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Advanced Spectral Analysis: Two-Dimensional Correlation Spectroscopy

Summary

We have developed an approach to characterize overlapping spectral data based on two-dimensional correlation spectroscopy (2DCOS). Using advanced 2DCOS, we characterized spectral signatures associated with structural changes in polymer and alkane melting and unraveled the mechanism of associated phase transitions.

Description

When a series of spectra are acquired as a function of controlled perturbation variable, such as temperature, pressure, electric field strength, concentration, and any arbitrary variables, a conventional spectral analysis is performed on the basis of intensity changes at a specific frequency. However, a highly overlapped spectral region is challenging for the single-frequency-based analysis. Two-dimensional correlation spectroscopy (2DCOS), which was originally proposed by Noda in 1986, compares spectral intensity variations at a frequency pair with respect to a perturbation variable. 2DCOS provides positive and negative correlations between two spectral components. It also provides specific sequences of certain spectral intensity changes and is used to enhance the spectral resolution of highly overlapped spectra. 2DCOS has been applied extensively to analyze various types of spectral data from Raman and IR to X-ray, UV-Vis, and fluorescence.

We have applied 2DCOS analysis methods to unravel spectral signatures of phase transitions occurring in polymers and other complex materials [1,2,3]. We also have developed theories to advance moving-window approaches to 2DCOS [4,5].

Image 1
Figure 1. Moving-window two-dimensional correlation spectroscopic (MW-2DCOS) analysis of n-C21H44 phase transition [1]. Two narrow overlapping peaks are identified in the Raman CH2 twisting band in the orthorhombic phase. The two peaks become a single peak in the rotator phase, which then broadens and shifts to a higher frequency in the amorphous phase.

 

Figure 2
Figure 2. Least squares moving-window (LSMW) analysis [5] of the Raman spectra of high-density polyethylene (HDPE) [2]. This result is consistent with the multi-thickness lamellae model, where multiple melting temperatures correspond to a multimodal thickness distribution of the orthorhombic lamellae in the PE crystalline phase.


PUBLICATIONS

[1] Y. Jin, A. P. Kotula, A. R. Hight Walker, K. B. Migler, and Y. J. Lee, Phase-Specific Raman Analysis of n-Alkane Melting by Moving-Window Two-Dimensional Correlation Spectroscopy, J. Raman Spectrosc. 47, 1375 (2016). https://doi.org/10.1002/jrs.4967

[2] Y. Jin, A. P. Kotula, A. R. Hight Walker, K. B. Migler, and Y. J. Lee, Raman Identification of Multiple Melting Peaks of Polyethylene, Macromolecules 50, 6174 (2017). https://doi.org/10.1021/acs.macromol.7b01055

[3] I. S. Ryu, X. Liu, Y. Jin, J. Sun, and Y. J. Lee, Stoichiometric analysis of competing intermolecular hydrogen bonds using infrared spectroscopy, RSC Advances 8, 23481 (2018). https://doi.org/10.1039/c8ra02919a

[4] Y. J. Lee, Analytical and Numerical Characterization of Autocorrelation and Perturbation-Correlation Moving-Window Methods, Appl. Spectrosc. 71, 1321 (2017). https://doi.org/10.1177/0003702816681169

[5] Y. J. Lee, Least Squares Moving-Window Spectral Analysis, Appl. Spectrosc. 71, 1894 (2017). https://doi.org/10.1177/0003702816685336

Created May 15, 2019