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

Aspirin, also known as acetylsalicylic acid (ASA), is not only a wonderful drug, but also a good glass former. Therefore, it serves as an important molecular system to study the near-arrest and arrested phenomena. In this paper, a high-resolution quasi-elastic neutron scattering (QENS) technique is used to investigate the slow dynamics of supercooled liquid and glassy aspirin from 410 down to 350K. The measured QENS spectra can be analyzed with a stretched exponential model. We find that (i)the stretched exponent β(Q) is independent of the wavevector transfer Q in the measured Q range and (ii)the structural relaxation time τ(Q) follows a power-law dependence on Q. Consequently, the Q-independent structural relaxation time τ ₀ can be extracted for each temperature to characterize the slow dynamics of aspirin. The temperature dependence of τ ₀ can be fitted with the mode-coupling power law, the Vogel-Fulcher-Tammann equation and a universal equation for fragile glass forming liquids recently proposed by Tokuyama in the measured temperature range. The calculated dynamic response function χ _T(Q, t) using the experimentally determined self-intermediate scattering function of the hydrogen atoms of aspirin shows direct evidence of the enhanced dynamic fluctuations as the aspirin is increasingly supercooled, in agreement with the fixed-time mean squared displacement 〈x ²〉 and the non-Gaussian parameter α ₂ extracted from the elastic scattering.