Natural Cellulose Fibers: Properties

molecules will extend fully to fit into an extendedchain crystal. The folded-chain configuration can thus be viewed as an energetic compromise. In the plant cell wall cellulose I can form because here the synthesis and crystallite formation take place simultaneously and there is no solution phase (see Natural Cellulose Fibers and Membranes: Biosynthesis). Regenerated, technical cellulose such as rayon always has the foldedchain configuration with lower values of modulus and tensile strength than native cellulose. Interestingly, cellulose II forms fibrils of the same diameter as cellulose I. This first led to the conclusion that they would be identical. This caused a lot of confusion as regards the crystal structure of native cellulose. It is not possible to measure the mechanical properties of native cellulose directly. In the plant cell it is always found together with different matrix substances and upon isolating it loses its native conformation. Thus, theoretical approaches were made to calculate these properties from the molecular structure and the interatomic forces. The resulting values range from 60 GPa (Treloar 1960) to 320 GPa (Gillis 1969). Experimental measurements have been made on cells consisting almost exclusively of cellulose, e.g., ramie fibers. More sophisticated studies have derived the modulus of cellulose from x-ray measurements on strained ramie fiber, where the specific strain of the cellulose fibrils was calculated from changes in the x-ray diffraction angles (Sakurada et al. 1962). The experimental values for bast fibers go up to 110 GPa (Meyer and Lotmar 1936). The theoretical values should be higher, since they do not take into account

Natural Cellulose Fibers: Properties Cellulose is the main component of the plant cell wall, with a share of 60–90%. At the same time, it is the main strengthening element of this natural composite material. The mechanical properties are quite remarkable: a cellulose filament of 1 mm diameter can hold more than 60 kg in weight—it has 80% of the strength of steel. Mechanical properties for a number of different fibers are given in Table 1. The cellulose is incorporated into the cell wall in form of thin fibrils, so called microfibrils or elementary fibrils, with an elaborate substructure (Fig. 1). Their diameter measures 35 AH . The native cellulose is an unbranched polymer of β-1-4-linked glucose residues arranged in linear, antiparallel chains, where every other glucose residue is rotated at approximately 180m. As a result, cellobiose is the structural repeating unit of the glucan chains in cellulose. This is in contrast to other glucan polymers such as starch where glucose is the repeating unit. In the native cellulose microfibril crystalline regions alternate with unordered, amorphous regions. From x-ray diffraction and negative staining with subsequent electron microscopic observation the length of the crystalline regions has been calculated to be 60 nm. Crystalline cellulose can take on two different polymeric structures—the extendedchain configuration (cellulose I) and the folded-chain configuration (cellulose II, Fig. 2). While the former is found in native cellulose and is the thermodynamically more stable form, the latter arises by crystallization from solution, where it is unlikely that the random-coil Table 1 Mechanical properties of different fibers. Fiber

Wet or dry

Hemp

Wet Dry Wet Dry Wet Dry Wet Dry Wet Dry Wet

Jute Flax Ramie Cotton Collagen Silk Rayon Fortisan, high strength, regenerated cellulose Lyocell Polyester

Wet Dry Wet Dry Wet Dry Dry Wet

Tensile modulus (GPa)

Tensile strength (MPa)

Extension to break (%)

35 70

920

1.7 3

60 27 95 19 80 7.5 27 2 0.3 0.4 4.4

4.3 7

860 880 840 1080 920 240–830 200–800 100 600 130 260 650 900 580 650 720 720

2.2 1.8 2.4 2.3 6–12 8–10 18–20 28 27 8 6 15 44

1

Natural Cellulose Fibers: Properties

Figure 1 Overview over the various levels of organization within plant cell walls (after Niklas 1984).

(a)

(b)

Figure 2 Polymer configurations: (a) extended-chain (b) foldedchain.

the amorphous regions of the microfibril but only the much stiffer crystallites. For theoretical calculations different models have been used concerning the accuracy of the employed molecular model and the interatomic forces taken into account. A very comprehensive model has been published by Gillis (1969) and is based on the real 3D anisotropic crystal lattice of cellulose in its native state. The molecular processes occurring upon straining are mainly interchain hydrogen bond deformations and alterations in the valence angle at the bridging 2

oxygen atoms between the monomers. The necessity of these two processes can be derived from the schematic representation of a unit cell depicted in Fig. 3. Straining will align the individual molecular strands and necessarily deform the hydrogen bonds in angle and length together with a change in the valence angle of the bridging oxygen atom. In tension, deformation of the bridging oxygen bond plays the most important role, accounting for about 50% of the deformation. These oxygen bridges also constitute the limiting factor with regards to strength: the multiple interchain hydrogen bonding is so strong that the failure of cellulose fibrils is not due to slipping of adjacent chains but to chain scission. In the plant cell wall the cellulose microfibrils are embedded in an aqueous medium, which affects their mechanical properties. The water penetrates the amorphous regions and competes for potential hydrogen bonding sites, loosening the interactions between adjacent crystalline regions. This effects an increase in both extensibility and strength of the fibrils, while the modulus of elasticity is decreased. A counteracting effect to the hydration is exerted by deposition of hydrophobic lignin in the cell wall, which waterproofs it. This opens the possibility of a specific adjustment of the mechanical properties. For bast fibers the difference between wet and dry in the modulus of elasticity amounts roughly to a factor of 3.5. In wood, which is heavily lignified, the factor is about half. A

Natural Cellulose Fibers: Properties are predominately oriented parallel to the longitudinal axis. Some cells, such as wood cells, also have to cope with tension, with bending, with negative pressure for water transport and with damping of vibration. Here the fiber axes are tilted for a trade-off between these different demands (Booker and Sell 1998).

Bibliography

Figure 3 Crystalline unit cell of inextensible glucose residues linked in the chain direction by oxygen atoms and cross-linked by hydrogen bonds (after Gillis 1969).

further modification of the cell walls’ overall mechanical properties is made on the structural level through orientation of the microfibrils. In cells subjected mainly to tension, such as fiber cells, the fibrils

Booker R E, Sell J 1998 The nanostructure of the cell wall of softwoods and its functions as a living tree. Holz als Roh- und Werkstoff 56, 1–8 Gillis P P 1969 Effect of hydrogen bonds on the axial stiffness of crystalline cellulose. J. Polym. Sci. A 2, 783–94 Meyer K H, Lotmar W 1936 Sur l’e! lasticite! de la cellulose. HelŠ. Chim. Acta 19, 68–86 Niklas K J 1984 Plant Biomechanics. University of Chicago Press, Chicago, IL Sakurada I, Nukushina Y, Ito T 1962 Experimental determination of the elastic modulus of crystalline regions in oriented polymers. J. Polym. Sci. 57, 651–60 Treloar L R G 1960 Calculations of elastic moduli of polymer crystals. III. Cellulose. Polymer 1, 290–303

L. Ko$ hler

Copyright ' 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials : Science and Technology ISBN: 0-08-0431526 pp. 5944–5946 3