Cellulose crystallites

It is interesting to note that in their first paper on cellulose (11) Meyer and Mark proposed a structural unit cell model which is classic and accepted, for the largest part, even today. They proposed a cellulose crystallite in which all... 

The cellulose crystallite size is highest in ulvophycean and certain chlorophycean algae (114-169A), lowest in vascular plants (49-62A) and intermediate in Aceiobacter (70-84A) (1). It appears that cellulose crystalline dimensions are independent of the type of terminal cellulose synthesizing complex. The idea that cellulose biosynthesis is not exclusively responsible for determining its crystalline dimensions has been proposed previously by Marx-Figini (52). 

Finally, American Viscose Corporation (61) has obtained a patent on graft copolymers from cellulose crystallites using ceric ion initiation. 

Crystallinity. Diffraction traces of the cellulosic materials are presented in Figures 1-4. Table I lists the crystallinity indices obtained from the diffraction patterns. Also included in Table I are the crystallite sizes of the unmilled materials obtained from measurement of the half widths of the 002 reflections. The 002 reflection is noticeably sharper in the case of the cotton cellulose. Crystallite size measures 54 A for the cotton cellulose compared to an average of 29 A from the wood celluloses. 

We envisage that any alteration in the active site would be accompanied by some modification of the tertiary structure of the enzymes (Cx), which, although possibly only very minor, may be sufficient to prevent the cooperation of Cx with Cx in a loose complex on the surface of the cellulose crystallite. Equally well, of course, there may be other differences between the various insoluble growth substrates that are important determinants of the structure of the active site. These differences could be either at the molecular or at the supermolecular level. 

Aqueous suspensions of cellulose microcrystalhtes obtained by acid hydrolysis of native cellulose fibers can also produce a cholesteric mesophase [ 194]. Sulfuric acid, usually employed for the hydrolysis, sulfates the surface of the micro crystallites and therefore they are actually negatively charged. Dong et al. performed some basic studies on the ordered-phase formation in colloidal suspensions of such charged rod-like cellulose crystallites (from cotton filter paper) to evaluate the effects of addition of electrolytes [195,196]. One of their findings was a decrease in the chiral nematic pitch P of the anisotropic phase, with an increase in concentration of the trace electrolyte (KC1, NaCl, or HC1 of < 2.5 mM) added. They assumed that the electric double layer on... 

Marinading can also transform texture. In animal tissues, dilute acid (e.g., acetic or citric) or salt solution destroys collagen-collagen interactions and so softens the fibres. Presumably in plant material, for which this treatment is also effective, it is the pectins that are altered, since the cellulose crystallites are too tightly bonded to be affected. 

Humidity and Heat. Moisture is crucial to the normal behaviour of cellulosic fibres. Under moderate conditions (relative humidity 45-65%) water is readily absorbed through the network of pores running through a fibre cell, it coats cellulose crystallites and acts as a plasticiser of the amorphous regions, disrupting inter-chain hydrogen bonds. Without this bound water the fibre would be permanently brittle, with an effective glass transition point way above room temperature. 

The salt-free crystalline polysaccharides reviewed by Bluhm et al. [15] are stabilized in characteristic crystalline unit cells by specific amounts of water. Two kinds of locations have been proposed for the water molecules one is unique, i.e., the water lies clustered in an existing interstitial cavity between double helices of B-starch. The other has water bound at specific sites within each unit cell. Additional water in this second type expands one or more unit cell dimension. This almost continuous expansion of the unit cell with increasing content of water may represent a more ordered aspect of the same interaction that occurs between water and accessible, disordered surfaces of celluloses crystallites (and other imperfectly crystalline polysaccharides). 

Cellulose, the hemicelluloses, and lignin sorb moisture to different extents (Figure 1). The hemicelluloses are more hygroscopic than cellulose, which is more hygroscopic than lignin (5). This means that lignin (noncrystalline and probably totally accessible), the hemicelluloses (all noncrystalline and nearly totally accessible), the noncrystalline portion of cellulose, and the surfaces of the cellulose crystallites are responsible for moisture uptake by the wood cell wall. 

The second Is the Incorporation of some technique to compensate for the fact that cellulose crystallites are not Infinite. This fact modifies the Intensities and should be Included before fine distinctions can be made with confidence. A whole pattern fitting technique as proposed by Fraser et al (25) may solve both problems. 

Direct measurements of cellulose orientation in fibers have yielded conflicting results. Evidence from x-ray diffraction studies of wood samples suggested that the cellulose crystallites are arranged randomly in the plane perpendicular to the fiber axis (30-33). Raman spectroscopic studies of cotton fibers dried under tenTTon, however, demonstrated that the methine C-H bonds are oriented preferentially perpendicular to the surface of the cell wall (34). Since the C-H bonds are perpendicular to the 002 plane, the orientation suggested by the Raman evidence differs from the alternating orientation in algal celluloses. 

J. F. Revol and D. A. I. Goring, Directionality of the fiber c-axis of cellulose crystallites in microfibrils of Valonia ventricosa. Polymer, 24 (1983) 1547-1550. 

Figure 2 Example of planar projection of one of the configurations of an ortho-para PF dimmer on the surface of a schematic cellulose crystallite showing a phenolic dimmer (a dihydroxy diphenyl methane) conformation of minimal energy and main dimer-cellulose hydrogen bonding. (From Ref. 8.)... 

Figure 6 Example of view along the cellulose crystallite axis of a ternary system similar to that in Fig. 4 but using a different photopolymerizable acrylic primer, namely trimethylolpropane triacrylate (TMPTA). (From Ref. 73.)...

Revol, J. F., Godbout, L., and Dong, X. M. (1994). Chiral nematic suspensions of cellulose crystallites phase separation and magnetic field orientation, LiguidO, 16(1), 127-134. 

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