Visualizing latex film formation using freeze-drying and scanning electron microscopy

Aqueous latex coatings transform from a suspension of discrete particles to a continuous polymer film during drying. Characterizing these microstructural changes is challenging given the transient nature of drying and the presence of water within samples. The present work proposes a freeze-drying preparation method that captures intermediate microstructural states during latex film formation so that they can be characterized by scanning electron microscopy (SEM) at room temperature. To develop this method, aqueous coatings prepared from diluted styrene acrylic latex (Primal DC-420, DOW Chemical) were deposited onto silicon substrates and dried just above the latex minimum film formation temperature (MFFT) of ~ 30°C. Coatings were then flash-frozen at various time points during drying, and then the ice within the frozen coating was sublimed under vacuum (P < 200 Pa). Specimens were stored and sputter coated with platinum at low temperature (<0°C) before SEM imaging. Surface and cross-sectional images showed multiple stages of film formation, enabled by a lateral drying front. Parallel cryogenic SEM studies verified that the freeze-drying method comparably captures microstructure after latex particles consolidate into a network. Studies with lower MFFT coatings, including those made from a different latex (Acronal S 790, BASF) or from Primal DC-420 with a coalescent (Texanol, Eastman Chemical), demonstrated the importance of storing and sputter coating specimens at temperatures less than the MFFT to maintain the microstructures for room temperature SEM imaging. Through the freeze-drying method, microstructures associated with all three stages of film formation (consolidation, compaction, and coalescence) were visualized, enabling the continued development and improvement of latex-containing coating formulations.