Elastic moduli of highly oriented polyoxymethylene

The five independent elastic moduli C 11 , C 12 , C 13 , C 33 , and C 44 for uniaxially drawn polyoxymethylene with draw ratio λ = 1‐16 have been measured from −60 to 120°C by an ultrasonic method at 10 MHz. Wide angle x‐ray diffraction and birefringence measurements indicate that the chains in the crystalline regions are fully aligned at λ = 5 but the degree of amorphous orientation increases steadily up to the highest draw ratio. Below the major amorphous γ relaxation (∼0°C at 10 MHz), only the axial longitudinal and tensile modulus, C 33 and E 0 , show large increases. At 120°C, however, all the moduli increase with λ, with E 0 , E 90 (transverse tensile modulus), C 44 (axial shear modulus) and C 66 (transverse shear modulus) increasing 10, 2.5, 3, and 2.5 times, respectively, as λ rises from 1 to 16. This improvement in mechanical properties may be attributed largely to the effect of taut tie‐molecules and crystalline bridges. Whereas the two‐parameter model of Seferis seems to be valid only below the γ relaxation, the Halpin‐Tsai equation can give a reasonable description of the mechanical behavior for the highly oriented samples (λ>5) over the entire temperature range.