FORMATION OF CAGE-LIKE PARTICLES BY POLY(AMINO ACID)-BASED BLOCK COPOLYMERS IN AQUEOUS-SOLUTION

When dissolved in N,N-dimethylformamide and then dialyzed against phosphate-buffered saline, A-B-A block copolymers composed of poly[N5-(2-hydroxyethyl)-L-glutamine]-block-poly(gamma-benzyl-L-glutamate)-block-poly[N5-(2-hydroxyethyl)-L-glutamine] form particles. The particles are cage-like structures with average diameters of 300 nm (average polydispersity, 0.3-4.5). They are stable in aqueous solution at 4-degrees-C for up to 3 weeks, at which time flocculation becomes apparent. Negative staining and freeze-fracture electron microscopy suggest that cage-like particles are formed by selective association of segregated micelle populations. A model of particle formation is presented in which B blocks form micelles in dimethylformamide. On dialysis against an aqueous solution, the extended A blocks then associate intermolecularly to form rod-shaped micelles, which connect the B block micelles. The result is a meshed cage-like particle. The implications of these observations on the aggregation behavior of polymeric surfactants in dilute solution are discussed.