The effects of phase morphology and adhesion between phases on fracture of ethylene-vinylalcohol copolymer nylon 6/12 copolymer blends

The effects of phase morphology and the adhesion between phases of ethylene-vinylalcohol copolymer(EVOH)/nylon 6/12 copolymer blends on the fracture properties were estimated. Films of the blends which were obtained by extrusion processing showed different phase morphologies depending on the composition of the nylon 6/12 copolymer. The morphology of the partially miscible blend (EVOH and nylon 6(f)-nylon(1-f) where f=0.8) was needle-like in appearance. On the other hand the immiscible blend (EVOH and nylon 6(f)-nylon12(1-f) where f=0.5) had equiaxed particles of nylon 6/12. The plastic deformation of films of the blends was observed using transmission electron microscopy. Deformation zones were observed for both blends but extensive debonding of particle interfaces was observed in the immiscible blend system. These observations are reinforced by our measurements of the interfacial fracture energy, G(c), between EVOH and nylon 6(f)-nylon12(1-f) made using a double cantilever beam test. G(c) decreases monotonically as 1-f increases. The fracture toughness of the partially miscible blend film measured at low temperature (-80 degrees C) was higher than that of EVOH alone and there was fractographic evidence of a larger crack tip plastic deformation zone. In contrast, the fracture toughness of the immiscible blend was lower than that of EVOH and there was fractographic evidence of extensive debonding of the second phase nylon particles. This result suggests that it is important to have good adhesion between phases to achieve the optimum fracture toughness of these polymer blends. (C) 1998 Chapman & Hall.