F. Kleinschmidt, M. Hickl, K. Saalwächter, C. Schmidt, H.
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 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.