Forest Products Laboratory

Title: Determination of Cellulose I Crystallinity by FT-Raman Spectroscopy

Author: Agarwal, U.P.; Reiner, R.S.; Ralph, S.A.

Date: 2009

Source: Proceedings of the 15th International Symposium on Wood, Fiber, and Pulping Chemistry, Oslo, Norway (June 15-18, 2009). Oslo, Norway : Congress-Conference AS, 2009: paper no. P-053_ISWFPC_Cellulose_Crystallinity.

Description: Two new methods based on FT-Raman spectroscopy, one simple, based on band intensity ratio, and the other, using a partial least-squares (PLS) regression model, are proposed to determine cellulose I crystallinity. In the simple method, crystallinity in semicrystalline cellulose I samples was determined based on univariate regression that was first developed using the Raman band intensity ratio of the 380 and 1096 cm^-1 bands. For calibration purposes, 90% crystalline Whatman CC31 and cellulose mixtures with crystallinities in the range 12-72% were used. When the intensity ratios were plotted against the theoretical crystallinities of the mixtures the plot showed a linear correlation (correlation coefficient R² = 0.995). Standard error calculated from five replicate Raman acquisitions indicated that the cellulose I Raman crystallinity model was reliable. Crystallinities of these cellulose mixtures were also calculated from x-ray diffractograms but these generated a correlation that was inferior compared to the one based on the Raman model. Additionally, using both Raman and x-ray techniques, sample crystallinities were determined from partially crystalline cellulose samples that were generated by grinding Whatman CC31 in a vibratory mill. The two techniques showed significant differences. In contrast to x-ray diffractograms, evidence of significant differences was present between the Raman spectra of low crystallinity celluloses and indicated that Raman spectroscopy was better suited to investigate differences between such samples.
In the second approach, a successful PLS (partial least squares) regression model for crystallinity, covering the 12-90% range, was generated from the same 7 calibration sample Raman spectra used in the simple method. The calibration model had a strong relationship between theoretical and predicted crystallinity values (R² = 0.995). The RMSEC (root mean square error of calibration) and RMSEP (root mean square error of prediction) values were 2.3 and 3.8%, respectively. RMSEP was the average prediction error, estimated in the validation stage and was subsequently validated with six independent ball milled Whatman CC31 samples. The regression coefficients obtained from the PLS model indicated that the cellulose bands, at 380 and 1096 cm^-1 were the major contributors in building a robust model for predicting cellulose I crystallinity.
It was concluded that either of the two Raman methods could be used for cellulose I crystallinity determination in cellulose samples.

Keywords:  Correlation, statistics, analytic chemistry, cellulose, chemistry, wood chemistry, X-rays, Raman spectroscopy, Fourier transform spectroscopy, spectrum analysis, regression analysis, standard error, partial least squares, band intensity ratio, cellulose I, crystallinity

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Citation

Agarwal, U.P.; Reiner, R.S.; Ralph, S.A.  2009.  Determination of Cellulose I Crystallinity by FT-Raman Spectroscopy.   Proceedings of the 15th International Symposium on Wood, Fiber, and Pulping Chemistry, Oslo, Norway (June 15-18, 2009). Oslo, Norway : Congress-Conference AS, 2009: paper no. P-053_ISWFPC_Cellulose_Crystallinity..