ASYMMETRIC DIBLOCK COPOLYMERS NEAR THE MICROPHASE SEPARATION - LIMITATIONS OF MEAN FIELD-THEORY

The microphase separation transition in five asymmetric diblock copolymers {poly(styrene-block-isoprene)} is studied using small angle X-ray scattering. In the disordered phase Leibler's random phase approximation together with a polydispersity correction yields a quantitative description of the scattering profiles. Close to the microphase transition we find strong deviations from mean field theory. The peak intensity and the apparent interaction parameter are smaller than expected. The Frederickson-Helfand theory explains these effects only qualitatively in terms of concentration fluctuations. The polymer coil size is found to increase when approaching the microphase transition.