Effect of single-walled carbon nanotubes on the transport properties of sulfonated poly(styrene-isobutylene-styrene) membranes
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Transport properties of polymer nanocomposite membranes (PNM) of sulfonated poly(styrene–isobutylene–styrene) (SIBS) were determined as a function of sulfonation level (0–89.7%), single-walled carbon nanotubes (SWCNT) loading (0.01, 0.1 and 1.0 wt%), and chemical functionalization (carboxylic acid, aminomethanesulfonic acid and p-aminobenzoic acid) for direct methanol fuel cells and chemical and biological protective clothing applications. Transport properties increased with sulfonation level, as the ionic domains interconnect; however, at very high sulfonation levels a morphological transition to a more complex structure caused a decrease in transport properties. Proton conductivity increased with water content, but was sensitive to the type of water present (bound vs. bulk), especially in the SWCNT. Water bound to the ionic domains had the most significant effect on the proton conductivity. Methanol permeability was very sensitive to the incorporation of SWCNT addition, since its transport mechanism seems more dependent on free-volume; increasing SWCNT loading reduced the methanol permeability further. The selectivity (i.e., proton conductivity/methanol permeability) of the studied membranes was determined and compared to SIBS and Nafion® 117, suggesting an optimum SWCNT loading (0.1 wt%) and functionalization (with terminal sulfonic groups), to balance the improvement in proton conductivity while reducing the methanol permeability. The selectivity of the membranes for vapor permeation of dimethyl methylphosphonate (DMMP) and water was best (up to 8.47) for the highest sulfonated membrane (SIBS 89.7) with the highest SWCNT loading (1.0 wt%) and the SWCNT functionalized with terminal sulfonic groups.