Melt miscibility and mechanical properties of metallocene linear low-density polyethylene blends with high-density polyethylene: influence of comonomer type

In this paper, the implications of melt miscibility on the thermal and mechanical properties of linear low-density polyethylene (LLDPE)/high-density polyethylene (HDPE) blends were assessed with respect to the influence of the comonomer type. The influence of the latter was examined by selecting one butene LLDPE and one octene LLDPE of very similar weight-average molecular weight (M-w), molecular-weight distribution (MWD) and branch content, keeping the comonomer type as the only primary molecular variable. Each of the two metallocene LLDPEs was melt-blended with the same HDPE at 190 degrees C in a Haake melt-blender. The rheological, thermal and mechanical properties were measured by the use of an ARES rheometer, differential scanning calorimeter and Instron machine, respectively. The rheological measurements, made over the linear viscoelastic range, suggested no significant influence of the branch type on the melt miscibility. The rheology results are in agreement with those obtained from previous transmission electron microscopy (TEM) and small-angle neutron scattering (SANS) studies. The dynamic shear viscosity and total crystallinity of the metallocene (m)-LLDPE blends with HDPE followed linear additivity. At small strains, the branch type has little or no influence on the melt miscibility and solid-state properties of the blends. Even the large-strain mechanical properties, such as tensile strength and elongation at break, were not influenced by the comonomer type. However, the ultimate tensile properties of the HDPE-rich blends were poor. Incompatibility of the HDPE-rich blends, as a result of the weak interfaces between the blend components, is suggested to develop at large strains. (c) 2005 Society of Chemical Industry.