PHASE-STRUCTURE AND MORPHOLOGICAL VARIATIONS INDUCED BY STRAINING IN INJECTION-MOLDED SAMPLES OF PA6 EVA BLENDS - INFLUENCE OF MOLECULAR-STRUCTURE OF EVA COPOLYMERS

The influence of molecular structure and composition of ethylene-co-vi nyl acetate (EVA) copolymers, differing in molecular mass and, for a g iven molecular mass, in vinyl acetate content (wt/wt), on morphologica l and structural changes undergone by injection-moulded samples of Nyl on 6 (PA6)/EVA blends during uniaxial tensile straining at room temper ature up to sample rupture and on properties of such materials was stu died. 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, i t was found that the original layered structure was strongly modified by straining. Three layers were, in fact, generated: a skin surface, w here 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 segregat ed in ellipsoidal-shaped domains having their major axis oriented alon g the draw direction. For the samples strained to break it was found t hat the PA6/EVA fibres showed a single glass transition temperature, w hose value is lower than that found for the fibre of plain PA6, but hi gher than that shown by PA6 phase in the unstrained samples; such a va lue, moreover, tends to be decreased on increasing the vinyl acetate c ontent along the EVA chain, irrespective of copolymer molecular mass. The calorimetric measurements performed revealed, moreover, a noticeab le increase of the observed melting temperature values of EVA phase in PA6/EVA fibres. In the PA6/EVA samples strained to break, only two di fferent layers were seen: an outer skin, free of EVA domains, and a co re where the EVA dispersed phase segregated in ellipsoidal-shaped doma ins. The values of stress at break, sigma(B), and of the elongation at break, epsilon(B), exhibited by PA6/EVA fibres were found to be lower than those shown by the plain PA6. The trends of sigma(B) and epsilon (B) values against the EVA vinyl acetate content showed, moreover, tha t sigma(B) and epsilon(B) values decreased with increasing vinyl aceta te content along the copolymer chain.