Relation between thermal effects and structural changes under deformation of thermoplastics

Abstract

The article reports thermal effects accompanying deformation of widely used thermoplastics: PE of different density, highly isotactic and stereodefective PP, PA6, PC. A number of structural methods (X-ray diffraction at large and small angles, DSC, IR spectroscopy, microscopy, volume measurements) were used to establish the factors underlying generation of heat under tension. In all the polymers studied, the greatest temperature and structural changes are observed in the second stage of deformation, when the neck is being formed. For polyolefins the temperature jump at the transition to the neck was found to correlate with the degree of crystallinity. Analysis of structural data showed that for these polymers the most important heat sources are the following structural processes: decrease in entropy of polymer chains due to orientation, appearance and growth of cavities, strain-induced crystallization of a part of amorphous phase.In the PEs, studied in this work, the crystallographic modification does not change under plastic deformation. In highly isotactic PP a part of initial α-crystals lose three-dimensional ordering and transform into the mesomorphic form during fragmentation of lamellae. In PA6 necking triggers transition from a less ordered γ-phase to a more stable α-modification, which proceeds in parallel with the crystallization of a part of the amorphous phase in the α-form. When the glassy amorphous PC is deformed, phase transformations, crystallization and pore formation do not occur. In this polymer, processes of internal friction are supposed to be the main cause of heat generation during deformation.