Mapping MXene Alignment under Shear: An In Situ SAXS Study of Size-Dependent Orientation Dynamics

Effective control over the alignment of 2D nanomaterials in solution is critical for optimizing their macroscopic properties. However, the fundamental factors governing this alignment remain poorly understood. This study addresses this gap by investigating the disorder-to-order transition states of Ti₃C₂T_x MXene sheets. We specifically examine the interplay among sheet sizes, concentrations, and shear rates in driving orientation. Our results reveal a strong size-dependent behavior. Large sheets (∼5 μm) at a high concentration (30 mg mL^-1) spontaneously align into stable liquid crystalline phases. Conversely, small sheets (∼250 nm) require a minimum shear rate (0.11 s^-1) to align but lose order rapidly, particularly at low concentrations (e.g., 25 s at 1 mg mL^-1). Fiber wet-spinning experiments further demonstrate how the shear rate directly dictates sheet alignment and, consequently, the electrical conductivity of the resulting fibers. These findings provide key insights for fabricating ordered structures from MXene dispersions and other anisotropic nanomaterials, highlighting the importance of understanding orientation dynamics to advance solution-based processing.