The thermal conductivity κ and diffusivity α of high-strength and high-modulus crystalline polymer fibers (polybenzobisoxazole (Zylon\circR) and polyethylene (Dyneema\circR)) and their fiber-reinforced plastics (FRPs) were measured in directions parallel and perpendicular to the molecular chain axis of the fibers. The main contribution to thermal conductivity was from phonon conduction along the molecular chains in both fibers and the phonon conduction was limited by boundary-like scattering over the temperature range of 10–260 K. From the analyses using a phenomenological model, the thermal conductivity anisotropy ratio (κ// fiber/κ⊥ fiber) of Zylon fiber was estimated to be 80 at 100 K, which was about two or three times larger than that of Dyneema fiber.

The thermal conductivity κ and thermal diffusivity α of high-strength polyethylene fibers, DyneemaTM, with various draw ratios were measured from 10 K to 260 K. κ and α increased with increasing draw ratio and α was nearly proportional to the square root of the tensile modulus E of the fiber. The estimated phonon mean free path l (40–60 Å) was almost temperature independent. Boundarylike scattering due to characteristic microstructures with comparable size to l may effectively limit the thermal conduction in these fibers.

ABSTRACT:High strength polyethylene fiber (Toyobo, Dyneema® fiber: hereinafter abbreviated to DF) has a negative thermal expansion coefficient. Relation between fiber structure and thermal strain of DF used as reinforcement of DF reinforced plastic (DFRP) for cryogenic use was investigated. The crystallinities and orientation angles of several kinds of polyethylene fibers having different modulus from 15 to 134Gpa (herein after abbreviated to DFs) were measured by NMR and X-ray. We obtained the parameters of the mechanical series-parallel model composed of crystal and amorphous by crystallinity and modulus. Thermal expansion coefficients of DFs were estimated by mechanical seriesparallel model. All DFs having different modulus showed negative thermal expansion coefficients in the temperature range from 180 to 300K, and absolute values of those markedly increased by increasing tensile modulus of DF. The estimated thermal expansion coefficients showed negative values, and thermal strains showed a similar curve to observed ones mostly. Average thermal expansion coefficients in the temperature range from 180 to 300K estimated by mechanical model agreed with the observed ones.

High strength polyethylene fiber (Toyobo, Dyneema 1 fiber, hereinafter abbreviated to DF) used as reinforcement of fiber-reinforced plastics for cryogenic use has a high thermal conductivity. To understand the thermal conductivity of DF, the relation between fiber structure and thermal conductivity of several kinds of polyethylene fibers having different modulus from 15 to 134 GPa (hereinafter abbreviated to DFs) was investigated. The mechanical series-parallel model composed of crystal and amorphous was applied to DFs for thermal conductivity. This mechanical model was obtained by crystallinity and crystal orientation angle measured by solid state NMR and X-ray. Thermal conductivity of DF in fiber direction was dominated by that of the continuous crystal region. The thermal conductivity of the continuous crystal part estimated by the mechanical model increases from 16 to 900 mw/cmK by the increasing temperature from 10 to 150K, and thermal diffusivity of the continuous crystal part was estimated to about 100 mm 2 /s, which is almost temperature independent. The phonon mean free path of the continuous crystal region of DF obtained by thermal diffusivity is almost temperature independent and its value about 200 Å . With the aforementioned, the mechanical series-parallel model composed of crystal and amorphous regions could be applied to DFs for thermal conductivity. V V C 2005 Wiley Periodicals, Inc.

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