Molecular mobility in water and glycerol plasticized cold- and hot-cast gelatin films

The molecular mechanisms that control functionality in gelatin films are poorly understood. We have used phosphorescence emission from erythrosin covalently attached to lysine residues to investigate the effect of plasticizer and physical cross-links on molecular mobility and oxygen diffusion in amorphous gelatin thin films. Phosphorescence emission energy and red-edge effects, which monitor the extent of matrix relaxation during the ∼0.5 ms probe lifetime, were modulated only slightly by either water or glycerol; the extent of matrix relaxation, however, was significantly greater in cold-cast films with, than in hot-cast films without physical cross-links. The phosphorescence intensity was well described using a stretched exponential in which the lifetime and the stretching factor, a measure of the lifetime heterogeneity, were the fit parameters. Lifetimes, which provide a measure of the rate of matrix collisional quenching, were essentially unaffected by hydration from 0 to 50% RH, slightly decreased from 50 to 84% RH, and greatly decreased at higher RH; rates of matrix collisional quenching were higher in cold-cast than in hot-cast films at all RH values. Glycerol, varied up to 50% by weight of gelatin, only slightly increased the rate of matrix collisional quenching. The rates of oxygen diffusion were higher in cold-cast than hot-cast films at 75 and 84% RH; there was no detectable oxygen diffusion at any glycerol content in dry films. Physical cross-links thus actually increased the molecular mobility of gelatin on the millisecond time scale and water and glycerol, despite their similarity as hydrogen-bonding molecules, had quantitatively different effects on the gelatin mobility.