Lattice modulus and crystallite thickness measurements in ultra-high modulus linear polyethylene

Measurements have been made of the apparent Young's modulus of the crystalline part of a number of ultra-high modulus polyethylene drawn tapes by observing the change in Bragg angle of the 002 X-ray reflection when the tapes are placed under stress. The variation with temperature has been measured and it is argued that the common limiting value of modulus reached by all samples at low temperature represents the true crystalline modulus and that the room temperature value, which is some 40% lower, is strongly suggestive of a morphology in which a considerable fraction of the material is non-crystalline and is located, from the mechanical point of view, essentially in parallel with the crystalline fraction. Accurate measurements of the linewidths of the 200, 020 and 002 X-ray reflections have been made in order to deduce the mean thickness of the crystalline elements in the three principal directions. The ‘a’ and ‘b’ thicknesses vary very little with draw ratio but the ‘c’ thickness increases at high draw ratios to a value more than twice as great as the (constant) long period determined from small-angle scattering.