Thermal response of cholesteric liquid crystal elastomers

The effects of temperature variation on photonic properties of cholesteric liquid crystal elastomers (CLCEs) are investigated in mechanically unconstrained and constrained geometries. In the unconstrained geometry, cooling in the cholesteric state induces both a considerable shift of the selective reflection band to shorter wavelengths and a finite degree of macroscopic expansion in the two directions normal to the axis of the helical director configuration. The thermal deformation is driven by a change in orientational order of the underlying nematic structure $S$ and the relation between the macroscopic strain and $S$ is explained on the basis of the anisotropic Gaussian chain network model. The helical pitch varies with the film thickness in an affine manner under temperature variation. The CLCEs under the constrained geometry where thermal deformation is strictly prohibited show no shift of the reflection bands when subjected to temperature variation. This also reveals the strong correlation between the macroscopic dimensions and the pitch of the helical director configuration.

Figure 1

Chemical structure of a monoacrylate chiral mesogen, miscible LC solvent, and cross-linker.

Figure 2

(a) Transmission spectra of the CLCE film in unconstrained geometry as a function of temperature for the incident light with right- or left-circular polarization ($R$* and $L$*, respectively). (b) Color of reflected light for $R$* from the CLCE film at each temperature.

Figure 3

(a) Central wavelength $\left({\mathrm{\Lambda }}_R\right)$, (b) minimum transmission of the reflection band $T_{min}$, and (c) full width at half minimum $\left(\mathrm{\Delta }\mathrm{\Lambda }\right)$ reduced by ${\mathrm{\Lambda }}_R$ as a function of temperature in the unconstrained and constrained geometries with a compressive strain of ${\varepsilon }_z=0.15$.

Figure 4

Macroscopic dimensions of the CLCE film as a function of temperature. The dimensions in the directions normal $\left(x\right)$ and parallel $\left(z\right)$ to the helical axis are reduced by those in the high-temperature isotropic state at $130{}^{\circ }\mathrm{C}$.

Figure 5

Relation between the central wavelength of the reflection band ${\mathrm{\Lambda }}_R$ and film thickness.

Figure 6

Relation between ${\lambda }_x$ and $\mathrm{\Delta }\mathrm{\Lambda }/{\mathrm{\Lambda }}_R$ for the CLCE film in the range $T_g\le T\le T_{CI}$. The solid line represents the fitted result of Eq. (3) assuming $S_B=a\mathrm{\Delta }\mathrm{\Lambda }/{\mathrm{\Lambda }}_R$ with $a=1.55$ for the data, excepting those near $T_g$.

Figure 7

Transmission spectra of the CLCE film as a function of temperature in the constrained geometry where thermal deformation is prohibited for the incident light with (a) right- or (b) left-circular polarization ($R$* or $L$*, respectively). The dotted lines depict the corresponding data for $R$* in the unconstrained geometry.