The thermal expansion of single-crystal texture linear polyethylene between 0 and ?190� C

Abstract

Thermal expansion of linear polyethylene in both macroscopically isotropic and single-crystal texture forms was measured between 0 and −190° C. Results are interpreted by treating the semicrystalline polymer as a simple two-phase composite material. The single-crystal texture specimen was prepared by drawing with constant width at 121° C followed by annealing, giving unique preferred orientations of each of the three crystal axesa, b andc (c parallel to the draw directionZ, a parallel to the sheet normalX andb parallel toY, the normal to the thin edge of the sheet). The thermal expansions parallel toX, Y andZ follow approximately the thermal expansions ofa, b andc vectors of the unit cell as determined by X-ray diffraction. An attempt is made to allow for the contribution to thermal expansion from amorphous regions, representing the oriented solid by a simple model in which crystal and amorphous regions are coupled in series in the draw direction and in parallel in the two perpendicular directions. This analysis requires the thermal expansion of amorphous polyethylene which was obtained by extrapolation of thermal expansion measurements on ten isotropic specimens whose crystal volume fractions lay between 0.81 and 0.49. It was observed that the single crystal texture specimen of crystal fraction 0.80 and an isotropic specimen of the same crystal fraction on cooling changed in volume by equal amounts. This fact together with the observation that the thermal expansion parallel toY of the single crystal texture specimen is equal to that of the isotropic specimen leads to important conclusions concerning the existence of microstresses in linear polyethylene caused by a change of temperature.