM9960600289

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Synthesis and gas sensing properties of poly [tetra( pyrrol-l-yl)~ilane]~ Phillip Evans,"Norman M. Ratcliffe,""James R.Smithband Sheelagh A. Campbellb"Department of Chemical and Physical Sciences, Faculty of Applied Sciences, University of the West of England (Bristol), Coldharbour Lane, Frenchay, Bristol, UK BS16 1Q Y Applied Electrochemistry Group, School of Chemistry, Physics and Radiography, Chemistry Division, University of Portsmouth, St. Michael's Building, White Swan Road, Portsmouth, UK PO1 2DT Conducting polymers such as polypyrrole and polythiophene offer a new approach to the design of modified electrodes and sensors. In the current work, the electrochemical and chemical polymerisation of tetra( pyrrol- 1-y1)silane is described. Resultant polymers with different anions have been characterised by electrochemical methods, XPS and microanalysis.Molecular geometry calculations suggest that both inter- and intra-molecular couplings are present in the film. Crosslinking of the polymeric matrix via P-linkages will result in a three-dimensional structure with a concomitant reduction in the degree of conjugation, accounting for the low film conductivity (CT ca. S cm-I). Preliminary results show that poly [tetra(pyrrol-1-yl)silane]is a promising material for the fabrication of gas sensors. It is unexpectedly sensitive to ammonia and trimethylamine gas when compared with polypyrrole and poly(N-methylpyrrole) prepared in a similar fashion. The properties of polypyrrole are being exploited for appli- cations in such diverse fields as cathode materials for rechargeable batteries,' selective membrane electrodes,2 elec- tromagnetic shielding material^,^ selective bio~ensors,~ ion-exchange chromatography resins,' biological markers6 and gas sensors.' There is a need for the syntheses of novel polypyrroles, polythiophenes and other conducting polymers for assess-ment as potential gas sensors for use in 'artificial noses'.Little information on the use of derivatised polypyrroles for gas sensor applications has appeared in the literature. Understanding these materials may offer new insights for molecular electronics and the fabrication of modified electrode^.^,^ Tetra(pyrro1- 1-y1)silane (1) consists of a central silicon atom, tetrahedrally coordinated to four pyrrole rings via the pyrrole nitrogen atoms (Fig.1). This compound was initially investigated as an intermediate for the chemical syntheses of 3-substituted pyrroles, as an extrapolation of the use of triisopropylsilyIpyrrole.'o,ll Electropolymerisation of 1 might be expected to occur predominantly through the a-positions, although some P,p'-couplings may also be present, causing a detrimental effect on cond~ctivity.l~*'~However, for many modified electrode appli- cations, electron transfer occurs at the underlying electrode surface rather than at the conducting polymer per se. Hence, the potentially higher porosity of poly(1) may offer greater scope for the physical entrapment of anions or other immobi- lised material for modified electrodes and sensor applications.In the current work, the electrochemical and chemical Fig. 1 Structure of tetra(pyrro1-1-y1)silane (1) ?Presented at the Second International Conference on Materials Chemistry, MC2, University of Kent at Canterbury, 17-21 July 1995. polymerisation of 1 is described and the application of the resultant polymer as a vapour sensor investigated. Experimenta1 Reagents Dichloromethane (Fisons, AR grade), tetrabutylammonium tetrafluoroborate (TBABF,, Fluka, Puris), lithium perchlorate (LiCIO,, BDH, ACS grade) and silver nitrate (Aldrich, AR grade) were used as received. Acetonitrile (Aldrich, HPLC grade) was distilled over P205and stored over alumina (Woelm N-Super 1).Pyrrole and N-methylpyrrole (Aldrich, AR grade) were redistilled immediately prior to use and stored under a nitrogen atmosphere at 0 "C.Light pttroleum (bp 40-60 "C; BDH, AR grade) was distilled over 4 A molecular sieves and stored over calcium chloride. Organic synthesis Preparation of tetra(pyrro1-I-y1)silane (I).14 Potassium (1.07 g, 0.027 mol) was cautiously added to a stirred cooled (0°C) solution of pyrrole (1.9 cm3, 0.027 mol) in light pet- roleum (bp 40-60°C) under a nitrogen blanket and left for 30 min. The mixture was slowly warmed to 65 "C to allow any residual potassium to react. The solution was then cooled to 0 "C and the white precipitate of potassium pyrrol-1-ide filtered off, washed with light petroleum (bp 40-60°C; 40 cm3) and dried in uucuo (40 "C, 1h, 0.95 mmHg) to yield potassium pyrrol-1-ide (2.23 g, 0.021 mol, 94%).All of this compound was suspended in light petroleum (bp 40-60 "C; 40 cm3) under nitrogen and cooled to 0 "C. Silicon tetrachloride (0.78 cm3, 6.8 x lod3mol) in light petroleum (7 cm3) was slowly added to the stirring mixture over a 20min period. Stirring was continued for a further 2 h and the product was recovered by Soxhlet extraction using light petroleum (bp 40-60 "C) as solvent. Tetra( pyrrol-1-y1)silane was recrystallised from light petroleum (bp 40-60 "C) to yield fine colourless needles (0.44g, 28%), mp 173 "C (uncorrected) (lit.,I4 173.4 "C); G,(CDC13) 6.32 (8 H, m, 8 x PH) 6.68 (8 H, m, 8 x aH); m/z 67 [(C,H,N)+], 80, 94 [(C4H4N)Si+], 106, 132, 146, 159 [(C4H4N)2Si+], 171, 199, 226 [(C4H4N)3Si+], 251, 265, 292, W+).J. Mater. Chem., 1996, 6(3), 295-299 295 Electrochemical studies Electrochemical studies were performed in a three-compart- ment divided cell l5 Platinum (disc area 0385 cm') and indium-tin oxide coated glass (IT0 glass) (thickness 100 nm, surface resistivity < 30 SZ m, Balzers High Vacuum Ltd , Milton Keynes, UK) were used as working electrodes The platinum electrode was polished prior to use with an alumina slurry (0 3 ym) Platinum gauze was used as the counter electrode All potentials were measured against an Ag/AgN03 reference electrode, consisting of a silver wire immersed in a solution of acetonitrile containing silver nitrate (0 01 mol dm-3) and TBABF, (0 1mol dmP3) This solution was separated from the surrounding electrolyte by a glass frit A Hi-Tek DT 2101 potentiostat coupled to a Hi-Tek PPRl wave form generator was used to generate the electrochemical signals Output was recorded on a Lloyd PL3 XYt recorder Charge passed during the experiments was measured with a Hi-Tek integrator Potentiodynamic and galvanostatic tech- niques were used to study electrochemical behaviour All solutions were freshly prepared and degassed with nitrogen for 15 min prior to each experiment For cyclic voltammetry experiments, the monomer together with the supporting elec- trolyte, TBABF, or LiClO,, were dissolved in either acetonitrile or dichloromethane (in the case of 1)and the potential cycled, typically from 0 to 2 V us Ag/Agf and the current monitored as a function of the applied potential For characterisation studies, films were grown from the same electrolyte composi- tions under constant current or potential conditions In the former, currents in the range 0 2-20 mA cm-2 were passed for between 10min to 5 h using a two electrode system and monitoring the potential as a function of time Potentiostatic growth was performed by stepping the potentials (El= 0, Ef= 200-5000 mV us Ag/Agf) Polymer redox behaviour was evaluated by cycling the films in the same electrolytes used for growth, but in the absence of monomer Polymers were grown on IT0 glass from a solution of the monomer (0 005 rnol dmP3) in dichloromethane [poly(l)] con- taining TBABF, (0 1mol dm-3) or acetonitrile (polypyrrole) containing LiClO, (0 1 rnol dmP3) by potential cycling between 0 and 2100 mV at a sweep rate of 5 mV s-l A total of 1 5 cycles were carried out, with termination of the final cycle at 2100 mV thus ensuring full doping of the film Excess supporting electro- lyte on film surfaces was removed by extensive rinsing with dichloromethane and the films dried under a stream of nitro- gen Samples were examined under a Vickers M41 Photoplan optical microscope and a JEOL JSM-35C scanning electron microscope (SEM) Films were gold sputtered for SEM obser- vation and film thickness measurements were made by viewing a cross-section of the film-substrate interface Conductivity measurements were performed by applying a known current through the film and measuring the resulting potential across the film In these experiments, a mercury drop of known area was used as a contact to the film surface Electrical conductivities of chemically synthesised polymers were measured on pressed pellets (5 tonnes) using the four- point-probe technique under dc conditions X-Ray photoelectron spectroscopy (XPS) studies were car- ried out using a VG Scientific ESCALAB Mk I1 instrument Lineshape analysis was performed on each peak in an attempt to resolve the broad signals A1-Ka radiation (1486 7 eV) was used as the X-ray source Binding energies were adjusted so that the main C( 1s) peak occurred at 285 00 eV and atomic percentages were calculated from the peak areas using standard atomic sensitivity factors l6 The atomic percentage for the C1 signal was calculated by overlapping the C1(2p3,,) and C1(2p1,,) peak areas, although binding energies refer to the C1( 2 ~ peak XPS studies of polypyrrole were also carried out for comparison purposes 296 J Muter Chem, 1996,6(3), 295-299 Molecular modelling studies Molecular geometry optimisation calculations were performed using the PM3 semi-empirical program Hyperchem@ Convergence was set to 001, iterations were limited to 50 and a PolakRibiere optimisation algorithm used Preparation of vapour sensors Tetra(pyrro1-1-y1)silane (1)(031 g, 0 001 mol) was dissolved in a few drops of dichloromethane and rapidly added to a stirred solution of copper(I1) bromide (0 91 g, 4 08 x mol) in acetonitrile-dichloromethane (1 1 v/v, 150cm3) then left for 70 min A black precipitate was formed which was collected and washed with copious amounts of acetonitrile until the eluent was clear The recovered polymer was dried zn vacuo (60 "C, 1h, 0 95 mmHg) and stored at 0 "C until required Polypyrrole and poly (N-methylpyrrole) were prepared in an identical fashion Yields poly( 1) 0 30 g, polypyrrole 0 07 g, poly(N-methylpyrrole) 0 08 g Sensor fabrication Sensors were constructed using gold on alumina interdigitated electrodes (GEC-Marconi, Wembley, UK) with a 125 pm gap between the electrode 'fingers' Poly( 1) (0 1 g) was ground in water (0 2 cm3) until a fine paste was obtained which was then transferred to the surface of the electrode and the water was allowed to evaporate in an oven regulated at 60 "C When dry, contacts to the interdigitated array were made using circuit board shell pins connected to lengths of wire Sensors with polypyrrole and poly(N-methylpyrrole) were fabricated as above Each sensor contained an approximate polymer loading of 10mg with a typical thickness of 67 ym (as determined by SEM) Sensing trials The sensing apparatus consisted of the coated electrode sus- pended in a flask of known volume into which a known concentration of analyte gas was introduced by means of a gas syringe Resistance change was monitored manually using a multimeter (Fluke73, Maplin Electronics Ltd ) Sensors were exposed to a range of ammonia and trimethyl- amine concentrations, ranging from 001 to 10% vapour by volume The maximum percentage change in resistance occur- ring during a 1min exposure was recorded The responses recorded were the average of three exposures at each gas concentration Recovery to the original baseline occurred within 20 min of removal from the vapour chamber Results and Discussion Electrochemical polymerisation The electrochemical behaviour of 1 in a TBABF, (0 1mol dm-3~dichloromethane electrolyte is shown in Fig 2 Monomer oxidation resulted in the formation of a smooth, blue-black homogeneous and very adherent film on the elec- trode surface The onset of pyrrole oxidation was observed at 450 mV On the return cycle, a nucleation loop at 745 mV, indicative of the formation of an electroactive film, was observed and a very broad reduction peak, corresponding to polymer dedoping, was present at 0 mV Increased currents for both processes were seen on the second scan, although by the third, these began to decrease The location of the polymer oxidation signal was not obvious but was thought to be under the broad monomer oxidation wave However, when the film was cycled in the same electrolyte in the absence of monomer, ~~)~only large capacitive currents were observed with no indication of redox behaviour The absence of a polymer oxidation peak has also been reported for 3-trimethyl~ilylthiophene~~~and In -0.5 I I I f -1000 -500 0 500 1000 E vs.AglAg' /mV Fig.2 Cyclic voltammogram of 1.Working electrode Pt disc (area 0.385 cm2); monomer conc.0.005 mol dm-3; v = 5 mV s-'; electrolyte 0.1 mol dm -3 TBABF,-dichloromethane; first scan (-), second scan (----), third scan (--. -.-)* this case was partly attributed to the high solubility of the polymer in its reduced form. Polymer characterisation Electrochemically polymerised 1 exhibited a conductivity of S cm-l with no enhancement in conductivity observed when the film was placed in an atmosphere of iodine for 24 h. The low conductivity observed together with the absence of redox behaviour suggests some disruption in the conjugation of the polymer which may be due to non-a,d-linkages between different pyrrole rings in adjacent monomers.The film could be removed from the electrode surface by abrasion, but was insoluble in acetone, ethanol, dichloromethane, THF, toluene and water. Fig. 3 shows an SEM image of poly(1) on IT0 glass. The film is essentially very smooth although higher regions appear to have a coral-like topography, quite different to that of p~lypyrrole.'**~~ Microanalysisof the chemically synthesised polymer Microanalysis data of chemically synthesised poly( 1) and polypyrrole are shown in Table 1. For polypyrrole, the C:N ratio was calculated to be 4 : 1, in agreement with the stoichio- metric ratio, with a Br :N ratio of 1:1, suggesting incorporation of one bromine for every monomer unit. In the case of poly( l), Fig. 3 SEM micrograph of the polymer formed from the electropolym- erisation of 1 on IT0 glass Table 1 Mi~roanalyslsdata of 1, poly( 1) and polypyrrole atom% compound element sample 1 sample 2 avcrage polypyrrole C H 28 55 120 28.25 1.16 28.40 1.18 8 29 8.26 8.28 51.17 50.64 50.91 0 22 0.22 0.22 41.53 41.72 4 1.62 3.03 3.05 3.04 11.05 11.16 11.11 19.14 18.97 1906 14.22 14.44 14 33 "Synthesised using copper(I1) bromide as oxidant.C :H and C :N ratios of 1.1:1 and 4.3 : 1 were obtained with a Br :N ratio of 1 :3.3, showing a similar dopant level to that measured for polypyrrole. X-Ray photoelectron spectroscopy studies of the electrochemically and chemically synthesised polymers The binding energies for carbon, boron (from the dopant tetrafluoroborate anion), nitrogen, oxygen and silicon observed in the XPS spectrum of the electropolymerised poly(1) film, together with atomic percentages, are summarised in Table 2. The degree of polymer doping can readily be obtained from the atomic percentage ratio of the N(1s) peak to that of the B( 1s) peak assuming four nitrogen atoms per monomer unit.Thus, it would appear that 2.1 BF,- anions are incorporated into the film for every monomer unit i.e. approximately one anion for every two pyrrole rings. The O(1s) signal at 532.50 eV can be attributed to the presence of carbonyl species, formed as a result of oxidation of the film.20.2' Two N(1s) signals were observed in the XPS spectrum of the electropolymerised film.These are significantly different from those previously reported for polypyrroles22 whereby a single signal may be resolved into a number of smaller overlap- ping signals. Since the atomic percentage ratios of the combined N( 1s) signal to the Si(2p) peak is 3.88, i.e. within experimental error of the theoretical value of 4.0 for that of the monomer, it was assumed that the nitrogen peaks are due to the pyrrole rings, not to tetrabutylammonium cation incorporation. The two nitrogen signals must therefore result from pyrrole rings in different chemical environments, although the exact nature of these is uncertain. One suggestion is the occurrence of inter- and intra-molecular couplings via the pyrrole rings, to form the polymer shown in Fig.4.The atomic percentage ratio of the two nitrogen peaks is close to 1 :1, in agreement with this structure. In addition, further oxidation reactions may then Table 2 XPS data of the film formed from the electropolymerisation of 1 and pyrrole ~~~~~ ~ ~ polyp yrrole POlY(1 ) signal binding energy/eV atom% binding energy/eV atom% --193.90 3.1 207.85 3.4 284.15 13.9 283.80 5.5 285.00 25.6 285.00 46.0 286.20 17.5 286.45 13.8 288.45 5.7 288.55 1.9 -291.75 0.9 400.30 4.7 399.95 2.8 401.10 4.2 402.40 3.2 403.30 1.8 101.75 1.6 532.80 19.9 532.50 7.2 -534.85 2.3 3. Mater. Chem., 1996, 6(3), 295-299 297 L -In Fig. 4 Proposed idealised structures for poly( l), polypyrrole and poly (N-methylpyrrole) occur via a-or P-linkages in the remaining uncoupled pyrrole rings leading to the formation of a three-dimensional structure.XPS of the chemically oxidised polymers was carried out to detect copper, the presence of which might affect sensor response. None was found in either poly( 1) or polypyrrole, whilst a small quantity, corresponding to 0.38 atom%, was detected in poly(N-methylpyrrole). This supports our suppo- sition that entrapped copper plays no part in the response of the polymers to volatile amines. Molecular modelling Molecular geometry optimisation calculations show four pyr- role units tetrahedrally positioned around the central silicon atom, with the plane of each neighbouring pyrrole ring oriented in such a way as to overcome steric hindrance between adjacent hydrogen atoms [Fig.5(a)]. This three-dimensional represen- tation of the molecule emphasises the availability of the pyrrole a-positions for monomer couplings. When two neighbouring pyrrole rings in 1 are intramolecularly coupled via their a-positions [Fig. 5(b)],no change in the enthalpy of formation (92 kJ mol-l) of this new structure results. This suggests that both intra- and inter-molecular couplings are energetically favourable in the polymerisation reaction. Thus, the polymer is likely to be highly crosslinked, a conclusion supported by electrochemical studies which show significantly reduced con- ductivity compared with polypyrrole. If the two remaining pyrrole rings are coupled in an intra- molecular fashion via the a&-positions, then a large increase in energy, to 167 kJmol-l, is observed due to strain on the tetrahedral geometry of the central silicon atom.Thus, we conclude that only one a,a'-intramolecular coupling may be present per monomer unit. Preliminary sensing trials Fig. 6(a) and (b)show response profiles of the three polymer sensors, polypyrrole, poly(N-methylpyrrole) and poly( l), to ammonia and trimethylamine vapours, respectively, in the concentration range of 0.01 to 10% vapour by volume. It can be seen that in both cases poly(1) yields a greater response than either of the other sensors. This is unexpected since our work and a previous report23 show that N-substituted pyrroles give a diminished response to ammonia vapour when compared with polypyrrole.A mechanism of H+ abstraction from the --NH of the pyrrole units by ammonia has been proposed as the basis for the reversible interaction between polypyrrole and ammonia.24 However, this mechanism cannot be the only process occurring and in the case of poly (N-methylpyrrole), the sensor response must be due to an interaction of the vapour with the charged polymeric backbone itself. From the XPS and microanalyses data we find no evidence to support any suggestion that entrapped copper ions may play any part in the responses observed. The reason for the high response of 298 J. Muter. Chem., 1996, 6(3),295-299 H P H Fig. 5 Molecular geometry of (a) 1 and (b) 1 containing one u,u'-pyrrole coupling poly( 1)to ammonia and trimethylamine is unclear at present.Steric arrangement of pyrrole rings in poly( 1) may permit greater interaction of the amine nitrogen electronlone pair with the charged polymeric conjugated backbone. This process may also be facilitated by the higher porosity of this polymer compared with polypyrrole, which would allow increased penetration of the analyte gas into the polymer matrix, thereby maximising its response. Conclusions Electropolymerisation of tetra( pyrrol-1-y1)silane results in the formation of a smooth and very adherent film on IT0 glass with a conductivity of S cm-l. These films appear to be highly doped with incorporation of approximately two BF4-anions per monomer unit. Molecular geometry calculations suggest that both inter- and intra-molecular couplings are present in the film.A three-dimensional structure is proposed in which fi-linkages are also present thus reducing the degree of conjugation and hence overall film conductivity. Preliminary results show that chemically prepared poly [tetra( pyrrol-1-yl)silane] is a promising material for the fabrication of gas sensing materials. It shows a superior response to ammonia and trimethylamine when compared with polypyrrole and poly(N-methylpyrrole) prepared in a similar manner. Such a sensor has potential application in a number of fields such as monitoring odours from agricultural buildings and activitie~,~~ food freshness monitoring26 and as a component of an electronic nose based sensor array system.27 !?! -t2 01 I I 0.1 1 2$ 100 0.01 K c-2 80 60 40 20 0 I I 0.1 1 10 vapour concentration (96) Fig.6 Response profiles of the three polymer sensors to a range of vapour concentrations of (a) ammonia and (b) trimethylamine.Poly( 1); 0poly(N-methylpyrrole); + polypyrrole. The authors thank Dr R. Ewen at the University of the West of England (Bristol) for the use of XPS facilities, Mr M. West (University of Bristol) for microanalyses and the EPSRC for financial support. References 1 B. 2. Lubentsov, G. I. Zvereva, Ya. H. Samovarov, S. M. Bystriak, 0.N. Timofeeva and M. L.Khidekel, Synth. Met., 1991,41, 1143. 2 P. C. T. Wong, B. Chambers, A, P, Anderson and P V. Wright, IEEE Con& Publ., Antennas and propagation, 1993,370,part 2,934.3 E. V. Thillo, G. Defieuw and W. dewinter, Bull. SOC. Chim. Belg., 1990,99, 981. 4 P. R. Teasdale and G. G. Wallace, Analyst, 1993,118,329. 5 C. J. Gow and C. F. Zukoski, J. Colloid Interface. Sci., 1990, 136, 175. 6 P. J. Tarcha, D. Misun, D. Finley, M. Wong and J. J. Donovan, in Polymer Latexes: Preparation Characterisation and Applications, ed. E. S. Daniels, E. D. Sudal and M. S. Elaasar, ACS Symp. Ser., 1992,492,347. 7 P. N. Bartlett, Electrochemical Sensors, Report commissioned for the DTI Chemical Sensors Club by The Laboratory of the Government Chemist, 1990. 8 A. Aviram, J. Am. Chem. SOC., 1988,110,5687. 9 J. Roncali, C. Thobie-Gautier, H. Brisset, J.-F.Favart and A. Guy, J. Electroanal. Chem., 1995,381, 257. 10 B. L. Bray, P. H. Mathies, R. Naef, D. R. Solas, T. T. Tidwell, D. R. Artis and J. M. Muchowski, J. Org. Chem., 1990,55,6317. 11 N. M. Ratcliffe and P. Evans, unpublished work. 12 J. R. Smith, S. A. Campbell and N. M. Ratcliffe, Bull. Electrochem., 1995, 11, 378. 13 J. Roncali, Chem. Reu., 1992,92, 711. 14 J. E. Reynolds, J. Chem. Soc., 1909,95, 505. 15 J. R. Smith, S. A. Campbell, N. M. Ratcliffe and M. Dunleavy, Synth. Met., 1994,63,233. 16 C. D. Wagner, L. E. Davis, M. V. Zeller, J. A. Taylor, R. H. Raymond and L. H. Gale, Surf. Interface Anal., 1981,3,211. 17 D. Adebimpe, P. Kathirgamanathan and M. Shepherd, J. Electroanal. Chem., 1993,348,447. 18 K. K. Kanazawa, A. F. Diaz, W. D. Gill, P. M. Grant, G. B. Street, G. P. Gardini and J. F. Kwak, Synth. Met., 1980, 1, 329. 19 J. Prejza, I. Lundstrom and T. Skotheim, J. Electrochem. SOC., 1982,129,1685. 20 S. Dong and J. Ding, Synth. Met., 1987,20, 110. 21 J. R. Smith, N. M. Ratcliffe and S. A. Campbell, Synth. Met., 1995, 73, 171. 22 J. G. Eaves, H. S. Munro and D. Parker, Polym. Commun., 1987, 28, 38. 23 P. Foot, T. Ritchie and F. Mohammad, J. Chem. Soc., Chem. Commun., 1988,1536. 24 G. Gustaffson, I. Lundstrom, B. Liedberg, C. R. Wu, 0. Inganas and 0.Wennerstrom, Synth Met., 1989,31, 163. 25 G. Huyberechts, M. Van Muylder, M. Honore, J. Desmet and J. Roggen, Sensors Actuators B, 1994, 18/19,296. 26 P.-M. Schweizer-Berberich, S. Vaihinger and W. Gopel, Sensors Actuators B, 1994,18/19,282. 27 J. W. Gardner and P. N. Bartlett, Sensors Actuators B, 1994, 18/19,211. Paper 5/04834F;Received 21st July 1995 J. Mater. Chem., 1996, 6(3), 295-299 299

ISSN:0959-9428    

DOI:10.1039/JM9960600295

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

~~~~ ~ ~ Electrorheological properties of polypyrrole prepared by the action of mineral acids on pyrrolef Geoffrey C. Teare and Norman M. Ratcliffe" Faculty of Applied Sciences, University of the West of England, Coldharbour Lane, Frenchay, Bristol, UK BSl6 1Q Y Polypyrrole was prepared in high yield by the polymerisation of pyrrole in strongly acidic conditions using hydrofluoric, hydrochloric (at 20 "C and 100 "C), hydrobromic, nitric and orthophosphoric acids. The polymer products incorporated significant amounts of the anion from the acid used. The particulate morphology of the polymer products chiefly consisted of agglomerated pm-sized spheres. The rheological properties of dispersions of polymers prepared from pyrrole using hydrochloric and hydrobromic acids, suspended in 1-chloronaphthalene-1-bromonaphthalene,were found to be electric-field dependent.Fluids prepared from the polypyrrole products derived from hydrofluoric, nitric and orthophosphoric acids did not exhibit an electrorheological response. The electrorheological (ER) effect involves the rapid and reversible modification of the flow properties of a fluid under the application of an external electric field, principally an increase in the viscosity and in the elastic modulus. There is considerable interest in these materials for possible uses as stop valves, clutches, brakes and dampers. Winslow first reported the ER effect in 1949,' using moist silica particles suspended in kerosene. Subsequently, other systems have been demonstrated to be ER a~tive.~-~ Most ER fluids consist of suspensions of inorganic materials such as aluminosilicates in a continuous phase of an insulating fluid.In addition, these systems generally require the presence of significant amounts of polar solvents, such as water, to enhance particle polaris- ation, for an appreciable ER response. There are problems of settling out and abrasion with these inorganic materials, and the presence of water limits the working temperature range. Recently, it has been demonstrated that semiconducting organic polymers such as polyaceneq~inones,~ polyaniline6 and poly (p-~henylene)~ doped with copper(I1) chloride or iron(m) chloride exhibit ER responses in the anhydrous state. These systems show considerable promise owing to the reduction of abrasion, and because of the wider range of fluids available for density matching with the dispersed phase.In this study it is demonstrated that materials prepared by the polymerisation of pyrrole in the presence of mineral acids possess electrorheological properties in the anhydrous state when dispersed in an appropriate fluid. These polymers are worthy of study owing to their ease of preparation and their low conductivities. Experimental Reagents Pyrrole and mineral acids were purchased from the Aldrich Chemical Company (Dorset, UK). Pyrrole was distilled prior to use. Acid solutions (7.5 mol dm-3) were prepared by dilution of the appropriate general reagent grade concen-trated acids.Characterisation Scanning electron microscopy (SEM) was carried out at the University of the West of England using an Hitachi S-450 scanning electron microscope, with an accelerating voltage of i Presented at the Second International Conference on Materials Chemistry, MC2, University of Kent at Canterbury, 17-21 July 1995. 10kV. Samples were prepared for microscopy by sputter coating with gold, using an Emscope SC500 sputter coating unit. Microanalyses were carried out by the Microanalytical Department, School of Chemistry, Bristol University. X-Ray photoelectron spectroscopy (XPS) studies were carried out using a VG Scientific ESCALAB Mk I1 instrument. Lineshape analysis was performed on each peak in an attempt to resolve the broad signals.Al-Ka radiation was used as the X-ray source. Bonding energies were adjusted so that the main C 1s peak occurred at 285.00eV and atomic percentages were calculated from the peak areas using standard atomic sensi- tivity factors. The rheologies of the suspensions were tested using continuous shear viscometry, using a Bohlin VOR rhe- ometer, with a measuring cell consisting of two 3 cm diameter, parallel stainless-steel plates, 1mm apart, electrically isolated from the rest of the apparatus by ceramic spacers. A potential was applied to the upper plate via a fine gauge insulated wire. A flexible metal strip, in contact with the lower plate, was used as an earth. Potential was supplied using a Time Electronics (Tonbridge, Kent) multifunction ac-dc calibrator 9822 power supply.The current drawn was measured using a Fluke 8010A digital multimeter. Conductivity measurements were obtained from pressed pellets (10 tonnes pressure for 1 min) of the powdered, dried material. Both sides of each pellet were gold coated and the resistance measured with a Fluke 8010A digital multimeter. Conductivities were calculated using the surface area and thickness of each pellet. Preparation of the polymers Product 1. An aqueous solution of pyrrole (250 cm3, 0.75 mol dm-3) was added rapidly to a stirred solution of hydro- chloric acid (1000cm3, 7.5 mol dm-3) at 20 "C. Stirring was continued for 12 h, after which time the resulting product was filtered off, extensively washed with distilled water (3000 cm3), and dried in vucuo at 70 "C for 5 h, yielding 7.89 g of a dark red-brown powder (product 1).Product 2. Product 1 (4.00 g) was heated in U~CUOfor 72 h at 180°C, yielding 2.81 g of a fine black powder (product 2). Product 3. An aqueous solution of pyrrole (250cm3, 0.75 mol dm-3) was added rapidly to a stirred solution of hydro- chloric acid (1000cm3, 7.5 mol dm-3) at 100"C. Stirring was continued for 10min, after which time the resulting product was filtered off. The same washing and drying procedures were carried out as for product 1, yielding 8.53 g of a brown powder (product 3). J. Mater. Chem., 1996, 6(3), 301-304 301 Product 4. An aqueous solution of pyrrole (250 cm3,0.75 mol dme3) was added rapidly to a stirred solution of nitric acid (1000 cm3, 7.5 mol dm-3) at 20 "C.The reaction continued for 8 h, after which time the precipitate was filtered. Washing and drying procedures were carried out as for product 1, yielding 10.56 g of a dark brown powder (product 4). Product 5. An aqueous solution of pyrrole (100cm3, 0.75 mol dm-3) was added rapidly to a stirred solution of hydro- fluoric acid (400 cm3, 7.5 mol dm-3) at 20 "C. No product was apparent after 24 h, and stirring was continued for 96 h, after which time the precipitate was filtered off, washed with distilled water (2000cm3) and dried in uucuo at 70 "C for 5 h, yielding 1.63 g of a fine brown powder (product 5). Product 6. An aqueous solution of pyrrole (50 cm3, 0.75 mol dm-3) was added rapidly to a stirred solution of orthophos- phoric acid (200 cm3, 7.5 mol dm-3) at 20 "C.No product was apparent on stirring for 24 h; on stirring for a further 72 h a dark red intractable gelatinous mass was obtained. Removal of acid was carried out by vigorous stirring of the gel in distilled water (1000 cm3), followed by decantation of the supernatant liquid. This process was repeated several times. The washed gel was filtered and dried in YUCUO at 70°C for 8 h, to yield a black powder. Residual acid was removed by stirring the solid product in distilled water (200 cm3), followed by filtration. This process was repeated several times. The solid product was then filtered and dried in U~CUOat 70°Cfor 5 h, to yield a black powder, mass 2.03 g (product 6).Product 7. An aqueous solution of pyrrole (44 Cm3,0.75 mOl dm-3) was added rapidly to a stirred solution OfhYdrobromic acid (177 cm3, 7.5 mol dm-3) at 20 "c.Stirring was continued for 8 h, after which time the precipitate was filtered, washed with distilled water (2000 m3)and dried in uucuo at 70 "C for 5 h, yielding 1.71 g of a fine brown powder (product 7). Preparation of suspensionsfor ER testing Samples of products 1-7 were ground in a vibratory ball mill for 40-60 min. The resulting particle size and morphology were determined by SEM. The suspending fluid used was a mixture of l-chloronaphthalene and l-bromonaphthalene, in the ratio 0.56: 1 v/v, with a density of 1.30 g ~m-~. This was found to be a suitable density match for the materials tested.For products 1-7, test fluids were prepared by suspending 0.60g of solid material in 2.00cm3 of the l-chloronaph-thalene-l-bromonaphthalene mixture (designated 30% sus-pensions). In addition, for product 1, suspensions were prepared with 0.20, 0.40 and 0.80 g of solid in 2.00 cm3 of fluid (designated lo%, 20% and 40% suspensions). Results and Discussion Although the oxidative polymerisation of pyrrole to produce polypyrrole has been widely studied,*-" there are few reports of the polymerisation of pyrrole mediated by mineral The properties of polypyrrole obtained by oxidative polymeris- ation and under acid conditions are different: the former is a highly conjugated material which in the oxidised form may have conductivities in excess of 10 S cm-', whereas the latter is insulating, with limited conjugation along the polymer chain because of ring opening and the presence of hydrogenated pyrrole units.Lamb and Kovacic13 studied the polymerisation of pyrrole with trichloroacetic acid. Rapid quenching of the reaction mixture was found to lead to the isolation of an oligomer as the major product, identified as 2,5-bis( 2-pyrrolyl) pyrrolidine. The authors suggested that the formation of polypyrrole in acidic solution proceeds through this trimer, 302 J. Muter. Chem., 1996, 6(3), 301-304 with protonated trimer species attacking pyrrole or trimer molecules. Polypyrrole was therefore assumed to essentially consist of alternating pyrrole and pyrrolidine units.Elemental analyses also indicated the presence of appreciable amounts of oxygen, with minor amounts of chlorine. Salmon et all4 obtained yellow, insulating films of polypyrrole by the treat- ment of an ethanolic pyrrole solution with aqueous sulfuric acid. From elemental analyses and IR spectra, it was suggested that the material consisted of chains of saturated or partially saturated pyrrolidine units and pyrrole units terminating in pyrrolidinone moieties. It was proposed that under the acidic conditions present during polymerisation, a proportion of the pyrrolidine nitrogens became protonated, leading to a cationic polymer backbone. Evidence for the incorporation of sulfate counter-ions in the polymer film was obtained from elemental analyses and IR spectroscopy.In our study, we have extended the range of acids shown to polymerise pyrrole. Aqueous pyrrole solutions were added to 7.5 mol dm-3 acids to obtain a final 6 mol dm-3 acid concentration. Brown polymer products were obtained using 6 mol dm -3 hydrochloric, hydrofluoric, hydrobromic and nitric acids at room temperature (products 1, 5, 7 and 4), and 6 mol dm-3 hydrochloric acid at 100°C (product 3). The reaction with 6 mol dmP3 orthophosphoric acid led to the formation of a red-brown gel, which on work-up gave a black powder (product 6). Polymer product formation was much slower using the weaker hydrofluoric and orthophosphoric acids. The material obtained by thermal degradati~n'~ of product 1 was also a black powder (product 2).In all cases the materials were insoluble in common solvents. Conductivities of compressed discs of all the polymers were less than S Cm-'. SEM images obtained for products 1, 2,4 and 7 showed a similar polydisperse spherical morphology, with particle diameters in the range 1-4 pm. The spheres appear to be fused together. Fig. 1 shows an SEM image of product 1. In contrast, the material obtained from polymerisation carried out in hydrochloric acid at 100 "C (product 3) consisted of irregular particles with an average size of 20 pm. Fig. 1 SEM image of product 1 (scale bar = 5 pm) There is clear evidence from the microanalytical data that acid treatment of pyrrole results in products which have an increase in the carbon :nitrogen ratio relative to the monomer starting material (Table 1).The use of orthophosphoric acid resulted in the greatest loss of nitrogen. Ring opening and subsequent loss of nitrogen from the polymer is the most likely explanation for these results. The high acid concentrations would also be expected to cause protonation of non-aromatic nitrogen atoms, with the concomitant incorporation of the respective anion into the polymer. XPS was undertaken for products 1 and 2. Chlorine was shown to be present solely in the anionic state. The inclusion of halogen, nitrate and phos- phate in the polymers was also supported by the micro- analytical data. The polymer products resulting from the acid polymerisation from pyrrole were thoroughly dried.They were then success- fully ground up in a ball mill to pm-sized particles. The rheological properties of these dispersions in the pres- ence and absence of an electric field were evaluated by trapping the dispersions, in turn, between two parallel stainless steel discs separated by a 1 mm gap, the rotation of one of the discs being electrically controllable. The electrorheological activity of the dispersions of each polymer in 1-chloronaphthalene-1-bromonaphthalene at a 30% loading (0.60 g in 2.00 cm3) was assessed at -a constant shear rate of 0.990 s-l, to determine which dispersions were worthy of further study. Measurements of shear stress were obtained in the absence of an electric field, and for ac (50 Hzj and dc electric fields of up to 500Vmm-l.Those fluids prepared from products 2, 4, 5 and 6 gave no response, even at the highest field strength. After testing suspensions of products 4 and 6 under dc fields, examination of the rheometer plates revealed the coating of particles on the cathode. The bulk of the fluid appeared depleted of the disperse phase. This effect must be due to electrophoresis. A similar effect has been noted for fluids based on zeolite particles.16 This is unlikely to account for the lack of ER response, since no response was observed with ac field conditions. Suspensions of products 1, 3 and 7 gave appreciable increases in developed shear stress with increasing field strength. The material derived from hydrochloric acid at room temperature (product 1j showed the greatest response, with the product derived from hydro- chloric acid at 100"C showing a lesser response.The material derived from the hydrobromic acid polymerisation of pyrrole showed a much smaller ER effect (Fig. 2). Clearly the differing methods of polymer preparation have a profound effect on the ER response of the dispersions. The lack of knowledge of the polymer structures, however, prevents structure/activity relationships being made. Cation exchange resins with different halide counter-ions have been utilised for ER research.17 The same trend of ER response was observed as for polypyrrole with different counter-ions, the greatest effect being observed for chloride, followed by bromide and then fluoride.Table 1 Elemental ratios (derived from microanalytical data) for a range of polypyrrole polymers elemental ratios product C H N 0" anion 1 4 5.97 0.81 0.41 0.33 (Cl)2 4 3.2 0.69 0.56 0.23 (Cl) 3 4 5.6 0.64 0.74 0.33 (Cl)4 4 3.58 1.1 2.28 -b 46 -40 _-35 -. r 15 -. ,(c) , 0 50 100 150 200 260 340 400 500 applied field strengtW mm-' Fig. 2 Shear stress us. applied electric field (dc) for products 1 (a), 3 (b)and 7 (c) (shear rate 0.990 s-l) The effect of different sample loading on the ER response was examined using 10, 30 and 40% suspensions of product 1 (Fig. 3). These data show clearly the increase in ER activity at greater solid loadings. This correlation is broadly in line with results obtained by other workers utilising different fluids.'* The most promising disperse phase (product 1) was selected for further study, using a 40% mass/volume loading.The current density drawn using this fluid under a range of dc electric fields is shown in Fig. 4.This parameter is of impor- tance with respect to Joule heating and power consumption. At the highest field tested (500 V mm-I), a current density of 0.85 mA cm-2 was obtained. In contrast, Gow and Zukoski6 quote current densities for polyaniline-based fluids (volume fraction 0.018, field strength 500 V mm-') of between and 1 mA cmP2, although it should be noted that the signifi- cance of this comparison is questionable owing to the difference in sample loadings. Meaningful comparisons of our results with current densities quoted in the literature cannot be made since most literature values have been obtained at higher electric field strengths.::f 17.0 +I J/ o 50 im 150 200 2w wx) 44a 600 appliedelectrlc fielfl mm-' Fig.3 Shear stress us. applied electric field (dc) for 30% suspensions of product 1 (shear rate 0.990 s-'): (a) 40% loading; (b) 30% loading; (c) 10% loading 09 -08 07 cu5 06 --U4 05 e E 04 U E 03 --L0" 02 -. 01 --4 5.14 0.7 0.86 0.42 (F)0--I " Oxygen values obtained. were obtained by difference. bNitrate ratio not Fig.4 Current density us. applied electric field (dc) for suspension of product 1 (shear rate 0.990 s-l) a 40% J. Muter. Chem., 1996, 6(3), 301-304 303 12 000 10000 52 4000 2 000 0 100 1000 10000 100 0oo 1000 000 shear rat&-' Fig.5 Shear stress us shear rate for a 40% suspension of product 1, at different applied ac electric fields (a) 600, (b) 500, (c) 400, (d) 200 V mm-', (e)no field Shear stress data as a function of shear rate were obtained for a suspension of product 1 (40% loading) in the absence of an electnc field and for ac fields of 200, 400, 500 and 600Vmm-' (Fig 5) From Fig 5 it can be seen that the behaviour of the fluid under each applied field is broadly similar, with an approximately linear increase in developed shear stress with increasing shear rate, over the range of shear rates tested The enhanced shear stresses, as defined by the difference between the developed shear stress with and without the electric field, is broadly independent of the shear rate over the range tested This, for instance, is in contrast to hydrated lithium poly(metha~ry1ate)'~ which shows a ca ten-fold reduction in shear stress with an applied field of 400 V mm-' for a change in shear rate from 0 2 to 6 s-' In our studies on fluids based on products 1, 3 and 7, it was found that for sample loadings of 10 and 20%, and for dc field strengths of below 200 V mm-', the response was reproducible and reversible, with a rapid increase in developed shear stress to a stable value However, for sample loadings of 30% and above, and for dc electnc fields of above 200 V mm-', the enhancement of shear stress upon application of the field was not constant, tending to decay with time This was most apparent for the case of a suspension of product 1 (40% loading, 500Vmm-' dc field, shear strain rate 0990s-'), where application of the field produced a rapid increase in shear stress to approximately 250Pa, the shear stress then falling to the zero-field shear stress over a period of 60 s This effect was not found in the case of ac electric fields, where repeated cycling of this suspension with an electric field of 750 V mm-l gave steady and repeatable enhanced shear stress values Conclusion Novel polymers were prepared by hydrofluoric, hydrobromic and orthophosphoric acid treatment of pyrrole Significant ER responses were observed for dispersions of polypyrrole obtained by the action of hydrochloric acid on pyrrole A lesser effect was observed for the polymer obtained by the hydrobromic acid-mediated polymerisation of pyrrole These materials represent a new class of polymers which may be used as components for the formulation of anhydrous ER fluids They merit further investigation because of their facile synthesis and their significant ER responses The authors would like to thank Dr R Ewen, at the University of the West of England (Bristol), for performing ESCA analy-ses, Dr D Patton at the University of the West of England (Bristol), for obtaining SEM images, and Mr M West at Bristol University for performing elemental analyses References W M Widow, J Appl Phys, 1949,20,1137 Y F Deinega and G V Vinogradov, Rheol Acta, 1984,23,636 H Block and J P Kelly, J Phys D, 1988,21,1661 K D Weiss, J D Carlson and J P Coulter, Journal oflntellzgent Material Systems and Structures, 1993,4, 13 5 H Block, J P Kelly, A Qin and T Watson, Langmua, 1990,6, 6 6 C J Gow and C F Zukoski IV, J Colloid Interface Scz, 1990, 136,175 7 T Shiga, A Okada and T Kurauchi, Macromolecules, 1993, 26, 6958 8 K K Kanazawa, A F Diaz, W D Gill, P M Grant, G B Street, G P Gardini and J F Kwak, Synth Met, 1980,1,329 9 J Mansouri and R P Burford, J Muter Sci ,1994,29,2500 10 S Pouzet, N LeBolay, A Ricard and F Jousse, Synth Met, 1993, 55-57,1069 11 M Dennstedt and F Voigtlander, Chem Ber ,1894,27,476 12 H A Potts and G F Smith, J Chem SOC, 1957,4018 13 B Lamb and P Kovacic, J Polym Scz Polym Chem Ed, 1980, 18,1759 14 M Salmon, K K Kanazawa, A F Diaz and M Krounbi, J Polym Sci Polym Lett Ed, 1982,20, 187 15 S J Hawkins and N M Ratcliffe, J Polym Chem Part A, 1996, in press 16 K Negita and Y Ohsawa, J Phys ZZ (France), 1995,5883 17 N Sugimoto, Bull Jpn SOCMech Eng, 1977,20,1476 18 P J Burchill, Muter Forum, 1991,15, 197 19 L Marshall, C F Zukoski IV and J W Goodwin, J Chem SOC Faraday Trans I, 1989,85,2785 Paper 5/04836B, Received 21st July, 1995 304 J Muter Chem, 1996,6(3), 301-304

ISSN:0959-9428    

DOI:10.1039/JM9960600301

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Effect of complexation with copper(@ on cured neat resin properties of a commercial epoxy resin using modified imidazole curing agents? Ian Hamerton,"" Brendan J. Howlin," John R. Jones," Shuyuan Liu" and John M. Bartonb "Departmentof Chemistry, University of Surrey, Guildford, Surrey, UK GU2 5XH bStructural Materials Centre, Non-metallics, Defence Research Agency, Farnborough, Hampshire, UK GUl4 6TD A commercial epoxy prepolymer (MY750) was cured with novel modified imidazole curing agents under both isothermal and dynamic scanning conditions. The incorporation of an imidazole-copper (11) chloride complex curing agent to the epoxy prepolymer effected full cure after an isothermal cure schedule and post-cure treatment. The thermal stability of polymers arising from the isothermal cure schedule were generally higher than those for the corresponding dynamic cure.For samples cured via the dynamic curing process, a lower heating rate resulted in superior thermal stability. The same findings were obtained for samples cured via the isothermal curing process where the lower initial cure temperatures were optimal. The results of the present study show that the addition of metal atoms to the polymer systems does not have an adverse effect on either the water absorption or the dielectric properties of the final product. The cured resin displayed comparable thermal stability and absorbed the same amount of water at saturation (and a marginally lower amount at lower curing agent loadings) to a similar sample cured with an unmodified imidazole adduct.Epoxy resins are of immense technological importance as they form the continuous phase that binds together many light- weight, tough composite materials. There are many factors governing the physico-mechanical properties of the final resin. For example, it has long been recognized that the chemical nature of the curing agent can have significant influence on the gel-time and the physical properties of the epoxy, largely because it determines both the morphology and the crosslink density of the growing network. The thermal stability and water absorption characteristics of the epoxy, for instance, can be markedly affected by small changes in the curing agent. Some imidazoles are highly effective, fast curing agents' and are added to commercial epoxy systems to catalyse the homo- polymerization of epoxide groups (in a polyetherification mech- anism) to yield a thermoset network.However, unmodified imidazoles have low storage stability when mixed with epoxy resins (cure occurs slowly at room temperature) making them unsuitable for use in one-pot compositions and therefore, much work has been carried out into stabilizing imidazoles for use as latent epoxy curing agents. One approach involves the preparation of transition-metal-imidazole complexe~~*~ which exhibit very good solubility in common ep~xides,~ good stab- ility at room temperature4 and a rapid cure at elevated temperature^.^,^ The addition of metal atoms to the polymer systems should not, ideally, have an adverse effect on either the water absorption or the dielectric properties of the final product.In this study the incorporation of an imidazole-copper(I1) chloride complex curing agent to an epoxy prepoly- mer effected full cure after an isothermal cure schedule and post-cure treatment. The cured resin displays comparable thermal stability and absorbs the same amount of water at saturation (and a marginally lower amount at lower curing agent loadings) to a similar sample cured with an unmodified imidazole adduct. The dielectric responses observed for this epoxy resin system are typical for the isothermal cure of an epoxy resin and an additional benefit is that the presence of copper@) salts could act as an additional probe for ionic conductivity.The preparative routes to the manufacture of these curing agents are described elsewhere.6 +Presented at the Second International Conference on Materials Chemistry, MC2, University of Kent at Canterbury, 17-21 July 1995. Experimenta1 Materials The commercial epoxy prepolymer MY750 (the structure is nominally represented by 3 in Fig. 1) was donated by Ciba- Geigy (Duxford, UK). Phenyl glycidyl ether (PGE), 2-ethyl-4- methylimidazole (EMI) and copper(n) chloride were obtained from Aldrich Chemical Company. Purities were determined using 'H NMR (with the exception of CuC12, for which elemental analysis was used) and the compounds were used as received. Sample preparation The preparative routes to the 1:1 adduct of phenyl glycidyl ether and 2-ethyl-4-methylimidazole (PGE-EMI) 2 and the corresponding metal complex Cu( PGE-EMI)4C12 1 are described elsewhere.6 Compositions containing 5 and 6.64% (by mass) of adduct 2 and 5.6 and 7.5% (by mass) of complex 1(in molar equivalence to the adduct) in MY750 were made.The adducts were dispersed directly in the resin, while the complexes were dissolved in dichloromethane prior to mixing (the solvent being removed under vacuum at 30°C for 4 h). Neat resins were obtained by the following route. The adduct 2-epoxy formulations were cured initially at 100 "C and post- cured at 150°C until no further increase in was observed from DSC measurements. The complex l-epoxy mixtures were cured initially at 120 "C and post-cured at 165 "C, again until a final Tg had been obtained.Apparatus Thermogravimetry (TG) measurements were made both iso- thermally and dynamically at a variety of heating rates (1, 5, 10, 15 and 20 K min-') under nitrogen (25 cm3 min-') using a Shimadzu TGA-50 calorimeter interfaced with a Shimadzu TA-501 thermal analyser. Samples (6&1mg) were run in an open aluminium pan. Dielectric measurements were carried out on a Solatron 1250 frequency response analyser. Neat resin and epoxy-curing agent mixtures were placed in a cell consisting of two pre- etched copper electrodes of active area 1 cm2-mounted on a glass fibre reinforced epoxy resin base. The samples to be J. Mater. Chem., 1996, 6(3), 305-310 305 formation CH3 2 +epoxy 3 ring-opened intermediate 4 4 t epoxy 3 5 polyetherification and network formation Fig.1 Structures of the compounds employed in this study and the proposed mechanism of epoxy cure involving the metal-imidazole complexes measured were injected into the cell and placed in a cryostat (DN1704) The curing temperature was 100°C for the adduct 2 and 165 "C for the complex 1 The frequency range used was 10-1-105 Hz The data were obtained as plots of relative permittivity and dielectnc loss at the frequencies given above us time Dielectric measurements on cured resin samples were carned out on samples of size 12 x 17 x 1mm3 A curometer designed at Strathclyde7 was used to monitor changes in the viscosity with time at 2 Hz The instrument was calibrated using Santovac-5 which was chosen because it exhibited a very high temperature-viscosity coefficient and formed a stable, supercooled liquid state, which has been studied extensively Water absorption measurements were performed on cured neat resin plaques (20 x 10x 1mm3) immersed in boiling water (15 cm3, reflux) over a period of 14 h, while the sample mass was monitored Results and Discussion Dielectric measurements made during the curing process and on cured resins Measurements were made on samples containing the epoxy 3 and complex 1 (7 5 and 6 5 mass%, respectively) curing iso- thermally at 165 "C,and the epoxy 3 and adduct 2 (5 mass%) curing isothermally at 100"C, three-dimensional plots were obtained The dielectric responses observed for this epoxy resin 306 J Mater Chem, 1996, 6(3), 305-310 system (Fig 2 and 3) are typical for the isothermal cure of an epoxy resin A particular feature of the resin systems studied in this work is that the presence of copper(r1) salts could act as an additional probe for ionic conductivity and, as expected, the mixtures display a distinguishable ionic conductivity for which the dielectnc loss is greater than for a conventional imidazole-cured epoxy resin The initial value of the dielectric loss (log E") is approximately 9, which is greater than that observed for a typical bisphenol A diglycidyl ether (BADGE) system (log E"= 8) At low frequency and during the initial stages of cure a large dielectric loss is observed which rapidly decreases as cure proceeds and can be attributed to blocking electrode effects lo l2 This major ionic conductivity process is revealed by the rapid decrease in dielectric loss Generally, in this region the ionic conductivity is caused by inherent impurit- ies (which are present from the preparation of the epoxy monomer) and this phenomenon dominates the dielectnc behaviour owing to the lower viscosity of the polymer The curing process leads to a reduction in the dc conductivity as a function of time [Fig 2(u) and (b)] The initial high conduc- tivity of the mixture progressively decreases with time as the behaviour becomes dominated by dipole orientation It should also be remembered that the cure of epoxy resins using imidazole compounds proceeds uza an etherification mechan- ism (Fig 1)which produces a certain amount of dipolar species (arising from the cations on imidazole rings and anions on the other end of the growing macromolecule) during the whole !5O '300 i250 Fig. 2 (a) Relative permittivity and (b)dielectric loss measured as a function of frequency and time for a mixture of a blend of epoxy 3 and curing agent 1 (5.6 mass%) at 165 "C; (c) relative permittivity and (d) dielectric loss measured as a function of frequency and time for a mixture of a blend of epoxy 3 and curing agent 2 (5.0 mass%) at 100°C course of cure.It has been postulated" that the chemical reaction leading to the formation of a gel may significantly inhibit the mobility of charge carriers, and hence that the conductivity might be used as an indication of gelation.However, if the conduction process is influenced by the segmen- tal mobility of chains forming the three-dimensional matrix, then changes in conductivity may also be indicative of vitrifi-cation. Unfortunately, in this case the isothermal cure tempera- ture (T,)required to effect cure for the epoxy4omplex mixtures (T,= 165 "C) is greater than the ultimate glass transition tem- perature for this system (q,=140 OC)I3 and consequently it is impossible for the vitrification transition to be observed for the whole process. In contrast, vitrification can be reached in the 3-2 system (q=112"C)if a T, of 100°C is used [Fig.2(c) and (41.As a result it was necessary to employ a complimen- tary technique (curometry) to analyse the cure process to yield more information concerning the gelation and vitrification. The measurements on cured resins were made by determin- ing the dielectric response against frequency and temperature. The traces showed no observable influence of the metal salt in the matrix (Fig. 3), the dielectric response being dominated by an a-relaxation between -10 and 300 "C. This phenomenon may arise because the tiny amount of metal salt, < 1%, in the system is essentially at a lower level than the interferent ion impurities and they are effectively constrained within the rigid network up to 300°C. Rheology measurements during the cure process and on cured resins Although the dielectric technique (DETA) is useful to monitor the physical changes (sol-gel-vitrification), it is probably not the most effective method to monitor gelation during cure.Dynamic viscosity measurements offer an efficient means of determining the gel-point, but our previous measurements6 could not fully examine the cure of the polymer in an undis- turbed state, because the constant shearing of the spindle effectively orders the molecules in the system, lowers entropy, and accelerates the cure process. Owing to the high shear rates of the spindle at around the gel-point, the network structure is broken down at the spindle-resin and/or cylinder wall-resin interface, making it impossible to monitor the cure after gelation has been reached.The curometer, developed in Strathclyde,' can carry out measurements under conditions which involve very little disruption to the cure mixture and allow measurement over a greater range of conversion. These J. Muter. Chem., 1996, 6(3),305-310 307 3500 1 r loo Fig.3 (a) Relative permittivity and (b)dielectric loss measured as a function of frequency and temperature for a cured mixture of a blend of epoxy 3 and curing agent 1 (7.5 mass%) measurements may even continue after vitrification has occurred so that the whole cure process may be monitored. In turn, the Tg may also be determined by examining the changes in rheological parameters in one of two ways: at cure tempera- tures above the polymer's Tg the changes in viscosity are measured as the temperature is decreased, while at cure temperatures below its Tg a temperature ramp is employed.Plots of the real and imaginary responses, and the computed viscosities for the cure are presented in Fig. 4-7. The raw data (real and imaginary responses) are shown in Fig. 4, an example in which the cure temperature is above the Tg of the polymer. Fig. 5 shows the viscosity data derived from Fig. 4. The onset of gelation occurs at about 2.8 h and the rheology value reached a plateau at about 7000 Pa s (for the polymer in a rubbery state) after cure at 165 "C for about 17 h. When the analysis temperature was decreased the viscosity gradually increased until the mixture solidified at its Tg to a value of about 11500 Pa s (Fig.6). Analysis in this manner revealed a Tg of 143°C which is in good agreement with the value measured by DSC (141 "C).The corresponding adduct mixture 3-2 showed a different viscosity behaviour during the cure process in which sol-gel-glass transitions were involved. The 308 J. Mater. Chew., 1996,6(3), 305-310 3000 a 2500 &.--a5 2000 a3 1500 J= Q .E 1000 500 0 0 I0000 20000 30000 40000 60000 60000 tls Fig. 4 Real and imaginary responses from curometry for a mixture of a blend of epoxy 3 and curing agent 1 (5.6 mass%) at 165 "C 10000 3I000 v) (0a .-2. 100 v) 0 .-> 10 I 1 1 I I I I I 0 10000 20000 30000 40000 50000 60000 tls Fig.5 Viscosity derived from curometry data for a mixture of a blend of epoxy 3 and curing agent 1 (5.6 mass%) at 165 "C 1:ooo i f--10000 a a .-'c 9000 0 v).-> 8000 7000 165 160 155 150 145 140 135 TI"C Fig. 6 Glass transition temperature derived from curometry data on cooling a mixture of a blend of epoxy 3 and curing agent 1 (5.6 mass%) from 165 to 135°C gelation started after 48 min and the viscosity value reached a plateau at about 1x lo6 Pa s (for the polymer in a solid state) after cure at 100°C for about 4 h (Fig. 7). Although the Tg obtained ( 112 "C)is higher than the cure temperature ( 100 "C) it is not the final Tg, which would require further post-cure treatment to achieve. Thermal stability of the cured epoxy resins In our earlier study we demonstratedi3 the importance of both the initial and the final stages of the cure schedule upon the nature of the network formed, and hence the effect upon the final properties of the cured resin.In particular, the magnitude 10000000 1000000 100000 -2 10000 -f I /I I I I I i 0 5000 10000 15000 20000 tis Fig. 7 Viscosity derived from curometry data for a mixture of a blend of epoxy 3 and curing agent 2 (5.0 mass%) at 100"C of Tgwas extremely sensitive to the cure schedule employed to cure the complex-cured epoxy resins. With reference to Fig. 1, earlier mechanistic studies of imidazole cure of epoxy resins14 have shown that the alkoxide (RO-) propagation is considered to be favoured at lower temperatures while the hydroxy (OH) propagation route is only active at higher temperatures, per- haps leading to different network structures.However, purely from a consideration of the molecular structure, it is not obvious how the two paths (of RO-and OH polyetherification) should lead to different types of ether-linked network. However, there is a difference between these two paths if one considers the position of the alkoxide (RO-) and hydroxy (OH) groups. RO-groups are generally located at terminal positions in the intermediate, which is produced by the initial attack of an RO-group on an oxirane ring at the terminus of the molecule, while the OH group is initially produced towards the middle of the BADGE oligomer3 (Fig.1). The geometrical structures resulting from these two paths may be slightly different: the product from the RO-etherification may result in a more linear structure, while OH etherification may result in a more branch-like structure. The influence of these two kinds of intermediate structures on the formation and final structure of the network cannot be gauged easily. The magnitude of this proposed effect may be expected to be small since the concen- tration of hydroxy groups is rather low and all the procedures were carried out under moisture-free conditions in the absence of acid contaminants. However, one point which is clear from the outcome of the results of our investigation is that this is certainly not the sole factor influencing the final structure and properties. As both the degree of cure and the network architecture have an influence on both the Tg and the thermo- oxidative stability of the polymer, in this study we will limit our analysis to the properties of specimens that have reached their ultimate conversion and yielded 'final' Tp values (Tgf).Dynamic TG experiments were performed on cured resin samples in a nitrogen atmosphere to assess the thermal stability of the cured epoxy polymer. In each case the mixture was scanned at a heating rate of 10 K min-l from ambient tempera- ture to 500°C after the final q was measured by DSC. The TG data were collected in the form of plots of residual mass us. temperature and a representative plot is shown in Fig.8. This overlay plot contains TG data from complex-containing epoxy resins cured both isothermally and using dynamic cure schedules. While some discrepancies were observed in the order of stability, in general those samples cured using a slower scanning rate were found to be more stable on subsequent rescanning of the cures resin (Le., as expected the thermal stability of the cured resin is proportional to its q).All plots from polymer samples cured isothermally displayed a similar profile to the TG data from samples cured using the dynamic cure schedules, indicating no significant differences in v)3 E 0 100.00 200.00 300.00 400.00 500.00 TI"C Fig. 8 TG plots of cured mixtures of epoxy 3 and curing agent 1 (7.5 mass%) after a variety of cure schedules: (a) 120°C, 16 h and 160"C, 12 h; (b) 1 K min-', 300°C; (c) 5 K min-', 300°C; (d) 10K min-', 300°C; (e) 15 K min-', 300°C; (f)20 K min-', 300°C.TG measure- ments were made under nitrogen at a heating rate of 10 K min-'. Note, TG data are shown offset, all data were initially 100%. the mechanism of degradation. It can be seen that those samples cured using a scanning schedule were found to be less stable on subsequent rescanning of the cured resin than those polymers cured using a comparable isothermal programme. The overall trend is that, as expected, samples exhibiting a higher final Tg value display superior thermal stability for both dynamically and isothermally cured resins. The epoxy resin cured with the imidazole adduct undergoes the onset of degradation (defined by the intersection of tangents in the mass loss data) at ca.408 "C (for a Tgf of 128 "C). This sample also gave a char yield (the residual mass remaining at 500°C) of 85%. If we now examine the properties of the complex-cured resin which has been fully cured (by an optimised isothermal cure schedule: 16 h at 120"C followed by a postcure of 8 h at 160"C), both the thermal stability (onset temperature and char yield) and the qf (140°C) are comparable with the figures from the adduct cured system (Le., an onset of 402°C and a char yield of 80%). These TG data appear to support the suggestion from the DSC experiment^'^ that the significant difference in the curing schedules is favour- ing one polymerization mechanism over another.Water absorption of the cured epoxy resins It is well known that the absorption of water in a thermosetting resin leads to a decrease in the value of Tg,and the mechanical and dielectric properties of the cured material, and this is particularly true of epoxy resins where a cured resin may sacrifice as much as 20 K of Tg for every 1% of moisture absorbed." Hence, it is of crucial importance to prepare high- performance materials which exhibit low levels of moisture absorption in a range of environments. In this study it was of particular interest to ascertain the effect of residual copper@) in the cured resin on the degree of water absorption. The immersion data for the epoxy 3 cured with both curing agents 1 and 2 show some interesting features.In the case of the higher curing agent loading both 2 (6.64%) and 1 (7.5%) display similar profiles [Fig. 9(a)]; saturation is reached at ca. 2.5 mass% after a period of ca. 8 h in boiling water. Again, in the case of the lower curing agent loading, both 2 (5%) and 1 (5.6%) display similar profiles, but in this case saturation is reached much more rapidly [ca. 4 h in boiling water, Fig. 9(b)]. In both cases saturation is reached at a level of ca. 2.5 mass%, indicating that the residual metal has no apparent effect on the eventual water uptake of the cured epoxy resin. These data have important ramifications in the possible use of these materials in microelectronic applications where the effect of moisture upon the electrical behaviour of the cured resin is of J.Muter. Chern., 1996, 6(3), 305-310 309 3 25 2 15 1 05 h s v c Y50 c 02 4 6 8 10 12 148 .c.s3 $ 25 2 15 1 05 0 0 2 4 6 8 10 12 14 th Fig. 9 Plots of water absorption (mass%) for cured epoxy resin samples after immersion in boiling water (a)Curing agents 1 (7 50 mass%, @) and 2 (664 mass%, O),(b) cunng agents 1 (5 60 mass%, 0) and 2 (500 mass%, 0) Table 1 Copper and chlonde content from ICP-MS analysis of water samples from immersion tests (after 14 h at reflux) curing agent (mass%) 35C1 content (ppb) Tu content (ppb) 2 (6 64) 2 122 5 106 1 (7 50) 2 585 134 535 1 (5 60) 2 935 298 060 prime concern It was found from an ICP-MS analysis of the water samples in which the metal-containing samples had been boiled, that these samples showed notably high copper contents when compared with the adduct-containing mixtures (Table 1) This suggests that the action of water, albeit under extreme conditions, caused some of the metal salt (CuCl,) which had been incorporated into the polymer to be leached from the resin network (to a level of ca 300 ppb) Conclusions Transition-metal-imidazole complexes have been prepared which exhibit very good solubility in common epoxides, good stability at room temperature and which effect a rapid cure at elevated temperatures The results of the present study show that the addition of metal atoms to the polymer systems does not have an adverse effect on either the water absorption or the dielectric properties of the final product The incorporation of 65 and 7 5 mass% of an imidazole-copper(I1) chloride complex curing agent to an epoxy prepolymer effected full cure after an isothermal cure schedule and post-cure treatment The cured resin displayed comparable thermal stability and absorbed the same amount of water at saturation (and a marginally lower amount at lower curing agent loadings) to a similar sample cured with an unmodified imidazole adduct Work continues to examine the great potential of this exciting family of curing agents The work of S L was generously supported by The Structural Materials Centre, Non-metallics, Defence Research Agency, Farnborough The commercial epoxy prepolymer was kindly donated by Mr Ian Gurnell and Mrs Debbie Stone of Ciba-Geigy (UK) Duxford, Cambridgeshire, UK We thank Professor Richard Pethrick and Dr David Hayward (University of Strathclyde) for their help with the operation of the curometer and DETA and advice concerning data interpret- ation At the University of Surrey we thank Dr Fadi Abou- Shakra for ICP-MS analysis References 1 W R Ashcroft, Curing Agents for epoxy resins, in Chemistry and Technology of Epoxy Resins, ed B Ellis, Blackie, Glasgow, 1993, ch 2,pp 58-59 2 R Dowbenko, C C Anderson and W H Chang, Ind Eng Chem Prod Res Dev, 1971,10,344 3 J M Barton, GB Pat 2135316B, 1984 4 J M Barton, I Hamerton, B J Howlin, J R Jones and S Liu, Polym Bull, 1994,33,347 5 J M Barton, G J Buist, I Hamerton, B J Howlin, J R Jones and S Liu, Polym Bull, 1994,33,215 6 J M Barton, G J Buist, I Hamerton, B J Howlin, J R Jones and S Liu, J Muter Chem , 1994,4,379 7 D Hayward, E Trottier, A Collins, S Affrossman and R A Pethnck, J Oil Colour Chem Assoc , 1989,452 8 A J Barlow, A Evnngsav and J Lamb, Proc R Soc London A, 1969,309,473 9 J Cochrane and G Harmon, J Phys E, 1972,547 10 J R MacDonald, Phys Rev, 1953,92,4 11 J R MacDonald, J Electrochem SOC , 1988,135,2274 12 W W Bidstrup, N F Sheppard and S D Sentuna, Polym Sci Eng , 1986,26,358 13 I Hamerton, B J Howlin, J R Jones, S Liu and J M Barton, 1995, Polymer, submitted 14 M S Heise and G C Martin, Macromolecules, 1989,22,99 15 W W Wright, Composites, 1981, 12, 201 Paper 5/04831A, Received 21st July, 1995 310 J Muter Chem , 1996, 6(3), 305-310

ISSN:0959-9428    

DOI:10.1039/JM9960600305

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

~~~~~ ~ Molecular modelling of the physical and mechanical properties of two polycyanurate network polymers? Ian Hamerton, C. Richard Heald and Brendan J. Howlin" Department of Chemistry, University of Surrey, Guildford, Surrey, UK GU2 5XH Elastic moduli and glass transition temperatures (T,s) of two polycyanurates, based on the dicyanates of bisphenol A and an oligomeric poly(ary1ene ether sulfone), have been predicted from molecular simulation. The simulated mechanical and physical parameters offer reasonable agreement with the experimental values. This is one of the first preliminary reports of the prediction of properties of a network polymer. The development of commercial cyanate (-0-CEN) chemis-try followed the discovery by Grigat and Putter in 1964 of a convenient and reliable preparation involving treatment of alkoxide or phenolate with cyanogen halide.' Several other methods of preparation are known, but are less widely appli- cable.' Alkyl cyanates readily isomerize to the corresponding isocyanate (-N=C=O), but aryl cyanates (1) do not rearrange and their chemistry is dominated by attack of nucleophiles on the cyanato ~arbon.~ A particularly important reaction is the cyclotrimerization to yield 1,3,5-triazines (or sym-triazines)(2), Scheme 1; the reaction is promoted by heat and a range of catalysts including protic acids, Lewis acids, bases and metal ions.4 The cyclotrimerization of a dicyanate gives rise to a network structure, and cyanate ester resins formed by homo- or co-polymerization of the dicyanate of bisphenol A, or closely related compounds, are an increasingly important class of high-performance polymers.2 The resins possess high glass transition temperatures ( 190-260 "C),show low dielectric loss behaviour and low moisture absorption, are tough, and show good peel strength; they are seen as potential replacements for epoxy resins and bismaleimides.2 In their range of applications as high performance materials the value of the cured resin < is extremely important as it largely governs the use temperature.Hence, an ability to predict this parameter would be of major importance in designing new materials. Previous work has demonstrated the feasibility of building molecular models of poly(ary1ene ether sulfone) s, poly(ary1ene ether ketone)s5-* and linear epoxy polymer^.^^'^ The results of this study have indicated that modelling of thermoset polymers in the bulk should give insights into the factors governing the observed physico-mechanical properties.The aim of the current study is to calculate physical and translion metal 0 catalyst-nonylphenol, heat * NAN cyclotrirnerization 044& X D Qx 1 2 Scheme 1 Polymerization reaction for the polycyclotrimerization reac- tion and monomer structures for the compounds studied. Bisphenol A dicyanate: X =4-NCO-C6H4-C(CH3)2-. Six ring poly(ary1ene ether sulfone) dicyanate: X =4-NCO-C,H4-C( CH3)2-C6H4-O-C6H,-S02- C,H,-O-C,H,-C( CH3)2-. +Presented at the Second International Conference on Materials Chemistry, MC2, University of Kent at Canterbury, 17-21 July 1995.mechanical properties from simulation and to compare these with experimental data for the bulk state. Calculations Molecular simulation A Silicon Graphics Indigo RS4000 running the computer program 'Professional POLYGRAF v3.2.1' (Molecular Simulations, Inc.) on IRIX v5.1.1.2 was employed to generate models of the monomer from crystal data." It was also used to model the polycyclotrimerization product of bisphenol A dicyanate (the unit shown in Fig. 1). The inherent symmetry of the models is used to duplicate the dihedral angles of interest. The generic force field Dreiding-I1 which was used has been described previously.12 Modelling of the polycyanurate of bisphenol A dicyanate (BPADC) under bulk conditions.A repeat unit containing the bisphenol A moiety linked to cyanurate ring was constructed with one head and two tails in the polymer module of POLYGRAF (Fig. 1).When this repeat unit was polymerised by removal of the tail atoms which are replaced by the head atoms a three-dimensional network polymer was constructed. Owing to the 200 atom limit in the ELASTICA module in this version of POLYGRAF, only five repeat units can be added before this limit is reached. Partial atomic charges were assigned by the Gasteiger method.13 This was then converted into an amorphous network at the experimental densityi4 of 1.26 g ~rn-~ at a temperature of 300 K. The method used to generate the amorphous system was the Monte Carlo tech- nique.15 Fifteen random struFtures were built into a cubic periodic cell of length 12.95 A and extended using periodic boundary conditions (PBC).Tail correction16 was performed to join the simulated cell to the imaginary cells. Structures were chosen where the head atoms of one structure were as close to the tail of the image as possible. Six structures which fulfilled these criteria were used in the remainder of the simulation. These structures were then minimised using conju- gate gradient^'^ at constant volume until energy convergence Fig. 1 The repeat unit of the bisphenol A polycyanurate. The hydrogen tail atoms (t) are removed and the fragment joined to the head atom of the next repeat unit (h).J. Mater. Chem., 1996, 6(3), 311-314 311 t H Fig.2 The repeat unit of the four ring poly(ary1ene ether sulfone) polycyanurate. The hydrogen tail atoms (t) are removed and the fragment joined to the head atom of the next repeat unit (h). was achieved. The energy convergence was defined as an energy change in two subsequent cycles amounting to less than 0.01 kcal mol-l.7 Canonical dynamics under both con- stant pressure and temperature conditions (NPT) were per- formed. The Nos& formulation18 was used to integrate the velocities over the time period of 1OOps after equilibration at the temperature concerned for the six remaining structures. Two simulation temperatures, 300 and 400 K respectively, were used. Calculation of the mechanical properties of bisphenol A polycyanurate. After the molecular dynamics simulations (MD) the six lowest energy structures from each simulation were extracted for further use.Their periodic images were unex- tended so that only the original cell and the structure remained. Constant volume minimisation was then carried out on these structures until energy convergence was achieved. During the MD stage the structure moved away from a cubic cell into an anisotropic form. In the ELASTICA module the triclinic function was used to calculate the elastic properties, due to the structure becoming anisotropic in the MD stage. Consequently, the elastic properties have to be averaged over many different forms of the bisphenol A dicyanate model so that the anisotropic nature can be overcome. Modelling of the polycyanurate of a six-ringed poly(ary1ene ether sulfone) PAES.Mechanical properties. The repeat unit used to build the six ringed PAES polycyanurate network is shown in Fig. 2. This consisted of adding another bisphenol A and a bisphenol S moiety to the bisphenol A polycyanurate model generated earlier. This model also had one head and two tails. Owing to the 200 atom limit only two repeat units of this polymer were allowed by the current version of the software. The network is built up from this ideal two monomer unit using the periodic boundary conditions. The density used was 1.20 g cmP3, the value being detFrmined from experiment. The periodic cell was of length 15.1 A and the structures to be used as input to the ELASTICA module were generated in the same manner as described in the previous section.Calculation of the glass transition temperature (Tg).The only limit on the size of the model to be used for determination is the amount of computer time that the simulation will take, unlike the elastic properties calculation. The compromise that we have chosen is to use six repeat units as described above (Fig. 3). Partial charges were assigned by the Gasteiger method13 and Monte Carlo simulation was used to build ten different models at a den$ty of 1.20 g cm-3 at a temperature of 3W0K, cell length 17.1 A. The starting volume was therefore 4983 A3. The model that was most amenable to tail correction was used for the rest of the simulation.PBC were imposed and extended to 26 image tails. The structure was energy minimised to convergence using constant volume minimisation. Canonical (NPT) dynamics were performed on this model starting at 800 K and dropping by 100 K every 250 ps. The final structure from each simulation was used as the starting structure for the next simulation. The temperature was decreased rather than increased in order to attempt to smooth out the effects of high energy transitions. The first 50ps of t 1 cal=4.184 J. 312 J. Muter. Chem., 1996, 6(3), 311-314 Fig. 3 Model of the six-ringed poly(ary1ene ether sulfone) polycyan- urate used in the Tg simulation each simulation was discarded as the structure was equilibrat- ing during this time period and the volume thermal expansion coefficient (VTEC) at each temperature was calculated using eqn.(1) VTEC=AV/V=(V-&)/V (1) where Vo is the original cell volume and V the average cell volume at a particular temperature. Results and Discussion Tests for equilibration and consistency The potential energy and volume were monitored over time and were seen to equilibrate after the first 50ps of the simulation under NPT conditions. Roe et a1.l’ have shown for thermoplastic polymers that the volume of the cell continues to decrease slowly over a nanosecond of simulation but that the majority of the decrease is over in about 50ps. Hence, there will be an error associated with the use of a volume after 250 ps, but this will be unlikely to have a major effect on the calculated properties.Mechanical properties of the polycyanurate of bisphenol A dicyanate This simulation was performed in order to validate the model- ling techniques used by correlating with an experimentally well determined and well known commercial polymer system. The elastic constants for the bisphenol A polycyanurate are given in Table 1 along with the experimental data. Experimental values were determined by us using standard mechanical testing procedures. The details of this will be published separ- ately.” The Young’s modulus from simulation was 4.04 GPa and that of experiment 3.39 GPa. Poisson’s ratio for the same system was 0.39 (simulated) and 0.35 (experimental).The only Table 1 Elastic constants of the bisphenol A polycyanurate obtained from the ELASTICA module in POLYGRAF ~ ~~~~~ mechanical property experimental result simulation result bulk modulus, B/GPa 3.79 &0.28 3.89 f2.04 Poisson’s ratio, u 0.35 k0.01 0.39 0.13 Young’s modulus, E/GPa 3.39 k0.16 4.04 2.14 Lame constant, A/GPa 2.95 k0.25 4.32k2.17 Shear modulus, G/GPa 1.25& 0.05 1.28& 0.75 constant to be above a 50% error margin was the Lame constant 2 (which has no physical meaning). In this and other simulation work21*22 the elastic constants are generally higher than the experimental values. Additionally, they are always at the upper limit of these values. One problem with the model was that it fails to address the formation of cage structures arising from intramolecular cyclisation (Fig.4) which have been reported by Fang et ~21.~~using mass spectral data and these structures may account for the low observed crosslink density of these polymer systems. Naturally, our model assumes a ‘perfect’ polymer lacking voids, defects or structures like those mentioned above. Mechanical properties of the polycyanurate of a six-ringed poly (arylene ether sulfone) The averaged results, together with standard deviations for the mechanical properties are given in Table 2. The Young’s modu- lus and Poisson’s ratio had simulated values of 3.64 GPa and 0.35 respectively. Experimental values for these parameters were 2.50 GPa and 0.33 respectively. It is interesting to note that the simulated results are higher than the experimental results but are of the same order of magnitude.The difference in the parameters is indicative of the limited nature of the model where the Young’s modulus of elasticity is higher in the simulation. In their simulation of a thermoplastic polymer system, Fan and Hsu~~have reported errors of between 20 and 40% on Young’s, bulk and shear moduli in a simulation of UDEL poly(ether sulfone). Their results were consistently Fig. 4 Diagram depicting the proposed cage structure comprising three bisphenol A dicyanate oligomers (after ref. 23) Table 2 Elastic constants of the six-ringed poly(ary1ene ether sulfone) polycyanurate obtained from the ELASTICA module in POLYGRAF mechanical property experimental result simulation result bulk modulus, B/GPa 2.53 k0.28 3.68 & 2.43 Poisson’s ratio, u 0.33 k0.02 0.35k0.19 Young’s modulus, E/GPa 2.50 0.04 3.64 & 1.86 Lame constant, A/GPa 1.91 k0.29 3.45 5 2.54 Shear modulus, G/GPa 0.89 0.10 1.12 k0.79 higher than experimental values although the Young’s modulus was lower.They concluded that the difference in results might arise from the idealised interactions between atoms and mol- ecules. Our results, which of course relate to a much more complex network, are still remarkably well reproduced. Physical properties of the polycyanurate of a six-ringed poly (arylene ether sulfone) The initial volume of the system was 4983.6 A3 and the results are given in Table 3.A plot of volume thermal expansion coefficient versus temperature was used to calculate the Tg (Fig. 5). The intercept of the best fit lines to the data gave a calculated Tp of 490 K. A plot of total energy versus temperature was also used to determine the Tg and this gave a value of 495 K (Fig. 6). The experimental value for this system is 413 K as determined by dynamic mechanical thermal analysis. This value is very low for a crosslinked p~lycyanurate~’ and prob- ably reflects the low degree of conversion achieved as a result of the cure schedule used. From Fig. 6 the volume thermal expansion coefficients for the glassy (a,) and liquid states (al) were calculated and from these the linear thermal expansion coefficient was determined.These are compared with the literature values2’ in Table 4. The good agreement between Table 3 Average energies, volume and change in volume (AVIV)values for the six-ringed poly(ary1ene ether sulfone) polycyanurate 100 807.4 5626.9 0.114 200 899.6 5884.9 0.153 300 1030.4 5798.2 0.140 400 1035.2 6217.3 0.185 500 1338.7 6264.3 0.204 600 1343.2 6283.3 0.207 700 1630.9 6525.5 0.236 800 1790.8 7093.1 0.297 0.351 0.05 0 100 200 300 400 500 600 700 800 TIK Fig. 5 Plot of AV/Vversus temperature for the six-ringed poly(ary1ene ether sulfone) polycyanurate model for the G determination. The straight lines of the plot are least square fits to the data points. I. I lbo 260 300 400 500 600 700 800 TIK Fig.6 Plot of total energy uersus temperature for the six-ringed poly(ary1ene ether sulfone) polycyanurate model for the G determi-nation. The straight lines of the plot are least squares fits to the data points. J. Muter. Chem., 1996, 6(3), 311-314 313 Table 4 Volume thermal expansion coefficient (VTEC) and linear thermal expansion coefficient (LTEC) in the glassy (a,) and liquid (al) states of the six-nnged poly(ary1ene ether sulfone) polycyanurate simulated VTEC expenmental VTEC simulated LTEC experimental LTEC a,/K a,/K 189x10 309 x 10 226x10p4 576 x 629x10 103 x 752x10 192x lop4 calculated and experimental values is indicative of the validity of the simulation, ag agrees more closely than does al indicating that the liquid transition is not handled as well These results indicate that simulated Tgs can be obtained accurately to within 70 K of the actual expenmental value, providing confi- dence in the prediction of the Tp of other network polymers It should be borne in mind that the simulated values represent the Tg of a ‘perfect’ polymer and the model does not take into account any defects, areas of crystallinity or voids which might yield a Tg at variance with the simulation Nevertheless, accurate results can be achieved with relatively simple models such as this It would also be possible with this model to perform further simulations around the simulated Tp value in order to determine more accurately the position of the large increase in volume thermal expansion coefficient and hence the position of Tg more precisely It should be noted that the simulated Tp was actually obtained first and corroborated by the experimental result, making it a true prediction and allowing no bias of the simulation to occur Conclusions The results of this work indicate the potential of molecular simulation in the determination of physical and mechanical properties of crosslinked polymers The relatively unsophisti- cated models give mechanical and physical properties that agreed well with literature values This level of accuracy found here for a thermoset system is comparable with that found in the literature simulations of linear polymers The major limi- tations on our models were the 200 atom limit imposed by the software With improvements in software and computing power more representative models are now possible The update to POLYGRAF, Cerius’ has a 20000 atom limit but at the time of writing does not have the facility to handle monomers with multiple heads and tails We wish to thank the Engineering and Physical Science Research Council for generously funding a research studentship for one of us (C R H ) At the University of Surrey we thank Mr R Whattingham (Materials and Science Department) for assistance in obtaining physical and mechanical measure-ments and Dr A S Deazle for his help with the use of the computational techniques References 1 E Grigat and R Putter, Chem Ber ,1964,97,3012 2 A W Snow, The synthesis characterisation and manufacture of cyanate ester monomers, in Chemistry and Technology of Cyanate Ester Resins, ed I Hamerton, Blackie Academic and Professional, Glasgow, 1994 3 E Grigat and R Putter, Angew Chem ,Int Ed Engl , 1967,6,206 4 D Martin, M Bauer and V A Pankratov, Russ Chem Rev, 1978, 47,975 5 I Hamerton, B J Howlin and V Larwood, J Mol Graphics, 1995, 13,14 6 I Hamerton, C R Heald and B J Howlin, Makromol Chem Theory Szmul, 1995, in the press 7 I Hamerton, C R Heald and B J Howlin, Molecular modelling of poly(ary1ene ether su1fone)s under bulk conditions, Modelling and Simulation in Materials Science and Engineering 1995, in the press 8 N Anscombe, I Hamerton, B J Howlin and I D H Towle, Polym Bull, submitted for publication 9 I P Aspin, J M Barton, G J Buist, A S Deazle, I Hamerton, B J Howlin and J R Jones, J Muter Chem, 1994,4385 10 J M Barton, G J Buist, A S Deazle, I Hamerton, B J Howlin and J R Jones, Polymer, 1994,35,4326 11 J M R Davies, I Hamerton, J R Jones, D C Povey and J M Barton, J Crystallogr Spectrosc Res ,1990,20,287 12 S L Mayo, B B Olafson and W A Goddard 111, J Phys Chem, 1990,94,8897 13 J Gasteiger and M Marsili, Tetrahedron, 1980,36, 3219 14 D A Shimp and W M Craig, 34th Int SAMPE Symp ,May 8-11 1989,34,1336 15 N Metropolis, A W Rosenbluth, M N Rosenbluth, A H Teller and B Teller, J Chem Phys , 1953,21, 1087 16 T A Weber and E Helfand, J Chem Phys ,1979,71,4760 17 R Fletcher and C M Reeves, J Comput, 1964,7, 149 18 S Nose and M L Klein, Mol Phys ,1983,50,1055 19 J R Roe, D Rigby, H Furayuia and H Takeuch, Comput Polym Sci ,1992,2,32 20 C R Heald, PhD Thesis, University of Surrey, 1995 21 C F Fan,T Cagin,Z M Chenand K A Smith, Macromolecules, 1994,27,2383 22 M Hutnik, A S Argon and U W Suter, Macromolecules, 1993, 26,1097 23 T Fang and D A Shimp, Prog Polym Sci ,1995,20,61 24 C F Fan and S L Hsu, Macromolecules, 1992,25,266 25 I Hamerton, Properties of unreinforced cyanate ester resins, in Chemistry and Technology of Cyanate Ester Resins, ed I Hamerton, Blackie Academic and Professional, Glasgow, 1994, p 209 Paper 5/04835D, Received 21st July 1995 314 J Muter Chem, 1996,6(3), 311-314

ISSN:0959-9428    

DOI:10.1039/JM9960600311

出版商:RSC    

年代:1996

数据来源: RSC