Spatiotemporal Structure Heterogeneity Evolution during Latex Film Formation Investigated by Operando Single-sided Nuclear Magnetic Resonance Relaxometry

Abstract

Understanding the film formation mechanism of waterborne latex is crucial for developing high-performance, eco-friendly coatings. However, the influence of various enviromental factors, i.e. temperature and wind, on spatiotemporal structure heterogeneity induced during drying complicates the establishment of the structure-process-property of coating. Here, we track a polyacrylate latex film formation using a custom-built single-sided nuclear magnetic resonance (NMR) hyphenated instrument. Two-dimensional correlation spectra reveal highly restricted water dynamics within the latex suspension, evidenced by a decreased self-diffusion coefficient D from 2.00×10⁻⁹ m²/s of pure water to 1.39×10⁻⁹ m²/s in suspension with a plummeted T₁/T₂ of 30, which is about 111 for pure water. For film drying under mild conditon (24 °C without airflow), dense particle packing induces strong capillary forces, generating anomalous fast-diffusion channels (D up to 31.05×10⁻⁹ m²/s) to accelerate water evaporation resulting in homogeneous structure along the thickness direction. Conversely, after introducing airflow, i.e. 35 °C with 10 L/min airflow, the accelerated drying rate (3.72 µm/min) drastically amplifies the spatiotemporal heterogeneity and triggers premature surface skinning. Such skin-core structure traps residual water inside and inhibit further coalescing of latex particles. The uncoalesced bottom layer thicknesses quantitatively predicted by non-destructive single-sided NMR (about 100, 200, and 400 µm), which is well consistent with SEM measurements (93, 240, and 409 µm, respectively). This work provides direct physical insights and theoretical guidance for the formulation and application of waterborne coatings during real service condition.