Purpose: To investigate the local and global mobility in amorphous sucrose and trehalose and their potential implications on physical stability.
Methods: Amorphous sucrose was prepared by lyophilization while amorphous trehalose was prepared by dehydration of trehalose dihydrate. The variation in the effective activation energy of alpha-relaxation through glass transition has been determined by applying an isoconversional method. Beta-relaxations were detected as shallow peaks, at temperatures below the glass transition temperature, caused by annealing glassy samples at different temperatures and subsequently heating at different rates in a differential scanning calorimeter. The effect of heating rate on the beta-relaxation peak temperature formed the basis for the calculation of the activation energy.
Results: alpha-Relaxations in glassy trehalose were characterized by larger activation energy barrier compared to sucrose, attributable to a more compact molecular structure of trehalose. The effect of temperature on viscous flow was greater in trehalose which can have implications on lyophile collapse. The size of the cooperatively rearranging regions was about the same for sucrose and trehalose suggesting similar dynamic heterogeneity at their respective glass transition temperatures. The activation energy of beta-relaxations increased with annealing temperature due to increasing cooperative motions and the increase was larger in sucrose. The temperature at which beta-relaxation was detected for a given annealing time was much less in sucrose implying that progression of local motions to cooperative motions occurred at lower temperatures in sucrose.
Conclusions: Trehalose, having a lower free volume in the glassy state due to a more tightly packed molecular structure, is characterized by larger activation energies of alpha-relaxation and experiences a greater effect of temperature on the reduction in the activation energy barrier for viscous flow. The pronounced increase in cooperative motions in sucrose upon annealing at temperatures below (T(g) -50) suggest that even a small excursion in temperature could result in a significant increase in mobility.