Thermodynamics of polymer solutions. Part 4.—Polyisobutylene and n-pentane

Abstract

Activities of n-pentane in solutions with polyisobutylene have been measured at 25°C over the range of concentrations, expressed as the segment fractions φ₂ of polymer, from 0.363 to 0.965. These results in conjunction with those of Baker et al.(with which our results agree) yield values of χ that increase from ca. 0.48 at φ₂= 0 to ca. 0.90 at φ₂= 1.0. Theoretical calculations match the experimental χ at high concentrations of polymer well, but overestimate them somewhat at lower concentrations. The excess volume is calculated to be –1.55 %, in good agreement with experiment. Both the partial molar enthalpy and entropy of dilution are predicted to be negative at high dilutions; the reduced partial molar non-combinatory entropy S^R₁/Rφ²₂ is calculated to be ca.–1.0. Thus, the existence of an LCST is predicted; the temperature at which phase separation occurs is not reliably calculable, however, owing to the precision that would be required on the one hand and the limitations of theory on the other. Results for the four systems treated in the present series of papers are compared.