Anisotropic Thermal Conductivity in Cross-Linked Polybutadienes Subjected to Uniaxial Elongation

Abstract

Anisotropic thermal transport in cross-linked polybutadienes deformed in simple elongation is investigated experimentally. Using a novel optical technique based on forced Rayleigh scattering, two components of the thermal diffusivity tensor are measured as a function of stretch ratio. The thermal diffusivity is found to increase in the direction parallel, and decrease in the direction perpendicular, to the stretch direction, with a maximum anisotropy of ∼25% relative to the equilibrium value. Measurements of the tensile stress as a function of stretch ratio were also made. These data were used to show that the thermal conductivity and stress tensors are linearly related or that the stress-thermal rule is valid. The value of the stress-thermal coefficient for the crosslinked polybutadiene systems was found to be consistent with previously reported values for both a cross-linked silicone rubber and polyisobutylene melts.

Investigation of Anisotropic Thermal Conductivity in Polymers Using Infrared Thermography

“…A lightly cross-linked cis 1,4-polybutadiene rubber similar to that used in a previous study [22] is used as test material. Properties of as-received polymer are summarized in Table 1 along with values of the plateau modulus Gn and entanglement molecular weight Me.…”

“…Assuming that pcp is independent of deformation [20], the anisotropy in thermal diffusivity is equiva lent to that of thermal conductivity. Using this technique, we have obtained components of the full thermal diffusivity (conductivity) tensor in polymers subjected to uni-axial elongation [21][22][23][24][25] and shear flow [26][27][28].…”

“…(2) is the product of keq, the equilibrium thermal conductivity, and Ct, the stress-thermal coefficient, which is analogous to the stress-optic coefficient. In the previous work [21][22][23][24][25][26][27][28], we have combined measurements of the thermal conductivity tensor (using FRS) and stress tensor in deformed polymeric systems to provide the first (and only) tests of the stress-thermal rule. For all systems considered thus far, the stress-thermal rule has been found to be valid.…”

“…For all systems considered thus far, the stress-thermal rule has been found to be valid. In addition, we have found that the stress-thermal coeffi cient, when normalized by the plateau modulus of the polymer Gn, appears to be relatively insensitive to polymer chemistry with a value CtGN~ 0.03 [21][22][23][24][25][26][27][28], In a more recent study [23], we evaluated both the stress-thermal and stress-optic rules, the latter with independent measurements of birefringence [33], on a crosslinked polyisoprene. Interestingly, we found that the stressthermal rule is valid at stress levels where the stress-optic rule fails.…”

“…A robust data analysis method is developed, which allows for measurement of two com ponents of the normalized thermal conductivity tensor. The method is applied to a lightly cross-linked polybutadiene network [22] for which independent measurements of the thermal conduc tivity tensor components have been made using the FRS tech nique. Results from both methods are used to evaluate the stress thermal rule.…”