Thermodynamics of Deformation of Thermoplastic Vulcanizates

The thermodynamic characteristics of deformation of thermoplastic elastomers obtained by the dynamic vulcanization of two crystallizable components polypropylene and ethylene-propylene-diene copolymer are studied by deformation calorimetry. The entropy and energy effects of deformation are determined for the original, oriented, and annealed (at 70°C) samples containing 20 and 29 wt % polypropylene. It is shown that stretching of the studied elastomers causes changes in thermodynamic parameters which are typical of thermoplastic elastomers and demonstrate the initial distortion of the original structure and its hardening on further deformation. It is ascertained that the reason behind the appearance of residual elongations and hysteresis phenomena consists in partial maintenance of structural changes after unloading, as confirmed by X-ray diffraction. Estimation of the linear thermal expansion coefficients calculated from thermal effects proves that, in the thermoplastic vulcanizates studied, the polypropylene phase is continuous, which implies the existence of interpenetrated network structures in them. It is found that stretching of the annealed sample with a high content of polypropylene is accompanied by the appearance of stress-induced crystallization. The strain dependences of the degree of crystallinity calculated from the relative internal energy changes correlate with the X-ray diffraction data.