Size‐exclusion chromatography of solutions of block copolymers in selective solvents

Solutions of polystyrene‐block‐poly(ethylene/propylene) and polystyrene‐block‐poly(ethylene/butylene)‐block‐polystyrene copolymers in 5‐methyl‐2‐hexanone and 4‐methyl‐2‐pentanone were studied by size exclusion chromatography. The association equilibrium was studied shifting from micelles to free copolymer chains by increasing the temperature. A remarkable tailing was observed in diblock copolymer micelle peaks that may be explained by the disturbance and restablishment of the association equilibrium during separation in the chromatographic column. This phenomenon was very small for triblock copolymers. In this case, micelle hydrodynamic volumes obtained from SEC curves were compared to those calculated from light scattering and viscosity measurements. The data agreement was very good. Comicellization was analysed by mixing two different micelle solutions and injecting the solution mixture at different elapsed times. The SEC curves showed an intermediate peak corresponding to mixed micelles. The results show that three micelle species coexist in solution during the first period after mixing, which suggests that comicellization proceeds via dissociation of pure micelles and association of both copolymer free chains to form uniform mixed micelles.