High pressure aging studies on the low-molecular weight glass-forming pharmaceutical – Probucol

In this work, broadband dielectric spectroscopy (BDS) has been applied to investigate the aging kinetics in a low-molecular weight pharmaceutical - probucol (PRO) at various pressure (p) conditions. Unexpectedly, we found that isostructural relaxation times (τiso, i.e., τα in the glass), as well as the stretch exponent (βag) calculated using the Casalini and Roland (CR) approach at 115 and 160 MPa, vary from the respective τiso and βag determined at 0.1 MPa. These results were different in regard to the data reported earlier for polymethylmethacrylate (PMMA). Further Fourier transform infrared (FTIR) measurements on the glass recovered from high-p experiments (pressure densified glass, PDG) allowed us to certify that most likely the change in the specific intermolecular interactions, namely the formation of weak, rarely observed O H⋯S hydrogen bonds might be responsible for that. Moreover, taking into account the values of βag (free parameter) obtained from the analysis of aging data using the CR approach, we estimated primitive relaxation times (τ0) from the Coupling Model, which were close to experimental Johari-Goldstein (JG) β-relaxation times (τJG-β). Findings reported herein may contribute to a better understanding of the aging phenomenon occurring in the pressurized H-bonded systems.