This has been compounded by a flood of conceptual articles applying chemistries but which lack the analytical rigor of traditional chemistry disciplines. 2010), which by nature involves regioselective surface chemistry, represents a significant challenge due to the infancy of the field and complexity of the materials. Chemical modification of nanocelluloses (Habibi et al. In addition, T 1 relaxation times in the solid state are typically very long, requiring labeling strategies to give sufficient signal-to-noise (S/N) for quantitative experiments. However, spectral resolution is rather limited using typical MAS probes, preventing the accurate separation and quantitation of different chemical species. High resolution, using ultra-fast magic angle spinning (MAS), and multidimensional experiments are possible for solid-state NMR. Solid-state NMR, in particular, has found utility in the quantification of the different crystalline phases in celluloses (Newman 1999 Kono et al. However, this process is often lengthy as a whole and its threads are difficult to bring together. Typically, a succession of direct and indirect methods are applied for this task, affording partial insights. This limitation has imposed researchers to rely on the poorer chemical resolution of solid-state techniques or indirect methods for characterization of samples, which contain a significant phase composition of crystalline cellulose.
This is in large part due to the poor solubility of cellulosic materials in common molecular solvents, preventing non-destructive solution-state analyses. Unlike in small-molecule-based chemical disciplines, with cellulosics, currently there is no established general quantitative analytical technique to accurately assess chemical changes with sufficient resolution. Surface chemical modification of cellulosic materials is a logical approach to tune the properties and, thus, applicability of these bio-renewable polymers (Klemm et al. The comprehensive signal assignment of the diverse set of cellulosic species in this study constitutes a blueprint for the direct quantitative structural elucidation of crystalline lignocellulosic, in general, readily available solution-state NMR spectroscopy. Quantitation using HSQC was possible, but only after applying T 2 correction to integral values. Quantitative heteronuclear single quantum correlation (HSQC) was applied in the analysis of key samples to assess its applicability as a high-resolution technique for following cellulose surface modification.
We utilize a series of model compounds and apply now classical (nitroxyl-radical and periodate) oxidation reactions to cellulose samples, to allow for accurate resonance assignment, using 2D NMR. Our method relies on the use of a readily accessible ionic liquid electrolyte, tetrabutylphosphonium acetate ():DMSO-d 6, for the direct dissolution of biopolymeric samples. Herein, we rigorously demonstrate a general quantitative NMR spectroscopic method for structural determination of crystalline cellulose samples. To that end, a diverse set of narrow analytical techniques are typically employed that often are time-consuming, costly, and/or not necessarily available on a daily basis for practitioners. The limited access to fast and facile general analytical methods for cellulosic and/or biocomposite materials currently stands as one of the main barriers for the progress of these disciplines.