Plasticity of semicrystalline flexible-chain polymers at the microscopic and mesoscopic levels

This review considers the achievements of the last 10-15 years in interpreting the micromechanisms of plastic deformation in semicrystalline polymers. The range of the objects in question is mostly limited to flexible-chain polymers with T g of their amorphous component always lower than T def . Under these conditions, the amorphous component behaves as the softest constituent of the material. The specific features characterizing the deformation of chain crystals in comparison with nonchain ones are analyzed. Published data make it possible to state that the plasticity of polymer crystals is governed by crystallographic mechanisms similar to those in nonpolymer crystals. In the overwhelming majority of cases, a computational analysis of the plasticity of chain crystals and semicrystalline polymers (within the concepts of the nucleation and motion of screw dislocations in them) provides a good agreement of the calculated and measured yield points and critical shear stresses, as well as their temperatures and strain rate behavior. It may be presently regarded as proved that the key structural parameter in the nucleation and motion of dislocations is the thickness of crystallites. All the principal deformation modes are considered for the amorphous component of a semicrystalline material. The data available demonstrate the important role of the combined deformations of the amorphous and crystalline components, which constrain each other. An important specific feature of deformation in the soft amorphous component is that the plasticity in it is locked (blocked) by the surrounding crystallites. After the locking strains are reached, all processes in the amorphous phase take place only together with crystallites and obey the development of plasticity in these crystallites. Such a locking of the amorphous phase deformation by crystallites usually takes place at relatively early stages of the sample plasticity. Such an important achievement as the preparation of textured quasi-single crystals of macroscopic dimensions (above all, polyethylene and polyamide-6) is described in detail. These textured quasi-single crystals are novel structures in polymer science. The amorphous component in them shows an unusual structure. In polyethylene, for example, the amorphous and crystalline components are mixed and have no interface. The material in textured quasi-single crystals is structurally coherent at the sample scale. The amorphous component has an increased degree of order; therefore, crystalline and amorphous regions in the materials can reach a good mutual structural adjustment and exchange crystallographic symmetry operations. Due to the latter fact, deformation micromechanisms similar to those in a perfect crystal are implemented in the amorphous component. Since textured quasi-single crystals have macroscopic dimensions, it was possible to cut samples with different orientations of crystallographic axes out of them and to measure their mechanical characteristics, thus obtaining the critical resolved shear stresses for various slip systems of the same material. Simulations of plastic deformation for semicrystalline polymers have received much attention. The model most successful at present is the microcomposite one; within this model, it became possible not only to obtain the stress-strain diagrams and deformation resistances of polyethylene for various loading geometries but also to predict the development of the texture evolution in this polymer under various deformation conditions. A comparison of the calculations and experiment shows good agreement. All the results accumulated by now make it possible to state that the plasticity of semicrystalline polymers with a soft amorphous component is controlled by the deformation of crystallites up to the largest strains. In this connection, a quantitative plasticity analysis of crystals and crystalline components of semicrystalline polymers must play an important role in the further deve