Surface Modification of TFC-PA RO Membrane by Grafting Hydrophilic pH Switchable Poly(Acrylic Acid) Brushes

Abstract

The grafting of pH-responsive poly(acrylic acid) (PAA) brushes was carried out on the surface of a commercial TFC-PA membrane using surface-initiated atom transfer radical polymerization (SI-ATRP). Poly(t-butyl acrylate) was polymerized through the SI-ATRP method followed by its acid hydrolysis to form PAA hydrophilic polymer brushes. Surface morphology, permeation flux, salt rejection, and pore sizes were investigated. The contact angle for water was reduced from 50° for a pristine membrane to 27° for the modified membrane due to a modification with the hydrophilic functional group and its brush on membrane surfaces. The flux rate also increased noticeably at lower pH values relative to higher pH for the modified membranes, while the flux remains stable in the case of pristine TFC-PA membranes. There is slight transition in the water flux rate that was also observed when going from pH values of 3 to 5. This was attributed to the pH-responsive conformational changes for the grafted PAA brushes. At these pH values, ionization of the COOH group takes place below and above pKa to influence the effective pore dimension of the modified membranes. At a lower pH value, the PAA brushes seem to permit tight structure conformation resulting in larger pore sizes and hence more flux. On the other hand, at higher pH values, PAA brushes appeared to be in extended conformation to induce smaller pore sizes and result in less flux. Further, pH values were observed to not significantly affect the NaCl salt rejection with values observed in between 98.8% and 95% and close to that of the pristine TFC-PA membranes. These experimental results are significant and have immediate implication for advances in polymer technology to design and modify the “switchable membrane surfaces” with controllable charge distribution and surface wettability, as well as regulation of water flux and salt.

Antifouling and Flux Enhancement of Reverse Osmosis Membrane by Grafting Poly (3-Sulfopropyl Methacrylate) Brushes

“…Modified membranes with grafted polymer brushes at different monomer concentrations are shown in Figure 2d-f. Thus, it has been concluded that polymer brushes have been successfully grown on the TFC-PA membrane by ATRP, as reported in previous studies such as those of Abbas et al and Wang et al [28,29].…”

“…FTIR spectra of PA, PA-NH 2 , and PA-Br are shown in Figure 3 a. Spectra of pristine polyamide are shown in Figure 3 a, the peak at 3095 cm −1 shows the aromatic stretching, which is due to the aromatic group in polyamide, the peak at 1689 cm −1 is designated to C=O, and the peak at 1599 cm −1 is that for NH bending of the peptide bond present within the polyamide structure [ 28 ]. Figure 3 a shows the spectra for PA-NH 2 and PA-Br.…”

“…Figure 3 a shows the spectra for PA-NH 2 and PA-Br. In the PA-NH 2 spectrum, the peak at 3321 cm −1 is assigned to N-H bond stretching due to the presence of an amine group; the broad peak results from weak N-H bond stretching, and the peak at 1590 cm −1 is due to the N-H bond bending [ 28 ]. A sharp peak shows that extensive bending has occurred in the bond.…”

“…The spectra of PA-Br is shown in Figure 4 a, and the peaks that are formed due to N-H stretching and bending have disappeared because of the conversion of PA-NH 2 to PA-Br; moreover, the peak at 1727 cm −1 shows the presence of C=O in the attached ethyl α bromoisobutyrate [ 28 ].…”

“…The PA-NH 2 bar corresponds to the attachment of an amine group on the TFC-PA membrane. As a result of membrane functionalization, a uniform and even surface shows a decreased average surface roughness value that gradually rises after the attachment of bromo initiator represented by PA-Br bar [ 28 ]. With increasing polymer brush growth, the average surface roughness is reduced as a result of the polymer brush layer forming observed in the previous study [ 49 ].…”