Polystyrene-poly(ethylene oxide) diblock copolymers form well-defined surface aggregates at the air/water interface

A polystyrene - poly(ethylene oxide) containing diblock copolymer, when spr ead at the air/water interface, spontaneously forms surface aggregates. Thi s surface aggregation is shown to be neither compression-induced, associate d with micellization in the spreading solvent, nor induced by the Langmuir- Blodgett film transfer process. We have previously found that such two-dime nsional surface aggregation occurs for diblock copolymers with a polystyren e block and a hydrophilic block of quaternized poly(vinylpyridine), poly(te rt-butyl acrylate), poly(n-butyl acrylate), or poly(dimethylsiloxane). The phenomenon has also been observed in films of polystyrene-poly(methyl metha crylate) by Rice and co-workers.' Indeed, whenever an appropriate imaging t echnique has been used, phase-separated domains with 30-100 nm length scale s have been observed, when amphiphilic diblock copolymers are spread at the air/water interface and transferred to solid substrates at appreciable sur face pressures. We therefore believe that the formation of surface aggregat es (often well defined) is a general phenomenon for hydrophobic-hydrophilic diblock copolymers. The implications of this phenomenon for the study of d iblock copolymers at the air/water interface are discussed, particularly in relation to studies using techniques which report properties averaged over the lateral dimensions of the film in question, such as specular neutron r eflectivity, specular X-ray reflectivity, and Brewster angle microscopy.