H-2 NMR CHARACTERIZATION OF PHENYLENE RING FLIP MOTION IN POLY(PARA-PHENYLENE VINYLENE) FILMS

Solid-state H-2 quadrupole echo nuclear magnetic resonance (NMR) spectra and measurements of H-2 spin lattice relaxation times have been obtained for films of poly (p-phenylene vinylene) deuterated in phenylene ring positions (PPV-d4). NMR line shapes show that all the phenylene rings of PPV undergo 180-degrees rotational jumps about the 1,4 ring axis ("ring flips") at 225-degrees-C. The temperature dependence of the H-2 line shapes show that the jump motion is thermally activated, with a median activation energy, E(a) = 15 kcal/mol, and a distribution of activation energies of less than +/- 2 kcal/mol. The jump rate was also determined from the magnitude of the anisotropic T2 relaxation associated with H-2 line shapes and from the curvature of inversion recovery intensity data. The experimental activation energy for jumps is comparable to the intramolecular potential barrier for rotation about phenylene vinylene bonds. H-2 NMR provides a method for determining the phenylene-vinylene rotational barrier in pristine PPV, and may potentially be used to study conjugation in conducting films.