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 °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(τ) 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 σc1 = 0.18 MPa, and the slip-stick regime occurs at σc2 = 0.25 MPa, independently of temperature. The values of the plateau modulus, G N° = 1.6 × 106 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.

SUMMARY: Dynamic viscoelastic and capillary extrusion rheometry measurements were carried out with a series of 13 metallocene catalyzed polyethylenes and copolymers of ethene and 1-hexene. The structural parameters were analyzed by size exclusion chromatography (SEC) and 13 C NMR, showing that the molecular weights range from M -w = 80 000 to 308 000, the polydispersity index from 2 to 3.5 and the degree of short chain branching (SCB) from 0 to 13.8 SCB/1 000 C. In order to extract the maximum information from the experimental data, the following rheological methods were used: a) Viscosity and relaxation time dependence on molecular weight M ). d) log G 9 versus log G 9 9plots. e) Storage compliance J 9 dependence on storage modulus G 9 . f) Phase angle d dependence on complex modulus G*. g) Relaxation spectra. h) Dependence of the exponent n of the power law model for the viscosity function g (_ c ) on of molecular weight. i) Analysis of the critical rate for sharkskin. These methods, except the last one, allow to separate the samples into three different groups, at least when low frequencies (below 10 -1 Hz) or times higher than 10 s are involved. The definition of these groups cannot be undertaken considering only the molecular parameters obtained by SEC and 13 C NMR. Analyzing our rheological results in comparison with long chain branched polyethylenes (LCB) and looking at the theoretical aspect of the dynamics of long branched chains, we assume that among our samples there are five linear (non-LCB, Group I) polyethylenes and two groups of slightly long chain branched polyethylenes, which differ in the number of branches.

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Dynamic viscoelastic and extrusion capillary results of metallocene based polyethylenes are analyzed. Three samples show very high viscosities at low frequencies and large relaxation times, which is a symptom of the presence of small amounts of long chain branching (LCB). A linear correlation is found between the sharkskin dynamics (periodicity) and a characteristic entanglement‐disentanglement time. It is found that this correlation does not hold for samples suspected of LCB.