Focus Review in 2017

A focused review of photoresist strategies that have been studied over the past few decades driven by the demands of Moore’s law. Selected results are discussed with emphasis on the choice of photoresist chemistry depending on the particulars of each radiation type or patterning strategy, while we present special architectures of photoresists that have attracted a great interest in the semiconductor field.

Organic polymers for hydrogen storage and their molecular design were reviewed. Porous organic polymers reversibly stored and released hydrogen through hydrogen physisorption on their highly porous structures. Polymers containing ketone or N-heterocycle groups fixed and stored hydrogen under atmospheric pressure through the formation of chemical bonds: the hydrogenated polymers released hydrogen in the presence of catalysts at mild conditions. The advantages of the polymers enable easy handling and provide portable hydrogen-carrying materials for use in, for example, a home/on-site or pocketable hydrogen-supply system.

This review focuses on the physicochemical characterization of polyion complex (PIC) micelles, particularly addressing the critical molecular factors to determine the self-assembly scheme of double-hydrophilic block copolymers composed of charged and non-charged segments into micelle structures. The process of PIC micelle formation provides new insights on the block copolymer self-assembly in thermodynamically equilibrium condition in aqueous medium, such as chain length recognition and transition from uPIC, which is the PIC of smallest neutralizing unit, to PIC micelle structures.

Diverse functionalized insulated conjugated polymers (ICPs) were synthesized via co-polymerization between insulated conjugated monomers and various functional units. The functional moieties strongly affected their π-conjugation, which provided sensitivity towards external stimuli. Chemical and physical inputs, such as redox, light, ions and gases, could be detected by modulating their optical and electrical properties. Moreover, the insulating structures inhibited undesired π–π interactions and prohibited thermal fluctuation in the conjugated backbones, enhancing the physical properties of the sensing materials. Accordingly, cooperative effects were observed between the insulation and functional moieties in functionalized ICPs.

Direct surface modification of polymer fibers and films by surface-initiated polymerizations has been investigated. The introduction of initiating sites on the polymer materials and successive polymerization produce surface-tethered polymer chains on the polymer surface. The surface-selective modification controls the surface properties such as wetting, lubrication and anti-fouling without sacrificing the bulk performances. Researches on grafting polymer chains to five types of polymers, poly(methyl methacrylate)-based copolymer, Br-containing polyolefins, poly(butylene terephthalate), poly(vinylidene fluoride-co-trifluoroethylene) and poly(ether-ether ketone) are reviewed.

DNA has been focused as a key component for the fabrication of metal nanoarchitectures due to its intrinsic properties and advantages, such as a well-ordered structure, rich chemical functionality and programmable base-pairing interactions, as well as the availability of multiple enzymes for manipulations. In this focus review, various approaches for DNA-templated metal nanoarchitecture fabrication are introduced. The approaches include DNA-mediated metal nanoparticle formation, DNA-templated conductive nanowire fabrication by metal depositions, sequence-selective metal deposition onto DNA for elaborate nanowire fabrication and DNA brushes as templates for use on solid substrates.

We review our recent studies on material designs based on element-blocks, which are defined as a minimum functional unit composed of heteroatoms, and particularly focus on the roles of element-blocks in these materials. To start, the development of ‘designable hybrids’ by employing polyhedral oligomeric silsesquioxane is illustrated. The following topics include solid-state luminescent materials containing o-carborane and group 13 element-blocks. Their highly efficient emission in the solid state and their photochemistry are explained. Furthermore, we mention chiral element-blocks containing optically active phosphine atoms. The unique properties of this series of element-block materials are illustrated.

Our recent studies on the nano- and micro-structured molecule-responsive hydrogels are summarized. The nano- and micro-structured molecule-responsive hydrogels exhibited rapid swelling/shrinkage behavior in response to a target molecule based on the association/dissociation of molecular complexes that act as crosslinkers. In addition, these hydrogels showed smart functions, such as autonomous molecule-responsive microchannel flow regulation and highly sensitive detection of a target molecule. The smart functions of nano- and micro-structured molecule-responsive hydrogels can provide tools for constructing sensors, microdevices and smart biomaterials.

Recently, we have developed a simple and green approach to surface wrinkling via lignification mimetic reaction and drying. A skin layer is synthesized on a chitosan film via immersion in a methanol solution containing a phenolic acid and subsequent surface reaction by horseradish peroxidase, mimicking wood lignification. A surface relief with micron-scale wrinkles is formed upon drying and as a result of inhomogeneous shrinkage. This focus review overviews this innovative approach and its detailed mechanism.

Hydrogel microspheres (microgels) are composed of crosslinked hydrophilic or amphiphilic polymer chains and exhibit fascinating properties. Their typical size range (<~10 μm) and their excellent properties promise a variety of attractive applications as for example templates, microreactors, catalysts, coatings, and drug carriers. Against this background, we have conducted systematic research on microgels including their synthesis, characterization, assembly, and applications. In this focus review, we summarize recent results of microgel research mainly from our group and those of our collaborators.

This focus review describes the utilization of M13 phage, a filamentous virus, for the development of a novel class of materials. Recently, the preparation of ordered structures composed of M13 phages based on liquid crystal formation has generated great interest as a means of utilizing the outstanding properties of phages for the development of novel soft materials. The combination of genetic engineering-based functionalization and liquid crystal formation can be effectively used to develop structurally regular hybrid materials composed of M13 phages and inorganic or organic materials. M13 phages can be used for various functional soft materials.

In this focus review, the basic principle of stimuli-cleavable polyrotaxane (PRX) comprising many cyclic molecules threaded onto a linear polymer chain capped with bulky stopper molecules via cleavable linkers are described. Additionally, recent progress in the use of β-cyclodextrin-threaded stimuli-cleavable PRXs as a therapeutic agent for Niemann–Pick-type C (NPC) disease is described. The lysosomal release of threaded β-cyclodextrins from PRXs leads to the formation of an inclusion complex with cholesterols accumulated in NPC disease, promoting extracellular excretion of cholesterols.

Although conventional polymer gels are known as mechanically weak materials, their fracture toughness can be effectively improved by introducing weak and brittle bonds into soft and stretchy polymer networks. This toughening method, denoted as the ‘sacrificial bond principle’, has been recently proposed. In this focus review, I describe some extremely tough gels prepared using this principle, e.g., double- or multiple-network gels with high water content featuring covalent sacrificial bonds, self-healing polyampholyte gels containing ionic sacrificial bonds and PDGI/PAAm gels based on hydrophobic sacrificial bonds exhibiting stress-responsive structural colors.

Zwitterions are organic salts whose cation and anion are covalently bridged. In the Focus Review, I describe the potential utility of these zwitterions as building blocks for self-organizing materials, such as liquid crystals and block copolymers. Owing to the unique characteristics of zwitterions to form homogeneous mixtures with certain acids and lithium salts, their self-organization behavior can be tuned by addition of these compounds. In particular, these zwitterion derivatives are useful for constructing bicontinuous cubic liquid-crystalline assemblies with three-dimensionally continuous periodic minimal surface.

This focus review introduces recent progress made in microphase-separated structures under three-dimensional (3D) confinement. Block copolymers spontaneously form unique structures when the polymer molecules are assembled in confined spaces. The polymers are frustrated because of the limited space for phase separation, resulting in morphologies that are more complex than those of bulk films. In addition to conventional parameters such as block ratio, molecular weight and interactions of constituent polymers, the confinement effect is a significant parameter for controlling the morphologies. Here I give an overview of experimental and theoretical results for spherical 3D confinement and discuss the prospects for this area of research.

By interaction between light and structural periodicity, structural coloration is caused. The artificially periodic materials are called photonic crystals and it has attracted great attention for optical applications. Recently, we prepared one-dimensional photonic crystal with multibilayered films consisting of azobenzene-containing polymers and polyvinyl alcohol and reported the on–off switching controlled by refractive index change depended on the molecular orientation states. In addition, we studied the birefringence properties of azobenzene-containing polymers based on their molecular design for the improvement of the switching speed and reflection intensity of photonic crystals.

The recent progress of our research on proton exchange membranes for fuel cell applications is reviewed. In particular, we have focused on the effect of the molecular structures of fluorine-free sulfonated aromatic polymers on membrane properties (e.g., proton conductivity, chemical and mechanical stability). We identified an optimized hydrophilic molecular design (sulfophenylene as SPP), that is, SPP simultaneously exhibited the highest proton conductivity and excellent mechanical stability over a wide range of humidities.

The author has recently focused on exploring novel possibilities of cellulose and amylose, which exist in great abundance on earth. In this review, the author describes the functionalization of cellulose and amylose to give novel chiral functions other than that as chiral stationary phases and their use in applications, such as asymmetric organocatalysts, chiral auxiliaries and chiral fluorescent sensors. Furthermore, the application of a saccharide-containing helical polymer, which can be prepared through polymerization of a glucose-based monomer, to a circularly polarized luminescence material is also described.

A new type of porous crystal, metal–organic framework (MOF), has been recently utilized for the preparation of network polymers. The obtained network polymers possess highly controlled structures derived from the molecular organization of MOF crystal. This focus review emphasizes the significance of integration of flexible organic polymers and rigid MOFs to attain shaped network polymers and organic–inorganic hybrid networks.