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Abstract
The arrangement of cellulose molecules in natural environment on the nanoscale is still not fully resolved, with longitudinal disorder in cellulose microfibrils (CMF) being one relevant question. Particularly the length of the dislocated cellulose segments in CMFs is still under debate. Using molecular dynamics simulations, we are first investigating the phenomenon of pseudo-recrystallization of dislocated cellulose regions after cleavage of CMFs. Based on our simulations we propose that 3–4 glucose residues bordering to each side of a cellulose nanocrystal are actually reorganizing to a quasi-crystalline state, which are corroborating recent analytical investigations reporting an increase in crystallinity after acid vapor hydrolysis of CMFs. Combining our molecular dynamics simulation results with these analytical data we can estimate the length of the dislocated cellulose segments in CMFs. We propose that, for the investigated sources of biomass (cotton and ramie), the dislocation lengths are between 3.1–5.8 nm equaling to 6–11 glucose residues in the cellulose crystallites.Graphic abstract
Highlights
Cellulose nanomaterials, often termed nanocellulose, are emerging materials and are integral parts of more sustainable and biodegradable lightweight composites with exceptional mechanical properties
As the tensile modulus of the fibrils depends on the amount of hydrogen bonds (Santiago Cintron et al 2011), the moduli of the dislocated regions larger than 10 glucose residues (GRs) would subsequently be almost equal to 76 GPa, as measured for the r10 model (Khodayari et al 2020b)
We have been addressing the recrystallization of amorphous regions in cellulose microfibrils (CMF) after cleavage, a process which has been proposed after observing an increase in crystallinity after acid vapor hydrolysis of CMFs (Kontturi et al 2016; Spiliopoulos et al 2021)
Summary
Often termed nanocellulose, are emerging materials and are integral parts of more sustainable and biodegradable lightweight composites with exceptional mechanical properties. The nanocellulose materializes in microfibrils (CMF), occasionally termed as cellulose nanofibrils, which form the smallest structural unit in plant cell walls They provide structural integrity and are capable to assemble into larger structures often called macrofibrils (Nishiyama 2009; Gibson 2012). Even longitudinal disorder in CMFs has been a major source for confusion, largely due to the adaption of the fringed-fibrillar models to cellulose In these models, CMF consists of alternating highly ordered crystalline and disordered ‘amorphous’ domains, which feature different reactivity and accessibility (Mark 1940; Hearle 1958). The fringed-fibrillar model was used to explain acidic hydrolysis of CMFs as it predicts different reactivities for the ’amorphous’ and crystalline domains, resulting in different kinetics with the LODP (levelling off degree of polymerization) being the end point. Washing the resulting CNC solid with water (Paakkonen et al 2018) did not give significant amounts of glucose (\1%), which is in clear contrast to the fringed fibrillar model (Kontturi et al 2016)
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