Low Frequency Mechanical Spectroscopy Study of Three Pyrrolidinium Based Ionic Liquids

Abstract

In this work we present our recent results on three ionic liquids (ILs), which share bis(trifluoromethanesulfonyl)imide (TFSI) as anion and have different pyrrolidinium based cations. By means of a combination of mechanical spectroscopy and thermal analysis, many of the physical processes occurring during cooling down from the liquid phase, can be studied. Depending both on the diverse cation and the different thermal history, crystallization from the melt or glass transition, cold-crystallization, solid-solid phase transitions and thermally activated processes are observed. In one of the ILs, which could be easily undercooled, a prominent thermally activated peak could be observed above the glass transition. The temperature dependence of the relaxation time is approximated by a Vogel-Fulcher-Tamman equation, as usual for fragile glass forming liquids, and the apparent activation energy of W = 0.36 eV with a pre-exponential factor of the relaxation time τ 0 = 1.7 · 10 −13 s were derived supposing jumps between asymmetrical potential wells. The kinetics of the crystallization processes have been studied in the framework of the Johnson-Mehl-Avrami-Kolmogorov theory and the Avrami parameters have been derived for both the crystallization from the melt and for the cold crystallization observed on heating.Keywords: Ionic liquids, dynamic mechanical analysis, differential scanning calorimetry W pracy przedstawiono najnowsze wyniki badań trzech cieczy jonowych (ILs), zbudowanych o anion bistriflimidowy (TFSI) i różne kationy pirolidyniowe. Za pomocą kombinacji spektroskopii mechanicznej i analizy termicznej można badać wiele procesów fizycznych występujących podczas schładzania z fazy ciekłej. W zależności od zróżnicowania kationów i różnic historii termicznej, można obserwować krystalizację cieczy lub zeszklenie, zimną krystalizację, przemiany fazowe w stanie stałym i procesy aktywowane termicznie. W jednej z ILs, którą można łatwo przechłodzić, obserwowano termicznie aktywowany pik powyżej temperatury zeszklenia. Zależność temperaturową czasu relaksacji aproksymowano równaniem Vogela-Fulchera-Tammana, co jest typowym podejściem stosowanym dla cieczy tworzących nietrwałe szkła, uzyskując pozorną energię aktywacji W = 0,36 eV i czas relaksacji τ 0 = 1.7 · 10 −13 s przy założeniu przeskoków pomiędzy asymetrycznymi studniami potencjału. Kinetykę procesu krystalizacji badano w ramach teorii Johnson-Mehla-Avramiego-Kołmogorowa a parametry Avramiego zostały uzyskane zarówno dla krystalizacji z cieczy, jak i zimnej krystalizacji obserwowanej przy ogrzewaniu.

A study of the conformers of the N,N-diethyl-N-methyl-N-propylammonium ion by means of infrared spectroscopy and DFT calculations

“…We already reported the comparison of DSC and DMA measurements both on pure ionic liquids and on an ionic liquid swelling a polymer membrane. We showed that DMA is a powerful tool to identify the temperatures of solidification and liquefaction of IL and to study the kinetics of these phase transitions [21,27]. In fact an increase of M is expected in correspondence with the solidification, while a strong decrease of M on heating indicates the liquefaction.…”

“…Dynamic mechanical analysis (DMA) measurements were conducted by means of a Perkin Elmer DMA8000 equipped with a liquid nitrogen cooling system and the material pocket accessory, in order to determine the solidification/liquefaction temperature [20,21]. The sample was spread into a stainless steel material pocket with dimensions of 30 mm by 14 mm.…”

“…2). Indeed, the elastic modulus, M, is extremely sensitive to the formation of a solid phase [21,27]. We already reported the comparison of DSC and DMA measurements both on pure ionic liquids and on an ionic liquid swelling a polymer membrane.…”

Relaxational Dynamics in the PYR14-IM14 Ionic Liquid by Mechanical Spectroscopy

“…Dynamic mechanical analysis was carried out using a PerkinElmer DMA 8000 instrument, following a procedure already used by us in previous works 2,7,8 : the sample, which is liquid at room temperature, was laid out into a stainless steel Material Pocket, supplied by PerkinElmer, with dimensions of 30.0 mm by 14.0 mm by 0.5 mm; the pocket, which is scored in the mid-point, was then folded in half and crimped closed to form a sandwich. Flexural vibration measurements were performed in the three-point bending configuration.…”

“…In particular, in pyrrolidinium based ILs, depending both on the ions composition of the material and the different thermal history, crystallization from the melt or glass transitions, cold-crystallization and solid-solid phase transitions were observed and the kinetics of the crystallization processes was studied in the framework of the Johnson-Mehl-Avrami-Kolmogorov theory 7 ; the Avrami parameters, namely the activation energy and the Avrami index, were derived by the measure of the modulus variation under isothermal and non-isothermal conditions for both the crystallization from the melt and the cold crystallization observed on heating, thus providing additional information with respect to the common calorimetric measurements 7 .…”