Secondary crystallisation and degradation in P(3HB-co-3HV): an assessment of long-term stability

Abstract

The effect of long-term storage, at a range of temperatures, on the thermal and mechanical properties of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(3HB-co-3HV)) is reported. P(3HB-co-3HV) was stored at −22, 7, 20, 50, 75, 100, 125 and 150 °C for 672 h. Below 100 °C, the observed increase in melting point, glass transition temperature, modulus and strength are due to the development of crystallinity via a secondary process and facilitated by increasing temperature. At 125 °C, the processes of secondary crystallisation and degradation occur in parallel and there is an interplay between these processes, with the effects of degradation masked by the continuation of secondary crystallisation. At 150 °C there is a deterioration in all the properties noted above and given the reduction in molecular weight (and discolouration), this can be attributed to degradation. The significant observation from these results is that the degradation process remains active well below the melting point and long-term stability of P(3HB-co-3HV) is not only affected by secondary crystallisation, but also degradation. In addition, as no significant changes to the thermal or mechanical properties were observed during storage at −22 and 7 °C it suggests that P(3HB-co-3HV) may be a suitable packaging material for frozen or refrigerated products. The effects of long-term storage of poly(3-hydroxbutyrate-co-3-hydroxyvalerate) were evaluated over a range of temperatures. As the storage temperatures were increased towards 100 °C, secondary crystallisation resulted increasing the melting point, Tg, modulus and strength. At 125 and 150 °C secondary crystallisation and degradation occurred simultaneously; the effects of degradation masked by secondary crystallisation. The significant observation is that the degradation process remains active at storage temperatures well below the melting point and long-term stability of P(3HB-co-3HV) is not only affected by secondary crystallisation, but also sub-melting point degradation.