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Dimitri Merger, Mahdi Abbasi, Juri Merger, A. Jeffrey Giacomin, Chaimongkol Saengow, Manfred Wilhelm
Simple Scalar Model and Analysis for Large Amplitude Oscillatory Shear

Appl. Rheol. 26:5 (2016) 53809 (15 pages)

This work presents a simple, scalar model for predicting a nonlinear shear stress response of a viscoelastic fluid in Large Amplitude Oscillatory Shear (LAOS) experiments. The model is constructed by replacing the viscosity in the well-known Maxwell model by a shear rate dependent viscosity function. By assuming the empirical Cox-Merz rule to be valid, this shear rate dependent viscosity function is specified based on the Maxwell expression for the complex viscosity. We thus construct a particular case of the White-Metzner constitutive equation. Numerical solutions as well as an asymptotic analytical solution of the model are presented. The results, analyzed for higher harmonic content by Fourier transform, are compared to experimental data of a viscoelastic solution of wormlike micelles based on cetyltrimethylammonium bromide. Good agreement is found for low frequencies, where viscous properties dominate.

Cite this publication as follows:
Merger D, Abbasi M, Merger J, Giacomin AJ, Saengow C, Wilhelm M: Simple Scalar Model and Analysis for Large Amplitude Oscillatory Shear, Appl. Rheol. 26 (2016) 53809.

Paridokht Mahallati, Hojjat Mahi Hassanabadi, Manfred Wilhelm, Denis Rodrigue
Rheological characterization of thermoplastic elastomers (TPE) based on PP and recycled EPDM

Appl. Rheol. 26:3 (2016) 33503 (7 pages)

The rheological behavior of thermoplastic elastomers (TPE) based on 50/50 recycled ethylenepropylene-diene monomer (r-EPDM)/polypropylene (PP) was studied to determine the effect of feeding strategy when preparing these blends using twin-screw extrusion. In particular, small and large deformation characterizations have been performed to better understand the relationships between sample preparation and final properties of the blends. It was found that small changes in blend morphology (particle size and interfacial adhesion) are better distinguished in rheological properties (melt state) under large deformation (LAOS and step shear) compared to small deformation (SAOS).

Cite this publication as follows:
Mahallati P, MahiHassanabadi H, Wilhelm M, Rodrigue D: Rheological characterization of thermoplastic elastomers (TPE) based on PP and recycled EPDM, Appl. Rheol. 26 (2016) 33503.

Susana Filipe, Alfons Becker, Vitor C. Barroso, Manfred Wilhelm
Evaluation of melt flow instabilities of high-density polyethylenes via an optimised method for detection and analysis of the pressure fluctuations in capillary rheometry

Appl. Rheol. 19:2 (2009) 23345 (12 pages)

An optimised method for the detection and analysis of the time dependent pressure associated with the development of melt flow instabilities during extrusion through a capillary die was developed and validated. The magnitude and frequency of the developed quasi-periodic distortions, as well as the pressure profiles along the die length, were found to depend on the MWD, topology, melt elasticity and uniaxial extensional flow properties. Both the onset and magnitude of strain hardening in uniaxial extension appear to be related to the onset for the development of melt flow instabilities under capillary flow. For a better understanding of the role of the extensional properties (namely that of a purely elastic instability) the Hencky strain to failure was also determined and correlated to the observed flow instabilities. Time resolution of the capillary rheometer was improved by a factor of 1000, pressure resolution by a factor of 100 compared to the original set-up.

Cite this publication as follows:
Filipe S, Becker A, Barroso VC, Wilhelm M: Evaluation of melt flow instabilities of high-density polyethylenes via an optimised method for detection and analysis of the pressure fluctuations in capillary rheometry, Appl. Rheol. 19 (2009) 23345.

Christopher Klein, Paul Venema, Leonard Sagis, Dagmar van Dusschoten, Manfred Wilhelm, Hans Wolfgang Spiess, Erik van der Linden, Salman S. Rogers, Athene M. Donald
Rheo-optical Measurements using Fast Fourier Transform and Oversampling

Appl. Rheol. 17:4 (2007) 45210 (7 pages)

Rheo-optics is a method that allows the analysis of optical properties, like birefringence and dichroism under steady and oscillatory shear. It is possible to correlate macroscopic mechanical responses with induced microscopic changes in the material. We describe how this method was improved several fold and implemented on a commercially available setup. However, the here presented ideas are applicable to any rheo-optical setup, based on modulation of the laser light. Additionally it does not need a lock-in amplifier and therefore reduces the costs of the setup.

Cite this publication as follows:
Klein C, Venema P, Sagis L, vanDusschoten D, Wilhelm M, Spiess HW, vanderLinden E, Rogers SS, Donald AM: Rheo-optical Measurements using Fast Fourier Transform and Oversampling, Appl. Rheol. 17 (2007) 45210.

T. Neidhöfer, M. Wilhelm, H.W. Spiess
Fourier-transform-rheology on linear polystyrene melts

Appl. Rheol. 11:3 (2001) 126-133

Large amplitude oscillatory shear (LAOS) was applied to a linear polymer melt in order to study the mechanical harmonic generation in the nonlinear regime. In the nonlinear regime mechanical harmonics at 3w1, 5w1, etc. are generated under oscillatory shear with a shear frequency w1. These higher harmonics can be analysed with respect to frequencies, amplitudes and phase if the time data of the torque is Fourier transformed. This experimental method (FT-Rheology) permits therefore to quantify the mechanical nonlinearities if sinusoidal shear rates are applied. This article describes the basic idea of this experimental method as well as the application to different anionically polymerized polystyrene melts. The dependence of the nonlinear, rheological properties with respect to the molecular weight Mn, the applied strain g0, the frequency w1 and the temperature was investigated. In addition to simple nonlinear theories we could also observe even harmonics at 2w1, 4w1, etc. in the FT-rheology spectra. The appearance of even harmonics was correlated with the appearance of a more complex nonlinear behaviour.

Cite this publication as follows:
Neidhö, fer T, Wilhelm M, Spiess HW: Fourier-transform-rheology on linear polystyrene melts, Appl. Rheol. 11 (2001) 126.


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