Design Rules for Enhanced Interfacial Shear Response in Functionalized Carbon Fiber Epoxy Composites

Advanced Textile-Based Wearable Biosensors for Healthcare Monitoring

With the innovation of wearable technology and the rapid development of biosensors, wearable biosensors based on flexible textile materials have become a hot topic. Such textile-based wearable biosensors promote the development of health monitoring, motion detection and medical management, and they have become an important support tool for human healthcare monitoring. Textile-based wearable biosensors not only non-invasively monitor various physiological indicators of the human body in real time, but they also provide accurate feedback of individual health information. This review examines the recent research progress of fabric-based wearable biosensors. Moreover, materials, detection principles and fabrication methods for textile-based wearable biosensors are introduced. In addition, the applications of biosensors in monitoring vital signs and detecting body fluids are also presented. Finally, we also discuss several challenges faced by textile-based wearable biosensors and the direction of future development.

Rational Design of Solid Polymer Electrolyte Based on Ionic Liquid Monomer for Supercapacitor Applications via Molecular Dynamics Study

The safety concern arising from flammable liquid electrolytes used in batteries and supercapacitors drives technological advances in solid polymer electrolytes (SPEs) in which flammable organic solvents are absent. However, there is always a trade-off between the ionic conductivity and mechanical properties of SPEs due to the lack of interaction between the ionic liquid and polymer resin. The inadequate understanding of SPEs also limits their future exploitation and applications. Herein, we provide a complete approach to develop a new SPE, consisting of a cation (monomer), anion and hardener from ions-monomers using molecular dynamics (MD) simulations. The results show that the strong solid-liquid interactions between the SPE and graphene electrode lead to a very small gap of ∼5.5 Å between the components of SPE and electrode, resulting in a structured solid-to-liquid interface, which can potentially improve energy storage performance. The results also indicated the critical role of the mobility of free-standing anions in the SPE network to achieve high ionic conductivity for applications requiring fast charge/discharge. In addition, the formations of hardener-depleted regions and cation-anion-poor/rich regions near the uncharged/charged electrode surfaces were observed at the molecular level, providing insights for rationally designing the SPEs to overcome the boundaries for further breakthroughs in energy storage technology.

Unravelling the Influence of Surface Modification on the Ultimate Performance of Carbon Fiber/Epoxy Composites

The overall performance of polymer composites depends on not only the intrinsic properties of the polymer matrix and inorganic filler but also the quality of interfacial adhesion. Although many reported approaches have been focused on the chemical treatment for improving interfacial adhesion, the examination of ultimate mechanical performance and long-term properties of polymer composites has been rarely investigated. Herein, we report carbon fiber (CF)/epoxy composites with improved interfacial adhesion by covalent bonding between CFs and the epoxy matrix. This leads to the improved ultimate mechanical properties and enhanced thermal aging performance. Raman mapping demonstrates the formation of an interphase region derived from the covalent bonding between CFs and the epoxy matrix, which enables the uniform fiber distribution and eliminates phase separation during thermal cycling. The covalent attachment of the CF to the epoxy matrix suppresses its migration during temperature fluctuations, preserving the mechanical performance of resulting composites under the thermal aging process. Furthermore, the finite elemental analysis reveals the effectiveness of the chemical treatment of CFs in improving the interfacial strength and toughness of silane-treated CF/epoxy composites. The insight into the mechanical improvement of CF/epoxy composites suggests the high potential of surface modification of inorganic fillers toward polymer composites with tunable properties for different applications.

Latent, Cross-Linkable Triazole Platform on a Carbon Fiber Surface for Enhancing Interfacial Cross-Linking within Carbon Fiber/Epoxy Composites

A long-running need in carbon fiber composite production is to ameliorate interfacial adhesion between the polymer and carbon fibers. Here, we present a convenient and feasible strategy for controlling the carbon fiber's surface in a continuous process: syntheses of click-modified silanes via copper(I)-catalyzed azide-alkyne cycloaddition reaction and grafting them onto fiber surfaces which prepare a latent curable platform under mild processes without postmodification. As 1,2,3-triazole moieties from the click reaction were added to the epoxy/dicyandiamide system, they triggered additional reactions in the later conversion stage; approximately, a 20% increase in the total reaction enthalpy compared to the system with no additives was obtained. We expected the enhanced cross-linking between the surface and matrix to expand the interfacial area, leading to reinforcements on interfacial adhesion and stress-transfer abilities within composites. The merit of the approach is well-demonstrated by conductive atomic force microscopy, showing that the interphase can be extended up to 6-fold when the triazole platform acts as curatives and serve as bridges after the epoxy cure. Consequently, the composite's interfacial shear strength and interlaminar shear strength were increased up to 78 and 72%, respectively. This work affords a reactive platform where a custom-tailored fiber/matrix interface can be designed by virtue of versatility in clickable reactants.

A Computational Procedure for Atomistic Modelling of Polyphosphazenes towards Better Capturing Molecular-Level Structuring and Thermo-Mechanical Properties

Poly(phosphazenes)(PZ) are versatile polymers due to their tunable properties that can be tailored for specific applications. Despite extensive experimental research, not all properties are tested, and the list of PZs studied via molecular simulations is limited. Further, a general procedure to generate and test PZ systems is lacking. We present an in situ polymerization procedure developed to make, test, and tune the thermo-mechanical properties of four PZs—poly(dichlorophosphazene)(PZ-DC), poly[bis(2,2,2-trifluoroethoxy)]phosphazene (PZ-TFE), poly(2,2,2-trifluoroethoxy-5,6-diazidohexanoxy) phosphazene (PZ-Azido), and poly(2,2,2-trifluoroethoxy-5,6-dinitratohexanoxy)phosphazene (PZ-Nitrato) via molecular dynamics simulations. The predicted thermo-mechanical properties (i.e., density and glass transition temperature) agreed with experimental values when a direct comparison of PZ systems was possible. This demonstrates the reproducibility and reliability of our procedure which will help understand the behaviour of PZs at the molecular scale.

Adsorption of Epoxy Oligomers on Iron Oxide Surfaces: The Importance of Surface Treatment and the Role of Entropy

Epoxy-based coatings are widely used in a range of industries as protective coatings. The performance of the final solid-polymer system is dependent on the physicochemical properties of the interface and the interaction between the polymer and the solid substrate. In this study, we perform atomistic molecular dynamics simulations to investigate the binding of a common component in epoxy resins, diglycidyl ether of bisphenol A (DGEBA), on two iron oxide surfaces, hematite (0001) and magnetite (100), and investigate the effect of surface hydroxylation on the binding energy. We show that adsorption of DGEBA on hematite is more favorable than on magnetite and that the adsorbed molecules are highly localized on the pristine hematite surface but mobile on highly hydroxylated hematite surfaces and magnetite surfaces irregardless of surface hydroxylation fraction. A high degree of hydroxylation significantly reduces the binding energy of DGEBA on hematite but not on magnetite. The free-energy calculations confirm the trends observed upon hydroxylation, but the magnitude of the potential of mean force is lower than the binding energy due to the entropic contributions. Therefore, it can be suggested that DGEBA will adsorb more strongly on a surface containing a higher content of hematite than magnetite and that the presence of hydroxyl groups will weaken this adsorption. The presence of hydroxyl groups increases mobility of the chains, which can affect the coating rigidity.

Building Tailored Interfaces through Covalent Coupling Reactions at Layers Grafted from Aryldiazonium Salts

Aryldiazonium ions are widely used reagents for surface modification. Attractive aspects of their use include wide substrate compatibility (ranging from plastics to carbons to metals and metal oxides), formation of stable covalent bonding to the substrate, simplicity of modification methods that are compatible with organic and aqueous solvents, and the commercial availability of many aniline precursors with a straightforward conversion to the active reagent. Importantly, the strong bonding of the modifying layer to the surface makes the method ideally suited to further on-surface (postfunctionalization) chemistry. After an initial grafting from a suitable aryldiazonium ion to give an anchor layer, a target species can be coupled to the layer, hugely expanding the range of species that can be immobilized. This strategy has been widely employed to prepare materials for numerous applications including chemical sensors, biosensors, catalysis, optoelectronics, composite materials, and energy conversion and storage. In this Review our goal is first to summarize how a target species with a particular functional group may be covalently coupled to an appropriate anchor layer. We then review applications of the resulting materials.

Moisture Ingress at the Molecular Scale in Hygrothermal Aging of Fiber-Epoxy Interfaces

Almost all applications of carbon fiber reinforced composites are susceptible to water aging, either via ambient humidity or through direct exposure to liquid water environments. Although the impacts of water aging in composites can be readily quantified via experimental efforts, details regarding the mechanisms of moisture ingress and aging, particularly at the incipient stages of aging under hygrothermal conditions, have proven challenging to resolve using experimental techniques alone. A deeper understanding of the factors that drive incipient moisture ingress during aging is required for more targeted approaches to combat water aging. Here, molecular dynamics simulations of a novel epoxy/carbon fiber interface exposed to liquid water under hygrothermal conditions are used to elucidate molecular details of the moisture ingress mechanisms at the incipient stages of the aging process. Remarkably, the simulations show that the fiber-matrix interface is not vulnerable to a moisture-wicking type of incipient water ingress and does not readily flood in these early stages of water aging. Instead, water is preferentially absorbed via the matrix-water interface, an ingress pathway that is facilitated by the dynamic mobility of polymer chains at this interface. These chains present electronegative sites that can capture water molecules and provide a conduit to transiently exposed pores and channels on the polymer surface, which creates a presoaked staging reservoir for subsequent deeper ingress into the composite. Characterization of the absorbed water is according to hydrogen bonding to the matrix, and the distributions and transport behavior of these waters are consistent with experimental observations. This work introduces new insights regarding the molecular-level details of moisture ingress and spatial distribution of water in these materials during hygrothermal aging, informing future design directions for extending both the service life and shelf life of next-generation composites.

Spatiotemporally Controlled Photoresponsive Hydrogels: Design and Predictive Modeling from Processing through Application

Photoresponsive hydrogels (PRHs) are soft materials whose mechanical and chemical properties can be tuned spatially and temporally with relative ease. Both photo-crosslinkable and photodegradable hydrogels find utility in a range of biomedical applications that require tissue-like properties or programmable responses. Progress in engineering with PRHs is facilitated by the development of theoretical tools that enable optimization of their photochemistry, polymer matrices, nanofillers, and architecture. This review brings together models and design principles that enable key applications of PRHs in tissue engineering, drug delivery, and soft robotics, and highlights ongoing challenges in both modeling and application.

Molecular Dynamics Simulation on the Interfacial Behavior of Over-molded Hybrid Fiber Reinforced Thermoplastic Composites

Hybrid fiber reinforced thermoplastic composites are receiving important attention in lightweight applications. The fabrication process of hybrid thermoplastic composites is that discontinuous fiber reinforced thermoplastics are injected onto the continuous fiber reinforced thermoplastics by over-molding techniques. The key issue during this process is to get a reliable interfacial bonding strength. To understand the bonding mechanism at the heterogeneous interface of hybrid thermoplastic composites which is difficult to obtain through experimental investigations, a series of molecular dynamic (MD) simulations were conducted in this paper. The influence of processing parameters on the interfacial characteristics, i.e., the distribution of interfacial high-density enrichment areas, radius of gyration, diffusion coefficient and interfacial energy, were investigated during the forming process of a heterogeneous interface. Simulation results reveal that some of molecule chains get across the interface and tangle with the molecules from the other layer, resulting in the penetration phenomenon near the interface zone. In addition, the melting temperature and injection pressure exhibit positive effects on the interfacial properties of hybrid composites. To further investigate the interfacial bonding strength and fracture mechanism of the heterogeneous interface, the uniaxial tensile and sliding simulations were performed. Results show that the non-bonded interaction energy plays a crucial role during the fracture process of heterogeneous interface. Meanwhile, the failure mode of the heterogeneous interface was demonstrated to evolve with the processing parameters.

Fiber with Butterfly Wings: Creating Colored Carbon Fibers with Increased Strength, Adhesion, and Reversible Malleability

Colored and color-changing materials are central to perception and interaction in nature and have been exploited in an array of modern technologies such as sensors, visual displays, and smart materials. Attempts to introduce color into carbon fiber materials have been limited by deleterious impacts on fiber properties, and the extension of colored fibers toward "smart composites" remains in its infancy. We present carbon fibers incorporating structural color, similar to that observed on the surface of soap bubbles and various insects and birds, by modifying the fiber surface through in situ polymerization grafting. When dry, the treated fibers exhibit a striking blue color, but when exposed to a volatile solvent, a cascade of colors across the visible light region is observed as the film first swells and then shrinks as the solvent evaporates. The treated fibers not only possess a unique color and color-changing ability but also can be reversibly formed into complex shapes and bear significant loads even without being encased in a supporting polymer. The tensile strength of treated fibers shows a statistically significant increase (+12%), and evaluation of the fiber-to-matrix adhesion of these polymers to an epoxy resin shows more than 300% improvement over control fibers. This approach creates a new platform for the multifaceted advance of smart composites.

Linking Nanoscale Chemical Changes to Bulk Material Properties in IEPM Polymer Composites Subject to Impact Dynamics

A synthesizable interfacial epoxy-polyurea-hybridized matrix (IEPM), composed of chemical bonded nanostructures across an interface width ranging between 2 and 50 μm, is a candidate for dialing-in molecular vibrational properties and providing high-impact dynamics resistance to conventional fiber(x)-reinforced epoxy (F/E), engendering an x-hybrid polymeric matrix composite system (x-IEPM- t_c). Atomic force microscopy and scanning electron microscopy elucidate the interfacial nanoscale morphology and chemical structure via reaction kinetics of curing epoxy (as a function of time, t_c) and fast-reacting (prepolymerized) polyurea. Nano-infrared spectroscopy (nano-IR) spectra, per non-negative matrix factorization analysis, reveal that simultaneous presence of characteristic epoxy and polyurea vibrational modes, within a nanoscale region, along with unique IEPM characteristics and properties following thermomechanical analysis and dynamic mechanical analysis (DMA), indicate chemical bonding, enabling IEPM reaction kinetics, as a function of  t_c, to control natural bond vibrations and type/distribution of interfacial chemical bonds and physical mixtures, likely due to the bond mechanism between -NCO in polyurea and epoxide and -NH₂ in epoxy hardener (corresponding to characteristic absorption peaks in nano-IR results), leading to enhanced IEPM quality (fewer defects/voids). Test results of ballistic-resistant panels, integrated with thin intermediate layers of x-IEPM-b- t_c, confirm that lower t_c significantly enhances loss modulus (∝ material damping and per DMA) in impact dynamics environments.

Positive effect of functional side groups on the structure and properties of benzoxazine networks and nanocomposites

Calculated glass transition temperature of the MDP-a resin obtained through tetra-functional and penta-functional polymerization paths.

Self-Extinguishing Resin Transfer Molding Composites Using Non-Fire-Retardant Epoxy Resin

Introducing fire-retardant additives or building blocks into resins is a widely adopted method used for improving the fire retardancy of epoxy composites. However, the increase in viscosity and the presence of insoluble additives accompanied by resin modification remain challenges for resin transfer molding (RTM) processing. We developed a robust approach for fabricating self-extinguishing RTM composites using unmodified and flammable resins. To avoid the effects on resin fluidity and processing, we loaded the flame retardant into tackifiers instead of resins. We found that the halogen-free flame retardant, a microencapsulated red phosphorus (MRP) additive, was enriched on fabric surfaces, which endowed the composites with excellent fire retardancy. The composites showed a 79.2% increase in the limiting oxygen index, a 29.2% reduction in heat release during combustion, and could self-extinguish within two seconds after ignition. Almost no effect on the mechanical properties was observed. This approach is simple, inexpensive, and basically applicable to all resins for fabricating RTM composites. This approach adapts insoluble flame retardants to RTM processing. We envision that this approach could be extended to load other functions (radar absorbing, conductivity, etc.) into RTM composites, broadening the application of RTM processing in the field of advanced functional materials.

Modification of Carbon Fibre Surfaces by Sulfur-Fluoride Exchange Click Chemistry

Technologies that enable surface modification are in high demand and are critical for the implementation of new functional materials and devices. Here, we describe the first modification of a carbon surface (in this case carbon fiber) using the sulfur-fluoride exchange (SuFEx) reaction. The parent sulfur (VI) fluoride moiety can be installed directly to the surface via electrochemical deposition of the fluorosulfate phenyldiazonium tetrafluoroborate salt, or by 'SuFExing' a phenol on the carbon surface followed by treatment of the material with SO2 F2 ; similar to a 'graft to' or 'graft from' functionalization approach. We demonstrate that these SuFEx-able surfaces readily undergo exchange with aryl silyl ethers, and that the subsequent sulfate linkages are themselves stable under electrochemical redox conditions. Finally, we showcase the utility of the SuFEx chemistry by installing a pendant amino group to the fiber surface resulting in interfacial shear strength improvements of up to 130 % in epoxy resin.

Interphase Engineering of a Cellulose-Based Carbon Fiber Reinforced Composite by Applying Click Chemistry

Given our possible future dependence on carbon fiber reinforced composites, the introduction of a renewable matrix might be advantageous for the vision of a sustainable world. Cellulose is a superior green candidate and provides exceptional freedom in composite design as the free OH groups can be conveniently functionalized to give tailor-made materials. To obtain a high-performing carbon fiber reinforced cellulose propionate composite, we accurately tailored the interfacial adhesion by invoking click chemistry. The synthetic strategy involved grafting of a phenylacetylene structure onto the carbon fiber surface, onto which O-acylated 6-azido-6-deoxycellulose and a number of aromatic azides could be covalently attached. Single-fiber fragmentation tests indicated that the lipophilicity and size of the substituent on the deposited structure played a crucial role in determining molecular entanglement and mechanical interlocking effects, as penetration into the cellulose propionate matrix was of utmost importance. Enhanced interfacial shear strength was obtained for the carbon fiber covalently functionalized with the cellulose derivative. Nevertheless, the greatest increase was observed for the derivative substituted with a compact and highly lipophilic CF3 substituent. In a broader sense, our study provides a synthetic platform to bind cellulose derivatives to graphitic surfaces and paves the ways towards the preparation of innovative cellulose-based carbonaceous materials.

Do Carbon Nanotubes Improve the Thermomechanical Properties of Benzoxazine Thermosets?

Fillers are widely used to improve the thermomechanical response of polymer matrices, yet often in an unpredictable manner because the relationships between the mechanical properties of the composite material and the primary (chemical) structure of its molecular components have remained elusive so far. Here, we report on a combined theoretical and experimental study of the structural and thermomechanical properties of carbon nanotube (CNT)-reinforced polybenzoxazine resins, as prepared from two monomers that only differ by the presence of two ethyl side groups. Remarkably, while addition of CNT is found to have no impact on the glass-transition temperature ( T_g) of the ethyl-decorated resin, the corresponding ethyl-free composite features a surge by ∼47 °C (50 °C) in T_g, from molecular dynamics simulations (dynamic mechanical analysis measurements), as compared to the neat resin. Through a detailed theoretical analysis, we propose a microscopic picture for the differences in the thermomechanical properties of the resins, which sheds light on the relative importance of network topology, cross-link and hydrogen-bond density, chain mobility, and free volume.

Carbon fibre reinforced cellulose-based polymers: intensifying interfacial adhesion between the fibre and the matrix

Interfacial interactions governing the interfacial adhesion between cellulose propionate and carbon fibre surface are placed under scrutiny to pave the way towards the development of green cellulose-based carbon fibre reinforced polymers. A range of molecular entities are deposited on the surface by initially grafting aromatic structures with appropriate functions via diazonium species followed by further derivatization of these entities. Cellulose propionate was also bound covalently to the surface via a tosylated derivative invoking its facile nucleophilic displacement reaction with surface-grafted amino functions. Significant increase in interfacial shear strength was obtained for the cellulose propionate-grafted carbon fibre composite as well as for the 4-(aminomethyl)benzene-functionalized sample, in the latter case possible hydrogen bonding took place with the cellulose propionate matrix. Furthermore, the positive effect of a highly lipophilic and yet compact –CF₃ substituent was also noted. In order to let the grafted structure efficiently penetrate into the matrix, steric factors, lipophilicity and potential secondary interactions should be considered. It needs to be pointed out that we provide the first synthetic strategy to covalently bind cellulose derivatives to a largely graphitic surface and as such, it has relevance to carbonaceous materials being applied in cellulose-based innovative materials in the future.

Interfacial adhesion of a cellulose propionate/carbon fibre composite is tailored providing a synthetic strategy to bind cellulose derivatives to graphitic surfaces.

Structured Ionomer Thin Films at Water Interface: Molecular Dynamics Simulation Insight

Controlling the structure and dynamics of thin films of ionizable polymers at water interfaces is critical to their many applications. As the chemical diversity within one polymer is increased, controlling the structure and dynamics of the polymer, which is a key to their use, becomes a challenge. Here molecular dynamics simulations (MD) are used to obtain molecular insight into the structure and dynamics of thin films of one such macromolecule at the interface with water. The polymer consists of an ABCBA topology with randomly sulfonated polystyrene (C), tethered symmetrically to flexible poly(ethylene-r-propylene) blocks (B), and end-capped by a poly(t-butylstyrene) block (A). The compositions of the interfacial and bulk regions of thin films of the ABCBA polymers are followed as a function of exposure time to water. We find that interfacial rearrangements take place where buried ionic segments migrate toward the water interface. The hydrophobic blocks collapse and rearrange to minimize their exposure to water. The water that initially drives interfacial reengagements breaks the ionic clusters within the film, forming a dynamic hydrophilic internal network within the hydrophobic segments.