A Dielectric and Vibrational Spectroscopy Study of the Confinement Effects on Ion Dynamics in a Methacrylate Based Polymerized Ionic Liquid within Nanoporous Silica Membranes

Abstract

In this study, the structures and dynamics of a low molecular weight ionomer confined at length-scales below which solvation layers have been shown to form for ionic liquids within nanoporous silica are investigated using broadband dielectric spectroscopy, Raman spectroscopy and 1H NMR. Silica membranes having 7nm unidirectional pores were prepared using well-defined electrochemical etching procedures in a 1:1 volume fraction ethanol:hydrofluoric acid electrolyte solution. In situ free radical polymerization of the low molecular weight ionic liquid polymer, poly-[(methacryloyloxy)ethyl]dimethylammonium bis[(trifluoromethyl)-sulfonyl]imide, was performed monitored with Raman spectroscopy to observe changes in ion coordination and anion conformational fluctuations with monomer conversion in bulk and within nanoporous silica. The free radical polymerization reaction kinetics were observed to be increased under confinement compared to bulk as previously reported, and resulted in a 5kg/mol polymer with narrow molecular weight distribution. From the dielectric spectra, the dc conductivities of the polymer under confinement were determined to be increased compared to the polymer in the bulk. Using a simple model accounting for porosity within the membranes, a calculated adsorbed layer at the interface of the pore wall with the ionic liquid was determined to decrease with polymerization, effectively counteracting the effects of immobilization of ions within the polymer backbone. Furthermore, the possible changes in free volume under confinement and with polymerization are discussed through detailed observation of the Raman spectra regarding anion conformational changes. These changes in free volume are shown to correlate to increased ion dynamics. The ion dynamics in these materials are further described in terms of the current understanding of interfacial electrostatic interactions of ionic liquids with solid surfaces and in conjunction with the effects of polymer dynamics under confinement.