Regenerated cellulose crystalline structure

Sisson has traced the evolution of current concepts of the crystalline part of cellulose structures. The fiber diagram obtained by X-ray diffraction is now known to be produced by a series of elementary crystals, called crystallites, which have a definite arrangement with respect to the fiber axis. It is also known that the crystallites in regenerated cellulose may be oriented to varying degrees with respect to the fiber axis and that the crystallites in regenerated cellulose and mercerized cotton differ from those in native fibers. These hydrate type crystallites appear to be more reactive chemically than the native type. 

In addition, cellulose undergoes changes in crystalline structure with relative ease. The most common modification is the conversion of cellulose I (i.e. la and 1/8) to cellulose II. This can be achieved by dissolution and regeneration or by simply treating cellulose I with sodium hydroxide. Cellulose II is usually considered to be more thermodynamically stable than biosynthesised cellulose I. However,... 

The degree of crystallinity of the fibers and the structure of the approximately 80% crystallinity, kraft with 60%, and regenerated cellulose fiber with around 50% show differing degrees of accessibility. A cotton-based paper does have longer life, under adverse conditions, than one made from rayon. However, given acid conditions, all the cellulose fibers finally do degrade and become brittle. 

In the crystalline part, the cellobiose units are closely packed to form Cellulose I in native cellulose fibres and Cellulose II in regenerated cellulose fibres. In Cellulose I the chain molecules are parallel to one another [16]. The folded chain occurs at Cellulose II, in the crystalline regions the chain molecules are antiparallel. Thus, the basis for helical structure for Cellulose I is preferably extended to the structure of Cellulose II [17]. 

Viscose rayon is inherently a weak fibre, particularly when wet, therefore it is highly susceptible to damage if enzymatic hydrolysis is not controlled. The enzymatic hydrolysis of viscose fibres causes a decrease of the intrinsic viscosity from 250 to 140 ml/g and an increase in crystallinity from 29 to 39% after 44 h [34]. Strong changes of the structure, however, are not typical for the enzymatic hydrolysis of cellulosic materials. Neither cotton nor wood pulp show an essential decrease of the DP during enzymatic hydrolysis [35-37]. The kinetics of the enzymatic hydrolysis of regenerated cellulose fibres before and after acid prehydrolysis changes the kinetics from a monophasic to a biphasic first order reaction [38]. 

As a result of these investigations it is generally agreed that naturally-occurring cellulosic fibres contain of the order of 60 to 70 per cent of molecules orientated in crystalline structure. The regenerated celluloses contain 30 to 40 per cent, Terylene 50 per cent, and nylon between 50 and 60 per cent. 

In native cellulose, the structure develops under conditions of thermodynamic equilibrium and occurs very slowly. For regenerated cellulose, however, not only must the structure be formed rapidly, but also the organization of the macromolecules by crystallization is constrained by the extent of tangling present in the solution. It was suggested by Baker [261] that the structure of cellulose derivatives could be represented by a continuous range of states of local molecular order rather than by definite polymorphic forms of cellulose. This view is supported by the observation that the x-ray diffraction pattern of rayon often reveals both cellulose II and IV components to an extent, depending on the conditions used to make the fiber. Hindeleh and Johnson [262] have described an x-ray diffraction procedure to measure crystallinity and crystallite size in cellulose fibers by which the relative proportions of cellulose II and IV in rayon can be determined. 

Since the details of the crystal structure are of very little importance with regard to the structure and the behaviour of cellulose gels, we shall forbear from discussing them. Neither the lattice structure nor the amount of crystalline substance in cellulose has, as yet, been found to be affected by ordinary mechanical deformation. The orientation of the crystallites changes, but it would seem as though the crystallites themselves remain, as a rule, unaffected. In all regenerated cellulose fibres the percent e of crystalline substance is astonishingly constant, a fact which remains to be explained... 

If regenerated cellulose fibres are formed by the deformation of swollen gels, there is no reason to expect that the molecular fringes between the crystalline regions should have equal length. This may be the reason for the relatively low tensile strenght of these objects (particularly in the wet state) as compared to that of native fibres like ramie and cotton. In the latter, formed by the action of life, a more regular structure may be expected. 

The structures of native and regenerated celluloses have been determined by H n.m.r. spectroscopy. When deuteriated DMSO was used, native cellulose, even in the swollen state, was devoid of a liquid-like mobile component, whereas regenerated cellulose contained much of a non-crystalline component with liquid-like segments. 

Nishikawa and Ono recorded the crystaUine nature of cellulose using the X-ray diffraction patterns from fiber bundles from various plants. Cellulose is known to exist in at least four polymorphic crystalline forms, of which the structure and properties of cellulose 1 (native cellulose) and ceUulose II (regenerated cellulose and mercerized cellulose) have been most extensively studied. As a first approximation, the crystal structure of cellulose I determined by X-ray diffraction can be described by monoclinic unit cell which contains two cellulose chains in a parallel orientation with a twofold screw axis (Klemm et al. 2005). Cellulose I has two polymorphs, a triclinic stmcture (la) and a monoclinic structure (IP), which coexist in various proportions depending on the cellulose source (Azizi Samir et al. 2005) (Nishiyama 2009). The la structure is the dominate polymorph for most algae (Yamamoto and Horii 1993) and bacteria (Yamamoto and Horn 1994), whereas ip is the dominant polymorph for higher plant cell wall cellulose and in tunicates. 

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