Design of High Performance Nanofiltration Membranes from Simple Amine-Modified Polyvinyl Chloride Ultrafiltration Membranes

A facile method for fabricating poly(vinyl chloride) (PVC)-based nanofiltration (NF) membranes from corresponding ultrafiltration (UF) membranes is presented. The UF membranes were prepared through grafting simple diamines, namely, ethylenediamine (EDA) or 1,4-diaminobutane (DAB) onto PVC followed by a nonsolvent-induced phase separation process. A direct immersion thermal annealing (DITA) method in n-heptane medium transformed the pore size of the selective layer of modified PVC UF membranes to the NF range, highlighting the importance of the modification step in reducing the free spaces between polymer chains. The PVC-EDA-UF membrane annealed at 50 °C for 40 s showed a pure water flux (PWF) of 51 L·m–2·h–1 and magnesium chloride, magnesium sulfate, and dye rejections of 91%, 68% and 95%, respectively. However, the PVC-DAB-UF membrane annealed at the same condition showed a higher PWF (213 L·m–2·h–1) and lower rejections (63%, 56% and 69%, respectively). Increasing the annealing temperature to 60 °C decreased the PWF, while increasing dye and ion rejections of both membranes. The lower PWF and higher rejections of EDA-modified PVC-NF membranes than those of the DAB-modified ones were attributed to the shorter alkyl linker of EDA, which allows more complete packing of the polymer chains during DITA. Finally, the use of simple amine modifiers as well as facile and mild amination reactions in this work will pave the way for industrial applications of PVC-based NF membranes.