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 anti-plasticized 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.