Study of Comicellization of Diblock Copolymers in a Selective Solvent

The size, aggregation number and other quantities which characterize the formation of micelles in a bidisperse solution of block copolymers which are chemically identical but have different block asymmetries has been studied in a selective solvent using LS and SANS techniques. Experimental results for the comicellization of two chemically identical block copolymers (high molecular weight, PS-PI, and a low molecular weight dPS-PI with a deuterated PS block) and two slightly incompatible copolymers (high molecular weight PS-HPI. and dPS-PI) in decane, a selective solvent for PI, are reported. Individually, each of the PS-HPI and PS-PI copolymers formed micelles with a PS core, which were stable over the entire temperature range covered by the experiments (25-70 degrees C). The small copolymer, dPS-PI, formed micelles with a highly swollen core at low temperatures and was molecularly dispersed above the critical micelle temperature (47 degrees C). Comicellization was observed for both pairs of copolymers studied over a broad range of mixing ratios. The mixed micelles were comprised of large copolymer (PS-HPI) or PS-PI and a portion of the small copolymer (dPS-PI). The amount of adsorbed dPS-PI copolymer increased with increasing weight fraction of this copolymer and decreased with increasing temperature. The critical comicellization temperature was estimated as 70 and 60 degrees C for the comicelles based on PS-HPI and PS-PI copolymers, respectively. The SANS data suggest that the small copolymer does not form a shell coating the PS core of the initial micelle, but is instead incorporated into the core where it forms a layer of mixed dPS and PS chains.