Claus Gabriel

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...

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.

Elongation-induced crystallization of a high molecular weight isotactic polybutene-1 melt compared to shear-induced crystallization

The extensional viscosity fixture (EVF) from TA Instruments was applied for elongation-induced crystallization (EIC) experiments on a high molecular weight isotactic polybutene-1 (PB-1). The results are compared to those previously obtained for shear-induced crystallization (SIC) on the same sample. After annealing the sample at a temperature well above the melting point to erase any memory effects, the sample is cooled to a temperature below the melting point (93–101°C). An increase of the transient elongational viscosity compared to the Trouton viscosity 3η(t) is used to define a crystallization onset time. Challenges caused by the chamber temperature control of the EVF in the ARES, like the delayed attainment of a stable sample temperature as well as the significant mismatch of the latter compared to the imposed nominal temperature, are addressed. By modifying the temperature program accordingly, it was finally possible to run shear and elongation experiments at a comparable sample temperature protocol to prove that quasi-quiescent crystallization occurs at the same time for both types of flow. At Hencky strain rates above 10−3s−1, the onset time decreases rapidly with increasing strain rate. Compared to SIC, the decrease in EIC is much stronger (e.g., at 10s−1 the onset time in elongation is two powers of ten shorter than that in shear) and approaches a constant Hencky strain eC=4 regime. A temperature change by 4°C had no significant effect on eC. Following a procedure introduced for SIC, a temperature-invariant plot of the onset time versus normalized flow rate was constructed containing both flow types.The extensional viscosity fixture (EVF) from TA Instruments was applied for elongation-induced crystallization (EIC) experiments on a high molecular weight isotactic polybutene-1 (PB-1). The results are compared to those previously obtained for shear-induced crystallization (SIC) on the same sample. After annealing the sample at a temperature well above the melting point to erase any memory effects, the sample is cooled to a temperature below the melting point (93–101°C). An increase of the transient elongational viscosity compared to the Trouton viscosity 3η(t) is used to define a crystallization onset time. Challenges caused by the chamber temperature control of the EVF in the ARES, like the delayed attainment of a stable sample temperature as well as the significant mismatch of the latter compared to the imposed nominal temperature, are addressed. By modifying the temperature program accordingly, it was finally possible to run shear and elongation experiments at a comparable sample temperature protocol...

Comparison of shear-induced crystallization behavior of PB-1 samples with different molecular weight distribution

Shear-induced crystallization behavior of three grades of PB-1 having different molecular weights and breadths of molecular weight distribution was investigated by means of rotational rheometry covering a shear rate range of 0.0001 up to 0.3 s−1 and temperatures between 99 °C and 107 °C. The up-shoot in viscosity after a certain time is used to define an onset time ton for both quasiquiescent and shear-induced crystallization. The plot of ton versus the shear rate can be converted into a temperature-invariant curve by dividing the y axis by quiescent onset time ton,q and multiplying the x axis by the square root of ton,q. The shape and location of the T-invariant curves for the three PB-1 samples are compared with respect to their molecular parameters. An additional normalization step by multiplying the abscissa by the square root of the characteristic retardation time of the melt yields a dimensionless representation of the T-invariant curve. Furthermore, by this procedure the three curves are shifted into one region, where they have the same threshold value, indicating the onset of shear-induced crystallization. A connection of this T-invariant curve to the morphology of the crystallites formed are addressed.Shear-induced crystallization behavior of three grades of PB-1 having different molecular weights and breadths of molecular weight distribution was investigated by means of rotational rheometry covering a shear rate range of 0.0001 up to 0.3 s−1 and temperatures between 99 °C and 107 °C. The up-shoot in viscosity after a certain time is used to define an onset time ton for both quasiquiescent and shear-induced crystallization. The plot of ton versus the shear rate can be converted into a temperature-invariant curve by dividing the y axis by quiescent onset time ton,q and multiplying the x axis by the square root of ton,q. The shape and location of the T-invariant curves for the three PB-1 samples are compared with respect to their molecular parameters. An additional normalization step by multiplying the abscissa by the square root of the characteristic retardation time of the melt yields a dimensionless representation of the T-invariant curve. Furthermore, by this procedure the three curves are shifted in...

Guidelines for checking performance and verifying accuracy of rotational rheometers: viscosity measurements in steady and oscillatory shear (IUPAC Technical Report)

Abstract The paper addresses techniques for checking the performance of rotational rheometers with cone–plate, plate–plate, or concentric cylinder geometry. We focus on the determination of the viscosity as a function of the shear rate and of the magnitude of the complex viscosity as a function of the angular frequency. After summarizing the relevant definitions and test modes, we show examples of measurements in the linear viscoelastic range, and applications of the Cox–Merz relationship. Sources of reference fluids with defined viscosities are presented, and their use in tests for verification of accuracy is demonstrated. Relevant issues, predominantly for Newtonian reference liquids, are the exploration of measurement limits, related either to the shear rate range or to reliably accessible viscosity levels. Viscoelastic reference samples are also discussed. Prerequisites for sample preparation and loading are addressed. In particular, we present recommendations based on experience from various laboratories. Finally, we discuss the problem of temperature calibration, presenting techniques that allow the determination of the true sample temperature for a given set temperature of the rheometer. This paper summarizes contributions from various industrial and academic laboratories.

DURABILITY TESTING ON MAGNETORHEOLOGICAL FLUIDS FOR CLUTCH AND BRAKE APPLICATIONS

A twin gap plate-plate shear cell, developed by BASF, meets the requirements for reliable durability testing on magnetorheological fluids. The possibility to test the effect of life time energy input to MR-formulations within a few days is demonstrated for commercial MRF formulations (Basonetic®). MRF durability measurements as well as further analytical characterization of the sheared MRF prove that magnetorheological fluids with high content of magnetisable particles, as required for clutch/brake applications, are able to withstand specific energy intakes of more than 5·10 J/m3. This remarkable result should encourage designers in industry and automotive to explore new MRF applications.