Cellulosic materials crystallinity

Cellulose is the main component of the wood cell wall, typically 40—50% by weight of the dry wood. Pure cellulose is a polymer of glucose residues joined by 1,4-P-glucosidic bonds. The degree of polymerization (DP) is variable and may range from 700 to 10,000 DP or more. Wood cellulose is more resistant to dilute acid hydrolysis than hemiceUulose. X-ray diffraction indicates a partial crystalline stmcture for wood cellulose. The crystalline regions are more difficult to hydrolyze than the amorphous regions because removal of the easily hydrolyzed material has Htde effect on the diffraction pattern. 

It has grown increasingly apparent that the non-crystalline portions of cellulose structures may play as important a role in the properties and behavior of cellulosic materials as the crystalline parts. X-ray diffraction studies have greatly extended knowledge of crystalline cellulose but in the case of the amorphous or disordered fraction the methods of study have necessarily been indirect and not completely reliable. 

The need of fully dependable information on the relative proportions of crystalline and amorphous cellulose in different cellulosic materials has given rise to a variety of approaches to the problem. These alternative methods in many instances fail to support X-ray findings and often show large discrepancies among themselves. It is the purpose of this... 

A series of estimates of non-crystalline cellulose in various types of cellulosic material is presented in Table II. These estimates were obtained by extrapolating the relatively flat portions of curves such as those given in Figure 2 to zero time and are admittedly first approxima-... 

The evidence presented fails to suggest the causes for the large variations in crystallinity estimates which have been reported for similar cellulosic materials. There is a possibility that the different methods may not measure precisely the same characteristic of the material. It also may be that relative crystallinity is not a fixed quantity in any case but depends on circumstances involved in the measurement, such as the amount of swelling. The estimates reached by different methods need to be reconciled. At present, crystallinity estimates which depend wholly or in part on X-ray diffraction seem to be much higher than those obtained by chemical methods. The fact is that X-ray diffraction methods are ideal for studies of the crystalline fraction but are necessarily indirect in application to the non-crystalline fraction. The converse is true for the chemical approach. Apparently a combination of diffraction and chemical methods may adjust the existing differences. 

It is probable that varying degrees of ordering of chains exist in a cellulosic material and that a sharp differentiation of crystalline and non-crystalline celluloses may not be feasible or even possible. Theoretically, the lateral surfaces of crystallites are amorphous but may have far less importance in determining such properties as strength, flexibility and extensibility than the non-crystalline cellulose which supplies continuity of structure in the direction of crystallite orientation. Yet properties like moisture absorption and swelling may be more dependent upon the amount of cellulose which exceeds a certain degree of disorder (permeability) than upon location. The definition of crystallinity may, therefore, be made ultimately in terms of practical objectives. 

Relative crystallinity undoubtedly influences such properties of cellulosic materials as rigidity, flexibility, plasticity and extensibility. Likewise the amount and reactivity of intercrystalline cellulose are major factors in common processing treatments such as bleaching, dyeing, pulping and wet finishing. Further refinement of measuring methods and the development of further correlations between crystallinity and fiber properties would contribute much to this important field. 

Cellulosic materials are quite variable from source to source, not only in cellulose, hemicellulose, and lignin content but also in the crystallinity of the cellulose. As a consequence, each natural substrate would be expected to have its own unique set of process conditions to optimize glucose yield and minimize secondary product contamination. The next section on kinetics of acid hydrolysis will examine this point. 

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. 

The effect of ball milling is similar in all cases. Within the first 10 min of milling, the 002 reflection is considerably broadened and appears slightly shifted to a smaller angle. This apparent shift is probably the result of the superposition of the broadened 002 peak upon the rising, broad, amorphous peak which centers about 18.5° (2 ). As ball milling continues, the crystalline characteristics decrease and the amorphous characteristics increase. After about 1 hr of milling time, the crystalline characteristics of all the cellulosic materials have disappeared. 

The use of Cl to represent the percentage of the crystalline component is unjustified among such a diversity of cellulosic materials, whose lignin components vary from 0 to about 30%. Cl, as used here, is not intended to represent the proportion of the crystalline component. Rather, Cl provides a means of quantifying the characteristics of the x-ray diffraction patterns. When applied to a set of similar samples, Cl is a convenient measure of the degree of lateral order. 

The cellulosic material S0 is composed of amorphous matter Sa, crystalline matter Sc, and nonhydrolyzable merts Sx, and their rates of enzymatic degradation are different. 

A more recent trend in polymer materials research is the hybridization of cellulosic polysaccharides with inorganic compounds natural and synthetic layered clays, silica, zeolites, metal oxides, and apatites are employable as nanoscale components. In addition, if mesoscopic assemblies such as liquid-crystalline ordering are used in the construction of new compositional systems, the variety of functionalized cellulosic materials will be further expanded. 

Cellulosic materials usually form crystal structures in part, and water cannot penetrate the inside of crystalline domains at room temperature. Native celluloses form crystalline microfibrils or bundles of cellulose chains 2-5 nm in width for higher plant celluloses and 15-30 nm for algal celluloses, which are observable by electron microscope. Almost all native celluloses have X-ray diffraction patterns of cellulose I with crystallinity indexes (Cl) 13] of about 40-95 %. 

Powdered cellulose has acceptable compression properties, although its flow properties are poor. However, low-crystallinity powdered cellulose has exhibited properties that are different from standard powdered cellulose materials, and has shown potential as a direct-compression excipient. ... 

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]. 

The crystallinic structure and lignin existence in cellulosic materials prevent attack by microorganisms. The growth of fungi is too slow in untreated raw materials for commercial production of cellulase. In order to design an economically viable technical process, these difficulties must be overcome by a suitable mechanical or chemical pretreatment. 

Pulverization can reduce the size as well as the crystallinity of cellulosic materials and increase the surface area and bulk density. It is also possible to separate part of the hgnin from carbohydrates which makes it easier for microorganisms to digest cellulose. Various equipment, such as a compression mill, a bead mill, an extruder, a roll mill and disc refiners, etc., can be used for pulverization. Unfortunately these methods tend to be very expensive and too energy intensive. For sohd-state fermentation, if the particles are too fine, the oxygen mass transfer will become a big problem therefore, hghtly crushed or just ground raw material will suffice. 

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