Cellulose vibrational spectra

Two classes of experiments were conducted. In both sets of experiments, fibers in which the cellulose chains are oriented parallel to the fiber axis were used. In the first class of experiments, the plane of polarization of the incident light was changed relative to the axis of the fibers by rotating the fibers around the optical axis of the microscope (see Figure 2a). The dependence of the band intensities on the polarization of the incident light was studied to determine the directional character of the vibrational motions. This information was used to advance the assignment of the Raman spectrum of cellulose. Spectra from Valonia, ramie, and mercerized ramie fibers, which have different allomorphic compositions, were compared to study the structural differences between the allo-morphs. 

Based on the number and location of the maxima and minima in the intensity vs. 0 curves, the bands in the Raman spectrum of native cellulose were divided into four categories. Table 1 summarizes the band classifications for those bands which were resolved well enough to be analyzed. The classifications provide information about the directional character of the vibrations. The four categories are described as follows ... 

Based on the number and location of the maxima and minima in the relationship between the band intensities and the polarization of the incident light relative to the chain axis, the bands in the Raman spectrum of cellulose could be divided into four groups. The about the direction of the vibrational motions in cellulose. The directions of the vibrations are such that the major change in polarizability associated with the motions is either parallel or perpendicular to the chain axis. Raman spectra recorded from deu-terated celluloses allowed the vibrational modes involving C-H and 0-H motions to be identified. These spectra demonstrated that most of the modes are complex coupled vibrations. Normal coordinate analyses of cellulose model compounds were done to determine the types of motion most likely to occur in each region of the spectrum. The calculations also suggested that the vibrational motions are very complex. The information from the normal coordinate calculations. 

The reactions of NaOH with cellulose in natural fiber yielded cellulose-ONa compound and removed impurities from the fiber surface [11,12]. This is confirmed by the FTIR spectroscopic analysis as shown in Figure 14.1. The FTIR spectrum of the raw wood clearly shows the absorption band in the region of3407 cm, 2917 cm and 1736 cm due to O-H, C-H and C = O stretching vibration respectively. These absorption bands are due to hydroxyl groups in cellulose, carbonyl groups of acetyl ester in hemi-cellulose and carbonyl aldehyde in lignin. 

The spectrum revealed the typical nitrile stretching vibrations at 2200 cm the -OH stretching vibrations at 3500 cm and the absence of the strong carbonyl band at 1720 cm This confirmed that the product was, indeed, a true cellulose - PAN graft pol3rmer. 

The Raman spectrum of cellulose has a pair of bands at 1122 and 1097 cm , and a few other peaks below 500 cm . As is normal with Raman spectra, frequencies associated with the hydroxy functionality are extremely weak in intensity. The most informative band is located at 899 cm , which confirms the (1-1,4-linkage of pyranose rings are, as discnssed above. In the case of a-pyranose compounds, such as D-glucose and sncrose, the Q — H vibration appears at around 825 cm (glncose) and 850 cm (sucrose). 

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