Recrystallization and size distribution of dislocated segments in cellulose microfibrils—a molecular dynamics perspective

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

Effect of Moisture on Polymer Deconstruction in HCl Gas Hydrolysis of Wood

“…Similar minor differences due to HCl gas or vapor hydrolysis have been detected in previous X-ray diffraction experiments, 16 , 18 and, for instance—the decrease of the 200 lattice spacing agrees with simulations of recrystallizing cellulose chains after cleavage. 19 Most importantly, however, the crystallinity index increased clearly (by 10–20%), which is a common result obtained for various cellulosic substrates subjected to HCl gas or vapor hydrolysis. 16 − 18 In the current case, it can be assigned to the removal of noncrystalline material and the small increase in crystallite dimensions.…”

“…Especially, studies on filter paper, 16 bacterial cellulose, 17 and hardwood cellulose nanofibers 18 have reported increases in cellulose crystallinity with minimal effects on the crystallite dimensions. This effect has been explained by the crystallization of cleaved cellulose chains either in the small disordered domains along the microfibrils 19 or on the hydrophilic surfaces of the cellulose crystals, 20 but the results are not fully consistent. In another work, the HCl vapor hydrolysis of cotton filter papers was found to decrease the water-holding capacity of the cell walls, which was at least partly related to tighter aggregation of cellulose microfibrils in the hydrolyzed samples.…”

Moisture uptake in nanocellulose: the effects of relative humidity, temperature and degree of crystallinity

“…Note that degree of crystallinity of cellulose can be quantified by the Ramachandran plot showing the distribution of the main dihedral angles about their glycosidic linkage (Khodayari et al 2021;French et al 2021). We calculated these distributions for the crystalline and amorphous cellulose for both TEMPOand CM-modified cellulose, see Figure S2.…”

The Impact of Surface Charges of Carboxylated Cellulose Nanofibrils on the Water Motions in Hydrated Films

“…At the nanoscale, CNFs are recognized as being highly crystalline with small segments of non-crystalline regions in the longitudinal direction. These areas were formerly called ″amorphous regions″ but modern accounts strongly suggest that they are too short to be consisting of bulky, amorphous cellulose as their length is allegedly somewhere between 1 and 5 nm. , Unlike the completely disordered cellulose polymer chains, which are more accessible to water molecules, the disordered segments do not likely contribute much to the water uptake of CNFs but they are more susceptible to certain chemical reactions, notably acid hydrolysis . The crystalline phase of CNFs is on the other hand the most compacted crystalline building blocks with practically no access to water infiltration and is also resistant to hydrolysis .…”