In this study, an experimental approach is used to characterize the geometrical and micromechanical properties of the shear transformation zones (STZs) in glassy polymers. Nanoindentation experiments have been conducted on both ascast and annealed poly(methyl methacrylate) (PMMA) at different strain rates and temperatures, utilizing continuous stiffness measurement (CSM) technique at room temperature indentations, and conventional loading rate control method for nanoindentations at elevated temperatures. Employing a homogeneous flow theory for analyzing the experimental data, the geometrical properties of the STZs are observed to be almost independent of the thermal history of the samples. While the transformation shear strain of the STZ in PMMA is found to be slightly smaller than that in glassy metals, the size of the STZ in this polymer is shown to be at least 10 times of that in metallic glasses. On the other hand, the activation energy of a single STZ is found to change drastically with annealing. In addition, analysis of the rate sensitivity of the shear flow stress reveals a remarkable transition at a certain strain rate which is believed to pertain to the β-transition. This transition is well-matched with a jump in the STZ activation energy at the same strain rate range; hence, the jump is referred to as the β-transition activation energy, which is found to be about 10% of the STZ nucleation energy for PMMA.

Due to the lack of the long-range order in their molecular structure, amorphous polymers possess a considerable free volume content in their inter-molecular space. During finite deformation, these free volume holes serve as the potential sites for localized permanent plastic deformation inclusions which are called shear transformation zones (STZs). While the free volume content has been experimentally shown to increase during the course of plastic straining in glassy polymers, thermal analysis of stored energy due to the deformation shows that the STZ nucleation energy decreases at large plastic strains. The evolution of the free volume, and the STZs number density and nucleation energy during the finite straining are formulated in this paper in order to investigate the uniaxial post-yield softening-hardening behavior of the glassy polymers. This study shows that the reduction of the STZ nucleation energy, which is correlated with the free volume increase, brings about the post-yield primary softening of the amorphous polymers up to the steady-state strain value; and the secondary hardening is a result of the increased number density of the STZs, which is required for large plastic strains, while their nucleation energy is stabilized beyond the steady-state strain. The evolutions of the free volume content and STZ nucleation energy are also used to demonstrate the effect of the strain rate, temperature, and thermal history of the sample on its post-yield behavior. The obtained results from the model are compared with the experimental observations on poly(methyl methacrylate) which show a satisfactory consonance.

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