Abstract
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)4PSS and PEI(PSS/PAH)4PSS 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.
Highlights
Polyelectrolyte multilayers (PEMs) are thin films formed upon the adsorption of oppositely charged polyelectrolytes in a layer-by-layer (LbL) fashion [1]
We have shown that PEMs can be swelled with aqueous ionic liquid (IL) solutions, the swelling is controlled by the IL’s hydrophobicity [22], and the corresponding IL uptake can be controlled by stimulating a charge-excess in PEMs [18]
At the concentrations of the IL and salt solutions employed for swelling, the salt affects the liquid viscosity rather strongly, which has to be taken into account in the Kelvin-Voigt model
Summary
Polyelectrolyte multilayers (PEMs) are thin films formed upon the adsorption of oppositely charged polyelectrolytes in a layer-by-layer (LbL) fashion [1]. The thickness and mechanical strength of PEMs can be varied within a wide range, and typically vary between nm to several μm and between a few kPa to several MPa [2,3,4,5,6], respectively This is possible by varying the number of deposited polyelectrolyte layers and/or intrinsic properties of polyelectrolytes, such as the charge density, molecular weight and structure [7,8,9,10,11,12,13]. Such tunable physical properties of PEMs make them suitable candidates for surface coatings [23], biological substrates [24,25] and electrochemical separators [26,27,28] For the latter type of application, PEMs combined with a suitable electrolyte solvent can tremendously improve the transport of ions through the multilayer. Several studies have reported the potential of ILs in battery systems
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