AN-RS-008
2025-02
Identification of monomers with Raman spectroscopy
Monitor the polymerization process from monomer to polymer
Summary
Polymers are comprised of macromolecules that are in turn made of numerous identical or similar structural units, referred to as monomers. The number of monomers, including additives or inhibitors such as benzoquinone – used to endow the polymers with specific properties – is enormous.
All polymer manufacturers use the same monomers and can profit from a quick raw material check before feeding them into the polymerization process. Raman spectroscopy provides a nondestructive, in-situ, real-time analytical method to monitor the polymerization process by tracking monomer consumption and polymer formation. Ultimately, Raman spectroscopy is a convenient and efficient tool for various industries related to the polymer sector.
Introduction
This Application Note demonstrates the convenient identification of monomers within seconds using Raman spectroscopy. Monomers such as styrene, various alkyl methacrylates, vinyl acetate, ethylene glycol, phenol, terephthalic acid, and urea, as well as additives or inhibitors like benzoquinone, can be identified rapidly and unambiguously.
A quick demonstration of the distinct spectra of different monomers and their respective polymer leads to an in-depth look at the polymerization reaction of Bakelite.
Experimental
Raman spectroscopy is a simple point-and-shoot nondestructive analysis technique that allows for fast and safe analysis with no sample preparation. In some cases, samples can even be analyzed in their original packaging.
A handheld 785 nm Raman device, featuring an automated workflow and Orbital Raster Scanning (ORS™), was used to collect basic monomer spectra.
The polymerization reaction of Bakelite was safely monitored by placing a laboratory Raman fiber-optic probe against the wall of a beaker containing the reactants, allowing real-time evaluation as the reaction progressed.
Monomer spectra
Figure 1 contains overlaid spectra of different monomers (and benzoquinone), demonstrating that Raman is both sensitive and high specific—it is very easy to distinguish the spectra of different materials.
Monomers and polymers
Monomers bind together to form polymers during the polymerization process. Real-time monitoring of the polymerization reaction with Raman spectroscopy is a powerful way to optimize and control the process and the resulting product. It is Raman’s specificity that permits monomers to be easily distinguished from their respective polymers. Figure 2 illustrates the spectral differences between polymer starting materials and products.
Polymerization monitoring with Raman
Bakelite is a thermosetting plastic created through the polymerization of phenol and formaldehyde. Figure 3 shows how the Raman peaks of phenol diminish as it reacts with formaldehyde to form a cross-linked polymer, while new peaks emerge due to changes in the vibrational environment. High Raman shift regions permit the observation of changes in the C-H stretching vibrations of phenol (2000–4500 cm⁻¹), providing insight into this reaction.
Conclusion
Real-time, on-site analysis using Raman spectroscopy enables polymer manufacturers to maintain product integrity through quality checks at every stage, from raw materials to final products. This ensures consistent quality, optimizes efficiency, and drives innovation in polymer manufacturing.
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