Mechanical relaxations and moduli of isotropic and oriented linear low‐density polyethylene

Abstract

AbstractThe mechanical relaxations of isotropic and oriented linear low‐density polyethylene have been studied between −180 and 90°C by the use of a torsion pendulum (1 Hz), a dynamic tensile apparatus (5–90 Hz), and a longitudinal wave attenuation technique (10 MHz). The five independent elastic moduli of the oriented samples have also been measured from −60° to 50°C by an ultrasonic method. Wide‐angle X‐ray diffraction and birefringence measurements reveal that the chains in the crystalline phase are fully aligned at draw ratio λ = 4, but the degree of amorphous orientation increases steadily, indicating that the number of well‐aligned tie molecules continues to increase up to the highest attainable draw ratio (λ = 6.1). The slight depression of the γ and β relaxation peaks upon drawing results from a lowering of molecular mobility in the amorphous and interfacial regions due to the constraining effect of taut tie molecules. At low temperature, the sharp rise in the axial Young's modulus E0 and slight drop in the transverse modulus E90 with increasing λ largely reflects the overall chain orientation. Just above the β relaxation, the stiffening effect of taut tie molecules leads to increases in all moduli including the shear moduli. The two‐phase Reuss (uniform stress) model provides an adequate description of the elastic anisotropy only at λ < 3, mainly because the internal stress distribution in a highly oriented sample is not uniform but corresponds to a situation intermediate between uniform stress and uniform strain field.