Bending creep behaviour of various polymer films analysed by surface strain measurement.

Understanding the temporal bending deformation of polymer films is key to designing mechanically durable flexible electronic devices. However, such creep behaviour under persistent bending remains unclear due to a lack of precise and accurate bending strain analysis methods. Herein, we quantitatively analysed the bending creep behaviour of various polymeric films using our developed strain measurement method that can precisely measure surface strain from optical diffraction. The surface strain measurement reveals that bending creep deformation differs depending on the polymer structure. The four-element Burgers model was employed to model the temporal strain increase on the bending surface successfully. By fitting the four-element model to the time course of the measured surface strain, we found that each polymer film has a different threshold surface strain for the appearance of bending creep deformation. Such disparity in the bending creep behaviour can be explained by the difference in strain energy density between the polymer films and their elastic model; polymer films with a small strain energy density difference show small bending creep deformation. The results obtained in this study contribute to the elucidation of the bending creep behaviour of polymer films and the development of flexible electronic devices operated under persistent bending.

Synthesis and Characterization of Side-Chain Liquid-Crystalline Block Copolymers Containing Cyano-Terminated Phenyl Benzoate Moieties.

Block copolymers, known for their capacity to undergo microphase separation, spontaneously yield various periodic nanostructures. These precisely controlled nanostructures have attracted considerable interest due to their potential applications in microfabrication templates, conducting films, filter membranes, and other areas. However, it is crucial to acknowledge that microphase-separated structures typically exhibit random alignment, making alignment control a pivotal factor in functional material development. To address this challenge, researchers have explored the use of block copolymers containing liquid-crystalline (LC) polymers, which offer a promising technique for alignment control. The molecular structure and LC behavior of these polymers significantly impact the morphology and alignment of microphase-separated structures. In this study, we synthesized LC diblock copolymers with cyano-terminated phenyl benzoate moieties and evaluated the microphase-separated structures and molecular alignment behaviors. The LC diblock copolymers with a narrow molecular weight distribution were synthesized by atom transfer radical polymerization. Small angle X-ray scattering measurements revealed that the block copolymers exhibit smectic LC phases and form cylinder structures with a lattice period of about 18 nm by microphase separation. The examination of block copolymer films using polarized optical microscopy and polarized UV-visible absorption spectroscopy corroborated that the LC moieties were uniaxially aligned along the alignment treatment direction.

Unidirectional Alignment of Surface-Grafted ZnO Nanorods in Micrometer-Thick Cells Using Low-Molecular-Weight Liquid Crystals.

Inorganic nanomaterials such as nanotubes and nanorods have attracted great attention due to their anisotropic properties. Although the alignment control of inorganic nanomaterials is key to the development of functional devices utilizing their fascinating properties, there is still difficulty in achieving uniform alignment over a large area with a micrometer thickness. To overcome this problem, we focused on liquid crystals (LCs) to promote the alignment of anisotropic nanomaterials, taking advantage of the cooperative motion of LCs. We present the uniform, one-dimensional alignment of ZnO nanorods along the direction of LCs in micrometer-thick cells by grafting nematic LC polymers from the nanorod surfaces to provide miscibility with the host LCs. Polarized optical microscopy and polarized UV-visible absorption spectroscopy revealed the unidirectional alignment of nematic LC polymer-grafted ZnO nanorods parallel to the nematic host LCs.

Quantitative analysis of bending hysteresis by real-time monitoring of curvature in flexible polymeric films.

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.

Effect of Crosslinkers on Optical and Mechanical Behavior of Chiral Nematic Liquid Crystal Elastomers.

Chiral nematic (N*) liquid crystal elastomers (LCEs) are suitable for fabricating stimuli-responsive materials. As crosslinkers considerably affect the N*LCE network, we investigated the effects of crosslinking units on the physical properties of N*LCEs. The N*LCEs were synthesized with different types of crosslinkers, and the relationship between the N*LC polymeric system and the crosslinking unit was investigated. The N*LCEs emit color by selective reflection, in which the color changes in response to mechanical deformation. The LC-type crosslinker decreases the helical twisting power of the N*LCE by increasing the total molar ratio of the mesogenic compound. The N*LCE exhibits mechano-responsive color changes by coupling the N*LC orientation and the polymer network, where the N*LCEs exhibit different degrees of pitch variation depending on the crosslinker. Moreover, the LC-type crosslinker increases the Young's modulus of N*LCEs, and the long methylene chains increase the breaking strain. An analysis of experimental results verified the effect of the crosslinkers, providing a design rationale for N*LCE materials in mechano-optical sensor applications.

Shape-Shifting Azo Dye Polymers: Towards Sunlight-Driven Molecular Devices.

The development of stimuli-responsive polymers is among the key goals of modern materials science. The structure and properties of such switchable materials can be designed to be controlled via various stimuli, among which light is frequently the most powerful trigger. Light is a gentle energy source that can target materials remotely, and with extremely high spatial and temporal resolution easily and cheaply. Reversible light-control over molecular mechanical properties in particular has in recent years attracted great interest due to potential applications as optical-to-mechanical conversion actuators and 'devices', enabling 'molecular robotic machines'. In this review, some recent examples and emerging trends in this exciting field of research are highlighted, covering a wide variety of polymer hosts that contain azobenzene photo-reversible switches. It is hoped that this review will help stimulate more interest towards the development of light-reversible materials for energy harvesting and conversion, and their successful incorporation into a wide variety of current and future high-tech applications in devices.

[Long-term prognosis of children with hypoxic encephalopathy].

OBJECTIVE: Reports of the prognosis of hypoxic encephalopathy in children have not been common. We investigated the prognoses in 35 children with hypoxic encephalopathy at more than one year from the onset. METHODS: The average age of onset was 5 years 8 months, and the present age was 12 years 6 months in all cases. The medical records were reviewed, and the clinical courses during the acute stage, the state of sequelae were investigated. RESULTS: The etiologies were drowning 12 cases, asphyxia in 6, heart diseases in 10, respiratory diseases in 2, cardiac arrest in 3, etc. Each etiology showed age-related characterisitics. All cases showed consciousness loss levels of triple-digits on the Japan coma scale. The sequelae comprised physical disabilities in 28 cases, mental disabilities in 30, epilepsy in 16, higher brain dysfunction in 12 especially visiospacial disturbance, etc. The onset of epilepsy was mainly within 3 months after onset of the hypoxic encephalopathy. The types of epileptic seizures were focal seizures in 14 cases and generalized seizures in 12. On average, 2.1 antiepileptic drugs, such as carbamazepine and valproate were prescribed. EEG, brain MRI and brain SPECT showed an extensive range of abnormalities. Severe disabilities depended on the following factors:(1) the etiologies such as asphyxia, congenital heart diseases, cardiac arrest, (2) the age of onset under 2 years or over 13 years, (3) long and severe consciousness loss during the acute stage. CONCLUSION: Though the sequelae of hypoxic encephalopathy were similar to those of acute encephalopathy, the former was more serious than the latter.

Dipalladium catalyst for olefin polymerization: introduction of acrylate units into the main chain of branched polyethylene.

A dipalladium complex with a double-decker structure catalyzes ethylene-acrylate copolymerization to produce the branched polymer containing the acrylate units in the polymer chain, not at the branch terminus. The cooperation of the two palladium centers, which are fixed in a rigid framework of the macrocyclic ligand, is proposed to have a significant dinuclear effect on the copolymerization.

Direct Surface Patterning of Microscale Well and Canal Structures by Photopolymerization of Liquid Crystals with Structured Light.

Precise control of the surface topographies of polymer materials is key to developing high-performance materials and devices for a wide variety of applications, such as optical displays, micro/nanofabrication, photonic devices, and microscale actuators. In particular, photocontrolled polymer surfaces, such as photoinduced surface relief, have been extensively studied mainly through photochemical mass transport. In this study, we propose a novel method triggering the mass transport by photopolymerization of liquid crystals with structured light and demonstrate the direct formation of microscale well and canal structures on the surface of polymer films. The wells and canals with depths of several micrometers and high aspect ratios, which are 10 times larger than those of previously reported structures, were found to be aligned in the center of non-irradiated areas. Furthermore, such well and canal structures can be arranged in two dimensions by designing light patterns. Real-time observations of canal structure formation reveal that anisotropic molecular diffusion during photopolymerization leads to a directed molecular alignment and subsequent surface structure formation. We believe that our proposed approach to designing microscale surface topographies has promising applications in advanced optical and mechanical devices.

Effect of Host Structure on Optical Freedericksz Transition in Dye-Doped Liquid Crystals.

The optical Freedericksz transition (OFT) can reversibly control the molecular orientation of liquid crystals (LCs) only by light irradiation, leading to the development of all-optical devices, such as smart windows. In particular, oligothiophene-doped LCs show the highly sensitive OFT due to the interaction between dyes and an optical-electric field. However, the sensitivity is still low for the application to optical devices. It is necessary to understand the factors in LCs affecting the OFT behavior to reduce the sensitivity. In this study, we investigated the effect of the host LC structure on the OFT in oligothiophene-doped LCs. The threshold light intensity for the OFT in trifluorinated LCs was 42% lower than that in LCs without fluorine substituents. This result contributes to the material design for the low-threshold optical devices utilizing the OFT of dye-doped LCs.

Alignment Control of Smectic Layer Structures in Liquid-Crystalline Polymers by Photopolymerization with Scanned Slit Light.

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.

Liquid crystalline 2D borophene oxide for inorganic optical devices.

Borophene has been recently proposed as a next-generation two-dimensional material with promising electronic and optical properties. However, its instability has thus far limited its large-scale applications. Here, we investigate a liquid-state borophene analogue with an ordered layer structure derived from two-dimensional borophene oxide. The material structure, phase transition features and basic properties are revealed by using X-ray analysis, optical and electron microscopy, and thermal characterization. The obtained liquid crystal exhibits high thermal stability at temperatures up to 350 °C and an optical switching behaviour driven by a low voltage of 1 V.

Photohardenable Pressure-Sensitive Adhesives using Poly(methyl methacrylate) containing Liquid Crystal Plasticizers.

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.

A Deformable Low-Threshold Optical Limiter with Oligothiophene-Doped Liquid Crystals.

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.

Novel Bending Sensor Based on a Solution-Processed Cu2O Film with High Resolution Covering a Wide Curvature Range.

A Cu2O film is prepared on a flexible polyethylene terephthalate substrate for a bending sensor using the spin-spray method, a facile and low-environmental-load solution process. The Cu2O bending sensor shows high sensitivity and high resolution not only over a wide range of curvatures (0 < κ < 0.21 mm-1) but also for very small curvature changes (Δκ = ∼ 0.03 mm-1). The bending response of the sensor exhibited a curvature change of high linearity with a good gauge factor (18.2) owing to the grain-boundary resistance and piezoresistive effects of the fabricated Cu2O film. In addition, the sensor possesses good repeatability, stability, and long-term (>30 days) and mechanical fatigue durability (1000 bending-release cycles). The sensor is capable of detailed monitoring of large- and small-scale human motions, such as finger bending, wrist bending, nodding, mouth opening/closing, and swallowing. In addition, excellent stability and repeatability of the monitoring performance is observed over a wide range of motion angles and speeds. All of these results demonstrate the potential of the flexible bending sensor based on the Cu2O film as a candidate for healthcare monitoring and wearable electronics.

Isomerization polymerization of 4-alkylcyclopentenes catalyzed by Pd complexes: hydrocarbon polymers with isotactic-type stereochemistry and liquid-crystalline properties.

Isomerization polymerization of 4-alkylcyclopentenes catalyzed by Pd-diimine complexes produces the polymers with trans-1,3-disubstituted cyclopentane rings located regularly along the polymer chain. The polymers with isotactic structure formed by using a C(2)-symmetric catalyst exhibited liquid-crystalline properties.

Correction to "DNA Brush-Directed Vertical Alignment of Extensive Gold Nanorod Arrays with Controlled Density".

[This corrects the article DOI: 10.1021/acsomega.7b00303.].

Scanning wave photopolymerization enables dye-free alignment patterning of liquid crystals.

Hierarchical control of two-dimensional (2D) molecular alignment patterns over large areas is essential for designing high-functional organic materials and devices. However, even by the most powerful current methods, dye molecules that discolor and destabilize the materials need to be doped in, complicating the process. 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, without alignment layers or polarized light sources. A set of 150,000 arrays of a radial alignment pattern with a size of 27.4 µm × 27.4 µm were successfully inscribed by SWaP, in which each individual pattern is smaller by a factor of 104 than that achievable by conventional photoalignment methods. This dye-free inscription of microscopic, complex alignment patterns over large areas provides a new pathway for designing higher-performance optical and mechanical devices.

DNA Brush-Directed Vertical Alignment of Extensive Gold Nanorod Arrays with Controlled Density.

Control over the orientation of metal nanorods is important for both fundamental and applied research. We show that gold nanorods (GNRs) can be aligned in a single direction by adsorbing positively charged GNRs onto a double-strand DNA-grafted substrate through electrostatic interaction. The ordered structure can be optimized by controlling the density of the positive charges on the surface of the GNRs. We found, in agreement with the results of theoretical simulation, that the resultant structure exhibits plasmonic properties that are dependent on the GNR orientation relative to the direction of an oscillating electric field. Our approach provides new insights into the polymer-assisted self-assembly of rod-shaped nanoparticles utilizing electrostatic interactions.

Suppressing molecular vibrations in organic semiconductors by inducing strain.

Organic molecular semiconductors are solution processable, enabling the growth of large-area single-crystal semiconductors. Improving the performance of organic semiconductor devices by increasing the charge mobility is an ongoing quest, which calls for novel molecular and material design, and improved processing conditions. Here we show a method to increase the charge mobility in organic single-crystal field-effect transistors, by taking advantage of the inherent softness of organic semiconductors. We compress the crystal lattice uniaxially by bending the flexible devices, leading to an improved charge transport. The mobility increases from 9.7 to 16.5 cm(2) V(-1) s(-1) by 70% under 3% strain. In-depth analysis indicates that compressing the crystal structure directly restricts the vibration of the molecules, thus suppresses dynamic disorder, a unique mechanism in organic semiconductors. Since strain can be easily induced during the fabrication process, we expect our method to be exploited to build high-performance organic devices.