This article explores the field of MXene/epoxy resin composites, their properties, and their applications.
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What are Epoxy Resins?
Resins are natural or synthetic viscous monomers or pre-polymers with a non-crystalline and brittle texture in solid form. Epoxy resins (ERs) in particular are resins with an epoxide functional group. They are either cross-linked with themselves by catalytic homopolymerization or with a wide range of co-reactants, or so-called hardeners, to form thermosetting polymers. This cross-linking or curing occurs through the epoxide or hydroxyl functional group. Some of such common hardeners are amines, acids, alcohols, phenols, and thiols. The epoxide ring can react with varieties of reactants through several paths to form epoxy resins with great versatility.
ERs have excellent mechanical properties, low shrinkage after curing, low residual stresses, and high thermal and chemical resistance. Thus, they are incorporated in the matrix of many reinforced polymer composites. However, in the purest form, ERs are brittle, and their lower fracture toughness, electrical non-conductivity, and poor thermal conductivity limit their application in modern electronic devices.
These can be improved by adding elastomers, silica, clays, and varieties of nanomaterials. More specifically, 2D nanomaterials are widely used as fillers or reinforcing materials to improve the desired electrical and thermal properties of ERs for application in electronic devices. The advantages of using 2D nanofillers are their high aspect ratio and specific surface area with the additional functionality driven by their quantum size.
2D Nanofillers for ERs
Graphene is currently the most famous and most-tested filler material in most electronic devices due to its high electrical and thermal conductivity, high modulus, and resistance. However, its application is limited to conducting composite materials. Meanwhile, hexagonal boron nitride (hBN) is an electrical insulator with good thermal conductivity.
Apart from conductors and insulators, dielectric materials have major roles in electronic devices. Dichalcogenides such as molybdenum sulfide (MoS2) are excellent dielectric materials.
A new class of 2D transition metal carbides, nitrides, and carbonitrides called MXenes have garnered a lot of attention as dielectric nanofillers in many electronic devices. These are layered nanomaterials with alternating layers of transition metal and carbide, nitrides, or carbonitrides. They have the general formula of M2X, M3X2, and M4X3, where M represents transition metals and X represents carbides, nitrides, or carbonitride.
Properties of MXenes
MXenes may have one or more terminal groups (Tx) on their surface based upon the synthesis process, such as oxygen (-O), hydroxyl (-OH), or fluorine (-F). -O and -OH groups impart hydrophilic characteristics to the MXene nanosheets. These functional groups also promote electric conductivity and electrochemical activities.
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Additionally, some of them exhibit biocompatibility and antibacterial properties. Hence, their applications have steadily grown up in energy storage, purification, electromagnetic interference (EMI)-shielding, and radio-wave-absorption devices. Usually, MXenes are synthesized by delamination of an intermediate third metallic layer. For example, Ti3C2Tx is synthesized by removing the aluminum (Al) layer from the Ti3AlC2 using strong fluorinated acid solutions.
MXenes/ER Composites
The primary reason MXenes/ER composites possess enhanced mechanical properties is attributed to the intercalation and covalent bonding of Tx groups on the surface of the MXenes with epoxy monomers. This significantly improves the toughness, impact strength, crack deflection, shear yielding, and flexural strength of the composite. However, too much of a concentration of MXene in ER could be detrimental due to the formation of too many defects in the thermosetting. Some studies show that the concentration of Ti3C2Tx MXene should be less than 5% in most ER.
Furthermore, reactants like methyl tetrahydro phthalic anhydride (MTHPA) are hardeners, and at the same time, they prevent agglomeration of Ti3C2Tx in ER matrix and decrease the glass transition temperature of the ER. Another nanomaterial, attapulgite 1D nanorods (ATP), is used in concentration < 1% to further increase the storage modulus of ER. In fiber-reinforced polymers (FRPs), MXene-coated fibers further enhance the interfacial interaction between fibers and ER.
MXene/ER composites exhibit excellent thermal conductivity and flame retardancy. Hence, they are the ideal materials for electrothermal devices, which are used to measure high-frequency currents. The thermal conductivity of ER can be further improved by incorporating silver nanoparticles (Ag NPs). Red phosphorous-coated MXene sheets are excellent flame retardants.
From a tribological perspective, MXenes such as Ti3C2 nanosheets are excellent antifriction and anti-wear agents in ER matrix. Polytetrafluoroethylene (PTFE) latex coated MXenes significantly reduce the coefficient of friction of the composite. This helps in preventing fatigue failure of the surface of the MXene/ER-coated device.
Reference and Further Reading
Giménez, R., Serrano, B., San-Miguel, V., Cabanelas, J., Recent Advances in MXene/Epoxy Composites: Trends and Prospects. Polymers 2022, 14, 1170. https://doi.org/10.3390/polym14061170
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