Self-assembly of liquid-crystalline block-copolymers for responsive nematic gels

Abstract

Liq. crystals (LCs) are combined with polymers to create soft, rapidly responsive, reversible gels using selfassembly of block copolymers having a very long side-group liq.-cryst. polymer (SGLCP) midblock with LCphobic endblocks. The unusually high mol.-wt. SGLCP domains used allow gels to form at relatively low concns. that are amenable to alignment by surface anchoring, shear, or magnetic fields. The high content of small-mol. liq. crystal (>95%) allows them to reorient quickly. At elevated temps., above the nematic to isotropic transition, the gel dissolves due to the sudden change in solvent quality for the end blocks. To understand the reversible self-assembly process, we examine diblock polymers that form responsive micelles rather than gels. Small angle neutron scattering (SANS) reveals that the micelles' anisotropic cores have long axes orthogonal to those of the SGLCP corona chains. Flipping the side group orientation from parallel to the backbone (side-on attached mesogens) to perpendicular to the backbone (end-on attachment) flips the anisotropy of both the core and the corona relative to the nematic director. Thus, SANS shows us that the innate orientation of the mesogenic side groups relative to the flexible backbone in the SGLCP block dictates a hierarchy of anisotropy. When the corona chains are SGLCPs having a prolate conformation due to side-on mesogen attachment, the corona mirrors their prolate shape, elongated parallel to the director; in contrast, the micelle core is oblate with its short axis along the director. For SGLCPs that are oblate due to end-on attached mesogens, the corona again has a similar shape and orientation relative to the director. Pairs of polymers with matched LC-phobic blocks have identical size and shape of the core: interfacial tension penalizes prolate cores and dominates over corona crowding in detg. the aggregation no. of the micelle core. Effects of the relative lengths of the coil and SGLCP domains will be described, along with implications for LC physics and technol.