Alignment Technology and Applications of Liquid Crystal Devices
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CA2261776A1 - Liquid crystal composition and alignment layer - Google Patents
Try again? Cited by. Back to tab navigation Download options Please wait Article type: Review Article.
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DOI: Download Citation: J. Giant surfactants for the construction of automatic liquid crystal alignment layers W. Yoon, K. Lee, D. Evans, M. McConney, D.
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- Vertical Alignment of Liquid Crystals Over a Functionalized Flexible Substrate | Scientific Reports;
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WOA1 - Liquid crystal device alignment - Google Patents
A Nature Research Journal. A simple and effective approach for vertical alignment of liquid crystals LCs over a functionalized transparent flexible substrate is described. Surface characterization of this commercially available plastic substrate through X-ray photoelectron spectroscopy XPS and attenuated total reflection infrared spectroscopy ATR-IR indicated that cellulose acetate is main component of the transparent substrate. This substrate was chemically functionalized with a suitable LC compound.
A trimethoxysilane terminated new rod-shaped mesogen is synthesized and covalently attached to the pre-treated film through silane condensation reaction. Remarkably, LC functionalized cellulose acetate films were found to be highly efficient in assisting a perfect homeotropic alignment of LCs for both, a room temperature N and a high temperature SmA phase over the entire area of the LC sample under observation indicating their superior aligning ability in comparison to their unmodified and octadecyltrimethoxysilane OTS modified counterparts.
The demonstrated method of surface modification of flexible polymer film is easy, surface modified substrates are stable for several months, retained their aligning ability intact and more importantly they are reusable with maximum delivery. Liquid crystals LCs are self-assembled soft materials with interesting applications in a variety of fields such as display industry, sensing, biomedical, photonics, optoelectronics, electronic conductors, photovoltaics etc 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8.
A pre-oriented well aligned sample of LC over a suitable substrate is highly crucial for most of these applications.
For example, homeotropic alignment wherein long axis of the LC molecules director are oriented perpendicular to the substrate surface 9 , or planar alignment where LC director lie parallel the substrate surface In general, a vertical alignment of LC at a solid-LC interface is often preferred for sensor applications. Molecular orientation in LCs is very sensitive to small perturbations and any binding events which change the orientation of the molecules will be propagated through several micrometers to provide detectable optical signals in the form of change in the textures, which can be observed under a polarizing optical microscopy POM.
Thus a pre-oriented uniform film of LCs supported on a substrate can be effectively utilized for LC based sensing which is simple, rapid and an inexpensive process. Upto date, LC-based sensors for gas-phase analytes such as organophosphonates, organoamines, aldehydes and organosulphur compounds biomolecules such as proteins, oligopeptides and DNA and microorganisms viruses, bacteria and mammalian cells have been described 4 , 5. The conventional method followed so far for the vertical alignment of LCs involves coating the substrates with suitable alkyl silanes 9.
However, there has been a continuous effort to improve upon the aligning technique for LCs. Several surface modification techniques employed to achieve effective vertical alignment of LCs include formation of self assembled monolayers SAMs on substrates, using Langmuir-Blodgett LB and Langmuir-Schaeffer LS methods, coating polyimide derivates containing hydrophobic long alkyl chain derivatives, irradiating with UV light, doping an organic compound or nanoparticles, or fabricating micro and nanostructures 11 , 12 , 13 , 14 , 15 , To name few, D.
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Walba et al. Hwang et al. In an another example, P. Noonan et al. LC alignment studies over blend polymer surfaces were also reported Self-organized thin films of molecules on water surface Langmuir films transferred to a solid substrate through Langmuir-Blodgett or Langmuir-Schaeffer techniques were also examined for the effective alignment of LCs. For example, Seki et al.
Mann and co-workers have described the perpendicular alignment of a nematic phase exhibited by a bent-core molecule using LS monolayers of BC LCs In an another approach, K. Kim et al.
Similarly, an automatic vertical alignment of nematic LCs over an ITO substrate by using carbohydrate-based giant surfactants as alignment layers was reported by D. Following a similar approach, I. Son et al. Same group have also reported a new strategy for the vertical alignment of LCs wherein, an in - situ photo-polymerization of alkyl acrylate monomer was carried out in a LC cell resulting in a polymer alignment layer on the ITO substrate We also reported an effective homeotropic alignment of bulk LCs over a functionalized ITO substrate It is interesting to point out here that, most of the approaches described so far for the LC alignment were carried out typically either on glass, indium tin oxide ITO or silica thin films and in all these studies, alignment of LC was demonstrated in a LC cell in which the oriented LC sample was confined between two surface modified substrates.
On the other hand, flexible substrates have gained significant attention in recent years, owing to their unprecedented properties such as light-weight, low-cost, thinness, easy processing and popularity of the devices incorporating flexible components since they are more convenient, interesting and easy to store 28 , 29 , 30 , 31 , Several plastic materials namely polyethylene terephthalate, polycarbonate, polyurethane, polydimethylsulfoxide etc.
As mentioned earlier an effective orientation of LCs is crucial for their applications. However, techniques for LC alignment over flexible substrates are very limited. Some of the proposed methods suffer from drawbacks such as polyimide alignment materials which require higher temperature, photo alignment which lack longtime stability of the alignment direction, vaccum treatment of the composite organic film structure on the plastic substrate which might bring unpredictable deformations and thermal nanoprint lithography which is expensive.
Only few examples known so far which describe the LC alignment over a polymer substrate. For example, Sekine et al. Mahilny et al. Further, H. Kang et al. The polymer was spin coated on ITO coated glass substrates and vertical alignment of LCs was demonstrated in LC cells made by sandwiching two of the polymer film deposited ITO slides using a spacer. While designing LC alignment layers for plastic substrates, several factors need to be considered such as chemical compatibility, low temperature process, good thermal and chemical stability of the modified substrate, reproducibility and the cost.
In this regard, herein, we describe a simple, robust, reproducible technique for the efficient vertical alignment of LCs over an economically viable, easily available flexible polymer substrate having a commercial name, overhead projector OHP film Fig. This film is essentially a transparent plastic film and cellulose acetate is supposed to be the main component of the film.
In order to probe the chemical nature of this substrate elemental composition and presence of functional groups , as-received OHP films were characterized using X-ray photoelectron spectroscopy XPS and infrared spectroscopy IR-ATR mode techniques described later which supported our claim, hence the plastic substrate is referred as cellulose acetate film in the rest of the content. Unlike the other two methods described for LC alignment over flexible polymer 38 , 39 , in which polymer alignment layer was deposited from a solution over a support plastic substrate or ITO , our approach involved chemical functionalization of surface of the polymer film in order to create permanent alignment layers on the flexible substrate.
Acetate films have remarkable features such as transparency, clarity, high mechanical strength, lightweight, stability under ambient conditions, breakage resistance, smooth finish, low cost and easy availability. Graphical representation of methodology followed for the surface functionalization of cellulose acetate based flexible polymer film with a LC molecule.
The polymeric substrate was modified with a monolayer of newly synthesized LC compound C containing a terminal trimethoxysilane moiety in order to obtain an effective LC alignment at the solid substrate — LC interface. The LC molecules chemically attached over flexible substrate were expected to provide necessary shape and symmetry compatibility for the bulk LC sample to be aligned.
https://olcasimas.tk Characterization of the unmodified and LC modified polymer substrates and versatility of the LC functionalized polymer substrate for the bulk LC alignment are presented. Interestingly, our approach meets several requirements such as ease of the method, surface modification at ambient conditions, good chemical and thermal stability of the modified substrate, long-time stability of the alignment, reproducibility, reusability and low-cost.
Herein, we described a simple, efficient and reproducible approach for the homeotropic alignment of LCs over a cellulose acetate based flexible film at substrate-LC interface. Although appropriate methods for alignment of LCs on different substrates such as glass, silicon wafer and ITO have been investigated in detail, similar studies over emerging, new-generation, interesting substrate material, namely flexible polymer are very limited in number.
In this regard, present work describes a promising strategy for effective use of an easily available, cost-effective cellulose acetate based polymeric substrate commercial name- OHP film , for homeotropic alignment of bulk LCs. The strategy involved the chemical functionalization of surface of the film with a newly synthesized LC compound C containing a terminal trimethoxysilane moiety through a monolayer formation, which in turn acted as permanent aligning layer for the bulk LC orientation. The motives behind the approach are as follows; chemically attached LC molecules on the surface are expected to provide necessary shape and symmetry compatibility for the bulk LC alignment and the chemical modification of the surface in comparison to the physical methods will provide a long term stability and assures reusability of the modified substrates.
LC compound C containing a trimethoxysilane terminal group required for functionalization of the cellulose acetate substrate was synthesized using the starting compound A following a pathway shown in Fig. S1 along with the analytical data obtained for the compound are provided in supplementary information SI. The compound A was esterified with allyl alcohol resulting in a vinyl terminated compound B.