Dynamic viscoelastic results of 23 noncommercial metallocene-catalyzed polyethylenes and poly(ethylene/1-hexene) copolymers, in the range 130−190 °C, are presented. The effects of well-determined structural parameters such as molecular weight, polydispersity, and degree of short chain branching (SCB), are analyzed. The molecular weight varies between 60 000 and 325 000, the polydispersity between 1.8 and 7.3, and SCB between 0 and 48.5 branches/1000 C atoms. It is observed that a group of 11 polymers displays rheological specific features which can be summarized as follows:  (a) higher dynamic viscosities at low frequencies than other polyethylenes and ethylene/1-hexene copolymers of similar molecular weight, polydispersity and SCB degree; (b) higher relaxation times than narrow molecular weight distribution polyethylenes of similar dynamic viscosities at low frequencies but similar relaxation times to those of broad molecular weight distribution; (c) higher values of elastic modulus, in comparison with polyethylenes of similar molecular weight, polydispersity, and SCB but of the same order of magnitude as those of broader molecular weight distribution; (d) higher activation energy of flow than linear polyethylenes of the same molecular weight, polydispersity, and SCB level. An analysis of the literature results leads us to suspect that the polymers which show a “dissident” behavior possess a certain very low degree of long chain branching (LCB). The analysis of the samples by SEC coupled with intrinsic viscosimetry reveals that some of these 11 polymers are long chain branched. However, this technique does not appear to be enough sensitive to detect very small amounts of LCB, and an alternative single rheological method, based on the effect of temperature on dynamic viscosity, is proposed to evaluate the possible presence of LCB.

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.

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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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Rheology is proposed as a tool to explore plasticized poly(vinyl chloride) (PVC) formulations to be used in the fused filament fabrication (FFF) 3D printing process and so manufactures flexible and ductile objects by this technique. The viscoelastic origin of success/failure in FFF of these materials is investigated. The analysis of buckling of the filament is based on the ratio between compression modulus and viscosity, but for a correct approach the viscosity should be obtained under the conditions established in the nozzle. As demonstrated by small amplitude oscillatory shear (SAOS) measurements, PVC formulations have a crystallites network that provokes clogging in the nozzle. This network restricts printing conditions, because only vanishes at high temperatures, at which thermal degradation is triggered. It is observed that the analysis of the relaxation modulus G(t) is more performing than the G″/G′ ratio to get conclusions on the quality of layers welding. Models printed according to the established conditions show an excellent appearance and flexibility, marking a milestone in the route to obtain flexible objects by FFF.

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