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

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 degrees 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 degrees 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. (C) 1998 Elsevier Science Ltd. All rights reserved.