Thermoelasticity in Open Systems

Abstract

Abstract The thermoelastic properties of polymer networks are usually obtained by studying the effect of temperature on the magnitude of the stress of samples subjected to uniaxial elongation. It is assumed that polymer networks are in the region of small elongation characterized by reversible deformation, and that the results obtained represent equilibrium values. The method makes it possible to characterize the network by a series of thermodynamic parameters, of which the ratio fu/f is used most frequently. This ratio, fu/f, expresses the ratio between the energetic component fu and the total elastic force f. The values of fu/f are technically important parameters which may serve as a measure of the “ideality” of an elastomer. For an ideal elastic polymer, i.e., the polymer the elasticity of which is entirely entropic in origin, it can be written fu/f = 0. The significance of thermoelasticity was markedly increased when the phenome-nological parameter fu/f was shown to be interpretable on the molecular level as a consequence of conformational changes in macro-molecular chains. By refinement of the molecular theory of rubberlike elasticity, a relation between fu /f and the temperature coefficient of the unperturbed dimensions of the chains d In r 0 2/dT (TCUD) was established [l, 2]