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)