## 2 Materials and Methods  
Polyamide thin-film composite membranes were prepared with lab-made support membranes via interfacial polymerization as described previously [26; 27]. In detail, a 10 wt% polysulfone (PSf) polymer solution was prepared with the solvent mixture consisting of N-methyl-2-pyrrolidone (NMP) and N, N-dimethylformamide (DMF) at a ratio of 1:7 to fabricate highly porous microstructured support (HPuS) membranes via non-solvent-induced phase separation (NIPS). The HPuS membrane was adopted as a support membrane due to its desired traits such as large surface pores and surface porosity leading to a short effective diffusion path length over an active layer [26], negligible sub-layer resistance, which does not affect the overall water flux [28], and excellent mechanical properties enough to withstand high pressure even for seawater reverse osmosis (SWRO) desalination [29]. HPuS membranes possess extremely high porosity and comparable mechanical strength at the same time, such that they are suitable to prepare highly permeable TFC-RO membranes without compromising salt rejection. This point allowed us to determine how the SEC varies at high water permeability ranges while increasing salt rejection by adjusting an operating parameter. A polyamide selective layer was prepared via interfacial polymerization on the top side of the HPuS support membrane using 2 wt% \(m\)-phenylenediamine (MPD) aqueous solutions without and with 1 wt% dimethyl sulfoxide (DMSO) and a trimesoyl chloride (TMC) solution (0.1 \(w\)/\(v\)%; solvent: \(n\)-hexane). The TFC membranes prepared without and with 1 wt% DMSO were marked as TFC-D0 and TFC-D1, respectively. 1 wt% DMSO can form a highly permeable polyamide active layer (5.14 L m\({}^{-2}\) h\({}^{-1}\) bar\({}^{-1}\); in brackish water reverse osmosis (BWRO)) with comparable salt rejection (98.3%; in BWRO) by inducing a smooth, thin, and slightly loose polyamide layer via the dipole-dipole interaction between DMSO and carbonyl oxygens of TMC [27]. Cross-sectional SEM images were taken to observe the morphologies of support membranes and measure a mean active layer thickness using FESEM (Field-emission scanning electron microscopy, JSM-7600F, JEOL, Japan). The average values of the active layer thickness were estimated using the thicknesses measured at five arbitrary positions, as described previously [30; 31; 32; 33]. The active layer roughness was evaluated with an atomic force microscope (AFM; NX10, Park Systems, Suwon, Korea). A Fourier transform infrared (FTIR) spectrometer (IR Prestige-21, Shimadzu, Japan) equipped with attenuated total reflection (ATR) was used to observe the change in the peak intensity at 950 cm\({}^{-1}\), as described previously [27; 34], in order to determine whether 1 wt% DMSO slightly decreased the crosslinking degree. The peak with a larger intensity at 950 cm\({}^{-1}\) indicates more unreacted acyl chloride groups on a polyamide active layer, implying a lower crosslinking degree. The membrane filtration test was carried out without a spacer to obtain the water flux (L m\({}^{-2}\) h\({}^{-1}\) bar\({}^{-1}\); LMH) and salt rejection (\(R\) = \(C_{p}\)/\(C_{f}\); %) under increasing applied pressure from 25 to 55 bar in a bid to explore the possibility of increasing salt rejection by adjusting operating pressures. In addition, this allows us to compare the benefits between a process approach and a material approach from the perspective of the SEC. \(C_{f}\) and \(C_{p}\) refer to the salt concentrations of the feed and permeate solutions, respectively. When \(C_{f}\) was used to calculate the \(R\), the concentration polarization phenomenon, which is variable depending on the use of a spacer and spacers' geometry [35], was excluded from consideration.