Effect of high molar mass species on linear viscoelastic properties of polyethylene melts

The molecular features and the linear viscoelastic melt properties of a series of polydisperse high density polyethylene are investigated. The main molecular characteristic of the materials studied is the presence of high molar mass species, which give rise to unusual values of the Mz/Mw ratio. This molecular particularity strongly affects to the linear viscoelastic melt properties as the Newtonian viscosity and the steady-state shear recoverable compliance. Both the third and the fourth moments of the molecular weight distribution affect to the values of these properties, which follow the trend expected by the reptation model. The evidences clearly prove the effect not only of the polydispersity index, but more interestingly of the shape of the molecular weight distribution on the dynamics of the systems. The behavior shown by the experimental samples studied in this work is interesting as the linear viscoelastic properties have become widely used, not only to test the molecular weight distribution dependence of the viscoelastic fingerprint and the processability, but also to assess the possible presence of long chain branching.