| The influence of molecular weight, comonomer type, and mixing temperature on the miscibility of LLDPE and LDPE is discussed. Adequate amounts of antioxidants were added during melt blending. Dynamic and steady shear measurements were carried out in a Rheometrics Mechanical Spectrometer 800. The mixing temperatures were chosen to dovetail with the recently reported high-temperature transitions in polyethylene (Hussein and Williams, Macromol. Rapid Commun., 19, 323-325, 1998). Blends of low-Mw LLDPE (butene) and LDPE are likely miscible as revealed by the dependence of their rheological properties on blend composition. Blends of high-Mw LLDPE (butene) and LDPE mixed at 190oC are only partially miscible; immiscibilty is likely to occur around the 50/50 composition and in the LDPE-rich blends. Blends are likely miscible in the LLDPE-rich range. Blends of high-Mw LLDPE (butene) and LDPE mixed at 220oC are almost immiscible. Miscibilty is likely to occur for the 70% LLDPE blend. Increasing the branch length from butene to octene slightly increased the miscibility of LLDPE/LDPE blends. It is suggested that the molecular order (see Hussein and Williams, J. Non-New. Fluid Mech., in press, 1999) and mismatch of the molecular conformations of different polyethylene structures provide an explanation for the immiscibility of polyethylenes. Agreement of the measurements of dynamic viscosity as function of composition and theoretical predictions of two-phase models (e.g., dilute emulsion model) was observed. |
