The fracture of poly(hydroxybutyrate) Part III Fracture morphology in thin films and bulk systems

Abstract

This paper is the final part of a three paper series describing the fracture and ageing behaviour of poly(hydroxybutyrate). In the first two parts conventional fracture mechanics methods were used to monitor changes both during the detrimental room temperature ageing process that occurs and after a subsequent annealing process that had been reported to reverse the ageing process. This paper reports on our studies of the morphology of fracture surfaces and how fracture proceeds in different ways in the original, ductile, “fresh”, material, the more brittle, aged material and the ductile, annealed material. We have used optical and electron microscopic techniques to examine fracture surfaces of samples which had already been well characterised by mechanical testing. The effect of ageing and high temperature annealing on the resultant fracture morphology is detailed for both thin films and bulk samples. We have found that PHB undergoes crazing before failure regardless of annealing history. We have studied the craze morphology using optical microscopy and scanning electron microscopy. Both aged and un-aged samples are found to deform in approximately the same manner, the primary difference on ageing being the volume of material that is plastically deformed. After high temperature annealing a different craze morphology has been observed. In thin films this is characterised by the formation of a dense zone of micro-crazes over a relatively large area. In bulk samples there is a distinct change in the resultant fracture surface. In both thin films and bulk systems there is an increased occurrence of fracture initiation in the spherulite cores after high temperature annealing which helps to extend the craze zone.