Investigation of filters and membranes

Recently one topic is dominating our life. The pandemic of coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

In this context several methods and strategies are discussed and recommended. However one recent aspect in the discussion is wearing masks in public. The discussion not only deals with the usage of masks, but also with their different types and properties. We remember terms like FFP1, FFP2 or FFP3. (Please read the infos of the responsible ministries.)

Behind all these actual themes industry and science are working intensively to deliver best quality of and knowledge about the products.

One important aspect in the field of health protection is the investigation of filters and membranes on a microscopic or even electron microscopic level.

Here, the FELMI-ZFE has got a broad expertise especially in the field of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and spectroscopic methods. Here you find the overview over several methods starting with mere imaging through to correlative electron microscopy.

Morphological investigation of filter materials and membranes with SEM

Figure 1: SEM images of membranes. Cross-sections of a) a flat-sheet membrane, b) a hollow-fibre membrane and c) a multipore membrane.

For the assessment of the morphology of material in the micrometer regime down to a resolution of some nanometers the SEM is used. The conventional SEM needs a preparation of electrically isolating specimens by applying a carbon or gold layer onto the surface. However, at FELMI-ZFE there are several SEMs available, which enable the direct imaging of isolating materials by a so called low vacuum mode (variable pressure mode, VP), where an imaging gas is used, which avoids charging of the material during investigation. The used microscopes are called Variable Pressure/ Environmental Scanning Electron Microscopes (VP-ESEM)

Reference: PhD thesis Manfred Nachtnebel (2017)

Investigation of wet materials

Figure 2: In situ drying of a microfiltration membrane in the ESEM, while the left images show the surfaceand the right images the cross-section and A the complete wet state throgh to the dry state at D.

The ESEM additionally enables the investigation of materials in wet state using water vapour as imaging gas during cooling of the specimen. Thus the behaviour of material in contact with water and even condensation and drying processes can be performed and imaged at high resolution.

Reference

Nachtnebel, H. Fitzek, C. Mayrhofer, B. Chernev, P. Pölt, Spatial localization of membrane degradation by in situ wetting and drying of membranes in the scanning electron microscope, J. Memb. Sci. 503 (2016) 81–89

3D reconstruction of filter materials

Figure 3: left side: SEM image of a cross-section of a membrane (bright areas) embedded in resin (dark areas) which undergos a SBEM investigation to enable the 3D-reconstruct of the whole structure (right side)

The combination of SEM and ultramicrotomy leads to a comparatively new method called serial block-face scanning electron microscopy (SBEM). Here, a specimen is cut with a diamond knife and after each cut it is imaged by an SEM. As a result a series of images are collected to a stack of micrographs, which finally can be reconstructed in three dimensions. This gives an optimized look into a porous material and open the way to simulations concerning e.g. the performance of a filter.

References

Zankel A., Kraus B., Poelt P., Schaffer M., Ingolic E. Ultramicrotomy in the ESEM, a versatile method for materials and life sciences. J. Microsc. 233 (2009) 140-8.

Reingruber, A. Zankel, C. Mayrhofer, P. Poelt. Quantitative characterization of microfiltration membranes by 3D reconstruction. J.Membr.Sci. 372 (2011) 66-74.

Automated particle analysis via energy dispersive X-ray spectroscopy (EDXS)

Figure 4: Automated particle analysis with SEM/EDXS of particles on a dust filter a), which provides good statistics b) or elemental information of every single particle c).

To obtain information about the elemental composition of the material under investigation, the combination of SEM and energy dispersive X-ray spectroscopy (EDXS) is widely used. Especially for selected areas or particles. A special option available at the institute makes it possible to obtain automatically elemental information of a huge number of particles at filters or other substrates. This provides good statistics about particle morphology (particle size distribution – PSD) and composition as well as elemental information about every single particle.

Vibrational spectroscopy: Infrared and Raman

Figure 5: top: IR-spectroscopic images of the material distribution of two different components (PES/PVP) at the cross-section of a membrane; bottom: representative IR-spectra at two different points at the membrane.

A further method to obtain information about the distribution of different materials is IR-spectroscopy. This especially enables the investigation of organic materials, like polymers, because it provides information about the molecular composition. Spectra can be obtained either at different points or at whole areas.

Recently the system RISE (Raman Imaging and Scanning Electron microscopy) was established at FELMI-ZFE, where a high resolution SEM is combined with a Raman microscope. Here on one position of a specimen a correlation of SEM information and chemical information can be gained.

References

https://www.youtube.com/watch?v=a7QPXjmmeJw

PhD thesis Manfred Nachtnebel (2017)

Schmidt R, Fitzek H, Nachtnebel M, Mayrhofer C, Schroettner H, Zankel A. The Combination of Electron Microscopy , Raman Microscopy and Energy Dispersive X-Ray Spectroscopy for the Investigation of Polymeric Materials. Macromolecular Symposia 2019; 384: 1–10. DOI: 10.1002/masy.201800237.