Structural Transitions During Formation and Rehydration of Proton Conducting Polymeric Membranes
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Abstract Knowledge of the transitions occurring during the formation of ion‐conducting polymer films and membranes is crucial to optimize material performances. The use of non‐destructive scattering techniques that offer high spatio‐temporal resolution is essential to investigating such structural transitions, especially when combined with complementary techniques probing at different time and spatial scales. Here, a simultaneous multi‐technique study is performed on the membrane formation mechanism and the subsequent hydration of two ion‐conducting polymers, the well‐known commercial Nafion and a synthesized sulfonated poly(phenylene sulfide sulfone) (sPSS). The X‐ray data distinguish the multi‐stage processes occurring during drying. A sol‐gel‐membrane transition sequence is observed for both polymers. However, while Nafion membrane evolves from a micellar solution through the formation of a phase‐separated gel, forming an oriented supported membrane, sPSS membrane evolves from a solution of dispersed polyelectrolyte chains via formation of an inhomogeneous gel, showing assembly and ionic phase separation only at the end of the drying process. Impedance spectroscopy data confirm the occurrence of the sol‐gel transitions, while gel‐membrane transitions are detected by optical reflectance data. The simultaneous multi‐technique approach presented here can connect the nanoscale to the macroscopic behavior, unraveling information essential to optimize membrane formation of different ion‐conducting polymers.Keywords:
Nafion
Small-angle X-ray scattering
Small-angle X-ray scattering
Characterization
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1 Physicochemical aspects of multilayered polyelectrolyte films Multilayered polyelectrolyte films are obtained by alternated depositions on a solid surface of polyanions and polycations~([1]). The driving force for the build-up results from the charge excess which appears on the top of the film after each new polyelectrolyte adsorption. The film top becomes thus positively (respectively negatively) charged when the film is ending by a polycation (respectively polyanion). Various polyelectrolyte systems hav...
Polyelectrolyte adsorption
Biomaterial
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The complexation of a charged hard sphere (macroion) with short and oppositely charged polyelectrolytes was studied by the use of a simple model system. Structural data of the macroion–polyelectrolyte complex and the surrounding solution were obtained by the use of Monte Carlo simulation technique. The number of polyelectrolyte chains as well as the linear charge density and the flexibility of the polyelectrolyte were varied, whereas the properties of the macroion were held constant. In all cases, the macroion and a polyelectrolyte carried an equal number of elementary charges. The simulations showed that for flexible and highly charged polyelectrolytes, a tight complex containing the macroion and one polyelectrolyte was formed independent of the number of polyelectrolytes present. However, for flexible but lower charged polyelectrolytes, a looser macroion–polyelectrolyte complex was established, and a complex involving two polyelectrolytes occurred when two or more polyelectrolytes were available. Finally, for the most rigid polyelectrolyte no complex was formed at all. Generally, an intramolecular contraction of the polyelectrolyte occurred upon complexation with the macroion.
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We report the observation of sharp volume phase transition in the non‐ionic N‐isopropylacrylamide gel. The finding demonstrates that the phase transition of gels may be general and not confined to ionic gels as was previously considered. It will allow us to separate the electrostatic effects and network‐structural effects on the phase transition.
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Monolayers of polyelectrolyte complexes are generated at solid surfaces using covalently attached monolayers of weak polyelectrolyte molecules ("polyelectrolyte brushes") exposed to aqueous solutions of oppositely charged polyelectrolytes. The adsorption of the charged polymers onto the brush-coated substrate is measured as a function of deposition time, pH value, salt concentration, and concentration of the polymer solution as well as the thickness of the surface-attached monolayer. It is shown that the adsorbed amount depends critically on the pH value of the solution and shows a pronounced maximum at intermediate salt concentrations. Upon exposure to an oppositely charged polyelectrolyte solution, the brush adsorbs large amounts of the polyelectrolyte and form thick layers in the same range as the thickness of the brush layer. Additionally, the thus-obtained layers are used as substrates for the formation of polyelectrolyte multilayers following a standard layer-by-layer approach. It is shown that the total film thickness scales linearly with the number of dipping cycles and that even the outermost layer is strongly correlated to the thickness of the brush layer which is used as the substrate.
Polyelectrolyte adsorption
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Polyelectrolyte materials exhibit unique properties and important applications in industry,due to their having the characteristic structure of polymer chain and electrolyte.However,polyelectrolyte would inevitably interact with other substances during their applications,which has remarkable influences on the properties of polyelectrolyte materials.This paper introduced the progress in studies on the interactions between polyelectrolyte and salt,surfactant or opposite charged polyelectrolyte in solution.Moreover,the influences of several factors,such as polyelectrolyte structure,charge density,hydrophobic property and concentration,on the interaction in polyelectrolyte complex systems were discussed.
Polyelectrolyte adsorption
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Nafion{sup Ri}s perfluorosulfonated ionomer membrane that is used as a solid polymer electrolyte in H{sub 2}/O{sub 2} fuel cells due to its excellent chemical stability and high ionic conductivity. This paper describes experiments designed to probe the morphology of water-swollen and air-dried Nafion{sup Rm}embranes using small angle X-ray scattering and fitting the resulting data to an interparticle model. For air-dried Nafion membranes the average diameter of ionic clusters was found to be 2.24 nm ; for wet Nafion membranes of the -SO{sub 3}H groups the average ionic cluster diameter was 4.10 nm. The minimum and maximum distances of separation between the surfaces of two adjacent hydrated clusters were 0.30 nm and 0.88 nm, respectively. These distances may be bridged by single hydrated ionic sites, but since there are several ionic sites per ionic core that reside outside the core, it is suggested that the connection between ionic cores most likely occurs via multiple single ion sites. 40 refs., 3 tabs., 4 figs.
Nafion
Ionomer
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Results of luminescence probe studies of solutions of the polyelectrolytes obtained upon dissolving Nafion are reported. The primary objective of this work was to study the nature of the interactions between these anionic polyelectrolytes and their counterions. Both 1100 and 1200 equivalent weight Nafion were studied; solutions of these Nafions were prepared using a procedure developed in this laboratory. These luminescence probe studies have shown that strong binding to the Nafion polyelectrolytes occurs when both hydrophobic and electrostatic interactions are possible. Hence, Nafion preferentially binds hydrophobic cations such as , methylviologen, and auramine O over simple inorganic cations such as H+, Na+, and Mg2+. The implications of this unusual ion exchange selectivity to transport properties of Nafion films and membranes are discussed.
Nafion
Polyoxometalate
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Layer by layer
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