Elastic moduli of ultradrawn polyethylene

The five independent elastic moduli C 11 , C 12 , C 13 , C 33 , and C 44 of oriented high‐density polyethylene with draw ratio λ from 1 to 27 have been determined from −60 to 100°C by an ultrasonic method at 10 MHz. At low temperature the sharp rise in the axial extensional modulus C 33 with increasing λ and the slight changes in the other moduli result from chain alignment and the increase in the number of intercrystalline bridges connecting the crystalline blocks. At high temperature (say, 100°C) the transverse extensional modulus C 11 , as well as the axial ( C 44 ) and transverse ( C 66 ) shear moduli, also show substantial increases, reflecting the prominent reinforcing effect of stiff crystalline bridges in this temperature region where the amorphous matrix is rubbery. If the crystalline bridges are regarded as the fiber phase, the mechanical behavior can be understood in terms of the Halpin–Tsai equation for aligned short‐fiber composites.