The efficient fractionation of crystalline polymers should be possible near the upper critical end point (UCEP) which, for a sharp polymer fraction, is connected to the triple point (melting point) of the polymer by a three-phase line along which solid-liquid-gas equilibrium exists. By operating at temperatures slightly below that of the UCEP, such fractionation can be performed under conditions where solid-supercritical fluid equilibrium exists. The advantages associated with supercritical fluid extraction (SFE) should therefore be gained. The fractionation and relevant phase equilibrium near the UCEP are to be studied for the system polyethylene-n-propane at about 100-140oC and 400-600 bar. Under these conditions, fractionation induced by differences in molecular weight, as well as in microstructural irregularity (short-chain branching), is expected to occur.
