Temperature relation of thermodynamic functions of combining polymer systems with upper and lower critical temperatures of mixing

Abstract

An apparatus was designed to measure vapour pressure over polymer solutions at high temperatures and pressures. Vapour pressures of solvents were measured above their boiling points for polyisobutylene (PI) solutions in iso-octane and benzene and polystyrene (PS) solutions in ethylbenzene and cyclohexane at temperatures of 25 to 165° in a wide range of concentration. Values of Δμ 1, Δμ 2 and average free Energies, enthalpies and entropies of mixing and combinatorial values of S c and non-combinatorial entropies of mixing S nc were calculated. It was shown that for systems only having a lower critical temperature of mixing, long before this value is achieved, the thermodynamic affinity between components decreases continuously with an increase in temperature and enthalpies and entropies of mixing are negative over the entire range of temperature and concentration. For systems with upper and lower critical temperatures of mixing (CTM) thermodynamic affinity varies with temperature according to a curve with a minimum, decreasing in absolute value on approximating CTM. On increasing temperature, enthalpies and entropies of mixing change sign (from positive to negative). It was shown that for solutions of PI in iso-octane the non-combinatorial entropy of mixing is responsible for the variation of entropy of mixing with temperature and depends on the interaction between components. ΔS nc < 0 and ∂ΔS nc/ ∂T < 0. ΔS c is always positive and independent of temperature.