Enthalpy and mechanical relaxation of glassy gelatin films

The structural relaxation during the ageing of glassy gelatin films containing 8, 12 and 17% water at 25 °C was studied through (i) the extent of relaxation enthalpy (ΔH) associated with physical ageing using Differential Scanning Calorimetry (DSC) and (ii) the changes in storage modulus (E′) measured isothermally in the bending mode at different frequencies using Dynamic Mechanical Thermal Analysis (DMTA). The E′ increased with increasing ageing time (ta) and measurement frequency. For a given water content, the rates of ageing extracted through time–ageing time superposition of E′ data was in agreement with the rate of increase of ΔH. The rate and extent of ageing increased with the water content.