Miho Aizawa

Alignment layer-free molecular ordering induced by masked photopolymerization with non-polarized light

Controlled and uniform molecular alignment can provide and enhance functionality in polymer films. We first report that masked photopolymerization with non-polarized light enables direct and precise control of molecular alignment without using a conventional molecular alignment layer. The photopolymerization of a mixture composed of an optically anisotropic acrylate monomer and an isotropic dimethacrylate crosslinker induces either unidirectional or complex molecular alignment, depending upon the shape of the photomask. Such molecular alignments are successfully achieved by shear stress arising from molecular diffusion, even when the photopolymerization is carried out at isotropic temperatures of both the monomer mixture and the obtained polymers.

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

Liquid crystals are 2D patterned with nonpolarized light by a new dye-free photoalignment method. 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.

Thermal and optical properties of multiblock macrocycles with hysteretic polymorphic transition

The thermal phase transition properties of macrocyclic molecules 1 and 2 consisting of aromatic 1,4-bis(phenylethynyl)benzene (BPEB) components and tetraethylene glycol (TEG) chains were investigated. 1 and 2 are structural isomers with a small difference at the connecting points of the BPEB units with TEG chains, where 1 has a lower structural symmetry than 2. 1 forms a crystal Cr1 upon cooling from the isotropic liquid. 1 at Cr1 shows a crystal-to-crystal polymorphic transition upon heating affording Cr2. Since the Cr1-to-Cr2 phase transition of 1 is an exothermic process, Cr2 is considered to be thermodynamically more stable than Cr1. Indeed, cooling from Cr2 shows no phase transition, and this hysteretic polymorphic transition allows the preparation of 1 in different crystalline states, namely Cr1 and Cr2, at room temperature. In contrast, 2 shows a reversible transition between a crystal and a nematic phase upon temperature changes. Based on the polymorphism of 1, switching of an optical property as a memory function is also demonstrated. Thus, the molecular structure of 1 with a lower structural symmetry than that of 2 is likely an important factor for the crystallization and its thermoresponsive polymorphic phase transition property.

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.

Unpolarized light-induced alignment of azobenzene by scanning wave photopolymerization

AbstractControlling the alignment of various functional molecules is important for the development of many next-generation, high-performance optical devices. However, there are some limitations in inducing molecular alignment using the current methods. We report herein the alignment control of azobenzene in a polymer film by a simple, new alignment-patterning technique based on a scanning wave photopolymerization (SWaP) concept. In this technique, molecular alignment was induced by the spatiotemporal control of the non-polarized light. A photoisomerizable azobenzene molecule, Disperse Red 1 (DR1), was doped into the photopolymerizable mixture, and it was successfully aligned along the direction of the neighboring mesogens; the alignment was induced by SWaP with unpolarized light. The alignment behavior showed that the degree of photoisomerization of the doped azobenzene moieties was proportional to the light intensity, and the unidirectional alignment of DR1 was achieved through optimization of the photopolymerization conditions. This finding indicates that SWaP could be employed as a novel and simple fabrication process for preparing a wide variety of highly functional optical devices requiring alignment control.The alignment of azobenzene molecule was induced by a new-alignment-patterning technique based on a scanning wave photopolymerization (SWaP) concept with unpolarized light. This finding indicates that SWaP could be employed as a novel and simple fabrication process for preparing a wide variety of highly functional optical devices requiring alignment control.

Direct induction of molecular alignment in liquid crystal polymer network film by photopolymerization

Liquid crystal (LC) is the promising material for the fabrication of high-performance soft, flexible devices. The fascinating and useful properties arise from their cooperative effect that inherently allows the macroscopic integration and control of molecular alignment through various external stimuli. To date, light-matter interaction is the most attractive stimuli and researchers developed photoalignment through photochemical or photophysical reactions triggered by linearly polarized light. Here we show the new choice based on molecular diffusion by photopolymerization. We found that photopolymerization of a LC monomer and a crosslinker through a photomask enables to direct molecular alignment in the resultant LC polymer network film. The key generating the molecular alignment is molecular diffusion due to the difference of chemical potentials between irradiated and unirradiated regions. This concept is applicable to various shapes of photomask and two-dimensional molecular alignments can be fabricated depending on the spatial design of photomask. By virtue of the inherent versatility of molecular diffusion in materials, the process would shed light on the fabrication of various high-performance flexible materials with molecular alignment having controlled patterns.