Rheology

Our research interests in this field are (mainly for polymer containing systems):

• Effects of changes in the molar mass and concentration of the polymer, temperature, shear rate and solvent
• Pressure influence on the viscosity
• Interrelation of rheological and thermodynamic properties. An intensively studied topic is the phase behaviour of flowing, polymer containing mixtures, namely shear induced mixing and demixing. This subject is of great practical importance during the processing of polymeric mixtures, for example the co-extrusion of polymer blends.
• Description and prediction of the viscosity of mixtures of both low and high molecular weight compounds with a new method developed on the basis of molecular surfaces and volumes of the components.
• Rheology of two-phase blends
• Electrorheology.

Equipment:

At atmospheric pressure most measurements are performed on the two rheometers Haake CV100 (shear rate controlled, also for electrorheology) and Carri-Med CSL 500 (stress controlled), respectively. Furthermore we have two pressure viscometers at our disposal, a rolling ball apparatus for low viscosities and a Searle-type viscometer for more viscous systems. A rheo-optical apparatus consisting of a searl type viscometer is used to measure simultaneously viscosity and turbidity. To study the morphology of two-phase-systems we use the optical shearing system CSS 450 made by Linkam Scientific Instruments, GB, in combination with a microscope Olympus BX 50.

Examples of scientific topics:

Polystyrene microgels: stationary shear and creep experiments

Microgels are small network particles with diameters smaller than 1 µm for which the inner structure resembles typical networks. Due to their molecular nature, they can not only swell but dissolve in solvents. (PS)_1:20 are polystyrene microgels, crosslinked via radical copolymerisation of styrene and m-diisopropenylbenzene in microemulsion with a cross-linking density of 1 cross-link per 20 monomer units.

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Fig. 1.A flow curve of a mixture of the microgel (PS)1:20 and 1,2-dichlorobenzene (DEB)

The viscosities η for shear rates < 10^-1s^-1 and < 10^-1 s^-1 were measured by creep experiments and stationary shear, respectively. In this system for low shear rates the viscosity increases with temperature. This inverse temperature behaviour results from the increasing degree of swelling of the microgels, as the temperature rises.

Literature: Effects on the viscosity, due to changes in the molar mass and concentration of the polymer, temperature, shear rate and solvent

Pressure influences:

Rheological measurements can be performed up to 2000 bar and 200 °C, applying shear rates up to 2000 s^-1. An example of our high pressure measurements is given in the following diagram.

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Fig. 2.The zero-shear viscosities of the system CO2/poly(dimethylsiloxane) as a function of the volume fraction φ of the polymer for three different molar masses; p = 200 bar and T=30°C

Literature : Pressure influences on the viscosity

Shear induced mixing or demixing

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Fig. 3.The shear effects on the phase separation for a ternary system with two highly incompatible polymers and a common solvent

This graph shows the shear effects on the phase separation for a ternary system made up of two highly incompatible polymers (polystyrene (PS), poly(n-butyl methacrylate) (PBMA)) and a solvent (cyclohexanone, CHO) which is thermodynamically good for both components. The results are presented for constant values of w*_PBMA, the weight fraction of PBMA in the blend PS/PBMA (w*_PBMA = w_PBMA /(w_PS + w_PBMA)) and of w_pol, the over-all weight fraction of the polymer (w_pol = w_PS + w_PBMA). The corresponding theoretical calculations (black curves, with slightly different w_pol) are also depicted in this figure. In the case of PBMA-rich solutions, a augmentation of the shear rate by 100 s^-1 increases the cloud point temperatures by approx. 20 °C (extension of homogeneous region / shear induced mixing) in an approximately linear manner. The shear effects depend pronouncedly on the composition of the blend and are very prominent for mixtures that contain more of the higher molecular weight PBMA and less of the lower molecular weight PS.

Literature : Interrelation of rheological and thermodynamic properties.
Literature : shear influences on phase behaviour.

Viscosity of 2-component-systems: Experiments and theoretical description

The viscosity of polymer/solvent systems is modelled theoretically as a function of composition under the premises that the dissipation of energy is taking place at the molecular interfaces and that the friction between solvent and solute varies with composition due to a change in the flow mechanism (drainage of coils). The resulting simple expression contains four system-specific parameters: a geometric factor γ, which accounts for the differences of the surface to volume ratios of the components; a viscometric interaction parameter α, which measures the friction between solute and solvent in the case of fully draining polymer coils, [η], the specific hydrodynamic volume of the polymer at infinite dilution (intrinsic viscosity), and [η] _Θ the specific hydrodynamic volume under theta conditions. In the equation shown in the diagram [η]_Θ is described by λ. The suitability of this model has been demonstrated for poly(dimethylsiloxane) dissolved in penta(dimethylsiloxane) and for the systems diethyl phthalate/poly(methyl acrylate) and diethyl phthalate/poly(vinyl acetate) . The last is given as example in the following figure.

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Fig. 4.Viscosity of 2-component-systems: Experiments and theoretical description

Literature: Systems consisting on low respectively high molecular weight compounds .

Rheology of two-phase blends

The figure shows a light microscopy image of a mixture of two immiscible siloxanes (PDMS / P(DMS-ran-MPS), containing 60 vol.-% of PDMS). The micrograph was taken under stationary conditions at a shear rate of 1 s^-1 by means of the optical shearing system CSS 450. The most striking feature in the blend morphology consists in the absence of individual droplets and in the existence of string phases.

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Fig. 5.Picture of string phases in a sheared polymer blend.

Literature : Rheology of two-phase blends.

Electrorheology:

The viscosity η of nematic liquid crystals measured in the presence of an electric field parallel to the gradient of the velocity shows a complex dependence both on magnitude E of the electric field and on the shear rate .

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Fig. 6.Viscosity of 4-(trans-4´-pentylcyclohexyl)-benzonitrile (PCH-5): Influences of shear rate and electric field.

Beside of low molecular weight nematics, for electrorheological experiments single phase polymer containing systems are investigated.

Literature : Electrorheology