Fabricating robust nanoporous materials and polymer electrolyte membranes from reactive block copolymers by metathesis reactions.

Abstract

Polymer membranes have been studied for several decades, and conventional utility of these membranes has been realized over a wide range of applications such as water purification, gas separation, and fuel cells. Separation performance of polymer membranes for these applications can be evaluated from different aspects including selectivity, material cost, and thermal and mechanical stabilities. This research focused on the design of crosslinked polymer membranes from reactive block copolymers, which could possess some attractive features such as controlled functionality and morphology. Novel block copolymers containing a chemically crosslinkable block and a chemically modifiable block were of particular interest. Metathesis reactions were employed to crosslink these copolymers with additional tough polymers such as polycyclooctene (PCOE) and polydicyclopentadiene (polyDCPD), to realize enhanced toughness in the resulting materials. First, a norbornene-functionalized polystyrene-polylactide (PNS-PLA) block copolymer was synthesized. A self-assembled blend containing this copolymer and DCPD was cured by metathesis reactions, and nanoporous monoliths with a cylindrical morphology were successfully produced after removing the PLA component. These monoliths exhibited pronounced mechanical and thermal stabilities superior to nanoporous polystyrene (PS). Second, polymerization induced phase separation (PIPS) during the ring-opening metathesis polymerization (ROMP) of DCPD in the presence of the PNS-PLA copolymer rendered continuous PLA nanodomains in a crosslinked PNS/polyDCPD matrix. Upon etching the PLA component, the resultant nanoporous membranes exhibited well-defined percolated nanopores and good thermal and mechanical stabilities. Preliminary diffusion measurements demonstrated potential utility of such membranes in ultrafiltration. Third, crosslinked polymer electrolyte membranes (PEMs) were fabricated from a PNS-poly(n-propyl-p-styrenesulfonate) (PSSP) block polymer and COE/DCPD via the PIPS scheme followed by deprotection of the PSSP block. These PEMs possessed a bicontinuous morphology and mechanical and thermal robustness. Select PEMs exhibited high proton conductivity similar to Nafion at high humidity and reduced methanol crossover. Tunable domain size and mechanical strength of the resulting PEMs are advantageous attributes of this preparation protocol. Additionally, the use of such PEMs for NH3 separation from gas mixtures was demonstrated. The appendix chapter represented our efforts to produce amine-functionalized polymer membranes from PNS-poly(dimethylaminoethylmethacrylate) (PNS-PDMAEMA) and COE by metathesis reactions, potentially useful for CO2 separation from gas mixtures.