Well-defined diblock copolymers of polystyrene and smectic side-chain liquid crystalline siloxanes have been prepared with a wide range of molecular weights (M, total from 20,000 to 165,000) and Liquid crystalline siloxane weight fractions (0.4-0.91). Two different types of block copolymers were examined, each series having a different mesogen attached to the siloxane block. Increasing the rigidity of the mesogen led to stronger microphase segregation between the PS and LCP blocks, and to a higher T,and LC clearing point for the LCP block. Samples with large LCP weight fractions (>0.8) and low T-g (< - 25 degreesC) LCP blocks were elastomeric at room temperature, presumably because of the high molecular weight of the siloxane block (80-130 K). As the mesogen choice and the block lengths were varied, four general types of morphologies were observed using transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS): hexagonally packed LC cylinders, alternating PS-LCP lamellae, weakly ordered PS cylinders, and hexagonally packed PS cylinders. The PS cylinder morphology persisted to unusually high LCP weight fractions.