Energy Storage in Cold Non-elastic Deformation of Glassy Polymers

Abstract

AbstractExperimental results on work W(ε), heat Q(ε) and stored energy U(ε) of deformation for glassy polymers such as linear PS, PC, PMMA, Polyimid, amorphous PET, thermotropic aromatic polyesters, Vectra™ for example, crosslinked epoxy are presented. All the data was obtained by a deformation calorimetry technique. Loading and unloading of samples were performed at room temperature with strain rate έ = 10-2 - 10-4 sec-1 under uniaxial compression up to engineering strains of εdef = 40-50%. During straining all polymers accumulate an excess of the latent energy U(ε). Elastic fraction of the energy is released completely at sample unloading and only residual Ures(ε) energy is conserved in samples. The latent energy Ures(ε) grows up to εdef =20-25% and levels off then. Shapes of the Ures(ε) curves are the same (S-shape) for all polymers. However, the saturation level is different for each polymer. The ratio U(ε)/W(ε) was also measured. It was found that at strains εdef < εy (εy - strain at the yield point) U(ε)/W(ε) ≈100%. I.e. all W is stored by sample in a form of U. The ratio decreases up to 60-30% for different polymers at higher strains. Release of the residual energy Ures (DSC measurements) and strain εres (thermally stimulated strain recovery technique) was measured for deformed and unloaded samples at heating. It was found that about 85-90% of Ures stored by samples is released in glassy state of polymers (below Tg). The Ures is related to a small fraction of εres, only to 7-10%. The rest of Ures and εres are recovered at the softening (devitrification) interval, around Tg. Computer modeling (molecular dynamics) of an isothermal shear deformation was performed for 2-dimentional two component atomic glass containing 500 Lennard-Jones particles of two different diameters. It was found that localized deformation events are of anelastic nature. The εan appears at early deformation stage in a form of localized shear events (transformations). Such events are nucleated in a sample and merged and united at later deformation stages, when concentration of the events becomes high enough. Finally, merged transformations form kind of shear band crossing entire sample. On the basis of experimental data and computer modeling the deformation mechanism for glassy polymers is proposed. The first stage of the process is the nucleation of “the carriers of non-elastic strain”, anelastic shear transformations (ASTs). All these ASTs are energetically excited. The concentration of the ASTs is responsible for the amount of Ures(ε) stored by a sample. It is suggested that such nucleation is the rate-controlling step in non-elastic deformation of any non-covalent glass. Saturation of the stored energy is defined by the reaching the steady state regime in carrier’s concentration. In this regime the rates of nucleation and termination (decrease of the stored local energy by AST) of carriers becomes equal. The termination proceeds spontaneously and easy (fast). The decrease of local energy of ASTs follows by local uncoiling of chains and by an appearance of new, extended chain conformers. However, such uncoiling is not the rate-controlling step for entire deformation process. Suggested mechanism very well describes all existing experimental facts. Deformation mechanisms for glasses seriously differ from that operating in rubbers and crystals.