The Anisotropy of Young's Modulus in Drawn Polyethylene

Abstract

The macroscopic physical properties of a partially crystalline polymer become anisotropic when the polymer is `drawn', i.e. when the polymer undergoes a large permanent deformation from its isotropic state. The anisotropy of refractive index, X-ray scattering etc. has been investigated by many workers. In this paper the anisotropy of Young's modulus in samples of polyethylene, drawn by simple tensile loading so as to have transverse isotropy, is studied experimentally at room temperature, 20°C. According to classical elasticity theory the Young's modulus of a transversely isotropic material can be defined completely in terms of three independent parameters. These have been chosen to be E0, E45 and E90, the Young's moduli at angles 0°, 45° and 90° to the symmetry axis respectively. It is found that: (a) the polar distribution of Young's modulus in highly drawn polyethylene (draw ratio 4.65) agrees well with the distribution predicted by classical elasticity theory; (b) at draw ratios up to approximately 1.2, E0, E45 and E90 all decrease from the isotropic value and E45 > E90 > E0; (c) at draw ratios greater than 1.2, E0 and E90 both increase with increasing draw ratio but E45 continues to decrease; (d) in the highly drawn material, draw ratio approximately 4.65, E0 similar, equals 1.5E90 similar, equals 12E45 similar, equals 5Eisotropic°