The development of microstructure during drying-induced phase inversion or dry casting of homogeneous water/acetone/cellulose acetate coatings, which evolve into asymmetric separation membranes, was witnessed using 'time-sectioning' cryogenic scanning electron microscopy (cryo-SEM). Coating specimens were prepared via the following sequential steps: uniformly coating or casting the polymer solution onto a substrate, drying with environmental control for a specific time, rapidly freezing the specimen in liquid cryogen, fracturing to reveal the coating cross-section, subliming briefly for topographical contrast, sputter-coating to prevent charging and cryo-SEM imaging. Specimens were created with different drying times and hence each specimen is called a 'time-section'. The earliest time-section, one from a coating soon after deposition, shows a featureless specimen, as expected for a homogeneous polymer solution. As drying proceeds, time-sectioning reveals first the nucleation of polymer-lean droplets dispersed within a polymer-rich matrix across a region bound by the free surface above and a phase separation front below. On further drying, this front travels down to the substrate; the polymer-lean droplets grow and coalesce, forming a smoothly interconnected phase, which eventually becomes the pore space of a honeycomb-like structure as drying progresses. Meanwhile at the free surface, a seemingly dense skin develops on drying, while a nodular intermediate layer appears between the thinner skin and the thicker honeycomb-like substructure. The images are analyzed with composition paths derived from theoretical modeling to elucidate the fundamentals of microstructure development in asymmetric membranes.
Bibliographical noteFunding Information:
The authors would like to express sincere thanks to Prof. Y. Talmon at the Technion University, Haifa, Israel for lending his expertise at critical junctures of this work. The authors acknowledge support from the National Science Foundation and the University of Minnesota's Industrial Partnership for Research in Interfacial and Materials Engineering (IPRIME) through its Coating Process Fundamentals Program.
- Cellulose acetate
- Dry cast asymmetric membrane
- Dry phase inversion
- Microstructure evolution
- Phase separation
- Time-sectioning cryo-SEM