Increased conductivity of polymerized ionic liquids through the use of a nonpolymerizable ionic liquid additive
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Martin Timusk, Kristjan Saal, Rünno Lõhmus, and Ilmar Kink University of Tartu, Institute of Physics and Estonian Nanotechnology Competence Center, 51014 Tartu, Estonia
Uno Mäeorg University of Tartu, Institute of Chemistry, 50411 Tartu, Estonia (Received 10 June 2013; accepted 20 September 2013)
In the present study, polymerizable ionic liquids (ILs), 1-[n-(methacryloyloxy)alkyl]-3methylimidazolium bromides (n 5 2, 6, 7, or 10), were synthesized in high yields. Moreover, the compounds obtained (n 5 6, 7, or 10) were used in the preparation of composite materials comprising a polymerized IL matrix and a nonpolymerizable IL additive, 1-ethyl-3methylimidazolium tetrafluoroborate ([EMIM][BF4]) in various proportions (up to 75% vol/vol of [EMIM][BF4]). The UV-radiation-initiated photopolymerization process was monitored in situ by measuring the resistivities of the mixtures. An increase in [EMIM][BF4] content in the composites led to an increase in the ionic conductivities of the materials while retaining their solid state at levels as high as 40% vol/vol of the [EMIM][BF4] content. The 40% vol/vol composites had conductivities of approximately 10 4 S/cm compared to the conductivities of 10 5 S/cm for the corresponding neat polymerized ILs. Above this [EMIM][BF4] content, the materials were sticky gels, and from 50% vol/vol onwards, entirely liquid.
I. INTRODUCTION
Ionic liquids (ILs) are a diverse group of salts that are liquid at ambient temperatures.1 The cation of the salt is generally an organic structure with a low symmetry, and the anion can be either organic or inorganic. ILs are the focus of intense research because of their extraordinary chemical and physical properties, such as negligible vapor pressure, thermal stability, and nonflammability2,3 combined with a high ionic conductivity (0.1 to 18 10 3 S/cm) and a wide electrochemical window (2–6 V).4 ILs also have diverse solvating properties. For example, ILs can disperse carbon nanotubes5 and dissolve biopolymers, such as cellulose.6 Numerous applications are encountered for ILs in different fields, such as organic synthesis,7 analytical chemistry,8 and material science.9 However, regardless of their unique properties, the exploitation of ILs can be complicated in certain applications because of their liquid state. The problems relate to the possible leakage of the liquid or difficulties in obtaining or maintaining the liquid in a predefined physical shape. These drawbacks can be overcome by applying methods for “solidifying” ILs while keeping their specific liquid state properties, which is a challenging task. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.330 3086
J. Mater. Res., Vol. 28, No. 22, Nov 28, 2013
http://journals.cambridge.org
Downloaded: 13 Mar 2015
To apply ILs in the solid state, different polymer gels having IL encapsulated into a polymeric three-dimensional network have been widely investigated.10 Composites of polymers and ILs are found to be versatile platforms for immobilizing ILs bec
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