Shoichi Kubo

Red-light-driven atom transfer radical polymerization (ATRP) using methylene blue as a photocatalyst has attracted considerable attention due to its high efficiency and oxygen tolerance. However, its application to functional polymers remains unexplored. Here we demonstrate the synthesis of liquid-crystalline polymers (LCPs) under ambient conditions using red-light-driven ATRP. Well-controlled polymerizations with high monomer conversions were achieved within several hours. The polymer growth could be precisely regulated by alternating the light irradiation on and off. Molecular weights were tuned by adjusting the polymerization conditions, affording LCPs with narrow molecular weight distributions. A clear relationship between molecular weight and LC behavior was observed. The method was extended to various mesogenic monomers and polymerizable groups, confirming its broad applicability. Moreover, controlled polymerization was achieved using accessible light sources such as sunlight and smartphone LED flashlights. This versatile approach enables efficient LCP synthesis and facilitates exploration of structure–property relationships in functional soft materials.

The precise alignment control of polymer main chains and liquid crystals (LCs) is effective for fabricating high-performance functional coatings. Conventionally, polymer chain alignment is regulated by mechanical stretching or flow fields; however, these approaches are inherently limited to one-dimensional alignment. Our recent discovery demonstrates that spatiotemporal photopolymerization using scanning light generates flow fields, a process termed scanning wave photopolymerization (SWaP). In this study, we explored the potential of SWaP for achieving precise microscale control over polymer main chain alignment. One-dimensional scanning of light induced uniaxial alignment of both polymer main chains and side-chain LC molecules. Thermal analysis of the resulting coating revealed that the high degree of polymer main-chain alignment in the coating recovered its original LC alignment upon cooling after thermal disordering, demonstrating excellent thermal stability. Furthermore, the most thermodynamically stable configuration of the polymer main chains depended on the LC phase. By designing the shape of the light, we successfully achieved two-dimensional alignment of polymer main chains at the microscale. This approach represents a foundational technology for the development of functional materials through molecular alignment control.

Atomically flat two-dimensional networks of boron are attracting attention as post-graphene materials. An introduction of cations between the boron atomic layers can exhibit unique electronic functions that are not achieved by neutral graphene or its derivatives. In the present study, we propose a synthesis strategy for ion-laminated boron layered materials in a solution phase, which enables the preparation of analogs by changing the alkali-metal species. The introduction of large cations extends the thermal range of the liquid-crystal phases because of weakened ionic interactions between borophene-like layers. An investigation of the capacitance of ion-laminated structures revealed a 10⁵-fold or better increase in capacitance when the borophene-like materials were introduced between electrodes. That is difficult for general materials since the dielectric constant of common materials is below several thousand.

The photoinduced molecular reorientation of liquid crystals (LCs) caused by their nonlinear optical responses has attracted much attention due to their large refractive index change, leading to promising applications in optical devices. This reorientation is typically induced by light irradiation above a threshold intensity and is temporary, with the initial orientation recovering unless the LCs are polymerized and cross-linked. Our report highlights the memory effect of molecular reorientation in LCs. The high-intensity laser beam irradiation of dye-doped LCs containing polymer-grafted ZnO nanorods resulted in the molecular reorientation of the LCs. The effect was maintained even after the light was turned off. This memorized molecular orientation functioned as a polarization-dependent microlens due to the self-phase modulation and self-focusing effect of the propagating light. The polarization selectivity of the microlens allows for innovative optical applications, including security printing and anticounterfeiting.

Molecularly aligned liquid-crystalline (LC) polymer films hold great promise for next-generation high-performance photonics, electronics, robotics, and medical devices. Photoalignment methods capable of achieving precise molecular alignment in a noncontact manner have been actively studied. Recently, we proposed the concept of using spatiotemporal photopolymerization to induce molecular diffusion and the resulting alignment, termed scanning wave photopolymerization (SWaP). The spatial gradient of the polymer concentration is the dominant factor in inducing the molecular diffusion and alignment of LCs. However, the effect of polymer concentration on molecular alignment behavior remains unclear. In this study, we performed SWaP at different exposure energies to modulate the polymer concentration during polymerization. We found that a certain polymer concentration was required to initiate the alignment. Furthermore, the phase diagram of the polymer/monomer mixtures and real-time observations during SWaP revealed that phase emergence and unidirectional molecular alignment occurred simultaneously when the polymer concentration exceeded 50%. Since SWaP achieves molecular alignment coincident with photopolymerization, it has the potential to revolutionize material fabrication by consolidating the multiple-step processes required to create functional materials in a single step.

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.

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.

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.