The elastic constants of ultradrawn polypropylene

The five independent elastic constants C 11 , C 12 , C 13 , C 33 , and C 44 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 f c and f a were also determined from wide‐angle x‐ray diffraction and birefringence; f c increases sharply at low λ and becomes saturated above λ = 8, but f a 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, C 33 and E 0 , show large increase. At 110°C, however, all the moduli increase with λ; E 0 , E 90 (tranverse tensile modulus), C 44 (axial shear modulus), and C 66 (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 f a . The results were analyzed in terms of the two‐phase models of Takayanagi and Seferis et al. The effects of annealing were also studied.