Craze plasticity and toughness of particulate block copolymer blends

Abstract Blends of KRO-1 Resin in high molecular weight polystyrene in which the K-Resin appears as dispersed spherical composite particles with the characteristic K-Resin morphology have been modified by incorporating into them additional polybutadiene of a molecular weight of 3 kg/mole (PB3K). It was found that small additions of PB3K into KRO-1 initiate first a morphological transformation resulting in particles with two coexisting morphologies of distorted rods and parallel lamellae. At a ratio of PB3K/KRO-1 of somewhat above 0.33 the transformation is complete and results in composite particles with a very regular concentric spherical shell morphology of PB and PS. The craze initiating effectiveness of composite particles peaks with this morphology, resulting in craze flow stresses as low as 8 MPa and strains to fracture in excess of 0.80 for a PB3K/KRO-1 ratio of 0.5. Larger volume concentrations of PB3K result in reprecipitation of free PB3K inside particles and this gives rise to a rapid degradation of toughness. The very effective craze initiating action of the spherical shell particles cannot be fully accounted for by the best techniques of numerical stress analysis of the elastic and thermal properties of the composite particles utilizing locally multiaxial craze initiation criteria that were developed for surface crazing in homo-PS. It is suspected that the craze initiation condition is locally relaxed around such composite particles by the presence of certain pre-existing ‘catalytic’ interface configurations that makes the nucleation of crazes from such particle interfaces more a heterogeneous nucleation phenomenon than a homogeneous nucleation phenomenon.