Liquid crystal alignment behaviours on poly(methyl methacrylate) having polyhedral oligomeric silsesquioxane groups

A series of poly(methyl methacrylate) derivatives containing polyhedral oligomeric silsesquioxane (POSS) groups (MCP#) were synthesised via free radical polymerisation (FRP) using methacryl isobutyl POSS (MA-POSS) and methyl methacrylate as monomers to investigate liquid crystal (LC) alignment property of these polymer films. The LC cells made from the films of the polymers having 100 mol% of MA-POSS units (MCP100) showed vertical LC alignment having a pretilt angle of about 90°. The vertical LC alignment behaviour on the MCP100 film was ascribed to the very hydrophobic MCP100 surface having the surface energy value smaller than about 23 mJ/m² generated by the nonpolar bulky POSS group. Good electro-optical characteristics, such as voltage holding ratio (VHR) and residual DC voltage (R-DC), were observed for the LC cells fabricated using MCP100 as a LC alignment layer.     

Liquid crystal alignment properties of poly(styrenesulphonate)/alkyltrimethylammonium complexes

We synthesised a series of poly(4-styrenesulphonate)/alkyltrimethylammonium (PSS-#Cx, # = 12, 14 and 16; x?=?80, 60, 40 and 20) complexes, where # is the number of carbon atoms in the alkyl groups in alkyltrimethylammonium bromide, and x is the molar content (%) of alkyltrimethylammonium moiety, using polymer analogous reactions to investigate their liquid crystal (LC) alignment properties. In general, the LC cell fabricated using the polymer film having a longer alkyl side group and a higher molar content of alkyl side group showed homeotropic LC alignment behaviour with a pretilt angle of about 90°. The homeotropic LC alignment behaviour was well correlated with the surface energy of the polymer films. Homeotropic LC alignment was observed when the surface energy values of the polymer were smaller than about 44.87 mJ/m².     

Liquid crystal alignment properties of 2-naphthoxymethyl-substituted polystyrenes

The liquid crystal (LC) alignment properties of LC cells fabricated with films of 2-naphthoxymethyl-substituted polystyrenes with different contents of naphthoxymethyl side groups were investigated. The polymer films exhibited good optical transparency in the visible light region (400–700 nm). The LC cells made from the unrubbed films of polymers having more than 57 mol%?of 2-naphthoxymethyl containing monomeric units showed homeotropic LC alignment with a high pretilt angle of about 90^o. Good electro-optical characteristics, such as the threshold voltage, response time, voltage holding ratio and residual DC voltage were observed for the LC cells fabricated with the polymer having 100 mol%?of 2-naphthoxymethyl containing monomeric units as an LC alignment layer.     

Liquid crystal alignment property of n‐alkylthiomethyl‐ or n‐alkylsulfonylmethyl‐substituted polystyrenes

We synthesized a series of n‐alkylthiomethyl‐substituted polystyrenes (#T‐PS, # = 4, 8, 12, and 16) and n‐alkylsulfonylmethyl‐substituted polystyrenes (#S‐PS, # = 4, 8, 12, and 16), where # is the number of carbon atoms in the n‐alkyl side group of the polymers, using polymer analogous reactions to investigate their liquid crystal (LC) alignment properties. In general, the LC cell fabricated using the polymer film having a longer n‐alkyl side group, a thioether linkage group, and a higher molar content of n‐alkyl side group showed homeotropic LC alignment behavior with a pretilt angle of about 90°. The homeotropic alignment behavior was well correlated with the surface energy of the polymer films; when the positive dielectric anisotropic LC (ZLI‐5900‐000 from Merck) and negative dielectric anisotropic LC (MLC‐7026‐000 from Merck) were used to fabricate the LC cells, homeotropic alignment was observed when the surface energy values of the polymer were smaller than about 25 and 32 mJ/m², respectively. Copyright © 2009 John Wiley & Sons, Ltd.     

Fabrication of vertically aligned liquid crystal cell without using a conventional alignment layer

We propose a novel method to fabricate a vertical alignment (VA) of liquid crystal (LC) molecules without using a conventional alignment layer such as polyimide film. The method produces the vertical alignment polymer layer (VAPL) by polymerisation of a monomer or mixed monomers including in the LC layer above T_NI of the LC material. The VA mode LC cell with the VAPL (VAPL-LC cell) produced from the mixed monomers of acrylic acid 4-(4?-octyloxy-biphenyl-4-yloxy)-butyl ester and 1,2-bis-(4-methacryloxy-phenyl)-2,2-dimethoxy-ethane-1-one exhibited enough level of alignment state and electro-optical property with high voltage holding ratio. We can expect that the VAPL-LC cell is useful for next-generation displays such as flexible liquid crystal displays because the method does not need the process including high temperature over 200°C.     

Study of dynamic light scattering in nematic liquid crystal and its optical,electrical and switching characteristics

In the present study, the polarisation-independent dynamic light scattering (DLS) in a nematic liquid crystal (LC) with a negative dielectric anisotropy has been compared in two operating modes by applying DC voltage or AC voltage to LC cells with a vertical and hybrid alignment. The attenuation of light transmittance and the DLS optical threshold without polarizers as well as a phase retardation of LC bend–splay deformation and a Fredericks threshold voltage with polarizers have been determined. Diffuse scattering of a light beam and broadband transmittance spectra of LC cells have been examined using the DC voltage in the interval of 0–40 V. Multi-domain structures and turbulent flows in the LC cells have been observed by polarisation optical microscope. The results show that the enhancement of the diffuse scattering of light in the LC cells correlate with ion density increase. The largest attenuation of the light intensity in the LC cells with a vertical alignment was 16.4 dB at a wavelength of 650 nm with a minimum decay time equal to 2.5 ms at DC voltage of 60 V. Our study has been shown that switching from a diffuse light scattering state to a transparent state can be twice accelerated by applying AC voltage with high frequency to LC cells.     

Electro‐optical characteristics in phase separated liquid crystal/photo‐curable acrylic monomer mixture system

One of the main objectives of the experiment was to achieve the vertical aligned (VA) effect. To accomplish this, we employed liquid crystal (LC)/photo‐curable acrylic monomers mixture systems to prepare vertical alignment copolymer film (VACOF) for LC molecules with the photo‐polymerization induce phase separation (PIPS) process. From previous experimental results, we successfully fabricated LC devices without the micro‐protrusion structure. After the application of a saturated voltage, the LC molecules actually exhibited such interesting phenomena as uniaxial orientation, uniform single‐domain display state, etc. In this study, to obtain VACOF with smooth surface, we similarly controlled appropriate experimental conditions such as UV light exposure intensity and curing temperature, and altered process parameters such as the cell thickness, chemical structure length of the main chain type biphenol acrylic monomer [to simulate the main chain function of the traditional vertical alignment type polyimide (PI)], etc. During the experiment, we discovered that regardless of the cell thickness, this photo‐alignment system would yield the VACOF instead of the polymer disperse liquid crystal (PDLC) film morphology. Another notable finding was that the contrast ratio was heavily influenced by the length of the main chain type acrylic monomer structure for LC/monomer mixture systems, with enhancement of up to ～56%. Therefore, we further investigated the display effects, electro‐optical properties, etc. for these two main chain type acrylic monomer systems with different lengths and cell thicknesses. Copyright © 2011 John Wiley & Sons, Ltd.     

Liquid crystal alignment properties of n-alkylsulphonylmethyl-substituted polyoxyethylenes

The liquid crystal (LC) alignment properties of LC cells fabricated with films of n-alkylsulphonylmethyl-substituted polyoxyethylenes (#S-PEO, #?=?4, 6, 7, 8 and 10), where # is the number of carbon atoms in the n-alkyl side groups having different n-alkyl chain length, were investigated as a function of the rubbing density. The LC cells made from unrubbed #S-PEO (# ≥8) films having more than eight carbon atoms in the n-alkyl side groups showed homeotropic LC alignment. The homeotropic LC alignment behaviour correlated well with the surface energy values of the unrubbed #S-PEO films; homeotropic LC alignment was observed when the surface energy values of the unrubbed #S-PEO films were smaller than about 21.62 mJ m^?2. The LC cells made from rubbed #S-PEO (# ≥7) films having more than seven carbon atoms with a rubbing density of 150 showed homeotropic LC alignment. It was also found that the tilt angle of the LCs on the rubbed #S-PEO films was affected not only by the n-alkyl chain length of the polymers, but also by the rubbing density, regardless of the surface energy value of the #S-PEO film.     

Effect of crosslinking polymer networks on the molecular reorientation and electro‐optical performance of in‐plane switching vertically aligned liquid crystal devices

The effects of crosslinking polymer networks (PNs) on the molecular reorientation and electro‐optical properties of vertically aligned (VA) liquid crystal (LC) devices are investigated by applying an in‐plane switching (IPS) electric field. Through the polymerization process, crosslinking PNs are developed on the substrate surface, effectively increasing the anchoring energy and governing the LC molecular reorientation. With its stronger anchoring effect, the PNs cell shows good light transmittance and excellent vertical alignment quality, as compared to the pure LC cell. Furthermore, the alignment transformation and transmittance bounce resulting from the transient process of LC molecular reorientation are eliminated when the cell is operated at high voltages. The rising‐time (t_r) and falling‐time (t_f) responses of the PNs cell are significantly improved, and around 36% improvement in the optical switching response is obtained. In addition, the dynamic gray‐level t_r and t_f responses of the PNs cell are enhanced by around 55% and 42%, respectively, at a low driving voltage (～12 V). This developed VA‐IPS LC/PNs cell benefits not only the LC molecular alignment but also the electro‐optical performance. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1123–1130     

Self-constructed stable liquid crystal alignment in a monomer-liquid crystal mixture system

Here, we demonstrate excellent liquid crystal (LC) vertical alignment without using an alignment layer printing process by introducing octadecyltrichlorosilane (OTS) into the LC mixture. Further, we investigated the alignment mechanism by analysing the surfaces of the substrates. The optimum concentration of OTS was found to be about 0.03 wt%, which is 1/100 of that in the previously reported polyhedral oligomeric silsesquioxane (POSS)–LC system. Moreover, the OTS–LC system exhibited a more stable LC alignment compared with the POSS–LC system. These differences may arise from the different strengths of surface–dopant interactions; that is, the covalent bond in the OTS–LC system and the van der Waals interactions in the POSS–LC system. We also demonstrated that the method can be used in a capillary tube, which may serve as a new method facilitating the application of LCs with curved surfaces.     

Fabrication of coloured liquid crystal device using photoluminescent biomolecular chlorophyll

We fabricated a coloured liquid crystal (LC) device using photoluminescent biomolecular chlorophyll. The chlorophyll molecule, which is inexpensive because of its abundance in nature, was doped in the LCs for manufacturing the coloured LC device. We confirmed the green colour property of the LC doped with chlorophyll dye using UV-vis spectroscopy. Although the LC cell filled with 0.5 wt% chlorophyll showed good vertical alignment and fast response time, the doping of LC medium with 1.5 wt% chlorophyll resulted in imperfect vertical alignment and slowed response time due to the aggregation of chlorophyll molecules at high concentration. Furthermore, the photoluminescence (PL) characteristics of the LC doped with chlorophyll were investigated using PL spectroscopy. The coloured LC cell doped with chlorophyll dye showed emission in the red wave number region under UV light. As the LC doped with chlorophyll exhibited good colour performance, conventional colour filter layers could be avoided with the employment of the proposed coloured LC device. By controlling the light source, it is possible to apply the advanced LC device for colour switching. Moreover, a full-colour-switching LC device can be realised using various biomolecular dyes that can emit other colours.     

In situ control of surface molecular order in liquid crystals using a localised polymer network and its application to electro-optical devices

A liquid crystal (LC) alignment technique has been developed that allows local control of the polar pretilt angle over the range of 0–90°. This was achieved through the formation of a polymer network localised in the vicinity of the LC cell substrates. The network was formed as a result of in situ UV-induced polymerisation of a photo-reactive monomer added at concentrations of 0.5–1%. Localisation of the polymer network at the LC–substrate boundary was achieved by the application of a high voltage before polymerisation. The resultant pretilt angle was determined by the voltage applied during the polymerisation and/or the duration of the voltage application before the polymerisation step. The desired pretilt angle could be set over a small area of the sample, which allows the fabrication of LC devices with spatially variable optical retardation. Using this method we fabricated a converging lens, a bi-prism, and a phase diffraction grating with resolution greater than 50 lines mm^?1.     

Dye-dependent studies on droplet pattern and electro-optic behaviour of polymer dispersed liquid crystal

In order to study the droplet pattern and electro-optic (EO) behaviour of polymer dispersed liquid crystal (PDLC) with the addition of dye, dichroic polymer dispersed liquid crystal (DPDLC) films were prepared using a nematic liquid crystal (NLC), photo-curable polymer (NOA 65) and anthraquinone blue dichroic dye (B2), in equal ratio (1:1) of polymer and liquid crystal (LC) by polymerisation induced phase separation (PIPS) technique. Dichroic dye was taken in different concentration (wt./wt. ratio) as 0.0625%, 0.125%, 0.25%, 0.5% and 1% of the LC mixture in DPDLC films. Initially, in an open circuit when there is no proviso for external electric field (0 V), LC droplets in polymer matrix exhibited bipolar pattern, though on closing the circuit with the increase of electric field pattern of droplets starts changing, LC molecules align along the direction of applied electric field and aligned completely relatively at higher field (30 V), which illustrate vertical radial pattern. Further, results show that the DPDLC film containing 0.0625% dye concentration with consistent average droplet size ~4.30 μm, exhibits the best transmission at lower operating voltage.     

Fabrication of photoluminescent liquid crystal device using an in situ self-assembled molecular layer of a pyrene derivative

An in situ self-assembled molecular layer of 1-pyrenesulfonic acid sodium salt as an alignment agent was formed on indium tin oxide substrates for vertically aligning liquid crystals (LCs). The thus-aligned LCs exhibited uniform vertical alignment under crossed polarisers. The electro-optical characteristic of the LC cell fabricated using this method exhibited better performance than those of conventional LC cells with a polyimide alignment layer. Because the proposed alignment method is a simple one and involves low concentrations of the alignment agent (0.05 wt%), it is highly cost-effective. Further, the pyrene derivative, when mixed with LCs, exhibited photoluminescence (PL) under ultraviolet light. Given that the proposed method resulted in highly vertically aligned LCs and the alignment agent exhibited PL, the method should find wide use in the fabrication of colour-filter-free LC displays.     

The conducting polymer electrolyte based on polypyrrole-polyvinyl alcohol and its application in low-cost quasi-solid-state dye-sensitized solar cells

The effect of polypyrrole (PPy) on the polyvinyl alcohol (PVA)-potassium iodide (KI)-iodine (I₂) polymer electrolytes has been investigated and optimized to use in a dye-sensitized solar cell (DSSC). The different weight ratios of PVA: PPy (93: 2, 91: 4, 89: 6, 87: 8, and 85: 10 wt%) polymer electrolytes (PE) were prepared by solution casting. Structural, complex formation and surface roughness of the prepared electrolytes was confirmed by X-ray diffraction, FTIR, and atomic force microscopy (AFM) respectively. Conductivity plots of all polymer films showed increasing trend with temperature and concentration of PPy. The activation energy of the optimized system found to be 0.871 kJ mol^?1. UV-visible spectrum was adopted to characterize the absorption spectra of the material revealed that increase in the absorbance with increasing PPy content and shifting the absorbance maximum towards lower energy. The indirect band gap decreased from 3.78 to 2.14 eV and direct band gap decreased from 3.88 to 2.71 eV. The EIS analyses revealed the lower charge transfer resistance of 3.029 Ω cm² at the interface between CE and PE. The excellent performance was observed in the fabricated DSSCs using PVA (85%)/PPy (10%)/KI (5%)/I₂ polymer electrolyte with a short-circuit current density of 11.071 mA cm^?2, open-circuit voltage of 0.644 V, fill factor of 0.575, and photovoltaic conversion efficiency of 4.09% under the light intensity of 100 mW cm^?2. Hence, the PPy content in polymer electrolyte influences the remarkable performance of low-cost DSSC.     

UV-induced vertical alignment liquid crystal cell with two species of monomers having vertical-alignment-induced side-chain group and phenanthrene group

We propose the method for formation of the vertical alignment polymer film by polymerisation of the monomers being dissolved in the liquid crystal (LC) material. For obtaining the vertical alignment, combination of the two monomers, 4-(4?-octyloxy-biphenyl-4-yloxy)-butyl ester (AOBBE) and 2,7-dimethacryloyl-oxy-phenanthrene (DMAPhen), was found to be useful.

The monomer DMAPhen initiates the polymerisation under UV light exposure, and the AOBBE unit induces the vertical alignment without generating any alignment defects. The monomer DMAPhen is useful for maintaining the high voltage holding ratio and low residual direct current voltage after UV light exposure because the molecules of DMAPhen do not remain in the LC layer.     

Electro-optical and dielectric study of multi-walled carbon nanotube doped polymer dispersed liquid crystal films

The self-organising property of nematic liquid crystals (LCs) was used to align multi-walled carbon nanotubes (MWCNT) dispersed in them. MWCNT not only well integrate in the matrix but also, even at very low concentration, have a detectable effect on the LC properties that can be very attractive for display applications. In the present work, MWCNT were doped (0–0.5% wt/wt) in two different types of LCs. These MWCNT doped polymer dispersed LC (CPDLC) films were studied comprehensively using fundamental techniques. Polarising optical microscope (POM) and scanning electron microscope (SEM) techniques used for morphological study reveal that the LC droplet size remains unchanged with increase in MWCNT concentration. The electro-optical (EO) study performed by increasing voltage in steps of 10 V up to 100 V at an optimised frequency of 200 Hz and at temperature 25°C shows that the low MWCNT concentration films show good optical response than the higher one. The dielectric behaviour of CPDLC films in the frequency range 20 Hz to 20 MHz was investigated using precision impedance analyser. The obtained data were modelled with Debye and Cole-Cole methods to calculate relaxation time and distribution parameter (α). The zero value of α indicates Debye type relaxation processes.     

Liquid‐crystalline organic–inorganic hybrid polymers with functionalized silsesquioxanes

Liquid‐crystalline (LC) hybrid polymers with functionalized silsesquioxanes with various proportions of LC monomer were synthesized by the reaction of polyhedral oligomeric silsesquioxane (POSS) macromonomer with methacrylate monomer having an LC moiety under common free‐radical conditions. The obtained LC hybrid polymers were soluble in common solvents such as tetrahydrofuran, toluene, and chloroform, and their structures were characterized with Fourier transform infrared, ¹H NMR, and ²⁹Si NMR. The thermal stability of the hybrid polymers was increased with an increasing ratio of POSS moieties as the inorganic part. Because of the steric hindrance caused by the bulkiness of the POSS macromonomer, the number‐average molecular weight of the hybrid polymers gradually decreased as the molar percentage of POSS in the feed increased. Their liquid crystallinities were very dependent on the POSS segments of the hybrid polymers behaving as hard, compact components. The hybrid polymer with 90 mol % LC moiety (Cube‐LC90) showed liquid crystallinity, larger glass‐transition temperatures, and better stability with respect to the LC homopolymer. The results of differential scanning calorimetry and optical polarizing microscopy showed that Cube‐LC90 had a smectic‐mesophase‐like fine‐grained texture. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4035–4043, 2001     

Super fast switching and low operating of liquid crystals sandwiched between ion beam-spurted ITO thin layers

Ion beam (IB)-spurted indium tin oxide (ITO) thin layers are used to align liquid crystals (LC) with a lower driving voltage. During IB spurting process, the microcrystals transforming to large crystals of ITO is intimated by the change of In (3d), Sn (3d) and O (1s) core level in XPS spectra and the surface topology modifications in SEM and AFM images, and IB-spurted ITO thin layers are comparably transparent and conductive compared with ITO thin layers. The increased interactions between LC and IB-spurted ITO thin layers together with the roughed surface topology of ITO thin layers are the main causes for LC alignment. The fast response and distribution of electrical dipoles to external voltage in LC causes LC’s extremely low threshold voltage drive; in addition, LC directly aligned on ITO thin layers free from alignment layers shield effect further decreases LC’s threshold voltage. 1.8-keV IB-spurted ITO thin layers are more appropriate to align LC with the threshold voltage of 0.4853 V and the rising time of 0.237 ms.