Ultrasonic measurements of the elastic moduli of ultradrawn polyoxymethylene

Abstract

AbstractWe have employed an ultrasonic method to measure from −40 to 60°C the five independent elastic moduli C11, C13, C33, C44, and C66 of polyoxymethylene with draw ratio λ from 1 to 26 prepared by continuous drawing under microwave heating. The elastic moduli are controlled by three major factors: molecular orientation in the crystalline regions, fraction of noncrystalline taut tie molecules, and void content. The steep rise in the axial extensional modulus C33 and axial Young's modulus E0 with increasing draw ratio results from the alignment of chains in the crystalline blocks and an increase in the number of disordered taut tie molecules. Below the γ relaxation (located at 0°C at our measurement frequency of 10 MHz), these two factors also give rise to a slight decrease in the transverse extensional modulus C11, Young's modulus E90 and shear modulus C66. At high temperature where the amorphous regions have very low modulus, the stiffening effect of taut tie molecules becomes dominant, leading to an increase in all moduli as λ increases from 1 to 10. At higher λ the void fraction increases appreciably, causing small decreases in E90, C11, and C66 at all temperatures.