Studies of the rheological properties of fractions of linear and branched polyethylenes have shown that the melt recovery of linear polyethylene fractions is very small and independent of molecular weight over a wide range. Fractions containing high degrees of long-chain branching, on the other hand, have high melt recoveries. The melt recovery of a fraction can therefore be used as an index of long-chain branching. Alternatively, if no long-chain branching is present, the melt recovery is a unique function of the molecular weight distribution.
The log of the critical shear rate is a linear function of the log melt viscosity of the fraction for both linear and branched polyethylenes. This would indicate that the critical shear rate of polydisperse samples would depend primarily on the weight-average or Z-average molecular weight of the polymer. This is confirmed by previous studies on polydisperse samples. It also appears that critical shear rate is highly dependent on the homogeneity of the' sample. Blends of the same fractions had quite different critical shear rates, depending on the procedure used to prepare them, even though their molecular weight distributions were identical. The change in viscosity with shear rate is not a unique function of molecular weight or melt Viscosity. Fractions of linear polyethylene show a greater change in viscosity with shear rate than branched fractions of similar low shear melt viscosity. This suggesta that the effect is related to chain entanglement or coordinated segmental motion. This effect is illustrated by blends of fractions.