Polyimide Alignment Layer Research Articles

Direct visualization and measurement of the extrapolation length on cooling toward the nematic–smectic-Aphase transition temperature

A herringbone "easy axis" pattern is scribed into a polyimide alignment layer for liquid-crystal orientation using the stylus of an atomic force microscope. Owing to the liquid crystal's bend elasticity K33 , the nematic director is unable to follow the sharp turn in the scribed easy axis, but instead relaxes over an extrapolation length L=K33/W2φ, where W2φ is the quadratic azimuthal anchoring strength coefficient. By immersing a tapered optical fiber into the liquid crystal, illuminating the fiber with polarized light, and scanning the fiber close to the substrate, a visualization and direct measurement of L are obtained on approaching the nematic-smectic- A phase transition temperature T NA from above. L is found to exhibit a sharp pretransitional increase near T NA, consistent with a diverging bend elastic constant.

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.

Intermediate pre-tilt angle control by a composite alignment thin film structure for liquidcrystal displays

We designed a patterned composite alignment thin film structure using a horizontalalignment polyimide (PI) layer and vertical alignment liquid crystal polymer (LCP) pillars.The LCP polymer precursor concentration was varied at 0–10% and the pillars wereintroduced by a photolithography process. Both single-sided and double-sided liquid crystaldisplay cells were assembled for a series of electro-optical characterization techniques. Thehorizontal PI alignment layer alone had a designated control of the pre-tilt angle of7° after the prescribed mechanical rubbing process. The pre-tilt angle was improved to24° when the LCP precursor concentration was 5%. It was further increased to61° at the concentration of 10%. In addition, the study on the electrical response time and graylevel variation demonstrated promising results for potential applications. The field-onresponse time was only 2.79 ms and the field-off response time was 0.35 ms for thedouble-sided liquid crystal display cells using a ramping voltage of 5.5 V. The effectivecontrol of the cell pre-tilt angle suggested that the display power consumption and responsetime would be greatly improved.

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.

Patterning-induced surface chirality and modulation of director twist in a nematic cell

A substrate coated with a polyimide alignment layer is scribed bidirectionally with the stylus of an atomic force microscope to create an easy axis for liquid-crystal orientation. The resulting noncentrosymmetric topography breaks two-dimensional inversion symmetry and results in a spatial amplitude modulation of an imposed twisted nematic state. This is observed optically as spatially periodic light and dark stripes. When the alignment layer is scribed unidirectionally the centrosymmetric topography maintains inversion symmetry, and no stripes are observed. The appearance of the twist modulation is consistent with a chiral term in the free energy.

Control of liquid crystal pretilt angle by anchoring competition of the stacked alignment layers

We proposed a method to control the pretilt angle of liquid crystals by stacking of a vertical alignment layer on a planar alignment layer. The pretilt angle can be controlled over the full range (0°–90°) depending on the thickness of the vertical alignment layer. We also proposed a numerical model to describe the physical mechanism based on the anchoring competition between liquid crystal, planar, and vertical polyimide alignment layers.

Flexible Liquid-Crystal Display Stabilized by Pixel Walls Directly Bonded via Ion-Beam Irradiation

We characterize a flexible self-assembled liquid crystal display (LCD) fabricated from a polyimide (PI) alignment layer with polydimethylsiloxane (PDMS) pixel walls. Ion-beam irradiation aligned liquid crystal molecules in the PI layer and bonded two flexible plastic substrates in a one-step assembly of the pixel walls. X-ray photoelectron spectroscopic analysis, Fourier transform infrared spectroscopy, and scanning electron microscopy provided chemical and physical evidence for the formation of stable chemical bonds between the PI layer and the PDMS pixel walls, in addition to the important maintenance of a uniform gap between the two substrates without the use of any epoxy resins or other polymers. Electro-optical performance of our flexible device was highly reliable, even under deformation conditions, and is comparable to that of conventional rigid glass substrate LCDs.

Controlling pre-tilt angles of liquid crystal using mixed polyimide alignment layer

Three approaches to controlling liquid crystal (LC) pre-tilt angle in a cell are demonstrated using a polyimide (PI) alignment layer on substrates, in the form of a mixture of horizontal (H) and vertical (V) polyimides. The concentration ratio of H- to V-PI, baking temperature, and rubbing strength influence the pre-tilt angle, and highlight parameters for controlling the pre-tilt angle of an LC cell. Finally, a variable polarization converter is demonstrated using this approach.

Orientational ordering of 4-pentyl-4′-cyanobiphenyl molecules evaporated on multi-layered polyimide film

Using optical second-harmonic (SH) generation and polarized absorption (PA) measurements, the orientational ordering process of 4-pentyl-4′-cyanobiphenyl (5CB) was studied during evaporation onto the alignment layer of polyimide (PI) Langmuir–Blodgett films. The intensity of SH signal increased in the beginning of the deposition, but it saturated after the first 5CB monolayer was formed. The PA measurement suggested that a layer-by-layer growth was induced after the formation of 5CB monolayer with accompanying the orientational ordering. The orientational order of the first 5CB monolayer was studied in terms of the orientational order parameters, S 1 = 〈cos θ〉 and S 2 = 〈(3cos 2 θ − 1)/2〉, using the SHG and PA measurements. It was found that S 1 decreased as the thickness of PI alignment layer increased from 0.4 to 4.4 nm, while S 2 was nearly constant. These results indicate that the Coulomb interaction between the permanent dipole of 5CB and its image dipole makes a significant contribution to the orientational ordering of deposited 5CBs.

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

28.2: The Molecular Orientation of Polyimide Alignment Layer used in TFT‐LCDs with NEXAFS Study

AbstractAromatic polyimide thin films are used as the alignment film in thin film transistor‐liquid crystal displays (TFT‐LCD's). The films play an important role in the initial alignment of the liquid crystal materials in LCD panels. Therefore, any structural or morphological deformation may lead to defect in the physical interactions thus malfunction of the display eventually. for example, an LCD is known to bear image sticking problem when it is assembled following 3 days or longer stagnancy in the process after main curing of the layer. In this paper, the molecular orientation of polyimide surfaces with respect to the stagnancy is investigated by means of surface sensitive Near Edge X‐ray Absorption Fine Structure (NEXAFS) spectroscopy. NEXAFS measurements clearly reveal preferred in‐plane and out‐of‐plane orientation of phenyl and C=O groups at the surface of the polyimide thin film in regard to the stagnancy after main curing. with longer stagnancy phenyl and C=O groups preferred in‐plane orientation. The preferred in‐plane orientation within the film can accelerate the image sticking problem in LCD's operating in PVA mode. Based on the result, vertical alignment of polyimide pendant group is found helpful for the better alignment of LC molecules in LCD's operating in the patterned vertical alignment (PVA) mode.

P‐162: A Study on Image Sticking of Photo Reactive Polyimide Alignment Layer for LCD

AbstractPhoto‐alignment methods have image sticking problems, which can be a severe defect to achieve high reliability of LCD panels. It is caused by unreacted photosensitive functional groups. To deactivate those, we introduced interfacial reaction using ethanethiol or bromine and demonstrated transformation from C‐C double bonds to C‐S or C‐Br using UV‐vis, XPS, PEM and AFM images.

V-shaped switching ferroelectric liquid crystal structure stabilized by dielectric surface layers

The " V -shaped switching" mode in high polarization ferroelectric liquid crystals was studied with the aim of stabilizing the monostable bookshelf structure with the spontaneous polarization parallel to the glass plates. The director field in such cells was confirmed to be sensitive to both the liquid crystal properties and the cell parameters. In cells with only polyimide alignment layers, hysteresis free switching was never obtained, with bistable and asymmetric monostable structures compromising the zero-field dark state and preventing an ideal, hysteresis-free analog response. By incorporating a SiO(2) layer between the ITO electrode and the polyimide, the undesired states were suppressed and essentially hysteresis-free switching was obtained for driving frequencies in the range 0.2-200 Hz . Cells rubbed only on one side give more uniform alignment than cells rubbed on both sides but their inherent asymmetry shifts the long-term dark state away from 0 V and causes the response to gray level voltage modulation to be slightly asymmetric. The formation of different types of states as a function of the values of the surface parameters, and the observed stabilization of the V -shaped switching structure by the dielectric surface layers, are in good agreement with an earlier analysis by Copic [Phys. Rev. E 65, 021701 (2002)].

Relationship between the Optical Properties of Immobilized Liquid-Crystalline Film and Polyimide Alignment Layer

The relationship between the molecular orientation of a rubbed polyimide film alignment layer and that of mesogens in liquid-crystalline (LC) films coated on the alignment layer before and after photopolymerization has been investigated using optical measurements. LC monomers were coated over the alignment layer and were aligned in one direction. The LC monomers were subsequently photocured. The inclination angle of the refractive index ellipsoid and the optical anisotropy of the immobilized LC film were found to be related to the optical properties of the rubbed polyimide films.

Air Cavity Generation for Interconnect and High Resolution Displays

A new approach for the wafer-level fabrication of silicon-VLSI-based liquid-crystal (VLSI/LC) microdisplays has been investigated. Fabrication of the LC cell gap is based on the same processes developed for creating ultra low-k dielectrics for VLSI interconnect. A dimensionally precise planar LC cavity is created by use of a sacrificial polymer placeholder in an all build-up process. The process flow for a reflective LC display is presented along with early tests using a polyimide alignment layer.

Variable liquid crystal pretilt angles generated by photoalignment of a mixed polyimide alignment layer

A method to generate any liquid crystal pretilt angle is proposed and demonstrated. This method is based on deep ultraviolet photoalignment of a layer consisting of nanostructured domains of horizontal and vertical alignment polyimides. Competition between the two types of domains produces the controlled pretilt angle. The obtained anchoring energy is reasonably large.

Relationship between Molecular Orientation of Rubbed Polyimide Alignment Layer and That of Liquid-Crystalline Polymer Film Coated on the Alignment Layer

The molecular orientation of the coating type of retardation film was investigated. The retardation film was created by photopolymerization of a liquid-crystalline (LC) monomer coated on a rubbed polyimide alignment layer. The relationship between the molecular orientation of a rubbed polyimide film and that of mesogens in a photopolymerized liquid crystal coated on the polyimide film has been investigated using optical measurements. It was found that the inclination angle of the refractive index ellipsoide and the optical retardation of photopolymerized LC films are strongly related to the optical properties of the rubbed polyimide film regardless of the rubbing conditions.

Photoluminescence and photoconductivity studies of oriented polyfluorene thin films

Photoluminescence and photoconductive properties of poly(9,9-dioctylfluorene) (F8) on rubbed polyimide (PI) alignment layers have been studied. The F8 thin films are highly oriented, which is manifested from polarized absorption, fluorescence, and photoconductivity spectra (the order parameter of F8 is 0.86). The ratio of photocurrent parallel to perpendicular directions to the rubbing direction (photoconductivity anisotropy) is 18 at 2.9 eV for F8. Effects of C 60 addition to F8 on the optical properties are also studied. C 60 addition to F8 significantly increases photoconductivity but decreases the order parameter (0.67) and the photoconductivity anisotropy (7 at 2.9 eV). Both F8 and C 60 doped F8 thin films exhibit similar photoconductive response and similar photoconductivity anisotropy (3–4) above 3.8 eV.

Influence of rubbing conditions of polyimide alignment layer on optical anisotropy of immobilized liquid crystal film

The relationship between the molecular orientation of a rubbed polyimide film (alignment layer) and that of mesogens in a photopolymerized liquid crystal (LC) coated on the film has been investigated using optical measurements. LC monomers were deposited on the alignment layer and were aligned in one direction. The LC monomers were subsequently photocured. Alignment layers under various rubbing conditions were prepared. It was found that the inclination angle of the refractive index ellipsoid and the optical retardation of photopolymerized LC films are strongly related to the optical anisotropy of the rubbed polyimide film. The photopolymerized LC film exhibited high optical anisotropy when alignment layers with an inclination angle of the refractive index ellipsoid smaller than 6° were used.

Flow behaviours of liquid crystal droplets on polyimide alignment layers

The flow behaviours of liquid crystal droplets on polyimide alignment layers and during assembling between two substrates have been directly observed. The droplet shape became elliptical with time on the rubbed polyimide layer, where the major axis of the elliptical droplet was parallel to the rubbing direction. Rubbing enhanced the wettability between the liquid crystal and the polyimide layer. During the assembling process, the liquid crystal droplets elliptically splayed out between two substrates assembled antiparallel. The liquid crystal molecules preferentially flowed parallel to the rubbing direction in a two‐step flow mode; the droplet diameter slowly increased at the first step, and then it rapidly increased at the second step. The two‐step flow of the droplet proved to be due to the thickness of the droplet dependent on the rubbing strength.