(226at) Thermal Transport in Cross-Linked Natural Rubber Subjected to Uniaxial Elongation

It is well established that when elastomers  are subjected to deformation, the orientation of polymer chain segments between cross links can be perturbed far from their equilibrium  conformation. In previous work we have used a novel optical technique called Forced Rayleigh Scattering (FRS) to measure deformation-induced anisotropy in thermal conductivity. We have found that there exists an apparently universal relationship between the thermal conductivity and stress tensors in both linear and cross-linked polymers. More recently, have developed a technique based on Infrared Thermography (IRT) and found consistency with our earlier FRS results. We have also used IRT to investigate the dependence of heat capacity on deformation in elastomers. This phenomenon is related to the longstanding question of whether there is an internal energy contribution to rubber elasticity, and more generally in polymeric liquids. By tracking the temperature evolution of stretched samples heated by a laser beam, we are able to measure heat capacity changes with respect to the equilibrium value as a function of elongation. We find that heat capacity increases with stretch ratio in lightly cross-linked cis 1,4- polyisoprene specimens subjected to uniaxial extension. We evaluate our results with a simple thermodynamic analysis of the internal energy contribution to tension based on classical rubber elasticity results. Additionally, we find that the deviation from the equilibrium value of heat capacity is consistent with an independent set of experiments comparing anisotropy in thermal diffusivity and conductivity employing FRS and IRT techniques.