Block copolymers of polystyrene and side-chain liquid crystalline siloxanes: morphology and thermal properties

Well-defined diblock copolymers of polystyrene and smectic side-chain liquid crystalline siloxanes have been prepared with a wide range of molecular weights (Mn 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 Tg and LC clearing point for the LCP block. Samples with large LCP weight fractions (>0.8) and low Tg (<−25°C) 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.