Thermomechanics of Glassy and Crystalline Polymers

Conventionally, the deformation of solids is treated within the framework of elasticity theory in terms of stresses and strains, i.e. in purely mechanical terms. Although experimental determination of stress-strain relationship provides important information concerning the deformation process, it is quite obvious that this conjugated variable pair is only one of several pairs which can be used to describe the response of a material on deformation. The necessity of using the thermodynamic instead of mechanical approach became evident when investigators began to consider deformation in terms of thermodynamic potentials, such as internal energy, free energy etc., rather than in terms of potential energy. The first law of thermodynamics shows that energy is conserved in all deformation processess either reversible or irreversible. Therefore, the mechanical response of any material reflects exactly that amount of energy which accompanied the deformation process as heat and/or changes of internal energy. What this means to an experimentalist is that the temperature or thermal variations brought about by adiabatic or isothermal processes have to be measured simultaneously with stresses and strains. The thermal effects and temperature variations accompanying deformation of solids are usually rather small and it is their correct measurement which constitutes the major difficulty in passing from mechanical to a thermodynamic approach.