Helmut Münstedt

Elongational behaviour of a low density polyethylene melt

SummaryThe steady-state elongational viscosityµs of melts of the low density polyethylene IUPAC A was investigated over six decades of strain rate. At low elongation rates the melt behaves as a linear viscoelastic liquid. TheTrouton relationµ0=3η0 is valid. The tensile compliance is one third of the shear compliance. With increasing strain rate the elongational viscosity goes through a pronounced maximum of the heightµs≈7µ0. Steadystate recoverable strains up toεr,s=2.16 have been measured. The results from tensile creep tests and constant stretching rate tests are in quantitative agreement. It is shown how the influence of the interfacial tension between the polyethylene sample and the surrounding silicon oil which is not negligible at small strain rates can be taken into account experimentally.ZusammenfassungAn Schmelzen des verzweigten Polyäthylens IUPAC A wurden die Dehnviskositätenµs im stationären Bereich über sechs Dekaden der Dehngeschwindigkeit gemessen. Bei niedrigen Dehngeschwindigkeiten verhält sich die Schmelze linear-viskoelastisch. Es gilt dieTrouton-Beziehungµ0 = 3η0. Die Dehnnachgiebigkeit beträgt ein Drittel der Schernachgiebigkeit. Mit wachsender Dehngeschwindigkeit durchläuft die Dehnviskosität ein ausgeprägtes Maximum der Höheµs ≈ 7 µ0. Im stationären Bereich wurden reversible Dehnungen bis zuεr,s = 2,16 gemessen. Die Meßergebnisse von Dehnkriechversuchen und Messungen mit konstanten Dehngeschwindigkeiten stimmen quantitativ überein. Es wird gezeigt, wie der Einfluß der Grenzflächenspannung zwischen der Polyäthylenprobe und dem umgebenden Silikonöl, der insbesondere bei niedrigen Dehngeschwindigkeiten nicht zu vernachlässigen ist, experimentell berücksichtigt werden kann.

New Universal Extensional Rheometer for Polymer Melts. Measurements on a Polystyrene Sample

A new universal extensional rheometer, which makes it possible to investigate the elongational properties of polymer melts, is described. Measurements under a wide variety of constant stretching rates and constant tensile stresses can be undertaken and recoil and relaxation experiments can be performed on a small amount of material. The operation modes of the apparatus are demonstrated on a polystyrene sample. For this polystyrene the viscosity and the recoverable strain in the steady state of elongation were measured over more than three decades of the tensile stress. At small stresses the recoverable strain is proportional to the stress, i.e., the compliance is constant and the viscosity gives a constant value which is three times the zero‐shear viscosity. At higher stresses the recoverable strain seems to level off and the viscosity runs through a maximum which lies 30% above the stress‐independent value.

Dependence of the Elongational Behavior of Polystyrene Melts on Molecular Weight and Molecular Weight Distribution

The elongational viscosity and the tensile compliance on four polystyrenes of different molecular weights and molecular‐weight distributions were determined by means of an extensional rheometer designed particularly for investigations on a small amount of polymer. The steady‐state elongational viscosity at small strain rates is three times the zero‐shear viscosity according to the Trouton relationship. The Trouton viscosity is proportional to Mw3.5 independent of the molecular weight distribution. For the steady‐state tensile compliance in the linear range of elongation no quantitative relationship with molecular data can be given but a strong increase of the compliance due to small amounts of a higher molecular weight component is observed. In the nonlinear range of elongation a small viscosity maximum as a function of tensile stress is observed for three samples whereas one sample shows an increase of the elongational viscosity up to values which are at least five times higher than the elongational visc...

Strain hardening of various polyolefins in uniaxial elongational flow

In this contribution correlations between the type of strain hardening of long-chain branched polyethylenes and the level of their zero shear-rate viscosities in comparison to linear polyethylenes are described. For polyethylenes of various branching structures four different types of strain hardening can be observed. Type I: strain hardening is approximately independent of elongational rate. Type II: strain hardening decreases with increasing elongational rate. Type III: strain hardening increases with increasing elongational rate. In addition to that, materials are found which do not show strain hardening within the experimental window (type IV). Qualitative correlations with the dependence of the zero shear-rate viscosity on the weight average molecular mass can heuristically be deduced. Polyethylenes of type IV fulfill the well-established relationship η0∼Mw3.4, samples of types I and II give higher zero shear-rate viscosities than the linear products. Type III samples generally exhibit zero shear-rat...

Stick and slip phenomena during extrusion of polyethylene melts as investigated by laser-Doppler velocimetry

The flow behavior of polyethylene melts in a slit die was investigated using laser-Doppler velocimetry. Two polyethylenes, a linear and a long-chain branched sample, were compared in order to get an insight into the influence of branching on stick and slip phenomena. For the long-chain branched polyethylene, velocity profiles were obtained which do not give any indication to wall slip in the range of shear rates applied. For the linear polyethylene, the velocity distributions in the three regions, commonly distinguished by different pressure-output relations in the literature, were measured. Pronounced wall slip velocities are detected at low apparent shear rates. The importance of this finding for the determination of viscosity functions is discussed. At higher output rates the well-known pressure oscillations are accompanied by velocity fluctuations of the same frequency but totally different shapes of amplitudes. Significant periodic features of the time dependence of the velocity are tried to be inter...

Rheological behavior of blends from a linear and a long-chain branched polypropylene

This paper investigates how the rheological behavior of a linear polypropylene is changed by blending with different amounts of a long-chain branched polypropylene. The zero shear-rate viscosities of the blends follows the logarithmic mixing rule between the two blend partners up to 50 wt % of the long-chain branched polypropylene. For the blend with 75 wt % long-chain branched polypropylene a deviation from the logarithmic mixing rule was found, which can be referred to a disentanglement of the long-chain branched fraction of molecules during the blend extrusion process similar to the findings for the extruded long-chain branched polypropylene. It is concluded that the linear polypropylene in the blends reduces the extrusion effect on the long-chain branched species in a way that for the blends with up to 50 wt % of the linear PP and no influence of the blend preparation process on the rheological behavior occurred. As the branching structure within the blends remains unchanged it could be shown that the...

Analytical and rheological characterization of long-chain branched metallocene-catalyzed ethylene homopolymers

Abstract The aim of this paper is to compare the results of analytical and rheological characterization techniques with respect to the analysis of long-chain branches (LCB) in polyethylenes. The materials investigated are metallocene-catalyzed ethylene homopolymers into which LCB were introduced by the selection of appropriate polymerization conditions. The samples were characterized analytically by 13C NMR, IR-spectroscopy, and size exclusion chromatography coupled with a multi-angle laser light scattering detector. The rheological characterization was performed using dynamic–mechanical measurements and creep and creep recovery experiments in shear. It was found that the analytical methods were in good qualitative agreement for the most highly branched sample whose LCB content was 0.12 LCB/1000 C as determined by 13C NMR spectroscopy. However, even a 10-times lower LCB content, which is almost beyond the detection limit of 13C NMR measurements, had a significant impact on the rheological behavior. Rheological experiments clearly indicated the presence of LCB by changes in the frequency dependence of dynamic–mechanical material functions and the molecular mass dependence of the zero shear-rate viscosity in comparison to linear polyethylenes.

Rheological properties and foaming behavior of polypropylenes with different molecular structures

This article investigates how rheological properties of polypropylenes with different molecular structures influence their foaming behavior. The molecular structure of the different polypropylenes is analyzed by size exclusion chromatography coupled with a light scattering detector, and by rheological means, such as the molar mass dependence of the zero shear-rate viscosity. The main focus of the rheological experiments is laid on the strain hardening and failure behavior of the melts in uniaxial elongational flow. For all linear polypropylenes investigated, a rupture of samples occurred before the maximum strain—accessible with the instrument used—was achieved. For the linear polypropylenes, a growth of the stress to rupture and the elongation at break were found with increasing molar mass, which go along with an increase of the expansion ratio in the foaming experiments. Besides the linear polypropylenes, which do not show strain hardening, several so-called high melt strength polypropylenes were invest...

Effects of bioactive glass and beta-TCP containing three-dimensional laser sintered polyetheretherketone composites on osteoblasts in vitro

Because of their excellent physical properties nonresorbable thermoplastic polymers have become more important for the field of reconstructive surgery. Aim of the present study was to investigate the effects of laser sintered polyetheretherketone (PEEK) with incorporated osteoconductive and bioactive bone substitution materials on osteoblasts in vitro. Human osteoblasts (hFOB 1.19) were seeded onto laser sintered PEEK samples containing nano-sized carbon black, beta-tricalciumphosphate (beta-TCP), and bioactive glass 45S5. Osteoblasts were investigated for cell viability, cell proliferation and cell morphology. A constant proliferation of osteoblasts could be observed on all samples with the highest values for bioactive glass containing samples at day 7 (OD 1.76 +/- 0.22) and day 14 (OD 3.75 +/- 0.31) and lowest values for beta-TCP containing probes throughout the study compared with the PEEK pure control group. Highest cell viability was observed for Bioglass containing probes (95.5 +/- 3.32)% whereas osteoblasts seeded on beta-TCP containing probes showed reduced viability (84.4 +/- 4.32)%. Laser sintered PEEK implants seem to be attractive candidates for use as bone substitutes for reconstructive surgery because of their biocompatibility, individual shape, and the possibility of compounding bioinert polymer powder with osteoconductive and bioactive materials which might benefit bone formation in vivo.

Effect of long-chain branching on the foaming of polypropylene with azodicarbonamide

In this article the influence of long-chain branching on the foaming behavior of polypropylene (PP) is investigated. Different branching contents are achieved by blending a linear PP (L-PP) and a long-chain branched PP (LCB-PP). Whereas, the L-PP does not exhibit any strain hardening in laboratory stretching experiments, blends with amounts of the LCB-PP higher than 2 wt% already show a pronounced strain-hardening behavior. The strain hardening increased with a growing amount of the long-chain branched PP. A laboratory scale foaming apparatus based on a capillary rheometer is developed for the foaming experiments. In foaming tests with azodicarbonamide as chemical blowing agent, a significant improvement of the foaming behavior with respect to a higher expansion ratio, a lower amount of connected cells, and a more homogeneous cell size distribution is found with increasing content of the LCB-PP up to a concentration of 50 wt%. At this concentration, the foaming behavior of the LCB-PP is reached. The results demonstrate that low amounts of long-chain branching can significantly improve the optimal foaming process of PPs with a chemical blowing agent, and that additions of the linear material up to 50 wt% to the LCB-PP do not have any influence on the favorable foaming performance of the long-chain branched PP.