Structural irreversibility, celluloses

Possible Cause of Structural Irreversibility Between Cellulose I and Cellulose II Families... 

The allomorphs and derivatives prepared from cellulose I and II in solid state could be transformed into cellulose I and II, respectively. The memory phenomenon of the original crystal structure should be due to a structural characteristic (chain conformation, chain polarity or others) of an individual chain that is common within each family and kept through the change of crystal structure. There were direct irreversible conversions between corresponding cellulose esters, Na-cellulose and cellulose IV prepared from cellulose I and II just like that between I and II. Accordingly, the structural characteristic should be the cause of the structural irreversibility between the I and II families. 

In conclusion, we think that the cause of structural irreversibility between the cellulose I and II families is an irreversible transformation between the skeletal chain conformations in the families. Although we expect that further studies of cellulose will provide clearer details of chain conformation, it is not likely that it will be possible to completely solve the structures on the basis of the limited amount of X-ray data available. 

Once dehydrated, the microfibrils are practically without functionality in ordinary food processing and preparation operations, because the inert microcrystallites are difficult for water to penetrate. The polymorphs, cellulose I and II (Blackwell, 1982 Coffey el al., 1995), are differentiated by their molecular orientation, hydrogen-bonding patterns, and unit-cell structure. Cellulose I is the natural orientation cellulose II results from NaOH treatment under tension of cellulose I with 18-45% alkali (mercerization). The I—II transition is irreversible. Mercerization strengthens the fibers and improves their lustre and affinity for dyes (Sisson, 1943). Sewing thread was relatively pure mercerized cotton until the advent of synthetic polymer fibers. 

One of the most special aspects of cellulose polymorphy is the transformation from I to II. The conversion of the parallel-packed cellulose I structures to an antiparallel cellulose II structure is interesting because it can occur without loss of the fibrous form. This transformation is widely thought to be irreversible, although there are several reports [231-233] of regenerated cellulose I. The observation that there are two different forms of cellulose III and of IV is also remarkable. The two subforms of each allomorph have essentially identical lattice dimensions and at least similar equatorial intensities. Other intensities are different, particularly the meridional intensities, depending on whether the structures were prepared initially from cellulose I or II. The formation of the III and IV structures is reversible and the preceding polymorph (I or II) results. 

Acid hydrolysis enhances the pore system by removing amorphous cellulose from the surface and revealing the macrofibrillar structure of cellulose fibres [5]. Drying results in an irreversible reduction of the pore volume as a result of the pores collapse arising from the capillary forces, a mechanism called hornification. 

Atalla and Van der Hart (11, 12) concluded, based on their Raman and NMR spectra, that the molecules in cellulose I and II have different conformations. Based on x-ray analyses, Sarko et al. (13i H) and Blackwell et al. (15, 16) both concluded that crystal structures of cellulose I and II were based on parallel and antiparallel packing, respectively, of chains that have similar backbone conformations. Sarko (17) concluded that the allomorphs in the I and II families were based on parallel and antiparallel chains, respectively. The irreversibility may arise from the increase in entropy when parallel packing is converted to antiparallel packing. 

The major difference between these two crystal structures resides In the chain packing polarity. As expected from the conversion studies and the Irreversibility of the cellulose I to Na-cellulose I transformation, the crystal structure of Na-cellulose I Is based on antiparallel chains (cf. Fig. 3). Because of the presence of Na Ions, which apparently form secondary bonds with the cel-... 

Accordingly, cellulose has an average molecular weight in the range of 300 000-500 000. One of the most interesting eharacteristics is that eellulose eonsists of several crystal polymorphs, with the possibility of eonversion from one form to another. Its six different polymorphs differ in unit cell dimensions and chain polarity, and are the principle component of all plant cell walls. The natural cellulose I has two different structures, lot and 1(3, while cellulose II is another important crystalline form of cellulose. The transformation of cellulose I to cellulose II is generally considered to be irreversible, because cellulose II is more stable than cellulose I. With proper chemical treatments, it is possible to produce cellulose III and cellulose IV. 

Wood a mixed polymer with lignin as a structural component. Deposition of lignin in the cellulose matrix of the cell wall is called lignification. It is in principle comparable to an irreversible swelling. Cellulose and lignin are physically and chemically bound together in W. 

Qualitative explanations for the hysteresis in sorption for cellulosic materials have been advanced. Urquhart [23] hypothesized that hysteresis could be caused by a differential availability of hydroxyl groups in cellulose during the adsorption and desorption branches of the RH cycle. A second explanation for hysteresis stems from the observed plasticity of the cellulose gels, which swells upon adsorption. Desorption results in some irreversible (plastic) deformation, which results in a higher moisture content of the structure as compared to the adsorption process. His theory postulated that the lost work during a stress strain cycle is equal to the lost work represented by the hysteresis loop between two relative humidity levels. 

Polyvinyl aleohol is rarely used in paint systems as irreversible struetures may be obtained with the gelling agent. For paints the structure may be achieved by using colloidal agents, sueh as hydroxyethyl cellulose. 

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