Tensile drawing of ethylene/vinyl-alcohol copolymers. Part 1. Influence of draw temperature on the mechanical behaviour

Abstract

The uniaxial tensile drawing of two vinyl-alcohol-rich ethylene/vinyl-alcohol copolymers differing in composition and melt flow index is studied as a function of draw temperature. A change of mechanical properties occurs in the strain range of the strain-hardening threshold depending on whether the draw temperature is above or below a critical value of about 100°C. A strong propensity for necking and concomitant longitudinal fissuration of the films take place above this temperature for the copolymer of lower vinyl-alcohol content. The vinyl-alcohol richer copolymer, having a higher melting point and a lower melt flow index, exhibits similar trends at higher temperatures. Bulk materials also display longitudinal fissures originating from transverse craze-like defects located along the neck shoulder. It is suggested that the drastic weakening of the van der Waals (v.d.w.) interactions of paraffinic nature between the hydrogen-bonded sheets of the monoclinic crystal structure, above 100°C, results in a strong mechanical anisotropy. This may perfectly account for the fissuring trend thanks to the easy glide of the weakly interacting sheets. In contrast, a strain-induced disorganization of the crystalline structure triggers improved drawability to the copolymers at low draw temperature, owing to the mechanical isotropy of the new mesomorphic structure type.