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[Display omitted] • Gelation of chitosan dissolved in an alkaline aqueous solution was investigated. • Ethanol-water mixtures were employed as the coagulant for the APS processes. • Gelling film in the ethanol-containing coagulant was in-situ characterized by OCT. • Film morphology was analyzed by combining OCT and network-based modeling. • Internal rejection of particles was correlated with the morphological formation. Gelation of chitosan dissolved in an alkali/urea aqueous solution promotes the mechanistic diversity of forming a membrane via aqueous phase separation (APS), which is conceptualized to highlight the advantages of using environmentally friendly solvents. When aimed at deepening the knowledge of regulating the structural formation, the alkaline dope of chitosan was employed in this study to investigate how the presence of ethanol in the coagulant could regulate the gelation behavior in APS and thereby change the particle-trapping mechanism. Both the gelling films and the particle-filtering membranes were in-situ characterized by optical coherence tomography (OCT) to compare the film-formation kinetics and the morphological variations in the resulting porous substructures, respectively. It was revealed that the ethanol-containing coagulant could induce the shrinkage of the gelling film and thereby favor the transverse coalescence of polymer-rich phases; that is, increasing the content of ethanol in the coagulant (over a range less than a critical value to avoid film deformation) would give rise to more tortuous substructures (for effectively capturing particles) primarily owing to the lower connectivity in the transverse direction. The theoretical analysis was supported by numerically evaluating the topological analog. This study not only confirms effectiveness of the ethanol-based regulation, but also establishes a paradigm for fabricating membranes with tailored topological characteristics. [ABSTRACT FROM AUTHOR]

Precipitating polymer in a completely aqueous environment is conceptualized as a process of aqueous phase separation (APS) and lends itself to the environmentally friendly fabrication of membranes. In an effort to effectively regulate the morphological formation in APS, it is of fundamental concern to elucidate how asymmetry can be developed in the absence of an unbalanced exchange of the 'nonsolvent' and solvent. Therefore, technical modifications were made by this study to essentially improve the in-situ characterization based on optical coherence tomography (OCT). In particular, a process of APS was modeled by passively contacting an acidic aqueous dope of chitosan with another aqueous solution of sodium hydroxide in a mini-coagulation bath, where the OCT scan was implemented in a high speed mode to capture the onset and evolution of gelation at the coagulant-dope interface with a temporal resolution of 6.25 ms. The quantitative analysis indicated that the gelation could be initiated at the interface in a period shorter than ∼10 ms and evolved in an oscillating way; a skin layer with a smaller critical size would be formed by damping the interfacial oscillations. This study not only sheds light on the APS-based fabrication of membranes, but also establishes a novel paradigm for in-situ characterizing the film formation via an OCT-based approach. [Display omitted] • Formation of asymmetry in APS was modeled by the gelation of chitosan. • OCT was used to in-situ characterize the initial gelation occurring at the interface. • Passive contact was employed to eliminate the delay resulting from immersion. • High speed scan mode enabled achieving a temporal resolution of 6.25 ms. • Interfacial oscillations in the gelation were correlated with the skin rejection. [ABSTRACT FROM AUTHOR]