Cryogenic Scanning Electron Microscopy of Early Stages of Film Formation in Drying Latex Coatings

Cryogenic scanning electron microscopy (Cryo-SEM) was used to examine the distribution of polystyrene (PS) and of poly (butyl methacrylate) (PBMA) latex particles during drying of waterborne suspension coatings. The suspensions were coated on silicon wafer substrates and after different times of drying were rapidly frozen in liquid ethane. Top surfaces and fracture surfaces affording cross-sectional views were examined on the cold stage. The cryo-SEM micrographs demonstrate the presence of abrupt consolidation fronts advancing from the edges of the coatings in the earliest stage of drying of, e.g. suspensions of 10 wt% PS and of 15 wt% PBMA latex. Micrographs successively farther from an edge show that layers of ordered close-packed particles build sequentially downward from the top surface as drying progresses. But depending on the conditions, layered ordering gives way deeper in the coating to inter mixed regions of disordering and ordering. With care about sublimation of frozen water from the surfaces viewed, the cryo-SEM technique is useful for elucidating the compaction process in early stages of drying.