Microscopic dynamics of the glass transition investigated by time-resolve 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-prapanol, 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 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.