The interplay between inter- and intra-molecular dynamics in a series of alkylcitrates

Abstract

The inter- and intra-molecular dynamics in a series of glass-forming alkylcitrates is studied by a combination of Broadband Dielectric Spectroscopy (BDS), Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR), Fourier-Transform Infrared (FTIR) spectroscopy and Differential Scanning Calorimetry (DSC). Analyzing the temperature dependencies of specific IR absorption bands in terms of their spectral position and the corresponding oscillator strengths enables one to unravel the intramolecular dynamics of specific molecular moieties and to compare them with the (primarily dielectrically) determined intermolecular dynamics. With decreasing temperature, the IR band positions of carbonyls (part of the core units) and H-bonded moieties of citrates show a red shift with a kink at the calorimetric glass transition temperature (Tg) while other moieties, whose dynamics are decoupled from those of the core units, exhibit a blue shift with nominal changes at Tg. The oscillator strength of all units in citrates depicts stronger temperature dependencies above Tg and in some, the ester linkage and H-bonded units show a change of slope at a temperature where structural and faster secondary relaxations merge. By that, a wealth of novel information is obtained proving the fundamental importance of intramolecular mobility in the process of glass formation, beyond coarse-grained descriptions.