OSTWALD RIPENING IN POLYETHYLENE BLENDS

The coarsening of phase-separated domains in a molten blend of two ethylene-octene copolymers has been observed by studying electron micrographs of rapidly quenched samples. Blends containing 10% of a 2 mol% ethylene-octene copolymer (with 90% of an 8 mol% ethylene-octene copolymer) were held in the melt for various times at a temperature at which phase separation was known to take place. The diameters of the regions rich in 2 mol% copolymer (the minority phase) were measured and shown to increase with the cube root of the time in the melt. It is argued that the coarsening occurs through the process of 'Ostwald ripening'. The results are similar to those previously reported by others on blends of polyethylene with hydrogenated polybutadienes and blends of polypropylene with ethylene-propylene rubbers. It is argued that the fact that coarsening is observed is strong confirmation that liquid-liquid phase separation does indeed occur in blends of linear with lightly branched copolymers. Hence, we conclude that the biphasic morphologies observed after quenching are not an artefact of crystallization but reflect genuine separation in the melt.