The elastic constants of ultradrawn polypropylene

Abstract

AbstractThe five independent elastic constants C11, C12, C13, C33, and C44 for uniaxially drawn polypropylene with draw ratios 1 < λ ≤ 20 have been measured from −40 to 130°C by an ultrasonic method (2.5 MHz). The crystalline and amorphous orientation functions fc and fa were also determined from wide‐angle x‐ray diffraction and birefringence; fc increases sharply at low λ and becomes saturated above λ = 8, but fa increases monotonically up to λ = 20. Below the glass transition temperature (ca. 60°C at 2.5 MHz) only the axial longitudinal and tensile moduli, C33 and E0, show large increase. At 110°C, however, all the moduli increase with λ; E0, E90 (tranverse tensile modulus), C44 (axial shear modulus), and C66 (tranverse shear modulus) increase 30, 3, 5, and 2.5 times, respectively, as λ increases from 1 to 20. The improvement in the mechanical properties can be attributed to the increase in the fraction of taut tie molecules which was monitored by fa. The results were analyzed in terms of the two‐phase models of Takayanagi and Seferis et al. The effects of annealing were also studied.