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Small-angle light scattering (SALS)

Monitoring density and orientation fluctuations within a sample, averaged over scattering volume; investigation of shear-dependent shape, and orientation of microstructure

Rheological measurements provide information about the macroscopic behavior, but cannot directly provide further information about the inner microscopic structure of a sample. The macroscopic behavior usually depends on its microscopic structure. Conclusions from purely rheological measurements on the microstructure are usually based on observations of model systems. The combination of small-angle light scattering (SALS) and a rheometer permits direct correlation between the rheological behavior and the microstructure of the sample. 

Background

Figure 1 Small-angle light scattering method

If light passes through a medium, a reduction in the intensity of light can usually be observed. The light is partly reflected at the surface of the medium, and the residual light either interacts with the medium or goes through it. Interactions of the photons of the laser light with particles can lead to elastic scattering. The light is deflected away from its original path, and it leaves the medium in a different direction, without energy transfer.

The density and orientation fluctuations within a sample, averaged over the whole scattering volume, can be well monitored by SALS as intensity distribution in the so-called inverse or momentum space (scattering vector).

When SALS is combined with rheology, the intensity distribution analyzed as function of scattering angles or scattering vector respectively, gives information on the change of microstructure due to shearing.

Measurement example (polymer blend)

An immiscible polymer blend containing 1 % polyisobutylene (PIB) in a polydimethylsiloxane (PDMS) matrix was subjected to shear in a combined rheo-SALS setup. The immiscible blend consists either of PIB droplets or PIB domains in PDMS. The PIB is an elastomer, produced by cationic vinyl polymerization of isobutylene and is often used in glues and sealing compounds.

The flow behavior of the polymer blend, subjected to logarithmic shear ramp from 1 s-1 to 200 s-1, is characteristic for unlinked polymers. For shear rates up to 20 s-1, the sample shows zero-shear viscosity. The shear force leads to orientation and deformation of the PIB domains but is counteracted by interfacial tension forces, which try to keep the droplets spherical, i.e the viscosity doesn’t change.

The scattering patterns obtained at rest and at low shear rates (images a and b in the figure below) show an isotropic angular distribution of the scattered intensity indicating a spherical shape of the PIB droplets in the PDMS matrix.

Increasing the shear rate, the shear forces across the droplets dominate the interfacial tension forces; the droplets start to deform and the viscosity decreases. Following the orientation and deformation of the PIB domains, the scattering patterns begin to change from a circular to an elliptical shape.

The higher the shear rate (images d and e), the more oriented and deformed the PIB domains become, leading to higher anisotropy of the scattering patterns.

SALS is an "inverse space" method: Large structures scatter light at small angles, whereas light scattering on small dimensions causes large scattering angles. Therefore, the deformation of the PIB domains in a shear flow direction results in light scattered at smaller angles in the shear direction, and at larger angles perpendicular to the shear direction. The light-scattering patterns change from circular to ellipsoid, and the ellipse is perpendicular to the shear direction.

Figure 2 PIB + PDMS blend under shear. Higher anisotropy of the scattering patterns correlates with stronger deformation and alignment of the PIB domains toward shear direction.

Typical test procedures

  • Flow curves
  • Amplitude sweeps
  • Frequency sweeps
  • Temperature sweeps (melting & crystallization)
  • Shear-induced crystallization at constant temperatures

Measurement equipment

The modular rheo-SALS system features both Peltier and electrical temperature control. A small moveable laser diode emits light, with a wavelength of 658 nm, which is transferred towards the sample (primary laser beam). A lens system collects the scattered light and transfers it directly onto the CCD (charge-coupled device) chip of a camera. A rotatable polarizer in front of the sample (for incoming beams) and analyzer behind the sample (scattered light) allow measurements with polarized and depolarized light. The optical train (SALS lower module) also contains the beam stopper and maintains the scattering angle as well as the light intensity undistorted. In addition to the laser diode, the optics can be moved as well, allowing a flexible focus on different points of the sample. The large range of scattering angles allows measurements of both small and large structures.

Figure 3 The rheo-SALS setup

Conclusions

A combination of rheological measurements and small-angle light scattering can help to understand a sample's rheological behavior. The macroscopic behavior can be explained with information from the scattering images and this could permit forecasting for comparable systems. A further detailed analysis of the light scattering images provides additional information about the sample.

Both small-angle light scattering (SALS) and microscopy are suitable for structure investigations in the micrometer-size range simultaneously with rheological measurements, since they do not require special preparation of the samples. The density and orientation fluctuations within a sample, averaged over the whole scattering volume, can be well monitored by SALS as intensity distribution in the so-called inverse or momentum space. On the other hand, microscopy images show individual structure elements displayed in real space.

Thus, although both microscopy and SALS result from density (or orientation) fluctuations within a sample, they are complementary methods and together with the information provided by rheological studies contribute to a complete characterization of samples.