Cellulose Raman spectroscopy

Principles and Characteristics The prospects of Raman analysis for structural information depend upon many factors, including sample scattering strength, concentration, stability, fluorescence and background scattering/fluorescence from the TLC substrate. Conventional dispersive Raman spectroscopy has been considered as a tool for in situ analysis of TLC spots, since most adsorbents give weak Raman spectra and minimal interference with the spectra of the adsorbed species. Usually both silica and cellulose plates yield good-quality conventional Raman spectra, as opposed to polyamide plates. Detection limits for TLC fractions... 

Changes in the conformation of the different forms of cellulose have been investigated157,158,195,196 by use of Raman spectroscopy. Celluloses I and II were found157 to have different, and distinct, molecular-chain conformations. No assignments of the frequencies were proposed, but the correlation between the spectra and the structure of celluloses was discussed. The major differences in the Raman spectra were observed below 800 cm-1, in the... 

Since there are numerous types of structural units in lignin, it is highly unlikely that a single technique will be sensitive to all of them. In addition to the standard analytical techniques, we have been applying Raman spectroscopy to the studies of lignin in pulps. This report focuses primarily on Raman spectroscopic studies of mechanical pulps. Previously, we applied this technique to the studies of celluloses, chemical pulps, and wood. 

UP Agarwal and N Kawai. Self-Absorption Phenomenon in Near-Infrared Fourier Transform Raman Spectroscopy of Cellulosic and LignoceUnlosic Materials. Appl. 

The CNTs, both single-walled nanotubes (SWNT) and multi-walled nanotubes (MWNT) prepared by chemical vapor deposition (CVD) and arc-discharge (AD) methods, respectively, were purchased from Iljin Nanotech, Co., Korea. For proton and electron irradiation experiments, CNTs sheets were prepared as shown in Figure 2 by filtration of the CNT solution mixed in dimethylformamide through a cellulose membrane (pore size 0.45 pm). The thickness of the CNT sheets was approximately 0.5 mm, and they were 47 mm in diameter. After drying in a vacuum oven at 80 °C for 24 hours, CNT sheets (Figure 2) were obtained. These sheets were used in the radiation experiments, and were used for analysis such as SEM, Raman spectroscopy and XPS without any further treatment. For a dispersion test, a CNT powder was used instead of the CNT sheets. 

Although applied to cellulose later than Raman spectroscopy, high-resolution solid-state NMR has provided perhaps the most important new insights regarding the structures of celluloses, particularly in their... 

We now consider Raman spectroscopy and deal with it at greater detail because it is an approach that is less familiar to research workers in the arena of cellulose science, and also because advances in instrumentation now allow acquisition in a few minutes of the Raman spectra that previously required very tedious efforts and many hours per sample. This in turn has resulted in many laboratories acquiring the necessary instrumentation. 

In Raman spectroscopy, in contrast, variations in the refractive index do not present a difficulty since the excitation frequency and the frequencies of the Raman scattered photons are far removed from any absorption bands. It is, therefore, easier to record meaningful Raman spectra from samples such as cellulose, even though they may cause a high level of Rayleigh scattering of the exciting frequency. 

Cellulose, which Is one of the most abundant organic substances found In nature, has been extensively studied by various techniques such as x-ray scattering, electron microscopy, IR and Raman spectroscopy, NMR spectroscopy etc. However, the crystal structure and noncrystalline state are not yet solved for cotton, ramie, bacterial and valonla celluloses which can be easily obtained in pure form. Cross-polarization/magic angle spinning(CP/MAS) C NMR spectroscopy is a promising new method to study these unsolved problems of cellulose, because this method is very sensitive to local molecular conformations and dynamics. 

More recently, a number of new structure sensitive techniques have been developed, and they have been applied to studies of cellulose. These include Raman spectroscopy and Solid State Nuclear Magnetic Resonance, in the experimental arena, and conformational energy calculations in the theoretical domain. These are more recent contributions and are the subjects of subsequent sections in this chapter and later chapters in these proceedings. 

Two classes of spectral studies have been applied for the first time during the past decade as the basis of structural studies of cellulose. These are Raman spectroscopy, and solid state NMR using the CP/MAS technique. Both have raised questions concerning the assumptions about symmetry incorporated in the diffractometric studies. And while they cannot provide direct information concerning the structures, they establish criteria that any structure must meet to be regarded as an adequate model. The information from spectroscopic studies represents one of the major portions of the phenomenology that any acceptable structural model must rationalize. 

With respect to the con arison between celluloses I and II, the spectral data leave little question that the molecular conformations are indeed different. The chapter by Wiley and Atalla sets forth some of the evidence based on Raman spectroscopy. The validity of the theoretical arguments developed in support of the hypothesis that two distinct conformations do indeed occur has been demonstrated through its application in studies of model compounds. The most con rehensive is a study of the vibrational spectra of the inositols (47), wherein spectra of seven of the isomers were investigated and the effects of conformational differences accounted for. 

Raman Spectra of Cellulose. In laser excited Raman spectroscopy, a sample is exposed to monochromatic light, and the scattered light is analyzed. The frequency of a small fraction of the scattered light is shifted relative to the exciting light. The magnitude of the frequency shift corresponds to the vibrational frequencies of the molecules in the sample. Therefore, Raman spectroscopy provides information similar to that provided by infrared spectroscopy. 

The variations in the refractive index can cause anomalous features in infrared spectra. In Raman spectroscopy, refractive index variations are not a problem, since the excitation frequency is far removed from any absorption bands. Therefore, it is easier to record Raman rather than infrared spectra from samples such as cellulose which scatter light strongly. 

Resonaace locations and mulciplicities are characteristic of the cellulose II allomorph, confirming the results of x-ray diffraction and Raman spectroscopy. 

The supermolecular structures of cellulose have been investigated extensively by many techniques including x-ray and electron diffrac-tometry, electron microscopy, IR and Raman spectroscopy, broad-line proton NHR and solid-state c-uuR. Nevertheless, many questions remain concerning the solid-state structures. 

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