MXene Rheology and Phase Behavior: Exploring the Effects of Sheet Size and Salt Addition

Abstract

The overall research goal of this dissertation is to understand the effects of sheet size and salt addition on the phase behavior and rheological properties of aqueous MXene dispersions. The charged surfaces of the MXenes, their irregular shape, and the polydispersity that ensues from their synthesis process complicates the extension of two-dimensional (2D) colloid fundamental knowledge to this recently discovered family of nanomaterials. This research provides an increased fundamental understanding of aqueous Ti3C2Tx MXene behavior through the tuning of sheet size, MXene concentration, and ionic strength. The knowledge gained of MXene lyotropic liquid crystalline behavior and associated rheological properties can be directly utilized to improve the processability of MXenes in fluid-phase manufacturing methods, such as direct ink writing. Through consideration of MXenes as a specific example within the family of 2D colloids, the knowledge gained through this work improves the overall current scientific understanding of 2D materials by providing new insights to classical colloid science questions. The first half of this research focused on investigating three size fractions of Ti3C2Tx MXenes: large MXene sheets with an average longest lateral dimension of ~3 µm, small MXene sheets with an average longest lateral dimension of ~0.3 µm, and a bimodal mixture of large and small MXene sheets. A series of concentrations of each MXene size fraction was studied through polarized optical microscopy (POM) and steady and oscillatory shear rheology to elucidate the effects of selective size polydispersity on lyotropic liquid crystalline phase transitions. Changes in microstructure and rheological properties as a function of MXene concentration and sheet size provided experimental evidence necessary to produce the first phase transition chart for aqueous size fractionated MXene dispersions. Dispersions of large MXene sheets were observed to readily form long-range liquid crystalline alignment at relatively low concentrations; however, they also exhibited a tendency toward aggregation as concentration was increased. Small MXene dispersions remained more stable throughout increasing concentration. Signatures of liquid crystallinity were not observed in small MXene dispersions, indicating that the smaller sheets were unable to form extended aligned structures. A 1:1 bimodal mixture of large and small MXenes served as a selectively polydisperse size fraction to compare to the more monodisperse large MXene and small MXene dispersions. The inclusion of small MXene sheets into dispersions of large MXenes resulted in increased stability while maintaining the ability to achieve liquid crystalline alignment. Signatures of liquid crystallinity in bimodal MXene dispersions occurred at slightly higher concentrations than that observed in monodisperse large MXene dispersions; however, the resistance toward aggregation is a desirable characteristic for fluid-phase processing. The second half of this work involved the addition of sodium chloride (NaCl) to increase the ionic strength of bimodal MXene dispersions. By investigating the effects of ionic strength on dispersion phase behavior, microstructure, and associated rheological properties, the effectiveness of utilizing NaCl as a rheological modifier to aqueous MXene dispersions and inks could be revealed. It was observed that low concentrations of NaCl could induce increased rheological properties, including yield stress and storage modulus (G’), in lower concentrations of MXenes. At higher concentrations of MXenes (0.80 vol % and greater) the addition of NaCl was observed to initially weaken the percolated structure of MXenes, resulting in lower rheological properties. However, slightly raising the ionic strength of these dispersions increased the rheological properties above that of the 0 M dispersions. As expected from fundamentals of colloid science, high concentrations of NaCl (0.5 M) resulted in dispersion instability and severe flocculation of MXene sheet. This behavior was observed across an order of magnitude of concentrations (0.10 – 1.00 vol %), indicating that low MXene concentrations are not resistant to flocculation. The results from this research produced a MXene/NaCl phase diagram consisting of nematic liquid crystalline phases.