Lattice deformation of cellulose and chitin nano-crystals
embedded in PVA matrix
Y. Nishiyama, I. Diddens, M. Müller
Centre de Recherches sur les Macromolécules Végétales (CERMAV-CNRS), BP53 -38041 Grenoble cedex 9,
France
1
Institut für Experimentelle und Angewandte Physik, Kiel University, Leibnizstraße 19, 24098 Kiel, Germany
Fibrous polysaccharide such as cellulose and chitin are important mechanical component in
plant cell walls and plant based materials such as wood or native fibres. The aim of the project is to
measure precisely some of the stiffness tensor elements of cellulose nanocrystals and related
substances. Crystal modulus have been estimated using natural fibres having high uni-axial
orientation such as ramie, However, the precision is limited due to the difficulty in evaluating the
stress applied to the crystals in natural fiber samples. Terrestrial plants have typical microfibril
width ranging from 3 – 6 nm and have substantial contribution of surface chains with different
molecular conformation from the crystal core and also limiting the spatial resolution of offmeridional reflections.
Samples were prepared from aqueous solution of poly vinylalcohol (PVA) and suspension
of sulphuric-acid-hydrolyzed cellulose crystals or extracellular chitin filament from diatom,
Thalassiosira weissflogii. The cellulose crystals we used have typical lateral dimension of 15 nm
and a length of a few nanometres. The chitin filaments have a lateral dimension of about 50 nm and
are much longer (undefined). The mixture of PVA solution and suspension was gelled by adding
sodium borate, and subsequently stretched and dried to obtain a thin oriented fibre with diameters
ranging from 50 to 400 μm. The sodium borate was washed out with methanol and the fibre was
dried at 150˚C.
Series of fibre diffraction data was recorded in situ during the tensile test at beamline A2
using a stretching device with a possibility of humidity control, equipped with a 5N load cell [1].
The whole device was tilted to bring meridional 0 0 4 to Bragg condition. The test was carried out
on three samples of cellulose I with different diameter, 1 sample of cellulose I, and 1 sample of chitin. For the -chitin, the humidity was also changed to achieve measurement on different hydrate
forms on the same specimen.
The recorded two-dimensional intensity data was corrected for the optical distortion of the
camera and the equatorial intensity profile of the crystal was extracted using the orientation
distribution function determined from the 0 0 4 profile. The peak positions were determined by
fitting the profiles with pseudo-voigt peak functions with a linear background. The 0 0 4 peak
position could be determined with best accuracy due to the small air scattering and small peak
width whereas equatorial peaks at lower angles suffered more from the statistics.
The fine peaks allowed us to determine the three-dimensional lattice deformation at
increasing tensile stress. Figure 1 shows the sample strain calculated from the movement of the
head and the crystal strain obtained from the displacement of 0 0 4 peak of cellulose I with the
measured tensile force. This sample had a cross section of 0.002 mm2 and cellulose content of 30
%. The lattice strains as a function of tensile load are plotted in figure 2. The matrix shows
significant relaxation behaviour above a macroscopic stress of 200 MPa but crystal deformation in
the chain direction was always proportional to the macroscopic tensile stress and very close to
macroscopic strain (about 80%). Furthermore, in one case a crystal strain up to 1.5% could be
measured for the first time (figure 2 right). Sample often broke either at the beam position or at the
sample end, so the range of measurement should be further extended by a more careful sample
mounting and reduction of exposure time.
Assuming that all the tensile force applied concentrate on the nano-crystals, a Young’s
modulus, C1111, of about 150 GPa and C1122 of 50 GPa, C1133 of 25 GPa were obtained for cellulose
I. The Poisson’s ratio is independent of any hypothesis on the load transfer, and was 0.33 for the
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[200]/[004] close to the reported value 0.37 [2]. The Young’s modulus is slightly higher than the
value obtained for ramie fibres (138 GPa) [3,4] and could be either due to the high crystallinity in
our sample or the contribution of matrix that could not be neglected. A further experiment
modulating the matrix modulus by changing the relative humidity is needed to confirm the real
elastic constants. In the case of -chitin, a softening of the PVA was observed at 100% R. H.
showing macroscopic creep behaviour. However, the crystal structure also changed from anhydrous
to hydrous form. Further experiments changing the mixing ratio of the matrix and crystals are
needed to confirm the effect of hydration on the elastic properties of -chitin.
Figure 1: Sample strain, measured crystal lattice strain and tensile force as a function of time.
Figure 2: Lattice strain of cellulose I as a function of tensile force. The cross section area and cellulose
content was 0.015 mm2 and 31% for the left, 0.002 mm2 and 30% for the right.
References
[1] I. Grotkopp, Influence of water on the mechanical properties of wood investigated using X-ray and
neutron scattering. PhD thesis, Kiel, 2006.
[2] K. Nakamura, M. Wada, S. Kuga, T. Okano, J. Polym. Sci. : Polym. Phys. 42, 1206 (2004)
[3] I. Sakurada, Y. Nukushina, T. Ito, J. Polym. Sci. 57, 651 (1962)
[4] T. Nishino, K. Takano, K. Nakamae J. Polym. Sci. : Polym. Phys. 33, 1647 (1995)
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