Polymers for biodegradable medical devices. VIII. Hydroxybutyrate-hydroxyvalerate copolymers: physical and degradative properties of blends with polycaprolactone.

The physical and degradative properties of polyhydroxybutyrate-hydroxyvalerate copolymer blends with polycaprolactone were investigated. Blends containing low levels of polycaprolactone (less than 20%) were found to possess a considerable degree of compatibility, whilst those with higher levels of polycaprolactone were incompatible and showed phase separation behaviour. This incompatibility was most marked in blends containing approximately 50% of each component. In blends containing low levels of polycaprolactone, processing conditions governed the ease of crystallization of polycaprolactone in the polyhydroxybutyrate-hydroxyvalerate matrix and thus the mechanical property of the blend. The degradation rate of these blends was found to be influenced by a complex set of factors, including temperature, pH and polycaprolactone content of the blend. Although crystallinity affected the mechanical properties of the blends, its influence on the hydrolytic degradation rate was masked by the large difference in the molecular weight of the polyhydroxybutyrate-hydroxyvalerate copolymers (MW approximately 300,000) and polycaprolactone. (MW approximately 50,000). The polyhydroxybutyrate-hydroxyvalerate/polycaprolactone blends were found to be much more stable to hydrolytic degradation than polyhydroxybutyrate-hydroxyvalerate/polysaccharide blends previously studied. Here the combined techniques of goniophotometry and surface energy measurements proved extremely valuable in monitoring the early stages of degradation, during which surface, rather than bulk degradation, processes predominate.