Taiki Hoshino

Understanding the dynamic fluctuations of anisotropic particles is essential for elucidating the mechanisms of order formation through self-organization. Because anisotropic particles possess both translational and rotational degrees of freedom, their dynamics are intrinsically more complex than those of spherical particles. Time-resolved coherent X-ray scattering has the potential to probe such complex dynamics. Translational motion can be evaluated by X-ray photon correlation spectroscopy (XPCS), whereas orientational information can be extracted using the angular X-ray cross-correlation function (AXCCF). In this study, we investigated dynamic fluctuations in a colloidal nanorod system at concentrations near the coexistence region of isotropic and liquid-crystalline phases using time-resolved coherent X-ray scattering at 17.4 kHz. XPCS analysis revealed a pronounced slowing down of translational motion as the isotropic phase approached the liquid-crystalline phase. To examine possible temporal fluctuations in orientational order near the coexistence region, we applied a statistical approach based on AXCCF and principal component analysis. Temporal variations were indeed observed in the AXCCF. However, quantitative analysis showed that the magnitude and distribution of these variations are largely explained by photon-counting statistics expected for coherent scattering under low-count conditions. Although the present experiment did not provide definitive evidence for intrinsic orientational fluctuations beyond statistical noise, the analysis establishes an important framework for evaluating the detectability of orientational dynamics in time-resolved coherent X-ray scattering experiments and provides practical guidance for future measurements and validation.

Polymer fracture is governed by plastic deformation around crack tips, yet its underlying molecular processes remain difficult to observe in amorphous polymers. Polyrotaxane glasses provide a rare opportunity to probe these processes because the orientation and intermolecular distance of the constituent ring molecules can be directly evaluated by X-ray scattering. Here, simultaneous synchrotron microbeam small- and wide-angle X-ray scattering (SAXS/WAXS) measurements were used to spatially resolve crack-tip structural evolution from molecular to mesoscale dimensions. WAXS revealed spatially continuous ring orientation and phase separation from axle chains extending away from crack tips, suggesting that these structural changes constitute key molecular processes underlying crack-tip plastic deformation, whereas SAXS detected spatially discrete nanovoids and craze-like structures. The distinct SAXS/WAXS distributions suggest that ring orientation and phase separation can develop prior to, or independently of, localized mesoscale damage. These findings provide molecular insight into crack-tip plasticity relevant to fracture resistance and molecular design of tough polymers.

Polymer networks near the glass transition temperature (Tg) exhibit mechanical behavior governed by intrinsic chain dynamics and local structural heterogeneity. In polymers derived from multiple monomer systems, Tg spans a broad temperature range due to differences in molecular structure and intermolecular interactions, making it difficult to compare deformation behavior under identical thermal proximity to Tg. Since Tg reflects the characteristic timescale of segmental relaxation, controlling Tg provides a direct way to position materials within a comparable dynamical regime relative to experimental timescales. Here, we show that incorporation of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) enables chemically distinct polyacrylate networks to converge to a narrow near-ambient Tg window, despite an initial Tg variation of approximately 20 °C. This Tg convergence establishes a unified segmental-dynamics regime across different polymer compositions. Within this framework, copolymerization between ethyl acrylate and 2-[(butylcarbamoyl)oxy]ethyl acrylate is used to systematically tune ion coordination and local structural heterogeneity without significantly affecting Tg. Although Tg is comparable, the materials exhibit pronounced differences in tensile deformation, including multi-stage yielding and wide variations in yield and fracture stresses and ductility. In situ SAXS reveals void formation at early deformation stages followed by structural reorganization associated with secondary yielding and stress whitening. These results demonstrate that local structural heterogeneity governs deformation behavior in near-glassy polymer networks when Tg is constrained, providing a basis for structure–property correlation under a fixed dynamical window.

Many structural materials suffer from a trade-off between stiffness and toughness because the same molecular interactions that form their rigid frameworks also restrict the plastic deformation needed to resist fracture. Here we demonstrate a failsafe design principle in which flexible polymers are embedded within the interstitial spaces of a rigid molecular framework. The framework maintains high stiffness under elastic loads, while the hidden polymers remain dormant until a local framework failure occurs. At that point, they activate as a "stretch margin", accommodating crack-tip plasticity without compromising the structural integrity of the framework. The length of this stretch margin is governed by interstitial polymer content: a lower ring loading leaves longer hidden chains that can be drawn out further, enlarging the plastic zone and enhancing toughness. At the same time, Young's modulus can be independently tuned through chemical modification of the framework, which alters the strength of intermolecular interactions that hold the rigid structure together. By treating interstitial spaces as reservoirs for a hidden, flexible phase, this approach establishes a molecular design strategy that decouples stiffness and toughness─two properties long believed to be inseparably linked. This concept highlights interstitial space engineering as a route to organic materials that combine rigidity with resistance to fracture, expanding the design space for high-performance structural polymers.

X-ray photon correlation spectroscopy (XPCS) is a powerful technique for evaluating microscopic dynamics using coherent X-rays. Detecting fast or small-scale dynamics typically requires strong illumination and wide-angle scattering detection; however, such conditions can cause non-negligible sample damage. This study presents a non-uniform pulse-interval XPCS approach that enables quantitative dynamical analysis with substantially reduced X-ray exposure. In conventional XPCS, continuous acquisition at uniform time intervals leads to long cumulative exposure, which can introduce radiation-induced artefacts. In this study, only 11 scattering images were recorded at non-uniform intervals, providing a broad range of delay times from a single measurement and enabling dense temporal sampling without increasing the exposure dose. The resulting dataset was analyzed using both XPCS- and X-ray speckle visibility spectroscopy (XSVS)-based schemes, and the results demonstrated that these two independent analyses yield consistent relaxation behaviors. The proposed approach offers an efficient framework for probing complex or non-Brownian dynamics in radiation-sensitive materials and expands the applicability of XPCS to soft and biological systems.

Understanding mesoscale structural dynamics of precipitation-strengthened alloys is essential for optimizing the mechanical performances of these alloys. Herein, we establish a multimodal coherent X-ray diffraction imaging framework for spatiotemporal mapping of mesoscale structural dynamics in precipitation-strengthened alloys. As a demonstrative application, we visualized the structural evolution in Mg ₉₇ Zn ₁ Gd ₂ during isothermal annealing at 700 K, revealing real-time dynamics of nucleation, growth, and coarsening. Ptychographic reconstruction enabled imaging of microstructural transformations across a wide field of view (~100 μm ² ) with temporal resolution spanning several hours. We observed decomposition of (Mg, Zn) ₃ Gd and concurrent precipitation and coarsening of long-period stacking ordered phases. To resolve local dynamics at finer spatiotemporal scales, we combined dynamic coherent diffraction imaging with X-ray photon correlation spectroscopy, targeting selected regions (~10 μm ² ) with time resolution down to tens of seconds. This approach revealed the rapid formation of nanoscale precipitates within 10 s after heating, followed by coarsening over several hundred seconds. Additionally, we applied optical flow analysis—a computational method to track motion patterns—to visualize and quantify the nucleation, growth, and coarsening kinetics. The abovementioned findings demonstrate the capability of in situ coherent X-ray techniques to acquire the real-time evolutions of mesoscale structures in complex materials. Our methodology offers a robust framework for investigating dynamic phenomena in diverse material systems, including metals, polymers, and functional nanomaterials, under realistic thermal or mechanical conditions.

The unique motility of mechanically interlocked polymers enables their mechanical properties to profoundly transform. This property has been exploited less in glassy materials than in rubbery materials. This study demonstrated that in the glassy state the rings must orient before sliding and clarified the requisite structural changes by the synchrotron microbeam X-ray diffraction mapping of a ductile cyclodextrin (CD)-based glassy polyrotaxane. After inducing neck formation and propagation by uniaxial tension, the strain-localized area was scanned, elucidating how the CD orientation and its correlation distance change. As necking approaches and local strain increases, the CD rotational axis orients considerably in the tensile direction. Near the neck inflection point, polymer sliding triggers a sudden structural transformation, forming a phase-separated structure between the CDs and polymers that toughens the neck. This strain-induced orientation preceding sliding appears to facilitate sliding. In the rubbery state, host molecules can orient freely with the guest polymer orientation, but glassy materials must be designed to facilitate host orientation to enable guest sliding with minimum molecular friction.