Molecular Mobility in Glass Forming Fananserine: A Dielectric, NMR, and TMDSC Investigation

Abstract

This study was conducted to characterize the molecular mobility of supercooled fananserine and derive from this analysis the non-Arrhenius and nonexponential properties of the primary alpha-relaxation. The use of three investigation techniques of the molecular mobility, namely, dielectric relaxation, modulated differential scanning calorimetry, and proton nuclear magnetic resonance, allowed us to describe the dynamic properties of supercooled fananserine on a wide range of frequencies and temperatures, ranging from the melting temperature T(m) = 372 K down to the glass transition temperature T(g) = 292 K. We emphasized the capacity of these three techniques to give a coherent set of information. We used the coupling-model theory to interpret the dielectric results. It allowed us to identify two relaxation processes (alpha and beta), corresponding to different molecular motions. The temperature evolution of the alpha-relaxation indicates that fananserine is a fragile glass former, as reflected by the steepness index value, m = 77. The temperature T(o) where the relaxation times diverge was also determined. The description of the dielectric relaxation data in terms of the Kohlrausch-Williams-Watt relaxation function has shown the existence of an additional low-amplitude relaxation process assigned to the so-called Johari-Goldstein process. Mainly concerned by the primary alpha-process directly involved in the glass formation, we derived from this analysis the characteristic features of this process and showed that supercooled fananserine is characterized by a strongly non-Arrhenius and nonexponential behavior.