Detection and quantitation of cellulose II by Raman spectroscopy

In cellulose materials, the cellulose II allomorph is often present either exclusively or in conjunction with cellulose I, the natural cellulose. Moreover, in regenerated and mercerized fibers (e,g., lyocell and viscose), natural cellulose adopts to the crystal structure cellulose II. Therefore, its detection and quantitation are important for a complete assessment of such materials investigations. In the Raman spectra of such materials, a band at 577 cm⁻¹ is typically observed indicating the presence of this allomorph. In the present study, to quantify the content of cellulose II, a calibration method was developed based on the intensity of the 577 cm⁻¹ peak relative to the 1096 cm⁻¹ band of cellulose. For this purpose, in addition to pure cellulose I and cellulose II samples (respectively, Avicel PH-101 and mercerized Avicel PH-101; hence referred to as Avicel I and Avicel II), a set of five samples were produced by mixing them in known quantities of Avicel I and Avicel II. The crystalline cellulose II contents of the samples were calculated based on the X-ray crystallinity of mercerized Avicel I. These seven samples were included in the calibration set and their Raman spectra were obtained. Subsequently, Raman intensity ratios I₅₇₇/I₁₀₉₆ were calculated by taking ratios of peak intensities at 577 and 1096 cm⁻¹. These ratios were plotted against the % of crystalline cellulose II present in the calibration set samples and the two were found to be linearly correlated (R² = 0.9944). The set-samples were also analyzed using XRD which were then compared with the Raman method developed here. Compared to XRD, the Raman method was found to be more sensitive at detecting and quantifying cellulose II. Additionally, several cellulose II containing materials were analyzed by the new Raman method.