Robust hydroxide conducting membranes are required for long-lasting, low-cost solid alkaline fuel cells (AFCs). In this study, we synthesize Nafion-based anion exchange membranes (AEMs) via amination of the Nafion precursor membrane with 1,4-dimethylpiperazine. This initial reaction produces an AEM with covalently attached dimethylpiperazinium cations neutralized with fluoride anions, while a subsequent ion exchange reaction produces a hydroxide ion conducting membrane. These AEMs possess high thermal stability and different thermal transition temperatures compared to Nafion, while small-angle X-ray scattering reveals a similar ionic morphology. The hydroxide ion conductivity of the Nafionbased AEM is fivefold lower than the proton conductivity of Nafion at 80 C and 90% relative humidity. More importantly, the hydroxide conductivity is insensitive to drying and rehydrating the membrane, which is atypical of other AEMs with quaternary ammonium cations. The high chemical and thermal stability of this hydroxide conducting Nafion-based AEM provides a promising alternative for AFCs. V C 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 50: [552][553][554][555][556][557][558][559][560][561][562] 2012 Additional AEM properties of importance for practical AFC operation are hydroxide conductivity and mechanical Additional Supporting Information may be found in the online version of this article.
Small-angle neutron scattering (SANS) measurements of syndiotactic s-PMMA polymers mixed with weakly attractive 1.0 nm diameter polyhedral oligomeric silsesquioxane (POSS) nanoparticles (NPs) show no observable changes in the chain radius of gyration R g, regardless of the polymer molecular weights, the amount of residual solvent, or the POSS NP loading and dispersion (from 0 to 20 vol %). In retrospect, these results are not surprising since scaling arguments imply that chain size in the concentrated region of the phase diagram of a polymer solution is ideal and independent of the polymer volume fraction ϕ, and only as the semidilute region is entered with decreasing concentration does the chain size for a good solvent begin to increase due to polymer excluded volume and then scales with concentration as ϕ–1/8. For typical polymer nanocomposites the NP concentrations are less than 50% v/v, so the polymers are still generally within the concentrated regions of their phase diagrams, where ideal chain conformations are observed for small molecule solvents. By combining the present results with previous results from the literature, we conclude that spherical NPs apparently have little effect on the conformations of polymer chains, especially in typical polymer nanocomposites that only incorporate moderate amounts of NPs.
Highly stable hydroxide conducting membranes are necessary for solid-state alkaline fuel cells to have long performance lifetimes. In this study, we used solid-state chemistry to synthesize Nafion-based anion exchange membranes (AEMs) with a variety of covalently attached cations, including trimethylammonium, trimethylphosphonium, piperazinium, pyrrolidinium, pyridinium, and quaternized 1,4-diazabicyclo[2.2.2]octane. Infrared spectroscopy confirms a partial asymmetric functionalization of all cations with the exception of pyridinium. The AEMs that were successfully synthesized all exhibited sufficient water uptake and conductivity. The effect of cation type on AEM chemical and thermal stability was investigated as a function of various conditions (e.g., hydration levels, temperature, and pH). High chemical and thermal stability was observed for all successfully synthesized AEMs with the exception of the trimethylphosphonium cation AEM.
Friction-induced shear stress and wear of poly [tetrafluoroethylene-co-(perfluoroalkyl vinyl ether)], also known as perfluoroalkoxy polymer (PFA), causes microstructural and chemical changes. These changes are essential to understand PFA as a tribological material. Tribological (friction and wear) experimentation coupled with differential scanning calorimetry, X-ray diffraction, and infrared spectroscopy was used to characterize the microstructure and chemical differences in PFA wear debris versus bulk. PFA wear debris transitioned from triclinic to hexagonal crystalline structure at lower temperatures and had increased crystallinity versus bulk PFA. Shear-driven molecular alignment was the likely cause of these microstructural changes. Reduced molecular weight of PFA wear debris was confirmed by the presence of free carboxylic acids. Reduced molecular weight is likely due to shear-driven chain scission of the PFA backbone within the amorphous region of the microstructure. Reduced molecular weight and increased crystallinity are observed in wear of PTFE and support the tribochemically driven mechanism for ultralow wear fluoropolymer−alumina composites.
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