Polarization holographic grating recording in the cholesteric azobenzene‐containing films with the phototunable helix pitch

The recording of polarization gratings in films of a cholesteric liquid crystalline polymer with different helix pitch was studied in detail. For this purpose, the cholesteric mixture of the nematic azobenzene‐containing copolymer with a chiral‐photochromic dopant was prepared. The utilization of such mixture has made possible to realize dual optical photorecording in one system, first due to the phototuning of the helix pitch by UV light and second the polarization grating recording process by exposure with polarized visible light. The diffraction efficiency strongly depends on the cholesteric helix pitch and films thickness: the increase of the confinement ratio d/p (where d, film thickness; p, helix pitch) results in growth of the diffraction efficiency. Comparison of the induction of polarization gratings in cholesteric, nematic (copolymer without chiral dopant), and amorphous (nonannealed) cholesteric films revealed that only the cholesteric films were characterized by significant oscillations in the diffraction efficiency signal as well as by the presence of the maximum in the first‐order diffraction efficiency in the initial stage of the grating recording process. It was found that in addition to the polarization grating surface relief gratings (SRGs) were also formed in the studied systems, however, the amplitude of the SRG inscribed in the cholesteric films was lower (～20 nm) compared to the grating amplitude obtained in nematic films (～60 nm). Moreover, increasing helix pitch resulted in a decrease of the SRG amplitude. The obtained experimental data demonstrate the great potential of cholesteric LC mixtures of such type for different applications as photoactive materials for photonics. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 773–781     

Flow-induced grating from cholesteric mixtures

In this work we present a new technique for obtaining large diffraction gratings (some cm) by means of a simple filling of cells having a planar treatment of their inner surfaces. A homogeneous mixture, composed of a cholesteric liquid crystal and a nematic liquid crystal monomer, was used. During the filling process, the flow induces a phase separation between the cholesteric liquid crystal and the liquid crystal monomer and, at the same time, the latter is oriented planar to the surfaces of the cell. Phase separation produces alternate arrays constituted by the cholesteric liquid crystal and the nematic liquid crystal monomer. Successive UV polymerization of these films yields a permanent grating. We have investigated the transmitted and first order diffracted beam efficiency for films obtained at different temperatures. The morphology of the films was studied by using an optical microscope equipped with crossed polarizers and by electron microscopy in order to control the shape of the arrays and the alignment of the oriented polymer.     

Flow-induced grating from cholesteric mixtures

In this work we present a new technique for obtaining large diffraction gratings (some cm) by means of a simple filling of cells having a planar treatment of their inner surfaces. A homogeneous mixture, composed of a cholesteric liquid crystal and a nematic liquid crystal monomer, was used. During the filling process, the flow induces a phase separation between the cholesteric liquid crystal and the liquid crystal monomer and, at the same time, the latter is oriented planar to the surfaces of the cell. Phase separation produces alternate arrays constituted by the cholesteric liquid crystal and the nematic liquid crystal monomer. Successive UV polymerization of these films yields a permanent grating. We have investigated the transmitted and first order diffracted beam efficiency for films obtained at different temperatures. The morphology of the films was studied by using an optical microscope equipped with crossed polarizers and by electron microscopy in order to control the shape of the arrays and the alignment of the oriented polymer.     

Transient holographic grating in azo-dye-doped liquid crystals with off-resonant light

Holographic gratings in nematic liquid crystals doped with Disperse Red 1 (DR1) were investigated using a linearly polarised He–Ne laser. Although the electric transitions of the samples were almost off-resonant, the diffraction of the transient holographic grating was found to exhibit a sinusoidal feature with changing the polarisation of recording beams, and the diffraction intensities and response time of the gratings were strongly dependent on DR1 concentration. The results revealed that the diffraction of holographic grating depends on the rate of trans–cis isomerisation of DR1 molecules. Moreover, the temperature dependence of diffraction indicated that the grating formation mechanism was attributed to photoinduced reorientation of photoisomerisation effect.     

Mesomorphic properties of non-symmetric three-arm chenodeoxycholic acid-derived liquid crystals

Six three-arm star-shaped liquid crystals (LCs) based on chenodeoxycholic acid (CDCA), termed as G-BH, G-YD, G-FD, G-DJ, G-DZ and G-BX, respectively, have been synthesised. CDCA was used as the chiral core and the nematic side arm, 6-(4-(ethylbenzoyloxy) phenoxy)-6-oxohexanoic acid, was chosen to be introduced into the two hydroxyl of CDCA to synthesise cholesteric LC (CDCA2EA) and different structures were introduced into the carboxyl group of CDCA to prepare the three-arm star-shaped LCs. Chemical structures and LC properties of the six three-arm LCs were characterised by FTIR, ¹H-NMR, differential scanning calorimetry, thermogravimetric analysis, X-ray diffraction and polarised optical microscopy. G-BH and G-DJ displayed cholesteric phase with a long helical pitch; G-BX displayed nematic phase; and G-YD, G-FD and G-DZ displayed cholesteric phase on heating and on cooling. These results indicated that not only the chiral core CDCA but also the structures of the side arms played an important role in inducing the cholesteric phase of the CDCA-derived LCs. G-DZ displayed selective reflection, a wider ?λ and red shift on cooling.     

Preliminary Communications. Micellar phases formed by a solution of l-serine hydrochloride decylester and orthophosphoric acid

Polarizing microscope studies showed that the isotropic solution composed of l-serine hydrochloride decylester and orthophosphoric acid forms micellar cholesteric, nematic and lamellar phases, whereas the solution of the optically inactive ester gives micellar nematic and lamellar phases. The phase transitions were tentatively assigned as the result of two concurrent reactions at which ortho-phosphoserine hydrochloride decylester and orthophosphoric acid monodecylester are produced. Dilution of the optically active and the optically inactive lamellar phases with water gave a cholesteric and a nematic phase, respectively. The nematic director is aligned perpendicular to the glass plate, whereas that of the cholesteric phase is aligned parallel.     

Synthesis and thermal properties of side-chain liquid crystalline polyethers with racemic and chiral backbone

Racemic and chiral [(4-cyano-4′-biphenyl)oxy] and [(4-methoxy-4′-biphenyl)oxy]methyloxiranes were prepared from racemic epichlorohydrin or racemic and chiral glycidols and polymerized in dimethylsulfoxide (DMSO) with Bu^tOK as the initiator system. Initial phase identifications were made by differential scanning calorimetry (DSC) and optical microscopy techniques and confirmed by X-ray diffraction measurements. Upon heating, all the monomers show only a crystal–isotropic phase transition. The racemic and chiral [(4-cyano-4′-biphenyl)oxy]methyloxiranes exhibit a nematic and a cholesteric monotropic phase, respectively. Methoxybiphenyl substituted polyethers are crystalline and insoluble in virtually all common solvents. Cyanobiphenyl substituted polyethers are soluble under the same experimental conditions and show enantiotropic liquid crystalline properties. The racemic polymer exhibits a nematic phase, while the optically active polymer forms a cholesteric phase.     

Influence of different nematic crosslinking unit on mesomorphism of side-chain cholesteric elastomers containing menthyl groups

The synthesis of two cholesteric monomers (M₁ and M₂), nematic crosslinking agent (C₁ and C₂), and the corresponding side-chain elastomers containing menthyl groups (P₁ and P₂ series) is described. The mesomorphism was investigated by differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction, and thermogravimetric analysis. The effect of the content of the different nematic crosslinking unit on the mesomorphism of the elastomers was discussed. M₁ and M₂ showed cholesteric and blue phases; C₁ and C₂ showed nematic phase. Because of the introduction of the nematic crosslinking unit, elastomers P_1-1−P_1-5 and P_2-1−P_2-5 exhibited cholesteric phase. With increasing the content of nematic crosslinking unit, T _g of the obtained elastomers revealed an increased tendency, and T _i of P₁ series firstly increased then decreased, while T _i of P₂ series decreased the mesomorphism of the corresponding elastomers when the content of nematic crosslinking unit was 12 mol.%.     

Synthesis and characterization of side chain cholesteric liquid crystalline polymers containing isosorbide as a chiral centre

A series of new side chain cholesteric liquid crystalline polysiloxanes was synthesized by grafting copolymerization of a mesogenic monomer (M₁) and a chiral monomer (M₂). The chemical structures of the monomers and polymers obtained were confirmed by FTIR, and ¹H and ¹³C NMR spectroscopy. The mesomorphic properties were investigated by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. The influence of the content of the chiral unit on phase behaviour of the polymers is discussed. Monomer M₁ showed nematic and smectic phases on cooling. The polymers P₁ and P₂ showed a nematic phase, P₃-P₅ showed cholesteric Grandjean texture, and P₆ and P₇ exhibited smectic short-rod texture. The polymers containing more than 7.2 mol % and less than 28.6 mol % of the chrial unit showed an induced cholesteric phase. Experimental results demonstrated that the glass transition, melting and clearing temperatures decreased with increasing content of the chiral unit.     

Molecular model for cholesteric polymers including biaxiality and chain flexibility

A mean field theory for cholesteric polymers based on the elastic model is presented. The effect of flexibility and the biaxial order and nematic order parameters are studied. The biaxial order and nematic order parameters are increasing functions of chirality and vary slightly with flexibility. Biaxiality is shown to be important for the transition temperature and the cholesteric pitch as well as the latent heat of transition.     

Photoinduced reorientation processes in thin films of photochromic LC polymers on substrates with a photocontrollable command surface

Orientation and reorientation processes that occur in nematic and cholesteric LC polymer systems under irradiation with plane-polarized light are studied. A copolyacrylate containing phenyl benzoate and azobenzene side groups is synthesized as a nematic polymer; the cholesteric mixture is prepared via doping of the nematic copolymer with the chiral dopant, the derivative of D-isosorbide. Thin layers of the azobenzene-containing photoorientant SD-1 are first used as orienting substrates for polymer liquid crystals. Thin layers of the copolymer and of the mixture are spin-coated on the substrate after irradiation of the photoorientant layer with polarized light. It is shown that after annealing phenyl benzoate and azobenzene side groups of the nematic copolymer orient strictly along the direction of orientation of surface molecules, whereas in the case of the cholesteric mixture, a partial formation of the helical structure is observed. It is demonstrated that all the systems under examination can experience the repeated cyclic reorientation of the cooperative type under irradiation and subsequent annealing of the films.     

Photochemical tuning of the helical structure of cholesteric liquid crystals by photoisomerization of chiral azobenzenes, and their structural effects

Several chiral azobenzene compounds having different chiral substituents were synthesized. A cholesteric phase was induced by mixing each chiral azobenzene compound with a host non-chiral nematic liquid crystal (E44). The helical twisting power (HTP) as well as the change in HTP by trans-cis photoisomerization of the chiral azobenzene compound was dependent on the structure of the chiral substituents. A compensated nematic phase was induced by combination of E44, a chiral azobenzene compound and a non-photochromic chiral compound. Reversible switching between the compensated nematic phase and cholesteric phase was brought about by trans-cis photoisomerization of the chiral azobenzene compound in the liquid crystalline systems. An azobenzene compound substituted with a menthyl group showed the highest efficiency as the trigger for the switching; this efficiency was related to the compactness of the chiral group substituted within the azobenzene core moiety.     

Copolyesters exhibiting a thermotropic cholesteric phase

Mesomorphic copolymers were synthesized by incorporating varying ratios of azelaic acid and (+)-3-methyl adipic acid into copolyesters based upon 4,4′-dihydroxybiphenyl. Introduction of the phenylene (+)-3-methyl adipate unit broadens the temperature range of the nematic phase of the azelate homopolymer and, at the same time, produces a chiral nematic (cholesteric) mesophase which exhibits various iridescent colors. Circular dichroism measurements were used to determine the pitch of the cholesteric liquid crystals. The pitch decreased gradually with increasing temperature, and the inverse pitch increased in direct proportion to the molar content of the units containing (+)-3-methyl adipate. The twisted cholesteric structure could be conserved by quenching to produce films with various colors at room temperature. Annealing these films at a temperature immediately below that of the crystal–mesophase transition improved the regularity of the cholesteric structure.     

Chirality sensing of chiral pyrrolidines and piperazines with a liquid crystalline dynamic helical poly(phenylacetylene) bearing ethyl phosphonate pendant groups

Stereoregular cis‐transoidal poly(phenylacetylene) bearing a phosphonic acid monoethyl ester as the pendant group (poly‐ 1 ‐H) was found to form a preferred‐handed helix upon complexation with various optically active pyrrolidines and piperazines in dilute dimethyl sulfoxide and water, and the complexes exhibited characteristic induced circular dichroisms (ICDs) in the UV‐vis region of the polymer backbone. The Cotton effect signs in water reflect the absolute configuration of the pyrrolidines. The sodium salt of poly‐ 1 ‐H (poly‐ 1 ‐Na) and poly‐ 1 ‐H in the presence of optically active amines formed lyotropic nematic and cholesteric liquid crystalline phases in concentrated water solutions, respectively, indicating the rigid‐rod characteristic of the polymer main chain regardless of the lack of a single‐handed helix, as evidenced by the long persistence length of about 18 nm before and after the preferred‐handed helicity induction in the polymer. X‐ray diffraction of the oriented films of the nematic and cholesteric liquid crystalline polymers exhibited almost the same diffraction pattern, suggesting that both polymers have the same helical structure; dynamically racemic and one‐handed helices, respectively. On the basis of the X‐ray analysis, a possible helical structure of poly‐ 1 is proposed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1383–1390, 2010     

Thermal tuning band gap in cholesteric liquid crystals

The phase of a liquid crystal (LC) changing from a nematic phase to a cholesteric (Ch) mesophase is achieved by adding different ratios of chiral dopants S811. By studying the transmission spectrum, we are able to measure the helical pitch in cholesteric phase. The pitch in the mixtures of nematic E7 and chiral dopants S811 as a function of the concentration of the dopant and temperature is investigated. The sensitivity of the selective reflection notch of the cholesteric phase to the thermal tuning depends strongly on the ratios of the chiral dopants. It reveals that the influence of temperature is more profound for those cholesteric liquid crystals (CLCs) which exhibit smectic A (SmA) at lower temperatures. When fitted using Keating's formula, the helical pitch calculated from our experimental results lies on the predicted curve. Optimised ratios of the mixture CLCs for the optimised reflection band with the specified wavelength ranging from 467 nm to 2123 nm are suggested.     

Molecular model for cholesteric polymers including biaxiality and chain flexibility

Abstract

A mean field theory for cholesteric polymers based on the elastic model is presented. The effect of flexibility and the biaxial order and nematic order parameters are studied. The biaxial order and nematic order parameters are increasing functions of chirality and vary slightly with flexibility. Biaxiality is shown to be important for the transition temperature and the cholesteric pitch as well as the latent heat of transition.     

Optical rotatory power of liquid crystal mixtures II mixtures of a nematogenic and a non-mesomorphic chiralic compounds

Addition of a chiralic eight-membered ring derivative of 1, 1′-dinaphthyl (I) to a nematic mesophase produces an optical rotatory power Φ which exceeds the optical rotation of the solute I by a factor of several orders of magnitude. The temperature dependence Φ (T) as well as other optical properties are characteristic of cholesteric liquid crystals. From this, evidence has been derived that non-mesomorphic chiral molecules transform a nematic into a cholesteric mesophase. The screw-sense of the helical arrangement depends on the chirality of the molecules added.     

Side chain cholesteric liquid crystalline elastomers: synthesis and phase behaviour

A series of new side chain cholesteric liquid crystalline elastomers (P-2-P-6) containing the nematic crosslinking monomer 4-(10-undecen-1-yloyloxy)benzoyl-4'-allyloxybenzoyl-p-benzenediol bisate (M-1) and the cholesteric monomer 4-cholesteryl 4-(10-undecen-1-yloyloxy)benzoate (M-2) were synthesized. The chemical structures of the monomers and elastomers obtained were confirmed by FTIR and ¹H NMR spectroscopy. Their liquid crystalline properties and phase behaviour were investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. The effect of the crosslinking units on phase behaviour is discussed. Elastomers containing less than 20 mol % of the crosslinking units showed elasticity, reversible phase transitions and cholesteric Grandjean texture. The experimental results demonstrated that the glass transition and isotropization temperatures of P-2-P-6 increased with the increasing concentration of crosslinking unit M-1.     

Nematic liquid crystal gyroids as photonic crystals

ABSTRACT

We demonstrate nematic and cholesteric liquid crystal (LC) gyroids and show their photonic properties as photonic crystals by using numerical modelling. The LC gyroids are designed as composite optical materials, where we take one labyrinth of passages to be a solid dielectric, whereas the other (complementing) labyrinth of passages is taken to be filled by chiral or achiral nematic LC, with the intermediate gyroid surface imposing homeotropic (perpendicular) surface anchoring. The nematic inside the gyroid matrix is shown to exhibit a variety of possible orientational profiles which are characterised by complex networks of topological defects – from ordered, semi-ordered, to completely disordered. The diversity of possible nematic states is shown to lead to a rich structure of photonic bands, which can be tuned by the LC volume fraction and the cholesteric pitch, including control over full – direct and indirect – band gaps.     

Photochemical tuning capability of cholesteric liquid crystal cells containing chiral dopants end capped with menthyl groups

In order to investigate the photochemical tuning capability of chiral monomers and polymers containing end-capped menthyl groups, a new series of chiral dopants was synthesized and added to commercially available nematic liquid crystals to induce cholesteric liquid crystal (LC) phases. The addition of chiral dopants with azo structure led to phototunability of the reflection colour of the LC cells. Photochromic variation of the LC cells due to photoisomerization of the azo compound was investigated. After photopolymerization of the monomers inside the cholesteric LC cells, the centre wavelength of the reflected band of the incident light was found to be fixed and the reflected bandwidth was broadened, resulting in a red shift. A schematic representation of both the photoisomerization of the azo dopants and its effect on variation of twisting pitches is proposed. Real image recording was performed using 365 nm UV through a mask with text. The top and side views of the morphological network structures of a fabricated cholesteric LC cell were investigated using scanning electron microscopy. The results of this investigation demonstrated that RGB reflected colours of LC cells can easily be achieved through the addition of the menthyl-containing synthesized chiral compounds to nematic LCs. The addition of synthesized AzoM helped further in recording the patterns onto cholesteric LC films using 365 nm UV exposure.