Effects of molecular weight distribution and branching on rheological properties of polyolefin melts

The objective of this work was to determine the relationships among molecular and melt parameters of polyolefins. The polyolefins studied are polypropylene, poly‐1‐butene, poly‐1‐hexene, poly‐1‐dodecene, these have regularly spaced short‐chain 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 short‐chain branches increases the energy of activation and the melt viscosity but decreases the non‐Newtonian ratio. As the length of the short‐chain 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.