Thermodynamics of solutions of hydrocarbons in low molecular weight poly(isobutylene): A gas chromatographic study

The reduced chemical potentials (chi*) of 20 normal and branched alkanes, cyclohexane, and three aromatic hydrocarbons at infinite dilution in poly(isobutylene) (PIB) were measured by gas liquid chromatography at five temperatures between 35 and 65 degrees C. Reduced partial molar residual enthalpies (kappa*) were calculated from the temperature dependence of chi*; they are positive for PIB + alkane systems but smaller than those obtained in former chromatographic studies. Although uncertainties on kappa* are at least 1 order of magnitude larger than those on chi*, binary X(12) Flory parameters obtained from kappa* display a good correlation with the structural parameter theta(e1), defined as the ratio of the number of hydrogen atoms on methyl groups to the total number of hydrogen atoms in the alkane molecule. Very poor or nil correlation exists between X(12) values obtained from chi* and theta(e1). The evidence is by no means conclusive, but in principle the chi* results obtained for PIB + alkane systems could be explained in terms of free volume contributions and the antipathy between methyl groups on the alkane molecules and the polymer side groups. Positive partial molar residual entropies were detected for the three aromatic hydrocarbons; their partial molar residual enthalpies are however highly positive, resulting in their poor solvent properties toward PIB.