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 Λ remains unknown. In this work, polymer fibers with different molecular arrangements (crystalline, liquid crystalline, and amorphous) were plastically deformed. Λ was measured at engineering strains ε = 0.2-2.3 using time-domain thermoreflectance. Λ 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 Λ₀ ≈ 0.27 Wm^-1 K^-1 for crystalline PE, Λ₀ ≈ 0.29 Wm^-1 K^-1 for liquid crystalline PBO, and Λ₀ ≈ 0.18 Wm^-1 K^-1 for amorphous PMMA. Λ of PE drops to Λ ≈ 0.14 Wm^-1 K^-1 at ε = 1.9; Λ of PBO drops to Λ ≈ 0.12 Wm^-1 K^-1 at ε = 2.1. We attribute the decrease of Λ with ε 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.