SOLUBILIZATION OF A HOMOPOLYMER IN A BLOCK-COPOLYMER

Blends containing styrene--butadiene diblock copolymer (.50 wt % styre ne content) and polystyrene of various molecular weights are studied b y light scattering, transmission electron microscopy, and small-angle X-ray scattering. The solubility of polystyrene in the styrene domain of the block copolymer is governed by the ratio of the homopolymer mol ecular weight to the block molecular weight. A finite solubility limit exists when this ratio exceeds approximately 1. The lamellar repeat p eriod increases linearly as more polystyrene is added, but the butadie ne layer thickness remains constant, signifying that the average inter facial area occupied by a copolymer junction point does not change wit h added polystyrene. This contrasts to the case found by others that t he average area per junction point increases when the added homopolyme r is smaller than the block size. Small-angle X-ray scattering pattern s obtained from samples having lamellar morphology are described by an idealized model in which layers of styrene and butadiene of randomly varying thicknesses with a diffuse interface between them are stacked parallel.