Cross-polarization dynamics and conformational study of variously sized cellulose crystallites using solid-state 13C NMR

Abstract

Cellulose forms crystalline fibrils, via biosynthesis, that can be just a few nanometers wide. The crystallinity is a structural factor related to material performance. Recently, many routes to isolate these fibrils as nanocellulose have been developed, and there exist various types of nanocellulose with different crystallinities. Quantitative assessment of the crystallinity of nanocellulose is thus essential to advance knowledge in the high performance and functionality of such materials. Solid-state 13C cross-polarization/magic-angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectroscopy is a strong tool to investigate the structural features and dynamics of solid cellulose. The crystallinity is often evaluated by using the NMR signal ratio of the C4 crystalline and noncrystalline regions as a crystallinity index (CI) value. To calculate the CI value, it is necessary to examine the dependence of the contact time (CT) for CP on the signal intensity and set the optimum CT at a maximum of the signal intensity. However, the dependence has not been investigated for evaluation of the CI value of various cellulose samples with different crystal sizes. Here, we optimized the CT for evaluation of the CI value of cellulose with different crystal sizes. The error in the CI at the optimized CT was ~ 3%. At the optimized CT, the structural change after surface modification by TEMPO-oxidation was also analyzed from the NMR spectra of the C6 region. The relationship between the CI value and the degree of oxidation shows that it is possible to evaluate the degree of oxidation from the NMR spectra irrespective of the crystallinity of cellulose. Furthermore, the C4-based CI value was linearly correlated with the C6-based trans-gauche (tg) ratio, which is approximated by a function, CI = 0.9 tg ratio.

Crystallite fusion in nanocellulose aggregates

“…CP/MAS 13 C NMR spectroscopy. CP/MAS 13 C NMR measurements were performed using a JEOL JNM-ECAII 500 spectrometer (JEOL Ltd., Tokyo, Japan) equipped with a 3.2-mm HXMAS probe and ZrO2 rotors at 125.77 MHz for 13 C. The samples were spun at 15000 Hz and the 90° proton decoupler pulse width, contact time, and relaxation delay were set to 2.5 μs, 2 ms, and 5 s, respectively 32 . Adamantane was used as the internal standard for the 13 C chemical shifts.…”

Nanocellulose: Resources, Physio-Chemical Properties, Current Uses and Future Applications

“…The density and porosity of nanocellulose can be measured using a helium pycnometer. Daicho et al (2020) reported the density of pure native celluloses, such cotton and tunicin, are approximately 1.60 g/cm 3 using helium pycnometer. In order to study the elemental composition on nanocellulose surface, X-ray photoelectron spectroscopy (XPS) technique could be used (Jordan et al, 2019).…”

“…In the NMR spectra, the C4 and C6 carbon atoms are distinguishable into crystalline and noncrystalline signals. The signal ratio of C4 crystalline and noncrystalline phases can be used for calculation of crystallinity index (CI) value of nanocellulose (Daicho et al, 2020). However, nanocellulose tends to agglomerate while drying, therefore solid-state characterization may reflect certain structural changes from drying and not necessarily the precise nature of the individual nanocellulose (Jiang et al, 2014).…”

Crystallite fusion in nanocellulose aggregates

“…CP/MAS 13 C NMR measurements were performed using a JEOL JNM-ECAII 500 spectrometer (JEOL Ltd., Tokyo, Japan) equipped with a 3.2-mm HXMAS probe and ZrO2 rotors at 125.77 MHz for 13 C. The samples were spun at 15000 Hz and the 90° proton decoupler pulse width, contact time, and relaxation delay were set to 2.5 μs, 2 ms, and 5 s, respectively. 31 Adamantane was used as the internal standard for the 13 C chemical shifts.…”