Cellulose conformational properties

The discussion is organized in the following order First the advantages of HRC scheme, relative to the industrial (i.e., heterogenous) process are briefly commented on second, the relevance of celMose activation and the physical state of its solution to optimization of esterification are discussed. Finally, the use of recently introduced solvent systems and synthetic schemes, designed in order to obtain new, potentially useful cellulose esters with controlled, reproducible properties is reviewed. A comment on the conformity of these methods with the concepts of green chemistry is also included. 

Naturally occurring cellulose is extremely mechanically stable and is highly resistant to chemical and enzymatic hydrolysis. These properties are due to the conformation of the molecules and their supramolecular organization. The unbranched pi 4 linkage results in linear chains that are stabilized by hydrogen bonds within the chain and between neighboring chains (1). Already during biosynthesis, 50-100 cellulose molecules associate to form an elementary fibril with a diameter of 4 nm. About 20 such elementary fibrils then form a microfibril (2), which is readily visible with the electron microscope. 

The most relevant property of stereoregular polymers is their ability to crystallize. This fact became evident through the work of Natta and his school, as the result of the simultaneous development of new synthetic methods and of extensive stractural investigations. Previously, the presence of crystalline order had been ascertained only in a few natural polymers (cellulose, natural rubber, bal-ata, etc.) and in synthetic polymers devoid of stereogenic centers (polyethylene, polytetrafluoroethylene, polyamids, polyesters, etc.). After the pioneering work of Meyer and Mark (70), important theoretical and experimental contributions to the study of crystalline polymers were made by Bunn (159-161), who predicted the most probable chain conformation of linear polymers and determined the crystalline structure of several macromolecular compounds. 

Chauve et al. [253] utilized the same technique to examine the reinforcing effects of cellulose whiskers in EVA copolymer nanocomposites. It was shown that larger energy is needed to separate polar EVA copolymers from cellulose than for the nonpolar ethylene homopolymer. The elastomeric properties in the presence of spherical nanoparticles were studied by Sen et al. [254] utilizing Monte Carlo simulations on polypropylene matrix. They found that the presence of the nanofillers, due to their effect on chain conformation, significantly affected the elastomeric properties of nanocomposites. 

All of the likely conformations of cellobiose, cellulose, and xylan are explored systematically assuming the ring conformations and IC-D-O-IC-4 ) angle for each pair of residues to be fixed and derivable from known crystal structures. The absolute van der Waals energies, but not the relative energies of different conformations, are sensitive to the choice of energy functions and atomic coordinates. The results lead to possible explanations of the known conformational stiffness of cellulose and Its solubility properties in alkali. The characteristics of xylan conformations are compared with cellulose. 

The unusual properties of xanthan undoubtedly result from its structural rigidity, which in turn is a consequence of its linear, cellulosic backbone that is stiffened and shielded by the trisaccharide side chains. The conformation of xanthan in solution is a matter of debate. It does appear that the conformation changes with conditions. 

This subject has been of continuing interest for several reasons. First, the present concepts of the chemical constitution of such important biopolymers as cellulose, amylose, and chitin can be confirmed by their adequate chemical synthesis. Second, synthetic polysaccharides of defined structure can be used to study the action pattern of enzymes, the induction and reaction of antibodies, and the effect of structure on biological activity in the interaction of proteins, nucleic acids, and lipides with polyhydroxylic macromolecules. Third, it is anticipated that synthetic polysaccharides of known structure and molecular size will provide ideal systems for the correlation of chemical and physical properties with chemical constitution and macromolecular conformation. Finally, synthetic polysaccharides and their derivatives should furnish a large variety of potentially useful materials whose properties can be widely varied these substances may find new applications in biology, medicine, and industry. 

Finally it may be remarked that the dynamic viscoelastic properties of plasticized cellulose derivatives seem to give no evidence of any unusual temperature dependence of the chain conformations. Thus, Landel and Ferry (162, 163, 164) successfully applied the method of reduced variables [see, for example, Ferry (6)] to various concentrated solutions of cellulosic polymers, and found that the temperature reduction factors were quite similar to those for other flexible polymers such as poly(isobutylene). 

Based on properties in solution such as intrinsic viscosity and sedimentation and diffusion rates, conclusions can be drawn concerning the polymer configuration. Like most of the synthetic polymers, such as polystyrene, cellulose in solution belongs to a group of linear, randomly coiling polymers. This means that the molecules have no preferred structure in solution in contrast to amylose and some protein molecules which can adopt helical conformations. Cellulose differs distinctly from synthetic polymers and from lignin in some of its polymer properties. Typical of its solutions are the comparatively high viscosities and low sedimentation and diffusion coefficients (Tables 3-2 and 3-3). 

The conformation parameter a (=A/Af, where Af is A of a hypothetical chain with free internal rotation) for cellulose and its derivatives lies between 2.8-7.5 2 119,120) and the characteristic ratio ( = A2Mb//2, where Ax is the asymptotic value of A at infinite molecular weight, Mb is the mean molecular weight per skeletal bond, and / the mean bond length) is in the range 19-115. These unexpectedly large values of a and Cffi suggest that the molecules of cellulose and its derivatives behave as semi-flexible or even inflexible chains. For inflexible polymers, analysis of dilute solution properties by the pearl necklace model becomes theoretically inadequate. Thus, the applicability of this model to cellulose and its derivatives in solution should be carefully examined. 

The chemical transformations that lead to the conversion of cellulose to mixed polysaccharides differing from cellulose in the conformation of the pyranose ring and the number and configuration of the hydroxyl groups of the repeating unit of the macromolecule, may exert a considerable effect on the structure of the material as well as on its important chemical properties (rate of acetylation and O-alkylation of OH groups, stability of the acetal linkage) and physicochemical indices (solubility of modified preparations of cellulose and cellulose ethers and esters). 

Photoresponsive membranes of cellulose-2,4-diacetate incorporating 6-nitro-l, 3, 3 -trimethylspiro-(2H-l-benzopyran-2,2 -indoline) have been prepared and chiroptical and fluorescence properties of optically active co-polymers of acenaphthalene with methyl acrylate/methacrylate have been investigated. Marked optical activity is induced in the aromatic units only for the co-polymer with methyl methacrylate. This difference in behaviour is associated with an overall higher main-chain flexibility and conformational freedom in acrylates compared with methacrylates. Photoisomerization in polyurethanes containing azo-links has been found to be dependent upon the thermal history of the polymer, and photoisomerization of linoleic acid and... 

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