Structural and thermodynamic features of the polyhydroxybutyrate physical gels

The structural and thermal properties of polyhydroxybutyrate (PHB) gels in dimethylformamide (DMF) are presented and discussed. The dilute and semi-dilute mixed PHB/DMF system undergoes a phase separation on cooling at a temperature 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-crystals, with the same unit cell of the pure crystalline PHB. Upon heating, two melting peaks are normally observed at T_m1 and T_m2 (always higher than T_gel). Shapes of the peaks, temperatures and enthalpies of melting depend slightly on scanning rate on heating but on the procedure of gel preparation. Reproducibility and reversibility have always been found with repeated thermal cycles. A thermodynamic phase diagram is therefore constructed and discussed, although the morphology of the system may be subject to the experimental conditions under which the gel is prepared.