Quasi-elastic neutron scattering studies of the slow dynamics of supercooled and glassyaspirin

Abstract

Aspirin, also known as acetylsalicylic acid (ASA), is not only a wonderful drug, but also agood glass former. Therefore, it serves as an important molecular system to study thenear-arrest and arrested phenomena. In this paper, a high-resolution quasi-elastic neutronscattering (QENS) technique is used to investigate the slow dynamics of supercooled liquidand glassy aspirin from 410 down to 350 K. The measured QENS spectra can beanalyzed with a stretched exponential model. We find that (i) the stretched exponentβ(Q) is independent of thewavevector transfer Q in the measured Q range and (ii) the structural relaxation timeτ(Q) follows a power-lawdependence on Q.Consequently, the Q-independent structural relaxation timeτ0 can beextracted for each temperature to characterize the slow dynamics of aspirin. The temperature dependenceof τ0 can be fitted with the mode-coupling power law, the Vogel–Fulcher–Tammann equationand a universal equation for fragile glass forming liquids recently proposed by Tokuyamain the measured temperature range. The calculated dynamic response functionχT(Q, t) using the experimentally determined self-intermediate scattering function of the hydrogenatoms of aspirin shows direct evidence of the enhanced dynamic fluctuations as the aspirinis increasingly supercooled, in agreement with the fixed-time mean squared displacement⟨x2⟩ and the non-Gaussianparameter α2 extracted from the elastic scattering.