Fibrils, elementary

Naturally occurring cellulose is extremely mechanically stable and is highly resistant to chemical and enzymatic hydrolysis. These properties are due to the conformation of the molecules and their supramolecular organization. The unbranched pi 4 linkage results in linear chains that are stabilized by hydrogen bonds within the chain and between neighboring chains (1). Already during biosynthesis, 50-100 cellulose molecules associate to form an elementary fibril with a diameter of 4 nm. About 20 such elementary fibrils then form a microfibril (2), which is readily visible with the electron microscope. 

Fig. 2. Molecular and fribrillar structure of cellulose, mf = Microfibril ef = Elementary fibril cn = Cristalline net...

Figure 4-14 Tentative structure of the walls of suspension-cultured sycamore cells. The wall components are in approximately proper proportions but the distance between cellulose elementary fibrils is expanded to allow room to present the interconnecting structure. There are probably between 10 and 100 cellulose elementary fibrils across a single primary cell wall. From Albersheim et al.n6...

Elastomers based on VDF and TFE-VDF-HFP consist of fine particles 16 to 30 nm in diameter in contrast to PTFE which has a rodlike microstructure in which the elementary fibrils are approximately 6 nm wide and the molecular chains are all extended.12 For example, the properties of a VDF-HFP elastomers such as their resilience and flexibility can be related to spherical domains with diameter approximately 25 nm that are interconnected.10 The diameter of these particles was found to be proportional to the molecular weight of the elastomer.12... 

FIGURE 5.3 Cellulose microfibril composed of four elementary fibrils, viewed along the chain axes. The unit cell is shown, and the chains on the elementary fibril surfaces are disordered. Chain ends within the elementary fibrils account for more disorder. The surfaces of the crystallites are related to the crystallographic planes shown in gray. 

They form a network of long, fine elementary fibrils about 10 m/x in diameter which seem to be almost as strongly adherent in the lateral direction as they are along the fiber axis. Partial digestion with elastase or ultrasonic treatment reveals systems of interconnected fibrils which are tangled and loosely twisted together to form dense cordlike fibers in ligamentum nuchae or thick membranes in arterial walls. 

Unit cells are the smallest repeatable spatial units within crystallites. A crystallite contains multiple unit cells in parallel arrangement giving rise to crystalline strands or elementary fibrils (O Fig. 8). These fibrils are quoted with cross-sectional dimensions of 2.5 to 4 and 12-14 nm in length. In cotton they can have dimensions of 10 to 40 x 50-60 nm. Elementary fibrils aggregate to form microfibrils. 

With regard to the present advances in the chemistry of cellulose an excellent review has been given by Shafizadeh (3). He also refers to the folded chain structure of cellulose (4, 5) and of the structure of elementary fibrils (6, ... 

Reports on the characterization and the make-up of the elementary fibrils and on their association while establishing the fiber structure - usually called fibrillar or fringed fibril structure - are present in the literamre [32]. 

Keratin has a crystallinity of about 30 Z. Since 1950,X-ray diffraction and electron microscopy have led to a model in which the structural elements are (13,14) the a-helix with a diameter of 10, the elementary fibril containing two to three helicoidal chains of 20 diameter, the microfibril or association of ten elementary fibrils through the amorphous regions (of 80 A diameter) and finally the fibrils or structures of several microfibrils within an amorphous matrix. 

In Figure 1 we see bacterial cellulose that has been grown by Haigler in the presence of dissolved carboxymethyl cellulose (CMC), which inhibits the aggregation of the most elementary fibrils into the ribbons that are usually observed without the CMC. The most subelementary nanofibrils in Figure 1 are of the order of... 

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