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≤T≤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° 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.