Polymer Selection for the HME Process

1 Thermodynamic Considerations - Drug-Polymer Solubility and Miscibility 

In the last few years efforts have been focused on developing theoretical models and methods for the prediction of drug-polymer interactions, and estimation of a drug s saturation solubility in the polymer matrix. A solubility of the drug in the polymer can be defined only in the case of an amorphous soUd solution, where the drug is molecularly dispersed into the polymer matrix, and not in the case of soUd suspensions. 

When the AG j is negative, mixing will occur. According to the Flory-Huggins theory, the Gibbs free energy of mixing is expressed as a function of an F-H interaction parameter,[29]  

The value of the F-H interaction parameter,can be estimated on the basis of melting point depression data collected by a suitable thermal analysis method, such as DSC  

Although the F-H interaction parameter, x expresses mainly the enthalpic contributions to miscibility, it is not completely free of entropic components, and varies with temperature, T, and composition, 9 [31]  

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