MAGNETIC PROPERTIES OF POLYACETYLENE COMPOSITES

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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�

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 T M par ESE, me- surés à l'aide d'une séquence de puises 90°-180° ont montré la même dépendance en température ; toutefois, les temps T M (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 T M 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. l l J

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~averaged by 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 pha9e2mem r time determined by a go0-180 0 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 m i n i m u m i s 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 a l s o o f 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 E N W R 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 E N W R 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.

PROTON TENSOR 13c TENSOR Site 1 A = -0.5 MHZ

x A = -1.3 MHz

A = -2.1 MHz

Y A =-1.3MHz

Y

Site 2

A = -1.1 MHz A = +2.1 MHz 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! / P C 14 6./. P A / P E are shown for plyacetylene/ -

polyetylene composites of two

>kr+ plyacetylene polyethylene 2.6% polyacetylene (left concentrations: column) in low-density and 14.6% polyacetylene in low density polyethylene (right column). Spectra are shown for two representative temperatures:

292K (top row) and 139K (bottom

>k*, ^row). All spectra were recorded without Zeeman modulation employing frequency modulation of the radio frequency field.

- 12 14 16 18 ;eUHZ;6 18 ~ 1 1 spectra scans are.from 10

MHz to 20 MHz.

Typical electron spin echo phase memory time data are shown in Figure 2.

While the temperature dependence of the phase memory times for 2 to 5%

polyacetylene/polyethylene composites is consistent with intrachain electron diffusion as is the case for t-(CH) films, the diffusion coefficient is substantially reduced for the comp%site relative to the pure t-(CH) films.

Indeed, the experimental T data shown in Figure 2 indicate that electron diffusion frequencies are %ways in the slow motion region.

In Figure 2, the effect of prolonged exposure to air (oxygen) is also demonstrated. If oxygen is assumed to influence the phase memory time by altering the electron diffusion coefficient, the results shown in Figure 2 would imply that exposure to oxygen lowers the activation energy for diffusion and increases the diffusion coefficient. This re ult would be in direct 3

contradiction to the suggestion of Nechtschein et al. that oxygen acts as a soliton pinning agent. A second possible explanation for the enhanced relaxation upon exposure to oxygen is increased Heisenberg spin exchange according to the relationship (T )-I

M measured = ('MI-iotion

' e x ' where ' e x is

JOURNAL DE PHYSIQUE

the exchange frequency. From Figure 2 it is clear that the effect of oxygen is the greeatest at the lowest temperature. Fortunately, the effects of oxygen appear less severe for (CH)dLDPE than for pure (CH) and in particular the structural deterioration appears retarded.

In conclusion, composites are not only of interest technologically but also appear to represent good model systems for the study of paramagnetic electrons in organic metals. For the case of (CH) /LDPE the electron wavefunction is essentially identical to that in pur$ (CH)x; however, the electron diffusion coefficient is found to depend strongly on the lattice.

The effects of exposure to molecular oxygen appear to be less dramatic than for pure polyacetylene and hence composites may represent a useful model system for study of certain of the interactions involving the paramagnetic electron in polyacetylene and molecular oxygen. Moreover, composites permit the systematic variation of polyacetylene concentration and hence interchain paramagnetic electron interactions.

---

Acknowledgements B

0

8

0 0

G

We wish to acknowledge suport from the Air Force Office of Scientific Research under grant number 82-NC-067.

- s +

- L .-

" 8,

0 ⁰ wlpasa

A IW Figure 2. Phase memory time,

k, 4 9' data are shown as a

0 unction of temperature for a 4%

--- ^{Ad o} polyacetylene in low density

8 polyethylene composite. The data

a o represented by circles are for a

A, $0 sample unexposed to oxygen while

a A " ~ ~ o 0 0 0 ~ ~

A A ~ ~ ~ t , t , ~ ~ ~ 8 ~ g ~ S z 0 o the data represented by triangles

w L 4 are for a sample exposed to air

0

1-

l - E M P I K I

for 6 days.

References

1. Dalton, L.R., Thomann, H., ~omkiewicz, Y., Shiren, N.S., and Clarke, T.C., Polym. Preprints, 3 , 86 (1982).

2. Thomann, H., Dalton, L.R., ~omkiewicz, Y., Shiren, N.S., and Clarke, T.C., Phys. Rev. Lett., 2, 533 (1983).

3 . Thomann, H., Dalton, L.R., Tomkiewicz, Y., and Clarke, T.C.,

"Double-ENDOR Measurement of Electron-~lectron Correlations in cis-(CH) ", to be submitted to Phys. Rev. Lett.

4. Galvin, R.E. and Wnek, G.E., Polym. Corn., 3 , 795 (1982).

5. Holczer, K., Boucher, J.P., Devreux, F., and Nechtschein, M.,

Phys. Rev., g , 1051 (1981).