Mechanical properties and transition temperatures of crosslinked-oriented gelatin: II. Effect of orientation and water content on transition temperatures

This second part of a systematic study of the properties of crosslinked-oriented gelatin involves the effects of orientation and water content on the glass transition temperature T_g and on the melting behavior. The samples were the same as those in the preceding study, and their transition temperatures were determined by both differential scanning calorimetry and dynamic mechanical thermal analysis. The crosslinked gelatin which had been room-conditioned showed two transition temperatures: the lower one was attributed to T_g of the water-plasticized gelatin, and the higher one was interpreted as T_g of dried gelatin superimposed by melting. A rather unusual situation arose because of the fact that the T_g and melting temperatures T_m, (217 and 230 °C, respectively) are so similar. Using water as plasticizer not only decreases T_g but produces imperfect crystallites which melt below the T_g of the system. The presence of the amorphous phase in the glassy state would presumably make it essentially impossible to define a melting point or crystallization temperature in the normal manner, as an equilibrium between crystalline and amorphous phases.