Viscoelastic relaxation attributed to the molecular dynamics of polyrotaxane confined in an epoxy resin network

Impact of cross-linking on the time-temperature superposition of creep rupture in epoxy resins

Epoxy resins are an important class of thermosetting resins, and their network structure, formed by the curing reaction of epoxy and amine compounds, plays a crucial role in determining material properties, including creep behavior. We here applied the time-temperature superposition (TTS) principle to analyze the creep behavior of epoxy resins with well-defined network structures that were systematically varied based on the length of the n-alkyl diamine used. The superposition of isothermal creep curves under small stress was achieved through horizontal and vertical shifting, regardless of the length of the n-alkyl diamine. The temperature dependence of the horizontal shift factor was well described by the Williams-Landel-Ferry equation. Creep rupture measurements under large stress conditions revealed specimen rupture, and the time to rupture was plotted against the imposed stress. These plots, acquired at various temperatures, could be superimposed through horizontal shifting. As the diamine length decreased-namely, the distance between cross-linking points-the temperature dependence of the horizontal shift factors deviated from the WLF equation and exhibited Arrhenius-type behavior. The deviation was associated with differences in the fracture process involving chain scission, which became more pronounced as the diamine length decreased. The insights gained in this study should be valuable for controlling creep response and predicting the long-term durability of epoxy resins.

Light Stimuli-Responsive Degradable and Tough Polymeric Materials with Movable Cross-Links

Both strong and easily dismantlable adhesive systems are required to realize a sustainable society by recovering and reusing substrates. Introducing topological cross-links with cyclodextrins (CDs) into adhesives can improve their adhesive strength. In this study, we prepared movable cross-linked poly(ethyl acrylate) (PEA-TAcγCD) with polymerizable CDs and acid-degradable bonds (TAcγCDAAmMe) for both strong and dismantlable adhesion. The O-amidomethyl bond, which links CD to a polymerizable functional group, can be degraded by mixing Brønsted acid. By combining PEA-TAcγCD with photoacid generators, we successfully controlled the mechanical properties by cleaving the movable cross-links upon light stimulation. The degradation mechanism of TAcγCDAAmMe by photoacid generators was confirmed by mass spectrometry. In addition, the cleavage of movable cross-linking points via light stimulation was demonstrated by both the alteration of mechanical properties and chain relaxation of the system, which were evaluated by utilizing tensile tests and dynamic mechanical analysis, respectively. Therefore, the light-responsive degradable elastomer appeared applicable as an easily dismantled on-demand adhesion system. Using light stimulation, the adhesion strengths with the same or dissimilar substrates were reduced. The easy dismantling of the adhesion system by applying the acid degradability of TAcγCDAAmMe enabled excellent adhesive properties derived from the movable cross-links and easy dismantling by light stimulation. Facilitating the disassembly, collection, and reuse of resources will contribute to the realization of a sustainable society.

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.

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.

Kinetics of the interfacial curing reaction for an epoxy-amine mixture

A better understanding of the chemical reaction between epoxy and amine compounds at a solid interface is crucial for the design and fabrication of materials with appropriate adhesive strength. Here, we examined the curing reaction kinetics of epoxy phenol novolac and 4,4'-diaminodiphenyl sulfone at the outermost interface using sum-frequency generation spectroscopy, and X-ray and neutron reflectivity in conjunction with a full atomistic molecular dynamics simulation. The reaction rate constant was much larger at the quartz interface than in the bulk. While the apparent activation energy at the quartz interface obtained from an Arrhenius plot was almost identical to the bulk value, the frequency factor at the quartz interface was greater than that in the bulk. These results could be explained in terms of the densification and orientation of reactants at the interface, facilitating the encounter of the reactants present.

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.

Dynamic behaviour of water molecules in heterogeneous free space formed in an epoxy resin

Although an epoxy resin is a stable material, it absorbs moisture over a long period of time, causing deterioration of its material properties. We here applied a full-atomistic molecular dynamics (MD) simulation to study where water molecules exist in an epoxy resin and how they dynamically behave. First, the curing reaction was simulated to obtain a network structure so that the time course of the density, and thereby the free space, in the resin were obtained. The results made it possible to discuss the formation and size distribution of the free spaces which were not connected to each other. Then, a few percent of water were inserted into the free space of the cured epoxy resin to examine the location and dynamics of their molecules. We found that several water molecules were clustered at a preferred site, where hydrogen bonds can be formed with hydroxy, ether and amino groups of the network, in the free space, and they heterogeneously moved from there to other sites.

Rotaxane-Based Difunctional Nitrile N-Oxide Crosslinker: Synthesis and Direct Introduction of Movable Crosslinking Points into Ethylene-Propylene-Butadiene Monomer (EPDM) Rubber

Incorporation of rotaxane scaffolds into the crosslinking points of polymer networks significantly affects their rheological and mechanical properties. The present study involves the synthesis of a new rotaxane-type crosslinker containing nitrile N-oxide functional groups on both the axle and wheel components. The prepared crosslinker is highly reactive; however, it can be isolated and applied in the crosslinking reaction of a commercially important polymer, namely ethylene-propylene-butadiene monomer rubber (EPDM), in the absence of additives and catalysts. Tensile tests reveal that compared to a network containing conventional crosslinking points, both breaking strength and strain of the network structure prepared herein are improved due to the incorporation of movable crosslinking points. The synthesized network structure also exhibits five times higher fracture energy. The developed post-crosslinking methodology for the direct introduction of movable crosslinking points into pre-formed polymers will be valuable in the production of rotaxane materials for various applications.