Mechanical properties and transition temperatures of crosslinked-oriented gelatin .2. 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 degrees 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.