Electro-optic Properties and Phase Behavior of Chiral-nematic Molecules

摘要

We will discuss electro-optic properties and phase-behavior of lyotropic chiral-nematic materials under electric fields. (Ⅰ) Fingerprint rolls of a chiral nematic between two antiparallel-rubbed ITO substrates are formed at low electric field strengths. This pattern can be photo-stabilized through UV polymerization of added monomers by means of an UV focal-conic Gaussian laser beam. The chiral nematic liquid crystalline molecules are still free to diffuse within the polymer-stabilized confining structure. The resulting stabilized pattern can be used as a highly diffractive electrically switchable polymer-stabilized cholesteric diffraction grating (PSCDG). The regular spatial variation of chiral-nematic material gives rise to strong light intensity in forward diffraction peaks, together with weaker intensity peaks of half the spacing from the polymer matrix. The electro-optic properties and dynamical modes of these gratings depend on the dynamics of the chiral-nematic liquid within the polymer matrix, which is found to depend on whether isotropic or mesogenic monomers are used. We found a strong effect of confinement on the dynamics of chiral-nematic order fluctuations in the mesogenic polymer network for scattering vectors along the helicoidal axis of the fingerprint-roll structures. (Ⅱ) Preliminary experiments on the phase-behavior of fd-virus suspensions under external electric fields will be shortly discussed. A phase diagram in the electric field-amplitude versus frequency plane will be presented. At low ionic strengths, electric field-induced polarization of the double layer induces an isotropic to chiral-nematic phase transition. At higher field strengths we observe melting of the chiral nematic state, which is probably due to either induced dipole-dipole interactions or the finite relaxation time of the double-layer charge density.