Kinetics of spontaneous change in the localized motions of D-sorbitol glass

Abstract

The dielectric relaxation spectra of D-sorbitol glass have been studied in real time during annealing at 221.1 K, which is 47 K below its T g of 268 K. As the glass structurally relaxes during annealing, features of the Johari-Goldstein ͑JG͒ relaxation change with time: ͑i͒ the relaxation strength decreases, ͑ii͒ the relaxation peak at 48 Hz shifts to a higher frequency, and ͑iii͒ the relaxation spectra become narrower. All seem to follow the relation p ϰ exp͓−͑kt͒ n ͔, where p is the magnitude of a property, k the rate constant, and t the time. The parameter n may well be less than 1, but this could not be ascertained. It is proposed that shift of the relaxation peak to a higher frequency and narrowing of the relaxation spectra occur when local, loosely packed regions of molecules in the glass structure collapse nonuniformly and the relaxation time of some of the molecules in the collapsed state becomes too long to contribute to the JG-relaxation spectra. Consequently, the half width of the spectra decreases, and the relaxation peak shifts to a higher frequency. Molecules whose diffusion becomes too slow after the local regions' collapse would contribute to the ␣-relaxation spectra and thus the net relaxation strength would increase on structural relaxation. It is argued that these findings conflict with the NMR-based conclusions that motion of all molecules in the glass and supercooled liquid contributes to the faster relaxation process.

Molecular Mobility, Thermodynamics and Stability of Griseofulvin’s Ultraviscous and Glassy States from Dynamic Heat Capacity

“…These contributions decrease during storage as the glass structure relaxes spontaneously to a state of lower fictive temperature T f (the temperature at which the glass and the liquid have the same properties) volume, enthalpy and entropy (32)(33)(34)(35)(36) because the vibrational frequency increases, the vibrational amplitude, the a-relaxation contribution and the JG relaxation contribution decrease-the decrease in the last one due to decrease in the number of molecules involved in localized motions (34)(35)(36)(37)(38)(39)(40)(41)(42). The glassy state formed by slow cooling has a lower T f and hence a lesser JG relaxation contribution to C p than that formed by normal cooling (33)(34)(35)(36)(37)(38)(39)(40)(41)(42).…”

Dielectric Relaxation and Crystallization of Ultraviscous Melt and Glassy States of Aspirin, Ibuprofen, Progesterone, and Quinidine

“…Therefore, it is expected to remain constant with decreasing fictive temperature during structural relaxation. It is a characteristic feature of the glassy state that the strength of the JG relaxation decreases on structural relaxation, 35,[78][79][80] that is, the number of mobile molecules in local regions and/or the size and number of local regions in the structure decrease. The effect of an increase in Z and t 0 , once nuclei have formed and grown to an appreciable size, would be to slow down the rate of crystal growth.…”

Local Mobility in Amorphous Pharmaceuticals—Characterization and Implications on Stability

“…This leads to a decrease in global mobility and an increase in the heterogeneity of cooperative motions. 66 On the other hand, b-relaxation times as well as their heterogeneity of distribution has been generally found to decrease, [67][68][69][70] i.e., the local motions become faster and more homogeneous. However, the distribution of b-relaxation times is weakly dependent on annealing.…”

“…The faster b-relaxation time upon annealing is counterintuitive but has been explained to be resulting from nonuniform collapse of ''islands of mobility''. 70 Due to such nonuniform collapse, the domains with longer relaxation times would be kinetically frozen thereby resulting in an overall narrowing of the distribution as well as its shift to shorter times. Annealing of amorphous pharmaceuticals has been shown to improve chemical stability.…”