Effect of Cation n-Alkyl Side-Chain Length, Temperature, and Pressure on the Glass-Transition Dynamics and Crystallization Tendency of the [CnC1Pyrr]+[Tf2N]- Ionic Liquid Family

This work focuses on the influence of nonpolar side-chain length on the glass-transition dynamics and crystallization behavior of a family of nonaromatic pyrrolidinium-based ionic liquids bearing the bis(trifluoromethanesulfonyl)imide counterion, [C(n)MPyrr](+)[Tf2N](-), where n varies from 4 to 8. A combination of different experimental techniques, including differential scanning calorimetry and shear-mechanical and dielectric spectroscopy, enabled us to investigate the fundamental relationship between charge transport and structural relaxation as well as its impact on the dynamic properties of the investigated samples on approaching the glass transition. In contrast to recent experimental findings reported for imidazolium-based analogues, we demonstrate no evidence of the slow supramolecular mode in the rheological and dielectric responses of the studied pyrrolidinium-cation-based ionic liquids. Finally, we studied the crystallization tendencies for each sample under both ambient and elevated pressures and further compared behaviors of the samples with good glass-forming ability under isochronal conditions, characterized by the same conductivity relaxation time,-rm. Pressure seems to not exert a pronounced impact on the overall crystallization rate for the tested samples. Obtained results so as to the proposed methodology may shed new light on the properties affecting the physical stability of ionic liquids at various thermodynamic conditions, which is a key prerequisite for their rational and efficient applications in different sectors of technology.