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, polybutadiene, 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 glasstransition temperature Tg , another crossover at a critical temperature Tc about 30–50 K above Tg 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 Tg . On the other hand, for the complex polymers with side chains, poly~vinyl acetate!, poly~methyl acrylate!, and poly~ethyl methacrylate!, the crossover at Tg is less pronounced than those for the monomers and the polymer without side chains. Moreover, although we could not distinguish any singularities above Tg for these complex polymers, we observed another kink below Tg , which may be attributed to the side-chain motions. @S0163-1829~97!02333-3#