Abstract
pH shift induced Aqueous phase separation (APS) is a novel and more sustainable water-based approach to create microfiltration, ultrafiltration, and nanofiltration membranes. APS allows for control over membrane pore size and structure in ways analogous to traditional non-solvent induced phase separation (NIPS). Unfortunately, existing APS approaches require extreme pH shifts (from pH 14 to pH 1) to obtain successful membranes, limiting their applicability for large scale production. Here we demonstrate that APS membranes, with tunable pore sizes ranging from ~80 nm to dense nanofiltration type, can be prepared using a mild pH shift (pH 12 to pH 4) based on the complexation of poly(styrene sulfonate) (PSS) and branched polyethyleneimine (PEI) in acetate buffer coagulation baths. The molecular weight of PEI, the concentration and the pH value of the buffer solution, and the concentration of glutaraldehyde cross-linking agent were systematically varied to control and optimize the membrane fabrication conditions. It was found that tight nanofiltration membranes having a molecular weight cut-off of ~200 g mol−1 and excellent salt (97% MgCl2) and micropollutant retentions (~96%) could be prepared alongside ultra/microfiltration type membranes with an average pore size of ~60 nm. These results indicate that APS membranes with tunable pore sizes can be prepared under mild pH conditions with excellent control over separation properties.
Highlights
Non-solvent induced phase separation, commonly referred to as non-solvent induced phase separation (NIPS), is by far the most commonly employed technique to prepare polymeric membranes
The poly(styrene sulfonate) (PSS)-PEI casting solution was prepared by mixing PSS (25%) and PEI (25%) solutions is a 1:2 ratio of PSS to polyallylamine hydrochloride (PAH) monomers
Several key pa rameters such as the molecular weight of PEI, the concentration and pH of the acetate buffer, and the concentration of GA crosslinker were varied to investigate the effect on membrane structure and morphology
Summary
Non-solvent induced phase separation, commonly referred to as NIPS, is by far the most commonly employed technique to prepare polymeric membranes. De Vos demonstrated that a weak poly electrolyte such as poly(4-vinylpyridine), which is soluble in water only at low pH conditions can be precipitated at high pH conditions to form porous films [11] These membranes exhibited limited me chanical stability and relatively poor control over the pore size [12]. In that version of APS, a homogeneous solution of a strong polyelectrolyte poly(sodium 4-styrenesulfonate) PSS, and a weak poly electrolyte polyallylamine hydrochloride (PAH) was prepared at ~ pH 14 and precipitated in a ~pH 1 coagulation bath Several parameters such as the molecular weight of the polyelectrolytes, solution concen tration, pH of the bath, salinity of the bath, and the amount of cross-linker can be tuned to obtain desirable membrane pore sizes [18, 19]. The findings of this work offer insight into the PSS-PEI system as one of the most promising APS approaches for the production of a wide variety of sustainable membranes
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