Search

Photoalignment control of hierarchical structures is a key process to enhance the properties of optical and mechanical materials. We developed an in situ molecular alignment method, where photopolymerization with the scanned slit light causes molecular flow, leading to two-dimensional precise alignment of molecules over large areas; however, the alignment control has been explored only on a molecular scale. In this study, we demonstrate this photopolymerization-induced molecular flow, enabling mesoscopic alignment of smectic layer structures composed of anisotropic molecules. Side-chain liquid-crystalline polymers were obtained from two different monomers with or without alkyl spacers by photopolymerization with one-dimensionally scanned slit light. The polymer with an alkyl spacer displayed mesogens aligned parallel to the scanning direction, while the polymer with no alkyl spacer resulted in perpendicular alignment of mesogens to the scanning direction, regulated by the alignment of the polymer main chain along the light scanning direction. Moreover, the polymerization with the scanned light aligned not only the mesogens but also mesoscopic smectic layer structures over large areas, depending on the structure and scanning pattern of light. We envision that such a simple polymerization technique could become a powerful and versatile alignment platform of anisotropic materials in a wide range of scales.

The photoinduced molecular reorientation of nonlinear optical materials is a promising approach to cause the nonlinear optical effect used for developing next-generation self-modulating and optical switching devices.

We report on the fabrication of oriented collagen films by drying a dilute collagen solution on a polydimethylsiloxane (PDMS) substrate with a micrometer grooved surface. The resultant films show optical and mechanical anisotropy along the groove direction, and have a high mechanical strength with Young's modulus of 2.3 GPa. This process might enable collagen materials to be applied to anisotropic biocompatible substrates, large-area functional scaffolds, and soft robotic materials.

Hardenable pressure-sensitive adhesives, which show pressure-sensitive adhesion state with weak adhesion strength in their initial semisolid state and general adhesion state with strong adhesion strength in their hardened state, are desirable in various industrial fields to improve efficiency of manufacturing and recycling products. Here we developed novel photohardenable pressure-sensitive adhesives triggered by photoplasticization of poly(methyl methacrylate) containing photoresponsive liquid crystal (nematic and smectic E) plasticizers at various ratios. It was found that photoplasticization, which is the photoinduced reduction of glass transition temperature and hardness of polymers, could be repeatedly induced by alternate irradiation with ultraviolet (UV) and visible (Vis) light in all mixtures, regardless of the phase structures of the photoresponsive plasticizers. Upon photoplasticization under UV-light irradiation, all mixtures exhibited glassy-to-rubbery transition to a pressure-sensitive adhesion state under appropriate conditions. Upon irradiation of the photoplasticized samples with Vis light, the samples recovered their initial hardened state, recovering the glassy nature with elastic moduli. The adhesion strength of the samples in the hardened state was significantly influenced by the phase structures of the plasticizers. When a photoresponsive plasticizer exhibited the smectic E phase, which is a highly ordered liquid-crystalline phase, the adhesion strength was remarkably larger than those of the case using the plasticizers showing nematic and crystalline phases. This result was reasonably explained in terms of the suppressed bleed-out of the photoresponsive plasticizers from the polymer and the good mechanical properties of the mixture stemming from the characteristics of the smectic E phase. Furthermore, through the reversibility of a photoplasticization process, we achieved a photoinduced reduction of the adhesion strength by UV irradiation of the samples in the hardened state. Photohardenable pressure-sensitive adhesives with reversibility has been developed using a commodity plastic just by adding the photoresponsive plasticizer showing the smectic E phase.

Flexibility, viscoelasticity and stress-strain relation in bending polymeric films are key factors in designing mechanically durable flexible electronic devices and soft robots. However, bending hysteresis, which appears as a precursor phenomenon of fracture and fatigue, remains unclear; no one quantitatively evaluated a bending curvature causing hysteresis. Herein, we report the bending hysteresis of polymeric films used as common substrates in flexible electronics by precisely monitoring bending curvatures. By real-time measuring curvatures of films upon bending and subsequent unbending, we have successfully determined the curvatures that cause the hysteresis. These curvatures also depend on a film thickness. Furthermore, we revealed that the occurrence of bending hysteresis is explained by bending strains that have a nonlinear relation with internal stresses. This enables us to predict strain limits that cause the bending hysteresis, based on a stress-strain curve of polymeric films.

Photochemical control of the hardness and fluidity of polymeric materials is applicable to the development of photoreversible adhesives. Here, we report that common polymers show photoplasticizatio...

In this study, we fabricated liquid crystal polymer networks (LCNs) containing a large amount of crosslinker by scanning wave photopolymerization, and examined the effect of crosslinker concentrati...

The irradiation of oligothiophene-doped liquid crystals (LCs) with a linearly polarized laser beam induces homeotropic-to-planar director reorientation. However, such molecular reorientation was ex...

Liquid crystals (LCs) have been utilized for the innovative optical devices because the molecular reorientation triggers a change in their optical properties. Among them, oligothiophene-doped LCs can induce the molecular reorientation by irradiation with a laser beam, which is useful for all-optical devices. However, the photoinduced reorientational behavior was observed only by irradiation with a high-intensity laser beam. In this study, we investigated the effect of incident light properties on photoresponsive behavior of oligothiophene-doped LCs by irradiation with a collimated laser beam from the perspective of improvement of the photoresponsive sensitivity of the molecular reorientation. As a result, the irradiation of oligothiophene-doped LCs with a collimated laser beam enhanced the sensitivity of the molecular reorientation.

Inorganic materials such as nanotubes and nanorods have attracted much attention due to their anisotropic properties. Although controlling the alignment of inorganic materials is able to enhance their functionality, macroscopic alignment over a large area remains a challenge. We have recently proposed a simple method for inducing unidirectional alignment of ZnO nanorods on a rubbed polyimide layer. In this method, ZnO nanorods grafted with liquid-crystalline (LC) polymers are aligned by cooperative interaction between the LC moieties in the grafted polymers and surrounding LC host molecules. In this study, we investigated the unidirectional alignment of surface-modified ZnO nanorods in nematic LCs in a micrometer-thick cells. Alignment of LC polymer-grafted ZnO nanorods along nematic LC host molecules has been revealed by polarized optical micrography and ultraviolet-visible absorption spectroscopy.

The control of molecular alignment patterns in liquid crystals is key to developing high-performance optical devices. In particular, two-dimensionally designed patterns have attracted much attention due to their potential application to novel optical devices such as a high efficiency polarization grating and a vortex converter. However, there remain challenges in obtaining molecular alignment patterns by a simple method. We have recently proposed a novel method for controlling the alignment of liquid crystals termed scanning wave photopolymerization (SWaP). In this method, a mass flow triggered by spatiotemporal photopolymerization causes shear stresses to anisotropic molecules, resulting in the generation of alignment patterns finely guided by the scanned light. In this study, we present the direct fabrication of polymer films with cycloidal molecular alignment patterns by SWaP.

We present a dye-free alignment patterning technique, based on a scanning wave photopolymerization (SWaP) concept, that achieves a spatial light–triggered mass flow to direct molecular order using scanning light to propagate the wavefront. This enables one to generate macroscopic, arbitrary 2D alignment patterns in a wide variety of optically transparent polymer films from various polymerizable mesogens with sufficiently high birefringence (>0.1) merely by single-step photopolymerization.

Surface bending strains in various flexible films is quantitatively analyzed by a surface-labeled grating method with a single-nanometer resolution. The real-time strain analysis we achieved has multiple benefits: high resolution, precision, and a wide range of measurable materials. The reliability of the measurements was confirmed using the modified Elastica theory. This method revealed that the cracking of hard coatings on the surface of PET films occurred at surface strains exceeding 1.45%, regardless of the film thickness and curvature. The multilayering of two PET layers with a soft PDMS layer between them reduced the surface bending strain by 50% compared with that of a single-layer film with the same thickness. This triple-layer film successfully suppressed the cracking of the hard coating and the breakdown of the OTFT. The surface-labeled grating method, therefore, is a practical tool that allows the analysis of surface bending strain in the elaborately designed materials.

Macroscopic and precise alignment control of functional molecules represented by liquid crystals (LCs) and polymers is the key to generating a new function and enhancing their performances. Among various alignment techniques, a photoalignment method offers the greatest potential to finely control molecular alignment because of the capability of micro- to nano-patterning with remote processes. Recently, we have proposed a novel photoalignment method based on a new concept of scanning wave photopolymerization (SWaP). This method utilizes molecular diffusion triggered by the localized photopolymerization, enabling to generate an arbitrary alignment patterns merely by single-step photo-irradiation. In this study, we fabricated liquid-crystalline polymer networks directed by SWaP.

To develop flexible devices that have mechanical durability, understanding the bending behavior of soft material components is quite important. However, measuring bending strain in soft materials has been limited to their surface due to experimental constraints. In addition to the surface strain analysis, internal strain analysis will further clarify the bending behavior of soft materials. In this study, we quantified internal strain in bending polydimethylsiloxane (PDMS) films, which are a common soft material, through the selective reflection of a cholesteric liquid crystal elastomer (CLCE). The strain analysis with the CLCE revealed that internal strains depend on the position of the bending PDMS films. This internal strain quantification of soft materials leads to the development of flexible devices with high mechanical durability.

Over the past decades, flexible electronics such as flexible liquid crystal devices composed of polymer film substrates have been dramatically growing. Understanding bending behavior of polymer films is the key to designing flexible electronic devices with high mechanical durability. Although various bending analysis methods have been proposed, they are still limited to macroscopic and qualitative analyses. Recently, we have newly proposed a method for analyzing the surface bending strain in flexible materials, termed surface labeled grating method. This method enables us to quantitatively evaluate the surface bending strain by monitoring the diffraction angle of a He-Ne laser beam that passes through a grating label attached on a sample. In this study, we measure the surface bending strain in polyethylene terephthalate films and reveal that tension and compression occurs in their outer and inner surfaces, respectively.

Molecular alignment control in polymer films is key to the development of high-performance materials with optical, electronic and thermal functions. Among molecular alignment techniques, a photoalignment method offers the fine and remote control of two-dimensional molecular alignment by the irradiation with linearly polarized light. We have recently proposed a novel photoalignment method based on the molecular diffusion caused by the polymer concentration gradient, termed scanning wave photopolymerization. In this study, we report specific polymerization behavior occurring during the process of the scanning wave photopolymerization. We investigate the effect of molecular diffusion on the photopolymerization behavior by measuring molecular weight of the polymers obtained under various photopolymerization conditions.

The control of the surface topology is important to utilize their high functionality for a wide variety of applications such as optics and mechanics. Several methods of controlling the surface topology have been proposed; in particular, the light irradiation can precisely control the surface topology in a non-contacting manner. However, conventional methods using the light irradiation need complicated processes with specific photoresponsive molecules. Recently, we revealed that patterned photopolymerization, based on a novel method of molecular alignment, induced surface relief structures on a film; yet, the relationship between the surface structures and molecular alignment has not been clarified. In this study, we made the detailed investigation of surface structures and molecular alignment of the film induced by patterned photopolymerization.

The Japanese Liquid Crystal Conference Online 2020 was held from 29 to 30 October 2020, organised by the Japanese Liquid Crystal Society. Because of COVID-19, the annual meeting, the Japanese Liqui...

Chiral-nematic liquid crystal (N* LC) elastomers exhibit mechano-optical responsive behavior. However, practical sensor applications have been limited by the intrinsic sensitivity of N* LC elastomers to environmental conditions, such as temperature. Although densely cross-linked LC network polymers exhibit high thermal stability, they are not proper for the mechanical sensor due to high glass transition temperatures and low flexibility. To overcome these issues, we focused on enhancing thermal stability by introducing noncovalent cross-linking sites via intermolecular interactions between LC molecules bonded to the polymer network. N* LC elastomers with a cyanobiphenyl derivative as a side-chain mesogen exhibited mechano-optical responsive behavior, with a hypsochromic shift of the reflection peak wavelength under an applied tensile strain and quick shape and color recovery owing to high elasticity. Notably, the N* LC elastomers showed high resistance to harsh environments, including high temperatures and various solvents. Interactions, such as π–π stacking and dipole–dipole interactions, between the cyanobiphenyl units can act as weak cross-links, thus improving the thermal stability of the LC phase without affecting the mechano-optical response. Thus, these N* LC elastomers have great potential for the realization of practical mechano-optical sensors.

Optical limiting is a phenomenon widely recognized as the potential application for a protector of human eyes and optical sensors from irradiation with lasers. However, a high optical limiting threshold and low flexibility have restricted such applications. Here, we report that oligothiophene-doped liquid crystals (LCs) function as a low-threshold optical limiter with deformability. Irradiation of dye-doped LCs with a continuous wave (CW) laser beam brings about the formation of diffraction rings, and the number of rings changes depending on the incident light intensity due to their photoinduced molecular reorientation. Utilizing such reorientation enables reversible optical limiting without additional multilayered optical components. In particular, an electric field application to a LC-based optical limiter decreases their optical limiting threshold from 2100 to 25 mW/cm2, and the threshold can be tuned by adjusting the applied voltage. Furthermore, the softness of LCs allows for the fabrication of the deformable optical limiter; optical limiting due to the molecular reorientation occurs even in largely bent states. The low-threshold and deformable optical limiter based on oligothiophene-doped LCs thus will enable one to develop the protector of eyes and optical sensors from glaring light-induced damage.

Flexible electronics and soft robotics have been drawing much attention. However, prevention of fracture of materials and devices in three-dimensional deformation remains a challenge. Design of strain in the materials and devices enables durable high-performance materials and devices. In this study, surface strain analysis of bending polymer films by a novel method called a surface labeled grating method is introduced.

Scanning wave photopolymerization of liquid crystal monomers enables arbitrary molecular alignment patterning due to molecular diffusion induced shear stress by spatially selective polymer production. However, the molecular alignment process has not been explored because of the difficulty of measuring such non-equilibrium state. In this study, we investigated molecular alignment behavior by directly observing scanning wave photopolymerization process with a polarized micrograph equipped with a dynamic light processor. Optical anisotropy was generated along with light scanning to trigger photopolymerization, indicating that molecules are aligned parallel to the light scanning direction.