Liquid-crystalline behavior of rod-coil diblock copolymers

The exact mean-field phase behavior of the Semenov-Vasilenko model for rod-coil diblock copolymers is studied by applying self-consistent field techniques. The behavior depends on three quantities: the rod/coil immiscibility chi N, the coil volume fraction f, and the ratio nu of the characteristic coil to rod dimensions. When chi N less than or similar to 5, the rods and coils mix producing a nematic phase, and at larger chi N, they microphase separate forming a lamellar phase. The nonlamellar phases expected at f greater than or similar to 0.7 are not treated here. The lamellar phase is typically a smectic-C structure with monolayers of tilted rods. A thorough understanding of the model is achieved by closely examining segment distributions and various contributions to the free energy. The tilt angle theta is generally controlled by a competition between rod/coil interfacial tension and stretching of the coils. Lowering f reduces the latter contribution, causing theta --> 0 and producing a continuous transition to a smectic-A structure. Beyond that, there is a tendency to form structures with the rods arranged in bilayers, but this is strongly suppressed by a large steric penalty. We suggest that small amounts of solvent can greatly alleviate this penalty, and therefore could significantly affect some aspects of the phase behavior. (C) 1998 American Institute of Physics. [S0021-9606(98)51534-6].