Simulation of elastic and plastic response in the glassy polycarbonate of 4,4'-isopropylidenediphenol

Abstract

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 density 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. IntroductionMicroetructures of the dense, glassy polycarbonate of 4,4/-isopropylidenediphenol (Bisphenol A polycarbonate (PC)) have been simulated and well characterized in three previous associated studies (referred to here as I, II, and III), displaying very good agreement with various experimental microscopic quantities.1-3 These PC microstructures have been used to investigate the mechanical behavior of PC ranging from the small-strain elastic to the large-strain plastic response of the structure. In this study first the elastic properties o...

Deforming glassy polystyrene: Influence of pressure, thermal history, and deformation mode on yielding and hardening

“…[14][15][16][17] More chemically realistic MD simulations of polymers have been carried out on amorphous polyethylene ͑PE͒, [18][19][20][21][22][23] PS, [24][25][26][27] and PC. 24,[26][27][28] Also other simulation techniques are applied to study the deformation of polymers, such as Monte Carlo algorithms or variants of energyminimization methods for PE-alike, 29 polypropylene, 30,31 poly͑oxypropylene͒, 32 PC, [33][34][35] and PE. 36,37 As the simulation studies are limited to only small time and length scales, numerical agreement is often only possible by means of extrapolation over orders of magnitude.…”

Plastic deformation of glassy polymethylene: Computer-aided molecular-dynamic simulation

“…In the last 20 to 30 years, many studies have been devoted to the computer aided simulation of plasticity in glassy polymers (see, for example, [28][29][30][31][32][33][34][35][36][37]). In most publications, the method of molecular dynamics has been used to study the conformational rearrangements in polymer chains and variations in the rates of such rearrangements under the action of an external force, the orientation of chain segments, the volume changes in elementary deformation events [31,[33][34][35][37][38][39], and some other problems associ ated with chain behavior.…”

“…In most publications, the method of molecular dynamics has been used to study the conformational rearrangements in polymer chains and variations in the rates of such rearrangements under the action of an external force, the orientation of chain segments, the volume changes in elementary deformation events [31,[33][34][35][37][38][39], and some other problems associ ated with chain behavior. However, in all the above cited publications, growth in the potential energy of a polymer glass during its deformation has been neglected, while structures responsible for this growth have not been identified.…”

Effects of thermal history on impact‐modified, high‐flow polycarbonate

“…Before a discussion of these results, a brief review on toughening mechanisms of typical polycarbonate with high M W and high impact strength (961 J / m ) may be in order. The high impact strength was variously attributed to shear yielding from the notch tip (2), compressive stresses on the surface (12), a correlated conformational interchange between two neighboring carbonate units (81, and secondary dynamic-mechanical relaxations (3-5, lo), and unique features of molecular structure (13,14).…”

“…Moet et al (15) reported that shear yielding first takes place in a region of stress-concentration localized in a poorly packed region between regions of higher packing density. According to Hutnik et al (13). the shear yielding involves a cooperative movement of many chain segments, and rearrangements in the carbonate and isopropylidene groups are more important for the total strain than rotation of phenylene rings on the average.…”