EFFECT OF POLYDISPERSITY ON THE DEMIXING PRESSURES OF POLYETHYLENE IN NEAR-CRITICAL OR SUPERCRITICAL ALKANES

Demixing pressures of a polyethylene sample of broad molecular-weight distribution ((M) over bar(w) = 121,000, (M) over bar(w)/(M) over bar(n) = 4.43) in n-butane and n-pentane are reported. The results are compared with the demixing pressures of polyethylene standards with narrower molecular-weight distributions. For these systems at the concentrations investigated, the observed demixing pressures (cloud points) are greatly influenced by the high molecular-weight fractions of the polymer. The observations are further substantiated by measurements of the demixing pressures of synthetic blends (i.e., mixtures in different proportions) of polymer standards with narrow molecular-weight distributions.