MEMBRANE FABRICATION: A GREEN APPROACH
Green method of synthesis helps to minimize or eliminate the environmentally hazardous substance in design, processability, and application of polymeric membranes. Since the functional and most important part of the filtration system is a membrane, solvent media and various methods, which can be used for the synthesis and fabrication, are summarized along with the precursors used.
There are many conventional solvents used in both the synthesis process of polymer and fabrication of membrane which is known to be hazardous. Also, the traditional method of synthesis create waste which is nonbiodegrad- able and thus pollute the environment. The following section discusses the green solvent or solvent systems which should consider replacing the so-used medium during the design and fabrication of the membranes. The solvent selection should be in a way that it compromises minimum with the efficacy of the product. Selecting precursors and methods for the synthesis process is also veiy important, and thus various methods in place of typical chemical methods have been discussed citing the examples for the fabrication.
GREEN SOLVENT IN THE PROCESS MEMBRANE FABRICATION
There are few examples which show the use of greener solvent in the fabrication of PI membranes. Soroko et al. showed the preparation of organic solvent nanofiltration (OSN) using PI by replacing the solvent mixture of DMF/l,4-dioxane (DX) with DMSO/acetone solvent, thus allowing to engineer PI OSN membrane to match the precise requirement for diverse application, and also greener membrane showed the alike performance in terms of rejection as compared with the PI OSN membrane prepared using DMF and DX. SEM results in Figure 11.4 show the formation of spongy like membrane and in Figure 11.5 show the rejection of PI membrane.7 In addition with above, this author also successfully replaced the cross-linking agent for membrane, that is isopropanol with water, without affecting the performance of membrane.8 Gao et al. fabricated the composite consisting of outerselective thin film (TFC) hollow fiber membrane. Authors focused on the removal of excess of w-phenylenediamine (MPD) and alkane solvent which was used with trimesoyl chloride in the process of synthesis of TFC and Figure 11.6 confirms the formation of hollow fiber.9 For the removal of the solvents, initially TFC hollow fiber membrane substrate was modified with polyethyleneimine (PEI) for the cross-linking and then using by the small water-soluble molecules like glutaraldehyde and epichlorohydrin. The results obtained displayed the PEI cross-linking may help in the removal of the additional MPD during interfacial polymerization and introduced the additional functional sites for the functionalization of GA and ECH, but earlier alkane treatment was required for the site generation.
FIGURE 11.4 SEM micrograph of cross-section of PI membrane prepared from the varying ratio of DMSO/acetone. (a) Ml 3/1, (b) М2 5/1, (с) М3 7/1, (d) M4 11/1, (e) M5 13/1. and (f) M6 15/1.
Source: Reprinted with permission from Ref. [7]. © 2011 Royal Society of Chemistry.
FIGURE 11.5 Effect on rejection of PI OSN membrane with respect to solvent system used in the polymer-dope solution at 30 bar.
Source: Reprinted with permission from Ref. [7]. © 2011 Royal Society of Chemistry.
FIGURE 11.6 FESEM micrograph of the M-PEI and GA hollow fiber; membranes whole cross-section.
Source: Reprinted with permission from Ref. [9]. © 2016 Elsevier.
The results also showed that the use of PEI reduces the fabrication cost. Performance of the membrane was increased in a way that enhanced the salt rejection especially (Na,S04) and NaCl, which can reach up to 90% with a pure water permeability (PWP) of 1.74 ± 0.01 L m“2 bar1 h-1. Modified membrane showed decent term stability up to 72 li of experiment with impressive thermal stability with broader range of operation temperature (5-60°C). Electrospinning is nowadays becoming an attractive technique used in membrane fabrication and its composite formation. Fabrication of the long-continued fiber with a diameter ranging in nanometer to few micrometers is one of the main advantages of the technique. Jiang et al. focused on the alternative of large amounts of toxic, flammable, and environmental unfriendly organic solvent used in electrospinning by replacing the solvents and solute system with water-soluble precursor solution of hydro- phobic nanofibers membrane which showed the decrease in porosity and water permeability.9 Instead of the use of DMF as a solvent for poly(amic acid) (PAA), author found the alternative by electrospinning of ammonium salt of PAA from water and in second step high temperature used for the imidization and removal of ammonia and the template polymer to prepare the PI nanofiber. The fiber diameter and the contact angle have been shown in Figure 11.7 of PAA-salt/polyethylene glycol (PEO) nanofiber and PI nanofibers imidized.10 The unanticipated benefit for using the salt leads to an inherent increase in the conductivity of the solution. The membrane obtained from the greener route showed the better high-temperature liquid filtration (100°C), with better thermomechanical stability and good enough to accomplish quantitative filtration of n on oxide microparticles which could be of future interest for oil filtration in engines. Filtration test results were also comparable with the PI obtained from the DMF solution.10
FIGURE 11.7 SEM micrograph of electrospim PAA-salt/PEO nanofibers (a, b) and PI nauofibers imidized (d. e). Diameter distribution of electrospun PAA-salt/PEO and PI nanofibers.
Source: Reprinted with permission from Ref. [10]. © 2016 American Chemical Society.
