Use of thermal analysis in the optimization of polymeric diffusion barriers in controlled release delivery systems

Abstract

Increasingly a variety of polymers are being used as ratecontrolling barriers in controlled release delivery systems. These diffusion barriers are usually in the form of thin films coated over the dosage form. The films are deposited from either solvent or water-based vehicles (latices or pseudolatices). Some recent applications involve placing the dosage form inside an injection molded polymeric barrier. Owing to the rigid nature and high glass transition temperatures (Tg) of most of the polymers being utilized pharmaceutically, plasticizers are used to soften (lower the Tg) the polymers. Sometimes a similar result can be accomplished by blending or modifying the polymers used. Thermal analysis is a very useful tool in the selection of the optimal type and use level of the plasticizer. The use of thermal analysis also aids in the proper selection of the application temperature for the barrier film and in the prevention of overplasticization. The latter may be particularly troublesome when the dosage form is required to pass an accelerated stability test (elevated temperature and/or humidity). The study presented here will discuss the use of differential scanning calorimetry (DSC) and thermal mechanical analysis (TMA) in the selection of the optimal plasticizer type and use level to achieve the desired properties in the polymeric barrier. The polymers discussed in this study will include: ethyl cellulose, cellulose acetate, cellulose acetate butyrate and cellulose acetate phthalate. The plasticizer selection and the resulting Tg's will be discussed and the optimal plasticizer use levels will be presented. Some of the unique problems encountered in the use of DSC versus TMA methods also will be discussed.