Outstanding Research: An Interview with Professor Naoto Nagai, Recipient of the 2025 Applied Spectroscopy William F. Meggers Award

The 2025 Applied Spectroscopy William F. Meggers Award has been presented to Professor Naoto Nagai of Niigata University, Japan, for his paper, “Azimuth Angle Dependence of Polarized Infrared Spectra of Injection-Molded Polyoxymethylene” (1,2). His research explores how molecular orientation in injection-molded polyoxymethylene (POM) affects polarized infrared reflection and attenuated total reflection spectra. By analyzing the directional dependence of these spectra, the study reveals how strong absorption and frequency dispersion of C–O vibrations influence spectral features, including shoulder wing generation. In this Spectroscopy interview, Professor Nagai shares insights into his findings and their significance for the spectroscopy community.

What motivated your study on the azimuth angle dependence of polarized infrared spectra in polyoxymethylene (POM)?

Prior to joining the university, I worked in both private and public sectors, specializing in failure analysis of industrial materials. I frequently used spectroscopic techniques for identifying the causes of various issues reported by companies. Among these techniques, infrared (IR) spectroscopy is a crucial tool owing to its ability to provide rapid evaluations, offering insights into molecular structures, crystallinity, and orientation — key factors in identifying the root causes of material failures.

In failure analysis, a comparison between defective samples and normal ones is essential, with a focus on the appearance or disappearance of small peaks, peak shifts, and changes in bandwidth.

I was asked to conduct a failure analysis of polyoxymethylene (POM), which led me to investigate this resin comprehensively. On analyzing the POM resin plates using attenuated total reflection (ATR) spectroscopy, a shoulder appeared just below 1000 cm⁻¹, suggesting the presence of a vibrational mode in that region. I reviewed several research papers to determine the assignment of this vibrational mode.Manystudies employed the KBr pellet method or Nujol mull method, and I noticed an intriguing pattern — it was reported that the spectral shape varied significantly between measurements even for the same sample. This variation was particularly pronounced around the shoulder region. However, no such variations were observed when the same sample was analyzed using Raman spectroscopy or X-ray diffraction, which perplexed me. Several interpretations have been proposed; however, none offered me a completely satisfactory explanation (3).

Subsequently, I subjected the injection-molded POM plates to polarized reflection measurements, aligning the measurement direction with the injection molding flow. I observed a reflectance of approximately 40% in the C–O stretching vibration region, which was quite broad and unusually high for a polymer. Considering my prior experience with respect to far-IR analysis of inorganic ionic crystals, I intuitively concluded this reflection to be closely resembling a Reststrahlen band. Subsequently, I confirmed the presence of a region, wherein the real part of the relative permittivity (or dielectric function) became negative, when I attempted to reproduce the reflection spectrum using an oscillator model. This finding suggested that the region exhibiting poor reproducibility in KBr measurements and the shoulder observed in the ATR spectra did not correspond to true absorption bands. Drawing from the analogy of inorganic ionic crystals, the apparent absorption observed in KBr measurements could be interpreted as radiative surface polaritons (4).However, this led to another question: What caused the shoulder in the ATR spectra? Because we were not actively measuring in the Otto configuration (5), it appeared unlikely that this signal resulted from (non-radiative) surface polaritons.

When employing spectroscopy to analyze industrial material failures, assigning peaks correctly is critically important. This motivated me to investigate the azimuthal dependence of the shoulder feature for clarifying the attribution of this shoulder.

Can you explain the significance of molecular chain orientation in injection-molded POM and how it affects IR spectra?

In general, the macroscopic properties of polymers, such as strength, are typically related to their structures. Because polymers exhibit hierarchical structures, their macroscopic properties are believed to be more closely associated with higher-order structures, including molecular orientation, rather than only their primary structures.

POM is a highly crystalline polymer, with a crystallinity of approximately 70–80%. During injection molding, the polymer chains strongly align along the direction of injection. Raman spectroscopy analysis of the in-plane orientation distribution revealed two components: one oriented along the injection direction and the other oriented perpendicularly, indicating that POM exhibits a characteristic bimodal orientation distribution. Injection-molded POM plates are relatively tough, which may be attributed to their bimodal network-like orientation distribution.

Recently, the remarkable polarization characteristics of POM, as observed from the infrared Reststrahlen region, have been investigated for potential use as a standard plate for enhancing the performance of polarized infrared cameras in outdoor applications. As these plates are prepared from resin, they can be manufactured at a relatively low cost, even at meter-scale sizes. In this application, precise control of the polymer orientation directly contributes to the effectiveness of the plate as a polarization standard (6).

What specific challenges arise when analyzing anisotropic polymeric materials like POM using polarized IR spectroscopy?

Crystalline polymers often undergo uniaxial orientation during fabrication processes such as injection molding. There are no issues when the polarization direction matches or is perpendicular to the orientation direction. However, when it does not align with the orientation direction, both the molecular main-chain and perpendicular components are observed in the corresponding spectrum.

In the reflection spectra, overlapping components are observed, wherein the s-polarized component converts to the p-polarized component and vice versa. This superposition introduces additional spectral features, and without a thorough understanding of this physical phenomenon, there is a risk of attributing these changes to variations in the chemical structure erroneously.

Therefore, it is advisable to first perform polarization measurements while rotating the sample to determine the principal axis direction. Aligning the subsequent polarization measurements with respect to this direction can help prevent difficulties in spectral analyses.

Can you elaborate on the role of azimuthal dependence in structural characterization?

The azimuthal dependence allows us to determine the orientation of the molecular main chain (crystalline principal axis) within the sample plane. In the case of injection-molded POM, the main chain aligns in the direction of the molten polymer flow from the gate. However, near the gate, the flowing polymer may circulate, indicating that the main chains may not always align along the expected direction. In this case, the reflectance of the Reststrahlen band decreases, allowing us to determine the angle at which the molten polymer flows out from the gate by the azimuthal dependence (7). Therefore, azimuthal dependence measurements play a crucial role in determining the orientation direction. Because the p-polarized spectrum contains information regarding the direction perpendicular to the surface, its analysis along with the azimuthal dependence of the s-polarized component could enable a three-dimensional orientation distribution analysis.

Furthermore, as shown in my paper (1), the angular dependence can be used to investigate the origin of unknown bands in uniaxial crystals.

Why is it challenging to obtain reproducible IR measurements using powder methods like KBr for POM?

The KBr method typically involves fine powdering of the material of interest, mixing with KBr, compression into a pellet, and its analysis using transmission spectroscopy. This method is widely accepted as a bulk measurement technique.

However, for materials such as POM, which exhibit large frequency dispersions and include regions wherein the real part of the relative permittivity (RP) is negative, radiative surface polaritons can result in apparent absorption. This is not a true absorption, but rather closer to scattering and can be considered a type of leakage mode. The frequency at which this mode appears depends on the shapes and sizes of the particles.

Because the KBr method involves processes such as powdering and pellet formation, which render it difficult to control the particle shape, size, and aggregation state, the data obtained generally lack reproducibility. For such materials, the KBr method should not be regarded as a bulk measurement technique, but rather as a method for examining surface effects (8).

Can you explain the role of real and imaginary components of relative permittivity (RP) in interpreting IR spectra?

The IR spectrum is obtained using different measurement modes, such as the KBr method, ATR, and reflection, yielding distinct signal shapes for the same material. This is because each measurement mode acts as a filter through which the material is observed.

Arguably, the most accurate representation of the true nature of a material is obtained from its RP spectrum (9). This function encapsulates the chemical structure and electronic state of the material. If the spectral shape of this function is known, ATR, reflection and transmission absorption spectra can be determined.

This function is generally complex, with its real and imaginary parts linked via causality. Furthermore, this pair is equivalent to the refractive index and extinction coefficient pair. Although not strictly accurate, the imaginary part of RP corresponds to absorption, whereas the real part corresponds to dispersion related to the refractive index. This complex function can typically be derived from the reflection spectra, which preserve the phase information.

In structural analysis, absorption spectra (imaginary part) are often preferred because they are easier to interpret. Therefore, the ATR and transmission methods, which produce spectra similar in shape to the absorption spectra, are commonly selected. However, for materials with strong polarity and large dispersion (real part), unexpected apparent peaks or shifts may occur at unanticipated frequencies, resulting in misinterpretation.

For instance, the broadening observed slightly below 1000 cm⁻¹ in the ATR spectrum of POM arises owing to optical mixing effects due to the anisotropic RP, where the frequency dispersion of the main-chain direction differs from that of the perpendicular direction. It does not correspond to an actual vibrational mode.

The assumption that every absorption peak in an IR spectrum corresponds to a vibrational mode can lead to misinterpretations, particularly when analyzing polar materials. This phenomenon is not an instrument-related artifact, but rather a physically meaningful artifact, which must be considered when interpreting the spectrum.

What is the significance of Yeh’s equation in predicting spectral shapes, and how does it compare to traditional models?

Several equations have been proposed for calculating the reflection spectra of anisotropic media, with matrix formulations extended to multilayer films being particularly well-known. Yeh also proposed such a formulation. These equations were possibly introduced for analyzing the properties of optical filters and devices (10).

The generalized matrix approach is particularly useful; however, Yeh’s equation, which incorporates azimuthal angles for uniaxial bulk crystalline media, provides a clearer relationship between RP and reflection, which are analogous to the Fresnel equations for isotropic media. This renders it particularly valuable for spectral analysis.

Despite the derivation of these equations, relatively few studies have attempted to compute the reflection spectra directly by inputting the RP spectra. For materials with a low frequency dispersion, the peaks corresponding to the RPs along the main-chain and perpendicular directions appear in a certain proportion, thereby rendering Yeh’s equation useful for the quantitative evaluation of the molecular orientation properties.

Moreover, when applied to materials with large frequency dispersions, such as POM, Yeh’s equation allows for the qualitative interpretation of peak shifts and the apparent generation or disappearance of peaks — phenomena that are difficult to explain using isotropic formalisms.

For materials with significant frequency dispersion, it is necessary to first consider the physical effects using Yeh’s equation before interpreting spectral changes in terms of chemical structural variations.

How does polarization conversion during reflection affect spectral interpretation?

No polarization conversion occurs when the main molecular chain axis aligns along the polarization direction or is perpendicular to it. However, if the main axis and polarization direction are misaligned, in addition to the angular distribution components, overlapping components are observed, wherein the s-polarized component converts to the p-polarized component and vice versa.

Depending on the azimuth angle, this converted component can result in an error that reaches approximately 10% of the reflection component along the principal axis direction compared with cases where no polarization conversion occurs. Consequently, there is a possibility that the spectra obtained at arbitrary azimuth angles cannot be explained using a simple vector summation of the principal axis directions in terms of intensity.

Why do ATR spectra exhibit azimuthal dependence, and how can this information be applied in practical material analysis?

The azimuthal dependence observed in the ATR spectra fundamentally arises owing to in-plane anisotropy. Typically, the ATR spectrum appears to be a weighted sum of the spectra along the principal axes corresponding to their respective directional contributions, at angles misaligned with the principal axes.

However, in materials with large dispersions, optical responses from each of the principal axis interfere, resulting in peak shapes and/or shiftsthat cannot be predicted using simple summation. As demonstrated in my paper (1), I used azimuthal dependence to assign features appearing in ATR spectra. This approach is effective for materials with large frequency dispersion.

Because the p-polarized component also contains information about the vertical component relative to the sample surface, the optical response along that principal axis also contributes to this interference effect.

In particular, for injection-molded or extruded polymer materials, molecular chain orientation significantly affects mechanical and optical properties. Therefore, this analytical approach is useful for quality control and material design.

In what way does mechanical processing such as stretching or extrusion impact POM’s spectral properties?

Mechanical processes such as stretching or extrusion tend to align the molecular backbones along the direction of the applied force. During injection molding, the oriented molecules are presumed to undergo folding. Conversely, stretching or extrusion further causes lengthening of the polymer chains, leading to an extended structure, wherein the reflectance increases significantly, sometimes reaching as high as 90%. The type of mesostructure the material adopts can be determined by analyzing the intensity and spectral shape of the reflection.

Notably, the reflectance of the extruded POM can become comparable to that of inorganic ionic crystals. In cases where the reflectance is extremely high, the presence of a Reststrahlen band is indisputable (11).

What inspired your interest in studying the spectroscopic properties of engineering plastics like POM?

As I mentioned in the first interview question, I was asked by a company to perform a failure analysis of POM; however, I could not pinpoint the cause, and the band assignments reported in the literature were not entirely convincing. Therefore, I decided to investigate the orientation distribution from the gate of an injection-molded plate. I performed in-plane mapping and observed a reflectance of approximately 40%, which is unusually high for a polymer. Hence, I was prompted to conduct research to determine the origin and characteristics of this high reflectance. Engineering plastics are used in a wide variety of industrial fields, which has sparked my interest in analyzing other resins as well. The most fascinating aspect about POM is that it exhibits a remarkably complex optical response despite having extremely simple functional group-level structural units.

Were there any unexpected findings during your study that led you to rethink your initial hypotheses?

Most certainly. This is something that many researchers may have had in common. In general, the IR response of organic molecules is generated owing to localized vibrations of functional groups. This is the foundation upon which IR spectroscopy is used for structural analysis based on the functional groups. However, when I obtained an unusually high IR reflectance for POM, I concluded that it was due to the stretching vibration of the C-O functional group. However, this high reflectance was speculated to be due to polaritons, which are generated by the strong coupling of molecular vibrations with light. The dipole moments of the C-O groups possibly interact with each other, leading to a phonon-like (or exciton-like) vibration, thereby resulting in polaritons. In other words, the vibrations are considered to be delocalized. Assuming that localized vibrations (such as C-H stretching) and delocalized vibrations coexist, it became considerably easier to understand the IR response of POM, which was something I had not expected initially. Initially, I found it difficult to believe that the real part of the RP could be negative for organic materials, and it took me a long time to arrive at the above interpretation (7).

What was the most challenging aspect of analyzing the azimuthal dependence of POM?

When dealing with materials such as POM, which exhibit significant anisotropy and frequency dispersion, I did not realize immediately that polarization conversion occurs during reflection. Therefore, it is necessary to use two polarizers, one on the incident side and one on the outgoing side, to properly measure and analyze the reflection. In addition, because this material has a bimodal nature, its incorporation into the calculation of the azimuth angle dependence of the spectrum and subsequent comparison with experiments proved challenging.

Moreover, I had to consider the imperfect contact between the prism and the sample while calculating the ATR spectrum, which meant inserting a gap into the model. When the principal axis and polarization direction do not match, polarization conversion occurs, leading to multiple reflections within the gap that sequentially cause polarization conversion. Considering this, I restricted my calculations to the principal axis alone while considering the gap.

In calculations based on the equations derived by Yeh, a quartic equation appears in the computation of the wavevector. Its solutions include both positive and negative values corresponding to each ordinary and extraordinary ray. Initially, I performed calculations using only the positive solutions. However, this approach resulted in discontinuities, which prevented us from obtaining a smooth reflection spectrum. Eventually, I realized that the negative solutions also need to be appropriately incorporated depending on the incident angle, azimuth angle, frequency, and RP to avoid this issue.

What advice would you give to researchers looking to investigate similar topics in polymer spectroscopy?

Spectroscopic research has been extensively conducted on polymers, and it may appear that there is no scope for further investigation. However, our investigation has revealed that there are still several phenomena that warrant further study. In addition to identifying the causes of failures in polymers used in industrial products, there is also a need to gather fundamental information on biopolymers and address environmental issues such as microplastics in the oceans. These are challenges that need to be approached from various perspectives.

Based on our previous research, one piece of advice would be to pay careful attention when analyzing large bands that appear in the IR region, such as those originating from the ether group vibrations and their surrounding regions.Within the scope of our research, POM exhibits the largest frequency dispersion among polymers; however, in the case of some other polymers, the real part of the RP becomes negative or materials with significant dispersion exist even if the real part does not become negative. Because the dispersion occurs slightly on the higher frequency side of the large peak, when new peaks originate around these large peaks or when discussing peak shifts, it is essential to verify whether these changes are due to chemical structural modifications or physical effects before proceeding.

What does being selected as the William F. Meggers Award winner for 2025, given to the author(s) of the most outstanding paper appearing in Applied Spectroscopy in 2024 mean to you personally?

I am deeply grateful to have received such a prestigious and honorable award. Applied Spectroscopy is a journal that publishes findings from various fields related to spectroscopy, including spectrometers and analytical methods. I have occasionally come across references to papers published in Applied Spectroscopy while reading classical textbooks on IR spectroscopy, such as those authored by Bellamy and Colthup (12,13). Because band assignment is a crucial aspect for applying IR spectroscopy to industrial fields, it is clear that this journal has contributed to these fields since its early days.

My research style may appear somewhat old-fashioned; however, I believe that it aligns with the original spirit of Applied Spectroscopy—considering the band assignments of organic materials. I have previously received this award in 2017, and I never expected to receive it again. However, I am extremely pleased to be able to further pursue my research from that time and publish some new findings.

I will be retiring from my university position next year, and receiving this award has reaffirmed the value of my research. I am immensely grateful to have been able to dedicate myself to research on spectroscopy.I would like to express my gratitude to the award committee members. Finally, I would like to say that I love this journal. Regardless of the institution the authors are affiliated with, the fairness in the review process and the outstanding and constructive comments provided by the reviewers significantly improved the manuscript, leading to its publication. I sincerely hope that this journal continues to thrive in the future.

References

(1) Nagai, N. Azimuth Angle Dependence of Polarized Infrared Spectra of Injection-Molded Polyoxymethylene. Appl. Spectrosc. 2024, 78 (2), 197–208. DOI: 10.1177/00037028231217005

(2) Applied Spectroscopy William F. Meggers Award Page. https://sas.memberclicks.net/william-f--meggers-award (accessed 2025-03-04).

(3) Terlemezyan, L.; Mihailov, M.; Schmidt, P. Conformational Changes of Poly(oxymerhylene) Induced by Pressure and Mechanical Treatment. Makromol. Chem. 1978, 179 (3), 807–813.

(4) Ruppin, R.; Englman, R. Optical Phonons of Small Crystals. Rep. Prog. Phys. 1970, 33 (1), 149–196.

(5) Raether, H. Surface Plasmons on Smooth and Rough Surfaces and on Gratings; Springer-Verlag: Berlin, 1988; pp 37–39.

(6) Laramée, A. W.; Roy, O.; Pellerin, C. Electrospun Materials with High Reflectance and Polarization Contrast for Sensing Applications in the Mid-Infrared Atmospheric Window. Appl. Polym. Mater. 2024, 6, 6997–7005.

(7) Nagai, N.; Okawara, M.; Kijima, Y. Infrared Response of Sub-Micron-Scale Structures of Polyoxymethylene: Surface Polaritons in Polymers. Appl. Spectrosc. 2016, 70, 1278–1291.

(8) Nagai, N.; Okada, H.; Hasegawa, T. Morphology-Sensitive Infrared Absorption Bands of Polymers Derived from Surface Polaritons. AIP Adv. 2019, 9, 105203.

(9) Kittel, C. Introduction to Solid State Physics, 8th ed.; John Wiley & Sons, Inc.: New Jersey, 2005; Chapter 14.

(10) Yeh, P. Optical Waves in Layered Media; John Wiley & Sons, Inc.: New York, 1998; Chapter 4.

(11) Nagai, N.; Okada, H.; Amaki, Y.; Okamura, M.; Fujii, T.; Suzuki, T.; Takayanagi, A.; Nakagawai, S. Anomalous High-Infrared Reflectance of Extruded Polyoxymethylene. AIP Adv. 2020, 10, 095201.

(12) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 3rd ed.; Chapman and Hall: London, 1975.

(13) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: San Diego, 1990.

About the Award Winner

Naoto Nagai, Recipient of the 2025 Applied Spectroscopy William F. Meggers Award

Naoto Nagai received a master’s degree from Niigata University in the field of theoretical solid state physics. He worked at Toray Research Center, Inc. (TRC) after graduation. He was a leader of the infrared spectroscopy team in TRC and received his PhD from Tohoku University while working at TRC. His main work was to resolve problems in the production processes or complaints from the companies in Japan using spectroscopic techniques such as infrared spectroscopy, photoluminescence, spectroscopic ellipsometry and terahertz spectroscopy. His main interests were the point defect analysis of semiconductor materials, characterization of dielectric films, analyzing polymer surfaces, and feasibility study of the application of the terahertz spectroscopy. When he moved to Industrial Research Institute of Niigata Prefecture (IRI-Niigata), which is one of the research centers organized by the local government in Japan, his main focus was the application of spectroscopy, including Raman spectroscopy and X-ray photoelectron spectroscopy, to the analysis of industrial materials. In 2021, he retired from IRI-Niigata as its chair and moved to Niigata University as a professor in charge of internships. While mediating between students and companies, he is also working on elucidating the peculiar behavior observed in the infrared spectra of polar polymers with large dispersions. He received the Advanced Analytical Technology Award from The Japan Society for Analytical Chemistry in 2005, and he was awarded the Meggers Award from the Society for Applied Spectroscopy in 2017.

About the Interviewer/Author

Jerome Workman, Jr. serves on the Editorial Advisory Board of Spectroscopy and is the Executive Editor for LCGC and Spectroscopy. He is the co-host of the Analytically Speaking podcast and has published multiple reference text volumes, including the three-volume Academic Press Handbook of Organic Compounds, the five-volume The Concise Handbook of Analytical Spectroscopy, the 2nd edition of Practical Guide and Spectral Atlas for Interpretive Near-Infrared Spectroscopy, the 2nd edition of Chemometrics in Spectroscopy, and the 4th edition of The Handbook of Near-Infrared Analysis. Author contact: JWorkman@MJHlifesciences.com ●