Ionic liquids confined in porous matrices: Physicochemical properties and applications

The molecular self-diffusion coefficients in nematic and smectic-$A$ thermotropic liquid crystals are measured using stimulated-echo-type ${}^{2}\mathrm{H}$ and ${}^{1}\mathrm{H}$ pulse-field-gradient spin-echo nuclear magnetic resonance (PGSE NMR) combined with multiple-pulse dipolar decoupling and slice selection. The temperature dependence of the principal components of the diffusion tensor in the nematic phase follows a simple Arrhenius relationship except in the region of nematic-isotropic phase transition where it reflects, merely, the decrease of the molecular orientational order. The average of the principal diffusion coefficients in the isotropic-nematic phase transition region is close to the diffusion coefficient in the isotropic phase. At the nematic\char21{}smectic-$A$ phase transition the diffusion coefficients change continuously. The results in nematic phase are best described in terms of the affine transformation model for diffusion in nematics formed by hard ellipsoids. In the smectic-$A$ phase the data are interpreted using a modified model for diffusion in presence of a periodic potential along the director.

Anisotropic self-diffusion in thermotropic liquid crystals studied by 1H and 2H pulse-field-gradient spin-echo NMR.

Polymer-dispersed liquid crystals (PDLCs) are liquid-crystal dispersions within a polymer matrix. These films can be changed from an opaque to a transparent state by applying a suitable alternating-current electric field. PDLCs have attracted the interest of researchers for their applications as light shutters, smart windows, and active displays. For such applications, electrochromic devices, which change color as a result of electrochemical reactions, have also become a recent focus of research. Herein, we report our preliminary results on bifunctional devices based on PDLCs that host electrochromic guest molecules. Such devices allow both an independent and fast switching from a scattering opaque state to a transmissive transparent state owing to liquid-crystal reorientation and a color change from white (pale yellow) to dark blue, due to either oxidation or reduction of the electrochromic molecules.

Electrochromic Polymer‐Dispersed Liquid‐Crystal Film: A New Bifunctional Device

Relaxation and the dynamics of molecules in the liquid crystalline phases

We present the measurements of the proton spin-lattice relaxation time ${T}_{1}$ of liquid crystal 4-$n$-octyl-${4}^{\ensuremath{'}}$-cyanobiphenyl (8CB) confined into randomly oriented $\ensuremath{\sim}15\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ pores of untreated porous glass. In the low kilohertz range the spin-lattice relaxation rate in the nanoconfined 8CB is about ten times larger than in the bulk. We show that the increase is mainly due to molecular reorientations mediated by translational displacements (RMTD). In the paranematic phase the power law describing the RMTD dispersion, ${({T}_{1}^{\ensuremath{-}1})}_{\mathit{RMTD}}\ensuremath{\propto}{\ensuremath{\omega}}^{\ensuremath{-}p}$, is well characterized by the exponent $p=0.5\ifmmode\pm\else\textpm\fi{}0.06$ and suggests an equipartition of diffusion modes with different wavelengths. The largest distance related to the decay of the orientational correlation function is about twice the diameter of the cavity. The situation is different in the nematic phase, where the orientational correlation is eventually lost at $\ensuremath{\sim}60\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in the direction along the pore, a distance corresponding roughly to the length of a pore segment in the glassy matrix. The exponent $p$ is between 0.65 and 0.9, depending on the temperature, which implies that in the nematic phase long wavelength modes are relatively more important---a consequence of the uniform director field along the pore. These observations are in agreement with the model of mutually independent pores with nematic director parallel to the pore axis in each segment. We point out that in strongly confined liquid crystals the proton NMR relaxometry does not provide the evidence of director fluctuations correlated over micrometer distances as was suggested earlier. The local translational diffusion of molecules within the cavities is found about as fast as in bulk.

G. Althoff

Papers

Surface-induced order and diffusion in 5CB liquid crystal confined to porous glass