Surface-mediated photoalignment of discotic liquid crystals on azobenzene polymer films

This paper describes a simple strategy for the formation of photoaligned and micropatterned discotic liquid crystal (DLC) film on the surface of photoirradiated azobenzene-containing polymer thin film. The key material for the surface-mediated photoalignment of the DLCs was poly[4-(4-cyanophenylazo)phenyl methacrylate] (pMAzCN). Optical anisotropy was generated in a pMAzCN film by oblique exposure to nonpolarized light which resulted in angle-selective photoisomerization and reorientation of the azobenzenes. Subsequent annealing of the film at 240 degrees C enhanced the photoaligned state of the p-cyanoazobenzenes due to strong intermolecular dipole-dipole interaction and semicrystalline nature of the pMAzCN. This combination of photoirradiation and subsequent annealing of the pMAzCN film made it possible to realize the surface-assisted orientation control of a DLC molecule, which displays both columnar (Col) and discotic nematic (N(D)) phases over 152 degrees C. When the pMAzCN film was exposed to linearly polarized light from the surface normal, the DLC molecules showed homeotropic orientation with the director perpendicular to the substrate surface. In the contrast, oblique irradiation of the pMAzCN film with nonpolarized light gave rise to tilted DLC orientation with well-ordered optical birefringence at the N(D) phase. Rapid cooling from the N(D) phase produced a well-aligned glassy N(D) state at room temperature, which was adequately stable for 10 months even though no covalent cross-linking among the DLCs was performed. The spatial orientation of photoaligned DLCs in both their bulk film and in their interface region was characterized by means of optical birefringence, X-ray diffraction, and fluorescence measurements. At the N(D) phase, the DLC molecules were aligned in a hybrid manner such that their tilt angles varied throughout the thickness of DLC film. The direction of tilted DLCs was opposite to the propagation of the actinic nonpolarized light. The photoaligned DLC films exhibited polarized fluorescence emission with an s-polarized/p-polarized intensity ratio of 4.1, despite the nonpolarized excitation of only DLC at outmost surface. These results indicate that the three-dimensionally aligned azobenzene moieties of the pMAzCN thin film were transferred to the tilted DLC molecules at air/DLC interface. Finally, we demonstrated micrometer-scale photopatterned orientation of DLC molecules on the pMAzCN surface by oblique nonpolarized irradiation of the film through a photomask.     

Studies of polymer ball type polymer dispersed liquid crystal films

'Reverse' or 'polymer ball' polymer dispersed liquid crystal (PDLC) samples were prepared by a photopolymerization induced phase separation method. A detailed study of the effects of the sample preparation parameters, such as curing time, curing intensity, and liquid crystal concentration are reported. It was found that by adjusting these parameters, we were able to change the morphology of these 'polymer ball' PDLC samples and thus change their optical characteristics. Incomplete polymerization of the PDLC samples results in a higher threshold voltage and a lower ON state transmission. When the amount of monomer is too low, the shape of the resulting polymer ball becomes irregular, and the sample has a larger threshold voltage and a larger saturation voltage.     

Surface-assisted photoalignment of discotic liquid crystals by nonpolarized light irradiation of photo-cross-linkable polymer thin films

In this article, we describe the surface-assisted photoalignment of discotic liquid crystals (DLCs) on thin films of photo-cross-linkable polymers with cinnamoyl moieties as the side chains. Oblique irradiation of the polymer thin films with nonpolarized UV light at 313 nm brought about inclined orientation of the cinnamoyl residues as a result of their direction-selective photoisomerization and photodimerization. The DLC molecules on the photoirradiated polymer films were aligned in a tilted hybrid manner. This means that the DLC directors are continuously altered from the substrate to the DLC film surface so as to minimize the elastic free energy. Interestingly, we found that the tilted direction of aligned DLC molecules is clearly influenced by the chemical structures of the cinnamate-containing polymers. When a poly(vinyl cinnamate) thin film was obliquely exposed to nonpolarized UV light, the DLCs were inclined to the direction opposite to the UV light propagation. In a keen contrast, the thin film of poly(methacrylate)s tethering cinnamoyl groups, which was obliquely exposed to nonpolarized UV light in advance, provided the tilting DLC direction in parallel with the light propagation. The results were supported by tilted orientation of calamitic (rod-shaped) liquid crystal on the obliquely irradiated polymer films. Such photoalignment behavior of the DLCs can be rationalized by anchoring balance between intermolecular interaction of the DLC molecules with the photodimers of polymer films and those with the remaining E-isomers of cinnamoyl side chains at the film interface. The present technique of DLC photoalignment opens promising ways not only to understand anisotropic physical properties of DLCs, but also to design and fabricate novel nanodevices for photonics and electronics applications.     

Striped pattern formation in polymer dispersed liquid crystal films

Composite films of polymer and liquid crystal (LC) have been prepared by a simple solution casting technique. The films obtained exhibit a polymer dispersed liquid crystal structure where LC droplets are dispersed in the polymer matrix. Casting the mixture of polymer solution and LC on a tilted substrate results in a specific texture in which arrays of LC droplets align parallel to each other, i.e. a striped pattern can be formed. The size of the droplets and the spacing between the lines are dependent on the substrate tilt angle and the preparation temperature. By using a dip-coating technique, a similar striped texture appears at a fast dipping velocity. From in situ observation of the stripe development, it is seen that the translation of the phase-separating region, accompanying the flow, plays a more important role in the formation of the striped patterns than the flow of the solution itself.     

Fluorescence behaviour of cyanobiphenyl liquid crystal molecules in liquid crystal/polymer composite films

The fluorescence behaviour of the liquid crystal, 4-cyano-4-pentylbiphenyl (5CB), in composite thin films prepared by the photopolymerization of 5CB/diacrylate mixtures, was investigated by means of three different excitation methods, in which the total-internal-reflection or surface-limited excitation method was used for analysis of the fluorescence from an ultra-thin interface layer ( 100nm) in contact with the substrate surface, whereas the fluorescence from the interior bulk was analysed by the through-film excitation method. It was found that intensity ratios of the monomer and excimer emissions of 5CB are significantly lower in the interface layer than in the interior bulk, depending upon photopolymerization conditions as well as upon the structures of the diacrylates used. Scanning electron microscopic observations and light-scattering measurements of some typical composite films showed possible relationships between morphological features and fluorescence characteristics depending upon the diacrylate structures and polymerization conditions. The different fluorescence behaviour has been discussed in terms of differences in mobility and/or aggregation degrees of 5CB molecules arising from dominant molecular interactions with the substrate and polymer surfaces.     

High on-state clarity polymer dispersed liquid crystal films

Abstract

Results are presented which show that the on-state clarity of a UV cured polymer dispersed liquid crystal (PDLC) film depends on the refractive index of the final polymer in the PDLC film, the ordinary refractive index of the liquid crystal, the solubility of the liquid crystal in the prepolymer and the rate at which the film is cured. Liquid crystal mixtures for use in PDLC films are chosen such that the ordinary refractive index of the liquid crystal is equal to the refractive index of the polymer matrix. It has been shown previously that a large quantity of liquid crystal remains dissolved in the polymer matrix, thus increasing the mismatch between the refractive index of the polymer and the ordinary refractive index of the liquid crystal and therefore reducing the on-state clarity. For liquid crystal mixtures which have high solubility in the prepolymer (>60 per cent) the mismatch in the refractive indices can be very large and the on-state clarity of the resulting film can be very poor (T _on<70 per cent). Results are presented which show that it is possible to increase the on-state clarity of such films by increasing the rate at which these films cure. If the liquid crystal is less soluble in the prepolymer (<45 per cent), a PDLC film formed from such a liquid crystal/ prepolymer system often has very good on-state clarity (T _on>75 per cent) be it cured slowly or quickly. Results are also presented which show that in order to achieve a true measure of on-state clarity it is necessary to use a small collection angle (<3°) in the detecting optics. If larger collection angles are used, the photodetector collects light which is scattered out of the specular beam, thus leading to a false measure of on-state clarity.     

Dielectric relaxation in polymer dispersed nematic liquid crystal films

Dielectric absorption studies for polymer dispersed liquid crystal (PDLC) samples of different nematic (4-hexyl-4'-cyanobiphenyl, 6CB) contents (20, 30, and 40 wt%) have been carried out in the frequency range from 10kHz to 10MHz. A method for evaluation of the fraction of the molecules which are not affected by the polymer surface anchoring forces in the nematic droplets is proposed.     

Striped pattern formation in polymer dispersed liquid crystal films

Composite films of polymer and liquid crystal (LC) have been prepared by a simple solution casting technique. The films obtained exhibit a polymer dispersed liquid crystal structure where LC droplets are dispersed in the polymer matrix. Casting the mixture of polymer solution and LC on a tilted substrate results in a specific texture in which arrays of LC droplets align parallel to each other, i.e. a striped pattern can be formed. The size of the droplets and the spacing between the lines are dependent on the substrate tilt angle and the preparation temperature. By using a dip-coating technique, a similar striped texture appears at a fast dipping velocity. From in situ observation of the stripe development, it is seen that the translation of the phase-separating region, accompanying the flow, plays a more important role in the formation of the striped patterns than the flow of the solution itself.     

Azobenzene polymer stabilized ferroelectric liquid crystals: simultaneous photopolymerization and photoalignment

A single linearly polarized irradiation was used to initiate cationic photopolymerization of an azobenzene-containing diepoxide monomer dissolved in a ferroelectric liquid crystal (FLC) and, at the same time, to induce fast trans-cis-trans photoisomerization cycles for the azobenzene chromophore which leads to a bulk alignment of the FLC in the absence of surface orientation layers. The spectral output of the light source is required to have separate UV and visible wavelengths, which allow for simultaneous photopolymerization of the monomer and photoalignment of the chromophore. Photopolymerization was allowed to take place in different phases of the FLC to reveal the effects on the FLC photoalignment; the interaction between the anisotropic azobenzene polymer network and the FLC host was also investigated using polarizing UV-Vis spectroscopy.     

Reverse-mode polymer dispersed liquid crystal films prepared by patterned polymer walls

This article proposes a methodology to prepare polymer dispersed liquid crystal (PDLC) films working in the reverse-mode operation, where the ion-doped nematic liquid crystals (NLCs) with negative dielectric anisotropy (Δε) were locked by polymer walls. On-state and off-state of films were controlled by an electric field. In the absence of an electric field, it appears to be transparent. In the field, the homogeneous alignment NLCs form dynamic scattering, giving rise to opaque. The effect of the cylindrical holes with different diameters of photo masks and liquid crystal Δε on the electro-optical properties and transmittance wavelength range of 400–3000 nm light of samples were investigated. It was found that it exhibited very good electro-optical characteristics, high contrast ratio and excellent infrared energy-efficient of films used as switchable windows.     

Photoinduced anisotropy and photoalignment of nematic liquid crystals by a novel polymer liquid crystal with a coumarin-containing side group

A new type of photo-crosslinkable methacrylate polymer liquid crystal (PLC) with a coumarincontaining mesogenic side group was synthesized and applied as the photoalignment layer for low molecular mass nematic liquid crystals. Linearly polarized ultraviolet light was directed onto a thin film of PLC under various exposure conditions. When a film was irradiated at room temperature, a small negative optical anisotropy was generated due to angular-selective photo-crosslinking. In contrast, when the film was exposed near the clearing temperature of the PLC, the induced anisotropy was positive due to thermally enhanced photoinduced reorientation of the side groups. The aggregation of the mesogenic groups was also observed when the irradiation was carried out in the liquid crystalline temperature range of the PLC. The LC alignment on the photoreacted film was greatly dependent on these irradiation conditions. It was made clear that the LC alignment was regulated by the interaction among the LC, the photo-crosslinked side groups and the remaining mesogenic side groups, and that the aggregated mesogenic groups inhibited the LC alignment.     

Photoinduced anisotropy and photoalignment of nematic liquid crystals by a novel polymer liquid crystal with a coumarin-containing side group

A new type of photo-crosslinkable methacrylate polymer liquid crystal (PLC) with a coumarincontaining mesogenic side group was synthesized and applied as the photoalignment layer for low molecular mass nematic liquid crystals. Linearly polarized ultraviolet light was directed onto a thin film of PLC under various exposure conditions. When a film was irradiated at room temperature, a small negative optical anisotropy was generated due to angular-selective photo-crosslinking. In contrast, when the film was exposed near the clearing temperature of the PLC, the induced anisotropy was positive due to thermally enhanced photoinduced reorientation of the side groups. The aggregation of the mesogenic groups was also observed when the irradiation was carried out in the liquid crystalline temperature range of the PLC. The LC alignment on the photoreacted film was greatly dependent on these irradiation conditions. It was made clear that the LC alignment was regulated by the interaction among the LC, the photo-crosslinked side groups and the remaining mesogenic side groups, and that the aggregated mesogenic groups inhibited the LC alignment.     

Highly transparent memory states in polymer dispersed liquid crystal films

Electro-optical properties of polymer dispersed liquid crystal (PDLC) films which have reversed morphology are investigated. Highly transparent memory states, for which transmittances exceed more than 80 per cent, are observed in these PDLC films. The saturation voltage V₉₀ can be decreased by a PTF (a phase transition with a field) operation and becomes 10 times lower than that without PTF operation. A contrast ratio of more than 600 is obtained in the memory state of a thick PDLC film.     

Copolymerizable initiator for improved polymer dispersed liquid crystal films

Abstract

The problems of photoinitiator contamination are addressed for the liquid crystal phase in polymer dispersed liquid crystal films formed by photopolymerization induced phase separation of liquid crystal from monomer solutions. Initiator contamination lowers the clearing point of the liquid crystal phase, and decreases the photostability and resistivity of the polymer dispersed liquid crystal. These problems are minimized by replacing the conventional photoinitiators with copolymerizable initiators which become incorporated in the polymer phase as it separates. Copolymerizable photoinitiators are studied and used to form polymer dispersed liquid crystals with higher clearing point liquid crystal phases, higher resistivity, and better photostability than polymer dispersed liquid crystals formed with conventional photoinitiators. These improvements provide very significant advantages for many polymer dispersed liquid crystal applications.     

Copolymerizable initiator for improved polymer dispersed liquid crystal films

The problems of photoinitiator contamination are addressed for the liquid crystal phase in polymer dispersed liquid crystal films formed by photopolymerization induced phase separation of liquid crystal from monomer solutions. Initiator contamination lowers the clearing point of the liquid crystal phase, and decreases the photostability and resistivity of the polymer dispersed liquid crystal. These problems are minimized by replacing the conventional photoinitiators with copolymerizable initiators which become incorporated in the polymer phase as it separates. Copolymerizable photoinitiators are studied and used to form polymer dispersed liquid crystals with higher clearing point liquid crystal phases, higher resistivity, and better photostability than polymer dispersed liquid crystals formed with conventional photoinitiators. These improvements provide very significant advantages for many polymer dispersed liquid crystal applications.     

Highly transparent memory states in polymer dispersed liquid crystal films

Abstract

Electro-optical properties of polymer dispersed liquid crystal (PDLC) films which have reversed morphology are investigated. Highly transparent memory states, for which transmittances exceed more than 80 per cent, are observed in these PDLC films. The saturation voltage V ₉₀ can be decreased by a PTF (a phase transition with a field) operation and becomes 10 times lower than that without PTF operation. A contrast ratio of more than 600 is obtained in the memory state of a thick PDLC film.     

Optical properties of stretched polymer dispersed liquid crystal films

The basic mechanisms determining the formation of optical anisotropy in stretched, thin polymer dispersed liquid crystal (PDLC) films with micron sized nematic droplets have been studied experimentally and the results analysed in terms of a proposed theoretical model. The experiments were performed on PDLC films with the bipolar nematic director configuration in the droplets, where the film transmittance, microscopic structure, and birefringence of the polymer matrix were studied. It is shown that the orientational ordering of bipolar nematic droplets, introducing the main contribution to the ability of stretched PDLC film to polarize the transmitted light, is strongly dependent upon initial droplet shape and the elastic properties of the polymer matrix. The 'anomalous' nematic director orientation is also observed in a portion of elongated droplets where the axes of bipolar configurations do not coincide with the major axes of the droplet cavities due to the presence of inclusions at the cavity walls. The effect of alternation of droplet size and shape upon stretching and the influence of optical anisotropy of the polymer matrix on film transmittance are analysed. On the basis of the results obtained, simple criteria for optimization of main PDLC polarizer performance are formulated.     

Effects of the fluorinated liquid crystal molecules on the electro-optical properties of polymer dispersed liquid crystal films

The different fluorinated liquid crystal (LC) molecules doped to E8 were used as LC component to prepare polymer dispersed liquid crystal (PDLC) films. The mass fraction of the LC mixture is fixed 50.0 wt%. Results indicate that doping 8.0 wt% fluorinated LC molecule ME3CP to E8 significantly reduced the driving voltage of the PDLC films, and the driving voltage reduced with the rise of mass fraction of ME3CP. Besides, the terminal flexible chain length of the fluorinated LC molecule influenced the LC mixture properties based on E8, such as the dielectric anisotropy, birefringence and viscosity of the LC mixture, and the morphology and the electro-optical properties of PDLC films were controlled not only by the physical properties of the LC mixture, but also by the terminal flexible chain length of the fluorinated LC molecule .     

Electro-optical effects of anchoring transition in polymer dispersed liquid crystal films

Two groups of polymer dispersed liquid crystal films have been studied, one with a fixed cell thickness but varying liquid crystal (LC) concentrations and the other with a fixed LC concentration but varying cell thicknesses. A sudden decrease in transmittance with increasing temperature was observed for films whose LC domain sizes were comparable to their cell thicknesses. In particular, spontaneous alignment of LC directors was observed below ∼20°C when LC domains were formed spanning the space between upper and lower indium tin oxide-coated glass substrates. With increasing temperature, this axially aligned homeotropic configuration changed gradually into homogeneous configurations. We believe that the sudden decrease in transmittance originated from the anchoring transition at the glass substrates and polymer walls. In addition, it has been found that the intensity of ultra violet irradiation has a strong effect on the director configuration of LC domains, and that the cure temperature affects the anchoring transition temperature significantly.