Effects of the degree of substitution in ethyl cellulose on the clustering of sorbed water

The sorption and diffusion of water in various types of ethyl cellulose that had a degree of substitution (DS) in the range 1.7-2.6 was investigated and compared to the properties of pure cellulose. The derivatives were characterized by thermal and thermomechanical methods. The water vapor sorption isotherms were analyzed by the Zimm-Lundberg function, which relates the volume fraction activity coefficient, gamma(1), of the solvent in condensed incompressible systems to the cluster integral, G(11), based on the expressions for G(11) originally introduced by Kirkwood and Buff. Using the Orofino equation, which expresses the tendency of solvent clustering in terms of a formal activity dependence of the Flory interaction parameter chi, the chi(0) parameter at zero solvent concentration was evaluated. Since Flory-Huggins thermodynamics cannot be applied unequivocally to systems exhibiting marked water clustering, chi(0) was identified to be the ''true'' Flory parameter. The clustering functions obtained and the chi(0) values showed a strong dependence on the DS, both clustering tendency and chi(0) values increasing with increasing DS. The diffusion coefficient of water and the glass transition temperature were discussed as a function of water clustering in the polymers. The results of the cluster analysis confirm Barrer's idea that the unusual concentration dependence of the diffusion coefficient of water in highly substituted ethyl cellulose can be related to a nonrandom molecular distribution of the sorbed water molecules. Accordingly, the effect of water clustering in pure cellulose appeared to be of less consequence than its plasticizing effect, while the opposite is found in highly substituted ethyl cellulose.