A differential scanning calorimetry of a polymethacrylate with azobenzene side chains at the first heating exhibited a scan different from the subsequent ones. Comparison of absorption spectra in an aqueous dispersion, a cast film and dilute solutions of mixtures of dioxane and isopropyl alcohol revealed that the azobenzene forms J-aggregates in polymer solid resulting in the first heating scan whereas the subsequent heating scans corresponds to a polymer conformation with monomeric side chains.