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RheologyOur research interests in this field are (mainly for polymer containing systems):
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
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.
The viscosities η for shear rates 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.
Literature : Pressure influences on the viscosity
Shear induced mixing or demixing
This graph shows the shear effects on the phase separation for a ternary system
Literature :
Interrelation of rheological and thermodynamic properties.
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)
Literature: Systems consisting on low
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.
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
Beside of low molecular weight nematics, for electrorheological experiments single phase polymer containing systems are investigated. Literature : Electrorheology |
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