Morphological and Viscoelastic Properties of Ultradrawn Polyethylene with Such a High Young's Modulus as Super-Strength-Steel

Abstract

Solution viscosity measurements have been used to define the optimum concentration for the ultradrawn films of high molecular weight polyethylene produced by gelation/crystallization from solution. For a molecular weight of 4×106 the optimum concentration was 0.5g/100ml. Dry gel films prepared from the solution of this concentration could be elongated at 135°C to a remarkably high draw ratio λ=300. The Young's modulus and the tensile strength, 202 and 6.2 GPa respectively, of the drawn film are among the highest reported for polyethylene ; the observed Young's modulus is consistent with the modulus of polyethylene in the chain axis direction. The origin of the high drawability was discussed in terms of the morphology of the dry gel film as studied by wide-angle X-ray diffraction, small angle X-ray scattering, and scanning electron microscopy. The facile drawability is interpreted as due to the transformation from a folded to a fibrous crystal. A suitable number of entanglements in the chains connecting crystal lamellas is necessary to induce the transformation.Temperature-dependence of complex dynamic modulus functions was measured in the range of frequency from 0.1 to 100 Hz in order to study crystal dispersions of polyethylene. At temperatures lower than 70°C, master curves of loss modulus are obtained through only horizontal shift of the modulus function along the logarithmic frequency axis. By contrast, the temperature-frequency superposition above 70°C requires the vertical shift as well as the horizontal shift of the function. The Arrhenius plots indicate that there exist two mechanical dispersions, one at higher and the other at lower temperature sides of the reference temperature of 70°C. The activation energies of these dispersions range from 98.3 to 104.2 kJ/mol (from 23.5 to 24.9 kcal/mol) and from 78.2 to 80.8 kJ/mol (from 18.7 to 19.3 kcal/mol), respectively. These results indicate that the dispersions belong to α1 and α2 mechanisms, respectively, which have been reported by a number of authors.