Study of polyacetylene and composites of polyacetylene/polyethylene by electron nuclear double resonance, electron nuclear nuclear triple resonance, and electron spin echo spectroscopies

Abstract

Composites of trans-polyacetylene (t-(CH)x) and low density polyethylene (LDPE) have been studied by Electron Nuclear Double Resonance (ENDOR), Electron Nuclear Nuclear Triple Resonance (TRIPLE), and Electron Spin Echo (ESE) spectroscopies. The results are compared with analogous studies conducted on pure t-(CH)x films and on samples containing mixtures of cis/trans isomers. ENDOR measurements indicate that the paramagnetic electron in all materials studied resides in a pure π molecular orbital. The measured spin densities at carbon of ∥0.06 and ∥0.02∥ for two inequivalent 13C nuclei along the polyene backbone imply that the defect spin density is spread over a polyene segment length on the order of 70 Å. The absolute magnitude of the spin densities, +0.06 and -0.02, obtained by TRIPLE indicate the existence of electron Coulomb correlation effects. Detailed analysis of ENDOR spectra places strict limits on the allowable soliton diffusion coefficient. ENDOR, ESE phase memory (TM), and ESE spin-lattice (T1e) relaxation times are observed to be a function of t-(CH)x concentration in the host material. The importance of other dynamic processes such as electron Heisenberg spin exchange, electron dipolar interactions, and nuclear spin diffusion, are considered as mechanisms for obtaining the observed spectral responses. The analysis indicates the need for caution in interpreting electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) data in terms of models which consider the only active dynamic process to be soliton diffusion.