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.

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The mechanical loading induced flow of glassy polymers is triggered by the nucleation of shear transformation units, and strongly depends on the initial microstructural state of the material. Therefore, investigation of the possible relationship between the microstructural state variables and plastic deformation is required for a better understanding of the macroscopic response of this class of materials during large deformation. In this study, free volume content is considered as a state variable and thermal treatment is selected as a process through which the accelerated and forced evolution of the free volume can be imposed. For two well-known glassy polymers, poly(methyl methacrylate) and polycarbonate, the free volume content alteration upon annealing is monitored via positron annihilation spectroscopy, and the changes of the micro-and macromechanical properties are also obtained by utilizing nanoindentation technique and employing the homogeneous amorphous flow theory. The correlation between the microstructural state variable, that is, free volume, and the micromechanical state variable, that is, shear activation volume, is then investigated. The results reveal opposite direction of alterations of free volume and shear activation volume with annealing temperature. Accordingly, the possibility of the existence of an interrelation between these two state variables is critically discussed.