<p style="text-align: justify">Polymeric glasses (PGs) are extensively used as high impact resistant, low density, and transparent materials in different industries as structural and non-structural elements. Nevertheless, their mechanical response at nanoscale under severe loading conditions with different prior thermal and mechanical histories is yet to be appropriately understood. Describing large deformation response of PGs has always been a serious challenge in the field of material science and solid-state physics. Unlike crystalline solids which have long-range structural coherence with dislocations as the mobile carriers of plasticity, amorphous polymers have virtually no significant order in their molecular structures resulting in a different mechanism of plastic deformation. After years of developments for models describing mechanisms of plasticity for PGs, It is now believed that nucleation of discrete irreversible rearrangements of atomic/molecular clusters upon yielding, which are called the shear transformation zones (STZs), yields the main rate-controlling event of the whole plastic deformation. However, there are yet many unknown aspects of the mechanism of such localized deformations to bring about the large-scale permanent plastic strain in the material upon large deformations. This study sheds light on some aspects of the nanostructural state of the material on the shear transformation sites. The mechanical behavior of PGs is strongly dependent on their nano- and microstructural states and is a function of their thermomechanical history. Thus, in this study, plastic deformation is selected as a mechanism to change the nanostructural state of the polycarbonate and poly(methyl methacrylate) as two widely used PGs, and the effect of such alterations on the characteristics of their flow units is investigated. Accordingly, nanoindentation technique is utilized to evaluate the flow stress of the two PGs subjected to different prior inelastic deformations. Digital Image Correlation (DIC), coupled with uniaxial tensile testing technique, has been used to precisely induce and monitor the plastic strain in the samples over the region intended for subsequent nanoindentation measurements. While the elastic part of the deformation induces reversible nanostructural reordering, it is the plastic part that brings about permanent/irreversible rearrangements. Hence, the measurements are only focused on the inelastically deformed and unloaded specimens. By incorporating the homogenous flow theory and strain rate dependency of the plastic flow in nanoindentation, activation volume of the localized STZs that mediate the plasticity is calculated. The results show that the shear activation volume, as the nanomechanical state variable of the material, decreases with inelastic deformation. Considering the interrelation between the shear activation volume and the free volume content as the nanostructural defect in PGs, this observation is well supported by the previously observed increase in defects upon plastic deformation for PGs. Besides, the variation of hardness and modulus with plastic deformation history of the polymer is presented and discussed in this paper. The reported data provides critical insight into the rejuvenation mechanism of plastic deformation as opposed to the physical aging process in PGs.</p>

Nanomechanics of Plastically Deformed Amorphous Polymers

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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