59th Annual Report on Research 2014

Ionic liquids continue to be a very vibrant area of research in organic, physical and polymer chemistry. The scientific and potential commercial impact of these materials has evolved from developing potential “green” solvent replacements to functional integration into polymeric materials. Polymerizable ionic liquids, in particular those containing the imidazolium group, have been found to exhibit enhanced thermal stability and ionic conductivities compared to analogous non-ionic materials and, as a result, many of these monomers have been incorporated into polymers. Applications of these polymers have included electroactive/conducting devices, gas separation and absorption membranes and even drug delivery systems for suppressing gene expression.

Since being awarded support from ACS-PRF in September of 2013, our group has been working in the area of triazolium-based ionic liquids and polymers. The triazolium cation exists in two isomeric forms (the 1,2,4- and 1,2,3-isomers), depending upon the substitution pattern of the three nitrogen atoms.  The initial attraction to these cations is the close relationship they have to the imidazolium ring, thus theoretically making many of the same synthetic functionalization strategies applicable.  Nitrogen-rich ionic liquids have gained a great deal of recent interest as energetic materials in applications such as propellants and explosives due to their thermal stability, low volatility, high densities and large, positive heats of formation.  The attraction to studying these materials in a polymeric framework is the combination of the high nitrogen content and large heats of formation due to the heterocyclic ring combined with the attractive properties of ionic liquids, including lower volatilities, higher densities and lower melting points. Progress on two PRF-supported projects is described in this narrative: (a) the incorporation of triazolium-containing monomers into polyester networks and (b) the development of fundamental structure-activity relationships, with a specific focus on physicochemical and thermal properties.

Triazolium-containing polyester networks

The incorporation of triazolium ionic liquids into polyester networks represents the main focus of our research group and of the ACS-PRF proposal.  In short, our goal is to prepare the 1,2,4- and 1,2,3-triazolium-based diacetoacetates 1 and 2 (shown below) and incorporate them into polyester networks using Michael addition polymerization techniques. Synthesis of these diacetoacetate monomers has progressed and the first monomer (structure 1) has been made and characterized (see scheme below).  In short, 1-(6-hydroxyhexyl)-1,2,4-triazole 3 and 6-bromohexanol 5 were prepared and acetoacetylated separately using tert-butylacetoacetate (tBAA).  The two acetoacetates 4 and 6 were then combined in a 1:1 ratio, neat, at 60 °C to afford the desired diacetoacetate 1,2,4-triazolium ionic liquid monomer 1.   

Reaction of 1,2,4-triazolium ionic liquid monomer 1 with 1,4-butanediol diacrylate was then completed under base-catalyzed conditions to generate our first triazolium-containing polyester network.  In order to achieve network formation, a 1.0:1.4 ratio of acetoacetate:acrylate was employed. A picture of the resulting flexible network is shown below along with a representation of the resulting network.  Current efforts are focused on performing thermal measurements of this network using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Once completed, 1,2,4-triazolium acetoacetate monomers which contain other counteranions such as nitrate [NO₃^-], tetrafluoroborate [BF₄^-] and bis(trifluoromethylsulfonyl)imide [Tf₂N^-] will be prepared, polymerized and analyzed in a similar fashion.

We have also completed a short study on the mechanical properties of several imidazolium-containing polyester networks. In June of 2014, two Murray State undergraduate chemistry majors accompanied the PI to Virginia Tech to perform mechanical testing using a dynamic mechanical analyzer (DMA).  The results showed that increasing the diacrylate concentration increased the crosslink density of the material, as reflected in an increased glass transition temperature (T_g) and rubbery modulus.  We also observed that use of weaker Lewis bases (such as [Tf₂N^-]) lead to higher rubbery moduli at elevated temperatures (> 100°C). These results serve as a baseline for the future testing of our triazolium-containing polyester networks described above. To date, three undergraduates have worked on this project, two of which were directly financially supported. 

Physicochemical and Thermal Properties of 1,2,4-Triazolium Ionic Liquids

In addition to the triazolium polyester networks described above, we have also been working on investigating the fundamental physicochemical properties and long-term thermal stabilities for a series of 1,2,4-triazolium ionic liquids 9.  As the ionic liquid library continues to grow, so does the need for discovering and analyzing new classes of materials. As a result, we recently published a study where temperature-dependent viscosities, densities and conductivities were determined for a series of 1-alkyl-4-methyl-1,2,4-triazolium [Tf₂N^-] ionic liquids.  In general, viscosities were found to increase with increasing alkyl chain length while densities and molar conductivities were found to decrease. Analysis of the ‘ionicity’ of the ionic liquids was determined using a Walden plot, which correlates molar conductivity and viscosity. The resulting linear relationship is then compared with an ‘ideal’ KCl line. Ionic liquids that exhibit properties near this ideal line are considered to be ‘good’, where the cations and anions are described as being in a loose, quasi-lattice arrangement. Our 1,2,4-triazolium ionic liquids were classified as ‘good’ ionic liquids as shown in the graph below, however they did not perform as well as the standard imidazolium ionic liquid [bmim][Tf₂N^-].

‘Isothermal’ TGA experiments have also been completed on these materials to more accurately portray the long-term stability of ionic liquids and to allow for an estimation of degradation rates at temperatures lower than the typical T_onset that is commonly reported from ‘temperature-ramped’ TGA experiments. In the isothermal TGA method, weight loss is monitored over a period of time at a constant temperature, yielding a linear correlation that follows pseudo-zeroth order kinetics. A sample comparison of the isotherms for 1-butyl-4-methyl-1,2,4-triazolium [Tf₂N^-] is shown below. Extrapolation of this data allows for determination of thermal stability at any temperature over any period of time.  The overall results indicated a general increase in long-term thermal stability as the alkyl chain length increased. To date, one undergraduate and one graduate (Masters-level) student have participated in this project, resulting in three research presentations at local and regional meetings and one peer-reviewed journal article.