Slow Molecular Mobility in the Amorphous Solid State of Fructose: Fragility and Aging

摘要

The molecular mobility of β -D-fructose was studied by thermally stimulated depolarization currents (TSDC) in the amorphous solid state. The amorphous solid samples were prepared in such a way that the tautomeric mixture was near the equilibrium composition. A broad secondary relaxation was observed, that merges, at high temperatures, with the alpha relaxation. The alpha relaxation temperature provided by the TSDC technique is T_g = 13 ℃ (at 4 K min~(-1)). The fragility index calculated from TSDC data is m = 34, significantly lower when compared with the values reported in the literature obtained from Dielectric Relaxation Spectroscopy (DRS). The physical significance of the fragility obtained by the 2 dielectric techniques is discussed. The influence of physical aging on the secondary relaxation in amorphous fructose was analyzed as the glass structurally relaxes. A complex behavior was observed such that the faster components {lower temperature) of the secondary relaxation are negligibly dependent on aging and may be ascribed to intramolecular modes of motion, while the slower motional modes (higher temperature) show a significant dependence on aging and correspond to the genuine Johari-Goldstein β-relaxation.