Lamellar Liquid Single Crystal Hydrogels:  Synthesis and Investigation of Anisotropic Water Diffusion and Swelling

The synthesis and characterization of anisotropic liquid single crystal hydrogels (LSCHs) via photoinduced radical polymerization of magnetically aligned samples in the lyotropic mesophase are reported, which are stable against tensile stresses in all three dimensions. The hydrogels exhibit a lamellar phase (Lα) in the swollen state. The high mechanical stability is achieved by using a new type of cross-linker. Monomer and cross-linker molecules have almost the same chemical constitution, varying only in the number of polymerizable groups. The cross-linker is incorporated perfectly into the liquid-crystalline phase structure of the lyotropic liquid-crystalline monomers, yielding a network which has covalent bridges between the lamellae. The anisotropic hydrogels are characterized by a variety of methods on micro- and macroscopic length scales. Swelling with the nonselective solvent toluene shows that cross-linking within the liquid-crystalline phase causes an anisotropic topology of the network, which shows a memory effect even in the isotropic phase. Time- and temperature-dependent pulsed field gradient diffusion NMR measurements yield a ratio of the order of 10:1 for the self-diffusion coefficients of D2O perpendicular and parallel to the layer normal. A step in the diffusivity across the lamellae at 312−314 K is interpreted by a disruption of the lamellae caused by elastic forces due to anisotropic network deformation as a function of temperature or by an increased porosity of the membranes in analogy to a lamellar-to-sponge transformation. Hygroelastic measurements, in which the length and width of the hydrogels are measured as a function of their controlled water sorption, show an anisotropic swelling behavior consistent with the structure of a lamellar phase. The isotropic-to-lamellar phase transformation upon increasing water concentration leads to a flattening of the polymer coil along the layer normal. Swelling with water in the lamellar phase is anisotropic.