Energy Storage In Cold Non-Elastic Deformation of Glassy Polymers

Experimental results on work W(epsilon), heat Q(epsilon) and stored energy U(epsilon) of deformation for glassy polymers such as linear PS, PC, PMMA, Polyimid, amorphous PET, thermotropic aromatic polyesters, Vectra T 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 epsilon = 10(-2) - 10(-4) sec(-1) under uniaxial compression up to engineering strains of epsilon(def) = 40-50%. During straining all polymers accumulate an excess of the latent energy U( e). Elastic fraction of the energy is released completely at sample unloading and only residual U-res(epsilon) energy is conserved in samples. The latent energy U-res(epsilon) grows up to epsilon(def) = 20-25% and levels off then. Shapes of the U-res(epsilon) curves are the same (S-shape) for all polymers. However, the saturation level is different for each polymer. The ratio U(epsilon)/ W(epsilon) was also measured. It was found that at strains epsilon(def) \u3c epsilon(y) (epsilon(y) - strain at the yield point) U(epsilon)/ W(epsilon) approximate to 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 U-res (DSC measurements) and strain epsilon(res) ( thermally stimulated strain recovery technique) was measured for deformed and unloaded samples at heating. It was found that about 85-90% of U-res stored by samples is released in glassy state of polymers (below T-g). The U-res is related to a small fraction of epsilon(res), only to 7-10%. The rest of U-res and epsilon(res) are recovered at the softening (devitrification) interval, around T-g. 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 epsilon(an) 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 U-res(epsilon) 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\u27s 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 forentire deformation process. Suggested mechanism very well describes all existing experimental facts. Deformation mechanisms for glasses seriously differ from that operating in rubbers and crystals

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

Molecular-dynamic simulation of low-temperature plastic deformation (T def = 50 K, T def/T g = 0.3) is studied for glassy polymethylene under the regime of active uniaxial compression and tension for a cell composed of 64 chains containing 100 -CH2 groups in each (as united atoms) and with periodic boundary conditions. Thirty-two such cells are created, and, in each cell, polymethylene chains in the statistical coil conformation are independently constructed. The cells are subjected to isothermal uniaxial compression at T def = 50 K by ¿ = 30% and by ¿ = 70% under uniaxial tension. In the course of loading, a s-¿ diagram is recorded, while the mechanical work spent on deformation, the changes in the overall potential energy of the system, and the contributions from various potential interactions (noncovalent van der Waals bonds, chemical links, valence and torsional angles) are estimated. The results are averaged over all 32 cells. The relaxation of stored potential energy and residual strain after complete unloading of the deformed sample is studied. The relaxation of stored energy and residual strain is shown to be incomplete. Most of this energy and strain is stored in the sample at the deformation temperature for long period. The conformational composition of chains and the average density of polymer glass during loading are analyzed. Simulation results show that inelastic deformations commence not with the conformational unfolding of coils but with the nucleation of strain-bearing defects of a nonconformational nature. The main contribution to the energy of these defects is provided by van der Waals interactions. Strain-bearing defects are nucleated in a polymer glass during tension and compression primarily as short-scale positive volume fluctuations in the sample. During tension, the average density of the glass decreases; during compression, this parameter slightly increases to ¿ ˜ 8% and then decreases. An initial increase in the density indicates that, during compression and at ¿ <8%, coils undergo compactization via an increase in chain packing. During compression, the concentration of trans conformers remains unchanged below ¿ ˜ 8% and then decreases. During compression, it means that in a glass, coils do not increase their sizes at strains below ¿ ˜ 8%. During tensile drawing, coils remain unfolded below ¿ ˜ 35%; at higher strains, coils become enriched with trans conformers or unfold. At this stage, the concentration of trans conformers linearly increases. The development of a strain-induced excess volume (strain-bearing defects) entails an increase in the potential energy of the sample. Under the given conditions of deformation, nucleation of strain-bearing defects and an increase in their concentration are found to be the only processes occurring at the initial stage of loading of glassy polymethylene. The results of computer-aided simulation are compared with the experimental data reported in the literature