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Abstract
In the present paper the effect of crystallization conditions on the melting temperature (Tm ) of chemically cross-linked gelatin is studied by means of differential scanning calorimetry (DSC) and microhardness (H) measurements as extension of our recent findings (Ref. [11]). A rather unusual situation when polymer crystals melt below the glass transition temperature (Tg ) is confirmed by DSC. This is highlighted on dry gelatin which is characterized by relatively high, and close to each other, Tg and Tm values (217 and 230°C, respectively). By depressing Tg , using water as plasticizer, rather imperfect crystallites are obtained which melt well below the Tm of dry gelatin. It is shown that H increases with temperature mostly due to the drying out of the room-conditioned gelatin. In the 160–180°C range H reaches values of about 390MPa. In this temperature range the imperfect crystallites melt (according to DSC), however without formation of a typical liquid phase (since H remains constant). It is expected that this direct crystal-glass transition can be used to obtain highly ordered polymer glasses. The unusual high H values as well as the low coefficient of thermal softening of gelatin, in contrast to those common for synthetic polymers, makes this material attractive for practical applications.