Hitherto Ignored Effects of Chain Length on the Flory-Huggins Interaction Parameters in Concentrated Polymer Solutions

Abstract

Equilibrium vapor pressures were measured for the systems toluene/poly(dimethylsiloxane) [two different polymers, 35-55°C] and cyclohexane/poly(vinyl methyl ether) [three polymers, 35-65°C] by combining a head-space sampler with a gas chromatograph. The Flory-Huggins interaction parameters X calculated therefrom as a function of the volume fraction φ 2 of polymer were complemented by literature data on inverse gas chromatography and by X values at high dilution from osmometry in the former case and from light-scattering measurements in the latter. These results demonstrate that the effects of differing chain lengths can still be felt at φ 2 > 0.5, in contrast to the general perception. This observation is tentatively explained in terms of changes in coil dimension taking place in the region of high polymer concentrations. Furthermore, X(φ 2 ) may exhibit a minimum even with the common polymers of present interest. In all cases the concentration dependence of X is described well by a series expansion of X with respect to φ 2 up to the fourth term, if two parameters are adjusted. The temperature dependence of X at different concentrations demonstrates that the heats of dilution generally increase with increasing φ 2 , whereas the noncombinatorial entropy of dilution decreases. This situation leads to a linear interdependence of the enthalpy and entropy part of X. With the system toluene/poly(dimethylsiloxane) one observes an inversion of the heat effects upon an increase of φ 2 from exothermal to endothermal at ca. 13 vol %.