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    a Viscoelastic Properties of Polyelectrolyte Multilayers Swollen with Ionic Liquid Solutions

    Year: 2019

    Journal: Polymers, Volume 11, AUG

    Authors: Parveen, Nagma; Jana, Pritam Kumar; Schoenhoff, Monika

    Keywords: Kelvin-Voigt modeling; polyelectrolyte multilayers; ionic liquid; quartz crystal microbalance with dissipation; thickness; viscoelasticity; concentration regimes; swelling

    Polyelectrolyte multilayers (PEM) obtained by layer-by-layer assembly can be doped with ionic liquid (IL) via the swelling of the films with IL solutions. In order to examine the mechanical properties of IL-containing PEM, we implement a Kelvin-Voigt model to obtain thickness, viscosity and elastic modulus from the frequency and dissipation shifts determined by a dissipative quartz crystal microbalance (QCM-D). We analyze the changes in the modeled thickness and viscoelasticity of PEI(PSS/PADMAC)(4)PSS and PEI(PSS/PAH)(4)PSS multilayers upon swelling by increasing the concentration of either 1-Ethyl-3-methylimidazolium chloride or 1-Hexyl-3-methylimidazolium chloride, which are water soluble ILs. The results show that the thickness of the multilayers changes monotonically up to a certain IL concentration, whereas the viscosity and elasticity change in a non-monotonic fashion with an increasing IL concentration. The changes in the modeled parameters can be divided into three concentration regimes of IL, a behavior specific to ILs (organic salts), which does not occur with swelling by simple inorganic salts such as NaCl. The existence of the regimes is attributed to a competition of the hydrophobic interactions of large hydrophobic ions, which enhance the layer stability at a low salt content, with the electrostatic screening, which dominates at a higher salt content and causes a film softening.
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