10. Shear Transformation Zones in Amorphous Polymers: Geometrical and Micromechanical Properties

Glassy polymers are extensively used as high impact resistant, low density, and clear materials in industries. Due to the lack of the long-range order in the microstructures of glassy solids, plastic deformation is different from that in crystalline solids. Shear transformation zones (STZs) are believed to be the plasticity carriers in amorphous solids and defined as the localized atomic or molecular deformation patches induced by shear. Despite a great effort in characterizing these local disturbance regions in metallic glasses (MGs), there are still many unknowns relating to the microstructural and micromechanical characteristics of STZs in glassy polymers. This chapter is aimed at investigating the flow phenomenon in polycarbonate (PC) and poly(methyl methacrylate) (PMMA) as glassy polymers and obtaining the mechanical and geometrical characteristics of their STZs. To achieve this goal, the nanoindentation experiments are performed on samples with two different thermal histories: as-cast and annealed, and temperature and strain rate dependency of the yield stress of PC and PMMA are studied. Based on the experimental results, it is showed that the flow in PC and PMMA is a homogeneous phenomenon at tested temperatures and strain rates. The homogeneous flow theory is then applied to analyze the STZs quantitatively. The achieved results are discussed for their possible uniqueness or applicability to all glassy polymers in the context of amorphous plasticity.