Cholesteric Films Research Articles

Color selectivity lent to a cholesteric liquid crystal by monitoring interface-induced deformations

The cholesteric liquid crystalline structure is omnipresent in living matter and concerns many applications in optics because of its property of selective light reflection. The color reflected by this structure depends on material parameters such as the molecular chirality or the concentration of chiral dopant, the helical pitch of the twisted structure and the optical indices. In the present publication, we show that the color may be selected simply by varying the annealing time of a cholesteric oligomer film with hybrid anchoring. Experimental and simulated transmittance spectra reveal the color shift. The three-dimensional representation of the structure is provided from the macroscopic scale to the mesoscale by combining complementary imaging techniques (optical microscopy, transmission and scanning electron microscopies, atomic force microscopy) to image the texture in different directions. We show that the color selectivity is due to controlled changes of the orientation of the helical axis with respect to the air–material interface. The optical behavior is described with a single parameter, which is the angle between the film surface normal and the orientation within the film of the helical axis. Potential applications in the field of chiro-optical devices concern self-assembled chiral microreflectors fabricated via a low-cost controllable thermal process. Additionally, the texture formation provides insight to aid the understanding of the exoskeleton morphogenesis of beetles like Chrysina Gloriosa.

Multi-wavelength laser from dye-doped cholesteric polymer films

A multi-wavelength laser is demonstrated using a dye-doped cholesteric polymer film whose reflection bandwidth is broadened with several oscillations. Due to the abrupt change of the density of state between oscillation peak and valley, each oscillation functions as a photonic band gap for generating a laser wavelength under the excitation of a pumping laser. As a result, a multiple wavelength laser is generated. Results indicate that the dye-doped cholesteric liquid crystal polymer film is a good candidate for fabricating broadband lasers such as white light lasers. Potential applications include experimental testing of laser materials, identification markers, information displays, and inertial confinement laser fusion.

Resonant modes in cholesteric liquid crystals with a Gaussian pitch profile

In this paper, we investigate the spectral properties of a cholesteric film presenting a pitch profile with a gaussian deformation. Using the Berreman 4 × 4 matrix formalism, we numerically obtain the transmission spectrum at normal and oblique light incidence as a function of width and the position of the deformation. Our results reveal that a pair of resonant modes emerges inside the main stop band of the transmission spectrum as the width of the deformation becomes comparable to the helical pitch length. The mechanism behind the emergence of the resonant modes is discussed. The case of a pitch profile with multiple gaussian deformations is also analyzed. At this configuration, a crossover from single to multiple band-gap pattern can be observed in the transmission spectrum, depending on the deformation parameters.

Mechanism of colour changes in stretchable cholesteric films

Large shifts of the selective reflection band and colour changes are achieved in highly viscous mixtures of cholesteric polymers and low molar mass liquid crystals subject to mechanical deformation. The liquid crystal mixture was sandwiched between two silicone strips and stretched. The colour of the material changed instantaneously during stretching, and the time for the colour to be completely restored increased with the viscosity of the polymer mixture. A quantitative model considering the material as an incompressible viscoelastic fluid is proposed. The model accurately describes colour changes (shift of the selective reflection band) and its relaxation back to the original spectral location.

Frequency Tunable Lasing in a Three Layer Cholesteric Liquid Crystal Cell

Novel combinations of liquid crystalline materials for liquid crystals lasers tuning are presented. The cell is based on a three layer system, with two cholesteric films sandwiching a third layer containing an isotropic mixture of a photoluminescent dye. One of the cholesteric films has a wide band gap and the other one presents a narrow band gap, allowing the laser emission tuning over a wide spectral range, from UV to near IR. Further, using mixtures containing cholesteryl esters, a temperature control can be achieved.

Micro Manipulation with Optical Responsive Cholesteric and Compensated Nematic Liquid Crystal

We have developed photoresponsive liquid crystalline films (cholesteric liquid crystal and compensated nematic liquid crystal) having azobenzene compound to manipulate micro particles on the surface by light irradiation. When few solid particles (polystyrene bead) were placed on the surface of the film, movement of the solid particles was observed upon irradiation of UV (365 nm) or visible light (>420 nm). Only rotational motion of the particles was observed on the cholesteric liquid crystal film. Both translation and rotation was observed on the compensated nematic liquid crystal film. So the mode of motion (either rotation or both translation and rotation) was changed by changing the film from cholesteric film to compensated liquid crystal film. The rotational direction of the solid was controlled by using chiral azobenzene compound with right or left handed twisting ability.

Ordering phenomena and photoorientation processes in photochromic thin films of LC chiral azobenzene-containing polymer systems

The processes of mesogen ordering in nematic and cholesteric side-chain polymer systems were studied by polarized optical spectroscopy. For this purpose, nematic azobenzene-containing copolyacrylate was synthesized and its cholesteric mixture with chiral dopant was prepared. Thin films of copolymer and mixture were prepared by spin-coating using polyimide-coated and rubbed glass and quartz substrates. Film thickness was comparable or less than helix pitch value. It was found that azobenzene and phenylbenzoate side groups of nematic copolymer are oriented after annealing along a rubbing direction. Cholesteric mixture demonstrates a more complicated behaviour, the direction of orientation of the side-chain groups is rotated at certain angle depending on the film thickness. Dichroism in azobenzene groups’ absorbance is lower for cholesteric mixture and decreases with film thickness increasing. A possible mechanism of ordering of the side-chain groups in these thin films was proposed based on the helical order formation. Photooptical switching of the orientation and an influence of the nonpolarized and polarized light on the cholesteric thin films were studied demonstrating a possibility of using such thin films of photochromic cholesterics as promising photoswitchable materials.

Characterization of Liquid Crystal Layer and Cholesteric Film by Renormalized Spectroscopic Ellipsometry

Renormalized transmission spectroscopic ellipsometry (RTSE) is used to evaluate the device parameters of liquid crystal display (LCD). To determine the reduced dielectric and elastic constants, threshold voltage, pretilt angle, cell gap, and surface polar anchoring energy coefficient, symmetrically oblique incidence transmission ellipsometry (SOITE) was applied to vertical-alignment (VA) LCD. It is suggested that the rubbing process on the alignment films for VA LCD does not disturb the measurement of the genuine surface polar anchoring energy coefficient. RTSE is also applicable to the determination of the total twist angle of the cholesteric film.

Liquid Crystalline Collagen: A Self-Assembled Morphology for the Orientation of Mammalian Cells

We report the creation of collagen films having a cholesteric banding structure with an orientation that can be systematically controlled. The action of hydrodynamic flow and rapid desiccation was used to influence the orientation of collagen fibrils, producing a film with a twisted plywood architecture. Adult human fibroblasts cultured on these substrates orient in the direction of the flow deposition, and filopodia are extended onto individual bands. Atomic force microscopy reveals the assembly of 30 nm collagen fibrils into the uniform cholesteric collagen films with a periodic surface relief. The generation of collagen with a reticular, "basket-weave" morphology when using lower concentrations is also discussed.

Thermally Recordable Cholesteric Liquid Crystalline Copolymers Containing Pendant Menthyl and Cholesteryl Groups

In order to investigate the effect of spacer length and linkages between the rigid mesogenic core and the terminal group on the molecular interaction and physical properties of polymers, a series of novel side chain liquid crystalline polymers were synthesized. These were composed of liquid crystalline monomers with six or eleven methylene segments as spacers, and chiral monomers end capped with menthyl or cholesteryl groups. Liquid crystalline phases of the polymers were investigated using differential scanning calorimetry and polarized optical microscopy, and confirmed with X-ray diffractometry. Color image recording of the synthesized polymer films was achieved using a thermal treatment, and then fixed by quenching. This investigation demonstrates that the introduction of carbonate linking groups between the rigid mesogenic core and terminal group decreases both the lateral molecular interaction and thermal stability of the liquid crystalline polymers. The RGB reflection colors of the cholesteric composite film could be tuned by varying the film temperature and applying an external field. The results of this investigation present significant scientific and practical contributions with respect to the development of unique cholesteric polymer materials.

Indane-1,3-dione and cholesterol containing butadiene derivatives: Photoresponsive liquid crystalline glasses for imaging applications

Synthesis, thermo-optical and glass forming properties of a series of photoresponsive luminescent mesogenic materials containing a butadiene moiety linked to a cholesterol group ( CBIN, CBIN8 and CBIN12) as well as colour imaging devices based on them are described. These molecules exhibited relatively high fluorescence quantum efficiency in the solid and liquid crystalline states. The cholesteric phase was observed to be the major phase in these molecules and the pitch of cholesteric helix was sensitive to external stimuli such as temperature and light, making it possible to tune their iridescent colour in the visible region. The colour reflected by the cholesteric films could be stabilized by converting them to the glassy state by sudden cooling from their cholesteric phase to ∼0 °C. Selective exposure of the material in its liquid crystalline phase to light resulted in trans– cis photoisomerization of the butadiene chromophore. Consequently the pitch of the irradiated portion changed depending upon the intensity of the illuminating light. The coloured images thus formed could be stored for long periods by converting the films to glasses by sudden cooling. The images stored in the glassy state were stable over long periods of time (>one year).

High Chiral Induction in Cholesteric Liquid Crystal of Aromatic Polyesters with Chiral Naphthyl Ethylamine Groups at Their Polymer Ends

Chiral aromatic liquid crystal (LC) polyesters were prepared by incorporating (R)-(+)-1-(1-naphthyl)ethylamine (NEA) to both their ends, and the helical twisting powers were determined for the cholesteric LCs in mixtures with the homologous nematic polymer. The chiral NEA groups in the polymer ends were found to act as an effective chiral inducer so that only a small chiral content of 2–5 wt % can induce the helical pitches, which are comparable to the wavelength of visible light. Together with the easy preparation of chiral polymers and the simple processing to polymer cholesteric films, the high induction of chirality gives the opportunity to produce large-area nanostructures at low cost, which could be applied to the development of optical devices.

Synthesis and characterization of optically active liquid crystalline polyacrylates containing mesogenic phenylbenzoate groups

A series of novel side chain liquid crystalline polyacrylates with pendant chiral groups were synthesized. It was found that monomers with electron releasing –OC4H9 terminal groups seem beneficial for the formation of liquid crystalline phases. Copolymerization of the monomers was carried out and the physical properties of the copolymers were investigated. All synthesized polymers revealed liquid crystalline phases and appeared highly thermally stable with decomposition temperatures (Td) at 10% weight loss greater than 384°C and about 50% weight loss occurred beyond 442°C under nitrogen atmosphere. Two miscible chiral compounds were also synthesized and used as chiral dopants to induce cholesteric liquid crystalline phases of polymers. Liquid crystalline phases of the polymers were investigated using DSC and XRD, and confirmed with POM technique. The optical properties of the induced cholesteric liquid crystalline polymers were investigated using UV–vis spectrometer. The appearance and the color variation of the polymer films before and after UV irradiation were also investigated. Typical helical morphology of the cholesteric liquid crystalline film was analyzed by SEM technique.

Synthesis and characterization of novel photoisomerizable liquid crystalline polymers containing cinnamoyl groups

AbstractA series of novel liquid crystalline monomers and polymers incorporating phenylbenzoate or phenylcinnamate segments as mesogenic cores have been synthesized to investigate the sensitivity of the photochromic cinnamoyl derivatives and to overcome the defects of the thermal instability of azobenzene. Their liquid crystalline, thermal, and photoinduced properties of all monomers and polymers were characterized. The polymers showed excellent solubility in common organic solvents such as CHCl3, toluene, and DMF and exhibited good thermal stability with decomposition temperatures (Td) at 5% weight loss greater than 340 °C and about 50% weight loss occurred beyond 430 °C under nitrogen atmosphere. The pitch length (about 574 nm) of the synthesized cholesteric polymeric film (CP2) was estimated using scanning electron microscopy. These photochromic polymers exhibited strong UV–vis absorption maxima at about 264 or 320 nm. Moreover, photo induced configurational E/Z isomerization further changed the π‐electron conjugation systems leading to a decrease at the π‐π* transition and an increase in the range of 300 nm to 400 nm for photochromic copolymers. The thermal stability of the Z‐structural segment was confirmed by heating the polymer at 50 °C for over 5 h. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1289–1304, 2008

Flexible area‐color reflective displays based on electric‐field‐induced blueshift in a cholesteric liquid‐crystal film

Abstract— An electrically controllable blueshift of the reflection band is observed in a cholesteric liquid crystal with either positive or negative dielectric anisotropy. The change in optical properties is a result of a two‐dimensional periodic undulation of the cholesteric texture, known as Helfrich deformation. This blueshift mechanism was used to demonstrate area‐color reflective displays in a cholesteric cell and a rollable polymeric film.

Color changing and lasing stretchable cholesteric films

Polymer materials able to change reflective properties due to mechanical deformation fundamentally challenge the theory of soft materials and are important for a number of emerging applications. The most promising of those are chiral lasers. In this communication, we report novel cholesteric materials that display large color change from far red to blue and a shift of the position of the selective reflection band under uniaxial strain from near infrared to ultraviolet. Optical pumping of these materials which are doped with laser dyes, leads to lasing at the wavelengths controlled by strain within the emission interval of laser dyes of 80 nm.

Organic Semiconductors with Helical Structure Based on Oligothiophene derivatives Exhibiting Chiral Nematic Phase

We have synthesized 2-[5-(4-[(S)-2-methylbutoxy]phenyl)-3-methyl]-2″′-propyl-2:5′-2′:5″-2″:5″′-quaterthiophene ( 10 ), a liquid crystalline semiconductor that exhibits a chiral nematic phase, and used the time-of-flight technique to determine its carrier mobility behavior. In the chiral nematic phase, the positive carrier mobility was 2 × 10−4 cm2/Vs at 120°C and the negative carrier mobility was 1 × 10−4 cm2/Vs, both of which we attribute to electronic processes. For the negative carrier, in addition to the fast electronic process, we also observed a slow carrier transport, having mobility on the order of 10−5 cm2/Vs, which we attribute to ionic conduction. Chiral dimers containing binaphthyl group exhibited stable glassy cholesteric phase and cholesteric thin films with selective reflection band in visible light area could be fabricated. They emitted circularly polarized light with dichroic parameter of 1.3 when UV light was illuminated.

Sidelobe suppression in chiral optical filters by apodization of the local form birefringence

Chiral optical filters are characterized by circular Bragg effects, including the preferential reflection and transmission of circular polarization states. The selective response to circularly polarized light is caused by stratified birefringent plates twisted into a helical arrangement, as seen in cholesteric liquid crystals and columnar thin films produced by oblique-angle physical vapor deposition. A refinement of the latter, glancing angle deposition employs substrate rotation to control the optical anisotropy of columnar thin films, and was used in this study to suppress the reflection sidelobes of chiral optical filters by modulating the local birefringence of helically structured thin films using an apodization function. Both theoretical simulations based on Berreman formalism, and experimental results involving evaporated TiO(2) thin films are presented and compared.

Composites comprising cholesterol and carboxymethyl cellulose

Whereby cholesterol presents one of the major fatty substances in human body, carboxymethyl cellulose is a water-soluble derivative of cellulose, the most abundant dietary fiber. Whereas on one hand in vivo precipitation of cholesterol is the major cause of atherosclerosis, dietary fibers are on the other hand known for their ability to clean the fatty plaque deposited on intestinal pathways, and prevent its build-up in other critical areas within the organism. In this work, a method for the preparation of a composite material comprising cholesterol and carboxymethyl cellulose from 1-hexanol/water biphase mixtures is reported. Specificity of the interaction between the composite components in the given conditions of synthesis inhibits the tendency of solid cholesterol to adopt typical plate- or needle-shaped morphologies. Instead, control of the thixotropic behavior of the constituent polymer phase leads to the formation of bubbling, multi-layered cholesteric films. In view of the major illnesses that involve biological precipitation of cholesterol crystals, these findings may be considered as pointing towards the interactional specificity of potential chemotherapeutic and/or nutritional significance. Scanning electron microscopy, thermal and diffractometric analyses were performed as parts of the characterization of the prepared material.

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.