A Recent Look at Molecular Spectroscopy

Spectroscopy magazine publishes a column titled, “Molecular Spectroscopy Workbench” that focuses on molecular spectroscopic techniques. Many of these articles are designed to be tutorial in nature, allowing readers an inside look at the interactions between electromagnetic waves and matter.

Create an illustration of a detailed molecular structure analysis, highlighting bonds and molecular geometry. | Image Credit: © PrusarooYakk - stock.adobe.com

Below is a compilation of four of our most recent Molecular Spectroscopy Workbench columns. Happy reading!

Monitoring Chemical Changes by Raman Spectroscopy

This article discusses the application of Raman spectroscopy in real-time monitoring of chemical reactions, with a particular focus on polymerization. Polymerization involves the loss of a carbon double bond as the polymer chain lengthens, and the strong signals from π electrons make it easy to track this reaction to completion (1). Despite its usefulness, setting up polymerization experiments for demonstration can be hazardous. The article aims to illustrate the chemical and spectral changes that take place during the curing process of a commercial epoxy (1).

Raman Spectra Used to Understand the Origins of Banding in Spherulites

This article examines the formation and characteristics of spherulites in polymers, which form crystal lamellae radiating from a nucleation site when the polymer is crystallized from the melt. When observed under a microscope with crossed polarizers, these spherulites display a distinctive Maltese cross pattern with banding (2). The lit regions, resulting from crystals growing in directions not parallel to the polarizers, often show banding because of rotations of the crystal lamellae (2). Given the sensitivity of polarized Raman spectra to crystal orientation, the article explores the relationship between the observed banding pattern and Raman polarization/orientation behavior (2). The investigation focuses on spherulites of poly(hydroxybutyrate-co-hydroxyhexanoate) (PHBHx) with different compositions (2).

Calibrating the Composition of a Copolymer

This article details the use of Raman spectroscopy in collaboration with Isao Noda to study the properties of the bioplastic polyhydroxybutyrate hexanoate (PHBHx). The properties of PHBHx, such as optical clarity, dyeability, flexibility, and thermal characteristics, depend on the percentage of hexanoate, which introduces propyl side branches to the polymer chain (3). These percentages influence the polymer's maximum crystallinity and, consequently, its physical and chemical properties (3). The article highlights the usefulness of Raman spectroscopy in accurately determining the composition of PHBHx, demonstrating a higher-than-expected precision in measuring hexanoate percentages (3).

Measuring the Crystallinity of PHBHx with Varying Amounts of Sidechains on a Benchtop Instrument

This article explores the use of current benchtop instruments with edge filters that provide Raman spectra down to 50 cm^-1 for studying the crystallization of polyhydroxybutyrate-hexanoate (PHBHx). These compact instruments simplify the process compared to larger setups. By averaging the signals from spherulites, the orientation effects are mitigated, facilitating the use of multivariate techniques to compare different samples (4). PHBHx polymers are gaining commercial interest due to their biodegradable nature and production via fermentation, positioning them as sustainable alternatives to non-biodegradable, petroleum-derived polymers (4). The article also explains that controlling the polymer's physical properties, such as crystallinity, is achievable by adjusting the molecular weight and the percentage of sidechains. Removing the effects of polymer chain orientation through spectrum averaging is essential for accurate spectral comparison across samples (4).

References

(1) Adar, F. Monitoring Chemical Changes by Raman Spectroscopy. Spectroscopy 2024, 39 (2), 8–10. DOI: 10.56530/spectroscopy.ia6269u7

(2) Adar, F. Raman Spectra Used to Understand the Origins of Banding in Spherulites. Spectroscopy 2023, 38 (11), 8–13. DOI: 10.56530/spectroscopy.wu2080k9

(3) Adar, F. Calibrating the Composition of a Copolymer. Spectroscopy 2023, 38 (6), 6–8. DOI: 10.56530/spectroscopy.xy2066v8

(4) Adar, F. Measuring the Crystallinity of PHBHx with Varying Degrees of Sidechains on a Benchtop Instrument. Spectroscopy 2023, 38 (2), 8–12. DOI: 10.56530/spectroscopy.bn2087n4