Consequences of Melt Memory and Thermal Degradation for Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) Crystallization

Crystallization in poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3HB-co-4HB)) is strongly influenced by the thermal history. Additive-free P(3HB-co-5% 4HB) was biosynthesized, and melt memory domains were identified from DSC crystallization measurements after processing for 3 min at temperatures between 152 and 212 °C. A broad self-nucleation domain (∼20 °C) was confirmed by polarized optical microscopy, whereas complete melt memory erasure led to sparse nucleation and slow crystallization. Thermal degradation, already present at low processing temperatures, substantially reduced the molecular weight, which was shown to affect spherulite growth rates and overall crystallization behavior. When melt memory was evaluated after thermal history removal (3 min at TDI) and recrystallization, the self-nucleation domain narrowed (∼10 °C). Increasing TDI further weakened memory due to degradation-induced molecular weight reduction. These results demonstrate that meaningful crystallization studies of P(3HB-co-4HB) require careful optimization of processing temperatures to balance melt memory effects with molecular weight retention within the narrow thermal processing window of this copolymer.