Alignment control of liquid crystals on surface relief gratings

Liquid crystal alignment layers of a high T_g polymer containing an azobenzene moiety are prepared by photofabrication of a surface relief grating (SRG). The interference pattern of a circular and linearly polarized Ar⁺ laser beam generated the surface relief grating and the morphology was detected by atomic force microscope. The optical anisotropy of the films was investigated by polarizing optical microscopy. The orientation of the optical axis of the film mainly depends on the direction of the initial polarization plane. Nematic liquid crystals were aligned parallel to the direction of the grating, but the pretilt angles of the liquid crystals were nearly zero. Irradiation with homogeneous linearly polarized light could also align liquid crystals, but this alignment capability was weaker than that of the SRG film.     

Controllable alignment of liquid crystals based on the surface relief gratings from a photocross-linkable organic monomer

The orientational control of liquid crystal (LC) molecules is essential for high-quality liquid crystal displays, and the photo-induced surface relief grating (SRG) is a facile and effective non-contact process. Here, SRGs with different period and depth were prepared with a photocross-linkable organic monomer 4-propyldiphenylacetylenecarboxylic acid cinnamyl ester (PDACE), and the LC alignment induced by SRG was studied. It is found that both the surface topography and the chemical nature of the surface are responsible for the LC alignment, which is strongly dependent on the groove geometry of the gratings. Furthermore, the patterned LC cell was fabricated with the patterned SRG surface. These results demonstrate that the planar, perpendicular and patterned orientations of LC can be easily photo-controllably obtained with PDACE, which have important applications in optical devices.     

Photo-fabrication of surface relief gratings on polymer films

Surface relief gratings were optically produced on a number of azobenzene-based polymer films. The surface grating formation was investigated by monitoring the diffraction efficiency and using atomic force microscopy. The effect of the structure of the chromophores on surface modulation was investigated. The surface deformation process depended on the polarization state of the writing beams. The localized variations of the light intensity and alteration of the resulting electric field polarization were essential writing conditions to the formation of the surface relief gratings. The surface pattern from straight edge diffraction established that the surface profile of the recorded gratings is proportional to negative gradient of the intensity pattern incident on the polymer film.     

Alignment of liquid crystals on ultrathin organized molecular films

Liquid crystal (LC) alignment techniques based on various kinds of ultrathin organized molecular films are reviewed. The mechanisms of LC alignment on the organized films are discussed. For the homeotropic alignment of LCs the main anchoring mechanism is due to the dipole–dipole interaction between polar groups of an aligning agent and LC molecules while the homogeneous alignment is mainly attributed to the orientation of polymer chains or polymer aggregates. An experimental system for an anchoring transition induced by a conformation change of aligning molecules is introduced. Finally the AFM experimental observations on the rubbed polymer films and its mechanisms are summarized.     

Alignment of carbon nanotubes in nematic liquid crystals

The self-organizing properties of nematic liquid crystals can be used to align carbon nanotubes dispersed in them. Because the nanotubes are so much thinner than the elastic penetration length, the alignment is caused by the coupling of the unperturbed director field to the anisotropic interfacial tension of the nanotubes in the nematic host fluid. In order to relate the degree of alignment of the nanotubes to the properties of the nematic liquid crystal, we treat the two components on the same footing and combine Landau-de Gennes free energies for the thermotropic ordering of the liquid crystal and for the lyotropic nematic ordering of carbon nanotubes caused by their mutually excluded volumes. The phase ordering of the binary mixture is analyzed as a function of the volume fraction of the carbon nanotubes, the strength of the coupling and the temperature. We find that the degree of ordering of the nanorods is enslaved by the properties of the host liquid and that it can be tuned by raising or lowering the temperature or by increasing or decreasing their concentration. By comparing the theory to recent experiments, we find the anchoring energy of multiwalled carbon nanotubes to be in the range from 10(-10) to 10(-7) N m(-1).     

Photonic control of surface anchoring on solid colloids dispersed in liquid crystals

The anchoring of liquid-crystal (LC) mesogens to the surfaces of colloids is an important factor in determining intercolloidal interactions and the symmetry of the ensuing colloidal assembly in nematic colloids. The dynamic control of surface anchoring could therefore provide a handle to tune the colloidal organization and resulting properties in these systems. In this article, we report our results on the study of thermotropic nematic LC (E7) dispersions of silica and glass microcolloids bearing photosensitive surface azobenzene groups. By the photoinduced modulation of the colloidal-LC interfacial properties, due to the trans-cis isomerization of azobenzene units, we tune the anchoring on silica colloids from homeotropic (trans-azobenzene) to homogeneous planar (cis-azobenzene) reversibly. In tune with the change in surface anchoring, the interparticle interactions were also dictated by dipolar and quadrupolar symmetries for homeotropic and homogeneous planar anchoring, respectively. In our experiments, we find that, in addition to the isomerization state of the surface-bound azobenzene units, the nature of the colloid plays a crucial role in determining the anchoring state obtained on applying photostimuli. We also study the LC anchoring on colloids as a function of the azobenzene surface density and find that beyond a threshold value the anchoring properties remain invariant.     

Photoinduced surface relief gratings in high-Tg main-chain azoaromatic polymer films

We have synthesized two classes of polyureas with mono- and bisazoaromatic groups in their main chains via reactions between isophorone diisocyanate and the corresponding diamines. Holographic gratings were fabricated on azoaromatic polyurea films prepared by spin-coating from solutions. The effect of high glass transition temperature and dipole moment of azo groups on the formation of gratings was investigated. Although the two polymers have relatively high glass transition temperatures (197 and 236°C), chromophore alignment was induced by laser beam irradiation at modest light intensities. Regularly spaced surface relief gratings on the polymer film were also recorded upon exposure to an interference pattern of two polarized argon laser beams. Erasure could be achieved by heating above T_g or by exposure to one of the beams in a manner similar to low-T_g side-chain azo polymers. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 283–289, 1998     

Alignment of guest-host liquid crystals with polarized laser light

Surface-mediated alignment of nematic liquid crystals with polarized laser light was reported recently [1]. In this communication we describe the alignment of a guest-host liquid crystal medium with polarized laser light. Liquid crystals in the illuminated region orient perpendicular to the direction of the laser polarization and remain aligned in the absence of laser radiation. The liquid crystals can be reoriented again by subsequent illumination. The kinetic feature of this surface-mediated liquid crystal orientation is characterized by the presence of coexisting liquid crystal regions of directors pointing away from the initial alignment.     

Alignment and alignment dynamics of nematic liquid crystals on Langmuir-Blodgett mono-layers

Mono-layers of stearic and behenic acids, deposited with the Langmuir-Blodgett technique, were used as aligning films in nematic liquid crystal cells. During the filling process the liquid crystal adopts a deformed quasi-planar alignment with splay-bend deformation and preferred orientation along the filling direction. This state is metastable and transforms with time into a homeotropic state once the flow has ceased. The transition is accompanied by formation of disclination lines which nucleate at the edges of the cell. The lifetime of the metastable splay-bend state was found to depend on the cell thickness. On heating, an anchoring transition from quasi-homeotropic to degenerate tilted alignment in the form of circular domains takes place near the transition to the isotropic phase. The anchoring transition is reversible with a small hysteresis.     

Alignment of liquid crystals on solution-processed HfZnO films via ion beam irradiation

Liquid crystal (LC) alignment layers were prepared by fabricating solution-processed HfZnO films, annealing them, and treating them with ion beam (IB) irradiation, and the effect of annealing temperature upon the resulting film properties was studied. Homogeneous LC alignment was achieved on IB-irradiated HfZnO films. Topographical changes were observed from field-emission scanning electron microscopy as annealing temperature increased. X-ray photoelectron spectroscopy analysis showed that IB irradiation resulted in oxidation of HfZnO surfaces, which caused the LCs to be oriented more uniformly. The best electro-optical characteristics observed corresponded to the annealing temperature of 200°C. The low optimal annealing temperature for fabricating the HfZnO films suggested that this material has remarkable potential for LCD applications.     

Alignment of liquid crystals induced by a photopolymerized self-assembled film

An alignment film derived from a photopolymerized self-assembled film may be used to orient nematic liquid crystals after irradiating the film with linearly polarized UV (LPUV). A photosensitive cationic amphiphile was first synthesized containing two double bonds and which could be polymerized by UV. A layer-by-layer self-assembled multilayer film was next prepared in an aqueous solution of the cationic amphiphile and poly(sodium 4-styrenesulphonate); the UV-Vis spectra showed that each layer of the LBL multilayer film was uniform. When the film was irradiated by LPUV, the photosensitive double bonds underwent [2+2] cycloaddition along the vector direction of LPUV. The polarized UV-Vis absorption spectra also provided evidence that the film was anisotropic, i.e. the photopolymerization was along a certain direction. The anisotropic film was used as an alignment layer for nematic liquid crystals, and observations under a polarizing microscope indicated that the alignment of the liquid crystals was good, as expected, and that the orientation direction of the liquid crystals was always perpendicular to the electric vector of the irradiating LPUV.     

Photofabrication of surface relief gratings from azobenzene containing perfluorocyclobutane aryl ether polymer

An azobenzene based perfluorocyclobutane (PFCB) aryl ether polymer, poly[4‐(phenylazo)triphenylamine‐1,2‐hexafluorocyclobutyl ether], was synthesized by using a palladium catalyzed amination reaction and the 2π + 2π cyclodimerization of 4‐[(trifluorovinyl)oxy]bromobenzene. This polymer was designed and synthesized to permit azo chromophores to be incorporated, thus introducing photoinduced dynamic behavior into the PFCB containing polymer structure. The polymer exhibited a T_g of about 122 °C, and the maximum absorption of the azo chromophores was 407 nm in the film state. A thin film was prepared through a spin coating process, and the rapid growth of diffraction efficiency was then achieved by irradiation with an interference beam. The efficient formation of holographic surface relief gratings was also verified from AFM images, which show regularly defined and aligned grating structures. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3525–3532, 2005     

Electro-nanopatterning of surface relief gratings on azobenzene layer-by-layer ultrathin films by current-sensing atomic force microscopy

The electro-nanopatterning and mechanism of pattern formation in azobenzene-containing layer-by-layer (LbL) ultrathin films is described using surface probe microscopy techniques. First, arrays of nanodots were patterned on these films to investigate applied time at constant voltage bias dependence in electro-nanopatterning. The anisotropic mass transport and polar alignment of the azobenzene-containing films were observed after applying the electric field and heating the sample locally with the cantilever tip. On the basis of this novel phenomenon, small-sized surface relief gratings (SRG)s and their alignment were fabricated and observed by current-sensing atomic force microscopy. The rate of mass transport for the polymer is mainly controlled by the applied time at constant voltage bias between the cantilever and the electrode/substrate.     

Fabrication of surface: relief gratings and their laser induced damage resistance properties

This work demonstrates a promising method for fabricating ZrO₂ surface–relief gratings by photosensitive sol–gel method combining with two-beam laser interference. UV-photosensitive ZrO₂ resist with high resolution are prepared by using metal–chelate complexes as precursors. Two-beam laser interference lithography is carried out by a Kr ion laser with a wavelength of 350.7 nm and ZrO₂ gratings with the minimum line-width of 150 nm are obtained. Laser induced damage resistance properties of ZrO₂ gratings are investigated by a Q-switched solid state laser with the wavelength of 527 nm and the pulse width of 8.65 ns as the incident source. The results show that the LIDT of ZrO₂ gratings on glass substrate heated at 600 °C is about 4.59 ± 0.06 J/cm².     

Molecular weight effect on the photoinduced birefringence and surface relief gratings formation of a methacrylate azopolymer

Samples of a methacrylic side-chain azopolymer were synthesized by a radical copolymerization of methyl-methacrylate with the methacrylic derivative of the commercial dye Disperse Red-13 (DR13). Copolymers with different molecular weight were obtained by varying the reaction time and the concentration of initiator. Samples with molecular weight averages (M_w) from 3 to 8 × 10⁴ g/mol and 1.4-1.7 polydispersity were obtained. The glass transition temperature of the samples increased linearly from the lower to the highest molecular weight. Optical quality cast films were prepared for all samples and photoinduced birefringence was successfully carried out in all samples as well as surface relief gratings could be inscribed in the cast films.     

New developments in the dye-doped polymer dispersed liquid crystals gratings: A review

In this study, the dye-doped polymer dispersed liquid crystals (PDLC) gratings techniques performed by the various research groups or being developed are briefly reviewed. Especially, the electrically switched diffraction and holographic gratings, have attained much attention by various research groups working in the PDLC-related display studies. The fabrication methodologies used for such grating texture, include like the conventional dye-doped PDLC grating, Azo-dye doped PDLC gratings, and lasing techniques etc., adopted for dye-doped PDLC gratings. The useful features and characteristics of their fabrication process of such gratings are discussed. Finally, some of the future perspectives on this particular research field are presented.     

Synthesis of novel three‐arm star azo side‐chain liquid crystalline polymer via ATRP and photoinduced surface relief gratings

A three‐arm star azo side‐chain liquid crystalline (LC) homopolymer, poly[6‐(4‐methoxy‐4‐oxy‐azobenzene) hexyl methacrylate] (PMMAZO), was synthesized by atom transfer radical polymerization (ATRP) method. The polymerization of 6‐(4‐methoxy‐4‐oxy‐azobenzene) hexyl methacrylate proceeded in a controlled/“living” way. A series of three‐arm star LC block copolymers (PMMAZO‐b‐PMMA) were also synthesized. The polymers were characterized by ¹H NMR, gel permeation chromatograph, and UV–vis spectra, respectively. The both polymers of PMMAZO and copolymers of PMMAZO‐b‐PMMA exhibited a smetic phase and a nematic phase. As concern to the PMMAZO, the glass‐transition temperature (T_g) and phase‐transition temperature from the smetic to nematic phase and from the nematic to isotropic phase increased with the increase of molecular weight (M_n(GPC)) of PMMAZO. The phase transition temperature of the block copolymers, PMMAZO‐b‐PMMA, with the same PMMA block was similar to that of PMMAZO. However, the T_g of the PMMAZO‐b‐PMMA decreased at low azo content and then increased with the increasing M_n(GPC) when azo content was above 61.3%. With illumination of linearly polarized Kr⁺ laser beam at modest intensities (35 mW/cm²), significant surface relief gratings formed on PMMAZO films with different molecular weights were observed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 777–789, 2008     

Alignment and switching of nematic liquid crystals embedded in porous chiral thin films

Porous thin films with engineered microstructures have been fabricated using glancing angle deposition (GLAD). GLAD films with chiral microstructures have been previously shown to exhibit unique chiral optical response. The pores of these films were embedded with (non-chiral) nematic liquid crystals (LCs) to produce a new composite optical material wherein the GLAD film induces chiral nematic-like LC orientation. We demonstrate here reversible electro-optic switching of the LC component of these hybrid films. Unaddressed, cells of GLAD/LC hybrid films show enhanced chiral optic response compared with the unfilled GLAD film. When addressed, the chiral optic response vanishes.