Swelling and Thermal Transitions of Polyelectrolyte Multilayers in the Presence of Divalent Ions
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2016 American Chemical Society. The effects of CaCl2, MgCl2, and Na2SO4 on the swelling response and the thermal transition of poly(diallyldimethylammonium chloride)/poly(styrenesulfonate) (PDAC/PSS) layer-by-layer (LbL) assemblies are investigated. For MgCl2 and CaCl2, two swelling regimes are observed depending on the salt concentration of the overlying solution. Below 0.17 M MgCl2 or CaCl2, swelling appears to arise from electrostatic repulsion within the multilayer. Above 0.17 M MgCl2 or CaCl2, the multilayer swells linearly with the concentration of the overlying solution due to doping; the degree of swelling in this linear regime depends on the hydration shell of the doping ions. Alternatively, swelling with no concentration dependence was observed for Na2SO4. Unlike the swelling behavior, the thermal transition temperature observed in hydrated films shifts to higher temperatures with increasing concentration of Na2SO4. However, when hydrated with CaCl2 or MgCl2 solutions, the transition remains relatively constant within error. Results are discussed in terms of the salt ion's hydration shell and Donnan exclusion.