ABSENCE OF PHASE-SEPARATION EFFECTS IN BLENDS OF LINEAR POLYETHYLENE FRACTIONS OF DIFFERING MOLECULAR-WEIGHT

Blends of linear polyethylenes of differing molecular weights (MWs) have been studied to try to determine whether there is any observable phase separation in the melt. Using four linear polyethylene fractions, of MWs between 2 x 10(6) and 2.5 x 10(3), blends were prepared and four blend systems investigated, the two high MW polymers being blended with each of the two lower MW materials in turn. The blends were studied by transmission electron microscopy and differential scanning calorimetry, techniques previously used to study blends of linear with branched polyethylenes. In these systems, for some compositions of blends of linear with branched polyethylene, quenched from some temperatures, phase separation on a scale of micrometres has been recorded. In contrast, no indications of phase separation were observed on examination of the blends of linear with linear polyethylenes. From this it follows that if there is any liquid-liquid phase separation in linear with linear polyethylene blends, taking place as a result of molecular weight differences, it cannot be detected by the methods used to detect liquid-liquid phase separation in linear with branched polyethylene blends; and, conversely, the phase separation observed in linear with branched polyethylene blend systems does not take place as a result of molecular weight difference alone.