Shear dependence of thermal conductivity in polyethylene melts

Thermal conductivity measurements with a modified Couette flow cell were obtained as a function of shear rate for two linear polyethylene melts of weight‐average molecular weights 27,300 and 56,700, respectively. The lower‐molecular‐weight polyethylene revealed a maximum decrease in thermal conductivity of 55 percent at 150 s−1. After shearing at 400 s−1, approximately 90 minutes was required to recover the value corresponding to the zero shear condition. This was considered consistent with molecular orientation into the flow direction during shear with a subsequent relaxation upon the removal of stress. The higher‐molecular‐weight polyethylene gave a similar decrease in thermal conductivity at 50 s−1. Unlike the lower‐molecular‐weight melt, an increase was observed at higher shear rates. Enhancement of energy transport via cluster flow mechanism was presented as a possible interpretation of these results. A theory of molecular orientation of liquid poly(dimethylsiloxane) (PDMS) under shear flow was previously developed from thermal conductivity and birefringence data of this material. An attempt to clarify the difference in behavior between the two melts examined in this work, and between the polyethylene melts and the PDMS previously studied is presented.