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 cellulos e that had a degree of substitution (DS) in the range 1.7-2.6 was inve stigated and compared to the properties of pure cellulose. The derivat ives were characterized by thermal and thermomechanical methods. The w ater vapor sorption isotherms were analyzed by the Zimm-Lundberg funct ion, which relates the volume fraction activity coefficient, gamma(1), of the solvent in condensed incompressible systems to the cluster int egral, 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 dependen ce of the Flory interaction parameter chi, the chi(0) parameter at zer o solvent concentration was evaluated. Since Flory-Huggins thermodynam ics 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 stro ng dependence on the DS, both clustering tendency and chi(0) values in creasing with increasing DS. The diffusion coefficient of water and th e glass transition temperature were discussed as a function of water c lustering in the polymers. The results of the cluster analysis confirm Barrer's idea that the unusual concentration dependence of the diffus ion coefficient of water in highly substituted ethyl cellulose can be related to a nonrandom molecular distribution of the sorbed water mole cules. Accordingly, the effect of water clustering in pure cellulose a ppeared to be of less consequence than its plasticizing effect, while the opposite is found in highly substituted ethyl cellulose.