(50g) Cross-Linked Mixed Matrix Membranes for Organic Solvent Nanofiltration (OSN) Consisting of P84 Polyimide and Carboxyl-Functionalized Multi-Walled Carbon Nanotubes

To design solvent-resistant nanofiltration membranes from polyimide materials, one must cross-link the polyimide membranes in order to improve the stability and resistance of the membrane in the harsh environments but incorporate porous fillers to open up the channels for a better solvent transport. A moderate annealing is also needed to manipulate the pore size for a better solute rejection. We have fabricated mixed matrix membranes (MMMs) consisting of carboxyl-functionalized multi-walled carbon nanotubes (MWCNTs-COOH) and P84 polyimide in this study for organic solvent nanofiltration applications, such as pharmaceutical, with the aid of cross-linking by 1, 6-hexanediamine (HDA). The incorporation of hydrophilic functionalized carbon nanotubes into P84 not only improves liquid sorption and transport but also enhances membrane porosity and pore size. Consequently, the permeances of water, ethanol, and isopropanol across the MMMs increases with an increase in MWCNTs-COOH loading up to 0.075 wt. %. However, a higher loading of carbon nanotubes may result in agglomeration and reduce the separation performance. The cross-linked MMM comprising 0.05 wt. % MWCNTs-COOH has a rejection of 85% to rose bengal molecules of 1017.65 Dalton with an ethanol permeance of about 9.6 (L/m² h bar) at 5 bar. A higher transmembrane pressure or a lower feed concentration show a better separation performance for the MMMs. Interestingly, the rejection of rose bengal in isopropanol solutions is higher than that in ethanol solutions (i.e., 99 vs. 85%) even though the former has a lower solvent flux than the latter. After thermal annealing at 150ºC in a 3/1 EG/PEG400 (weight ratio) solution, the resultant membranes show very improved rejections to small dyes. The cross-linked and then annealed MMM comprising 0.05 wt. % MWCNTs-COOH has a rejection of almost 100% to Safranin O dye molecules of 350.85 Dalton with an ethanol flux of about 4.6 LMH at 20 bar.