Mechanical Properties and Microstructure of Drawn High-Density Polyethylene Films by Unilateral Roller Drawing Method.

Abstract

The unilateral roller-drawing (URD) method was tested to prepare high-modulus and high-strength polyethylene films. Samples were heated locally for a short period during the drawing by this method, in which only the bottom-edge of a roller was soaked with a part of the film in silicone of high temperatures. The deformation aspects of macroscopic shape and microscopic structure as well as the mechanical properties of the films drawn by the URD method were studied under the various conditions of draw-temperature (Td) and draw-ratio (λ). The values of breaking stress and Young's modulus reached 1.0GPa and 40.0GPa, respectively, at Td=120°C and λ>25. The results obtained from wide-angle X-ray diffraction indicated orientations of (110) and (200) planes as well as high degrees of c-axis orientation along the draw-direction. On the basis of small-angle X-ray scattering, it was suggested that the two-phase structure composed of crystalline and amorphous regions changed into the mono-phase structure with the extended-chain crystals when λ was increased. The folding index was evaluated from IR spectra to show that chain unfolding appreciably occurred when λ was larger than 15 and Td was in the region of between 120 and 135°C. The endothermic peak in DSC caused by the melting of the sample was found to shift to the higher temperature region with increasing λ. It was confirmed from the scanning electron microscopic observation on the fractured surface that the brittle fracture occurred for the sample drawn by the URD method, while the uniaxially cold-drawn films showed the ductile-fracture appearance. It was considered from these results that the URD method was useful to prepare high-density polyethylene films with superior mechanical properties such as high-strength and high-modulus.