INFLUENCE OF A LIQUID-CRYSTALLINE BLOCK ON THE MICRODOMAIN STRUCTURE OF BLOCK-COPOLYMERS

We report on the phase behavior and microdomain structure of two types of diblock copolymers containing a liquid crystal (LC) block joined t o a flexible coil block. Consideration of the symmetry groups of the l iquid crystalline phases and of the block copolymer microdomain struct ures provides a rationale for predicting the possible types of liquid crystalline block copolymer morphologies. Both previously reported and newly discovered structural types are identified. Possible organizati onal schemes are developed for the mesogens and periodic disclination defects with respect to the intermaterial dividing surfaces separating the liquid crystalline and flexible coil domains. The first type of c opolymer investigated has a rod-like LC block whereas the second type copolymer has a side chain LC block. Five different rod-coil diblocks based on poly(hexyl isocyanate-b-styrene) P(HIC-b-S) were synthesized by anionic polymerization. Wavy lamellae, zig-zag and arrowhead microd omain morphologies corresponding to smectic-C and smectic-O structures were observed depending on the composition. These layered phases have the director (PHIC chain axis) tilted at various orientations with re spect to the layer normal. Side-chain LC diblocks based on functionali zed poly(isoprene-b-styrene) P(I-b-S) were also investigated. These po lymers were synthesized using polymer analogous chemistry from P(I-b-S ) precursors. Three different mesogenic groups were attached to the PI blocks: one based on biphenyl benzoate and two based on azobenzene. T he microdomain structures found for the functionalized poly(isoprene s ide-chain LC-b-styrene) P(ILC-b-S) diblocks are typical of traditional coil-coil diblocks (lamellae and cylinders). However, these morpholog ies possess an additional smectic layering of the mesogens within the microdomains of the LC block. In the case of the rod-coil diblocks, th e transformation from an initially isotropic state to the final microp hase separated solid state occurs via nematic and then smectic liquid crystalline states, whereas for the side-chain LC-coil cases, the micr ophase separation transition occurs prior to development of orientatio nal order. The long-range microdomain order of LC block-coil block cop olymers can extend over very large distances due to the influence of t he orientational ordering of the LC block.