Dependence of the thermal conductivity on mechanical stress in polymers

Abstract

SynopsisAn examination of the theory of thermal conductivity of aniorphous dielectrics as applied to polymeric materials indicates that it is reasonable to expect that the conductivity is stress dependent. An experimental investigation was undertaken to determine the validity of this hypothesis for a number of plastics at temperatures below their respective glass transition points. Poly(methy1 methacrylate), nylon, and Delrin were chosen as representative of a wide range of percentage crystallinity and were tested a t compressive stresses up to 140 kg./cm.2 and temperatures between 4 and 38°C. The results indicate that the conductivity may increase as much as 20% and that the dependence on stress is a function of temperature and the type of polymer examined.

Molecular origins of anisotropy in the thermal conductivity of deformed polymer melts: stress versus orientation contributions

“…We make three important observations about these results: they are consistent with the stress-thermal rule, the sign is opposite that of melts, and the magnitude of the dimensionless stress-thermal coefficient is two orders of magnitude smaller than that of the melt. This is in contradiction with earlier, more invasive experiments 28 which showed a 10% effect under compression. Decidedly, stresses are unlikely to give significant contribution to the anisotropy in the thermal conductivity of polymer melts.…”

“…The connection between molecular properties and thermal conductivity in dense amorphous systems is still not completely clear, 26 and explanations have been attempted for some time for polymers. 27,28 Crystalline materials exhibit thermal conductivity consistent with the phonon theory of Peierls, where phonons are quasi-particles from quantum mechanical vibrations. For amorphous systems, the meanfree-path is too short to be physically meaningful, however.…”

“…26 Hence, the stress anisotropy itself might lead to the anisotropy in thermal conductivity and the STR. 28 Here we present experimental results to measure what effect stress itself might have on the anisotropy in thermal conductivity. In the first experiment, we create samples that have oriented polymers without significant stress.…”