Comparison of the rheological properties of metallocene-catalyzed and conventional high-density polyethylenes

Dynamic measurements in a plate-plate system and steady state flow experiments in a capillary die are presented for conventional high-density polyethylenes (HDPEs) and a new type of polyolefin. The latter, the so-called metallocene-catalyzed HDPEs, are characterized by their low polydispersity and the total absence of branching. The metallocene-catalyzed materials show a different rheological behavior than commercial polyethylenes, which can be summarized as follows: (a) Higher viscosities than conventional HDPEs of the same molecular weight. The dependence of the viscosity on the molecular weight follows a power law equation with an exponent of 4.2 for metallocene catalyzed and 3.6 for conventionals. (b) For high molecular weight materials, the storage modulus overcomes the loss modulus (G' > G'') at 190 degrees C in all frequency ranges. However, for conventional HDPEs, G'' > G' at the same temperature and frequency range. (c) At long relaxation times, the values of H(tau) spectra of metallocene-catalyzed samples are significantly higher than those which correspond to a conventional sample of practically the same molecular weight. (d) Metallocene-catalyzed HDPEs are difficult to process, as sharkskin and slip-stick effects take place at very low shear rates. The onset of sharskin takes place at sigma(c1) = 0.18 MPa, and the slip-stick regime occurs at sigma(c2) = 0.25 MPa, independently of temperature. The values of the plateau modulus, G(N) degrees = 1.6 x 10(6) Pa, and the corresponding molecular weight between entanglements, M(e) = 1830, found for the metallocene-catalyzed materials, are very similar to those found for conventional polyethylenes. However, the activation-energies of flow of the new polymers (7-9 kcal/mol) are slightly higher than those of conventional HDPEs.