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Rheometry combined with Polarized Light Imaging

Visualization of shear stress in the sample

Mechanical material properties, as measured by rheological testing, strongly depend on microstructural changes under shear deformation and temperature conditions. Thus, the combination of optical techniques with rheological measurements is of high interest in the correlation of microstructural properties with the macroscopic behavior of the material. Different optical methods such as small-angle light scattering (SALS), microscopy (polarized light, fluorescence, confocal), spectroscopy (NIR, IR, Raman), birefringence, as well as pure visualization techniques can be employed.

Different to SALS, microscopy, or spectroscopy methods, polarized light imaging enables the observation of the complete sample area while sheared in the rheometer. The integrated behavior of macromolecules and nanoparticles can thus be detected.

Background

Shear-induced polarized light imaging (SIPLI) allows observation of flow-induced effects like shear-induced crystallization processes, orientation of polymer chains, or particles in various polymer solutions, suspensions, or polymer melts.

The method is based on the phase difference of light passing through optical active materials and provides information on optical path boundaries between optical isotropic and anisotropic structures.

Birefringence (or double refraction) is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive).

Birefringence can be related to:

  • orientation of molecules
  • orientation on molecule segments
  • orientation of anisotropic (non-spherical) particles
  • deformation of particles
  • break-up of particles
  • relaxation of orientation and deformation
  • stress (stress-optical rule)

Measurement example (cellulose solution)

The figure below illustrates the combined rheo-optical measurements for a highly concentrated gel-like cellulose solution. In order to visualize the stress distribution as experienced by the sample during the flow, the shear rate was logarithmically increased from 1 s-1 to 1,000 s-1 via a cone-and-plate measuring geometry. The tests were performed at 25 °C.

Figure 1: Viscosity curve of the cellulose solution with SIPLI images indicating orientation of the chains with the flow direction.

At the beginning of the test, the cellulose chains are randomly aligned; increasing the shear rate leads to a decrease of the viscosity due to increasing orientation of the cellulose chains. This is confirmed optically by the appearance of the Maltese cross which becomes more pronounced with increasing shear rate.

Typical test procedures

  • Flow curves over a large shear rate range
  • Step shear tests
  • Amplitude sweeps
  • Frequency sweeps
  • Three interval thixotropy tests (3iTT)
  • Temperature sweeps 

Measurement equipment

Polarized imaging principle

White or monochromatic light travels through a polarizer to the beam splitter, where it is deflected towards the sample so that the sample is illuminated with polarized light. 

The reflected light passes a second polarizer (called analyzer), where the polarization can be set perpendicular or parallel to the first polarizer.

The image of the illuminated sample is transferred telecentrically to the chip of a CCD camera, allowing the recording of changes in sample structures induced by the shear forces of the rheometer leading to, e.g., birefringence.

While parallel-plate and cone-plate geometries up to 50 mm in diameter can be used, an area with a radius of 25 mm is directly optically monitored by the SIPLI setup.

Rotatable analyzer for parallel and orthogonal polarization

Polarizer and analyzer set parallel: optical view of the whole sample

  • Control during loading: enough sample volume, tracking of air bubbles or impurities trapped into the gap
  • Monitoring of the sample during thermal treatments: partial melting or crystallization
  • Observation of edge fracture and fracture propagation

Polarizer and analyzer set orthogonal (cross-polarization)

  • Monitoring of shear-induced polarization effects
  • Monitoring of sample residual stress during or after gap setting (e.g. polymer melt)
  • Black images when sample is relaxed and normal force is very low
  • Measurements of stress distribution under shear
  • Following stretching, deformation and/or orientation of the molecules along the direction of shear or relaxation after cessation of shear

More information and application examples can be found in the following literature:

  • Mykhaylyk et.al. JOURNAL OF POLYMER SCIENCE, PART B: POLYMER PHYSICS 2016, 54, 2151–2170. Applications of Shear-Induced Polarized Light Imaging (SIPLI) Technique for Mechano-Optical Rheology of Polymers and Soft Matter Materials, doi.org/10.1002/polb.24111
  • Hausmann et al: ACS Nano 2018, 12, 6926−6937. Dynamics of Cellulose Nanocrystal Alignment during 3D Printing, doi.org/10.1021/acsnano.8b02366
  • Parisi et al. ACS Macro Lett. 2020, 9, 7, 950–956: Shear-Induced Isotropic–Nematic Transition in Poly(ether ether ketone) Melts, doi.org/10.1021/acsmacrolett.0c00404
  • Dunderdale et al. NATURE COMMUNICATIONS | (2020) 11:3372 Flow-induced crystallization of polymers from aqueous solution, doi.org/10.1038/s41467-020-17167-8