Melting of gelatin crystals below glass transition temperature: A direct crystal-glass transition as revealed by microhardness

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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 390 MPa. 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.
DGICYT (Grant PB94-0049), Spain. It is a pleasure to acknowledge the partial financial support provided by the National Science Foundation (USA) through Grant INT-95 14149 (Division of International Programme), as well as by the Bulgarian Ministry of Education, Science and Technology (Contract TH-7 14). We also wish to thank NEDO’s International Joint Research Programme, Japan, for the generous support of this investigation. One of us (S. F.) wishes to acknowledge the tenure of a sabbatical grant from DGICYT, Spain