Effects of molecular weight distribution and branching on rheological parameters of polyethylene melts. Part I. Unfractionated polymers

Abstract It is well known that the rheological behavior of polyethylene melts is affected by at least four variables: (1) molecular weight, (2) molecular weight distribution, (3) long‐chain branching and its distribution, and (4) short‐chain branching. Of these, the first three appear to have the largest effects. In the present paper an attempt is made to determine the effect of molecular weight distribution by rheological studies of polymers having similar molecular weight distribution by rheological studies of polymers having similar molecular weights and degrees of branching, but varying considerably in their molecular weight distributions as determined by fractionation. The rheological parameters studied were melt recovery, non‐Newtonian behavior, critical shear rate, and melt strength. It is shown that the melt recovery increases uniformly as the molecular weight distribution broadens. The degree of non‐Newtonian behavior, as measured by the exponent n of the power law, also increases with distribution breadth and is particularly affected by the amount of low molecular weight polymer present. Melt strength increases in a similar manner. Critical shear rate is inversely related to the breadth of the molecular weight distribution and is particularly dependent on the molecular weight of the highest fractions. The log of the critical shear rate is inversely proportional to the melt index recovery.