Review of Recent Research into Cellulosic Whiskers, Their Properties and Their Application in Nanocomposite Field

Elastic modulus and stress-transfer properties of tunicate cellulose whiskers.

The current structural models of the cellulose microfibril as well as its mechanical and thermal properties are reviewed. The cellulose microfibril can be considered as a single thin and long crystalline entity with highly anisotropic physical properties. The contribution and limit of different methods employed such as electron microscopy, infrared spectroscopy, X-ray scattering and diffraction, solid state nuclear magnetic resonance spectroscopy, and molecular modeling are also discussed.

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Structure and properties of the cellulose microfibril

Microfibril angle (MFA) is perhaps the easiest ultrastructural variable to measure for wood cell walls, and certainly the only such variable that has been measured on a large scale. Because cellulose is crystalline, the MFA of the S2 layer can be measured by X-ray diffraction. Automated X-ray scanning devices such as SilviScan have produced large datasets for a range of timber species using increment core samples. In conifers, microfibril angles are large in the juvenile wood and small in the mature wood. MFA is larger at the base of the tree for a given ring number from the pith, and decreases with height, increasing slightly at the top tree. In hardwoods, similar patterns occur, but with much less variation and much smaller microfibril angles in juvenile wood. MFA has significant heritability, but is also influenced by environmental factors as shown by its increased values in compression wood, decreased values in tension wood and, often, increased values following nutrient or water supplementation. Adjacent individual tracheids can show moderate differences in MFA that may be related to tracheid length, but not to lumen diameter or cell wall thickness. While there has been strong interest in the MFA of the S2 layer, which dominates the axial stiffness properties of tracheids and fibres, there has been little attention given to the microfibril angles of S1 and S3 layers, which may influence collapse resistance and other lateral properties. Such investigations have been limited by the much greater difficulty of measuring angles for these wall layers. MFA, in combination with basic density, shows a strong relationship to longitudinal modulus of elasticity, and to longitudinal shrinkage, which are the main reasons for interest in this cell wall property in conifers. In hardwoods, MFA is of more interest in relation to growth stress and shrinkage behaviour.

/pdf/microfibril-angle-measurement-variation-and-relationships-a-524gkdmy8w.pdf

Microfibril Angle: Measurement, Variation and Relationships – A Review

Summary: The average lateral dimensions of cellulose microfibrils in paper sheets can be determined from the width of X-ray Bragg peaks at half the maximum height. The method assumes it is the finite size of the crystallite which is the main contributor to peak broadening in the directions orthogonal to the long axis of the crystallite. This assumption is examined in the context of samples from different botanical sources, of varying crystallinity and cellulose content and the various instrumental sources of broadening X-ray powder patterns. Excellent fits to the data from paper samples can be obtained by using five Voigt functions to the powder diffraction patterns, for the 101, , 021, 002 and 040 reflections. Of these reflections, the 020 is the clearest, and the best able to determine the lateral dimension of the crystallite. Results indicate that the size of microfibrils is dependent on the botanical source of the fibres.

On the Interpretation of X-Ray Diffraction Powder Patterns in Terms of the Nanostructure of Cellulose I Fibres

The morphology of single native cellulose (flax) fibres was investigated by means of simultaneous small-angle scattering and fibre diffraction using an X-ray microbeam. The azimuthal hroadenings both of the equatorial small-angle scattering streak and of Bragg reflections are found to be due to a misalignment of the crystalline cellulose microfibrils with respect to the fibre axis. Diffuse scattering on the layer lines and the equator of the fibre diffraction diagram indicates the presence of disordered cellulose between the cellulose microfibrils and of defects inside the crystallites.

Combined X-ray microbeam small-angle scattering and fibre diffraction experiments on single native cellulose fibres

The elementary diffusion jump of Fe atoms in the ordered intermetallic alloy FeAl is studied in a coordinated effort of atomistic experimental techniques, Monte Carlo simulation and abinitio electron theory. The experiment demands that the elementary diffusion jump is a jump into an antistructure site on the Al sublattice which is occupied for a much shorter time than the sites on the Fe sublattice. The diffusion path can be followed by Monte Carlo simulations which can perfectly explain the experiments. Since ab-initio theory yields a very low concentration of Al vacancies it is suggested that correlated jumps of two atoms prevent the creation of a fully developed Al vacancy.

Microscopic Diffusion Mechanism of Iron in FeAl Revisited by New Methods

Cellulose, the most abundant structural polymer in nature is strongly hydrophilic. Adsorbed water molecules are found to relax on the time scale of picoseconds far below the melting temperature of bulk water. The relaxation times are considerably longer than in bulk supercooled water. The liquid component is stable towards crystallization. With decreasing temperature it gradually freezes into a distinct amorphous polymorph of water which bears resemblance to low-density amorphous ice.

Relaxational motion and ice formation of water adsorbed to cellulose

The arrangement of the crystalline cellulose microfibrils in native flax fibers was studied by means of position-resolved microbeam X-ray small-angle scattering (μSAXS) at the ESRF. The nearly perfect alignment of the microfibrils parallel to the fiber axis could for the first time be determined directly on a single fiber; a tilt angle distribution with a mean hwhm of 3.5° was measured. The lateral organization (i.e., diameter and mean distance) of the microfibrils showed no significant variation across the fiber on a length scale of 4 μm (beam size at sample position). The advantages of the μSAXS technique are discussed by comparing it to a conventional SAXS experiment on flax fibers.

Direct observation of microfibril arrangement in a single native cellulose fiber by microbeam small-angle X-ray scattering

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Combined X-ray microbeam small-angle scattering and fibre diffraction experiments on single native cellulose fibres

Microscopic Diffusion Mechanism of Iron in FeAl Revisited by New Methods

Relaxational motion and ice formation of water adsorbed to cellulose

Direct observation of microfibril arrangement in a single native cellulose fiber by microbeam small-angle X-ray scattering