Lyotropic liquid-crystalline elastomers

Abstract

The linkage of nonionic amphiphiles via their hydrophobic ends to the monomer units of a polymer backbone leads to broad hexagonal (H₁) and lamellar (L_α) phase regimes in binary mixture with water. Cross-linking of these linear polymers yields rubber-like samples with elastomeric properties, which do not dissolve, but swell with a certain amount of water to form lyotropic mesophases. Mechanical deformation of samples in the mesophase causes a reversible alignment of the hexagonal or lamellar domains. Stressing of the gels during the synthesis of the elastomers enables to lock-in polymer anisotropy. As a result, spontaneously well-aligned LC-samples are produced by swelling with water. The type of alignment depends on the symmetry of the mesophase and the mechanical field. Due to the scaling between macroscopic dimensions and local anisotropy, changes of micellar shape and mesophase structure are visualized as changes of the sample length. This is measured by the hygroelastic method, a combination of simultaneous measurements of water sorption and sample length. It can be shown that the phase transformation between isotropic (L₂) and L_α-phase, which is driven by the change of partial vapor pressure of H₂O at constant temperature, is accompanied by a discontinuous lengthening of the network in the direction of the stress applied during synthesis. Due to its highly anisotropic swelling behavior, the sample length remains nearly constant within the L_α-127-3-phase, although water is absorbed and the volume increases.