Relation between Photochromism of Chromophores and Free Volume Theory in Bulk Polymers

Abstract

AbstractThe relation between the relaxation behaviour of photochromic aromatic azo benzenes and spiropyrans in a series of amorphous polymers and the polymer properties has been investigated. The distinctive differences of the rate of thermal isomerization found below and above Tg and also compared to solution can be explained by molecular properties of the polymer matrix, i. e., the overall chain segmental mobility, particular chain segmental motions and free volume effects. — The temperature dependency of the relaxation of the photochromes can be described by a WLF‐equation, log aT = ‐C1(T ‐ Tg)/(C2 + T ‐ Tg). The numerical values of the constants C1 and C2 differ from the „universal”︁ values, but for a given family of polymers a single set of parameters is sufficient for both types of photochromes. The deviations in the constants are ascribed to an unequal distribution of the free volume through the matrix caused by the photochromic foreign molecules. — The anomalies in the thermal cis‐trans relaxation of the aromatic azo chromophores in glassy matrices are correlated with restrictions in the fluctuation of the free volume. A temperature T0 is determined at which the contribution of translational motions to the fluctuations in the free volume is widely frozen in.