The previously determined molecular structure model of the dense, glassy polycarbonate of 4,4′-isopropylidenediphenol (“Bisphenol A polycarbonate” (PC)) has been used in conjunction with the structure-probing techniques of Theodorou and Suter and those of Mott et al. developed for atactic polypropylene (PP) to determine the small-strain elastic constants and study the mechanisms of large-strain plastic deformation of PC. The calculated elastic moduli of the microstructures all fall above the experimentally reported range but are not considered unreasonable in view of the relatively small size of the simulation cell and the rigidity of the PC repeat unit. For large-strain deformation, the ensemble-average stress–strain response exhibits a linear region parallel to the elastic loading line up to about 6–7% shear strain, where yielding is found to occur. Large-strain plastic shearing consists of a series of reversible elastic loading steps followed by irreversible plastic rearrangements. This behavior and its molecular interpretation are closely similar to what was found by Mott et al. in PP. The response of the ensemble shows a decrease in initial hydrostatic pressure with increasing strain, indicating that the initial state of the material has a strong tendency to densify upon plastic deformation. It is shown that the plastic response of the material is at least partly responsible for the pressure decrease that occurs during the deformation. This shear-induced pressure response in PC is opposite to that found in PP, where an initial dilatant response was found. Analysis of the stress drops indicated that plastic response is a result of shear transformations with an average transformation shear strain of 1.2%. Contrasting this finding with experimentally determined shear activation volumes leads to the conclusion that the shear transformations in bulk PC must occur in clusters of diameter of about 10.1 nm, indicating that the cooperative movement of many chain segments are involved in a single plastic relaxation event. The molecular movements associated with plastic flow were investigated. The applied strain increments could be partitioned into large phenylene ring rotations, carbonate group reorientations, and isopropylidene group rearrangements during the plastic events. However, none of these specific movements were found individually dominant. They appeared rather as kinematically necessary contributions to the overall transformation strain in the ca. 10.1-nm clusters.
All Science Journal Classification (ASJC) codes
- Organic Chemistry
- Polymers and Plastics
- Inorganic Chemistry
- Materials Chemistry