Thermophysical behaviour of solid polymers in simple elongation: A structural and molecular interpretation

Thermodynamic aspects of reversible simple extension of solid polymers have been considered in terms of the conventional equation of state and equations have been obtained for the thermodynamic functions. It is shown that simple deformation of solids is accompanied by inversion of internal energy which is controlled by coefficient of thermal expansion. Work, heat and internal energy as functions of strain have been determined by deformation calorimetry for the typical glass-like and crystalline polymers and it has been found that in uniaxially oriented crystalline polymers at aboveT _g the internal energy undergoes inversion due to the negative coefficient of thermal expansion. It has been demonstrated that the thermoelastic behaviour of two-phase crystalline polymers is controlled by the volume (irrotational) elasticity of amorphous regions rather than by shape elasticity typical of rubber elasticity. From this position, a thermophysical analysis of the deformation of the basic models of oriented crystalline polymers and combined investigation of the thermal phenomena and structural changes in oriented PE and PP have been carried out. It has been shown that the Peterlin-Prevorsek model which implies existence of both intra- and interfibrillar amorphous regions quite adequately account for the thermophysical and structural effect observed in tension of the oriented specimens in the original and annealed state. Thermoelastic properties of super-oriented crystalline polymers have also been discussed in brief.Dedicated to Professor Dr. F. H. Müller