60th Annual Report on Research 2015

Of all the PILs that have been researched, those containing the imidazolium cation have provided the greatest utility insofar as their synthetic versatility is concerned.  In many cases, imidazolium-containing polymers have been found to exhibit improved electrochemical and thermal properties when compared to non-ionic analogs. Despite these advancements, there exists a need to provide more flexibility with regards to specific application requirements by exploring other cation motifs. As a result, 1,2,3- and 1,2,4-triazolium-containing PILs have been targeted by our research group as an area of interest. Progress on two ACS PRF-supported projects is described in this narrative: (a) the synthesis, polymerization and characterization of triazolium-containing PILs and, as a secondary focus, (b) the development of fundamental structure-activity relationships of triazolium-containing ionic liquids.

Ionic Liquids in Polymer Design and Function

The main thrust of this ACS-PRF grant is to synthesize a series of triazolium-containing polyester networks and investigate their thermal, mechanical and conductive properties.  Towards this end, the PI and his group have prepared a series of 1,2,3- and 1,2,4-triazolium-containing acetoacetate monomers and incorporated them into a covalently crosslinked polyester network through the use of Michael addition chemistry.  As a part of this study, three structural features were varied:  (a) the cation (imidazolium, 1,2,3-triazolium, 1,2,4-triazolium), (b) the acrylate:acetoacetate ratio (to control crosslink density) and (c) the counteranion.  Each polymer film was characterized thermally by differential scanning calorimetry (DSC) in order to determine the glass transition temperature (T_g) and thermogravimetric analysis (TGA) in order to determine thermal stability.  It was determined that a rise in the T_g occurred with an increase in crosslink density but the T_g was found to decrease by employing larger, non-coordinating counteranions such as triflate [OTf] and bis(trifluoromethylsulfonyl)imide [NTf₂].  Changing the cation did not significantly influence the T_g or thermal stability of the polymer networks.

With the recent acquisition (fall 2014) of a dynamic mechanical analyzer (DMA), PI Miller and his students have been able to further characterize these networks for their mechanical properties.  In general, the rubbery plateau modulus increased with increasing crosslink density but decreased with the use of larger, bulkier counteranions such as [OTf] and [NTf₂]. Ultimately, we have found that the thermal and mechanical properties of these networks can be easily tailored by varying the monomer ratio (crosslink density) and/or the counteranion.

The conductivity of three of the films with variable cation have also been determined using a 4-electrode, in-plane conductivity cell (constants held:  1.4:1.0 acrylate:acetoacetate ration and [NTf₂] counternation).  We found that the 1,2,3-triazolium- and imidazolium-containing polyester networks performed analogously and both were superior to the 1,2,4-triazolium-containing system.  Work continues on trying to improve upon the conductivity of these films by further manipulating the polymer backbone as well as inclusion of free ionic liquid into the film (i.e. formation of a gel polymer electrolytes). To date, two undergraduates and one Masters student have worked on this portion of the grant, resulting in one peer-reviewed publication.

Triazolium Ionic Liquids:  A Fundamental Study

In addition to the polymeric studies above, we have also been investigating ‘fundamental’ dialkyl 1,2,4- and 1,2,3-triazolium ionic liquids in an attempt to create a head-to-head comparison with analogous imidazolium ILs published in the literature.  Towards this end, the alkyl chain length and counteranion were varied for a series of 1,2,3- and 1,2,4-triazolium ionic liquids.  Temperature-dependent physicochemical properties (density, viscosity, conductivity) were determined and a Walden plot analysis was conducted in order to determine the ‘ionicity’ for each IL (ionicity refers to the ability of a material to conduct an electrical current).  In a head-to-head comparison, given the same alkyl chain length and counteranion [NTf₂], it was discovered that the 1,2,3-triazolium system performed analogously to reported imidazolium IL  while the 1,2,4-triazolium IL did not perform as well, presumably due to the increased Lewis acidity of the ring.  Ultimately, an ionic liquid has a high degree of ionicity the closer it is to the “ideal” dilute KCl line (dashed line).

Thermal properties and stabilities were also determined for each of the 1,2,4- and 1,2,3-triazolium ILs studied. Differential scanning calorimetry (DSC) indicated that the T_g/T_m increased with increasing alkyl chain length (increased van der Waals interactions) but decreased when larger, non-coordinating anions were employed. Thermogravimetric analysis (TGA) was used to determine relative, short-term (temperature-ramped studies) and long-term (isothermal studies) stabilities of each IL.  The use of longer alkyl chain lengths and/or larger, non-coordinating anions resulted in higher thermal stabilities and higher activation energies for thermal decomposition (E_a).  1,2,3-Triazolium ILs were more thermally stable than the analogous 1,2,4-triazolium ILs, presumably due to the increased Lewis acidity of the 1,2,4-triazolium ring.  To date, three undergraduates have contributed to this part of the grant, resulting in two peer-reviewed publications.