Structural and thermodynamic features of the polyhydroxybutyrate physical gels

The structural and thermal properties of polyhydroxybuterate (PHB) gels in dimethylformamide (DMF) are presented and discussed. The dilute and semi-di lute mixed PHB/DMF system undergoes a phase separation on cooling at a temp erature T-gel, producing a gel in which the crystalline fraction is of the order of 50-60% and the diffraction pattern is that of disperse nano-crysta ls, with the same unit cell of the pure crystalline PHB. Upon heating, two melting peaks are normally observed at T-ml and T-m2 (always higher than T- gel). Shapes of the peaks, temperatures and enthalpies of melting depend sl ightly on scanning rate on heating but on the procedure of gel preparation. Reproducibility and reversibility have always been found with repeated the rmal cycles. A thermodynamic phase diagram is therefore constructed and dis cussed, although the morphology of the system may be subject to the experim ental conditions under which the gel is prepared.