THE EFFECT OF PRESSURE ON THE DETERMINATION OF THE FLORY-HUGGINS CHI PARAMETER BY VAPOR-PRESSURE MEASUREMENTS

The pressure experienced by polymer solutions during vapour pressure measurements varies with the composition of the solution at fixed temperature. The effect of this pressure variation upon the determination of the Flory-Huggins-chi-parameter through vapour pressure or vapour sorption measurements is examined. It is found that the effect is negligibly small at room temperature, but that it may become worth considering near lower critical solution temperature (LCST) conditions. Thus, for the system polystyrene-toluene, which exhibits a LCST at 277-degrees-C, consideration of the pressure variation effect during dissolution leads, when the weight fraction of polymer exceeds 0.9, to an estimated decrease in the value of chi of the order of 10%. The equivalent effect on light scattering determinations of the chi-parameter is also briefly discussed.