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

This paper is the final part of a three paper series describing the fractur e and ageing behaviour of poly(hydroxybutyrate). In the first two parts con ventional fracture mechanics methods were used to monitor changes both duri ng the detrimental room temperature ageing process that occurs and after a subsequent annealing process that had been reported to reverse the ageing p rocess. This paper reports on our studies of the morphology of fracture sur faces and how fracture proceeds in different ways in the original, ductile, "fresh", material, the more brittle, aged material and the ductile, anneal ed material. We have used optical and electron microscopic techniques to ex amine fracture surfaces of samples which had already been well characterise d by mechanical testing. The effect of ageing and high temperature annealin g on the resultant fracture morphology is detailed for both thin films and bulk samples. We have found that PHB undergoes crazing before failure regar dless of annealing history. We have studied the craze morphology using opti cal microscopy and scanning electron microscopy. Both aged and un-aged samp les are found to deform in approximately the same manner, the primary diffe rence on ageing being the volume of material that is plastically deformed. After high temperature annealing a different craze morphology has been obse rved. 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 di stinct change in the resultant fracture surface. In both thin films and bul k systems there is an increased occurrence of fracture initiation in the sp herulite cores after high temperature annealing which helps to extend the c raze zone. (C) 1999 Kluwer Academic Publishers.