SOLID-STATE THERMOCHROMIC TRANSITION IN POLY(2-METHOXYPHENYLENE VINYLENE)

The crystalline, conjugated polymer poly(2-methoxyphenylene vinylene) undergoes a solid-state thermochromic transition at about 350-degrees- C. The optical absorption maximum shifts hypochromically as the colour of the film changes from orange to olive-green during the transition. This effect can be correlated with an endothermic event, as seen in d ifferential scanning calorimetric measurements. Polarized infra-red sp ectra reveal that movements of the phenylene rings about the chain axi s are involved in the transition. The polymer deviates from the low-te mperature rigid-rod planar structure during the transition, but conjug ation is restored at temperatures above the transition. A molecular mo tion mechanism is proposed that involves phenylene ring wagging motion relative to the ring-ring axis as the temperature increases. At the t ransition temperature, the rings undergo a 180-degrees flip about the chain axis. This motion causes a temporary and reversible decrease of effective conjugation along the backbone of the polymer, and the colou r change is interpreted as a consequence of such phenylene ring motion s.