Intrachain dynamics and interchain structures of polymers: A comparison of polyacetylene, polyethylene, polyaniline, and poly(paraphenylene vinylene).

Abstract

X-ray data for trans-polyacetylene (CH${)}_{\mathit{x}}$ in the range 129 KT513 K show that the a parameter expands twice as fast as b, accompanied by a 6\ifmmode^\circ\else\textdegree\fi{} increase in the setting angle. Polyethylene (${\mathrm{CH}}_{2}$${)}_{\mathit{x}}$ exhibits a similar behavior (but in the opposite sense), such that for both materials the interchain packing becomes closer to that of a two-dimensional triangular lattice at high temperature, as would be expected for close-packed disks. In contrast, poly(paraphenylene vinylene) and the emeraldine-base form of polyaniline both show small, nearly isotropic interchain thermal expansions, and the packing does not approach close packing of disks at high T. We suggest that for polymers consisting of relatively flat and rigid chains, the average interchain structure reflects the T-dependent interchain dynamics, specifically rigid-chain rotational modes about the chain axes. This effect is absent in materials for which ring torsion modes are the lowest-energy deformations. The coupling between rotational and translational degrees of freedom, and between intrachain dynamics and the average interchain structures, is discussed in light of the planar-rotor model of Choi et al. and the rotator phases observed in finite-length alkanes.