Yoshinobu Tsujii

A multimodal operando measurement system integrating a surface force apparatus, optical interferometry, and Raman spectroscopy was developed to investigate layering, molecular-state evolution, and wear at concentrated polymer brush (CPB) interfaces in a good solvent. The system enables simultaneous monitoring of mechanical responses, interfacial gap, and molecular behavior at solid–liquid interfaces under both compression and sliding. Operando measurements during compression revealed that CPBs exhibit a hierarchical layered structure consisting of dilute, semidilute, middle, and concentrated layers, each characterized by distinct transition gaps in mechanical responses (Fz, ϕz) and molecular concentration profiles. Raman spectroscopic analysis further showed that the CPB-derived full width at half-maximum, ΓCPB, exhibits a nonmonotonic dependence on applied load. This behavior is interpreted as a Raman-detectable signature of compression-induced molecular-state changes in the confined CPB layer, reflecting changes in molecular mobility and local structural heterogeneity. The transition from a highly swollen, relatively mobile state to a deswollen, more constrained molecular state was closely associated with the onset and acceleration of CPB wear. Layer-resolved wear analysis, achieved by alternately performing contact and friction tests, demonstrated that CPB wear proceeds via a random chain-scission mode, in which the dilute layer is continuously regenerated while the concentrated layer gradually diminishes. Multimodal operando measurements during sliding revealed that this random scission is associated with localized tensile-strain-like deformation imposed on polymer chains under frictional shear in the glassy-like confined state. These results establish a unified operando framework linking hierarchical layering, compression-induced molecular-state changes, and wear mechanisms at polymer brush interfaces and provide design principles for achieving durable, superlow-friction polymer brush lubrication systems.

The interfacial assembly and rearrangement of nanomaterials are critical for stabilizing air–water/oil–water interfaces. Cellulose nanofibers (CNFs) are promising renewable bio‐based solid surfactants that form stable interfacial layers separating the fluid interface. However, the correlations between microstructural features (e.g., defects, orientation, buckling) and physicochemical properties of the interfacial layer remain unclear. This study attempted to understand the interfacial behaviors of CNFs with different hydrophobicity through a common mechanism. First, nanofiber monolayers were fabricated on the water surface of a Langmuir trough. Three different regions corresponding to gaseous, liquid expanded, and liquid condensed films were determined from the characteristic points of surface pressure isotherms. The film structures and their surface dilatational storage/loss moduli exhibited significant changes across these three regions. Overall, we propose that the interfacial behaviors of nanofiber monolayers can be organized by macroscopic wettability of nanofibers which is readily measurable. These results provide insights into the interfacial stabilization mechanism of fibrous nanomaterials and pave the way for applications in functional Pickering emulsions/foams.

The orientation and electro‐optical properties of in‐plane switching liquid crystal displays (IPS LCDs) filled with a newly developed ferroelectric nematic liquid crystal (N_FLC), 3FM‐C4T, are investigated. The 3FM‐C4T‐based IPS LCDs exhibit a uniform, homogeneous orientation and robust orientation stability, equivalent to those of nematic LCs. In addition, unlike conventional nematic LCDs, a fast‐response impulse‐type transmission efficiency (TE) modulation occurs in LCDs with 3FM‐C4Ts, in which the TE changes only for a moment when the polarity of the applied voltage reverses. The novel impulse‐type TE modulation is identified for the first time and differs from the impulse‐type emission observed in self‐luminous displays in the sense that the TE increases and decreases during an ON signal. This phenomenon occurs because the polarization vector of the 3FM‐C4Ts, which are oriented symmetrically with no voltage, becomes asymmetric under the application of an electric field below the coercive electric field, resulting in shielding voltage for the 3FM‐C4T layer via the polarization of the thin polyimide alignment layer and a corresponding decrease in the effective applied voltage to the 3FM‐4CTs. The proposed LC device effectively improves the image quality of LCDs and is useful for many other applications requiring fast‐response impulse‐type TE and polarization modulation.

Rod-shaped cellulose nanocrystals (CNCs) are typically obtained by hydrolyzing cellulose nanofibers bearing an formyl group at their reducing end. In this study, we prepare CNCs bearing densely grafted polymer brushes of different molecular weights (asymmetrically polymer-brush–decorated CNCs; aPB-CNCs). aPB-CNCs exhibit good dispersion in organic solvents and form a monolayer at the air/water interface. The compression of this monolayer induces a phase transition of a long polymer brush at the reducing end and a change in the orientation of the CNC core due to repulsive interactions. A CNC is identified as a key intermediate, possessing dormant moieties for reversible addition–fragmentation polymerization and atom transfer radical polymerization at the reducing end and other surfaces, respectively (bifunctional dormant CNC). This functional CNC facilitates the polymerization of a wide range of monomers and enables the synthesis of aPB-CNCs with diverse chemical properties. The developed synthetic route not only provides a basis for exploring the ordered structure of nanoparticles but is also useful for modifying the reducing ends of CNCs.

Tribology is the study on friction, wear and lubrication for contacting and moving two solid materials with fluid lubricants. Friction can be found easily in our life and should be reduced, and lubricants are a powerful answer for this purpose. The mechanism behind friction and lubrication has been primarily analyzed by the Stribeck curve, which should be 1) interpretable, 2) non-homogeneous in variances and 3) a mixture of Stribeck curves further. We propose a machine learning approach, Estribec, to estimate the Stribeck curves from observed data in tribology. Estribec is, considering all above three characteristics, a finite mixture model, with a component of a piecewise function, where each piece is a comprehensible simple (primarily, linear) function with a unique variance. Entirely our method keeps linear time complexity for all processes, including parameter estimation and prediction. Empirical results with synthetic data showed Estribec achieved favorable predictive performance against Gaussian process regression (GPR) and its tree variant (TGPR). Importantly Estribec ran always around 20 to 150 times faster than GPR and TGPR, which would be a sizable difference for conducting high-throughput experiments. Finally Estribec showed latent properties of real data which would be useful for tribology research.

Concentrated polymer brushes (CPBs) are known to exhibit excellent lubrication properties. However, the frictional behaviors of CPBs vary, depending on their preparation and operating conditions. In order to understand such complicated properties, it is necessary to determine their structures and correlate them with their properties, during shear motion. In this study, we employed surface forces and resonance shear measurement (RSM) as well as refractive index measurement using fringes of equal chromatic order (FECO) for studying the structure of the CPBs of poly(methyl methacrylate) (PMMA) in toluene. The obtained elastic (k_s) and viscous (b_s) parameters based on the RSM for the PMMA-PMMA were higher than those obtained for PMMA-silica over the entire distance range. With the increasing shear amplitude on the PMMA-PMMA under an applied load, the b_s value first increased and then decreased while the k_s value monotonically decreased. These behaviors were consistent with those of the thicker CPBs reported in a previous paper (Soft Matter, 2019). Thus, the dynamics of the CPBs under the applied load and shear were not dependent on the thickness of the polymer brushes in this case. The density distribution of the swollen PMMA brushes along the distance in the thickness direction of the brush layer was estimated by using the measured refractive index values, showing that the fraction of the PMMA brushes in the outer region from the surface (20% in the thickness) was ca. 10%. This lower density region near the surface of the swollen CPBs enabled them to interpenetrate with each other. Changes in the refractive index value under shear were observed, indicating that the interpenetrated PMMA chains were pulled out with increasing shear amplitude. These results demonstrated that broader applications of CPBs are possible by regulating the friction between them under different operating conditions, even for usually lubricious CPBs.

Concentrated polymer brushes (CPBs), which are significantly denser and thicker than conventional semidilute polymer brushes, have received increasing attention in the field of tribology because of their superlow friction properties. However, despite numerous studies aimed at enhancing CPBs for mechanical applications, the relationship between the specific layered structure and lubrication mechanisms of CPBs is still not completely understood. In this study, to reveal the relationship, simultaneous time-resolved measurements of the interfacial gap, static mechanical response, and dynamic mechanical response of the CPB at the contact interface were conducted using optical interference and precise force measuring methods. Two types of tests (i.e., the "indentation" and "sliding" tests) were alternately performed on a glass substrate coated with the CPB against a steel ball immersed in an ionic liquid. The indentation tests measuring the time-resolved interfacial gap and changes in static and dynamic mechanical responses quantitatively confirmed the presence of dilute, middle, and concentrated layers in the CPB. In the sliding tests, the wear of the CPB was detected by observing a decrease in the interfacial gap at the contact interface. Moreover, the thickness of the dilute layer remained constant with sliding, whereas the thicknesses of the other layers decreased, indicating that the dilute layer was continuously formed due to sliding. Therefore, CPB wear occurs randomly at the friction interface alongside the formation of a dilute layer with low density and stiffness on the surface.