Spontaneous bilayer phase separations of spin-coated polymer blend thin films: A neutron reflectivity study

This study investigates the spontaneous formation of phase-separated bilayer structures of polymer blend thin films upon spin coating. Neutron reflectivity is used to measure the bilayer structure of deuterated polystyrene ( d PS) and polymethylmethacrylate (PMMA) blend thin films with various molecular weights ( M w ). The results indicate that the binary polymer blend forms a well-defined phase-separated bilayer structure of a dPS-rich layer on top of a PMMA rich layer with clear interfaces between the layers. The interfacial width is in reasonable agreement with the results of self-consistent field theory modified to account for capillary waves. The blend with higher M w exhibits broader interfaces than the theoretical predictions, probably due to the reduced mobility of mixed polymers in the solvent during spin coating. Formation of thin blend films with parallel phase separation can provide a viable route to potential applications of the active layer in bilayer heterojunction devices for polymer photovoltaic applications.