MICELLIZATION OF POLY(ETHYLENE OXIDE) POLY(PROPYLENE OXIDE) BLOCK-COPOLYMER IN AQUEOUS-SOLUTION - EFFECT OF POLYMER IMPURITIES

The effect of polymer impurities on the micellization in aqueous solutions of a PEO-PPO-PEO triblock copolymer [PEO and PPO are poly(ethylene oxide) and poly(propylene oxide), respectively] was modeled with a mean-field lattice theory for multicomponent mixtures of copolymers with internal states occurring in heterogeneous systems. A PEO-PPO diblock copolymer and PEO and PPO homopolymers were considered as impurities. The critical micellar concentration (cmc), the aggregation number, and the fraction of impurities as well as the spatial distribution of segments in the micelle have been investigated. In particular, the presence of the diblock impurity reduces the cmc and increases the aggregation number. At the cmc, the diblock impurity aggregates more readily than the triblock copolymer, and the fraction of the impurity in the micelles could be a factor of 4 larger than the stoichiometric fraction. The width of the PO/EO interface in the micelles was not affected by the presence of the diblock impurity. The PPO homopolymer also reduces the cmc but to a lesser extent. The existence of polymer micelles increases the solubility of PPO by several orders of magnitude by solubilization of PPO in the hydrophobic core of the micelle. Despite the low solubility of PPO in aqueous solution, ca. 10% of the micellear volume could consist of solubilized PPO. The amount of PPO solubilized per micelle increases strongly with temperature and with the volume fraction of free PPO but only weakly with the volume fraction of the triblock copolymer. The effect of the PEO impurity on the micellization was negligible. It was also concluded that the repulsive interaction among the PEO headgroups determines the growth of the micelles.