(603g) Highly Proton-Conductive Polyelectrolyte Membranes with Supramolecularly Suppressed Water Swelling
Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556-5637
Polyelectrolyte membranes based on sulfonated multiblock copolymers with alternating hydrophilic and hydrophobic blocks hold great potential as alternatives to benchmark Nafion®for fuel cell applications, due to their much better proton conductivity at low relative humidity (RH) levels and thermal stability. Previous studies suggest that long hydrophilic sequences (>10,000 g/mol) are necessary to effect sharp nanophase separation and form continuous proton conducting nanochannels enabling high proton conductivity at low RH, which, however, is always accompanied with excessive water swelling resulting in deterioration of dimensional stability and mechanical robustness of the membranes.
In this study, we report an innovative supramolecular strategy to suppress excessive water swelling by introducing triptycene structure units into the hydrophobic sequences of multiblock copolymers, which induce strong supramolecular interactions with the hydrophilic sequences via the so-called chain threading and interlocking effectively constraining the polyelectrolyte membranes from excessive swelling. Specifically, a series of triptycene-containing poly(arylene ether sulfone) multiblock copolymers were synthesized via a coupling reaction between phenoxide-terminated hydrophilic oligomers (BPS100) and fluorine terminated triptycene-containing hydrophobic oligomers (TRP0). The molecular weight (ranging from 5,000 to 20,000 g/mol) and end group functionality of oligomers were precisely controlled by adjusting the stoichiometric feeding ratio of monomers. Additionally, the ion exchange capacity (IEC) of the multiblock copolymers was controlled by coupling oligomers with even and uneven block lengths. The resulting multiblock copolymers yielded tough and ductile membranes via solution casting. In general, the multiblock copolymer membranes showed increasing water uptakes and proton conductivities with increasing block length. Anisotropic swelling behavior was observed for all the copolymer membranes suggesting nanophase separated lamellae-like membrane morphology. Most importantly, for copolymers with long hydrophilic sequences (> 10,000 g/mol), despite of their expectedly high water uptake, they all showed greatly reduced swelling ratios as compared to Nafion® and reported nontriptycene-containing multiblock copolymers. This observation indicates that the supramolecular chain threading and interlocking interactions induced by the triptycene units in the hydrophobic sequences are instrumental in suppressing membrane swelling and improving dimensional stability. For example, the multiblock copolymer with hydrophilic block length of 15,000 g/mol had a water uptake of 107%, an excellent proton conductivity of 0.155 S/cm in R.T. liquid water (almost 3 times that of a Nafion® 212) and a volume swelling ratio of just 27% (more than 65% reduction when compared to Nafion® 212 and previously reported multiblock copolymer systems). In addition, a super high liquid water proton conductivity (R.T.) of 0.224 S/cm was achieved by a multiblock copolymer with uneven block length (i.e., 10,000 g/mol of BPSH100 and 7,000 g/mol of TRP0). These multiblock copolymer membranes hold tremendous potential for applications in proton exchange membrane fuel cells and water desalination membranes.