Implementation of the Pulley Effect of Polyrotaxane in Transparent Bulk Polymer for Simultaneous Strengthening and Toughening

Cyclodextrins-Based Polyrotaxanes: From Functional Polymers to Applications in Electronics and Energy Storage Materials

The concept of polyrotaxane comes from the rotaxane structure in the supramolecular field. It is a mechanically interlocked supramolecular assembly composed of linear polymer chains and cyclic molecules. Over recent decades, the synthesis and application of polyrotaxanes have seen remarkable growth. Particularly, cyclodextrin-based polyrotaxanes have been extensively reported due to the low-price raw materials, good biocompatibility, and ease of modification. Hence, it is also one of the most promising mechanically interlocking supramolecules for wide industrialization in the future. Polyrotaxanes are widely introduced into materials such as elastomers, hydrogels, and engineering polymers to improve their mechanical properties or impart functionality to the materials. In these materials, polyrotaxane acts as a slidable cross-linker to dissipate energy through sliding or assist in dispersing stress concentration in the cross-linked network, thereby enhancing the toughness of the materials. Further, the unique sliding-ring effect of cyclodextrin-based polyrotaxanes has pioneered advancements in stretchable electronics and energy storage materials. This includes their innovative use in stretchable conductive composite and binders for anodes, addressing critical challenges in these fields. In this mini-review, our focus is to highlight the current progress and potential wider applications in the future, underlining their transformative impact across various domains of material science.

Anomalous segmental dynamics of supercooled polyrotaxane melts: A computer simulation study

Polyrotaxanes, which consist of mechanically interlocked bonds with rings threaded onto soft polymer chains, exhibit unique mechanical properties and find applications in diverse fields. In this study, we investigate the anomalous segmental dynamics of supercooled polyrotaxane melts using coarse-grained molecular dynamics simulations. Our simulations reveal that the presence of rings effectively reduces the packing efficiency, resulting in well-contained local motion even below the glass transition temperature. We also observe variations in dynamical free volume, characterized by the Debye-Waller factor, which shows a minimum at a ring coverage of 0.1 on threading chains. Such a non-monotonic dependence on coverage shows great consistency in structural relaxation time and dynamic heterogeneity. Specifically, the high segmental mobility of threading linear chains at large coverage can be attributed to the increased dynamical free volume due to supported rigid rings. However, such anomalous segmental dynamics is limited to length scales smaller than one ring size. Beyond this characteristic length scale, the diffusion is dominated by topological constraints, which significantly reduce the mobility of polyrotaxanes and enhance the dynamic heterogeneity. These findings offer microscopic insights into the unique packing structures and anomalous segmental dynamics of supercooled polyrotaxane melts, facilitating the design of advanced materials based on mechanical interlocking polymers for various applications.

Fabrication of an Artificial Pulley Based on Electrospun Composite Polyurethane/Polycaprolactone Nanofibers for Hand Surgery: Structural, Mechanical, In Vitro, and In Vivo Examinations

Injuries to the hand's flexor pulley system can be debilitating, causing pain and restricting movement of the affected finger(s). The creation of a biocompatible artificial pulley could potentially alleviate some of the complications associated with current surgical treatments. In this study, a biocompatible artificial pulley was fabricated by using polycaprolactone (PCL) and polyurethane (PU) in the form of an electrospun nanofiber structure. All scaffolds were structurally analyzed using FESEM imaging, porosity, FTIR, and DSC examinations. Mechanical properties were evaluated, and in vitro studies were conducted on the degradation rate, swelling ratio, and toxicity. Immune response to fabricated scaffolds was evaluated by implanting them under the skin of rats for further pathological examination. All scaffolds exhibited a nanoscale structure and high porosity without any undesirable functional groups. The 25% PCL scaffold showed 17%, 20%, 80%, 17%, and 70% significant increases in Fmax, final stress, final strain, Young's modulus, and elongation percentage, respectively. In fact, the PCL25% scaffold demonstrated more than 100% improvement in mechanical properties compared to those of A2 and A4 natural pulleys. Additionally, all scaffold structures showed cell viability similar to that of the control sample. The study suggests that scaffolds made of 25% PCL hold promise as effective artificial pulleys for reconstructing the flexor tendon pulley system in cases of injury.

Artificial Spider Silk with Buckled Sheath by Nano-Pulley Combing

The axial orientation of molecular chains always results in an increase in fiber strength and a decrease in toughness. Here, taking inspiration from the skin structure, artificial spider silk with a buckled sheath-core structure is developed, with mechanical strength and toughness reaching 1.61 GPa and 466 MJ m-3 , respectively, exceeding those of Caerostris darwini silk. The buckled structure is achieved by nano-pulley combing of polyrotaxane hydrogel fibers through cyclic stretch-release training, which exhibits axial alignment of the polymer chains in the fiber core and buckling in the fiber sheath. The artificial spider silk also exhibits excellent supercontraction behavior, achieving a work capacity of 1.89 kJ kg-1 , and an actuation stroke of 82%. This work provides a new strategy for designing high-performance and intelligent fiber materials.

Preparation of stereocomplex and pseudo-polyrotaxane with various cyclodextrins as wheel components using triblock copolymer of poly(ethylene glycol) and polylactide

The ABA-type triblock-copolymers (BCPs) of polylactide (PLA) and poly(ethylene glycol) (PEG) were synthesized as axle components for rotaxane formation. It is known that α-cyclodextrin (CD) exists near the PEG moiety in pseudo-polyrotaxane (PPRX), and the PLA moiety can form a stereocomplex (SC), by mixing with L- and D-isomers. In this study, various CDs, including β-CD and γ-CD, were used as wheel components, and effects of CD structures on both PPRX and SC formations were studied. The solubility of CDs is influenced to form the PPRX, resulting in differing numbers of CDs in the axle. PPRX structures were investigated by ¹H NMR, NOESY, and DOSY, and SC structures were investigated by FT-IR and XRD. Their thermal properties were also evaluated by DSC and TGA, to consider the physical properties of the simultaneous formation of PPRX and SC. This study gave insight into the complicated host-guest and polymer-polymer interactions.

Sliding Cyclodextrin Molecules along Polymer Chains to Enhance the Stretchability of Conductive Composites

The demand for stretchable electronics with a broader working range is increasing for wide application in wearable sensors and e-skin. However, stretchable conductors based on soft elastomers always exhibit low working range due to the inhomogeneous breakage of the conductive network when stretched. Here, a highly stretchable and self-healable conductor is reported by adopting polyrotaxane and disulfide bonds into the binding layer. The binding layer (PR-SS) builds the bridge between polymer substrates (PU-SS) and silver nanowires (AgNWs). The incorporation of sliding molecules endows the stretchable conductor with a long sensing range (190%) due to the energy dissipation derived from the sliding nature of polyrotaxanes, which is two times higher than the working range (93%) of conductors based on AP-SS without polyrotaxanes. Furthermore, the mechanism of sliding effect for the polyrotaxanes in the elastomers is investigated by SEM for morphological change of AgNWs, in situ small-angle x-ray scattering, as well as stress relaxation experiments. Finally, human-body-related sensing tests and a self-correction system in fitness are designed and demonstrated.

Post-Cross-Linking of Collagen Hydrogels by Carboxymethylated Polyrotaxanes for Simultaneously Improving Mechanical Strength and Cell Proliferation

To improve the mechanical properties of collagen hydrogels, which are widely utilized as biomaterials, post-cross-linking of collagen hydrogels was performed using polyrotaxane (PRX) as a cross-linker. Herein, carboxymethyl group-modified PRXs (CMPRs) composed of carboxymethylated α-cyclodextrins (α-CDs) threaded along poly(ethylene glycol) (PEG) capped with bulky stoppers were used to cross-link via reaction with the amino groups in the collagen. Four series of CMPRs with different α-CD threading ratios and axle PEG molecular weights were used for the post-cross-linking of the collagen hydrogels to verify the optimal CMPR chemical compositions. The post-cross-linking of the collagen hydrogels with CMPRs improved the swelling ratios and mechanical properties, such as viscoelasticity and tensile strength. Among the tested CMPRs, CMPRs with an axle PEG molecular weight of 35,000 (PEG35k) resulted in better mechanical properties than CMPRs with a PEG10k axis. Additionally, the cell adhesion and proliferation were greatly improved on the surface of the collagen hydrogels post-cross-linked with CMPRs with the PEG35k axle. These findings suggest that the molecular weight of an axle polymer in CMPRs is a more important parameter than the α-CD threading ratios. Accordingly, the post-cross-linking of hydrogels with PRXs is promising for improving the mechanical properties and biomaterial functions of collagen hydrogels.

Dynamically Cross-Linked Polymeric Binder-Made Durable Silicon Anode of a Wide Operating Temperature Li-Ion Battery

The colossal volumetric expansion (up to 300%) of the silicon (Si) anode during repeated charge-discharge cycles destabilizes the electrode structure and causes a drastic drop in capacity. Here in this work, commercial poly(acrylic acid) (PAA) is cross-linked by hydroxypropyl polyrotaxane (HPR) via reversible boronic ester bonds to achieve a water-soluble polymeric binder (PAA-B-HPR) for making the Si anode of the Li-ion battery. Slidable α-cyclodextrins of modified polyrotaxane are allowed to move around when the unwanted volume variation occurs in the course of lithiation and delithiation so that the accumulated internal stress can be equalized throughout the system, while the reversible boronic ester bonds are capable of healing the damages created during manufacturing and service to maintain the electrode integrity. As a result, the Li-ion battery assembled with the Si anode comprised of the PAA-B-HPR binder possesses outstanding specific capacity and cycle stability within a wide temperature range from 25 to 55 °C. Especially, the Si@PAA-B-HPR anode exhibits a discharge specific capacity of 1056 mA h/g at 1.4 A/g after 500 cycles under a higher temperature of 55 °C, and the corresponding capacity fading rate per cycle is only 0.10%. The present work opens an avenue toward the practical application of the Si anode for Li-ion batteries.