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Dielectric spectroscopy combined with rheology

Dielectric spectroscopy is an investigation technique based on the study of a material’s response to an applied electric field. It is possible to investigate dielectric as well as conductive samples. Combining dielectric spectroscopy with mechanical spectroscopy (e.g. rheology) provides information on the structure and behavior of the analyzed data. In addition to rheological data (viscosity, modulus), the influence of mechanical deformation on the conductivity, capacity, and permittivity can be investigated. Dielectric spectroscopy combined with rheology can be carried out with constant or variable frequency of the electric field. Besides typical applications like the detection and monitoring of structural changes in curing reactions, dielectric spectroscopy combined with rheology can be helpful in the following fields of application:

  • Polymers: degree of polymerization, water content, determination of glass transition temperature
  • Pharmaceuticals: thermotropic phase transition, aging (e.g. gel-type drugs), microstructure of (semi-solid) emulsions
  • Food: moisture content, behavior of food after reheating
  • Cosmetics: aging, stability improvements, ingredient orientation

Background

Orientation of polar molecules without and with electric field

If a material contains polar molecules, they will be randomly distributed when no electric field is applied (unpolarized). Applying an electric field will polarize the material, and the molecules will be orientated. In an alternating electric field, the molecules will change their orientation according to the frequency. The lower the frequency of the applied electric field, the more easily all molecules can follow. If the frequency is increased, larger molecules will not be able to re-orientate completely, while smaller still do.

Measurement example

The following figure illustrates the creep test for a carbon black composite. Carbon black clusters are dispersed in a polymer matrix. The carbon black induces dielectric properties in the composite. During shape recovery (after being subjected to a constant load for 135 s), the rheological data (strain) show slow mechanical relaxation of the long chain polymers, while the dielectric data (conductivity) show fast electrical relaxation of carbon black clusters.

See more measurement examples as part of the e-learning: DRD

Figure 1: Creep test of a carbon black composite. Rheological data (top) and dielectric data (bottom).

Typical test procedures

  • Flow curves at different frequencies
  • Amplitude sweeps / creep tests at different frequencies
  • Frequency sweep (dielectric) under constant rheological conditions
  • Curing reactions at different frequencies
  • Temperature sweeps at different frequencies

Measurement equipment

In recent years, combined rheological methods (see the application report: Dielectric spectroscopy combined with high-temperature rheology) such as dielectric spectroscopy with rheology have become popular since they enable simulation of real-world application requirements.

To investigate dielectric properties simultaneously to rheology, a rheometer equipped with a Dielectric-Rheological Device (DRD) is used. The temperature concept of the DRD is Peltier-based or convection-based. The measuring system is a parallel-plate geometry for common tests or a disposable geometry for curing reactions. The convection-based setup is available in single-drive and TwinDrive mode.

Temperatures up to 600 °C and down to -160 °C can be applied. The frequency range depends on the LCR meter; usually applications work in a range from Hz to MHz in combination with a rheometer. The LCR meter of the DRD is synchronized with the rheometer by the software (trigger impulse). The rheometer can be equipped with various LCR meters. Different contact options are available to perform rotational as well as oscillatory tests.

You can find further information in our e-learning course on dielectric rheology.

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