The effects of absorbed water on the properties of amorphous mixtures containing sucrose

Purpose. To measure the water vapor absorption behavior of sucrose-poly(vin yl pyrrolidone) (PVP) and sucrose-poly(vinyl pyrrolidone co-vinyl acetate) (PVP/VA) mixtures, prepared as amorphous solid solutions and as physical mi xtures, and the effect of absorbed water on the amorphous properties, ie., crystallization and glass transition temperature, T-g, of these systems. Methods. Mixtures of sucrose and polymer were prepared by co-lyophilization of aqueous sucrose-polymer solutions and by physically mixing amorphous su crose and polymer. Absorption isotherms for the individual components and t heir mixtures were determined gravimetrically at 30 degrees C as a function of relative humidity. Following the absorption experiments, mixtures were analyzed for evidence of crystallization using X-ray powder diffraction. Fo r co-lyophilized mixtures showing no evidence of crystalline sucrose, T-g w as determined as a function of water content using differential scanning ca lorimetry. Results. The absorption of water vapor was the same for co-lyophilized and physically mixed samples under the same conditions and equal to the weighte d sums of the individual isotherms where no sucrose crystallization was obs erved. The crystallization of sucrose in the mixtures was reduced relative to sucrose alone only when sucrose was molecularly dispersed (co-lyophilize d) with the polymers. In particular, when co-lyophilized with sucrose at a concentration of 50%, PVP was able to maintain sucrose in the amorphous sta te for up to three months, even when the T-g was reduced well below the sto rage temperature by the absorbed water. Conclusions. The water vapor absorption isotherms for co-lyophilized and ph ysically mixed amorphous sucrose-PVP and sucrose-PVP/VA mixtures at 30 degr ees C are similar despite interactions between sugar and polymer which are formed when the components are molecularly dispersed with one another. In t he presence of absorbed water the crystallization of sucrose was reduced on ly by the formation of a solid-solution, with PVP having a much more pronou nced effect than PVP/VA. The effectiveness of PVP in preventing sucrose cry stallization when significant levels of absorbed water are present was attr ibuted to the molecular interactions between sucrose, PVP and water.