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Overview: Ion-exchange membranes are important in a number of applications, such as fuel cells and water purification. Self-assembled nanostructures, such as those obtained with neutral-polyelectrolyte block copolymers for example, are ideal for fabricating membranes. Here the concentration of the ionic groups provides good transport properties, while the confinement of ionic domains limits membrane swelling improving the membrane selectivity. The focus of this research has been on the synthesis and fabrication of new nanostructured ion-exchange membranes. Methods have been developed which allow for ease and flexibility in both the synthesis of polymers and the fabrication of membranes.

Research Highlights: In the first year of this project the major accomplishments were first, the preparation of poly(trithiocarbonate) reversible addition fragmentation chain transfer (RAFT) agents for the polymerization of (AB)_n multiblock copolymers and second, the direct polymerization of neutral-sulfonated AB block copolymers through the synthesis of hydrophobic alkylammonium styrene sulfonate monomers. This past year research concentrated on the synthesis and characterization of long-chain poly(dimethyl n-alkylammonium styrene sulfonate) polymers. It was found that these polymers could act as organogelators for the preparation of thermoreversible gels in organic solvents. The gelation behavior was dependent on the structure of the polymer where gels were stabilized at higher temperature and in solvents of increasing polarity by increasing the backbone degree of polymerization and/or the length of the alkyl side-chains. As an example the behavior of poly(dimethyl octadecylammonium styrene sulfonate) (PSS-NC18) in benzene is presented below.

Three PSS-NC18 polymers with molecular weights of 9.4, 18.3, and 43.5 kDa and polydispersity indices of 1.3-1.4 were prepared by RAFT polymerization. The chemical structure of PSS-NC18 is shown in Figure 1a. At room temperature these polymers were insoluble in benzene. However, they could be dissolved at elevated temperatures and upon cooling self-supporting gels were formed that did not flow when inverted as shown in Figure 1b. Figure 1c shows measurements of T_gel vs. concentration determined by the temperature at which the gels first flowed when inverted during heating.

Figure 1. (a) Structure of PSS-NC18, (b) 18.3 kDa PSS-NC18/benzene gels at 2.5, 5, 10, and 20 wt/vol% polymer (L to R), (c) T_gel vs concentration for PSS-NC18 in benzene .

            The morphology of the gels was investigated by scanning electron microscopy (SEM) on freeze-dried xerogels. Figure 2 shows two freeze dried xerogels obtained from 10 wt/vol% polymer in benzene using either the 18.3 kDa or 43.5 kDa PSS-NC18. Both samples had network-like morphologies, however there is a clear molecular weight dependence of the morphology where the low molecular weight polymer contains interconnected cylindrical struts, while the high molecular polymer has a sheet-like morphology. These network morphologies are believed to be driven by the amphiphilic nature of the PSS-NC18 polymers. An analogy can be made to small molecule surfactants where at high temperature the polymer is soluble in solvent, however at lower temperature reverse-micellar aggregates are formed to shield the ionic groups from the non-polar organic solvent.

Figure 2. Xerogels of (a) 18.3 kDa and (b) 43.5 PSS-NC18 prepared from 10 wt/vol% PSS-NC18/benzene gels.

Significance of the Research: A new organogelator system has been developed and is useful as an additive to modify the viscoelastic properties of solvents. In addition, the network morphology of these organogelators could template new porous membrane materials. Future work will explore the gelation and polymerization of organic monomers, where after polymerization pores or polyelectrolyte threaded pores will be generated by removal of the organogelator or solely the surfactant side-chains, respectively. Success of this approach would provide a new method to fabricate ion-exchange membranes with concentrated percolating ionic domains. This approach has the advantage of potentially decoupling the pore morphology (sheets, cylindrical or spheres) from the ion concentration. Therefore, confined ionic channels would be obtained at much higher polyelectrolyte loading levels than in comparable block copolymer systems. In neutral-ionic block copolymers high polyelectrolyte block volume fractions invariably lead to an unconfined ionic matrix, which exhibit poor resistance to swelling and low solvent selectivity.

Impact of PRF Funding: These funds from the PRF have been instrumental in supporting the PI's research group at the start of his career. One graduate student (Yuqing Liu) has been supported for two years. Travel support from this project allowed him to present a poster and an oral presentation at the 2009 and 2010 spring National American Chemical Society meetings, respectively. Two papers have been published from this work in peer-review journals.