Crystalline state cellulose

Study of the structure of cellulose (Figure 22.2) leads one to expect that the molecules would be essentially extended and linear and capable of existing in the crystalline state. This is confirmed by X-ray data which indicate that the cell repeating unit (10.25 A) corresponds to the cellobiose repeating unit of the molecule. 

There have been a lot of studies of cholesteric films and gels in order to exploit their potential as specific optical media and as other functional materials. Most of the preparations were achieved by modification or improvement of previous attempts to immobilize the cholesteric structure of cellulose derivatives into the bulky networks either by crosslinking of cellulosic molecules with functional side-chains in the liquid-crystalline state [203], or by polymerization of monomers as lyotropic solvents for cellulose derivatives [204-206],... 

The most important alternative crystalline form is cellulose II. This form can result from treatment of cellulose in concentrated alkali, such as 23% NaOH, followed by rinsing in water. This is also the main form that results from crystallization of dissolved cellulose, such as regeneration of rayon. Supercritical water can also effect the transformation [216]. The treatment of cotton in milder alkali, for industrial mercerization, amounts mainly to disruption and decrystallization rather than transformation to crystalline II. Cellulose II can occur as the native state when the normal biosynthesis and subsequent crystallization is disrupted [217-219]. 

Solid-state cellulose can also be noncrystalline, sometimes called amorphous. Intermediate situations are also likely to be important but not well characterized. One example, nematic ordered cellulose has been described [230]. In most treatments that produce amorphous cellulose, the whole fiber is severely degraded. For example, decrystallization can be effected by ball milling, which leaves the cellulose as a fine dust. In this case, some crystalline structure can be recreated by placing the sample in a humid environment. Another approach uses phosphoric acid, which can dissolve the cellulose. Precipitation by dilution with water results in a material with very little crystallinity. There is some chance that the chain may adopt a different shape (a collapsed, sixfold helix) after phosphoric acid treatment. This was concluded because the cellulose stains blue with iodine (see Figure 5.12), similar to the sixfold amylose helix in the starch-iodine complex. 

The response of the cotton fiber to heat is a function of temperature, time of heating, moisture content of the fiber and the relative humidity of the ambient atmosphere, presence or absence of oxygen in the ambient atmosphere, and presence or absence of any finish or other material that may catalyze or retard the degradative processes. Crystalline state and DP of the cotton cellulose also affect the course of thermal degradation, as does the physical condition of the fibers and method of heating (radiant heating, convection, or heated surface). Time, temperature, and content of additive catalytic materials are the major factors that affect the rate of degradation or pyrolysis. 

Polymer crystallization may also complicate the transition from the amorphous state to the intermediate liquid crystalhne state. Even small crystalline fractions in a polymer prevent free movement of macromolecules This is the case, for example, with cellulose. Cellulose fibres, obtained via cellulose xanthate (viscose), in the process of forming partially crystallize. Although the amorphous fraction in these fibres is large (up to 70-75 %), at a short-term heating of the fibres above the glass-transition temperature (240-260 °C) only slight self-elongation of fibres is observed, which can be attributed to the transition to the liquid crystalline state at the expense of the amorphous fraction... 

Recently, cellulose and its derivatives attracted researchers attention again. Besides the work by Aharoni who confirmed the appearance of the liquid crystalline state for cellulose acetate mentioned above, a number of other works appeared in which liquid crystalline state was established for hydroxypropyl cellulose . Finally, Chanzy et al. have obtained the results indicating the possible transition to the mesophase of cellulose itself, and not only its derivatives. 

In solid state, cellulose exists in mostly crystalline state. This state has been the subject of extensive studies for at least 100 years, and several important aspects need to be pointed out at the onset. Crystallinity is neither uniform (options exist) nor static (crystallinity can be lost as well as gained in relation to molecular mobility), nor is it permanent (conditions have been identified under which transitions of order take place). Cellulose crystallinity manifests itself through the existence of distinctive X-ray diffraction patterns. These patterns allow the determination of the overall dimensions of unit cells which are spatial units that represent the... 

The transition from crystalline to melt state, which is normal for crystalline polymers, is not observed with cellulose under normal conditions. It appears that the secondary bonds giving rise to the crystalline state are too strong and too numerous to be broken by a rise in temperature. Thermal degradation (beginning at ca. 180 °C) precedes melting under atmospheric pressure conditions. Nevertheless, a plastic deformation interpreted as melting has recently been reported for cellulose fibers exposed to laser radiation in a highly confined (pressurized) space [43]. The fracture surface of a thermoplastically deformed cellulose disc is shown in e Fig. 10. 

Zugenmaier, P. Structural investigation on some cellulose derivatives in the crystalline and liquid crystalline state. In Cellulose. Structure, Modification, and Hydrolysis, Young, R.A., Rowell, R.M., Eds. Wiley New York, 1986 221-245. 

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. 

In a wood fiber, the cellulose portion of the microfibrils is the only major component found in a crystalline state. A cellulose microfibril is a long thread-like aggregate of cellulose chains of about 200 A wide and a few microns long. 

Polymers are considered macromolecular chains of high molecular weight formed by monomers bonded covalently. They can be obtained from natural sources or from synthetic processes through different polymerization routes [1]. A number of both natural and synthetic polymers are able to form regular arrangements, that is, crystalline entities, under either quiescent or deformed states. Cellulose and natural... 

The phase transition from disordered states of polymer melt or solutions to ordered crystals is called crystallization-, while the opposite process is called melting. Nowadays, more than two thirds of the global product volumes of synthetic polymer materials are crystallizable, mainly constituted by those large species, such as high density polyethylene (HOPE), isotactic polypropylene (iPP), linear low density polyethylene (LLDPE), PET and Nylon. Natural polymers such as cellulose, starch, silks and chitins are also semi-crystalUne materials. The crystalline state of polymers provides the necessary mechanical strength to the materials, and thus in nature it not only props up the towering trees, but also protects fragile lives. Therefore, polymer crystallization is a physical process of phase transition with important practical relevance. It controls the assembly of ordered crystalline structures from polymer chains, which determines the basic physical properties of crystalline polymer materials. 

The powder X-ray diffraction patterns of porous crystalline cellulose (PCC) -10% ethenzamide (EZ) mixtures before and after storage of the mixtures for 1 month at 40°C and 0, 40.0, and 97.0% relative humidity are shown in Fig. 3 [7]. In the freshly prepared mixture (A), X-ray diffraction peaks were observed at 20 = 14.5, 19.3, and 25.3° that were attributable to EZ crystals. Following storage at 0 and 40.0% RH (represented by patterns B and C in Fig. 3), the X-ray diffraction peaks of EZ crystals disappeared. It was found that the mixing of EZ with PCC under dry conditions led to the transformation of crystalline EZ into the amorphous state. EZ molecules would be adsorbed physically onto the pore surface of PCC. In the case of 97.0% RH (Fig. 3D), X-ray diffraction peaks of EZ crystals were still observed EZ remained in the crystalline state under this condition. Matsumura et ai. [8] reported that coexisting water vapor caused a decrease in the adsorption of methanol onto porous materials. At 97.0% RH, the maximum pore diameter for water condensation was calculated as 42 nm. All capillaries of PCC were filled with water at 97.0% RH, and molecules of EZ had little chance to adsorb onto the surface of PCC. These results indicated that the indispensable condition for amorphization of EZ by mixing with PCC was storage under dry conditions. 

In its natural state, cellulose is highly crystalline in structure. It is a polydisperse linear stiff-chain homopolymer composed of the glucose building blocks which form hydrogen-bonded supramolecular structures. These strong hydrogen bonds are responsible for the stiff, linear shape of the cellulose polymer chains [18]. 

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