Study of multiple relaxations in crown ethers by dielectric relaxation spectroscopy

An investigation was carried out on the dipole dynamics of low molecular weight crown ether series and related compounds by dielectric relaxation spectroscopy (DRS). An unexpected discovery regarding two main relaxations observed by DRS and calorimetric (DSC) measurements have been analyzed using neat and solvent-present formulations over a wide range of frequency and temperature. The presence of nonpolar, nonreactive, fully miscible solvent (o-terphenyl, OTP) produced a marked opposite effect on the apparent activation energy of these two relaxations. Interestingly, the location of the lower of these two transitions in the temperature domain was found to be practically unaffected by the presence of OTP, while a higher temperature relaxation changes its location systematically as a function of solvent concentration. To explore the molecular origin of these findings, a large number of cyclic compounds of similar structure have been analyzed by DRS, DSC, and molecular simulation. As a result of this effort, a conformational model that accounts for multiple relaxations observed in this family of materials is introduced.