Thermodynamics of polyisobutylene–<i>n</i>‐alkane systems

Abstract

Abstract The solubility parameter theory and the molecular theory of polymer solutions developed by Prigogine and his collaborators are used to treat two phenomena not predicted by better‐known theories: the negative (exothermic) heats of mixing occurring in some nonpolar polymer—solvent systems at room temperature, and the lower critical solution temperature (L.C.S.T.), recently found by Freeman and Rowlinson, above which immiscibility occurs. It is found that a negative heat of mixing and a L.C.S.T. must occur at high enough temperatures when there is a difference in chain length between polymer and solvent (and if the critical endpoint of the solution does not intervene). The results of the cell theory of Prigogine et al. are presented for mixtures of a long‐chain polymer with a series of homologous short‐chain solvents, the free energy and enthalpy parameters χ and κ being given by: where r A is the number of segments in the solvent polymer, and A and B are constants. Heats of mixing have been obtained calorimetrically at room temperature for polyisobutylene with solvents in the n ‐alkane series and with some others. The heats with the n ‐alkanes are negative below C 14 , and are in good agreement with the theory which is also applied to the L.C.S.T. found by Freeman and Rowlinson for some of the same systems. Values of A and/or B obtained here compare with those found elsewhere from viscosity and swelling measurements and from the pure n ‐paraffin equation of state.