Thermal Conductivity in the Radial Direction of Deformed Polymer Fibers

Thermal conductivity of polymer fibers in the axial direction has been extensively studied while thermal conductivity in the radial direction Lambda remains unknown. In this work, polymer fibers with different molecular arrangements (crystalline, liquid crystalline, and amorphous) were plastically deformed. Lambda was measured at engineering strains epsilon = 0.2-2.3 using time-domain thermoreflectance. Lambda decreases with increasing strains for polyethylene (PE) and poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers and is independent of strain for poly(methyl methacrylate) (PMMA) fibers. The extrapolated thermal conductivity at zero strain is Lambda(0) approximate to 0.27 Wm(-1) K-1 for crystalline PE, Lambda(0) approximate to 0.29 Wm(-1) K1 for liquid crystalline PBO, and Lambda(0) approximate to 0.18 Wm(-1) K-1 for amorphous PMMA. Lambda of PE drops to Lambda approximate to 0.14 Wm(-1) K-1 at e = 1.9; Lambda of PBO drops to Lambda approximate to 0.12 Wm(-1) K-1 at e = 2.1. We attribute the decrease of Lambda with e in crystalline and liquid crystalline fibers to structural disorder induced by plastic deformation. The combination of structural disorder and phonon focusing effects produces a thermal conductivity in deformed PE and PBO fibers that is lower than amorphous PMMA.