Mechanism of thermal decolorization of benzospirans in some amorphous polymeric matrices

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Abstract
The photochromic effect has been investigated for three compounds of the benzospiran group dissolved in amorphous polymers: poly(methyl methacrylate), poly(n‐butyl methacrylate), poly(vinyl acetate), and poly(vinyl n‐butyrate). The kinetics of the thermal bleaching reaction above Tg of the matrix follow a first‐order equation due to the averaging of free volume distribution related to the diffusion of segments in viscoelastic state. A more complex mechanism of decolorization below Tg has been considered from the point of view of unequal, discrete distribution of environments in which the photochromic molecules exist in the glassy matrix. A change of the activation energy and the mechanism of color decay on passing through Tg is not a rule (PVB), which shows, that for a polymer having long, flexible chains, secondary glass transition phenomena play a decisive role. In the case of glassy polymers (PMMA), the photochromic effect of benzospirans may be employed to determine Tβ. It seems, that in addition to steric restrictions for trans–cis isomerization in the decolorization process one must consider the interactions of photochromic molecules with the matrix as well as their chemical nature.