Photo-reversible liquid crystal alignment using azobenzene-based self-assembled monolayers: comparison of the bare monolayer and liquid crystal reorientation dynamics

Photosensitive surfaces treated to have in-plane structural anisotropy by illumination with polarized light can be used to orient liquid crystals (LCs). Here we report a detailed study of the dynamic behavior of this process at both short and long times, comparing the ordering induced in the bare active surface with that of the LC in contact with the surface using a high-sensitivity polarimeter that enables detailed characterization of the anisotropy of the active surface. The experiments were carried out using self-assembled monolayers (SAMs) made from dimethylaminoazobenzene covalently bonded to a glass surface through a triethoxysilane terminus. This surface gives planar alignment of the liquid crystal director with an azimuthal orientation that can be controlled by the polarization of actinic light. We find a remarkable long-term collective interaction between the orientationally ordered SAM and the director field of the LC: while an azobenzene based SAM in contact with an isotropic gas or liquid relaxes to an azimuthally isotropic state in the absence of light due to thermal fluctuations, an orientationally written SAM in contact with LC in the absence of light can maintain the LC director twist permanently, that is, the SAM is capable of providing azimuthal anchoring to the LC even in the presence of a torque about the surface normal. We find that the short-time, transient LC reorientation is limited by the weak azimuthal anchoring strength of the SAM and by the LC viscosity.     

Twofold light and magnetic responsive behavior in nanoparticle-lyotropic liquid crystal systems

We demonstrate the dual magnetic and light responsive nature of hybrid mesophases constituted by Fe(3)O(4) nanoparticles dispersed in lipid-based lyotropic liquid crystals (LC). When subjected to an external magnetic field in the mesophase isotropic state, the nanoparticles aggregate and orient along the magnetic field direction, and upon cooling the system through the disorder-order transition the aggregates drive the orientation of the mesophase via heterogeneous nucleation; furthermore, order-disorder transitions in the lipidic mesophase can be triggered by Fe(3)O(4)-induced photothermal effect under visible light exposure. Both the orientational order and the photothermal effect of the hybrid mesophase can be tuned by the nanoparticle content, offering a general route for controlled assembly of complex fluids with combined magnetic and light responsiveness.     

Smectic liquid crystal alignment using mechanically rubbed n-octadecylsiloxane self-assembled monolayers

In recent years a variety of techniques has appeared for the fabrication and manipulation of self-assembled monolayers (SAMs). This development now offers new tools for the study and control at the molecular level of the interaction of liquid crystals (LCs) with solid surfaces, a research area of great importance for liquid crystal applications. In this paper we show that mechanically rubbed octadecylsiloxane SAMs generate a novel surface alignment of LCs in which the in-plane surface anisotropy usually accompanying rubbing is operative, but only for smectics in which the mean molecular long axis, [ncirc], is tilted from the layer normal. On our SAMs smectic phases align with the layers parallel to the SAM surface, and in tilted smectics the surface component of [ncirc] is along the rubbing direction. This anisotropy is absent in the nematic phases which align with [ncirc] strictly normal to the surface. This behaviour can be understood in terms of a rubbed SAM monolayer surface, which is low energy, molecularly smooth, and rendered anisotropic by the rubbing. UV irradiation of rubbed SAMs gave excellent planar alignment ([ncirc] parallel to the surface). This type of control over LC alignment has not been previously reported.     

In situ self-assembled photo-switchable liquid crystal alignment layer using azosilane monomer-liquid crystal mixture system

Self-assembled monolayers (SAMs) are a unique approach for the liquid crystal (LC) alignment in electro-optical applications such as displays. Herein, a new methodology for photo-switchable LC alignment layer using an azosilane monomer and LC mixture system in the absence of any other foreign alignment layer is presented. The azosilane monomer spontaneously separated from the host LCs, and formed a stable monolayer network on the substrate surface. Data from X-ray photo-electron spectroscopy (XPS), spectroscopic elipsometry (SE), water contact angle and LC alignment studies confirmed that, in the azosilane and LC mixture system, azosilane makes an in situ SAM that is capable of photo-switchable LC alignment layer on glass and indium tin oxide (ITO) substrates. The LCs are aligned with respect to change in the photo-isomerisation of the azo molecule.     

Crosslinked liquid crystal polymers from liquid crystal monomers: Synthesis and mechanical properties

Difunctional acrylates and methacrylate monomers have been made which are high order smectic liquid crystal (or crystalline) at room temperature. This report discusses materials with the following structure: F–S–M–S–F, where F is a functional group, acrylate or methacrylate (A or M); S is a spacer (CH₂)_n(n), and M is a mesogen—in this case 4,4′-dioxybiphenyl (B). They are codified as BnA or BnM where n is the number of methylenes in the spacer. High conversion with high T_g can be obtained when polymerizing in the smectic state because the reactive end groups are concentrated in a small volume and can react well with little or no diffusion. B2A, B3A, B6A, B11A, and B3M were polymerized in the smectic state and compared to polymers made at temperatures where the monomers were isotropic. High conversion was obtained below final T_g—even then, probably because the polymers were ordered. All the polymers were studied by WAXD and dynamic mechanical spectroscopy. Solid-state NMR on B3A showed that there was very high conversion of the double bonds at all temperatures. B3A photopolymerized in the smectic state (60–76°C) produced a crystalline polymer with T_g = 185°C (1 Hz). When photopolymerized at 85°C, above the isotropization temperature (T_i), a poorly organized polymer was obtained with a T_g of 155°C (1 Hz). Monomers with an odd number of methylene groups as spacers were crystalline after polymerization. With an even number of methylene groups, they lost most of their crystallinity on polymerization below T_i, but retained a low order smectic structure. Similar structures were obtained with all the monomers when they were polymerized above T_i. There was little effect of polymerization temperature on T_g when the spacers had an even number of methylene groups. © 1993 John Wiley & Sons, Inc.     

In situ self-assembled homeotropic alignment layer for fast-switching liquid crystal devices

We propose a novel method for homeotropic alignment of liquid crystals (LCs) utilising in situ self-assembly of a low concentration of 4-(4-heptylphenyl)benzoic acids that form hydrogen bond with the indium tin oxide (ITO) substrates. Stable homeotropic alignment in the LC device is achieved with a simple mixing process of benzoic acid derivative in LC media, and it yields electro-optical performance similar to that achieved with the conventional alignment method using polyimides. It is experimentally confirmed that an ultrathin self-assembled molecular layer of 4-(4-heptylphenyl)benzoic acid formed by hydrogen bonding on ITO substrate makes it possible to attain a reliable homeotropic alignment of LCs. Furthermore, this simple approach provides a cost-effective and stable LC alignment layer with fast response time and thermal stability.     

Synthesis and liquid crystal properties of mixed alkynyl-alkoxy-triphenylenes

The synthesis and liquid crystalline properties of some new multisubstituted triphenylenes are reported. The materials bear combinations of alkyne and alkoxy substituents and were designed to probe the structural factors controlling mesophase formation, type and stability in triphenylene discotics. A combination of factors contributes to mesophase stability in these systems. Compounds bearing two alkynes can support, and indeed enhance, columnar mesophase behaviour, but mesophase behaviour is completely suppressed when four or six alkynes are present. A twinned structure linked by a diacetylene bridge shows a stable nematic phase, but conversion to a saturated bridge destroys all mesophase behaviour.     

Synthesis and mesomorphic properties of trifluorobenzoate liquid crystal

Four series of trifluorobenzoate liquid crystals have been synthesized. Their phase transition temperatures have been also measured by texture observation in a polarizing microscope and confirmed by DSC. The influence of the lateral fluoro-substitution and triple bond has been also discussed.     

'Smart' light-reflective windows based on temperature responsive twisted nematic liquid crystal polymers

'Smart' windows which reversibly increase their reflectivity upon heating are attracting considerable attention as devices for maintaining comfortable indoor environmental conditions. In this work, twisted nematic semi-interpenetrating liquid crystal networks which lose their order upon heating are sandwiched between reflective linear polarizers. This 'smart' window reversibly decreases its transmission from about 50–10% over a wavelength range between 400 and 1100 nm upon heating, resulting in the window becoming darker and reflecting more light. This 'smart' window is potentially interesting for energy saving window applications where variation between privacy and visible light transparency states is required.     

Novel polypyromellitimides and their liquid crystal aligning properties

Various aromatic poly{3,6‐bis[(4‐n‐alkyloxy)phenyloxy]‐pyromellitimide}s [poly(Ar‐C_m‐PMDA)s, m = 1, 4, 8, 12] were prepared in film form by routine two‐step condensation of 3,6‐bis[(4‐n‐alkyloxy)phenyloxy]pyromellitic dianhydrides (C_m‐PMDAs) with various aromatic diamines. After characterization of their chemical structures their solution, thermal and liquid crystal (LC)‐aligning properties in terms of pretilt angle at various rubbing densities were measured and discussed with respect to their backbone structures. All polyimide films showed excellent thermal stability and homogeneous LC alignment, but the poly(p‐phenylene‐C₁₂‐PMDA) exhibited completely homeotropic alignment while the pretilt angle values of poly(p‐phenylene‐C₈‐PMDA) varied with varying rubbing density.     

Dielectric and electro-optical properties of a photosensitive liquid crystal

The temperature dependencies of the dielectric and electro-optical properties of a pure photo-ferroelectric liquid crystal have been investigated, in the chiral smectic C (SmC*) phase, with and without ultraviolet (UV) illumination. The SmC* dielectric Goldstone mode characteristics, the spontaneous polarization, the tilt angle and the rotational viscosity are affected by UV irradiation. Under increasing UV light intensities the dielectric strength Δ?_G increases whereas the relaxation frequency f_G , the spontaneous polarization P _s, the electro-optical rotational viscosity γ_eo and the tilt angle θ decrease. The twist elastic constant K ₃ and the Goldstone mode rotational viscosity γ_G of the studied compound are deduced from dielectric experimental results. Good agreement was found between both viscosities. The observed dielectric behaviours are controlled by the elastic constant and the rotational viscosity variations.     

Transfer of lipid and phase reorganisation in self-assembled liquid crystal nanostructured particles based on phytantriol

The internal structure of dispersed liquid crystal nanostructured particles of the V(2) and H(2) phases, termed cubosomes and hexosomes respectively, is integral to their application in the pharmaceutical, agricultural and food industries. However the nanostructure is susceptible to change upon incorporation of other lipids and hence it is important to understand the potential for interparticle lipid transfer for such particles when they encounter a particle of dissimilar lipid content. Using time resolved synchrotron small angle X-ray scattering, we have investigated the transfer of material between cubosomes composed of phytantriol with three different particle types of dissimilar composition, (i) hexosomes and (ii) emulsified microemulsion composed of phytantriol and vitamin E acetate, and (iii) cubosomes prepared from glycerol monooleate. It was found that material was transferred between the different dispersed nanostructured particles, with the transfer being caused by compositional ripening. Somewhat counter-intuitively the transfer was bidirectional with phytantriol being more rapidly transferred than the minor component vitamin E acetate. The greater lipophilicity of vitamin E acetate supports previous studies suggesting greater mobility for the less lipophilic components, regardless of the more efficient transfer route to achieve uniform composition. When particles comprising lipids with similar lipophilicities were mixed, the transfer was limited and did not achieve completion; a phase change between cubic nanostructures required to achieve complete mixing provides an apparent barrier to further compositional ripening. The conclusions from this study provide additional support to lipid transfer mechanisms, and highlight some subtleties in using dissimilar lipid mixtures in e.g. food applications.     

A novel photoalignment film for ferroelectric liquid crystal based on self-assembled monolayer method

A novel strategy based on self-assembly technology was devised for design of photosensitive material as a ferroelectric liquid crystal (FLC) alignment layer. This development offers new tools for the study and control at the molecular level of the interaction of FLCs with solid surfaces. The photoreactive material was self-assembled to the substrate by covalent bond linkage due to a special chemical adsorption reaction. Through ester bond linkage, a cyano group with strong polarity was introduced to be terminus of the film. Under irradiation of linearly polarised ultraviolet light, an optically anisotropic self-assembled film was easily obtained. The irradiated film was demonstrated to result in homogenous alignment of FLC by optical transmittance measurements and polarising optical microscopy images of a FLC cell at different rotation angles. The alignment quality of the FLC on this self-assembled monolayer film is comparable to that of commercial rubbed polyimide film. Furthermore, it was also found that the fine alignment of the FLC may be related to the smoothness of the self-assembled film surface owing to its polar end.     

Processing and properties of blends with liquid crystal polymers

Experimental data on the processing behavior and on the rheological and mechanical properties of blends with a liquid crystal polymer as one component are presented. The blends with low amounts of LCP show easier processability and lower viscosity than the thermoplastic matrix. The elastic modulus is also improved. The reduction of viscosity has been attributed to the lower pressure entry, due to the formation of fibrils and to the immiscibility of the two phases.     

Synthesis and liquid crystal properties of bipyridine-containing aromatic polyamides

A series of novel aromatic polyamides containing 2,2′-bipyridine moiety were synthesized by polycondensation of 2,2′-bipyridine-5,5′-dicarboxylic acid ( 2 ) with various aromatic diamines in hexamethylphosphoramide (HMPA) containing lithium chloride. The resulting polyamide solutions in 98% sulfuric acid and in HMPA-LiCl exhibited lyotropic liquid crystal phases. The phase transition behaviors were studied by polarizing microscopy and X-ray diffraction. The polyamides also formed metal complexes with cis-dichlorobis(bipyridine)ruthenium dihydrate [cis-Ru(bpy)₂Cl₂ · 2H₂O] which was supported by changes in electronic spectra.     

Synthesis of thermotropic liquid crystal polyimide and its properties

Thermotropic liquid crystal polyimide which has neither an ester linkage nor a carbonate linkage was prepared by the polymerization of 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA) and 1,3-bis[4-(4′-aminophenoxy)cumyl)]benzene (BACB). This polyimide shows the liquid crystal phase at 549-593 K. Mixing this liquid crystal polyimide or copolymerizing BACB decreases the melt viscosity of the thermoplastic polyimide (Aurum). © 1994 John Wiley & Sons, Inc.     

Polarization and dielectric properties of an antiferroelectric liquid crystal

The spontaneous polarization and dielectric properties of a new antiferroelectric liquid crystal (AFLC) exhibiting several intermediate phases between the SmCA and SmC* phases are investigated. A low frequency ferri Goldstone mode and a higher frequency ferro Goldstone mode have been observed over a certain range of temperatures. The effect of d.c. bias voltage on these modes is examined. The results confirm the existence of an FiLC phase with q 1/2 between SmCgamma and SmC*. These also show the co-existence of the FiLC phase with SmC* over a narrow range of temperatures above the FiLC phase. The phase sequence for this material is found to contain SmCA, SmCgamma, FiLC, (FiLC coexisting with SmC*), SmC*, SmA phases on heating and SmA- SmC* SmCgamma- SmCA- phases on cooling.     

Atomic Force Microscopy of freely suspended liquid crystal films transferred to octadecyltriethoxysilane self-assembled monolayers

Thin freely suspended films of 4-(2-methylbutyl)phenyl 4'-octylbiphenyl-4-carboxylate (8SI) in the smectic C phase were transferred to self-assembled monolayer coated glass and imaged by atomic force microscopy. Upon rapid cooling to room temperature, the smectic C quenches into a lamellar phase, exhibiting islands and single smectic layer steps. The surface of such films can be imaged with molecular resolution, revealing a partially disordered lattice of fixed molecular positions. Over a period of several days these films undergo a transition to a crystal phase having a different film morphology.