Abstract
The exceptional stiffness of cellulose chains in solution is discussed in relation to the configuration and the possible conformations of the chains and to postulated intrachain hydrogen bonds. The cellulose chains have a strong tendency to aggregate into partly crystalline fibrils, in native plant celluloses appearing as flat ribbons of about 100 Å. Width and of indefinite length. Electron microscopy of thin sections of plant cell walls has given good evidence that these fibrils are of native origin they are embedded in hemicellulose, lignin, etc. and not artefacts from the preparation of specimens. The crystallinity of the fibrils and their accessibility to, say, swelling or hydrolysis varies with the biological origin of the material and is also affected by the pretreatment. The higher lattice order and the lower accessibility of cotton cellulose fibrils compared with wood cellulose are particularly well studied. When hydrolysed with dilute mineral acid, the native cellulose fibrils (`micelle strings’) are attacked at certain points and degraded into rodlike fragments (`micelles’) . The fact that extraction of hemicellulose from wood holocellulose and subsequent drying decreases the length of the resulting micelles is discussed in relation to lattice distortion in the fibrils (formation of disordered regions and slip planes) due to the collapse (`crushing’) of the cell wall.
Plant fibre celluloses with a low content of hemicellulose (such as cotton hair and ramie) form a group with a higher degree of lattice order than does the wood cellulose group, which also includes straw and cotton stalks. There is experimental evidence that the wood cellulose chains in purified pulps contain a larger number of irregularities like carboxyl or aldehyde groups, than do cotton cellulose chains.
Sulphate pulp cellulose shows a somewhat higher degree of resistance to swelling in caustic soda than does sulphite pulp cellulose. Practically no difference between these pulps is found, however, if the fibres are prehydrolysed with dilute sulphuric acid or if the swelling tests are performed with concentrated phosphoric acid. This indicates that the differences between sulphite and sulphate pulps (‘the sulphate effect’) is mainly confined to the accessible (non-crystalline) regions of the cellulose fibrils.
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