NONPLANARITY AND RING TORSION IN POLY(P-PHENYLENE VINYLENE) - A NEUTRON-DIFFRACTION STUDY

Elastic neutron-scattering data from all-hydrogen and selectively deuterated poly(p-phenylene. vinylene) (PPV) samples from 296 less-than-or-equal-to T less-than-or-equal-to 616 K are reported. These data, in combination with structure factor refinements, support a nonplanar structural model of the phenylene rings. Increased temperature is accompanied by a gradual rise in both the degree of nonplanarity and in the associated displacements from this mean position. There is also measurable anisotropy in the equatorial thermal expansion coefficients combined with a small 2-degrees drop in the polymer chain setting angle, thereby indicating only a weak approach toward a high-temperature rotator phase. This is in contrast to the pronounced variations seen in polyacetylene and polyethylene, which lack local ring torsional degrees of freedom.