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Structure and Dynamics of Self-Assembled Liquid Crystalline Gels

Rafael Verduzco, Neal Scruggs, and Julia A. Kornfield. Chemical Engineering, California Institute of Technology, 1200 California Blvd., Mail Code 210-41, Pasadena, CA 91125

Block copolymer self-assembly yields materials organized on a variety of length scales. Here, we demonstrate that block copolymers self-assembled in a liquid crystalline solvent yield gels whose morphology is dependent on the order of the liquid crystalline phase. These gels also reveal fascinating properties that arise from the coupling of liquid crystalline order to polymer elasticity. ABA triblock copolymers are synthesized by grafting a liquid crystalline mesogen onto the midblock of an anionically synthesized polystyrene- 1,2 polybutadiene- polystyrene triblock copolymer. Self-assembled gels are prepared by swelling the high molecular weight ABA block copolymer with the small-molecule nematic liquid crystal (LC) solvent 4-pentyl-4'-cyanobiphenyl (5CB). The A blocks are nemato-phobic and micro-phase separate to form a self-assembled polymer network. By studying a model series of copolymer with differing endblock and midblock lengths we can tune the location of the structural transitions and infer the relevant molecular parameters for gel formation. Dynamic mechanical analysis reveals the dramatic changes in the gel structure across the nematic-isotropic transition temperature. Neutron scattering provides information on the structural length scales present in the material. Dynamic light scattering reveals novel relaxation behavior that arises from the coupling of the polymer network to the liquid crystalline order. Self-assembled liquid crystalline gels represent a new class of potentially useful materials that provide insight into the coupling between liquid crystalline order and polymer network morphology.