Micelle Formation and Gelation of (PEG P(MA-POSS)) Amphiphilic Block Copolymers via Associative Hydrophobic Effects

A series of well-defined amphiphilic di- and triblock copolymers have been synthesized, using atom transfer radical polymerization, with poly(ethylene glycol) (PEG) and poly(methacrylisobutyl polyhedral oligomeric silsesquioxane) P(MA-POSS) as the hydrophilic and hydrophobic blocks, respectively. The detailed self-assembly behavior of the amphiphilic macromolecules in aqueous media was studied using both static and dynamic light scattering (SLS and DLS) techniques. The evolution of block copolymer micelle formation in THF/water mixture (20/80 v/v) was monitored as the THF evaporated from the solvent mixture. Initially the block copolymer chains existed as unimers in solution, followed by the formation of smaller aggregates (R(h) < 2 nm) after 30 min, eventually growing in size to reach an equilibrium size when all the THF evaporated within 24 h. The micelles formed by the block copolymers were found to be kinetically unstable (not frozen); i.e., they tended to revert to individual copolymer chains on dilution. The hydrodynamic radii, R(h), of the micelles varied with the degree of polymerization (DP) of the hydrophobic P(MA-POSS); for example, for PEG(5K)-b-P(MA-POSS), an increase from R(h) 13.3 +/- 1.1 nm to R(h) similar to 17.5 +/- 1.4 nm was observed with a nominal change in the DP of P(MA-POSS) from 4 to 6. The micelles formed by the triblock copolymers (P(MA-POSS)-b-PEG(10K)-b-P(MA-POSS)) were comparable in size to the diblock copolymer micelles; e.g., R(h) similar to 14.0 +/- 1.3 nm was found for P(MA-POSS)4-b-PEG(10K)-b-P(MA-POSS)(4). The micellar structures created by the triblocks in aqueous media were flowerlike, where the PEG middle block adopted a loop conformation in the micelle corona. In addition to micelles, larger aggregates formed by P(MA-POSS)-b-PEG(10K)-b-P(MA-POSS) were also detected in solution. The larger aggregates may suggest a contribution from some PEG blocks adopting an extended conformation with one end dangling in solution, causing gelation at higher copolymer concentrations via intermicellar interactions. The P(MA-POSS)(4)-b-PEG(10K)-b- P(MA-POSS)(4) formed a gel in water at 8.8 wt % copolymer concentration. No gel formation by diblock copolymers was observed; however, the addition of a small amount of triblock copolymer to an aqueous solution of diblock copolymer results in gel formation. Finally, rheological behavior of the obtained gels was also investigated.