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Magnetic Properties of Polyacetylene Composites H MAGNETIC PROPERTIES OF POLYACETYLENE COMPOSITES H. Thomann, L. Dalton, M. Galvin, G. Wnek, Y. Tomkiewicz To cite this version: H. Thomann, L. Dalton, M. Galvin, G. Wnek, Y. Tomkiewicz. MAGNETIC PROPERTIES OF POLYACETYLENE COMPOSITES. Journal de Physique Colloques, 1983, 44 (C3), pp.C3-313-C3- 316. 10.1051/jphyscol:1983361. jpa-00222713 HAL Id: jpa-00222713 https://hal.archives-ouvertes.fr/jpa-00222713 Submitted on 1 Jan 1983 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL DE PHYSIQUE Colloque C3, supplément au n°6, Tome 44, juin 1983 page C3-313 MAGNETIC PROPERTIES OF POLYACETYLENE COMPOSITES H. Thomaim*, L.R. Dalton*, M.E. Galvin**, G.E. Wnek**and Y. Tomkiewiez*** *Dept. of Chemistry, Univ. of Southern California, Los Angeles, CA 90089-0482, U.S.A. **Dept. of Materials Science and Engineering, M.I.T., Cambridge, MA 02139, U.S.A. ***IBM T.J. Watson Research Center, Yorktown Heights, NY 10598, U.S.A. Résumé - Des composites du polyéthylène basse densité, LDPE, et du trans-polyacétylène, t-(CH)x, préparés par polymérisation de l'acé­ tylène dans des films de LDPE imprégnés de catalyseur Ziegler-Natta, ont été étudiés par double résonance (ENDOR) et échos de spins élec­ troniques (ESE) et les résultats ont été comparés avec ceux obtenus dans les mêmes conditions sur des films de trans-polyacétylène. Les mesures par ENDOR indiquent que la fonction d'onde, une orbitale w pure fortement délocalisée, est essentiellement la même dans les composites que dans le polyacétylène pur mais qu'à une température donnée le coefficient de diffusion du spin électronique est signi- ficativement réduit. Les temps de mémoire de phase TM par ESE, me­ surés à l'aide d'une séquence de puises 90°-180° ont montré la même dépendance en température ; toutefois, les temps TM (donc le temps de corrélation de la diffusion de spin électronique) des com­ posites sont substantiellement plus longs à chaque température. Ces résultats impliquent que le coefficient de diffusion de spin est réduit et l'énergie d'activation augmentée dans les composites com­ paré au polyacétylène pur. Les temps TM des composites apparaissent moins sensibles que ceux du polyacétylène pur à une exposition pro­ longée à l'oxygène ; toutefois, une exposition prolongée réduit le temps de mémoire de phase et l'énergie d'activation "effective" du ou des processus dynamique(s) qui déterminent la relaxation de pha­ se par modulation des interactions magnétiques. Abstract - Composites of low density polyethylene, LDPE, and trans-Polyacetylene, t-(CH) , prepared by polymerization of acetylene in LDPE films impregnated with a ziegler-Natta catalyst, have been studied by Electron Nuclear Double Resonance (ENDOR) and Electron Spin Echo (ESE) spectroscopies and the results compared with analogous studies conducted on trans-polyacetylene films. ENDOR measurements indicate that the wavef unction, a highly delocalized pure ir-orbital, is essentially the same in composites as in pure polyacetylenes but at a given temperature the electron spin diffusion coefficient is substantially reduced. ESE phase memory times, T , measured by a 90 -180 pulse technique exhibited the same general Junction dependence on temperature; however, T times (hence electron spin diffusion correlation times) for the composites were substantially longer at each temperature. These results imply a reduced spin diffusion coefficient and increased activation energy in the composites compared to pure polyacetylene. Phase memory times for the composites appear less sensitive to prolonged exposure to oxygen than is the case for pure polyacetylene; however, prolonged exposure does shorten phase memory times and reduce the "effective" activation energy for the dynamic process or processes which determines phase relaxation by modulation of the magnetic interactions. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983361 JOURNAL DE PHYSIQUE Electron nuclear double resonance (ENDOR) and electron spin echo (ESE) spectroscopies provide a means of avoiding the complications due to inhomogeneous broadening in the analysis of EPR spectra and permit direct measurement of the hyperfine interactions (hence the e ectron wavefunction) and electron dynamics in samples of pure polyacetylene. llJ For pure polyacetylene, the ENDOR tensors given in Table I were determined. These tensors, together with TRIPLE resonance experiments which determine the relative sign of the spin densities at various carbon positions along the polyene backbone, establish that the paramagnetic electron exists in an extensively delocalized pure IT-orbital characterized by alternating spin densities at carbon of +0.06 and -0.02. For samples of trans-polyacetylene at ambient temperatures the hyperfine interactions are effe~tively~averagedby fast electron diffusion characterized by a hopping frequency of 10 Hz. In the fast motion region where the motional frequencies exceed the frequencies of the magnetic interactions modulated by the motion, the electron 0 pha9e2mem r time determined by a go0-180 ESE pulse sequence is given by TM = (2n A TI-' :here A is the frequency of the modulated magnetic interaction and is the correlation time for the motional process. On the other hand, in the slow motion region where motional frequencies are much less than magnetic interactions, TM a r. These theoretical results predict a minimum in the graph of T versus T (or versus temperature). Such a minimum is experimenta?ly observed for samples of trans-polyacetylene. The detailed analysis of ESE data is consistent with the results of ENDOR experiments; indeed, identical activation energies (0.034 eV) are obtained for films of trans-polyacetylene. A potential means of obtaining improved structural properties for conducting polymers is to prepare composites of conducting polymers such as polyacetylene, poly-paraphenylene, or polyparaphenylene sulfide with nonconducting polymers having desired structural properties. Composites are also of considerable theoretical interest in that the effects of intermolecular interactions upon the electron wavefunction and of lattice dynamics upon electron diffusion can be ascertained. Moreover, the concentration of conducting polymer can be systematically varied to determine interchain electron interaction. We have prepared a variety of composite samples of conducting polymers and structural polymers such as low and high density polyethylene, polystyrene, polybutadiene, polybenzothyiazoles, etc. Preliminary spectroscopic results indicate that in addition to possessing interesting electrical and structural properties these materials may represent important model systems for the characterization of the role of solitons and polarons in organic metals. We report here tQe study of composites of polyacetyelne and low density polyethylene (LDPE) which we have examined by ENDOR and ESE techniques for polyacetylene compositions ranging from 2.6% to 14.5% and for temperatures ranging from 4.2 to 298K. Typical ENDOR spectra are shown in Figure 1. At low polyacetylene concentrations, the low temperature limiting spectra are essentially fdytical to those observed for pure polyacetylene films at low temperature ' and the hyperfine interactions giving rise to the structuring of the ENWR spectra at a given temperature are substantially less than for films of pure transpolyacetylene at an equivalent temperature. Indeed,the ENDOR spectra were observed to correspond to the slow motion regime for all temperatures at which the composites were examined. The ENWR results may imply that the paramagnetic electron wavefunction in the low temperature limit is determined by intrachain interactions and is little influenced by the lattice. On the other hand, electron diffusional dynamics are observed to depend strongly upon the lattice. TABLE I. PROTON AND 13c TENSORS FOR POLYACETYLENE PROTON TENSOR 13c TENSOR Site 1 A = -0.5 MHZ x A = -1.3 MHz A = -2.1 MHz A =-1.3MHz Y Y A = -1.1 MHz A = +2.1 MHz Site 2 A -1.8 MHz Ax -3.1 MHz X = = A = -7.0 MHz A = -3.1 MHz Y Y A = -3.9 MHz A = +7.0 MHz From Figure 1, it is clear that exchange interactions start to be observed at the highest polyacetylene concentrations as is evident by the enhanced distant ENDOR line and by the temperature dependence of the ENWR spectra. Comparison off the spectra with those of pure polyacetylene may imply a different relative orientation of polyacetylene chains in composites as compared to pure plyacetylene which in turn leads to an enhanced exchange interaction. Figure 1. Typical ENDOR spectra 2 6% PC! / PC 14 6./. PA/PE are shown for plyacetylene/ - polyetylene composites of two plyacetylenepolyethylene2.6% polyacetylene (leftconcentrations: column)in low-density and >kr+ 14.6% polyacetylene in low density polyethylene (right column). Spectra are shown for two representative temperatures: 292K (top row) and 139K (bottom row). All spectra were recorded without Zeeman modulation employing frequency modulation of the radio frequency field. - >k*,12 14 16 18 ;eUHZ;6 18 ~11 spectra
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