Drug-polymer mixture

The free energy of mixing per mole of drug-polymer mixture can be calculated as follows ... 

Problems in the accuracy of the measured Tend values arise due to the polymer s high viscosity that slows the mixing and dissolution process down. This is most prominent at temperatures close to or below the glass transition temperature (Tg) of the drug-polymer mixture as molecular mobility is reduced. The mixing and dissolution processes at temperatures close to the Tg may become so slow that equilibrium is not reached during the time frame of a DSC scan. These measurements lead to a wrong assumption of the equilibrium solubility. Therefore, solubility measurements close to or below the Tg of the mixture are not feasible with DSC measurements due to the time frame of the experiments. 

Fig. 2.10 Schematic of a temperature versus weight fraction diagram the straight line represents the Tg of the drug-polymer mixture, the curved solid line represents the measured Tend values, and the dashed curve line represents the extrapolated Tend values of drug in polymer. The intersection marks the solubihty at Tg for a saturated solution. (Adapted from Tao et al. 2009)...

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