Molecular mobility and the glass transition in amorphous glucose, maltose, and maltotriose

We have used measurements of the phosphorescence intensity decay of the triplet probe erythrosin B, dispersed in amorphous glucose, maltose, and maltotriose at probe:sugar mole ratios of ∼1:10⁴, to monitor the molecular mobility of the sugar matrix in the glass and melt around the glass-transition temperature (T_g). Intensity decays were well fit using a stretched-exponential decay model in which the Kohlrausch-Williams-Watts lifetime τ and the stretching exponent β are the physically meaningful parameters. When normalized to the glass-transition temperature, the erythrosin lifetime decreased in the order glucose > maltose > maltotriose. Analysis of the lifetime provided an estimate of the collisional quenching constant for deexcitation of the triplet state (k_TS0); k_TS0 increased in the order glucose < maltose < maltotriose over the range of T-T _g from about -40 to 50°C. Although approximately constant in the glass, the magnitude of β decreased around T_g in the order glucose > maltose > maltotriose, indicating that the lifetime heterogeneity increased in the order glucose < maltose < maltotriose. These data demonstrate that both the average rate of matrix molecular mobility and the width of the distribution of matrix mobility rates around the glass transition increase with an increase in the molecular size of the sugar in this homologous series.