Photochemistry in polymer solids: 12. Effects of main-chain structures and formation of hydrogen bonds on photoisomerization of azobenzene in various polymer films

Abstract

The free-volume distribution in various polymers and the effect of hydrogen bonds on the free-volume distribution were examined by using a photoisomerization of azobenzene as a photosensitive probe. Poly(methyl methacrylate) (PMMA), poly(vinyl alcohol) and phenoxy resin films containing molecularly dispersed azobenzene were irradiated over a wide temperature range (77–340 K). The final cis fraction for the trans to cis photoisomerization of azobenzene decreases with decreasing temperature in PMMA, and smaller fractions are produced than in polycarbonate at the same temperature. These differences are due to the fact that the local free-volume fluctuation of the methylene main chains of PMMA is smaller than that of the aromatic chains of polycarbonate. Since the side chains of PMMA compared to the main chains are larger than those of polycarbonate, the local free volume around the main chains is filled up by the side chains. Hence, the final cis fraction of azobenzene in PMMA is small. Furthermore, the final cis fraction in poly(vinyl alcohol) is smaller than that in PMMA. Since the hydrogen bonds among the polymer chains in poly(vinyl alcohol) restrict the local molecular motion at low temperatures, the azobenzene molecule is tightly seized by the matrix polymer chains and the free-volume fluctuation is restricted in poly(vinyl alcohol). The kinetic model considering the local fluctuation of the free volume was also applied to the results for PMMA, poly(vinyl alcohol) and phenoxy resin, and the parameters for the model in these polymers are discussed.