NMR Relaxometry Accessing the Relaxation Spectrum in Molecular Glass Formers

Area of Science:

• Condensed Matter Physics
• Chemical Physics
• Materials Science

Background:

• The molecular dynamics governing the glass transition in liquids remain debated, with ongoing discussion on universality versus specificity.
• Traditional methods like dielectric spectroscopy and light scattering have limitations in characterizing all molecular motions.

Purpose of the Study:

• To investigate the universality and specificity of molecular dynamics during the glass transition using advanced NMR techniques.
• To compare NMR findings with results from other spectroscopic methods and elucidate differences in observed relaxation dynamics.

Main Methods:

• Field-cycling nuclear magnetic resonance (NMR) relaxometry to probe both translational and rotational molecular motions.
• Isotope-specific NMR studies to selectively analyze molecular entities or components in liquid mixtures.
• Comparison of NMR-derived dynamical susceptibilities with data from dielectric spectroscopy and light scattering.

Main Results:

• NMR confirms the excess wing as a generic feature of liquids nearing the glass transition, though its prominence varies with method sensitivity.
• Translation is more retarded than rotation in hydrogen-bonded liquids, distinct from the Debye process in alcohols.
• NMR dynamical susceptibilities for structural (α) relaxation often align with light scattering, differing from dielectric spectra of polar liquids.
• The Cole-Davidson shape of α-relaxation is observed in ionic liquids and salt solutions, with varying width parameters.
• Nanoscopic confinement and dynamical disparity in mixtures broaden the low-frequency flank of the α-relaxation.

Conclusions:

• NMR relaxometry is a powerful, versatile tool for studying molecular dynamics in glass-forming liquids, complementing other techniques.
• NMR's isotope specificity allows detailed analysis of complex systems, including mixtures and confined liquids.
• The study highlights both universal relaxation features and system-specific dynamics, advancing the understanding of the glass transition.