Effet des proprietes thermodynamiques des melanges de solvants sur les dimensions de Chaines macromoleculaires

In a binary solvent mixture, the χ1 2 interaction parameter has a marked influence upon the intrinsic viscosity of a polymer. If χ 1 2 is positive, the [χ] value is higher than the weighted average of the viscosities measured in the pure constituents. When the latter are non‐solvents of the polymer, and the mixture dissolves it, the so‐called cosolvency‐effect requires a positive value of χ1 2. If χ 1 2 is negative, the [χ] value is found lower than the average of the viscosities measured in the pure constituents. Demixtion can even occur when in a given composition range the mixture of two good solvents behave like a precipitant (χ1 2 <0). Athermal mixtures (χ1 2 = 0) exhibit no such effect, and behave like a pure solvent. To determine the unperturbed dimensions of a polymer coil in various binary solvent media, the Stockmayer‐Fixman extrapolation was used. It is known that for a given polymer the same Kχ value is found in any pure solvent. In binary mixtures the Kχ values are found higher or lower than this unique value, depending upon the sign of χ1 2. These qualitative results remains valid as long as selective adsorption of the better solvent on the chain remains low. If this is not the case, account has to be taken of the fact that composition inside the coil is not identical with the average composition of the mixture. A pure solvent in which molecules have a tendency to associate behaves like a solvent mixture in which χ1 2 is negative: the Kθ value is found lower than the unique Kθ value for this polymer in all other pure solvents.