synopsisIt is well known that the rheological behavior of polyethylene melts is affected by at least four variables: ( 1 ) molecular weight, (9) molecular weight distribution, (3) long-chain branching and its dietribution, and (4) short-chain branching. Of these, the first three appear to have the largest effects. I n the present paper an attempt is made to determine the effect of 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 parsmeters 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.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.Melt strength increases in a similar manner.
synopsisThe objective of this work was to determine the relationships among molecular and melt parameters of polyolefins. The polyolefins studied are polypropylene, poly-lbutene, poly-1-hexene, poly-1-octene, and poly-1-dodecene ; these have regularly spaced shorechain branches. Conclusions from previous work, as well as some new data, on polyethylene are given. As the molecular weight increases, the critical shear rate decreases but the melt viscosity and non-Newtonian ratio increase. As the molecular weight distribution broadens, the critical shear rate decreases, whereas the normal forces and the non-Newtonian ratio increase. Increasing the number of shorechain branches increases the energy of activation and the melt viscosity but decreases the non-Newtonian ratio. As the length of the shortrchain branches increases, the non-Newtonian ratio increases, but the melt viscosity, critical shear rate, and energy of activation decrease. Increasing the number of long-chain branches decreases the non-Newtonian ratio, but the normal forces and the melt viscosity increase. Such information allows the polymer chemist to design a polyolefin molecule having the critical melt properties required for a given production technique.
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