Homogeneously Dispersed Polyrotaxane in Epoxy Adhesive and Its Improvement in the Fracture Toughness

Construction of Slide-Ring Polymers Based on Pillar[5]Arene/Alkyl Chain Host-Guest Interactions

Slide-ring polymers exhibit distinctive mechanical properties, making them highly promising for applications in emerging fields such as energy storage devices and smart sensing. However, existing slide-ring polymer systems primarily rely on hydrophilic-hydrophobic interactions to achieve ring-axle interlocking in aqueous phases. This reliance limits the construction of slide-ring networks mainly to water-soluble polymers, excluding a diverse range of lipophilic polymers. Therefore, it is crucial to introduce efficient construction strategies that facilitate interpenetration in organic solvents, enabling the development of diverse slide-ring polymers and expanding their range and applications. Herein, by utilizing the pillar[5]arene/alkyl chain host-guest interactions, we successfully facilitated the interpenetration of a pillar[5]arene and poly(caprolactone), enabling the efficient construction of two slide-ring polymer networks in organic solvents. One of these two slide-ring polymers demonstrates a unique network deformation mechanism along with outstanding mechanical properties compared with the control covalently cross-linked polymer network, including maximum stress (4.43 vs 1.98 MPa), maximum strain (1285 vs 330 %), and toughness (35.4 vs 3.92 MJ/m3). More importantly, this strategy of making slide-ring polymers is highly versatile, given the wide range of macrocyclic arenes and alkyl chain-containing polymers it can accommodate.

Next-Generation Structural Adhesives Composed of Epoxy Resins and Hydrogen-Bonded Styrenic Block Polymer-Based Thermoplastic Elastomers

Structural adhesives are currently applied in the assembly of automobiles, aircraft, and buildings. In particular, epoxy adhesives are widely used due to their excellent mechanical strength and durability. However, cured epoxy resins are typically rigid and inflexible; thus, they have low peel and impact strength. In this study, tough cured epoxy adhesives were developed by mixing a liquid epoxy prepolymer (EP) and polystyrene-b-polyisoprene-b-polystyrene (SIS). SIS is a block polymer-based thermoplastic elastomer (TPE) composed of polystyrene (S) soluble in liquid EP and polyisoprene (I) insoluble in liquid EP, where S and I have a glass transition temperature that is higher and lower than room temperature, respectively. In addition, cured adhesives tougher than the cured adhesives containing SIS were prepared by mixing liquid EP and SIS with hydrogen-bonding groups in the I block (h-SIS). Transmission electron microscopy (TEM) observations revealed mixed S/cured EP domains, with a d-spacing of several tens of nanometers, and cured EP domains, with diameters of one hundred to several hundred nanometers, that were macroscopically dispersed in the I or hydrogen-bonded I matrix of the cured adhesive containing SIS or h-SIS. The lap shear, peel, and impact strength of cured neat EP (EP*) were 23 MPa, 45 N/25 mm, and 0.62 kN/m, respectively. Meanwhile, the cured adhesive containing 16.5 wt % SIS exhibited the slightly lower lap shear strength of 17 MPa compared to that of cured EP*, whereas the peel and impact strength of the cured adhesive with SIS were 61 N/25 mm and 7.1 kN/m, respectively, both higher than those of EP*. Furthermore, the lap shear strength of the cured adhesive containing 15.5 wt % h-SIS was 21 MPa, which was similar to that of cured EP*. The cured adhesive with h-SIS also exhibited an excellent peel strength of 97 N/25 mm and an impact strength of 14 kN/m which was 22 times higher than that of cured EP*. Therefore, mixing liquid EP and SIS improved the cured adhesive properties and flexibility of the cured epoxy adhesives compared to the cured adhesive composed of neat EP, and further enhancement of the adhesive properties was achieved by mixing liquid EP and h-SIS with hydrogen-bonding groups instead of mixing with SIS.

Asynchronous Ring Opening of Cyclic Carbonate and Glycidyl Ether Induced Phase Evolution Towards Heat-Free and Rapid-Bonding Superior Epoxy Adhesive

Structural adhesives that do not require heating are in high demand in the automotive and electronics industries. However, it remains a challenge to develop robust adhesives that rapidly achieve super adhesion near ambient temperature. Herein, a room-temperature curable, fast-bonding, and super strong epoxy-based structural adhesive was designed from the perspective of cross-scale structure, which lies in threefold pivotal aspects: (i) high branching topology of glycerol carbonate-capped polyurethane (PUGC) increases the kinetics of the ring-opening reaction, contributing to fast crosslinking and the formation of abundant urethane and hydroxyl moieties; (ii) asynchronous crosslinking of epoxy and PUGC synergistically induces phase separation of PUGC within the epoxy resin and the resulting PUGC domains surrounded by interpenetrated shell serves to efficiently toughen the matrix; (iii) abundant dynamic hydrogen bonds including urethane and hydroxyl moieties, along with the elastomeric PUGC domains, dissipate energy of shearing force. As a result, the adhesive strength rapidly grows to 16 MPa within 4 hours, leveling off to 21 MPa after 7 hours, substantially outperforming commercial room-temperature curable epoxy adhesives. The results of this study could advance the field of high-performance adhesives and provide valuable insights into designing materials for efficient curing at room temperature.

Strain-Induced Orientation of Host Rings that Determines the Sliding of Guest Polymers and Plasticity of Glassy Polyrotaxane

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.

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.

Improvement in Cohesive Properties of Adhesion Systems Using Movable Cross-Linked Materials with Stress Relaxation Properties

A strong, tough, and stable adhesion system used in various environments must be developed. A long-lasting adhesion system should effectively perform in the following five aspects: adhesion strength, toughness, energy dissipation property, self-restoration property, and creep resistance property. However, these properties are difficult to balance using conventional adhesives. Here, a new topological adhesion system using single-movable cross-network (SC) materials [SC(DMAAm) Adh] was designed. 3-(Trimethoxysilyl) propyl acrylate was used as the anchor, N,N-dimethyl acrylamide (DMAAm) was used as the main chain monomer, and γ-cyclodextrin (γ-CD) units acted as movable cross-links. The movable cross-links provided SC(DMAAm) Adh with energy dissipation properties, thereby improving its toughness. The γ-CD units also acted as bulky stoppers that provided a high adhesion strength and self-restoration properties. Moreover, the combination of the movable cross-links and bulky stoppers provided creep resistance to SC(DMAAm) Adh. The performance of the adhesion systems under different mobilities of the polymer chains was examined by adjusting the water content. In proper water-containing states, all mechanical properties of SC(DMAAm) Adh were better than those of the adhesion systems using homopolymers [P(DMAAm) Adh] and polymers with covalent cross-linking points [CP(DMAAm) Adh].

Insights into Mechanical Dynamics of Nanoscale Interfaces in Epoxy Composites Using Nanorheology Atomic Force Microscopy

Interfacial polymer layers with nanoscale size play critical roles in dissipating the strain energy around cracks and defects in structural nanocomposites, thereby enhancing the material's fracture toughness. However, understanding how the intrinsic mechanical dynamics of the interfacial layer determine the toughening and reinforcement mechanisms in various polymer nanocomposites remains a major challenge. Here, by means of a recently developed nanorheology atomic force microscopy method, also known as nanoscale dynamic mechanical analysis (nDMA), we report direct mapping of dynamic mechanical responses at the interface of a model epoxy nanocomposite under the transition from a glassy to a rubbery state. We demonstrate a significant deviation in the dynamic moduli of the interface from matrix behavior. Interestingly, the sign of the deviation is observed to be reversed when the polymer changes from a glassy to a rubbery state, which provides an excellent explanation for the difference in the modulus reinforcement between glassy and rubbery epoxy nanocomposites. More importantly, nDMA loss tangent images unambiguously show an enhanced viscoelastic response at the interface compared to the bulk matrix in the glassy state. This observation can therefore provide important insights into the nanoscale toughening mechanism that occurs in epoxy nanocomposites due to viscoelastic energy dissipation at the interface.

Network Formation and Physical Properties of Epoxy Resins for Future Practical Applications

Epoxy resins are used in various fields in a wide range of applications such as coatings, adhesives, modeling compounds, impregnation materials, high-performance composites, insulating materials, and encapsulating and packaging materials for electronic devices. To achieve the desired properties, it is necessary to obtain a better understanding of how the network formation and physical state change involved in the curing reaction affect the resultant network architecture and physical properties. However, this is not necessarily easy because of their infusibility at higher temperatures and insolubility in organic solvents. In this paper, we summarize the knowledge related to these issues which has been gathered using various experimental techniques in conjunction with molecular dynamics simulations. This should provide useful ideas for researchers who aim to design and construct various thermosetting polymer systems including currently popular materials such as vitrimers over epoxy resins.

Fracture Behavior of Polyrotaxane-Toughened Poly(Methyl Methacrylate)

The fracture behavior of polyrotaxane (PR)-modified poly(methyl methacrylate) (PMMA) was investigated. PR is a supramolecule with rings threaded onto a linear backbone chain, which is capped by bulky end groups to prevent the rings from de-threading. The ring structure is α-cyclodextrin (CD), and it can be functionalized to enhance its affinity with the hosting polymer matrix. Adding only 1 wt % of PR containing methacrylate functional groups (mPR) at the terminal of some of the polycaprolactone-grafted chains on CD promotes massive crazing, resulting in a significant improvement in fracture toughness while maintaining the modulus and transparency of the PMMA matrix. Dynamic mechanical analysis and atomic force microscopy studies reveal that mPR strongly interact with PMMA, leading to higher molecular mobility and enhanced molecular cooperativity during deformation. This molecular cooperativity may be responsible for the formation of massive crazing in a PMMA matrix, which leads to greatly improved fracture toughness.

Mussel-Inspired Epoxy Bioadhesive with Enhanced Interfacial Interactions for Wound Repair

The balance between high mechanical properties and strong adhesion strength is crucial in designing and preparing a bio-based hydrogel adhesive for wound closure. Although the adhesion performance of bioadhesives has been remarkably improved by modification with catechol groups, their mechanical properties are yet to meet the biomedical requirements. In this study, mussel-inspired epoxy bioadhesives (CSD-PEG) were synthesized based on catechol-modified chitosan oligosaccharide (CSD) and polyethylene glycol diglycidyl ether (PEGDGE) through nucleophilic substitution. Notably, the CSD-PEG adhesive showed high mechanical and adhesion strengths, which were up to 50.7 kPa and 136.7 kPa, respectively. It was confirmed that a certain amount of the epoxy and catechol groups provided multiple interfacial interactions among the adhesives, substrates, and polymer chains for enhancing the performance of adhesives. The adhesives showed good binding and repairing effects for wound closure and favorable biocompatibility in vivo. The prepared CSD-PEG adhesives are expected to be a promising candidate for surgical tissue repair, wound closure, and tissue engineering fields. STATEMENT OF SIGNIFICANCE: Current reported adhesives composed of biopolymers generally suffer from poor mechanical properties or weak tissue adhesiveness. Therefore, to achieve simultaneously high mechanical and adhesion properties in a bio-based adhesive for wound closure is a big challenge. In this study, mussel-inspired adhesive hydrogels (CSD-PEG) were prepared based on catechol-modified chitosan oligosaccharide (CSD) and polyethylene glycol diglycidyl ether (PEGDGE). The tensile strength and adhesive strength of CSD-PEG on porcine skin reached 50.7 kPa and 136.7 kPa, respectively, which were higher than those for most reported biopolymeric adhesives, mainly due to the multiple interfacial interactions between the catechol and epoxy groups. The CSD-PEG bioadhesives also showed good binding and repairing effects for wound closure and tissue regeneration in vivo.

Entropy-driven segregation in epoxy-amine systems at a copper interface

The composition of an epoxy resin at the interface with the adherend is usually different from that in the bulk due to the enrichment of a specific constituent, a characteristic called interfacial segregation. For better adhesion, it should be precisely understood how epoxy and amine molecules exist on the adherend surface and react with each other to form a three-dimensional network. In this study, the entropic factor of the segregation in a mixture of epoxy and amine at the copper interface before and after the curing reaction is discussed on the basis of a full-atomistic molecular dynamics (MD) simulation. Smaller molecules were preferentially segregated at the interface regardless of the epoxy and amine, and this segregation remained after the curing process. No segregation occurred at the interface for a combination composed of epoxy and amine molecules with a similar size. These findings make it clear that the size disparity between constituents affects the interfacial segregation via the packing and/or translational entropy. The curing reaction was slower near the interface than in the bulk, and a large amount of unreacted molecules remained there. Finally, the effect of molecular shape was also examined. Linear molecules were more likely to segregate than round-shaped ones even though they were similar in volume. We believe that these findings, which are difficult to obtain experimentally, contribute to the understanding of the interfacial adhesion phenomena on a molecular scale.

Environmentally friendly recycling system for epoxy resin with dynamic covalent bonding

Recycling of epoxy resin and its composites is extremely difficult due to its thermoset nature. In this study, we proposed the environmentally-friendly recycling system of epoxy resin with dynamic covalent bonding in the assistance of cysteine-containing tripeptide, so-called glutathione. The glutathione attached on the epoxy resin and resulted in the cleavage of dynamic disulfide bonds of epoxy resin through thiol-disulfide exchange reaction between the thiol group of glutathione and disulfide bonding of epoxy resin, followed by the scission of epoxy networks. Therefore, the degraded epoxy residue was dissolved into chloroform. Finally, this resulting product could be reused as reagent for preparation the new epoxy materials with approximately 90% of initial mechanical strength via regeneration of disulfide bonding through heating. This work demonstrated the different aspect to understand the decomposition and recycling of thermosetting networks and the wide application under more environmentally friendly condition.

Polythiourethane Covalent Adaptable Networks for Strong and Reworkable Adhesives and Fully Recyclable Carbon Fiber-Reinforced Composites

The development of adhesives with superior optical and mechanical performance, solvent resistance, and reworkability is gaining increasing attention in recent years. However, traditional materials do not possess reprocessability and healing characteristics for sustainable development. Here, a superior dynamic polythiourethane (PTU) adhesive with high reprocessability was developed by introducing covalent adaptable networks (CANs). Specifically, dynamic thiocarbamate bonds (TCB) were used to prepare PTU CANs, which showed dramatically enhanced malleability and recyclability. The Young's modulus of the material was 2.0 GPa and the tensile strength was 62.7 MPa. The reprocessing temperature of CANs was reduced to 80 °C while more than 90% of their mechanical properties were retained, even after being reprocessed several times. Moreover, the highly transparent and water-resistant PTU CANs featured an excellent bonding property and reworkability for various materials including glass, with a lap shear strength of 2.9 MPa, metal (5.1 MPa), and wood (6.3 MPa), compared with commercially available adhesives. Additionally, carbon fiber-reinforced composites constructed with PTU CANs were capable of being fully recycled and reused. Importantly, laminated glass with a toughened PTU-PU elastomer interface exhibited an outstanding impact fatigue-resistance behavior, sustaining thousands of impacts. These features demonstrate that PTU CANs show great potential as sustainable materials.

Effect of a heterogeneous network on glass transition dynamics and solvent crack behavior of epoxy resins

In general, it has been widely accepted that the physical properties of an epoxy resin are strongly dependent on how it is prepared. However, a clear understanding of the mechanisms of the relationship at a molecular level has yet to be achieved. We here studied the glass transition dynamics and fracture behavior of four epoxy resins, which were pre-cured at different temperatures and well cured under the same conditions. Fourier-transform infrared spectroscopy revealed that the reaction kinetics for an epoxy-amine mixture were strongly dependent on the pre-curing temperature. The glass transition temperature of epoxy resins with the same cross-linking density was dependent on the pre-curing temperature. Dielectric relaxation spectroscopy and dynamic mechanical analysis revealed that the fragility index of the epoxy resin decreased with increasing pre-curing temperature, indicating that the network structure formed in it became more heterogeneous with increasing pre-curing temperature. Once the epoxy resin was immersed in a good solvent, it was partly swollen and was then macroscopically fractured. The fracture was initiated by the crack generation in an un-swollen region of the resin due to the stress induced upon swelling. The immersion time required to reach the fracture decreased as the extent of the heterogeneity increased. The knowledge here obtained should be useful for understanding and controlling fracture toughness of epoxy resins, leading to the furtherance of their functionalization.

Prediction and optimization of epoxy adhesive strength from a small dataset through active learning

Machine learning is emerging as a powerful tool for the discovery of novel high-performance functional materials. However, experimental datasets in the polymer-science field are typically limited and they are expensive to build. Their size (< 100 samples) limits the development of chemical intuition from experimentalists, as it constrains the use of machine-learning algorithms for extracting relevant information. We tackle this issue to predict and optimize adhesive materials by combining laboratory experimental design, an active learning pipeline and Bayesian optimization. We start from an initial dataset of 32 adhesive samples that were prepared from various molecular-weight bisphenol A-based epoxy resins and polyetheramine curing agents, mixing ratios and curing temperatures, and our data-driven method allows us to propose an optimal preparation of an adhesive material with a very high adhesive joint strength measured at 35.8 ± 1.1 MPa after three active learning cycles (five proposed preparations per cycle). A Gradient boosting machine learning model was used for the successive prediction of the adhesive joint strength in the active learning pipeline, and the model achieved a respectable accuracy with a coefficient of determination, root mean square error and mean absolute error of 0.85, 4.0 MPa and 3.0 MPa, respectively. This study demonstrates the important impact of active learning to accelerate the design and development of tailored highly functional materials from very small datasets.

Self-sorting assembly of a calixarene/crown ether polypseudorotaxane gated by ion-pairing

A polypseudorotaxane composed of a calix[5]arene-diammonium supramolecular polymer and dibenzo-24-crown-8 wheels self-assembles only in the presence of superweak counterions