Separating the effects of sparse long-chain branching on rheology from those due to molecular weight in polyethylenes

Abstract

The effects of sparse (<1 branch per chain) long-chain branching (LCB), molecular weight (MW), and molecular weight distribution on the shear rheological properties of commercial polyethylenes are often convoluted. In this paper a method for separating the effects of sparse LCB in metallocene-catalyzed polyethylenes (mPE) from those of molecular weight and its distribution based on time–molecular weight superposition is proposed. Four metallocene polyethylenes with degrees of long-chain branching [i.e., M of the arm (Ma) is greater than that for the onset of entanglements, Mc] as determined from dilute solution measurements ranging from zero (linear) to 0.79 LCB/104 CH2, along with a conventional Ziegler–Natta polymerized linear low-density polyethylene (LDPE), and a tubular free-radical polymerized LDPE are investigated. In general, it is observed that sparse LCB (for levels < 1.0 LCB/104 CH2) increases the zero shear viscosity, η0 (e.g., by a factor of 7) and decreases, but even to a greater degree, the critical shear rate (γ̇c) for the onset of shear thinning (e.g., by a factor of 100). The breadth of the molecular weight distribution just affects γ̇c but not η0 for the range of data used in this study. Furthermore, the dynamic storage modulus G′ shows similar enhancement at low frequencies as viscosity does, while the primary normal stress difference coefficient, Ψ1,0, exhibits a greater dependence on long-chain branching than that predicted from the zero-shear viscosity enhancement. The results for the mPEs are consistent with recent molecular theories for randomly branched molecules in that it is the spacing between branch points and not the number of branches at a point that is important. Furthermore, the results are consistent with the idea that the branches are located on the longest chains, and hence, have the greatest effects on the longest relaxation modes.