Two-dimensional self assembly of polystyrene-b-poly (butyl-methacrylate) diblock copolymers

We report a study of the two-dimensional self assembly of polystyrene-poly(n-butyl methacrylate) and polystyrenepoly(t-butylmethacrylate) nonionic diblock copolymers at the air-water interface. Langmuir film balance experiments, transmission electron microscopy and atomic force microscopy show that both of these diblocks form surface micelles. Starfish, rod and planar morphologies are observed, similar to those formed by copolymers where one block is ionic. The boundaries between the various morphologies in he nonionic systems occur at lower PS (polystyrene) content than in the ionic system. A plateau, indicative of a phase transition, is seen in isotherms when the PBMA block exceeds a certain length. The temperature dependence allows us to estimate the entropy change associated with the transition. Hysteresis effects are seen on cycling the compression and expansion of the films, and the phase transition is shown to be reversible. We suggest that the transition is associated with a pressure-induced reorientation of the ester side chains. These studies further establish the broad generality of the surface micellization phenomenon and assist us in our understanding the relationships between polymer conformations, polymer surface properties and the self-assembly properties of polymeric surfactants. (C) 1998 Elsevier Science B.V.