Controlled degradation of polyhydroxybutyrate via alcoholysis with ethylene glycol or glycerol

It was shown that controlled degradation of poly-[(R)-3-hydroxybutyrate] (PHB) can be achieved by alcoholysis with two types of alcohol in the presence of a catalyst. PHB oligomers terminated with free hydroxyl groups were prepared in this way. Molecular weight of the prepared samples was studied with three methods: SEC analysis with polystyrene calibration, SEC analysis using universal calibration, and viscometry. The data lead to the conclusion that SEC analysis using polystyrene calibration is a suitable method for monitoring degradation. The degradation proceeds by random chain scission and the molecular weight was decreased by almost two orders of magnitude depending on the alcoholysis conditions. The crystallinity and melting temperature, Tm, of PHB after alcoholysis, investigated by differential scanning calorimetry (DSC) show the independence of crystallinity on molecular weight and a decrease in Tm with decreased molecular weight. Time dependence of mechanical properties of selected samples (mechanical ageing) reveals that mechanical properties change with time for degraded samples in a similar way as for the parent polymer.