A comparative study of UV-irradiation-induced refractive index modulation in two analogous monodomain nematic side-chain liquid-crystalline elastomer materials is conveyed. In one of them, mesogenic azobenzene derivatives are incorporated as pendant co-monomers, and in the other as crosslinking units. The dependence of the optical diffraction properties on the polarization state of the probe beam reveals that diffraction is predominantly of a bulk origin for both materials. For prolonged UV exposures, the material with pendant azobenzene derivatives exhibits practically constant diffraction properties, while the material with cross-linking azobenzene units shows a profound decrease of diffraction efficiency with increasing exposure time. The difference is attributed to photoinduced alignment of the azobenzene molecules, which is much stronger in the material with the crosslinking azobenzene units.
COBISS.SI-ID: 2603620
We investigate light-induced patterning of a monodomain side-chain liquid crystal elastomer (SC-LCE) doped with light-sensitive azobenzene moiety in the temperature region close to the nematic-paranematic phase transition. We show that a strongly nonlinear relationship between the concentration of the cis isomers of the azomesogens and the refractive index modification of the material, which is characteristic for the phase transition region, results in nonmonotonous time dependence of the diffraction efficiency of a probe beam. From this effect we determine the sensitivity of the nematic transition temperature on the molar fraction of the cis isomers. The relation between the cis isomer molar fraction and nematic order also provides a possibility for recording hidden holograms, which can be made visible by cooling the sample from the paranematic to the nematic phase.
COBISS.SI-ID: 2533988
We describe the complex time dependence of the buildup of force exerted by a clamped photoelastomer under illumination. Nonlinear (non-Beer) absorption leads to a bleaching wave of a significant cis isomer dye concentration deeply penetrating the solid with a highly characteristic dynamics. We fit our experimental response at one temperature to obtain material parameters. Force-time data can be matched at all other temperatures with no fitting required; our model provides a universal description of this unusual dynamics. The description is unambiguous since these are clamped systems where gross polymer motion is suppressed as a possible source of anomalous dynamics. Future experiments are suggested.
COBISS.SI-ID: 2603876
The kinetics of holographic recording of diffraction grating into thin films of a lightsensitive monodomain liquid crystal elastomer shows an irregular oscillatory behavior attributed to contraction of the film during the recording process. At temperatures far below the nematicisotropic phase transition, the spontaneous erasure kinetics exhibits exponential relaxation with relaxation time following the Arrhenius activation law. However, at temperatures close to the nematicisotropic phase transition, the erasure process shows an interesting nonmonotonic behavior that we attribute to the nonlinear relation between the concentration of the phototransformed chemical groups and the nematic order parameter.
COBISS.SI-ID: 25765927
The orientational order parameters of UBd(4) [1,4bis(undec10en1 yloxy)benzened(4)] in the ZLI1167 nematic mixture are studied by means of H2 NMR at variable temperature. UBd(4) is a typical crosslinker in liquid crystal elastomers (LCEs). We infer that the biaxial order parameter of the crosslinker in the LCE matrix is low enough to be neglected. The crosslinker UBd(4) is a reliable probe to monitor the nematic order in the LSCEUBd(4) sample.
COBISS.SI-ID: 25573927