Heterogeneity During the Formation of Waterborne Barrier Coating Revealed by Cryogenic Transmission Electron Microscopy

Imaging the film formation process of waterborne barrier coatings in situ with nanoscopic resolution is very challenging, which limits the understanding of the underlying mechanisms and rational design of the materials. Here this challenge is tackled using in situ cryogenic transmission electron microscopy (cryoTEM) in combination with electron tomography (cryoET), which allows 3D imaging of the process with <1 nm resolution. By monitoring the film formation process of poly(ethylene-co-methacrylic acid) (EMAA) ionomer dispersion, onion-like nano-aggregates are captured. These aggregates can be removed by weakening the interactions between EMAA particles via adding amino alcohol coalescing agents or increasing the EMAA neutralization degree, which improves the barrier property of the coating simultaneously, indicating the importance of these heterogeneities to the material performance. The study benefits a better understanding of the formation kinetics of waterborne coatings, and demonstrates cryoTEM as an efficient method for studying the film formation process in situ.