Fraunhofer Institute for Structural Durability and System Reliability LBF
Fraunhofer Institute for Structural Durability and System Reliability LBF

Confocal Raman microscopy

Combination of Raman spectroscopy with confocal microscopy

Confocal Raman microscopy (CRM) is a powerful analytical tool for the analysis of all types of polymer samples with high spatial resolution. By combining Raman spectroscopy with confocal microscopy, CRM detailed chemical images of polymers are obtained down to the submicrometer range. In polymer research, CRM reveals the chemical composition and the phase separation of polymer composites, films, coatings as well as the identification of additives and impurities.

Raman microscope at the Fraunhofer LBF, type WiTec Alpha 500

Non-destructive and three-dimensional

The main advantages of CRM are the non-destructive approach and the ability to map chemical information in three dimensions. Often used in quality control, materials research and the development of new polymer materials, CRM provides insight into properties such as crystallinity, molecular orientation and chemical bonding without affecting the sample.

In general, CRM is suitable for organic and synthetic polymers and provides the high spatial and chemical resolution necessary to understand complex polymer behavior, quality control and the development of new polymeric materials:

  1. Polymer films and coatings: Thin films and coatings on substrates can be analyzed for chemical composition, homogeneity and film thickness. CRM is ideal for analyzing multilayer coatings, additives and impurities or voids in films.
  2. Polymer blends and composites: CRM can map the distribution of different polymers or fillers within a composite, revealing phase separation, filler distribution and interface characteristics between the different components.
  3. Biodegradable polymers and biomaterials: CRM can investigate biodegradable polymers, e.g. in medical devices and drug delivery systems. It helps to evaluate the degradation process, chemical composition and stability under different conditions.
  4. Crystalline and semi-crystalline polymers: CRM can recognize differences in crystallinity and molecular orientation in polymers such as polyethylene (PE) and polypropylene (PP), which is crucial for understanding mechanical and thermal properties.
  5. Polymer fibers and textiles: CRM is used to analyze synthetic fibers (such as PP, nylon and polyester) in textiles. It can map the distribution of sizing, dyes, additives and molecular orientation within the single fibers.
  6. Microplastics and environmental samples: CRM identifies microplastics in environmental samples and analyzes their chemical composition, size and degradation state.
  7. Cross-linked and cured polymers: For thermoset polymers (e.g. epoxies) CRM helps to determine the degree of cross-linking and chemical homogeneity, which is important for the evaluation of curing efficiency and material stability.
  8. Conductive polymers and electronic materials: CRM can analyze conductive and semiconductive polymers used in electronics, revealing chemical composition, doping level and distribution of conductive fillers.
  9. Polymers with additives: CRM is excellently suited for the detection and analysis of additives such as plasticizers, stabilizers, pigments and flame retardants within a polymer matrix.

Beispiele:

a) SEM image, b) Raman spectra, and c) chemical maps of the core and shell of microcapsules in polyoxymethylene (POM).
a) Raman spectra and b) chemical maps of the characteristic bands of halogen-free flame retardants in polypropylene.
a) Raman spectra and b) chemical maps of a polymethyl methacrylate (PMMA) film containing a titanium dioxide (rutile) agglomerate.
a) Raman spectra of polypropylene (PP) (red) and glass fiber (blue), b) distribution within the PP matrix, and c) cross-section of the glass fiber; the Raman maps can be used to determine the fiber orientation.
a) Raman spectra and b) chemical maps of the halogen-free flame retardants PPM triazine and ammonium polyphosphate (APP) in a polypropylene sample during thermal aging.
Raman mapping of a plastic film composed of multiple layers of different polymers (multilayer). Both the identity and the thickness of the individual layers can be characterized using confocal Raman microscopy (CRM).
Raman maps of the surfaces of 4 different PP samples. The samples contain varying amounts of recycled material (0% “virgin,” 30%, 50%, and 100%). For each sample, an area of 50 × 50 μm was measured with a resolution of 50 × 50 spectra. The samples show an increasing amount of carbon black (in red) on the surface, proportional to the recycled content.

Basics

In CRM, a laser is focused on the polymer sample, exciting molecular vibrations that are specific to the chemical bonds in the material. When the laser light interacts with the sample, a small portion of the scattered light shifts in wavelength according to the vibrational energy levels of the polymer's molecules - a phenomenon known as the Raman effect. This shifted light is collected and analyzed to produce a Raman spectrum that provides detailed information about the chemical composition and structure of the sample.

As a key feature, CRM uses a confocal pinhole aperture so that only light from a very small volume of the sample reaches the detector. By scanning the sample in three dimensions, CRM thus creates a high-resolution chemical map that enables in-depth analysis of polymer properties such as phase distribution, crystallinity and chemical composition without the need for time-consuming mechanical sample preparation. This high spatial resolution makes CRM particularly valuable for understanding complex polymer structures, quality control and the investigation of material behavior under different conditions.

Publications:

  • Baade, G., Geertz, G., Güttel, R., Friedland, J., Polymeric Reactors for Catalytic Reactions beyond Earth: Proof of concept, 29 November 2024, Version 1, https://doi.org/10.26434/chemrxiv-2024-51pdl
  • Bunjes, A., Arndt, J.-H., Geertz, G., Barton, B., Characterization and chemometric modelling of mechanically recycled polypropylene for automotive manufacturing, Polymer, Volume 249, 2022, https://doi.org/10.1016/j.polymer.2022.124823
  • Zou, M., Barton, B., Geertz, G., Brüll, R., Accurate determination of the layer thickness of a multilayer polymer film by non-invasive multivariate confocal Raman microscopy, Analyst, Volume 144, 2019, https://doi.org/10.1039/C9AN00664H

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