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Macrovoids formed in asymmetric membranes by non-solvent-induced phase separation (NIPS) critically influence membrane performance; however, it has remained experimentally unclear whether their formation is governed by a single dominant mechanism or by multiple competing mechanisms. Here, we employ high- resolution X-ray microcomputed tomography (micro-CT) to directly visualize and quantitatively analyze macrovoids in three dimensions over a sufficiently large field of view.
Three-dimensional micro-CT observations reveal the coexistence of two distinct types of macrovoids with fundamentally different initiation locations and geometries: surface macrovoids that initiate at or near the membrane surface with sharp initiation tips, and internal (sub-skin) macrovoids that nucleate beneath the skin layer with rounded initiation geometries. Quantitative analysis of depth-dependent structural parameters demonstrates that the number density, size distribution, and spatial arrangement of these macrovoid types strongly depend on fabrication conditions, such as polymer concentration and dope-solution composition, and that competitive growth between surface and internal macrovoids governs their final morphologies.
These three-dimensional structural characteristics cannot be resolved by conventional two-dimensional characterization techniques. The present results provide direct experimental evidence that multiple, process-condition-dependent physical mechanisms govern macrovoid formation during NIPS, and establish X-ray micro-CT as a powerful experimental approach for elucidating phase-separation phenomena and guiding the rational design of asymmetric membranes.

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• Creative Commons BY Attribution 4.0 International
  

Keyword: nonsolvent-induced phase separation (NIPS), macrovoid, X-ray microcomputed tomography (micro-CT), three-dimensional structural characterization, membrane formation mechanism

Date published: 2026-05-22

Publisher: American Chemical Society (ACS)

Journal:

• ACS Applied Polymer Materials (ISSN: 26376105) vol. 8 issue. 10 p. 7307-7315

Funding:

• Ministry of Education, Culture, Sports, Science and Technology JPMXP1122714694
• Core Research for Evolutional Science and Technology JPMJCR23L2

Manuscript type: Author's version (Accepted manuscript)

MDR DOI: https://doi.org/10.48505/nims.6430

First published URL: https://doi.org/10.1021/acsapm.6c00505

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Updated at: 2026-07-30 08:24:15 +0900

Published on MDR: 2026-07-30 10:31:02 +0900