Communication pubs.acs.org/cm
Glycerol-Based Bicontinuous Cubic Lyotropic Liquid Crystal Monomer System for the Fabrication of Thin-Film Membranes with Uniform Nanopores Blaine M. Carter,† Brian R. Wiesenauer,‡ Evan S. Hatakeyama,† John L. Barton,§ Richard D. Noble,*,† and Douglas L. Gin*,†,‡ †
Department of Chemical & Biological Engineering, and ‡Department of Chemistry & Biochemistry, University of Colorado, Boulder, Colorado 80309, United States § Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, United States S Supporting Information *
KEYWORDS: lyotropic, liquid crystal, bicontinuous cubic, glycerol, polymer, thin-film, nanoporous, membrane
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in the phase.4,6 This produces supported membranes with an active separation layer that is completely infused in the support material and as thick as the support itself (ca. 40 μm4−6 vs the ≤0.1-μm-thick active layers of commercial reverse osmosis (RO)7 and NF membranes8), resulting in water fluxes too low to be practical. A thin separation layer is needed for high fluxes, since thicker layers have more resistance to flow. Attempts to make thinner QI films via solution-casting and related methods were unsuccessful due to the compositional sensitivity of the Q phases.4 Evaporative water loss during processing and/or retention of residual casting solvent led to phase changes.4 As with composite RO and NF membranes, the ideal configuration for this type of system (especially for commercial application) is a thin-film composite (TFC) architecture (i.e., a very thin separation layer on top of a more porous support). This configuration yields the highest water flux while providing the mechanical stability needed to withstand the pressures required for passage through the separation layer.7 TFC membranes have recently been formed using cubic thermotropic (i.e., solvent-free) LC monomers;9 however, only one example of a TFC LLC membrane has been reported (i.e., a HII system with nonaligned cylindrical nanopores and low flux).10 To get the best LLC membrane performance, what is needed is a Q monomer phase with 3D nanopores that can be easily fabricated into a TFC configuration with retention of the desired phase composition and structure. Herein, we present a new imidazolium-based gemini LLC monomer (3) that forms a cross-linkable QI-phase with the low volatility and environmentally benign solvent, glycerol, instead of water. This 3/glycerol QI monomer phase can be readily fabricated into nanoporous TFC membranes via solutioncasting from MeOH to form defect-free thin films on porous supports, with minimal glycerol loss and retention of the QI composition and nanostructure after MeOH evaporation and photopolymerization (Figure 1). After prefiltration to exchange
yotropic liquid crystal (LLC) networks are nanoporous polymer materials formed by the in situ cross-linking of reactive amphiphiles (i.e., surfactants) that self-organize in water or other polar solvents into ordered, phase-separated assemblies.1 They contain periodic, uniform-size, nanoscale solvent regions/pores (i.e., ≤1 to 10 nm) that have different possible geometries and are lined by the amphiphile headgroups, thus allowing for pore control and functionalization.1 Because of these features, LLC networks have been shown to be valuable for many applications.1a,c One important application area where LLC networks have been found to have great potential is membrane-based water purification/desalination.2 The ability of cross-linkable LLC systems to generate polymers with monodisperse, sub-nanometer, aqueous pores enables the removal of molecular contaminants from water (0.27 nm diameter) with high selectivity.3 Also, the ability of LLCs to access different nanopore geometries and produce materials with high pore density is essential for high water filtration throughput (i.e., flux).3 In particular, bicontinuous cubic (Q) LLC networks have been found to be the most desirable for this application.2 In addition to high pore density, Q phases have 3Dinterconnected nanopores that do not require alignment for high throughput, unlike those in lower-dimensionality LLC phases (i.e., the 1D cylindrical hexagonal (H) and 2D lamellar (L) phases).2 Q phases are classified as type I (i.e., normal) or type II (i.e., inverted) depending on whether the hydrophilic− hydrophobic interface curves away or toward the water regions.1 Recently, we showed that water nanofiltration (NF) membranes based on cross-linked type I bicontinuous cubic (QI) phases can be formed. The first system was based on a gemini phosphonium monomer (1) with water (0.75 nm pores),4,5 while a second system was based on a more easily synthesized, gemini ammonium monomer with water (2) (0.86 nm pores).6 Both materials exhibit molecular-sieving capabilities; however, membrane fabrication was only possible via heating and pressing the QI monomer/water mixtures into a porous support, followed by radical photocross-linking to lock© 2012 American Chemical Society
Received: June 28, 2012 Revised: October 18, 2012 Published: October 30, 2012 4005
dx.doi.org/10.1021/cm302027s | Chem. Mater. 2012, 24, 4005−4007
Chemistry of Materials
Communication
that this system can be cross-linked in the QI phase, as confirmed by retention of the PLM and PXRD features (Figure 2). The black PLM textures were unchanged, and the
Figure 2. PXRD profiles of bulk QI-phase films of 3/glycerol/HMP (79.7/19.8/0.5 (w/w/w)): (a) before, and (b) after photocrosslinking. Inset: PLM optical textures (50×).
characteristic 1/√6 and 1√8 d-spacings showed very little change except for a slight increase in position after crosslinking, indicating a small unit cell expansion. FT-IR studies on the pre- and postpolymerized films showed >90% diene conversion (see the Supporting Information).4 Given glycerol’s properties, there should be minimal evaporative loss during thin-film solution processing if an appropriate volatile casting solvent is used. To prepare TFC membranes, a MeOH solution containing 60 wt % [79.7/19.8/ 0.5 (w/w/w)] 3/glycerol/HMP was roll-cast onto the surface of porous, asymmetric poly(ether sulfone) (PES) support films using a wire-wound rod. After mild heating to evaporate off the MeOH and photocross-linking at 70 °C (see the Supporting Information for details), scanning electron microscopy (SEM) indicated that the resulting membranes had defect-free coatings with an average thickness of 3 μm (Figure 3a). PXRD (Figure 3b) and FT-IR data (see the Supporting Information) confirmed that the TFC membranes had top layers with a QI structure and a high degree of polymerization.
Figure 1. Monomer 3, its QI phase with glycerol, and the formation of cross-linked QI-phase TFC membranes.
the glycerol in the pores with water, these TFC membranes were found to reject uncharged molecular solutes consistent with size-exclusion through uniform 0.96 nm pores but with absolute water fluxes ca. 10 times greater than prior meltinfused QI membranes. The rejection of smaller hydrated salt ions was found to be at the high level of RO membranes (≥98%) due to (charged pore−charged solute) repulsive interactions. To overcome the processing problems encountered in the earlier QI monomer systems, our approach was to substitute the water used for LLC phase formation with a lower volatility solvent. This alternative solvent should also be water-miscible (to enable facile flush-out) and nontoxic (in case traces remain in the membranes). Several polar organic solvents (formamide and its derivatives, N-methylsydnone, ethylene glycol (EG), glycerol, propylene carbonate)11,12 and ionic liquids (ILs)13,14 have been reported to form LLC phases (including Q phases) with select surfactants. Unfortunately, attempts to form LLC phases using 1 and 2 with many of these solvents were unsuccessful. To obtain better compatibility/LLC phase behavior with these solvents, a new gemini monomer (3) containing larger, more polarizable (i.e., “softer”) imidazolium headgroups was designed and synthesized (see the Supporting Information for synthesis details). Initially, a series of six gemini imidazolium monomers with different 1,3-diene tails and headgroup spacers was synthesized and screened for LLC behavior in different solvents using the solvent penetration scan technique with polarized light microscopy (PLM).15 Monomer 3 in this series was found to form Q phases with the broadest range of nonaqueous solvents (formamide, glycerol, and ethylammonium nitrate), in addition to pure water (see the Supporting Information). Since glycerol is a water-miscible, nontoxic solvent with low volatility (normal bp, 290 °C; vp @ 20 °C,
