Deformation of cholesteric elastomers by uniaxial stress along the helix axis

Abstract

The deformation of cholesteric elastomers by mechanical stress applied parallel to the helix axis is studied by calculation of the free energy density. The Frank elasticity contribution is taken into account. A chiral solvent, present at cross-linking time, is in general considered to be replaced after cross-linking by a solvent with different chirality. Two special cases considered are zero and unchanged solvent chirality, the first known as that of imprinted cholesteric elastomers, the latter equivalent to intrinsic cholesteric elastomers with chemically attached chiral groups. Depending on material parameters and imposed strain, the director can show a tilt towards the helix axis up to the maximum tilt, corresponding to a nematic state. In case of intrinsic elastomers with low conformation anisotropy, direct transitions from untilted to nematic states can be induced by straining. The helix structure of the director field is coarsened with an average wavenumber different to that of the information inscribed in the network at cross-linking time, if this lowers the average free energy density. Switching between different states can be achieved with electric fields of reasonable values applied parallel to the helix axis. Spectra of the reflection of polarized light are calculated.