Study on the preparation method of porous membranes with single pore size distribution and tailored pore sizes by the induced swelling effect of pre-set graft side chains

Abstract

To improve the sieving accuracy of material separation, it is necessary to develop a porous membrane with tailored pore size and narrow pore size distribution. Polyvinylidene fluoride (PVDF), the mainstream membrane material, is a semi-crystalline polymer. On the basis of the high elastic state of the PVDF molecular chain in the amorphous region at room temperature, a novel method was proposed to prepare porous membranes with single pore size distribution and tailored pore sizes in this paper. First, 1,3-bis (tris(hydroxymethyl)methylamino) propane (Bis-Tris propane) was grafted onto the PVDF-CTFE main chain during casting solution preparation, and the membranes with the thin and nonporous skin layers were prepared by non-solvent-induced phase separation (NIPS), which had almost no water flux. The membrane with the nonporous skin layer produced a water flux of 35.3 L ∙ m−2∙ h−1∙ bar−1 after soaking in the alkaline glycine solution. FTIR and EDX tests demonstrated that small molecular glycine can be guided into the membrane matrix by the affinity with the polar groups on the pre-set side chains, inducing the reactive swelling effect, which led to the formation of new through holes. Subsequently, the membrane with the nonporous skin layer was initially soaked in ethanol to form pores in advance, then the induced reactive swelling of pre-set graft side chains was carried out. The water flux of obtained membrane was significantly up to 96.9 L ∙ m−2∙ h−1∙ bar−1, proving that the pre-pore forming process successfully strengthened the reactive swelling-caused pore generation effect. Furthermore, based on the enhanced swelling-caused pore generation strategy, amino acids with different molecular sizes were induced to the reactive swelling effect in the amorphous regions. The mathematical fitting showed a linear correlation between the average pore size of the membrane and the molar hydration volume of amino acids, and the membrane was single pore size distribution, verifying that controllably tailored membrane pore size can be realized by the reactive swelling effect of the reactive small molecule with different molecular sizes. Compared to some membranes with single pore size distribution prepared by track etching and anodic oxidation method, the membranes prepared by this method had higher water flux. The water contact angle result clarified that the membrane surfaces and pore channels had excellent hydrophilicity. The reactive swelling did not change the crystallinity and porosity, so the mechanical strength of the membrane did not deteriorate.