| Summary: | Poly(hydroxybutyrate) is a thermoplastic material synthesised naturally by bacteria. Contemporary interest in this material arises from its biodegradability and biocompatibility but, unfortunately, its widespread use is restricted at present because it suffers from embrittlement after it has been moulded. The extension-to-break falls, and the semi-crystalline polymer becomes stiffer. The origins of the embrittlement - or ageing - phenomenon on the microstructural level are investigated in this thesis. The results obtained from the wide-angle X-ray scattering, small-angle X-ray scattering, thermal analysis and molecular modelling are consistent with the operation of a reeling-in process. Tie-molecules or loose loops of the amorphous phase are shortened with concomitant relaxation of a fold on the other side of the crystal. Annealing treatments produce a coarser lamellar morphology. Annealed material is still susceptible to ageing, but the extent to which reeling-in occurs is independent of the anneal temperature. These coarser starting structures are less affected by the limited amount of reeling-in which then takes place. While annealing does not reduce the absolute extent of the ageing, cold-rolling treatments do. Cold-rolling may therefore be an alternative way of reducing the severity of the embrittlement in objects of suitable shape.
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