Influence of molecular structure on rheological properties of polyethylenes

Abstract

A conventional long-chain branched polyethylene (LDPE) and an octene copolymer (LLDPE) have been characterized in elongation using an apparatus which allows creep and stressing experiments with high accuracy. Whereas the LDPE shows a strain-hardening behavior known from literature, the elongational properties of the LLDPE are surprising. In the stressing experiments the strain-hardening is much more pronounced at lower strain rates than at higher ones. Two distinct plateaus of the strain rate are found in creep experiments at smaller stresses, indicating that two different processes have to be taken into consideration. The elongational viscosity calculated from the constant strain rate at short times is in good agreement with 3 obtained from shear experiments. The viscosity determined according to the second plateau in strain rate increases with declining stress. The recoverable compliance is smaller for the LLDPE than for the LDPE at higher stresses. This result reflects the differences in the molecular mass distribution of the two polyethylenes. At lower stresses the recoverable compliance of the LLDPE becomes higher than that of the LDPE. A hint to an explanation of the unusual elongational behavior of the LLDPE is obtained from an analysis of the molecular structure which shows the existence of a linear component of a relatively high molecular mass within the short-chain branched material. The results are discussed assuming a separate phase of the linear molecules within the matrix of the short-chain branched polyethylene of lower molecular mass.