The effect of melt-processing on the degradation of selected polyhydroxyacids: polylactides, polyhydroxybutyrate, and polyhydroxybutyrate-co-valerates

A series of resorbable polyhydroxyacids were studied by size exclusion chromatography (SEC), viscosity measurements, and differential scanning calorimetry (DSC) for the effect of the injection moulding process on the molecular weight and thermal properties of the polymers. The polymers studied were polylactides (PLA), polyhydroxybutyrate (PHB) and polyhydroxybutyrate-co-valerate (PHB/VA) (5–22% VA content). All polylactides underwent extensive degradation, in the range of 50–88%, and an increase in molecular weight distribution (MWD) following injection moulding at temperatures ranging from 130 to 215°C. In contrast to the polylactides, the decrease in molecular weight of the PHB and PHB/VA polymers after injection moulding at temperatures from 135 to 160°C was less dramatic, in the range of 4–53%. This was accompanied by a decrease of the MWD. No evidence for low molecular weight non-volatile degradation products was observed. Injection moulding led to a decrease in the melting temperature (T_m) and the heat of melting (ΔH_m) of PLA. Conversely, the moulding process did not significantly affect the melting temperature and heat of melting of polyhydroxybutyrate/valerates.