A new class of “structure-by-design” polymer membranes for organic solvent nanofiltration with controllable selectivity

Membrane based separations offer an inexpensive and energy efficient pathway to purification. However, their use is often stifled during separation of organics due to low selectivity. This can be attributed to the inherent limitations of the synthesis methods employed, namely, interfacial polymerization and phase inversion. In this work, a new class of structure-by-design membranes are synthesized by surface modification of crosslinked polyimide using graft polymerization with a series of methacrylate monomers with differing chain lengths and chemistries. Graft polymerization of long chain monomers resulted in increased selectivity (α∼3.7 for 80-20 mol.% MeOH-toluene and α∼6 for 95-5 mol.% toluene-triisopropyl benzene (TIPB) for both test solutions as a result of increased stiffness and order (using GIWAXS). The observed selectivities renders these membranes competitive for organic solvent nanofiltration applications. In addition, pure solvent permeabilities were measured. The performance of these membranes was correlated with brush properties such as degree of grafting (using ATR-FTIR), surface polarity (contact angle measurements), stiffness (using QCM-D) and brush configuration in different solvent environments (using MD Simulations). MD simulations demonstrate that in mixtures containing hydrophobic species, polymers with longer hydrophobic side chains well more and form fuller matrices leading to increased solvent accessible surface area. GIWAXS results showed increased order with longer side chain length. These tunable membranes offer promise for organics separation.