Liquid Crystal Polymer Gels

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Micrograph of LC physical gel The molecules of a liquid crystal (LC) tend to cooperatively orient in a preferred direction giving rise to useful properties such as birefringence, dielectric anisotropy, diamagnetic anisotropy, and orientational elasticity that are forbidden by symmetry in isotropic liquids. The coupling of order with fluidity makes LCs particularly intriguing materials because their orientation-dependent properties can be influenced by readily accessible fields. Perhaps the best-known example is the LCD where modest electrical fields induce drastic changes in the LC’s optical properties.

Picture taken from Nature Materials 3, 139-140 (2004) Liquid crystalline gels are formed by dissolving a triblock copolymer in a small molecule LC solvent (green ovals). LC side groups on the midblock (red ovals) confer solubility on the polymer, but LC-phobic endblocks (blue spheres) aggregate to avoid the solvent forming physical crosslinks. Just as LCs respond to relatively small electric fields, polymeric gels respond to modest mechanical stresses. When the two are combined, the polymer’s elasticity is coupled to the LC’s orientational order and the result is a material with unique electro-optic and mechano-optic properties. Such materials were first hypothesized by P. G. de Gennes almost forty years ago and a great deal of theory has since been developed. However, attempts to create model systems by which to test such theories have had limited success due to heterogeneity of network structure or the inability to access a dilute regime of polymer concentration. To create dilute LC gels that are homogeneous and well-characterized, we use block copolymer self-assembly: we have synthesized polymers having very long LC midblocks flanked by LC-phobic endblocks so that when dissolved in a small molecule LC the endblocks spontaneously aggregate to form a physical gel.

These anisotropic soft materials are currently being studied in our group so that we may understand a number of phenomena unique to such systems. Of particular interest is the gels’ dynamic response to electric fields, their behavior when mechanically deformed, and the structure and length scales associated with the polymeric network.

Publications on LC polymers from our group are referenced here.

Associated with this project:
• Neal Scruggs
• Raphael Verduzco