An improved X-ray diffraction method for cellulose crystallinity measurement
Introduction
The presence of crystallinity in cellulose is one of the most important characteristics contributing to its physical, chemical and mechanical properties (Andersson et al., 2003, Ryu et al., 1981, Moon et al., 2011, Tanahashi et al., 1989, Weimer et al., 1995). Crystallinity index (CrI) is a parameter commonly used to quantify the amount of crystalline cellulose present in cellulosic materials and has also been applied to interpret changes in cellulose structures after physicochemical and biological treatments (Ai-Zuhair, 2008, Andersson et al., 2003, Cao and Tan, 2005, Lavoine et al., 2012, Hall et al., 2010). Analytical methods to determine CrI include 13C nuclear magnetic resonance (NMR) spectroscopy, Fourier transform (FT)-IR spectroscopy, and X-ray diffraction (XRD). Among these, XRD is the most prevailing method currently employed (Agarwal et al., 2010, Bansal et al., 2010, Deraman et al., 2001, Evans et al., 1995Kljun et al., 2011, Liitia et al., 2000, Liitia et al., 2003, Park et al., 2010, Teeaar et al., 1987).
There are three methods commonly applied to calculate the CrI of cellulose based on the XRD data (Bansal et al., 2010, Cao and Tan, 2005, Driemeier and Calligaris, 2011, French and Santiago Cintrón, 2013Park et al., 2010, Rietveld, 1969, Rowe et al., 1994Ruland, 1961, Segal et al., 1959Tanahashi et al., 1989, Thygesen et al., 2005). The first was developed by Segal et al. (1959), based on the ratio of the height of the (2 0 0) peak (I2 0 0) and the height of the minimum (IAM) between the (2 0 0) and (1 1 0) peaks. This is the simplest and most frequently used technique. However, as the exact amount of crystalline fraction is proportional to the peak area rather than its height, the peak height calculation presents an obvious flaw to accurate measurement of CrI. In addition, a recent study (French & Santiago Cintron, 2013) has shown that the CrI obtained using this method is dependent on crystallite size and cellulose polymorph, for materials with the same fraction of crystalline cellulose. The second method is based on peak deconvolution of crystalline and amorphous peaks. The crystalline cellulose is represented by several intense peaks at , (1 1 0), (1 0 2), (2 0 0), and (0 0 4) for cellulose Iβ. The positions and areas of these peaks may be arbitrarily chosen (Garvey, Parker, & Simon, 2005) or calculated from a known crystal structure. The amorphous fraction is typically modeled by a single broad peak and CrI is calculated from the ratio of the area of the crystalline peaks to the total area. The accuracy of this method is challenged by the difficulty in selecting peaks that correctly correspond to the actual diffraction contributed by each fraction. The third common technique is the amorphous subtraction method first described by Ruland (1961) and later modified by Vonk (1973). The method fits an intensity profile of an amorphous component which is scaled so that it remains just below all the observed intensity from the experimental sample pattern. The CrI is determined from the ratio of the area above the amorphous profile to the total area. The amorphous profile is obtained either from a polynomial function or a pattern measured from experimentally prepared material believed to be entirely amorphous (e.g., ball-milled cellulose, regenerated cellulose, xylan or lignin powder) (Bansal et al., 2010, Thygesen et al., 2005).
Detailed review of these methods is given by Park et al. (2010). These wide angle X-ray scattering (WAXS) based methods are reliable and commonly used for comparative determination of CrI between the cellulosic samples from same sources. Yet there is still a challenge in quantification and comparative analysis of CrI among cellulose samples obtained from different origins and process methods. The crystallites of natural celluloses are small in size (<10 nm) compared to typical crystalline solids like metals or ceramics. Because of the small crystallites and the variations in crystalline structures within cellulose chains, XRD analyses of cellulose always result in broad diffraction peaks which are difficult to distinguish from the diffractions of amorphous cellulose and background scattering. This is the main reason behind the long-term-challenge to apply XRD to obtain an exact quantification of cellulose CrI, especially when comparing samples from different origins or sources.
Recognizing the lack of accurate crystallinity measurement method, Driemeier and Calligaris (2011) have recently developed new protocol to improve CrI measurement by systematically considering preferred fiber orientation, incoherent scattering, moisture, and other compositional deviations. However, this method is based on transmission data from capillary samples which is a relatively uncommon instrument set up. It requires a separate measurement of an empty capillary and uses a complex background which could substitute for amorphous cellulose. The objective of this study is to modify existing XRD procedures to improve the reliability of WAXS for CrI determination using general XRD instrumentation. The Rietveld method was chosen due to its broad applicability to all cellulose polymorphs with an available crystal structure and its consistency in treating all the crystalline peaks. Although we used commercial Rietveld software in this study, the same functionality is widely available in a range of other programs, some of which are available free of charge. The following approaches were taken in this study: (1) select the model substrates which closely represent the nanolevel size cellulose crystallites and amorphous cellulose; (2) determine the diffraction background in the absence of amorphous material; (3) determine a suitable 2θ range for better coverage of amorphous and crystalline diffraction.
Section snippets
Substrate preparation
The brown stock kraft pulp was first prepared from poplar wood chips by cooking with a mixture of NaOH and Na2S solution at 170 °C for 160 min. Bleached kraft pulp (BKP) was prepared by subsequent treatment of brown stock kraft pulp using acid sodium chlorite solution at room temperature for 24 h following a procedure previously described (Ju, Engelhard, & Zhang, 2013). Nanocrystalline cellulose was prepared by acid extraction of BKP (Hamad & Hu, 2010). In brief, 64.5% sulfuric acid was used to
Results and discussion
To attain suitable representative peaks for the crystalline component of cellulose, nanocrystalline cellulose (NCC) prepared from bleached wood pulp (Dong, Revol, & Gray, 1998) was used as a model compound in this study. In nature, cellulose does not occur as an individual molecule, it is composed of crystalline and amorphous components. During NCC preparation, the amorphous regions of cellulose are removed inducing a transverse cleavage of cellulose fibers into rodlike nanoparticles (Habibi,
Conclusions
In conclusion, we have shown an improved XRD method for better quantification of cellulose CrI. The method utilizes a simple 3-parameter background model obtained from a representative crystalline sample and extension of the 2θ angle to 75°. This new method uses three distinct amorphous peaks identified from representative amorphous samples to calculate CrI instead of using only one representative amorphous peak. This method enables a more reliable measurement of CrI of cellulose Iβ as well as d
Acknowledgments
Funding for this research was provided by National Science Foundation (award number 1067012). The X-ray diffraction research was performed in EMSL, a national scientific user facility sponsored by the U. S. Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory in Richland, Washington.
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