Failure mechanisms in compatibilized blends of linear low-density polyethylene and polystyrene

The tensile failure of blends of linear low-density polyethylene (LLDPE) and polystyrene (PS) compatibilized with block copolymers ofstyrene (S) and butadiene (B) or hydrogenated butadiene (EB) has been studied. Compatibilizers were compared at the 5 wt% level in blends with equal amounts of LLDPE and PS by volume. Because of the lower viscosity of LLDPE, the morphology consisted of spherical PS particles dispersed in an LLDPE matrix. The stress-strain curve of the compatibilized blends was composed of an initial linear region, followed by a region of decreasing slope to a second linear region with a small positive slope. The yield point was defined by the intersection of the two linear regions. A modified yield-strain approach was used to predict the yield stress of the compatibilized blends. As a result of good adhesion, yielding of the matrix was constrained by the rigid PS particles to a region determined by the yielding angle, φ. Most of the blends exhibited a yield strain in the range of 1.2 to 1.6%. The calculated yielding angle of 70° or less was in accord with predictions for a rigid sphere in a plastic matrix. The higher yield stress of blends with crystalline styrene-hydrogenated butadiene and styrene-fiydrogenated butadiene-styrene compatabilizers, as compared to blends with noncrystalline compatibilizers, resulted from their higher modulus. The exception was the blend compatibilized with Kraton G, which had a yield strain of 1.9% and a yielding angle of 79°.