SOLID-SOLUTION FORMATION AND FRACTIONATION IN QUASI-BINARY SYSTEMS OF POLYETHYLENE FRACTIONS

Solid solution formation and fractionation during crystallization were studied in two quasi-binary systems of polyethylene fractions by means of differential scanning calorimetry, X-ray scattering (wide and small angle), and microscopy. The molecular weights (Mn) of these systems were: (1) 980 and 1790, and (2) 1790 and 20 × 103. At cooling rates lower than 20 °C/min fractionation occurred during the solidification process. The composite polymeric solids thus produced exhibited a eutectic-type melting point/composition diagram with the eutectic point located close to the axis of the constituent with the lowest molecular weight. These diagrams could be described with the well-known relation for the melting point depression according to Flory and Huggins, if the Flory-Huggins parameter had the value -0.02. Upon quenching to temperatures lower than 80 °C a continuous series of solid solutions were generated from the quasi-binary melts. The overall enthalpies of fusion of both the solid solutions and separate solid phases were found to depend linearly on the volume fraction of one of the components. The slowly cooled mixtures showed two SAXS-long periods that corresponded with the long periods in the pure fractions. The solid solutions were characterized by a new long spacing intermediate in length between the long periods of the fractions in the system (1) and exceeding those of the pure fractions in the system (2). The results are discussed in terms of structural differences between the solid solutions and the separate solid phases. © 1979, American Chemical Society. All rights reserved.