PHASE-SEPARATION BEHAVIOR AND CREW-CUT MICELLE FORMATION OF POLYSTYRENE-B-POLY(ACRYLIC ACID) COPOLYMERS IN SOLUTIONS

Static light scattering was used to study the phase separation behavio r of homopolystyrene and polystyrene-b-poly(acrylic acid) (PS-b-PAA) i n DMF as a function of added water content. It was found that the crit ical water content (cwc), at which phase separation starts, depends on both the polymer concentration and the molecular weight. The higher t he polymer concentration and the molecular weight, the lower the cwc. For PS homopolymer, phase separation involves the precipitation of the polymer chains. In the copolymer solution, phase separation results i n the formation of regular crew-cut micelles consisting of a PS core a nd a PAA corona; thus, it is preferably referred to as microphase sepa ration. The change of the micelle fraction as a function of water addi tion can be estimated from the relationship between the cwc and the in itial copolymer concentration. The influence of added electrolytes, i. e., NaOH, HCl, NaCl, CaCl2, or Ca(Ac)(2), on the self-assembly process of the copolymers in DMF was also explored. Since the micelle cores a re highly swollen by DMF in the early stages of their formation, the s tructures are labile. As the added water content increases, the cores become gradually less swollen and the mobility of the polymer chains i n the cores decreases. A study of polymer chain exchange among the mic elles was performed by mixing two solutions of micelles of different s izes at different water contents and studying the micelle core size di stribution by transmission electron microscopy. It is shown that the c opolymer chain exchange within a 1 day period is significant at a wate r content of 6 wt %, but becomes negligible when the water content is increased to similar to 11 wt %.