Liquid Crystalline Alignment

The relationship of glass transitions to mobility and isomerization in confined polymer systems aroused much interest. It was influenced by need for alignment in liquid crystalline flat panel displays, because in these displays films of polyimides are widely used. The surfaces are usually treated to produce uniform alignment of the liquid crystals into suitable “pretilt” angles. The treatments consisted of rubbing process with velvet fabrics. Search for new methods, however, led to develop ment of molecular structures that undergo alignment upon irradiation with linearly polarized UV light [222, 223]. Polymer-stabilized liquid crystals are low-molar-mass liquid crystal. Their bulk align ment or their texture is stabilized by a polymer network. Such polymer network is usually in low concentration [222, 223]. Several types of polarized-light-induced liquid crystalline aligning of molecules were reported in the literature [223]. One photo-alignment material is poly (vinyl cinnamate). The polymer and its copolymers were reported to have the ability to align in thin films in the direction perpendicular to the axis of the linearly polarized ultraviolet light. This photo-alignment mechanism has not been fully elucidated at present. A drawback to using poly (vinyl cinnamate) and its copolymers is a low glass transition temperature. As a result, they remain mobile after treatment and chain orientation. Other materials with higher Tg are, therefore, needed. Among the most promising ones are polyimides. They form liquid crystal alignment layers in flat panel displays and possess good optical transparencies, adhesion, heat resistance, and dimensional stability, and are good insulators. There are various reports in the literature about preparations of soluble photo-reactive polyimides with cinnamate chromophore side groups. Thus, it was reported by Lee et al. [224] that they prepared a photo-reactive polyimide with cinnamate chromophores side groups:

This polyimide is claimed to be thermally stable up to 340C and has a glass transition temperature of 181C. Also, it was demonstrated that the cinnamate chromophores, upon irradiation with linearly polarized ultraviolet light, undergo both photo-isomerization and dimerization. In addition, the light exposure induces anisotropic orientation of the polymer main chains and of the cinnamate side groups in the film. The irradiated films align homogeneously the nematic liquid crystal molecules along one direction at an angle of 107 with respect to the polarization. The liquid crystalline alignment was found to be thermally stable up to 200C. It was also reported [225] that photo-reactivity of side-chain liquid crystalline polymers can align liquid crystals both in a parallel mode or perpendicularly, depending on the degree of the photoreaction of the polymers. Presumably, this particular polymer can multiphoto-align the liquid crystal pattern without a change of the direction of the linearly polarized UV light. The chemical structure of such an aligning polymer is depicted as follows:

where n = 2or6. It was concluded[226],therefore, that the liquid crystals align both parallel and perpendicular to the incident E direction on the photocross-linked polymer film by changing the degree of the reacted cinnamoyl group. That can be controlled by irradiation time. A bias-tilt angle between the liquid crystals director and the substrate is also realized by controlling the irradiation angle of the light [226]. Another approach to liquid crystalline alignment is based on photo-isomerization of azo compounds in polymeric materials or as part of the polymer structure [227]. In recent years, investigation of the use of azobenzene-containing polymers for liquid crystalline alignment became quite thorough because of the potential application in holographic storage as well as optical and photonic use [228–230]. The photo-alignment of liquid crystalline polymers containing azobenzene groups has an advantage of local variation of the orientation order due to pixel-wise irradiation. This is a process that is reported to take place via angular-dependent excitation, a series of cis-trans photo isomerization cycles, and rotational diffusion within the steady state of the photoreaction. This results in the photochromic side group becoming oriented perpendicularly to the electric field vector of the incident light and establishing an oblate order in the films. Thus, studies of exposure of films of liquid crystalline polymers with azobenzene side chains to linearly polarized light of 436 nm [231] show successive occurrences of uniaxial in-plane orientations, followed by out-of-plane orientations of azobenzenes [231]. Two kinds of orientation modes were observed. These are possibly extreme cases, when linearly polarized light with the electric vector parallel to the xz-plane comes along the z-direction. One is the uniaxial in-plane orientation of the azobenzene with a dipole moment parallel to the x axis from the x- to y-direction and the other is out-of-plane (homeotropic) one toward the z-direction. Marked dependence of photo orientation processes on film temperatures was observed. In-plane orientation was generated in the glassy state. Photo-orientation at higher temperatures, slightly below the transition temperature between smectic and nematic phases, gives rise to distinct transformations from in-plane orientation at the early state to successive out-of-plane reorientations [232]. These orientations can be illustrated as follows:

Also, it was reported [233], that irradiation of a liquid crystalline polyester with azobenzene side chains with light of an argon laser (514, 488, or 351 nm) results in orientation of 4-cyano-40 alkoxy azobenzene side groups. The polyester is characterized by smectic and nematic phases and shows a strong tendency to form J-aggregates. The orientation process is cooperative, whereas the light-induced orientation of the photochromic moiety causes an ordering of the alkylene spacers and even of the main-chain segments into the same direction. It was concluded [226] that the most probable mechanism of this two-step process is the angular-selective transformation of the bulky trans-isomers to the rod-like cis-isomeric formed by the red light. The aligned cis-azobenzene side groups become strongly J-aggregated. Very high values of dichroism of about 0.8 and birefringence of about 0.3 were generated as a result of this combination of the photo-induced orientation process and the thermotropic self-organization, which takes place simultaneously under the irradiation conditions. The process results in a uniaxial prolate order of the film, whereas conventional photo orientation leads to a biaxial oblate order [93]. Thus, the direction of photo-induced orientation and the type of the three-dimensional orientation order can be controlled by the wavelength of the irradiating linearly polarized light. This can also be done in the same film of a smetic polyester with 4-cyano-40-alkoxyazobenzene side group [233]. Zhao and coworkers [234, 235] reported that an azobenzene polymer network can also optically align ferroelectric liquid crystals. This was done by dissolving two chiral dimethacrylate and one chiral diacrylate monomers containing azobenzene groups in a commercial ferroelectric liquid crystal host. The monomers were illustrated as follows:

The monomers were then thermally polymerized and simultaneously irradiated with linearly polarized light. Two of the monomers were able to induce bulk alignment of the liquid crystals in direction perpendicular to the polarized light. Monomer #1 was effective in concentrations as low as 1%. It was also concluded from the experimental evidence that the photo-induced bulk alignment of the ferroelectric liquid crystals may take place by a mechanism that is different from one that takes place in achiral azopolymers. In addition, Zhao and coworkers [236] reported photo-induced alignment of ferroelectric liquid crystals using azobenzene polymer networks of polyethers and poly epoxides. Bulk alignment was achieved by polymerizing several divinyl ethers and diepoxide monomers bearing an azobenzene moiety. Here too, thermal polymerizations were conducted in solution with in the ferroelectric liquid crystals, while exposing the reaction mixture to linearly polarized irradiation. The monomers can be shown as follows:

Polymerization of these monomers was achieved by cationic mechanism. The monomers were also found capable of inducing and stabilizing bulk alignment of the liquid crystals. Zhao and coworkers [234–237] concluded, however, that the mechanism of action might be different from the one obtained with chiral azobenzene polymethacrylates. Instead, the results suggest to them that the mechanism might be based on formation of an anisotropic azobenzene polyether or polyepoxide network. In still another subsequent work, Zhao and coworkers [237] used block copolymers composed of polystyrene and liquid crystalline azobenzene-containing polymethacrylate copolymers as a model system

to investigate the confinement effects on the photo-alignment, photochemical phase transition, and thermochromic behavior of the azobenzene polymer. The study showed [237] that when confined in the microphase-separated domains in the diblock copolymers, the azobenzene polymer behaves differently than it does as a homopolymer free from confinement. The confinement effects are manifested by (1) decreased photo-induced and thermally enhanced orientation of azobenzene mesogenic groups in different aggregation states, (2) slower transformation from a liquid crystalline phase to the isotropic state triggered by the trans–cis photo-isomerization and slower recovery of the liquid crystalline phase after the thermally induced cis-trans back isomerization, and (3) severely reduced and even suppressed changes in the aggregation states of azobenzene groups on heating, which is at the origin of the thermochromic property. The common cause of these confine ment effects is the restriction imposed by the confining geometry on either an order-disorder or a disorder-order reorganization process involving the motion and rearrangement of azobenzene groups [237]. Zettsu and Seki reported [238] preparation of a group of azobenzene-containing polymers that can be used in photo-induced surface relief formations. These are soft liquid crystalline azobenzene containing copolymers of acrylate with methacrylate monomers bearing oligo (ethylene oxide) chains. The copolymers display a smectic liquid crystal phase at room temperature. After preexposure to ultraviolet light, thin films of the liquid crystalline polymers show highly sensitive photo-induced material transfer to generate the surface relief structures. The typical exposure dose required for full polymer migration is as low as 50 mJ/cm².

The inscribed surface relief structures can be rapidly and fully erased either by irradiation with incoherentnonpolarizedultravioletlightorbyheatingclosetotheclearpointofthesoftliquidcrystalline polymers. It is also possible to chemically cross-link the polymers with mixed vapors of hydrogen chloride and formaldehyde after surface relief inscription. This results in a drastic improvement of the shape stability, maintaining the structure athightemperaturesupto250C.After cross-linking, the trans to-cis photo-isomerization readily proceeds without any modification of the surface morphology and can, therefore, be applied to the photo-switchable alignment of nematic liquid crystals [238].

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اخبار العتبة العباسية المقدسة

قسم صناعة الشبابيك يواصل أعمال الإدامة لأحد أبواب حرم مرقد أبي الفضل العباس (عليه السلام)

ضمن فعاليات ملتقى النورين.. المجمع العلمي يقيم ندوة فكرية في بابل

المجمع العلمي يحيي ليلة القدر الأولى في مقام الإمام المهدي (عجّل الله فرجه)

مركز السيد جعفر الحلي الطبي يقدم الاستشارات والخدمات الصحية في ليالي القدر المباركة

المزيد

الآخبار الصحية

خمسة مؤشرات صحية مهملة قد تنقذ حياتك

متى تكون الحمى طبيعية؟

التدخين الإلكتروني يهدد قلبك.. دراسة جديدة تكشف مخاطر ارتفاع ضغط الدم

دراسة تكشف فائدة بعض أنواع الشاي في محاربة الاكتئاب

المزيد

الآخبار التكنلوجية

الجهاز المناعي يظهر دورا غير متوقع في صحة العظام

ابتكار من جامعة الشارقة منخفض التكلفة للحماية من الزلازل

اكتشاف تأثير غير متوقع للكاكاو على وظائف الدماغ

باحثون يرصدون أثر تغيّر المناخ في دم البشر

المزيد

مواضيع ذات صلة

Polymer-Based Solar Cells

Photo-Conducting Polymers That Are Not Based on Carbazole

Photoconductive Polymers Based on Carbazole

Photo-Conducting Polymers

Application to Optical Data Storage

Changes in Viscosity and Solubility of Polymeric Solutions

Photo-Isomerization of the Azo Group

Photo-Isomerization of the Olefinic Group

Copolymers of Methacrylates with Cholesteric Monomers

Polymers with Norbornadiene Moieties

Photo-Responsive Polymers

Polymers Designed to Cross-link Upon Irradiation with Laser Beams