Polymer swelling during dissolution

Nonerodible systems. In the second matrix system, the matrix does not change during dissolution (insoluble, no disintegration, and no swelling). Polymers that are hydrophobic or cross-linked polymers often are used for the matrix. The drug solid is dissolved inside the matrix and is released by diffusing out of the matrix. Both dissolution and diffusion contribute to the release profile of this type of matrix systems. The mathematical expression for this system can be derived from the following equation ... 

Unlike classic solvent-developed negative resists such as cychzed rubber/ bisazide resist, which swell during development, the positive DNQ/novolac resists do not appreciably swell during their development in aqueous alkaline solutions. It should be emphasized that the dissolution of novolac is not exactly the same as that of the usual case II mass transfer kinetics in that it is not physical relaxation of the polymer molecules that is rate controlling. In case II dissolution, the rate-determining events occur at the polymer-gel interface. ... 

That it is not polymer relaxation but drug dissolution rate, or drug solubility, which accounts for membrane formation can be seen from Figure 15 During the short acetone swelling phase the 0X-loaded core expands, even as a sharply defined drug depleted outside layer is formed complete dissolution occurs only after "v 90 minutes. 

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