Photo-optical properties of photopolymerizable cholesteric compositions

Compositions consisting of nematogenic diacrylate monomer and chiral dopants capable of forming the cholesteric mesophase and of photopolymerizing were obtained and studied. For the first time, the change of optical properties and cholesteric helix pitch during photocrosslinking of diacrylate under UV irradiation (365 nm) was investigated. The kinetics of photopolymerization was studied and several stages of this process were observed. Cholesteric photopolymerizable blends containing chiral photosensitive dopant capable of E–Z isomerizing under UV irradiation (313 nm) were studied. The decrease of the helix twisting power of the dopant during photoisomerization was observed resulting in untwisting of the helix and shift of the selective light reflection peak into the long-wavelength region of the spectrum. The possibility of the photoregulation of optical properties of such blends with following fixation of structure and of these properties by means of photopolymerization under UV irradiation (365 nm) was demonstrated.     

Synthesis and characterization of side-chain cholesteric elastomers derived from an isosorbide crosslinking agent

New liquid crystalline monomer 4-(4-ethoxybenzoyloxy)biphenyl-4′-[(10-undecylen-1-yloxy)-4′-ethoxy]benzoate (M ₁ ), chiral crosslinking agent isosorbide di-(10-undecylen-1-yloxybenzoate) (M ₂ ), and the corresponding elastomers were prepared. The chemical structures of M ₁ and M ₂ were characterized by Fourier transform infrared and ¹H-nuclear magnetic resonance. The mesomorphic properties and phase behavior were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X-ray diffraction measurements. M ₁ exhibited typical threaded texture and droplet texture of nematic phase. The use of chiral crosslinking agent in the polymer networks could induce cholesteric phase. The elastomers containing less than 10 mol% of the chiral crosslinking units showed elasticity, reversible phase transition, wide mesophase temperature ranges, and high thermal stability. For the elastomers P ₂ –P ₄ , the glass transition temperature (T _g) increased; clearing temperature (T _i) and mesophase temperature range (ΔT) decreased with increasing content of the crosslinking unit.     

Electrically induced anchoring transition in cholesteric liquid crystal cells with different confinement ratios

Reorientation of cholesteric liquid crystal induced by the electrically controlled ionic modification of surface anchoring within the cell with confinement ratio exceeding 1 has been studied. The change of homeotropic surface anchoring to the planar one on the electrode-anode substrate under the action of DC voltage causes the formation of the modulated hybrid-aligned cholesteric layer in the cell. Optical texture of the liquid crystal layer with such an orientation structure is the linear periodic stripes. Homogeneity of emerging optical texture depending on the confinement ratio as well as on the prehistory of voltage application has been considered. It has been found that the ionic modification of surface anchoring results in total transformation of the diffraction pattern observed after the laser beam passing through the sample.     

On the existence of the thermomechanical terms of Akopyan and Zel’dovich in cholesteric liquid crystals

We revisit a theoretical paper of Akopyan and Zel’dovich about the thermomechanical coupling terms in nematic liquid crystals. We show that the expressions of these terms given by these authors must be corrected to satisfy the Onsager reciprocity relations, a point already stressed by Pleiner and Brand in 1987. We then extend this calculation to the cholesteric phase and show that there are no additional terms in the uniaxial approximation of this phase. Finally, we give the correspondence between the Akopyan and Zel’dovich terms and those calculated by Pleiner and Brand in 1996 by making a different choice for the forces and the fluxes in the theory.     

An experimental and computational study of calamitic and bimesogenic liquid crystals incorporating an optically active [2,2]-paracyclophane

Two liquid-crystalline materials containing an optically active (R)-4-hydroxy-[2,2]-paracyclophane group were prepared, one in which the chiral group is a bulky terminal unit and one in which it forms part of a terphenyl-like mesogenic unit. Both materials exhibit monotropic chiral nematic phases. Partial phase diagrams were constructed for mixtures of both materials with 5CB, allowing us to extrapolate pitch lengths and helical twisting power values (HTP) for each material. The HTP value of the material with a ‘locked’ paracyclophane is 70% higher than that of a ‘free’ paracyclophane and this is rationalised as being due to the reduction in conformational freedom of the former material relative to the later.     

Mesomorphic properties of side-chain cholesteric liquid-crystalline elastomers

New monomer cholesteryl 4-(10-undecylen-1-yloxybenzoyloxy)-4′-ethoxybenzoate (M₁), crosslinking agent biphenyl 4,4′-bis(10-undecylen-1-yloxybenzoyloxy-p-ethoxybenzoate) (M₂) and a series of side-chain cholesteric elastomers were prepared. The chemical structures of the monomers and elastomers obtained were confirmed by element analyses, FT-IR, and ¹H NMR. The mesomorphic properties and thermal stability were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X-ray diffraction measurements. The influence of the content of the crosslinking unit on the phase behavior of the elastomers was examined. M ₁ showed cholesteric phase, and M ₂ displayed nematic phase. The elastomers containing less than 12 mol% of the crosslinking units revealed reversible mesomorphic phase transition, wide mesophase temperature ranges, and high thermal stability.     

侧链聚硅氧烷液晶高分子的合成与表征及应用研究

液晶高分子既具有独特的液晶性 ,又具有高分子的良好材料性能 ,引起了人们的广泛注意[1～ 9] .侧链液晶高分子大多可以作为功能材料 ,对它们的研究有很大的理论与现实意义 .以往报道的此类化合物的介晶基元大多是通过烷氧基与间隔基相连[10 ] .我们以催化活性很高的铂络和物为催化剂 ,通过硅氢加成反应制备了间隔基与介晶基元通过酰氧基相连的两种侧链聚硅氧烷液晶高分子 ,并对它们的性质进行了初步表征 .发现它们具有很好的液晶性 .已有研究表明侧链聚硅氧烷液晶在气相色谱分离结构近似的物质方面 ,具有易涂渍、选择性及热稳定性优于低分子…     

Cholesteric bonded stationary phases for high-performance liquid chromatography: a comparative study of the chromatographic behavior

The properties of four cholesteric bonded stationary phases differing in the nature of the spacer and the end-capping were assessed using simple chromatographic tests based on the retention of nonpolar compounds and of planar or nonplanar probe solutes. All cholesteric columns showed a hydrophobicity close to that of conventional octadecyldimethylsilyl (ODS) materials. Non-end-capped cholesteric bonded phases showed greater selectivity than ODS ones and both end-capped cholesteric bonded phases exhibit behavior intermediate between that of the non-end-capped original material and that of the ODS bonded phase.     

Cholesteric gratings induced by electric field in mixtures of liquid crystal and novel chiral ionic liquid

Formation of gratings upon alternating electric field in compositions of highly birefringent liquid crystal (LC) with novel chiral ionic liquid (CIL) enclosed in 10-µm-thick cells is studied. The compatibility of two organic components of the mixture is investigated. The grating is formed in 1–5% CIL mixtures and causes significant changes of the transmission of polarised light through the cells. Transmission of polarised λ = 633 nm light through the 5% CIL sample is studied with respect to frequency (1 Hz–3 MHz) and amplitude (up to 10 V) of alternating voltage and the results are used for sketch a voltage–frequency phase diagram. Two possible ways of formation of the gratings with perpendicular orientation to that formed from initial state are presented. Two twisting axes in the molecular alignments of the cholesteric LC gratings with perpendicular directions are proposed. Optical switch based on four different states of LC including two gratings with perpendicular directions is proposed. Moreover, behaviour of the rotation of a grating induced by AC field in hybrid planar-homeotropic cell is studied in two frequency regimes and rotation by more than 90° upon change of the AC field amplitude is observed.