Macrophase-separation in binary blends of symmetric polystyrene-polybutadiene diblock copolymers

Binary blends of compositionally symmetric polystyrene-polybutadiene diblock copolymers are investigated as a function of chain length ratio and blend composition using small-angle neutron scattering. Three different low molar mass copolymers were blended with a high molar mass copolymer. The results are related to the theoretical phase diagram put forward by M. W. Matsen (J. Chem. Phys. 103, 3268 (1995)). The conditions for macrophase-separation are established, an observed asymmetry of the lamellae rich in long copolymers is discussed and the variation of the lamellar thickness with the volume fraction of short chains in the one-phase state is compared with theoretical predictions.