Morphology of plastic deformation and fracture of a biodegradable biopolymer

Abstract

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is becoming a hope for the development of sustainable products because it is produced from renewable resources, is completely biodegradable and can be molded as a petrochemical thermoplastic. The mechanical behavior and biodegradability of this semi-crystalline material strongly depends on its morphology. Particularly, in this paper we show how the amorphous matrix and the spherulites of PHBV behave under tension until failure. To achieve this goal, we prepared an acid etching agent to reveal details in the spherulitic structure of compressed molded sheets. Micro-tensile tests, performed with a home-made mini-testing machine set on the stage on an inverted polarized optical microscope, allow to follow morphological changes while loading and to determine the different crack propagation mechanisms. Crazing and shear bands are the strain mechanisms operating within the amorphous matrix of PHBV. In the closely packed spherulitic morphology, crack propagation may be: along the circumferential planes of the spherulites, trans-spherulitic along radial or circumferential planes or through the sharp boundaries between adjacent spherulites.