Microscopic dynamics of the glass transition investigated by time-resolved fluorescence measurements of doped chromophores

The microscopic dynamics of several monomeric and polymeric glass-forming materials has been investigated by time-resolved fluorescence measurements of doped malachite green molecules in a wide temperature region. For monomers, 1-propanol, propylene glycol, and glycerol, and a polymer without side chains, poly- butadiene, the temperature dependence of nonradiative decay time of doped malachite green molecules behaves in a similar way through the glass-transition region. Besides a kink around the calorimetric glass-transition temperature $T_g,$ another crossover at a critical temperature $T_c$ about 30–50 K above $T_g$ has been clearly observed. This experimental finding is in agreement with the prediction of the mode-coupling theory that a dynamical transition exists well above $T_g.$ On the other hand, for the complex polymers with side chains, poly(vinyl acetate), poly(methyl acrylate), and poly(ethyl methacrylate), the crossover at $T_g$ is less pronounced than those for the monomers and the polymer without side chains. Moreover, although we could not distinguish any singularities above $T_g$ for these complex polymers, we observed another kink below $T_g,$ which may be attributed to the side-chain motions.