New aspects of ionic linear polysiloxanes

Polysiloxanes exhibit better properties than petroleum-derived polymers, such as gas permeability, biocompatibility, and weather resistance. However, because most linear polysiloxanes maintain a fluid state at room temperature, they have been used in flexible materials such as oils, greases, and shampoos. Achieving a variety of mechanical properties with polysiloxane can lead to the proposal of new material design guidelines and the creation of new materials. In addition, because polysiloxane is synthesized from silicate minerals, which are an abundant resource, the preparation of polysiloxane materials that can replace petroleum-based polymers can contribute to the SDGs.

We recently found that linear polysiloxanes, which have amine hydrochloride in all monomer units, exhibit a hardness value comparable to that of petroleum resins. This polysiloxane is hygroscopic, and its hardness changes depending on the humidity (moisture absorption). The large modulus change of the dry resion was up to 100 million times that of the moist region. Polysiloxanes also show strong adhesion to glass under dry conditions, overturning the notion that linear polysiloxanes are flexible. We are currently developing and  applying various functional materials to exploit these novel properties.

Sci. Rep., 11, Article number: 17683 (2021). [link] Open access

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The chemical modification of ionic linear polysiloxane on the substrate surface yields ultrathin films with a thickness of approximately 1 nm, whose moisture absorption behavior differed from that of bulk films. Furthermore, upon patterning the ultrathin film as dots, water droplets significantly accumulated in the ultrathin film area. Thus, position-selective water retention is possible by exploiting the abovementioned phenomenon. The ultrathin film also provides the unique feature; an amphiphilic surface.

J. Fiber Sci. Technol.,  78, 169-177 (2022). [link] Open access

Linear polysiloxanes containing amine hydrochloride in all monomer units also self-assemble under moderate humidification. While the order of the self-assembled structures of water-soluble compounds generally decreases with humidification, our polymer showed the highest order with moderate humidification. We denote this unique phenomenon as "humidity-induced self-assembly (HiSA)" and have studied the HiSA mechanism by changing the material design.

In situ nanostructural observations of the liquid crystalline cell

Liquid crystalline (LC) compounds exhibit strong molecular cooperation, which allows the orientation of LC molecules to change beyond the micrometer scale. A familiar application of this property is LC displays. However, despite the worldwide popularity of LC displays, the behavior of LC compounds near the interface is still largely unknown, and new findings are still being reported.

We recently developed a new evaluation method for the structure of LCs by transmitting lab-scale X-rays in the in-plane direction of the LC cell to evaluate the nanostructure inside the cell. When low-molecular-mass LCs (5CBs) were injected into a glass cell coated with an LC photoalignment film and analyzed using this method, we found that the LC photoalignment film underwent a structural transition to a higher-order LC phase upon contact with the 5CBs. We can now evaluate areas that were previously black boxes and have started vigorously researching the behavior of LC compounds at interfaces from new perspectives.

Humidity-induced liquid-crystalline phase transition

Surfactants are environment-responsive, lyotropic liquid crystals that self-assemble into a variety of nano-periodic structures depending on the temperature and concentration of the solvent. When a surfactant is added to a system where a sol-gel reaction of a metal alkoxide occurs, the self-assembled structure of the surfactant is fixed by the inorganic oxide. This technique has been used to synthesize organic-inorganic mesostructured materials with various structures and compositions. However, it is difficult to change the self-assembled structure of the surfactant after organic-inorganic hybridization because the environmental responsiveness of the surfactant is lost upon completion of the sol-gel reaction.

Focusing on inorganic materials that exhibit hygroscopicity, we proposed a "humidity-induced phase transition (HiPT) method" in which the self-assembled structure of surfactants can be manipulated by humidity even after organic-inorganic hybridization. By applying the HiPT method to liquid crystals, we successfully deveoped a film in which different nanostructures can easily coexist and and proposed a new photolithography technique. Recently, we explored new theories of lyotropic liquid crystals by combining functional lyotropic liquid crystals, such as chromonic liquid crystals, with the HiPT method.

Crystallization-induced nanodot formation

Self-assembled structures of organic materials such as liquid crystals and block copolymers are often used as templates for mesostructures. Typical methods include mesoporous silica synthesis and block copolymer lithography. To date, the focus has been on the self-assembly of organic materials. 

We are currently developing a new mesostructure fabrication method by focusing on the self-assembly of inorganic materials. Recently, we reported a simple technique to fabricate a uniform nanodot array of titanium dioxide via two-dimensional shrinkage in the amorphous-to-crystalline (anatase) phase transformation during the calcination of a precursor film. Increasing the calcination temperature from 400 to 600 °C increased the crystal diameter from approximately 10 to 20 nm, while mixing the precursor film with a thermotropic liquid crystalline amphiphilic block copolymer resulted in dot arrays with a more uniform particle size.

In situ grazing-incidence X-ray diffraction/scattering/reflectivity measurements

We are also developing a method for in situ grazing-incidence X-ray diffraction/scattering/reflectivity (GI-XRD/GI-SAXS/XRR) on a lab-scale system. In combination with homemade attachments for temperature and humidity control, light irradiation, and voltage application, various in situ measurements have become possible, leading to many collaborative research projects. Recently, we showed that racemic compounds are arranged in a two-dimensional hexagonal pattern between clay layers and disk-shaped molecules with chirality at the molecular center form a racemic stacking structure. Collaborative research in a wide range of fields is still underway.