Phase structure and morphological variations induced by straining in injection-moulded samples of PA6/EVA blends: influence of molecular structure of EVA copolymers

Abstract

The influence of molecular structure and composition of ethylene-co-vinyl acetate (EVA) copolymers, differing in molecular mass and, for a given molecular mass, in vinyl acetate content (wt/wt), on morphological and structural changes undergone by injection-moulded samples of Nylon 6 (PA6)/EVA blends during uniaxial tensile straining at room temperature up to sample rupture and on properties of such materials was studied. Direct correlations between molecular structure and composition of EVA copolymers and the shape and size of domains of dispersed phase and ultimate properties of PA6/EVA materials have been drawn. For the dumb-bell-shaped specimens strained just beyond the yielding point, it was found that the original layered structure was strongly modified by straining. Three layers were, in fact, generated: a skin surface, where no domains of EVA dispersed phase were observed, an outer layer, where the EVA domains were elongated along the draw direction assuming mainly cylindrical shape, and a core where the EVA phase was segregated in ellipsoidal-shaped domains having their major axis oriented along the draw direction. For the samples strained to break it was found that the PA6/EVA fibres showed a single glass transition temperature, whose value is lower than that found for the fibre of plain PA6, but higher than that shown by PA6 phase in the unstrained samples; such a value, moreover, tends to be decreased on increasing the vinyl acetate content along the EVA chain, irrespective of copolymer molecular mass. The calorimetric measurements performed revealed, moreover, a noticeable increase of the observed melting temperature values of EVA phase in PA6/EVA fibres. In the PA6/EVA samples strained to break, only two different layers were seen: an outer skin, free of EVA domains, and a core where the EVA dispersed phase segregated in ellipsoidal-shaped domains. The values of stress at break, σB, and of the elongation at break, ɛB, exhibited by PA6/EVA fibres were found to be lower than those shown by the plain PA6. The trends of σB and ɛB values against the EVA vinyl acetate content showed, moreover, that σB and ɛB values decreased with increasing vinyl acetate content along the copolymer chain.