Conformation of cellulose

Fig. 2 Schematic representation of cellulose structures in solution Part A shows the fringed micellar structure. Parts B and C show possible chain conformations of celluloses of different DP. For high molecular weight cellulose, C, intra-molecular hydrogen bonding is possible...

Figure 3. Isolated chain conformations of cellulose predicted by MM2(85) (left) and PS79 (middle). The conformation on the right is that of the crystal structure of cellulose I (3). Hydrogen bonds are shown by dashed lines.

From another point of view, it can be seen that approximate conformity of cellulose nitrates to both of the asymptotic relations (63) and (89) for [ /] and (SF), respectively, implies that the number 0 should remain essentially constant at its asymptotic value 0, given by Eq. (57). Since it is difficult to assess the polydispersity of cellulose nitrate fractions, the constancy of 0, rather than its numerical value, should be of prime importance in testing the consistency of our conclusions. Doty, Schneider and Holtzer (59) and Newman and Flory (198) both originally reported, that 0 decreased systematically with decreasing molecular weight. Later Holtzer, Benoit and Doty (726) preferred to regard 0 as essentially constant at about 1.9 1021, somewhat smedler... 

Influence of Physical Structure. The hydrolytic behavior of cellulose is much influenced by its physical structure and lateral order [121-132]. Wood cellulose was hydrolyzed twice as fast as cotton [125]. Hydrolysis rate was significantly increased by physical or chemical pretreatment, with the effect depending on the source of cellulose. Hill and coworkers [127,128] reported that mercerization increased the hydrolysis rate of cotton (by 40%) and of ramie (7%), whereas the opposite effect was observed for linen and a-cellulose samples showing an approximately 30% reduction. Based on kinetic analysis, they concluded that the end-attach model proposed by Sharpies [121] can only be applied to the cellulose II structure and not to the cellulose I crystallite. Thus, the conformation of cellulose is also a significant factor affecting its reactivity and possibly the hydrolytic mechanism as well. 

Fig. 31. Conformation of cellulose chain = projection of the monomer unit on chain direction, A/2 = 2.7 A = length of an effective bond about which rotation is possible, 6 = 1.45 A length of an effective bond about which rotation is impossible...

The wettability of various wood fibres was studied in [173], including bleached and unbleached, and alkyl ketene dimer sized and non-sized fibres. An improvement of the wettability with an increase of the surfactant concentration, except nonionics, was observed for all types of fibres. It has been noted in [174] that the electrokinetic potential of fibres determines considerably the efficiency of their washing and dying. Alkali mercerisation of cotton influences not only the fine structure, morphology and conformation of cellulose molecules, but also the negative electrokinetic potential of the cotton fibres. Based on this, the selection of mercerisation conditions due to changes in the NaOH concentration will allow to... 

It is proposed in this paper that the molecular flexibility, or in the case of cellulose, the lack thereof, is one of the fundamental factors that determines the suitability of a polymer for use as a fiber. Another factor is the set of interactions between molecules in the crystalline material. Those interactions, which depend on the accessible molecular shapes, diminish solubility, especially if both hydrogen and hydrophobic bonds are formed, as in the case for the common conformation of cellulose. The relatively limited range of shapes (a definition of stiffness) not only keeps the cellulose chains in a conformation that retains the interchain attractions but also minimizes increases in entropy in solution. 

In addition to their diffractometric studies reported in prior publications, they add in their contribution to the present symposium analyses of the infrared spectra as well as analyses of the CP-MAS 13c NMR spectra. Their thesis is not inconsistent with the proposals of Atalla and coworkers concerning differences between the conformations of celluloses I and II. However, Hayashi and coworkers go beyond this by proposing that the differences in conformation can be preserved in the course of heterogeneous derivati-zation reactions, and also in the process of generating the other allomorphs of cellulose, namely celluloses III and IV, from the two primary allomorphs 1 and II. 

Figure 4 shows the chain conformations of cellulose I ("bent") and of cellulose II ("bent and twisted") proposed in our previous work (28) on the crystal structure of cellulose II. These models of cellulose I and II have one and two kinds of Intrachain (03 -05) hydrogen bond, respectively. The number of 0-H stretching peaks in... 

Fig, Chain conformations of cellulose proposed by X-ray analysis 28). A bent form chain for cellulose I, B bent and twisted form chain deviated from 2 for cellulose II. There are two kinds of the distance of 03 - 05 Internal rotation angle around glucosidic linkage is according to Jone s expression 32). 01 - 01 distance is 5.16A, C - C,... 

Although the crystal stmcture of cellulose has been investigated for almost a century, certain details remain to be deeiphered. For an overview of the complexity eoneeming different crystal structures and conformations of cellulose and their transitions, see Ref 37 and the artiele by Perez and Samain in this volume. 

The structure and arrangement of cellulosic chains play an important role in the formation of liquid crystals. At present, neither the conformation of cellulosics nor the solvent bound to the chain in the case of a lyotropic mesophase are known for these liquid-crystalline systems. Nevertheless, these structural features form the basis for a discussion of structural and thermodynamic aspects. Information on cellulosics is available for the two borderline cases next to the LC state, i.e., for the solvent built-in solid state as well as for the pure solid state, obtained by X-ray, NMR, and potential energy analysis on one side, and for the semi-dilute state from light-scattering experiments on the other side. These data have to be evaluated for a discussion of possible structures and models in liquid-crystalline phases. 

A basic understanding of the structure and behavior of liquid-crystalline cellulosics has yet to evolve. From a conceptual point of view, the chirality of the cellulosic chain is most sensitively expressed in the super-molecular structure of the cholesteric phase, which may be described by the twisting power or the pitch. At present, no information is available about domains or domain sizes (correlation lengths) of supermo-lecular structures. The chirality in the columnar phases has not been addressed at all. The principal problem, i.e., how does chirality on a molecular or conformational level promote chirality on the supermolecular level, has not been solved. If this correlation were known, it would enable the determination of the conformation of cellulosic chains in the mesomorphic phase and the development of models for the polymer-solvent interactions for lyotropic systems. On the other hand, direct probing of this interaction would provide a big leap towards an understanding of lyotropic phases. 

Next, we have examined the relationship between the chemical shifts and the molecular chain conformation. The versatility of chain conformation of cellulose molecules is expressed in three torsion angles as shown in Fig. 14 (j> and xp, rotations around the /3-l,4-glycosidic linkages and the rotation of methylol side groups around the C5-C6 bonds. In order to find out the relationship between the chemical shifts of cellulose samples and the torsion angles, the... 

Persistence length a is a measure of the rigidity of the conformation of cellulose tricarbabilate in dioxane, which can be determined by a Kratky plot of small-angle neutron scattering. Plot the following data in the Kratky form and determine the persistence length a of the polymer under study ... 

Studies of the molecular conformation of cellulose sulphate in gels indicated that there are no double helices present. Stereo views of the cellulose sulphate molecule are shown in Figure 1. 

Molecular modeling has been demonstrated to be a useful tool in the characterization of many different types of chemical systems, including bulk materials. Many properties can be computed with an accuracy that is comparable to experimental capabilities [6]. Simulation of properties is especially important for systems that are challenging to study experimentally due to their limited solubility in common solvents. In some other cases, the experiment itself may be difficult due either to sensitivity to sample preparation or to ambiguities in the interpretation of the results. When the system consists of a relatively small number of atoms, both traditional ab initio and density functional methods can be employed. In the case of sugar molecules and disaccharides, a number of studies have been carried out to develop force field methods [7] to be used in molecular dynamics simulations while studying the conformations of cellulose and its interactions with water and other molecules [8-16]. 

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