Tailoring the interface in graphene/thermoset polymer composites: A critical review

Infusion Simulation of Graphene-Enhanced Resin in LCM for Thermal and Chemo-Rheological Analysis

The present numerical study proposes a framework to determine the heat flow parameters-specific heat and thermal conductivity-of resin-graphene nanoplatelets (GNPs) (modified) as well as non-modified resin (with no GNPs). This is performed by evaluating the exothermic reaction which occurs during both the filling and post-filling stages of Liquid Composite Moulding (LCM). The proposed model uses ANSYS Fluent to solve the Stokes-Brinkman (momentum and mass), energy, and chemical species conservation equations to a describe nano-filled resin infusion, chemo-rheological changes, and heat release/transfer simultaneously on a Representative Volume Element (RVE). The transient Volume-of-Fluid (VOF) method is employed to track free-surface propagation (resin-air interface) throughout the computational domain. A User-Defined Function (UDF) is developed together with a User-Defined Scaler (UDS) to incorporate the heat generation (polymerisation), which is added as an extra source term into the energy equation. A separate UDF is used to capture intra-tow (microscopic) flow by adding a source term into the momentum equation. The numerical findings indicate that the incorporation of GNPs can accelerate the curing of the resin system due to the high thermal conductivity of the nanofiller. Furthermore, the model proves its capability in predicting the specific heat and thermal conductivity of the modified and non-modified resin systems utilising the computed heat of reaction data. The analysis shows an increase of ∼15% in the specific heat and thermal conductivity due to different mould temperatures applied (110-170 °C). This, furthermore, stresses the fact that the addition of GNPs (0.2 wt.%) improves the resin-specific heat by 3.68% and thermal conductivity by 58% in comparison to the non-modified thermoset resin. The numerical findings show a satisfactory agreement with and in the range of experimental data available in the literature.

Model Approach to Thermal Conductivity in Hybrid Graphene-Polymer Nanocomposites

The thermal conductivity of epoxy nanocomposites filled with self-assembled hybrid nanoparticles composed of multilayered graphene nanoplatelets and anatase nanoparticles was described using an analytical model based on the effective medium approximation with a reasonable amount of input data. The proposed effective thickness approach allowed for the simplification of the thermal conductivity simulations in hybrid graphene@anatase TiO2 nanosheets by including the phenomenological thermal boundary resistance. The sensitivity of the modeled thermal conductivity to the geometrical and material parameters of filling particles and the host polymer matrix, filler's mass concentration, self-assembling degree, and Kapitza thermal boundary resistances at emerging interfaces was numerically evaluated. A fair agreement of the calculated and measured room-temperature thermal conductivity was obtained.

Recyclable Monolithic Vitrimer Foam for High-Efficiency Solar-Driven Interfacial Evaporation

With the exponentially rapid development of solar-driven interfacial evaporation, evaporators with both high evaporation efficiency and recyclability are highly desirable to alleviate resource waste and environmental problems but remain challenging. Here, a monolithic evaporator was developed based on a dynamic disulfide vitrimer (a covalently cross-linked polymer network with associative exchangeable covalent bonds). Two types of solar absorbers, carbon nanotubes and oligoanilines, were simultaneously introduced to enhance the optical absorption. A high evaporation efficiency of 89.2% was achieved at 1 sun (1 kW m-2). When the evaporator was applied to solar desalination, it shows self-cleaning performance with long-term stability. Drinkable water with low ion concentrations satisfying the drinkable water levels of the World Health Organization and a high output (8.66 kg m-2, 8 h per day) was obtained, revealing great potential for practical seawater desalination. Moreover, a high-performance film material was obtained from the used evaporator via simple hot-pressing, indicating excellent fully closed-loop recyclability of the evaporator. This work provides a promising platform for high-efficiency and recyclable solar-driven interfacial evaporators.

Poly (vinyl alcohol)/chitosan/polyethylene glycol-assembled graphene oxide bio-nanocomposites as a prosperous candidate for biomedical applications and drug/food packaging industry

The graphene oxide (GO) nanoplates and polyethylene glycol-decorated GO (GO-PEG nano-hybrid) were synthesized and characterized by FTIR, Raman, XRD, AFM, FE-SEM-EDAX and MTT assay. Obtained results confirmed the graphite oxidation and also assembly of PEG upon GO plates. The MTT assay indicated that GO-PEG nanohybrid enhanced biocompatibility to cells compared to the GO. The GO-PEG nanohybrid was introduced to the polyvinyl alcohol/chitosan carbohydrate (PVA/CS) blends. The bio-nanocomposite were prepared by simple casting method. The GO-PEG nanohybrids demonstrated a significant role in improving thermal, mechanical and antibacterial properties. Accordingly, bio-nanocomposites containing modified GO (PVA/CS/GO-PEG) exhibited higher glass transition temperature (Tg), Young's modulus, tensile strength, elongation at break and antibacterial properties than nanocomposites containing pure GO (PVA/CS/GO). The biodegradation outcomes indicated that the highest weight loss and degradability is related to the bio-nanocomposite containing modified GO (PVA/CS/GO-PEG), which was also confirmed by FE-SEM micrographs. Therefore, PVA/CS/GO-PEG bio-nanocomposites can be a suitable candidate for biomedical applications (tissue engineering, wound dressing) and food-drug packaging industry.

Tuning the interface in epoxy-based composites and laminates through epoxy grafted graphene oxide enhances mechanical properties

Improved dispersion together with enhanced interfacial adhesion of the reinforcement is the key to superior structural properties in polymer nanocomposites. Herein, graphene oxide (GO) is employed to reinforce epoxy, and in order to improve the interfacial adhesion, epoxy chains were grafted directly onto GO prior to composite preparation. The functionalized GO sheets were systematically characterized using FTIR, TEM, Raman spectroscopy, XRD, and XPS. The epoxy composites with GO and epoxy grafted graphene oxide (Ep-g-GO) were prepared with the addition of only a small amount (0.5 wt%) of GO using a mechanical stirrer coupled to a bath sonicator. This strategy resulted in an impressive increase in mechanical properties, 40% in storage modulus, 70% in hardness, 39% in fracture toughness, and 8% in tensile strength, as compared to neat epoxy. In addition, the modified composites were thermally stable up to 300 °C as inferred from the thermogravimetric analysis. The enhanced properties of the composites further led to investigating the effect of Ep-g-GO on epoxy/carbon fiber (CF) laminates. Interestingly, incorporation of 0.5 wt% Ep-g-GO resulted in improved interfacial adhesion between GO and the epoxy matrix, which enhanced the tensile strength by 12% and inter-laminar shear strength by 9% as compared to neat epoxy/CF laminates. This study clearly demonstrates the positive effect of the tailored interface, offered by Ep-g-GO, on the mechanical properties of epoxy composites and epoxy/CF laminates.

Thermal interface materials with graphene fillers: review of the state of the art and outlook for future applications

We review the current state-of-the-art graphene-enhanced thermal interface materials for the management of heat in the next generation of electronics. Increased integration densities, speed and power of electronic and optoelectronic devices require thermal interface materials with substantially higher thermal conductivity, improved reliability, and lower cost. Graphene has emerged as a promising filler material that can meet the demands of future high-speed and high-powered electronics. This review describes the use of graphene as a filler in curing and non-curing polymer matrices. Special attention is given to strategies for achieving the thermal percolation threshold with its corresponding characteristic increase in the overall thermal conductivity. Many applications require high thermal conductivity of composites, while simultaneously preserving electrical insulation. A hybrid filler approach, using graphene and boron nitride, is presented as a possible technology providing for the independent control of electrical and thermal conduction. The reliability and lifespan performance of thermal interface materials is an important consideration towards the determination of appropriate practical applications. The present review addresses these issues in detail, demonstrating the promise of graphene-enhanced thermal interface materials compared to alternative technologies.

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.

Self-Supported Reduced Graphene Oxide Membrane and Its Cu2+ Adsorption Capability

Graphene stratiform membrane materials have been recently applied to heavy metal removal in aqueous systems via adsorption due to their high mechanical strength, chemical stability, and other properties. We applied reduced graphene oxide (rGO) alone as an adsorbent to remove heavy metal ions from wastewater. Self-supported rGO membrane was prepared using a green reduction method with sodium hydrosulfite. We used the Raman spectra to observe the structure of the rGO membrane. The morphology of the self-supported membrane was measured by a scanning electron microscope. The Cu²⁺ adsorption performance was measured in terms of pH, reaction time, metal ion concentration, and temperature. The maximum Cu²⁺ adsorption capacity of the rGO membrane was found to be 149.25 mg/g. The adsorption process followed a pseudo-second-order kinetic model, and adsorption isotherms were simulated by the Freundlich model.

Tailoring graphene reinforced thermoset and biothermoset composites

The surge of knowledge among researchers pertaining to the excellent properties of graphene has led to the utilisation of graphene as a reinforced filler in polymer composites. Different methods of graphene preparation, either bottom-up or top-down methods, are important requirements of starting materials in producing reinforced properties in the composites. The starting graphene material produced is either further functionalised or directly used as a filler in thermoset polymer matrixes. An effective interaction between graphene and polymer matrixes is important and can be achieved by incorporating graphene into a thermoset polymer matrix through melt mixing, solution mixing or in situ polymerisation processes. In addition, by taking into consideration the importance of green and sustainable composites, the details of previous work on graphene reinforced bio-thermoset polymer matrixes is discussed. The resultant mechanical and thermal properties of the composites were associated to the chemical interaction between the graphene filler and a thermoset matrix. Exploration for further variations of graphene polymer composites are discussed by taking the reinforcement properties in graphene composite as a starting point.

Evolving Strategies for Producing Multiscale Graphene-Enhanced Fiber-Reinforced Polymer Composites for Smart Structural Applications

Graphene has become an important research focus in many current fields of science including composite manufacturing. Developmental work in the field of graphene-enhanced composites has revealed several functional and structural characteristics that promise great benefits for their use in a broad range of applications. There has been much interest in the production of multiscale high-performance, lightweight, yet robust, multifunctional graphene-enhanced fiber-reinforced polymer (gFRP) composites. Although there are many reports that document performance enhancement in materials through the inclusion of graphene nanomaterials into a matrix, or its integration onto the reinforcing fiber component, only a few graphene-based products have actually made the transition to the marketplace. The primary focus of this work concerns the structural gFRPs and discussion on the corresponding manufacturing methodologies for the effective incorporation of graphene into these systems. Another important aspect of this work is to present recent results and highlight the excellent functional and structural properties of the resulting gFRP materials with a view to their future applications. Development of clear standards for the assessment of graphene material properties, improvement of existing materials and scalable manufacturing technologies, and specific regulations concerning human health and environmental safety are key factors to accelerate the successful commercialization of gFRPs.

Graphene oxide modified carbon fiber reinforced epoxy composites

The interfacial interaction between the fiber and matrix is the most important factor which influences the performance of the carbon fiber-epoxy composites. In this study, the graphitic surface of the carbon fibers was modified with graphene oxide nanomaterials by using a spray coating technique which is an easy, cheap, and quick method. The carbon fiber-reinforced epoxy matrix composites were prepared by hand layup technique using neat carbon fibers and 0.5, 1 and 2% by weight graphene oxide (GO) modified carbon fibers. As a result of SEM analysis, it was observed that GO particles were homogeneously coated on the surface of the carbon fibers. Furthermore, Young's modulus increased from 35.14 to 43.40 GPa, tensile strength increased from 436 to 672 MPa, and the elongation at break was maintained around 2% even in only 2% GO addition.

Nano-modified epoxy: the effect of GO-based complex structures on mechanical performance

The application of nanofillers (NFs) in multicomponent polymer systems is accompanied by important structure-directing effects that are more marked in partially miscible systems, such as polymer-modified epoxy. This study deals with rubber-modified epoxy using different combinations of GO and amine-terminated butadiene-acrylonitrile copolymer (ATBN), including in situ and pre-made grafting. Moreover, GO grafted via planar epoxy groups or solely edge-localized carboxyls was used. It is shown that the grafted ATBN chains promote the assembly of GO-g-ATBN into nacre-mimicking lamellar structures instead of usual exfoliation in thermoplastics. This complex structure of elastically embedded GO leads to the best mechanical performance. It is obvious that a small concentration of the grafted polymer exceeds the contribution of a higher concentration of separately added ATBN. The results highlight the important effect of the degree of grafted chains and geometry of the internal structure of the self-assembled arrays and their effect on the mechanical performance.

ATBN-grafted GO forms nacre-mimicking lamellar structures in epoxy; the effect of grafting geometry on the structure/property relationship is highlighted.

The journey of self-healing and shape memory polyurethanes from bench to translational research

In this critical review, we have enlisted a comprehensive summary of different approaches that have been used over the past decade to synthesize self-healing polyurethanes including “close then heal” and “shape memory assisted self-healing” concept.

Poly(ether ether ketone)-Grafted Graphene Oxide "Interconnects" Enhance Mechanical, Dynamic Mechanical, and Flame-Retardant Properties in Epoxy Laminates

Herein, the interface of epoxy and graphene oxide (GO) is tailored through hydroxylated poly(ether ether ketone) (HPEEK). The resultant modification (HPEEK-g-GO) improved the interfacial adhesion between epoxy and carbon fiber (CF) in the laminates. This strategy resulted in improved tensile strength, modulus, and storage modulus by 8, 10, and 26%, respectively, with respect to control samples (epoxy/CF laminates). The HPEEK-g-GO was thoroughly characterized using spectroscopic techniques and was infused using vacuum-enhanced resin infusion technology into the epoxy/CF laminates. To address the challenges involved with primary agglomeration, the composite formulation was subjected to mechanical stirring coupled with bath sonication throughout the mixing process. The improved structural properties in epoxy/CF laminates were attributed to HPEEK-g-GO "interconnects", which provided the necessary reinforcement owing to better interfacial adhesion with the CF mat as inferred from the fracture surface morphology assessed using SEM. In addition, the epoxy laminates containing HPEEK-g-GO also showed flame-retardant properties along with good thermal stability. The electromagnetic interference shielding capability of the modified laminates was also evaluated in the frequency range of 12-18 GHz. It was observed that the laminates exhibited a shielding effectiveness of -50 dB. Thus, this strategy offers some promise in fabricating epoxy/CF laminates with multifunctional properties through HPEEK-g-GO "interconnects".

Facile silane functionalization of graphene oxide

The facile silane functionalization of graphene oxide (GO) was achieved yielding vinyltrimethoxysilane-reduced graphene oxide (VTMOS-rGO) nanospheres located in the inter-layer spacing between rGO sheets via an acid-base reaction using aqueous media. The successful grafting of the silane agent with pendant vinyl groups to rGO was confirmed by a combination of Fourier-transform infrared (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The structure and speciation of the silane-graphene network (nanosphere) and, the presence of free vinyl groups was verified from solid-state magic angle spinning (MAS) and solution 13C and 29Si nuclear magnetic resonance (NMR) measurements. Evidence from Scanning Electron Microscopy (SEM), High-Resolution Transmission Electron Microscopy (HRTEM) and TEM-High-Angle Annular Dark-Field (TEM-HAADF) imaging showed that these silane networks aided the exfoliation of the rGO layers preventing agglomeration, the interlayer spacing increased by 10 Å. The thermal stability (TGA/DTA) of VTMOS-rGO was significantly improved relative to GO, displaying just one degradation process for the silane network some 300 °C higher than either VTMOS or GO alone. The reduction of GO to VTMOS-rGO induced sp2 hybridization and enhanced the electrical conductivity of GO by 105 S m-1.

Interfacial Architecture Constructed Using Functionalized MWNT Resulting in Enhanced EMI Shielding in Epoxy/Carbon Fiber Composites

In this work, we have attempted to improve electromagnetic interference (EMI) shielding and mechanical behavior of epoxy/carbon fiber (CF) composite, simultaneously, in the presence of functionalized carbon nanotubes. It is well understood that properties of composite depend on the interface between the filler and matrix. Considering this basic understanding, functionalized carbon nanotubes/epoxy nanocomposites were impregnated into a bidirectional carbon fiber (CF) mat and, further, various mechanical and EMI shielding behaviors were studied. Multiwalled carbon nanotubes were functionalized with branched poly(ethyleneimine) (b-MWNT) to tailor the interface of epoxy/CF composites. Laminates with two layers of CF were fabricated with functional MWNT modified epoxy. Scanning electron microscopy was used to analyze the microstructure of epoxy/CF laminates. Lap shear test was performed to analyze adhesion between the modified epoxy and carbon fiber. Further dynamic mechanical analysis in the temperature range of 30-160 °C was performed. Thermal degradation of composites was studied using a thermogravimetric analyzer. Electrical conductivity of laminates was measured using a four-point method on an Agilent probe station. EMI shielding effectiveness (SE) was measured for 0.5 mm-thin laminates in the Ku band. The b-MWNT modified epoxy/CF composites showed excellent SE_T of ca. -60 dB and SE_A of ca. -50 dB, which are of commercial importance. Compared to unmodified epoxy/CF, b-MWNTs/epoxy/CF exhibited 200% increment in EMI SE_T and 35% enhancement in storage modulus due to the improved interface between the epoxy matrix and carbon fiber.

Block Copolymer Elastomer with Graphite Filler: Effect of Processing Conditions and Silane Coupling Agent on the Composite Properties

The control of morphology and interface in poly(styrene-ethylene/butylene-styrene) (SEBS) composites with graphitic fillers is extremely important for the design of piezoresistive sensors for body motion or flexible temperature sensors. The effects of a high amount of graphite (G) and silane coupling agent on the morphology and properties of SEBS composites with anisotropic mechanical properties are reported. The physical and chemical bonding of silane to both G and SEBS surface was proved by EDX and TGA results; this improved interface influenced both the thermal and mechanical properties of the composite. The vinyltriethoxysilane (VS) promoted the formation of char residue and, being tightly bound to both SEBS and G, did not show separate decomposition peak in the TGA curve of composites. The mechanical properties were measured on two perpendicular directions and were improved by both the addition of VS and the increased amount of G; however, the increase of storage modulus due to orientation (from 5 to 15 times depending on the composition and direction of the test) was more important than that provided by the increase of G concentration, which was a maximum of four times that obtained for 15 wt % graphite. A mechanism to explain the influence of G content and treatment on the variation of storage modulus and tan δ depending on the direction of the test was also proposed.

Three-Dimensional Graphene Foam-Filled Elastomer Composites with High Thermal and Mechanical Properties

To meet the increasing demands for effective heat management of electronic devices, a graphene-based polymeric composite is considered to be one of the candidate materials owing to the ultrahigh thermal conductivity (TC) of graphene. However, poor graphene dispersion, low quality of exfoliated graphene, and strong phonon scattering at the graphene/matrix interface restrict the heat dissipation ability of graphene-filled composites. Here, a facile and versatile approach to bond graphene foam (GF) with polydimethylsiloxane (PDMS) is proposed, and the corresponding composite with considerable improvement in TC and insulativity is fabricated. First, three-dimensional GF was coated with polydopamine (PDA) via π-π stack and functional groups from PDA reacted with 3-aminopropyltriethoxysilane (APTS). Then, the modified GF was compressed (c-GF) to enhance density and infiltrated with PDMS to get the c-GF/PDA/APTS/PDMS composite. As a result, these processes endow the composite with high TC of in-plane 28.77 W m^-1 K^-1 and out-of-plane 1.62 W m^-1 K^-1 at 11.62 wt % GF loading. Besides, the composite manifests obvious improvement in mechanical properties, thermal stability, and insulativity compared to neat PDMS and GF/PDMS composite. An attempt to use the composite for cooling a ceramic heater is found to be successful. Above results open a way for such composites to be applied for the heat management of electronic devices.

Facile and green preparation of biobased graphene oxide/furan resin nanocomposites with enhanced thermal and mechanical properties

A facile and green approach was developed to prepare biobased graphene oxide (GO)/furan resin nanocomposites by directly transferring GO from water dispersion into furan resin.

Controlling dielectrical properties of polymer blends through defined PEDOT nanostructures

The paper reports the crucial role of the morphology of poly(3,4-ethylenedioxythiophene) (PEDOT) nanostructures on the thermal and dielectric properties of polymer blends prepared thereof.

Graphene/3C-SiC Hybrid Nanolaminate

In this work, we demonstrate a one-step approach to create graphene/3C-SiC nanolaminate structure using microwave plasma chemical vapor deposition technique. Layer-by-layer arrangement of thin 3C-SiC layers and graphene sheets is obtained with the thicknesses of the individual 3C-SiC layers and graphene sheets being 5-10 nm and 2-5 nm, respectively. An intimate contact between 3C-SiC and the graphene sheets is achieved and the nanolaminate film shows a high room temperature conductivity of 96.1 S/cm. A dedicated structural analysis of the nanolaminates by means of high-resolution transmission electron microscopy (HRTEM) reveals that the growth of the nanolaminates follows an iterative process: preferential graphene nucleation around the planar defects at the central region of the SiC layer, leading to the "splitting" of the SiC layer; and the thickening of the SiC layer after being "split". A growth mechanism based on both kinetics and thermodynamics is proposed. Following the proposed mechanism, it is possible to control the layer thickness of the graphene/3C-SiC hybrid nanolaminate by manipulating the carbon concentration in the gas phase, which is further experimentally verified. The high electrical conductivity, large surface area porous structure, feasible integration on different substrates (metal, Mo; semiconductor, Si and 2H-SiC; insulator, diamond) of the graphene/3C-SiC hybrid nanolaminate as well as other unprecedented advantages of the nanolaminate structure make it very promising for applications in mechanical, energy, and sensor-related areas.