MXene polymer nanocomposites
Authors:   , Carey Michael S. , Barsoum Michel W. , Alvarez Nicolas J 
Published: October 2023

Discovered in 2011, MXenes are a unique family of hydrophilic, conductive, two-dimensional (2D) materials, derived from the layered ternary M[n+1]AX[n], or MAX, phases, where "M" is an early transition metal, "A" is a group IV-V element and "X" is carbon and/or nitrogen. Selective etching of the "A" layer from the MAX phases yields MXenes. In the composite literature, many polymer hosts have been reported, however most of these are hydrophilic polymers. Furthermore, nearly all these reports utilize MXene colloidal suspensions. In this work, we have explored the use of multilayer, ML, MXene in non-aqueous polymer systems, including nylon-6, alkylamine cured epoxies, flexible epoxies based on polyether amine curing agents, linear low density polyethylene LLDPE, 3D printed methacrylates, and covalent adaptable thiourethane, TU, networks (vitrimers). The goal in all cases was to produce polymer/MXene nanocomposites with enhanced properties. One leitmotiv of this work was to start with MLs in order to render our protocols scalable. In only the TU case, did we start with a colloid suspension. To work with hydrophobic polymers, it was essential to render the ML MXenes hydrophobic as well. To do so we treated the MLs with di(hydrogenated tallow)benzyl methyl ammonium chloride, DHT, a low-cost, long shelf life quaternary alkylammonium surfactant. Cationic exchange of Li⁺ with DHT dispersed and stabilized the MLs in nonpolar solvents, such as chloroform, decalin, hexane, cyclohexane, toluene and p-xylene. The organophilic Ti₃C₂T[z] suspensions were stable for at least 10 d with no signs of oxidation. This method was utilized to make solution processed linear low-density PE NCs, starting with MLs. It was found that mechanical reinforcement of the host LLDPE loaded with only 1.12 vol. % DHT functionalized Ti₃C₂T[z] resulted in an increase in elastic moduli, E, from 2.5 to 2.8 MPa and an increase in the maximum tensile strength, from 13.8 to 18.3 MPa. For non-DHT (untreated) MXene composites, a slight decrease in E to 2.45 MPa and increase in maximum tensile strength to 16.4 MPa were observed. Next, we prepared nylon-6 based NCs, loaded with ~ 1.9 and 5.0 vol. % Ti₃C₂T[z] MLs. By first exchanging the cations present between the MLs, with 12-aminolauric acid (12-ALA). This step facilitated the intercalation of monomer, [epsilon]-caprolactam, and catalyst, 6-aminocaproic acid between the MLs. Heating to 250 °C initiated the ring opening polymerization and resulted in a fully exfoliated nylon-6/MXene NCs with superior water barrier properties. An EMI SE of ~ 7 dB is achieved with 6.9 wt.% 12-Ti₃C₂T[z], and about ~19 dB with 17.4 wt.% 12-Ti₃C₂T[z], which is approaching the commercial standard. Utilizing DHT and ALA modified MLs, epoxy NCs loaded with 5.0 wt. % MLs (1.74 vol. %) were prepared. The relative water permeability of the epoxy resin is reduced by up to 90 % with a load of only 5 wt. % (1.74 vol. %) Ti₃C₂T[z]. Comparison with our relative permeability, (R < 0.1) with other NCs in the literature indicates our values are remarkably low. Additionally, the equilibrium solubility of water is uniformly lowered in our NC samples. To overcome the limitations of mechanical reinforcement in linear alkylamine cured epoxy NCs, we prepared flexible, conductive DHT-Ti₃C₂T[z] MXene epoxy NCs, loaded with 1.2, 7.2 and 19 vol. % DHT-Ti₃C₂T[z] flakes. Jeffamine D2000 was utilized to initiate the curing reaction in the presence of intercalated, organophilic MLs resulting in rubbery MXene epoxy NCs with significantly improved mechanical properties. Additionally, the incorporation of DHT-MXene to the epoxy matrix resulted in NCs conductivities of > 0.01 S·cm⁻¹, with a filler volume of 1.2 vol% and up to 0.08 S·cm⁻¹ at 7.2 vol.% filler. Similarly, to previous work, the water barrier performance is dramatically improved by the addition of the DHT-MXene. To realize the sustainable large-scale production of MXene, we addressed the volume of fluorinated wastewater produced in the washing process during Ti₃C₂T[z] synthesis, which drastically reduces the amount of wastewater generated, specifically from 640 mL·g⁻¹ to 160 mL·g⁻¹, a reduction of 75% through a simple neutralization process where the isolated sediment obtained after etching is treated with a small amount of sodium bicarbonate. Additionally, cryolite can be formed from this process, which provides insight into the chemistry of the etching reaction and allows for the recovery of fluorine from the spent etching liquid. The cryolite obtained can offset the cost of MXene production and provides a quantitative tool for calculating etching efficiency which would allow for the optimization of etching parameters such as time, temperature, pH and reagent concentrations. In another set of experiments, we demonstrated the successful production of methacrylate based MXene NCs via 3D digital light processing (DLP) printing. These NCs were produced with unmodified MXene MLs and produced with various degrees of exfoliation, achieved by employing a variety of processing techniques, namely simple mechanical mixing/stirring (m-MXene) and probe-sonication (p-MXene) of MLs in DA2 resin. We found that the addition of m-MXene to the methacrylate resin resulted in a 45% reduction in the equilibrium water saturation, despite little change in the diffusivity. However, with p-MXene, the saturation level was reduced by 60%, and the diffusivity was decreased substantially. Finally, a covalently adaptive thiourethane, TU, network was developed using inexpensive and industrially available reagents. This resin can be formulated rapidly with "click-chemistry" and the resin size can be controlled through the addition of a non-solvent such as acetone. The TU resin can be processed into granules which are then easily coated with 2D Ti₃C₂ MXene using colloidal suspensions and acid induced crashing of the nanosheets. This effectively wraps the TU particles with a continuous coating of MXene which adheres and follows the complex geometry of these TU granules. Bulk nanocomposite samples can be made by hot-pressing these coated powders at 90-125 °C for 5-10 min. These MXene containing NC samples have a unique microstructure where low loadings of MXene can produce a percolation pathway at an ultra-low percolation threshold of 0.14 vol.%. This morphology lends itself to conductive NCs, where a conductivity of 100 S/m is achieved with just 0.58 vol.% Ti₃C₂, increasing up to 600 S/m with 10 wt.% (2.3 vol.%). The presence of MXene improves the elastic modulus from 1.2 to 6 MPa, an increase of 400%, along with an increase of toughness from 3.5 to 11 MJ/m³, with just 2.5 wt.% (0.6 vol.%) Ti₃C₂T[z]. Furthermore, the vitrimer transition temperature is not significantly impacted by the addition of MXene, suggesting that the re-healing nature of this network will be maintained when MXene is incorporated.