Colloid & Polymer Science Colloid Polym Sci 271:30 37 (1993) A comparative study between cosolvent and cononsolvent ternary polymer systems through the preferential adsorption coefficient C. M. G6mez, R. Garcia, V. Soria and A. Campos Departament de Quimica Fisica, Universitat de Valencia, Spain Abstract: Sorption equilibrium of polystyrene and poly (dimethyl siloxane) in mixed solvents has been monitored by means of the preferential adsorption coefficient experimentally determined from intensity light scattering measure- ments. The pairs of solvents used to dissolve each polymer sample have been selected for the purpose to perform cosolvent and cononsolvent ternary polymer systems. Flory-Huggins formalism including ternary interactions has been used to predict the sorption equilibrium for the cosolvent system and, for the first time, for cononsolvent ones. Moreover, the proportionality between binary and ter- nary interactions, recognized by Pouchly, is also corroborated for both ternary polymer systems. From a thermodynamic standpoint, the sorption equilibrium has been described by approaching the behavior of the "bulk solvent" to the binary liquid mixture through the excess Gibbs free energy. Key words: Ternary polymeric systems - preferential adsorption coefficient - cosolvent - cononsolvent - ternary interactions - excess free energy - intensity light scattering Introduction Preferential adsorption coefficient 2 is an equili- brium property dealing with the composition of a binary solvent close to and far from the polymer chain. The basic understanding of this pheno- menon, first observed by Ewart et al. [1], was achieved during the early 1960s [2-9]. Several methods have been developed to determine experi- mental 2 values [10], light scattering combined with differential refractometry being the most fre- quently used for synthetic polymers in organic me- dia [11, 12]. Moreover, some theoretical expres- sions for 2 have been performed in the framework of Flory-Huggins [13, 14] and Flory-Prigogine-Pat- terson [15] theories of polymer solutions in mixed solvents. In order to make plausible the interpreta- tion of experimental data, not sufficiently sup- ported by the above theories in their original form, a phenomenological ternary interaction parameter gT has been introduced [16-19]. In this way, Pouchly and Zivny [20] stated that a propor- tionality between gr, dependent on the solvent composition, and the solvent-solvent interaction parameter g12 takes place for any ternary polymer system (TPS). This proportionality has been widely verified for cosolvent systems [21], and also very recently [22] for cononsolvent ones. From a thermodynamic point of view, a non- ideal binary liquid mixture can be characterized by the excess Gibbs free energy G E which can adopt positive or negative values. The determination of this magnitude is of great importance in interpret- ing data in terms of molecular interactions, in particular, to study the thermodynamic behavior of a polymer chain dissolved in a binary solvent mix- ture. In this respect, as has been shown by Dondos and Patterson [23], there is a relationship between the sign of G r and the thermodynamic behavior of the polymer. Thus, it can be used as a criterion to predict a priori the solvency in TPS, i.e., binary mixtures with GE> 0 behave as good solvents (cosolvent systems), while those with GE< 0 are poor solvents (cononsolvent systems). The aim of the present work is to study the sorption equilibrium phenomenon in cosolvent and K 102