Section 6.12 - Damage Mechanisms in Amorphous Glassy Polymers: Crazing

Abstract

This chapter deals with the explanation of damage mechanisms in amorphous glassy polymers. It discusses the basic microstructure of glassy polymers and briefs up on crazing versus micro shear bands. Any mechanical damage mechanism is closely dependent on the microstructure of the material. In polymers these mechanisms are dominated by the long and flexible macromolecules. Since crazes arise mainly in amorphous glassy polymers, or at least in the glassy phase of multiphase polymers, the basic structural concepts of this type of material is summarized briefly in this chapter. A polymer is a large condensed assembly of macromolecules, which in the solid state may have two quite different microstructures. The first is a glassy disordered structure in which the smallest elementary volume representing the material has nearly the size of a monomer. The macromolecules are connected to one another through physical entanglements or irreversible chemical bonds, and consistent mechanical properties are attained when each macromolecule is connected to at least two others. In this microstructure the basic parameter is the molecular weight between entanglements. The second solid state is a semicrystalline structure in which the macromolecules are arranged on more or less regular small rigid lamellae with flexible amorphous connecting macromolecules. Further, a comparison has been made between crazing and micro shear bands.