• F. Kleinschmidt, M. Hickl, K. Saalwächter, C. Schmidt, H. Finkelmann.
  Lamellar Liquid Single Crystal Hydrogels: Synthesis and Investigation of Anisotropic Water Diffusion and Swelling.
  Macromolecules. 38, 9772-9782 (2005).

  Abstract

  The synthesis and characterization of anisotropic liquid single crystal hydrogels (LSCHs) via photo-induced radical polymerization of magnetically aligned samples in the lyotropic mesophase is reported, which are stable against tensile stresses in all three dimensions. The hydrogels exhibit a lamellar phase in the swollen state. The high mechanical stability is achieved by using a new type of crosslinker. Monomer and crosslinker molecules have almost the same chemical constitution, varying only in the number of polymerizable groups. The crosslinker 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 non-selective 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 coefficients for the self diffusion 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 is 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.

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