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Cis–Trans Isomerization of Azopolyimides in the Solid State
Published in Andreea Irina Barzic, Neha Kanwar Rawat, A. K. Haghi, Imidic Polymers and Green Polymer Chemistry, 2021
Photochromic PIs can be applied as the photoalignment liquid crystal layers. These types of materials can be an interesting alternative for the rubbing technique, which uses PIs as layers for the alignment of liquid crystal. Rubbing of polymers turned out to be a very convenient technique and it is now widely used in both small scientific labs and large LCD factories.52 The rubbing technique has advantages like it is less expensive and easy control method. Despite these advantages, this technique has some serious drawbacks that became appeared to be crucial for the production of LCDs and miniature LC telecommunication devices. These drawbacks follow from the contact type of rubbing technology. Additional difficul-ties connected with the precise control of the rubbing characteristics over huge substrate areas. Using this method is also problematic in the case of projection displays because rubbing traces become visible when the former image is magnified. Other problems may arise when rubbing is used in miniature telecommunication devices where there is a need to align LC in the thin gaps of the light waveguides. Finally, this method in principle cannot be used to align LCs in closed volumes. Limitations of the rubbing technique caused the search for alternative methods of LC alignment. The interesting method is the application of photosensitive materials in which photoalignment of liquid crystal is based on irradiation of the photosensitive polymer layer by the laser beam.51–53 Photo-ordering can be used in devices where it is impossible to use mechanical techniques, that is, in the production of glass microtubes, photonic glass fibers, and planar optical fibers.54 Oriented high-purity LC semiconductor layers can be used in photovoltaic cells, organic light-emitting diodes (OLEDs), or organic thin-film transistors (OTFTs).51
Bulk-mediated in-situ homogeneous photoalignment induced by reactive mesogen containing diphenylacetylene moiety
Published in Liquid Crystals, 2020
Rui He, Pushan Wen, Yang Ye, Eunche Oh, Shin-Woong Kang, Seung Hee Lee, Myong-Hoon Lee
In this study, we took account of the diphenylacetylene structure, which exhibits an irreversible [2 + 2] photodimerization reaction rather than trans/cis photoisomerisation upon UV irradiation. We designed and synthesised a reactive mesogen (abbreviated as DAT) containing a diphenylacetylene moiety in the core and two polymerisable acrylate groups at both ends, which showed a smectic A phase at between 17.9°C and 119.2°C. A small amount of DAT was mixed with conventional host LC, and the resulting mixture was injected into a sandwich cell without an alignment layer. We demonstrated the polyimide-free in-situ homogeneous alignment of LCs by irradiating the LPUV on the cell at above TNI of the LC mixture. This gave rise to the photopolymerisation of acrylate groups in the DAT reactive mesogen, and simultaneously, [2 + 2] photodimerization of diphenylacetylene moiety, which generated an anisotropic polymer layer on the surface of both substrates inducing homogeneous alignment of LCs, as illustrated in Figure 1. The resulting irreversible homogeneous LC alignment layer exhibited excellent thermal and UV stabilities. Furthermore, by this approach, we were able to fabricate an in-situ homogeneously aligned IPS cell (DAT-IPS) without using a pre-treated alignment layer. In this work, we describe the synthesis and characterisation of DAT, the fabrication processes of DAT-IPS cells, and their electro-optic properties. We believe that this simple in-situ fabrication method is one of the potential alternatives to the conventional alignment technique to achieve a low-cost, eco-friendly and effective alignment in LCDs. Additionally, this in-situ photoalignment technique possesses a great potential for the fabrication of polarisation-selective-patterned devices or advanced flexible LCDs.
Simultaneous formation behaviour of surface structures and molecular alignment by patterned photopolymerisation
Published in Liquid Crystals, 2019
Sayuri Hashimoto, Miho Aizawa, Norihisa Akamatsu, Takeo Sasaki, Atsushi Shishido
Control of molecular alignment plays an important role in functionalisation and performance improvement of materials [1]. Especially, alignment control of liquid crystals (LCs) has drawn much attention as it imparts mechanical and optical functions [2–4]. Such functional materials have been applied to electronic, photonic, mechanical, and medical devices. For instance, Broer et al. [5] reported that dynamic modulation of the surface topology is achieved by the alignment patterning of cholesteric LCs, which provides functions such as switchable wettability, adhesion control and modulation of optical properties. These examples rely on a thermally responsive system. They also proposed a photoresponsive system to control the surface topology by ultraviolet (UV) light irradiation [6–8]. For aligning LC molecules, mechanical processes, which are stretching in a certain direction and rubbing the surface of polymer films, are practically used. Mechanical alignment methods are simple and show high versatility, but have problems such as contamination of dust, generating static electricity and difficulty of fine control of complex alignment patterns. Thus, a new molecular alignment process with light irradiation has been developed. In a photoalignment method, molecules are aligned depending on the polarisation direction of the incident linearly polarised light that causes photochemical reactions of photoisomerisable, photocrosslinkable or photodegradable compounds [9–12]. Ichimura et al. [13] firstly demonstrated a photoreactive alignment layer, referred to as a ‘command surface’. In this system, LC alignment can be reversibly controlled between homeotropic (out-of-plane) and homogeneous (in-plane) states by using the photoisomerisation of an azobenzene which is caused by the irradiation with linearly polarised light over the surface [14]. Recently, Seki et al. [15,16] developed a new photoalignment method to control the molecular alignment using free-surface molecular command systems, and they succeeded in complex alignment patterning by coating an azobenzene polymer by introducing an ink-jet printing system. As well as using azo-based polymers, a photoalignment process using other photoresponsive materials such as a cinnamate has also been developed [11,17–19]. In addition, the photoalignment method has the significant strong point of the complex alignment patterning by a control of the polarisation direction of the incident light. Then, this photoinduced molecular alignment method has been widely utilised for producing photonic and mechanical devices [2,20–28]. For example, the demonstration of the thermally activated dynamic control of the surface topography in the LC elastomer coatings along the pre-programmed alignment patterns has recently been reported [29]. In this system, the micrometre-sized alignment patterns were generated by the photoalignment process, and the inscribed two-dimensional patterns cause a three-dimensional dynamic profile change. However, even with a wide variety of functional materials, the photoalignment method has some challenges such as requirement of polarised light, and necessity of photoresponsive molecules.