Iα to Iβ mechano-conversion and amorphization in native cellulose simulated by crystal bending

Cellulose, Jun 2018

The bending of rod-like native cellulose crystals with degree of polymerization 40 and 160 using molecular dynamics simulations resulted in a deformation-induced local amorphization at the kinking point and allomorphic interconversion between cellulose Iα and Iβ in the unbent segments. The transformation mechanism involves a longitudinal chain slippage of the hydrogen-bonded sheets by the length of one anhydroglucose residue (~ 0.5 nm), which alters the chain stacking from the monotonic (Iα) form to the alternating Iβ one or vice versa. This mechanical deformation converts the Iα form progressively to the Iβ form, as has been experimentally observed for ultrasonication of microfibrils. Iβ is also able to partially convert to Iα-like organization but this conversion is only transitory. The qualitative agreement between the behavior of ultrasonicated microfibrils and in silico observed Iα → Iβ conversion suggests that shear deformation and chain slippage under bending deformation is a general process when cellulose fibrils experience lateral mechanical stress.

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Iα to Iβ mechano-conversion and amorphization in native cellulose simulated by crystal bending

Ia to Ib mechano-conversion and amorphization in native cellulose simulated by crystal bending 0 Y. Ogawa Y. Nishiyama K. Mazeau (&) Univ. Grenoble Alpes , CNRS, CERMAV, BP53, 38000 Grenoble Cedex 9 , France 1 P. Chen Wallenberg Wood Science Center, KTH Royal Institute of Technology , 56-58 Teknikringen, SE-10044 Stockholm , Sweden 2 P. Chen (&) State Key Laboratory of Pulp and Paper Engineering, South China University of Technology , 510640 Guangzhou , China 3 A. E. Ismail Department of Chemical and Biomedical Engineering, West Virginia University , Morgantown, WV 26505 , USA The bending of rod-like native cellulose crystals with degree of polymerization 40 and 160 using molecular dynamics simulations resulted in a deformation-induced local amorphization at the kinking point and allomorphic interconversion between cellulose Ia and Ib in the unbent segments. The transformation mechanism involves a longitudinal chain slippage of the hydrogen-bonded sheets by the length of one anhydroglucose residue (* 0.5 nm), which alters the chain stacking from the monotonic (Ia) form to the alternating Ib one or vice versa. This mechanical deformation converts the Ia form progressively to the Ib form, as has been experimentally observed for ultrasonication of microfibrils. Ib is also able to partially convert to Ia-like organization but this conversion is only transitory. The qualitative agreement between the behavior of ultrasonicated microfibrils and in silico observed Ia ? Ib conversion suggests that shear deformation and chain slippage under bending deformation is a general process when cellulose fibrils experience lateral mechanical stress. Bending; Chain slippage; Kink; Molecular dynamics simulation; Allomorphic conversion Introduction In recent years, cellulose nanofibers (CNF) and nanocrystals (CNC) have received increased interest in materials science (Habibi et al. 2010; Grishkewich et al. 2017) . These cellulose nanomaterials are obtained by disintegrating cellulosic fibers in the plant cell wall through chemical or mechanochemical treatments. These materials may experience various stress conditions in industrial processes and in use that may alter the inner structure of the cellulose nanocrystals through dislocations (Hidayat et al. 2012) and allomorphic transitions. Understanding the mechanical response of crystalline cellulose to various external forces is important for fundamental understanding and for development of more effective processing. Processing cellulose by ultrasound is a scalable process currently used in many laboratories. It is often observed that sonicated cellulose fibrils have a tortuous morphology, showing numerous kinks and can be accompanied by an allomorphic transformation (Briois et al. 2013) from the triclinic cellulose Ia to the monoclinic Ib form. We recently carried out molecular modeling to mimic a 3-point bending test of cellulose Ib crystals (Chen et al. 2016) . Upon bending, the crystal developed a sharp kink identical to that observed experimentally for processed celluloses (Kekaelaeinen et al. 2012; Wang et al. 2012; Zeng et al. 2012; Martoia et al. 2016) . We found shear deformations are very important when the bending load is applied on the hydrophobic face, for which the bending rigidity is also the lowest. In this study, we applied this bending protocol to produce kinks in molecular models of pure Ia and pure Ib cellulose crystals on their hydrophobic face. We followed the evolution of the internal atomic structure to investigate the molecular details of allomorphic transition and morphological changes. Results obtained on short crystals (about 20 nm in length) of Ia and Ib will be shown first, followed by results obtained on a long crystal (about 80 nm in length) of Ia. Computational methods Model construction The cellulose microfibril models of Ia and Ib were built from crystal structures (Nishiyama et al. 2002, 2003) refined from X-ray and neutron diffraction data. The dominant hydrogen bond network, pattern A, was considered. The constructed models consisted of 40 chains (5 9 8) of 40 residues each, exposing the 100 and 010 surfaces for Ib and the 110 and 110 surfaces for Ia, as shown in Fig. 1. The two models have approximately square cross sections with similar dimensions of * 3.12 nm 9 * 4 nm 9 * 21 nm. To estimate if the size of the crystals affects the results, we have also considered a long crystal initially in the Ia organization. Except for the length of the chains, which have degree of polymerization 160 to produce a crystal of 80 nm in length, this crystal is identical to the one already described. Each crystal model was energy minimized (EM) and then equilibrated by molecular dynamics (MD). EM was performed using the steepest descent method followed by the conjugated gradient method, with the convergence criterion being a maximum force of 10 kJ mol-1 nm-1. In the dynamics process, the temperature was slowly increased from 0 to 300 K ov (...truncated)


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Pan Chen, Yu Ogawa, Yoshiharu Nishiyama, Ahmed E. Ismail, Karim Mazeau. Iα to Iβ mechano-conversion and amorphization in native cellulose simulated by crystal bending, Cellulose, 2018, pp. 1-11, DOI: 10.1007/s10570-018-1860-x