Molecular motions in sucrose-PVP and sucrose-sorbitol dispersions: I. Implications of global and local mobility on stability

Purpose:To characterize molecular mobility by dielectric spectroscopy and determine the effect of additives on α-and β-relaxation times in amorphous sucrose solid dispersions. Methods:Sucrose was co-lyophilized with either PVP or sorbitol. The lyophiles were subjected to dielectric spectroscopy and differential scanning calorimetry. Results: The additives did not have an appreciable effect on the calorimetric T_g. However, dielectric spectroscopy revealed pronounced effects on global mobility (α-relaxation) , which correlated with the crystallization tendency of sucrose. The systems were characterized by two β-relaxations, and the relaxation times as well as their temperature dependence were influenced by the additive. Although sorbitol acted as a plasticizer of sucrose with respect to global mobility, it antiplasticized sucrose in terms of local motions. PVP, on the other hand, acted as an anti-plasticizer with respect to both global and local mobility. The slower β-relaxation in amorphous sucrose was found to correlate with the α-relaxation and was identified as the Johari-Goldstein relaxation. Conclusions:Amorphous systems with identical calorimetric T_g could have significantly different mobility and physical stability as revealed by dielectric spectroscopy. Additive effect on global mobility cannot be a predictor of the effects on local mobility. Additives could also be used to inhibit local mobility.