The crystalline, conjugated polymer poly(2-methoxyphenylene vinylene) undergoes a solid-state thermochromic transition at about 350°C. The optical absorption maximum shifts hypochromicaily 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 differential scanning calorimetric measurements. Polarized infra-red spectra reveal that movements of the phenylene rings about the chain axis are involved in the transition. The polymer deviates from the low-temperature rigid-rod planar structure during the transition, but conjugation is restored at temperatures above the transition. A molecular motion mechanism is proposed that involves phenylene ring wagging motion relative to the ring-ring axis as the temperature increases. At the transition temperature, the rings undergo a 180 ° flip about the chain axis. This motion causes a temporary and reversible decrease of effective conjugation along the backbone of the polymer, and the colour change is interpreted as a consequence of such phenylene ring motions.
Посилання на статтю:
Solid-state therrnochromic transition in poly (2-methoxyphenylene vinylene) / Wenbin Liang* and Frank E. Karasz // Polymer. – 1993. – Vol 34. – P. 2702-2706.