Effect of temperature, crystallinity and molecular chain orientation on the thermal conductivity of polymers: a case study of PLLA

The crystallinity of semicrystalline polymers and molecular orientation of polymer have long been considered to be significant influencing factors on the thermal conductivity of polymer materials, but more clear-cut understanding on their impact on the thermal conductivity is still needed. In this work, poly-l-lactide (PLLA), whose crystallinity and orientation can be adjusted in a wide range, is selected to discuss the effect of degree of crystallinity and orientation on the thermal conductivity of PLLA. Meanwhile, the influence of temperature on the thermal conductivity is also discussed. PLLA compression-molded samples were heat-treated at 120 °C to tune the crystallinity of the samples, while the degrees of orientation were tuned by stretching the amorphous PLLA bars at 60 °C to different strains. It is found that environmental temperature of application affects the thermal conductivity obviously and the glass transition temperature of polymers shows a strong influence on the thermal conductivity of PLLA. Below T_g, the thermal conductivity of PLLA with different crystallinity increases with temperature and when the temperature is higher than T_g, the thermal conductivity of PLLA with different crystallinity decreases remarkably. It is also demonstrated that the thermal conductivity of PLLA increases with the increase in crystallinity, and the tensile strain linearly increases the thermal conductivity in the direction of molecular orientation and decreases the thermal conductivity in the perpendicular direction, which are in agreement with other semicrystalline polymers that has been reported.