MOLECULAR STRUCTURE AND RHEOLOGY RELATIONSHIP OF POLYETHYLENES

摘要

The structure-rheology relationship is investigated in three polyethylenes namely high density polyethylene (HDPE), a metallocene linear low density polyethylene with no chain branching (mLLDPE) and a metallocene polyethylene containing long chain branching (mLLDPE-LCB). Shear and extensional rheology measurements were carried out in the linear viscoelastic regime and correlated to the molecular weight, molecular weight distribution and long chain branching. Shear rheology showed that HDPE exhibits a viscosity profile whereby the Newtonian behavior is not completely attained as shown by the slope of the storage modulus in the terminal region. mLLDPE was found to possess the longest and well-defined Newtonian region and the highest transition to the non-Newtonian region. In the presence of long chain branching (LCB), the terminal region is not apparent while the onset of shear thinning is decreased. Such behavior can be related to the effects of MWD and LCB and was corroborated using extensional viscosity measurements, which showed slight deviation from the LVE envelope for broader molecular weight distribution and strain hardening in the presence of long chain branching.