Thermal history effects on crystallisation and melting of poly(3-hydroxybutyrate)

The effect of thermal history on morphology, melting and crystallisation behaviour of bacterial poly(3-hydoxybutyrate) (PHB) have been investigated by using differential scanning calorimetry (DSC), temperature modulated DSC (TMDSC) and hot stage optical microscopy (HSOM). Various thermal histories were imparted by crystallisation with different continuous and modulated cooling techniques that involve cool-heat and cool-isothermal segments. The subsequent melting behaviour revealed that all samples experienced secondary crystallisation during heating and the extent of secondary crystallisation was varied depending on the treatment. PHB samples crystallised under slow to moderate continuous cooling rates were found to exhibit double melting behaviour due to melting of DSC scan-induced secondary crystals, but to a lesser extent. samples subjected to a fast cooling rate gave amorphous PHB, while considerable secondary crystallisation/annealing took place under modulated cooling conditions. Additionally, the effect of melting conditions was also investigated by applying various continuous and modulated programs. The overall melting behaviour was rationalised in terms of recrystallisation and/or annealing of crystals. Interestingly, the samples analysed by temperature modulation programs showed a broad exotherm before the melting peak in the non-reversing curve and a multiple melting reversing curve, verifying that the melting-recrystallisation and remelting process is also operative. HSOM studies supported that the radial growth rate of the PHB spherulites was significantly varied upon the crystallisation conditions.