Raman spectra celluloses

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 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). 

In this section, we examine the internal stress within the cotton fibers treated with SMPU and DMDHEU. Eichhom and Young (Eichhom et al, 2001 Eichhom and Yoimg, 2001,2003 Eichhom eta/., 2003) showed that the internal stress of cellulose fibers can be measured by shifts of C-O-C vibrations in the Raman spectrum an increase in the wave intensity indicates a reduction of the internal stress, and vice versa. Using the strain-induced shift in the special Raman peak, incorporated with micro-Raman spectroscopy, is a technique unique to micro-mechanics. A Raman spectrum of imtreated cotton is shown in Fig. 10.15. The C-O-C peaks are 1096 and 960cm , depending on the internal stress of the cellulose chain. 

The structural differences between Ig and Ig are not understood yet. Atalla (18) compared the Raman spectra of various native celluloses with different Ig to Ig ratios. He also compared the native cellulose spectra with the spectrum of cellulose II. The spectra of... 

The IR and Raman spectra of cellulose acetate are presented in Reference Spectrum 60. A decrease (or even disappearance) in band intensity of the... 

FT-Raman has been used as an alternative to TG techniques to determine filler content in HDPE/ CaC03 composites and provides comparable results [400]. As most pigments (apart Ifom carbon-black) and glass are poor Raman scatterers, in principle Raman spectra are obtainable Ifom these samples without removal of the fillers or difficult sample preparation. Conventional visible Raman spectroscopy has failed in attempting to analyse dyesmffs. Conventional Raman spectra of dyed textiles tend to be dominated by the (fluorescent) spectrum of the dye [401]. Consequently, FT-Raman spectroscopy may be a more useful tool for direct observation of low levels of dyestuffs in polymeric materials. Indeed, by using NIR excitation dramatic improvements in the Raman spectra of these dyes can be achieved [392]. FT-Raman was quite useful for the discrimination of differently dyed cotton-cellulose fabrics with the bifunctional reactive dye Cibacron C, provided that the interpretation was facilitated by chemometrics [402]. Schrader et al. [403] have used FT-Raman spectra to distinguish non-destructively the main dye components in historical textiles. Bourgeois et al. [401] have successfully used FT-Raman in the characterisation of... 

Details are given of the determination of apparent absorption coefficients and optimal sample thickness of PE, PETP, cellulose, and cyclohexane by FT-Raman analysis. A relationship was developed that permits the optimal sample thickness for FT-Raman measurements to be determined by measuring the absorbance spectrum of a sample whose thickness is known. Results are discussed in the context of scattering theory, sampling considerations, and spectral artifacts. 9 refs. 

---