Thermomechanics of polymer networks

Abstract

Calorimetric determinations of the total energy exchange in polymer networks provide information about both thermodynamic and molecular quantities characterizing the deformation process and, therefore, have a fundamental importance in investigating rubber elasticity. The thermomechanical behaviour of the chemically crosslinked polymer networks, filled networks, rubberlike thermoelastoplastics and crystalline networks are discussed. Thermomechanics of the crosslinked networks is considered from the point of view of the interchain entropy and energy contributions to the free energy of deformation and the temperature coefficient of the unperturbed chain dimensions. The comparison with the results obtained on isolated macromolecules demonstrates that the classical Gaussian theory of rubber elasticity quantitatively predicts the intrachain entropy and energy contributions at simple deformations of the networks and their independence of the deformation (at small and moderate deformations). The interchain changes of internal energy, vibrational entropy and volume resulting form the deformation are also considered and it has been concluded that they support the theory only at small deformations. Analysis of the entropy and energy effects resulting from the simple extension of the stress-softened networks filled with different fillers shows that, in many cases, the entropy and energy contributions are dependent on the concentration of the fillers, which contradicts the classical theory of rubber elasticity. Some reasons for the dependence are considered. Thermomechanical studies of SBS ands SIS block copolymers with a hard block content of below 40 % show that the energy contributions accompanying uniaxial extension are independent of the hard block content and degree of deformation. The energy contributions for diene blocks coincide well with the results for chemically crosslinked diene networks. On the other hand, the thermomechanical behaviour of the segmented polymers with the small molecular weight of the soft blocks and the large content of the hard block is determined not only by intrachain conformational changes but by intermolecular changes, both in the soft and hard blocks. Some possible deformation mechanisms which lead to such thermomechanical behaviour are considered. Although it is widely accepted that the free energy of the uniaxial deformation of the two-phase crystalline networks is purley intrachain, our calorimetric investigations show that the thermodynamics of the deformation of these networks is controlled by interchain changes in the amorphous regions. To support this conclusion some thermomechanical results for oriented and unoriented crystalline networks are considered.