Effect of polymer matrix on the cis‐trans isomerization of azobenzene residues in bulk polymers

Abstract Several acrylates and methacrylates were copolymerized with two 4‐aminoazobenzene methacrylic derivatives. The photochemical and thermal cis‐trans isomerization about the azolink was investigated in the bulk polymers. All the copolymers show photochromic behaviour without fading; the photostationary states upon irradiation with UV‐light of the appropriate wavelengths were similar in bulk and in solution. The thermal cis‐trans isomerization follows simple first order kinetics in the rubbery specimens as in solution. Below the glass transition temperature T g , the isomerization can only be described by two simultaneous first order reactions. Depending on the conditions of irradiation and on the reaction temperature, a portion of the azoaromatic compound reacts anomalously fast. The temperature dependence of the relaxation time of the isomerization reaction in all the polymer films investigated can be represented by a single WLF‐type graph; the corresponding Arrhenius plot is curved above T g . The findings are interpreted by considering the effects of the free volume and of the chain segmental mobility on the cis‐trans isomerization. A rotational and a translational relaxation mechanism is discussed for the normal and the anomalously fast reaction below T g .