The relaxation processes and boson peak (BP) behavior of two ammonium-based ionic liquids (ILs) were investigated by means of Incoherent Quasi-Elastic and Inelastic Neutron Scattering techniques (QENS and INS) at different temperatures (180 K – 300 K) and wave vector transfers (0.3 A-1 – 2.0 A-1), aiming to investigate the spatially heterogeneously dynamic contribution to the glass and melting transitions, and their relationship with the excess of vibrational density of states compared to the Debye’s prediction (e-VDoS) at low energies. The samples, tributylmethylammonium bis(trifluoromethylsulfonyl)imide (N1444TFSI) and trimethylbutylammonium bis(trifluoromethylsulfonyl) (N1114TFSI) were studied under heating from the glass to the liquid state after an almost instantaneous cooling to 180 K. Both ILs experienced cold crystallizations. Figure 1 presents the difference between the INS of N1114TFSI glass and crystal at 180 K and 210 K, respectively. We used the Kohlrausch−Williams−Watts (KWW) stretched exponential to probe the relaxation time and the Damped Harmonic Oscillator (DHO) function to analyze the BP and crystal vibrational modes. The BP parameters corroborated with previous studies by Raman spectroscopy1. The BP intensity increases and the BP energy decreases by increasing the temperature. We obtained the relaxation times of the glassy, crystal and supercooled liquid states (between 1 ns and 100 ps) and to the liquid states (from 20 ps to 1 ps). The results were compared with previous structural analyses using X-ray diffraction (XRD). Besides the diffusion process of the liquid state, our study correlates the relaxation processes with the spatial heterogeneity and provides direct evidence on the structural origins of the slow relaxations in ionic glass.

Quasi-elastic and inelastic neutron scattering studies of glass-forming ammonium-based ionic liquids at low temperatures

255th National Meeting (2018)

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