Cellulose nanocrystallites

Figure 19.13 Schematic of the chiral nematic ordering of cellulose nanocrystallites (a) and POM images (b-d) (scale bar = 100 mm). (Reprinted with permission from Ref. [200]. Copyright 2010, Macmillan Publishers Ltd.)...

Microfibrillar structure of cellulose. Cr, elementary nanocrystallite A, amorphous noncrystalline domain. 

Parallel orientation of the microfibrils in the S2 layer under an acute angle toward the fiber axis imparts to natural cellulose fibers increased axial mechanical properties. The microfibrils of the cell wall consist of elementary nanofibrils, and each such nanofibril is built of ordered nanocrystallites and low ordered noncrystalline (amorphous) domains statistically alternated along the fibril (Figure 9.3). 

The shape of nanocrystallites of natural cellulose is a subject of discussion. In several studies the cross-sectional shape of the crystallites was depicted as a square or rectangle. However, recent studies have shown that the most likely cross-sectional shape of the crystallites of natural cellulose is a hexagon (Ding and Himmel, 2006 loelovich, 1991 Yang et al., 2011). Three groups of planes (100), (110), and (110) are located on the surface of crystallites, allowing the co-crystallization process of adjacent crystallites in different lateral directions (loelovich, 1991). The co-crystallization process observed during extraction and hydrolysis of cellulose leads to an increase in lateral sizes of crystallites. 

A two-phase model containing crystalline and noncrystalline domains is currently used to describe the stmctural organization of cellulose (Krassig, 1993). Further investigations also revealed the presence on the surface of crystallites a paracrystalline fraction that must be taken into consideration in an improved model of cellulose structure (loelovich et al., 2010). Statistically alternated nanocrystallites along with nanoscale noncrystalline domains are integral constituents of long and thin elementary fibrils and their bundles—microfibrils. 

Ye S, Zhang D, Liu H, Zhou J (2011) ZnO nanocrystallites/cellulose hybrid nanofibers fabricated by electrospinning and solvothermal techniques and their photocatalytic activity. J Appl Pol5on Sci 121 1757-1764... 

To characterize the supermolecular structure of cellulose, the primary structural parameters (type of crystalline allomorph, crystallinity, paracrystallinity and amorphicity, and orientation of nanofibrils, nanocrystallites, and nanoscale non-crystalline domains, as well as porosity of cellulose) should be determined. These structural parameters can affect physicochemical, chemical, biochemical, physical and mechanical properties of cellulose materials. 

Cellulose is a nanostructured natural polymer that has a complicated multilevel structural organization. The linear cellulose macromolecules joined by hydrogen bonds form the nanocrystallites, nanoscale non-crystalline domains and elementary nanofibrils. The elementary nanofibrils are aggregated into bundles called... 

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