Energy Effects of Deformation in Low-Crystallinity Ethylene-Octene Copolymers

Using the method of deformation calorimetry based on the simultaneous precision registration of mechanical and thermal effects, the mechanism of uniaxial deformation of low-crystallinity ethylene-octene copolymers (α = 4–18%) characterized by unique stress–strain properties is studied. It is shown that changes in the internal energy play a considerable role in the deformation of the studied copolymers. Changes in the entropy and internal energy during deformation are determined. Intra- and intermolecular energy contributions are estimated, and their ratio is shown to be controlled by both the level of crystallinity and the value of strain. It is found that intermolecular changes, which are associated with destruction of the initial structure and which grow with an increase in the degree of crystallinity, prevail at the initial step of deformation, while the region of large deformations is controlled by intrachain conformational transitions occurring in amorphous regions. An analysis of intermolecular energy changes, including cyclic ones, demonstrates the absence of intense effects that are generally caused by stress-induced crystallization; however, the possibility of low-order ordering should not be excluded.