ON THE EQUATION OF STATE OF PLASTICIZED ETHYL CELLULOSE OF VARYING DEGREES OF SUBSTITUTION

The thermodynamic behavior of various types of plasticized ethyl cellu lose with a degree of substitution (DS) in the range of 1.7 less than or equal to DS less than or equal to 2.5 is presented. Three structura lly related polymer-plasticizer systems with different polarities were investigated, namely ethyl cellulose (DS 2.5)-glycerol tributyrate, e thyl cellulose (DS 2.1)-tributyl citrate, and ethyl cellulose (DS 1.7) -diethyl tartrate. The change in the specific volume of the materials was measured over a temperature range of 20-190 degrees C and a pressu re range of 10-100 MPa, and the scaling parameters of the Simha-Somcyn sky and the Flory-Orwoll-Vrij equations of state were determined and c ompared. Good agreement with experiment was obtained for both equation s of state. The cohesive energy density (CED) and, in the case of the Simha-Somcynsky equation, the occupied volume fraction (y) of the amor phous mixtures were calculated at thermodynamic equilibrium, that is a t temperatures above the relevant glass transition temperature (T-g). The T-g values of the binary mixtures were determined by the dynamic-m echanical method and corrected empirically to yield the low temperatur e limit of the thermodynamic equilibrium domain. The occupied volume f raction is strongly dependent on presssure and temperature and, above T-g, decreases with increasing amounts of the plasticizer component. A s a result of the comparatively narrow pressure range considered in th e evaluation of the volumetric data, the magnitude of y was found to b e hardly comparable for the three systems investigated, although it ac curately reflects the change in the specific Volume in each case. The CED generally appears to correlate with the hydrogen bond density of t he mixtures and consequently increases with decreasing DS of the polym er component. The CED provides valuable information for the descriptio n of intermolecular forces between the segments in the modeled quasi-l attice and may be capable of approximating the size distribution of th e unoccupied volume. Hence, both parameters are of outstanding importa nce in the study of factors that affect the diffusion coefficient of s mall apolar gas molecules in materials applied as film coatings in pro duct forms of pharmaceutical agents and food additives. In this contex t, the results of the present contribution were used to discuss the ox ygen permeability of plasticized ethyl cellulose in a subsequent paper .