Crazing in semicrystalline thermoplastics

The deformation processes in crystalline polymers have been studie ever since the discovery of chain folding in 1957. Since then, scientists have been intrigued by the different steps of the transformation of the folded-chain lamellar structure of single crystals or of macroscopically isotropic, often spherulitic, polymers into fibrous morphologies (see Refs. 1 and 2 for early reviews). The importance of molecular tilt, of inter- and intralamellar slip, and of micronecking were rapidly recognized 1–4]. In this paper, we discuss the analogies and differences with respect to crazing of glassy amorphous polymers. Obviously, there is an extensive body of literature on the micromechanics of crazing (see the reviews in Refs. 5–9). On the basis of these studies, it has been established that crazes in amorphous polymers are well-defined regions with approximately planar boundaries that extend perpendicular to the direction of maximum principal tensile stress and that contain highly stretched and voided material 7]. However, crazelike features have also been observed in many semicrystalline polymers (polyethylene PE], isotactic polypropylene IPP], isotatic polystyrene IPS], polyoxymethylene POM], polyamide 6 and 66 PA6 and PA66], polycarbonate PC], polyethylene terephthalate PET], polybutylene terephthalate PBT], polyvinylidene fluoride PVDF], and polyether ether ketone PEEK]). They are designated in the literature 3–10] as micronecks, true crazes, fibrillar deformation zones (DZs), or simply as crazes since they correspond well to the above definition.