An evaluation of the properties of enteric coating polymers: measurement of glass transition temperature.

When submitted to X-ray crystallography, two enteric coating polymers, cellulose acetate phthalate and polyvinyl acetate phthalate, were found to be essentially amorphous in structure. Values for the glass transition temperature, Tg, of each polymer have been obtained using both a surface microindentation technique and differential scanning calorimetry. The effect on this parameter of an increasing concentration of a plasticizer, diethyl phthalate, has also been determined. Measured values for Tg have been compared with predicted values obtained using a suitable mixture-rule model: the surface microindentation technique values were closer to the predicted.