Self-organization of diblock copolymers at polymer blend interfaces

Diblock copolymers can be used as emulsifiers to optimize the mechanical properties of polymer blends. We investigate the interface in thin polyparamethylstyrene (PMS)/polystyrene (PS) bilayer films with and without the presence of diblock copolymer P(S-b-PMS). Components are incompatible, but close to miscibility. Neutron reflectivity (NR) measurements were first carried out with PMS/PS bilayers at varying molecular weights and temperatures. The fits of the data reveal interface widths in the range between 9.4 and 19.6 nm. This corresponds in the framework of mean field theory to segment interaction parameters (χ) between 0.0033 and 0.0058 and a mean segment length (b) of 0.8 nm and is in good agreement to SANS investigations. For higher molecular weights the presence of diblock copolymer in homopolymer blends results in the segregation of the diblock copolymers to the interfaces and to an increase of the interface width between the two layers, which is in good agreement to other polymer systems. In nearly compatible systems, on the other hand, no significant segregation of the copolymer to the interface was observed.