Creep recovery behavior of metallocene linear low-density polyethylenes

The rheological behavior of two metallocene linear low-density polyethylene s (mLLDPE) is investigated in shear creep recovery measurements using a mag netic bearing torsional creep apparatus of high accuracy. The two mLLDPE us ed are homogeneous with respect to the comonomer distribution. The most int eresting feature of the two mLLDPE is that their molecular mass distributio ns are alike. Therefore, as one of the mLLDPE contains long-chain branches, the influence of long-chain branching on the elastic properties of polyeth ylene melts could be investigated. It was found that long-chain branches in crease the elasticity of the melt characterized by the steady-state recover able compliance. The long-chain branched mLLDPE has a flow activation energ y of 45 kJ/mol which is distinctly higher than that of the other mLLDPE. Th e shear thinning behavior is much more pronounced for the long-chain branch ed mLLDPE. A discrepancy between the weight average molecular mass M-W calc ulated from size exclusion chromatography measurements by the universal cal ibration method and the zero shear viscosities of the two mLLDPE was observ ed. These observations are discussed with reference to the molecular archit ecture of the long-chain branched mLLDPE. The rheological properties of the long-chain branched mLLDPE are compared with those of a classical long-cha in branched LDPE. It is surprisingly found that the rheological behavior is very much the same for these two products although their molecular mass di stributions and presumedly the branching structures differ largely.