Rheological properties of bimodal polyethylenes produced with silica nanoparticle supported catalysts

The rheological properties of bimodal ultra-high-molecular-weight polyethylenes (UHMWPEs) are investigated with an emphasis on the influence of the catalyst support. Bimodal UHMWPEs were produced in a single reactor with double support (DS) and single support (SS) catalyst systems. Dynamic measurements showed that the SS samples have a higher storage modulus at low frequencies than the DS samples with the same weight average molecular weight. After extrusion, the storage modulus of the bimodal DS samples increased to an identical value. The results indicate that the SS sample has a higher homogeneity than the DS sample. The steady-shear viscosities were measured to determine zero shear viscosities (η₀) of the bimodal samples. The results show that the SS samples follow the relation η₀ = KM_w ^α with α = 3.19. However, the DS samples exhibit a substantial deviation from this relation especially in the intermediate M_w range around 1000 kg mol⁻¹. The reason for the deviation is attributed to the heterogeneity in DS samples. The results demonstrate that the DS systems do not produce bimodal PEs with an intimate degree of mixing of short and long polymer chains even if silica nanoparticles are used as the support.