Enthalpy relaxation in binary amorphous mixtures containing sucrose

Purpose. To compare the enthalpy relaxation of amorphous sucrose and co-lyo philized sucrose-additive mixtures near the calorimetric glass transition t emperature, so as to measure the effects of additives on the molecular mobi lity of sucrose. Methods. Amorphous sucrose and sucrose-additive mixtures, containing poly(v inylpyrrolidone) (PVP), poly(vinylpyrrolidone-co-vinyl-acetate) (PVP/VA) de xtran or trehalose, were prepared by lyophilization. Differential scanning calorimetry (DSC) was used to determine the area of the enthalpy recovery e ndotherm following aging times of up to 750 hours for the various systems. This technique was also used to compare the enthalpy relaxation of a physic al mixture of amorphous sucrose and PVP. Results. Relative to sucrose alone, the enthalpy relaxation of co-lyophiliz ed sucrose-additive mixtures was reduced when aged for the same length of t ime at a comparable degree of undercooling in the order: dextran approximat e to PVP > PVPNA > trehalose. Calculated estimates of the total enthalpy ch ange required for sucrose and the mixtures to relax to an equilibrium super cooled liquid state (Delta H-infinity) were essentially the same and were i n agreement with enthalpy changes measured at longer aging times (750 hours ). Conclusions. The observed decrease in the enthalpy relaxation of the mixtur es relative to sucrose alone indicates that the mobility of sucrose is redu ced by the presence of additives having a T-g that is greater than that of sucrose. Comparison with a physically mixed amorphous system revealed no su ch effects on sucrose. The formation of a molecular dispersion of sucrose w ith a second component, present at a level as low as 10%, thus reduces the mobility of sucrose below T-g, most likely due to the coupling of the molec ular motions of sucrose to those of the additive through molecular interact ions.