Aggregation behaviors of a polystyrene-b-poly(methyl methacrylate) diblock copolymer at the air/water interface

Aggregation behaviors, in particular, surface micelle formation, of a diblock copolymer A-B, where the A block is polystyrene (PS, surface inactive and water insoluble) and the B block is poly(methyl methacrylate) (PMMA, surface active, but water insoluble), at the airwater interface have been examined. The block length ratio of PS:PMMA was 140K:646K. With the aid of a Langmuir film balance and an atomic force microscope (AFM), surface pressure isotherms, as well as the hysteresis and temperature effects on the isotherms and aggregation numbers, were determined, and a self-consistent picture of the surface morphology and the aggregation phenomena was obtained. Analogous to the critical micelle concentration in a bulk solution of a surfactant, the surface aggregation of the block copolymer took place only at a certain surface mass density and resulted in small circular micelles with an aggregation number around 6-7, as determined by AFM. The small aggregation number is attributed to the longer block length of PMMA relative to that of PS.