Synthesis and application of brush membranes for organic solvent nanofiltration

Abstract

Rate driven membrane-based separation offers a potential pathway to replacement of equilibrium driven processes such as chromatography and distillation for a plethora of applications such as organic solvent separation and biologics purification. Surface modification of membranes offers an inexpensive, tunable and convenient pathway to expand the use of current commercial membranes to a wider array of applications. A new class of polymeric membranes comprising the formation of selective bottle brush membranes on non-selective modified crosslinked polyimide support membranes are synthesized, characterized, and tested here. Their attractive features include broad flexibility of performance based on structure-and-chemistry-by-design that is currently impossible with current commercial synthesis methods and excellent competitive performance with different organic feed solutions. Crosslinked polyimide supports were modified by graft polymerization using Single Electron Transfer- Living Radical Polymerization (SET- LRP). Monomers chosen include apolar alkyl methacrylates with differing side chain lengths (C2, C6, C18) and functional polar monomers such as hydroxyethyl methacrylate (HEMA) and aminoethyl methacrylate (AEMA). These functional monomers were crosslinked using a 1- or 2-step crosslinking strategy to increase stiffness and the performance of these membranes were studied using different feed solutions namely (i) organic dyes from ethanol (ii) methanol-toluene mixtures and (iii) toluene- TIPB mixtures. Graft polymerization of alkyl methacrylates could be controlled for near complete rejection of organic dyes from ethanol. For methanol-toluene mixtures, the highest selectivity among alkyl methacrylates was obtained with the longest side chain (C18) with a value of ~3.7. Shorter chains led to selectivity between 1 and 1.5. Graft polymerization of AEMA followed by crosslinking in a 2-step pathway gave a maximum selectivity of ~12. Selectivity between 1.5 and 4 were obtained for one step crosslinking with HEMA with a decrease of this value as a result of crosslinking brought forth by the synthesis of a more open brush network. The maximum selectivity obtained for the toluene-TIPB mixtures was between 5 and 6 for graft polymerized alkyl methacrylates and 2-step crosslinked brush membranes. The stiffness of the grafted and crosslinked brush networks was measured using a Quartz Crystal Microbalance with Dissipation (QCM-D) and this stiffness was positively correlated with performance of membranes for the first time, to the best of our knowledge. Proof of modification was established and quantified using ATR-FTIR. Additional morphological studies were made with microscopy (Scanning Electron Microscopy and Atomic Force Microscopy). While higher selectivity for this separation has been reported in the literature, accounting for both the permeability and selectivity, these membranes are highly competitive for organic solvent nanofiltration applications. Further work can be done to improve the support and graft polymerization of copolymers using SET-LRP to incorporate the effect of chain length and functionality in a single step.