Microstructure evolution in dry cast cellulose acetate membranes by cryo-SEM

Sai S. Prakash, Lorraine F. Francis, L. E. Scriven

Research output: Contribution to journalArticle

15 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)328-338
Number of pages11
JournalJournal of Membrane Science
Volume283
Issue number1-2
DOIs
StatePublished - Oct 20 2006

Keywords

  • Cellulose acetate
  • Dry cast asymmetric membrane
  • Dry phase inversion
  • Microstructure evolution
  • Phase separation
  • Time-sectioning cryo-SEM

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