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

Binary blends of compositionally symmetric polystyrene-polybutadiene dibloc k copolymers are investigated as a function of chain length ratio and blend composition using small-angle neutron scattering. Three different low mola r mass copolymers were blended with a high molar mass copolymer. The result s 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 copoly mers is discussed and the variation of the lamellar thickness with the volu me fraction of short chains in the one-phase state is compared with theoret ical predictions.