Fibril, microfibril

Since neither elementary fibrils, microfibrils, or larger structures are visible in untreated cotton or wood fibers, and because similar plant fibers treated in different ways and different fibers treated in similar ways have... 

The forces are proportional to the lateral surface area of fibrils and microfibrils. Since they increase with better, more perfect contact of adjacent elements, one would expect a higher resistance to the displacemnt of microfibrils than to that of fibrils. Microfibrils do not vary much in their draw ratio they are packed more closely, and to some extent they are connected by inter-fibrillar tie molecules. Fibrils vary much more in their draw ratio and in their outer boundary there are a great many structural defects caused by the irregular ends of microfibrils. 

Supramolecular level (nm). The aggregation of the cellulose macromolecules into elementary fibrils, microfibrils and macrofibrils are described in this level. Its intermolecular interactions and crystal lattice are also discussed. 

Fig. 14. Fibril/microfibril structure of PBZO fibers Reprinted from Ref. 34, by permission of Marcel Dekker, Inc.

Investigation of thinner c/.v-polyacetylene films revealed large numbers of even smaller fibrils, microfibrils, with a diameter of 2-3 nm. On the basis of the crystal structure of the unit cell of m-polyacetylene. the number of polyacetylene chains contained in and therefore making up the microfibrils was deduced. This ranged from about 13 chains (for 2 nm microfibrils) to 60 (for 3 nm microfibrils). Since then, these microfibrils have been regarded as the basic morphological unit of the fibril hypothesis. On this basis the fibrils discovered earlier are made up of straight microfibrils. 

Table 5 compares the tensile properties of Vectra A950 in the form of dispersed fibers and droplets in the matrix by injection molding, microfibril by extrusion and drawing [28], injection molded pure thick sample and pure thin sample, and the pure drawn strand [28]. As exhibited, our calculated fiber modulus with its average of 24 GPa is much higher than that of the thick and thin pure TLCP samples injection molded. It can be explained that in cases of pure TLCP samples the material may only be fibrillated in a very thin skin layer owing to the excellent flow behavior in comparison with that in the blends. However, this modulus value is lower than that of the extruded and drawn pure strand. This can be... 

Another type of fibril substructure in PET fibers, besides the microfibrillar type already discussed, is the lamellar substructure, also referred to as the lateral substructure. The basic structural unit of this kind of substructure is the crystalline lamella. Formation of crystalline lamellae is a result of lateral adjustment of crystalline blocks occurring in neighboring microfibrils on the same level. Particular lamellae are placed laterally in relation to the axis of the fibrils, which explains the name—lateral substructure. The principle of the lamellar substructure is shown in Fig. 2. 

Figure 2 The lamellar substructure of a fibril. (a) Reciprocal positions of crystalline lamellae as a result of fiber annealing. (b) The situation after relaxation of stress affecting TTM. ai.2 - average angle of orientation of TTM CL - crystalline lamellae CB - crystalline blocks (crystallites) mF -border of microfibrils and F - fibril. In order to simplify it was assumed that (1) there are the taut tie molecules (TTM) only in the separating layers, (2) the axis of the fibril is parallel to the fiber axis.

From the foregoing it is clear that indentation anisotropy is a consequence of high molecular orientation within highly oriented fibrils and microfibrils coupled with a preferential local elastic recovery of these rigid structures. We wish to show next that the influence of crystal thickness on AMH is negligible. The latter quantity is independent on 1 and is only correlated to the number of tie molecules and inter-crystalline bridges of the oriented molecular network. 

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. 

Cellulose microfibrils make up the basic framework of the primary wall of young plant cells (3), where they form a complex network with other polysaccharides. The linking polysaccharides include hemicellulose, which is a mixture of predominantly neutral heterogly-cans (xylans, xyloglucans, arabinogalactans, etc.). Hemicellulose associates with the cellulose fibrils via noncovalent interactions. These complexes are connected by neutral and acidic pectins, which typically contain galac-turonic acid. Finally, a collagen-related protein, extensin, is also involved in the formation of primary walls. 

The final mechanism of stress relief is thermomechanically activated chain scission. Primary bond breakage can be homolytic, ionic or by a degrading chemical reaction. It is worthwhile to note that the relative slippage of chains, microfibrils and fibrils reduces or prevents the mechanical scission of chains in quasi-isotropic polymeric solids. In other words, chain scission is an important mode of fracture only in highly oriented thermoplastic fibers or in thermosets. 

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