Rubbed Polyimide Alignment Layer Research Articles

Surface alignment of nematic liquid crystals by direct laser writing of photopolymer alignment layers

ABSTRACT We demonstrate the fabrication of good quality surface alignment layers on glass by Direct Laser Writing method using a 2-photon polymerisation technique. We use commercially available photosensitive resins to print alignment layers by scanning the focal point of a femtosecond laser near the resin-glass interface. This results in down to ~ 100 nm thin alignment layers that provide good planar anchoring of 5CB and MLC13300, with the easy axis of alignment along the scanning direction. The azimuthal anchoring strength is ~ 5 × 10−6 J/m2 and is an order of magnitude weaker compared to commercial rubbed polyimide alignment layer. The threshold voltage for Fréedericksz transition in a 90° twisted nematic cell is slightly increased compared to conventional rubbed polyimide for printed alignment layers. The turn-on switching time is longer for printed layers compared to polyimide alignment layers, whereas the turn-off time is shorter for printed alignment layers. The advantage of this new method is in its flexibility, as we demonstrate printing of complex surface alignment patterns with alignment layer thickness below 100 nm.

Light-Powered Self-Sustained Oscillators of Graphene Oxide/Liquid Crystalline Network Composites Showing Amplitude and Frequency Superposition.

Light-activated self-oscillators have drawn enormous attention for their potential applications in mobile machines, energy harvesting, signal modulation, etc. Herein, we report one graphene oxide (GO)/liquid crystalline network (LCN) actuator that presents a unique light-activated oscillation with amplitude and frequency superposition. The GO/LCN composite film is prepared by the one-step polymerization of LC monomers, which favors a splay orientation in LC cells made by gluing together two glass sheets, one coated with photothermal agent GO and the other coated with a rubbed polyimide alignment layer. Owing to the asymmetric contraction/expansion, changing the cutting direction gives rise to notably different actuation behaviors for GO/LCN composite films. Moreover, it twists a little during the deflection process as a result of experimental error during the cutting process, which may cause the strip to be cut inaccurately. When the composite film is embedded in a self-shadowing system, it produces an unconventional hybrid oscillation mode upon near-infrared light irradiation, i.e., bending and twisting oscillation coupled. Furthermore, when the aspect ratio of the film decreases, the twisting mode is suppressed and the actuator changes from a coupled mode to a single bending mode. The proposed strategy may extend the application of GO/LCN composite materials and enrich light-activated self-oscillating behaviors.

Effect of alignment methods on liquid crystal performance in millimeter wave devices

We investigate the effect of different alignment methods on the performance of dual frequency liquid crystal-based phase shifters operating at 30 GHz to determine the optimal alignment method for such devices. We measure the response time, total phase shift and repeatability of devices with the liquid crystal alignment created by a rubbed polyimide alignment layer deposited on the metallization elements, devices with the alignment created by direct rubbing of the metallization elements, and devices with no initial treatment for alignment. For these tests, we used a commercial, dual frequency liquid crystal. We find that the devices with a polyimide alignment layer in an anti-parallel rubbing orientation produce the best results; however, the other devices produce acceptable results due to the nature of the dual frequency liquid crystal and they may be alternatives to polyimide in certain applications.

Polyimide-free homogeneous photoalignment induced by polymerisable liquid crystal containing cinnamate moiety

ABSTRACTWe designed and synthesised a reactive mesogen (CRM) containing cinnamate moiety in the core and two acrylate groups at both ends. The structure was characterised by 1H NMR and FT-IR, and its liquid crystalline property was investigated by DSC and POM. The compound showed a nematic phase between 175.2 – 97.5 oC, followed by a smectic C phase between 97.5 and 57.9 oC during the cooling cycle. We proposed a fabrication method to achieve uniaxial homogeneous alignment of liquid crystals by irradiating the CRM-doped LC mixture in a sandwich cell of in-plane switching mode (CRM-IPS) with linearly polarised ultraviolet light. The results indicated that the CRM-IPS cell showed excellent initial dark state with a good alignment state, electro-optical performance and durability. This confirmed that the photo-induced alignment of LCs by CRM possesses outstanding alignment capability compared to the conventional rubbed polyimide alignment layer. We expect that this fabrication method is a promising candidate for cost-effective, green-manufacturing, and high-quality alignment method for the manufacturing of high-resolution liquid crystal displays (LCDs).

Effect of alignment layer on polymer-dispersed liquid crystal random laser

The surface orienting effect of rubbed polyimide alignment layer on the random lasing (RL) properties of dye-doped polymer-dispersed liquid crystal (DDPDLC) was studied experimentally. Two groups of experiment were done and nematic liquid crystal E7 was used. In the first group, the samples were 25 μm non-oriented DDPDLC sample (N-DDPDLC) and 25 μm one-side oriented DDPDLC sample (P-DDPDLC). In the second, the samples were 10 μm N-DDPDLC and 10 μm P-DDPDLC sample. The results showed that compared to the N-DDPDLC, P-DDPDLC has a lower threshold, and the 10 μm P-DDPDLC has a lower threshold compared to the 25 μm P-DDPDLC. Apparently, the P-DDPDLC cell has stronger RL spike. The oriented layer changed the phase separation structure of DDPDLC which formed hierarchical structure so as to enhance the scattering intensity of system. It is a good way to reach high efficient random laser emission by tailoring the size and morphology of PDLC through surface orientation and Polymerization-induced phase separation process.

Anchoring strength measurements of a lyotropic chromonic liquid crystal on rubbed polyimide surfaces

ABSTRACTTechniques to achieve planar anchoring of the director of a lyotropic chromonic liquid crystal (LCLC) have been developed only recently. These techniques range from the very weak anchoring achieved by simply scratching the glass with a very fine abrasive to the very strong anchoring achieved with lithographic techniques. A possible alignment technique is to use rubbed polyimide (PI) alignment layers as is routinely done to align thermotropic liquid crystals (TLCs). The anchoring strength of the LCLC disodium cromoglycate (DSCG) on different rubbed PI alignment layers is measured using a /2 twist cell and optical polarisation techniques. Although the anchoring strength is not as large as can be achieved with lithographic techniques, it is larger than scratched glass for one PI. Just as important, rubbed PI provides a much more uniform surface than is possible with scratched glass. Interestingly, addition of a small amount of surfactant to the LCLC, which results in improved alignment, does not increase the anchoring strength. Weak anchoring of LCLCs can also be achieved with a photo-patterned dye surface layer. Finally, atomic force microscopy measurements of the surface combined with prior theoretical work linking surface features and anchoring strength predict anchoring strengths consistent with experiment.

반사형 타원계를 이용한 러빙된 Polyimide 배향막의 초미세 광학 이방성 정밀 측정

We developed a reflection ellipsometer so that one can measure the extremely small optical anisotropy of a rubbed polyimide alignment layer, without being disturbed by the residual anisotropy of the substrate. The optical anisotropy of the alignment layer was measured as rubbing strength was increased, and the measured anisotropy was compared to the retardation obtained by using a transmission-type ellipsometer, to confirm the reliability of our reflection ellipsometer. With this measured anisotropy we could verify the formation of an alignment layer by rubbing, and could quantitatively evaluate the formation of the alignment layer. Keywords: Ultra-small optical anisotropy, Reflection ellipsometry, Rubbed polyimide, Alignment layerOCIS codes: (120.2040) Displays; (120.2130) Ellipsometry and polarimetry; (310.5448) Polarization, other optical properties

다른 파일밀도를 가지는 LCD용 러빙포의 제조 및 폴리이미드 배향막에 대한 러빙효과 분석

Abstract: The mechanical rubbing of a polyimide (PI) layer with a velvet cloth has been dominantly used to induce auniform alignment of liquid crystal molecules in the manufacturing process of liquid crystal displays (LCDs). The rub-bing process is affected by its process parameters and also by the nature and properties of the rubbing cloth used. Wefabricated the rubbing cloths with various pile densities and different weaving structures and then investigated how theparameters were related to the effect of the cloth on the rubbing process and the properties of rubbed PI alignment lay-ers (ALs). As the pile density increases at same rubbing process parameters, the degree of molecular orientation of therubbed AL and its surface roughness increase with an increase of the contact area between the rubbing cloth and theAL surface. Furthermore, a more uniform rubbing is also achieved. The results in this work showed that higher piledensity of the rubbing cloth induces a better rubbing effect, which is indeed more favorable to the LCD process. It wasalso observed that the weaving can exert some influence on the rubbing process.Keywords: rubbing cloth, pile density, rubbing process, polyimide, liquid crystal display

Directed self-assembly of nematic liquid crystalline polymers on a rubbed polyimide alignment layer

Amphiphilic diblock copolymers PEO-b-PMA(4OPB) comprising hydrophilic poly(ethylene oxide) (PEO) and hydrophobic nematic liquid crystalline polymethacrylate [PMA(4OPB)] subunits were synthesized by atom transfer radical polymerization. The liquid crystalline property and microphase-separated structure were investigated by differential scanning calorimetry, polarized optical microscopy, atomic force microscopy, and X-ray diffraction analysis. Unimodal PEO-b-PMA(4OPB) with 114 ethylene oxide repeating units and phenylbenzoate moieties as methacrylate side chain mesogens showed a nematic phase when the methacrylate repeating unit was around 57. Hexagonal cylinders of the PEO subunits were formed in nematic phase and supercooling states. The thin film on a rubbed polyimide alignment layer showed a microphase-separated structure independent of the rubbing direction, while horizontally aligned cylinders were observed in the presence of small nematic liquid crystalline molecules of 4-cyano-4′-pentyloxybiphenyl (5OCB). The assistance of nematic liquid crystalline molecules was necessary for directed self-assembly of the nematic liquid crystalline diblock copolymer in accordance with the rubbing direction of the polyimide alignment layer.

Surface-Assisted Unidirectional Orientation of ZnO Nanorods Hybridized with Nematic Liquid Crystals

Inorganic semiconductor nanorods are regarded as the primary components of optical and electrical nanoscale devices. In this paper, we demonstrate the unidirectional alignment of monolayered and dispersed ZnO nanorods on a rubbed polyimide alignment layer, which was achieved by a conventional liquid crystal alignment technique. The outermost surfaces of the ZnO nanorods (average diameter 7 nm; length 50 nm) were modified by polymerization initiator moieties, and nematic liquid crystalline (LC) methacrylate polymers were grown by atom transfer radical polymerization. By regulating the densities of the polymerization initiator moieties, we successfully hybridized LC-polymer-grafted ZnO nanorods and small nematic LC molecules. The LC-polymer-modified ZnO nanorods were hierarchically aligned on the substrate via cooperative molecular interactions among the liquid crystal mesogens, which induced molecular orientation on the rubbed polyimide alignment layer.

개선된 투과형 타원계를 사용한 러빙된 Polyimide 배향막의 초미세 위상지연 정밀 측정

시료를 회전시키는 기존의 시료회전형 타원계에서 시료를 고정시키고 편광자모듈을 회전시키는 모듈회전형 타원계로 구조를 변경하고 광원교체와 회전모듈 동기화, 배경 미세이방성 보정 등을 통해서 위상지연값 측정정밀도를 <TEX>$3{\sigma}$</TEX> <0.005 nm 까지 향상시켰다. 개선된 모듈회전형 타원계를 사용하고 RVD(Retardation Vector Difference) 방법을 도입함으로써 1.0 nm 정도의 잔류이방성을 가지는 유리기층의 효과를 배제한 러빙된 Polyimide 배향막의 순수 배향효과만에 의한 초미세 광학이방성을 정밀하게 측정하는 방법을 제시하였다. 순수 러빙으로 인한 배향막의 위상지연값은 0.05 nm 에서 0.15 nm 정도의 크기를 가짐을 확인하였다. The precision of retardation measurement has been improved upto <TEX>$3{\sigma}$</TEX> <0.005 nm after improvements are made to the conventional transmission ellipsometer. Improvements are made such that, i) the polarizer module instead of the sample stage is rotated, ii) the light source is replaced, iii) the starting points of two rotating modules are accurately synchronized, and iv) the fine background retardation is compensated. Together with the newly introduced RVD (Retardation Vector Difference) method, the improved instrument is successfully applied to characterize the ultra small optical birefringence of the rubbed polyimide alignment layer, after the residual retardation due to glass substrate whose magnitude is about 1.0 nm is properly subtracted. It is verified that the net retardation of the alignment layer ranges from 0.05 nm to 0.15 nm.

옥타[8-{4-(4'-시아노페닐아조)펜옥시}]옥틸 그리고 옥타[8-{4-(4'-시아노페닐아조) 펜옥시카보닐}]헵타노화 이당류의 열방성 액정과 광화학적 상전이 거동

Octa[8-{4-(4'-cyanophenylazo)phenoxy}]octyl and octa[8-{4-(4'-cyanophenylazo)phenoxycarbonyl}]heptanoated disaccharide derivatives were synthesized by reacting cellobiose, maltose, and lactose with 1-{4-(4'-cyanophenylazo) phenoxy}octylbromide or 1-{4-(4'-cyanophenylazo)phenoxycarbonyl}]heptanoyl chloride, and their thermotropic liquid crystalline and photochemical phase transition behavior were investigated. All the {(cyanophenylazo)phenoxy} octyl disaccharide ethers (CADETs) formed monotropic nematic (N) phases, whereas all the {(cyanophenylazo) phenoxycarbonyl}heptanoated disaccharide esters (CADESs) exhibited enantiotropic N phases. Compared with CADETs, CADESs showed higher isotropic (I)-to-N phase transition temperatures. Photoirradiation of the disaccharide derivatives in a glass cell or in a cell with a rubbed polyimide (PI) alignment layer at a N phase resulted in disappearance of the N phase due to trans-cis photoisomerization of azobenzene, and the initial N phase was recovered when the irradiated sample was kept in the dark because of cis-trans thermal isomerization and reorientation of trans-azobenzenes. The rates of the photochemical N-I and the thermal I-N phase transition of disaccharide derivatives in a cell with a rubbed PI alignment layer were faster than those in a glass cell, and were significantly different from those observed for the monomesogenic compounds containing cyanoazobenzene and the 4-{4'-(cyanophenylazo)phenoxy}octyl glucose and cellulose ethers. The results were discussed in terms of difference in cooperative motion of azobenzene groups due to the flexibility of the main chain, the number of mesogenic units per repeating units, and the distance between the azobenzene groups.

Asymmetrically Tilted Alignment of Rigid-Rod Helical Polysilanes on a Rubbed Polyimide Surface

The homogeneous alignments of helical rod-like polysilanes on a rubbed polyimide alignment layer were investigated by polarized optical microscopy (POM) and atomic force microscopy (AFM) analyses. The POM and AFM observations determined that polysilanes with a series of aliphatic side chains helically arranged around the main chains were tilted to the right and left by 33° from the rubbing direction when the handedness of the side-chain helical array is left and right, respectively. It is interesting to note that the side-chain arrays run perpendicular to the rubbing direction on the polyimide surface, which is different from intuitive "knob and hole" packing of the extended polyimide chain and the helical grooves between the side-chain arrays surrounding the polysilane backbone. More surprisingly, both right- and left-tilting smectic domains were simultaneously observed with an equal probability for an achiral polysilane, which apparently has the interconverting right- and left-handed helical segments separated by helical reversals. This might be the first observation of the chiral segregation of dynamic helical polymers.

LCD 제조용 러빙포 물성에 따른 러빙된 폴리이미드 배향막의 특성 및 러빙효과 분석

러빙공정에서는 러빙 시 공정변수와 배향막의 특성 그리고 러빙포의 물성이 중요한 변수로 작용한다. 이러한 변수들은 러빙밀도(rubbing density)와 러빙력(rubbing force)에 의해서 결정되는 러빙된 정도(크기)를 통하여 배향막의 배향성에 영향을 미치게 된다. 본 연구에서는 러빙 변수 중 러빙포의 물성이 러빙밀도(길이)와 러빙력에 미치는 영향에 대하여 알아보기 위하여 다른 물성(파일 밀도, 강직도)을 가지는 러빙포를 이용하여 러빙효과(정도)를 분석하였다. 러빙포의 파일 밀도가 커지고 강직도가 강할수록 러빙된 배향막의 이방성과 표면조도가 커졌으나 러빙효과의 형태는 서로 달랐다. 러빙포의 파일밀도는 러빙에 관여하는 파일의 수와 밀접한 관련이 되어 있어 러빙하는 밀도(길이) 영향을 미치지만 강직도는 러빙밀도(길이)보다는 러빙력과 보다 밀접하게 관련성이 있었다.

Studies of the Directional Property on Rubbed Alignment Films by Rubbing Condition

The near-edge X-ray absorption fine structure(NEXAFS) and its polarization dependence on Carbon(C) K-edge, is used to obtain information on the molecular orientation at the polyimide(PI) surface. Rubbed PI alignment layer was controlled the pile contact impression and stage speed. We could estimate the alignment angle α of the molecules with the surface normal of the PI molecules through the intensity change of C˭C π* in NEXAFS C K-edge spectrum, depending on the photon incident angle θ = 20°, 55° and 90°. As increasing the depth of the pile contact, the angle α was linearly decreased. The stage speed dependency was shown an irregular pattern.

Room-temperature processing of π-conjugated oligomers into uniaxially oriented monodomain films on coumarin-based photoalignment layers

Photoalignment layers comprising Polymer 1 were prepared using linearly polarized UV-irradiation for chloroform- vapor annealing of p-conjugated oligomer films, both proc- esses conducted at room temperature. The resultant uniaxially oriented monodomain films exhibited S ¼ 0.74 (at Pr ¼ 0.90) and 0.82 (at Pr ¼ 0.95) in OF and OF2T films, respectively, apparently limited by film dewetting in comparison to S ¼ 0.82 6 0.01 from thermal annealing on rubbed polyimide alignment and Polymer 1 photoalignment layers. The time to arrive at maximum S values varied from 5-10 s to 6-8 min on rubbed polyimide alignment layers and Polymer 1 photoalignment layer, respectively, because of favorable p-p interactions enhanced by rubbing. In contrast, PF2T could not be oriented on either type of alignment layers after annealing under satu- rated chloroform vapor up to 14 h. Annealing of an OF2T film under saturated chlorobenzene vapor at room temperature per- mitted lyotropic nematic mesomorphism to be observed in situ, which is equivalent to thermotropic nematic mesomor- phism as the driving force behind thermal annealing. V C 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 725- 731, 2011

Determination of azimuthal anchoring strength in twisted nematic liquid crystal cells using heterodyne polarimeter

Two external-field-free methods are presented for measuring the azimuthal anchoring strength in twisted nematic liquid crystal (TNLC) cells. For asymmetrical TNLC samples, the twist angle is derived from the phase of the detected signal in a phase-sensitive heterodyne polarimeter and is then used to calculate the weak anchoring strength directly. The measurement resolution which is found to be about 0.01 μJ/m(2) makes the present method sensitive enough for the LC-based bio-sensing application. Using the proposed method, the weak azimuthal anchoring strength of a composite liquid crystal mixture (40% LCT-061153 + 60% MJO-42761) in contact with a plasma-alignment layer is found to be 7.19 μJ/m(2). For symmetrical TNLC samples, the liquid crystals are injected into a wedge cell, and the two-dimensional distributions of the twist angle and cell gap are extracted from the detected phase distribution using a genetic algorithm (GA). The azimuthal anchoring strength is then obtained by applying a fitting technique to the twist angle vs. cell gap curve. Utilizing the proposed approach, it is shown that the strong anchoring strength between a rubbed polyimide (PI) alignment layer and E7 liquid crystal is around 160 μJ/m(2) while that between a rubbed PI alignment layer and MLC-7023 liquid crystal is approximately 32 μJ/m(2).

Preparation of Zigzag Free Surface Stabilized Ferroelectric Liquid Crystals: Filling the Liquid Crystal in the Right Direction

The effects of surface characteristics of rubbed polyimide alignment layer on the product of zigzag free ferroelectric liquid crystals have been studied. Rubbing causes anisotropy in wettability on the polyimide surface. The flowing direction of the liquid crystal during cell filling is found to influence the molecular alignment in the resultant ferroelectric liquid crystal layer. Better aligned ferroelectric liquid crystal layer with fewer zigzag defects can be produced when the ferroelectric liquid crystal is allowed to flow into the cell toward the direction in which the alignment surface exhibits higher hydrophobicity.

Effects of size, capping agent, and concentration of CdSe and CdTequantum dots doped into a nematic liquid crystal on the optical and electro-optic properties of the final colloidal liquid crystal mixture

To investigate size and capping agent-dependent alignment and electro-optic effects of quantum dots (QDs) in nematic LCs, mixtures of a nematic phenylpyrimidine LC (LC1) doped with hexadecylamine-capped CdSe and thioglycolic acid-capped CdTe QDs were prepared and investigated by polarized optical microscopy between plain, untreated glass slides and in ITO-coated glass cells with rubbed polyimide alignment layers as well as by electro-optic analysis. The five investigated CdSe QDs, varying in size from 2.5 to 5.2 nm, show fluorescence emission maxima ranging from 480 to 610 nm, and the four CdTe QDs, varying in size from 3.2 to 4.0 nm, emit light from 530 to 610 nm depending on their size. The CdSe QDs favoring homeotropic anchoring of the nematic LC induce vertical alignment at 1 and 2 wt% in LC1 between plain glass slides and only at 1 wt% in the planar cells, except for CdSe QD emitting at 590 nm (CdSe590), which induces vertical alignment independent of the substrate only at 1 wt%. Concentrations above 2 wt% of the CdSe QDs in LC1 show planar alignment mainly due to aggregation of the QDs in LC1 at these higher concentrations. On the contrary, the CdTe QDs favoring planar anchoring of the nematic LC induce homeotropic alignment of LC1 at concentrations above 2 wt% between plain glass slides. All QDs are effective in reducing the threshold voltage (Vth) and the splay elastic constant (K11). The most drastic reduction over the entire temperature range, however, was observed for the smallest CdSe480 QDs, which showed the least tendency of all investigated QDs to aggregate in LC1.

Photomobile Polymer Materials: Towards Light‐Driven Plastic Motors

As light is a good energy source that can be controlled remotely, instantly, and precisely, light-driven soft actuators could play an important role for novel applications in wideranging industrial and medical fields. Liquid-crystalline elastomers (LCEs) are unique materials having both properties of liquid crystals (LCs) and elastomers, and a large deformation can be generated in LCEs, such as reversible contraction and expansion, and even bending, by incorporating photochromic molecules, such as an azobenzene, with the aid of photochemical reactions of these chromophores. Herein we demonstrate new sophisticated motions of LCEs and their composite materials: a plastic motor driven only by light. If materials absorb light and change their shape or volume, they can convert light energy directly into mechanical work (the photomechanical effect) and could be very efficient as a single-step energy conversion. Furthermore, these photomobile materials would be widely applicable because they can be controlled remotely just by manipulating the irradiation conditions. LCEs show an anisotropic order of mesogens with a cooperative effect, which leads them to undergo an anisotropic contraction along the alignment direction of mesogens when heated above their LC-isotropic(I) phase transition temperatures (TLC-I) and an expansion by lowering the temperature below TLC-I. [1, 13–18] The expansion and contraction is due to the microscopic change in alignment of mesogens, followed by the significant macroscopic change in order through the cooperative movement of mesogens and polymer segments. It is well known that when azobenzene derivatives are incorporated into LCs, the LC-I phase transition can be induced isothermally by irradiation with UV light to cause trans–cis photoisomerization, and the I-LC reverse-phase transition by irradiation with visible light to cause cis–trans back-isomerization. This photoinduced phase transition (or photoinduced reduction of LC order) has led successfully to a reversible deformation of LCEs containing azobenzene chromophores just by changing the wavelength of actinic light. Although the photoinduced deformation of LCEs previously reported is large and interesting, it is limited to contraction/expansion and bending, preventing them from being used for actual applications. Herein we report potentially applicable rotational motions of azobenzene-containing LCEs and their composite materials, including a first lightdriven plastic motor with laminated films composed of an LCE film and a flexible polyethylene (PE) sheet. The LCE films were prepared by photopolymerization of a mixture of an LC monomer containing an azobenzene moiety (molecule 1 shown in Scheme 1) and an LC diacrylate with an azobenzene moiety (2 in Scheme 1) with a ratio of 20/ 80 mol/mol, containing 2 mol% of a photoinitiator in a glass cell coated with rubbed polyimide alignment layers. The photopolymerization was conducted at a temperature at which the mixture exhibited a smectic phase. The glasstransition temperature of the LCE films is at about room temperature, allowing the LCE films to work at room temperature in air, as the films are flexible enough at this temperature. We prepared a continuous ring of the LCE film by connecting both ends of the film. The azobenzene mesogens were aligned along the circular direction of the ring. Upon exposure to UV light from the downside right and visible light from the upside right simultaneously (Figure 1), the ring