Chemometric analyses of the 1H-13C cross-polarization build-up of celluloses NMR spectra: A novel approach for characterizing the cellulose crystallites

Chemometric analyses of 13C CPMAS NMR spectra (without signal curve-fitting) recorded on celluloses from various origins and by varying the contact time of the cross-polarization process, indicated a direct relation between the crystallinity of cellulose and the PCA scores. The complex exponential increase of CPMAS signals was shown to be directly dependent on the cellulose crystallinity. Estimation of a crystallinity index was possible using the proton spin diffusion time values estimated by adjustment of the experimental NMR spectra obtained while cross-polarization built-up. Calculation of the corresponding lateral dimensions of cellulose crystallites has been carried out taking into account a variable thickness of the microfibrils surface layer. Using a multilinear regression method with a constraint of positivity (MLRC), modeling of the observed spectra of celluloses from various origins has been considered as a linear combination of available NMR spectra of pure forms (Iβ, Iα and amorphous forms). The concentration of these pure forms allowed a crystallinity index to be calculated as well as estimating the proportion of the cellulose Iβ allomorph. The proposed procedures based on chemometrics are rapid, robust, even on impure samples. They need no assumption about samples and very few manipulations of the NMR signals (no curve-fitting).