Self-assembly of mixed amphiphilic triblock copolymers in aqueous solution

A 1:1 (weight ratio) mixture of two commercial amphiphilic triblock copolymers, F127 (E99P69E99) and B20-5000 (E45B14E45) in aqueous solution, was studied by using static light-scattering and dynamic lightscattering techniques. E, P, and B are oxyethylene, oxypropylene, and oxybutylene, respectively, with the subscript denoting the number of segment units in the block. Both triblock copolymers tend to self-assemble into starlike micellar structures in aqueous solution because of the poorer solubility of their middle blocks at room temperature. Due to the difference in their thermodynamic parameters of the micellization process and in their micellar sizes, the mixed block copolymer system provides a suitable model for studying the self-assembly behavior of mixed, miscible triblock copolymers in aqueous solution. At temperatures below 27 degrees C, the E45B14E45 block copolymer has a much lower cmc than that of E99P69E99 Therefore, the micelle formation of the mixed polymer solution was determined by the cmc of E45B14E45. E99P69E99 chains gradually joined into the micelles at higher polymer concentrations. At temperatures above 33 degrees C, E99P69E99 had lower cmc values. Therefore, it micellized first, and then E45B14E45 chains gradually joined to form mixed micelles. At temperatures in between, where the two kinds of copolymers had similar cmc values, both unimers contributed to the cmc value in the mixed solution. In a special case, at the temperature where the two kinds of polymers had the same cmc value, mixed micelles comprised 50 wt % of each kind of polymer chains, with a cmc value equal to the sum of half of each of the two cmc values in its own pure polymer solution. The cmc value of the mixed polymer solution is the same as the cmc value of either of the two copolymer solutions. Other basic parameters of the mixed micelles, for example, association number (n(w)) and hydrodynamic radius (R-h), were also measured. The results are discussed in the text.