Micro-fibrils

In order to supplement micro-mechanical investigations and advance knowledge of the fracture process, micro-mechanical measurements in the deformation zone are required to determine local stresses and strains. In TPs, craze zones can develop that are important microscopic features around a crack tip governing strength behavior. For certain plastics fracture is preceded by the formation of a craze zone that is a wedge shaped region spanned by oriented micro-fibrils. Methods of craze zone measurements include optical emission spectroscopy, diffraction... 

Collagen fibres (a) and hydroxyl apatite crystals (b) together form larger units, the so-called micro-fibrils (c). It should be pointed out here that the collagen fibres are helically intertwined and consequently... 

We have shown by our ISO brightness and Ak measurements that the lignin was attacked by the reactive species created by the y irradiation. The results indicate that the reactive species created attack indiscriminately the phenolic hydroxyl or coniferalde-hyde groups and the quinone groups. The decrease in physical properties is associated with die attack on the micro fibrils cellulose. However, at 3000 krad/h, the increase in the ISO brightness is smaller than the one observed around 300 krad/h for the TMP, while the physical properties of the pulp are affected more profoundly at 3000 krad/h than at 300 krad/h. The physical properties of the P-TMP behave similarly, while the increase in brightness is negligible at 3000 krad/h for P-TMP. 

In the last 25 years, there have been efforts to reduce wood fibers in size. As a first step, in the early 1980s Turbak et al. [18] developed a micro-fibrillated cellulose (MFC). Today, there are different ways to produce materials with controlled fiber diameters. 

FIG. 2.15 Tie molecules in the amorphous layer between subsequent crystal blocks of the micro-fibril. At low strain (a) a single tie molecule (A), at medium strain (b) two tie molecules (B), and at the highest strain (e) three molecules (C) are stretched up to the rupture point (after Peterlin, 1971). 

In some respects semi-crystalline polymers are similar to filled reinforced systems (crystallites, embedded in amorphous matrix) in the same way highly oriented semicrystalline polymers are similar to fibber-reinforced systems (micro-fibrils embedded in... 

The polymer is about 0.8 nm in its maximum width and 0.33 nm2 in cross-sectional area, and can contain about 10,000 glucose residues with their rings in the same plane. In the cell wall these polymers are organized into micro-fibrils that can be 5 nm by 9 nm in cross section. These microfibrils apparently consist of an inner core of about 50 parallel chains of cellulose arranged in a crystalline array surrounded by a similar number of cellulose and other polymers in a paracrystalline array. Microfibrils are the basic unit of the cell wall and are readily observed in electron micrographs. Although great variation exists, they tend to be interwoven in the primary cell wall and parallel to each other in the secondary cell wall (Fig. 1-13). 

Barber and Meylan (1964) developed a model based on matrix-micro fibril interaction. In its simplest form their model ignores the different characteristics of the various layers in the cell wall. Instead it assumes that the behaviour of the wood is determined by the thick, dominant S2 layer, where the microfibrils can be inclined to the fibre axis at some angle between 5° and 50° (Figure 4.4). 

Extended chain crystals can be obtained by crystallization under high pressures or under uniaxial extension. So-called shish-kebob structures are frequently observed when crystallizing polymers under orientation, e.g. in stirred solutions and polymer processing from the melt. Similar to lamellae and (micro) fibrils treated in previous chapters, these structures can be routinely visualized and analyzed by AFM. 

Cellulose is found not to be uniformly crystalline. However, the ordered regions are extensively distributed throughout the material and these regions are called crystallites [27]. Cellulose exists in the plant cell wall in the form of thin threads with an indefinite length. Such threads are cellulose micro-fibrils, playing an important role in the chemical, physical, and mechanical properties of plant fibers and wood. 

The plasma membrane of the plant cells has been considered the likely site for the synthesis and assembly of cellulosic micro-fibrils [28]. These micro-fibrils are found to be 10-30 nm wide, less than this in width, and indefinitely long containing... 

These are covered with a sheath of para-crystaUine polyglucosan material surrounded by hemicellulose [29]. In most natural fibers, these micro-fibrils orient themselves at an angle to the fiber axis called the micro-fibril angle. The ultimate mechanical properties of natural fibers are found to be dependent on the microfibrillar angle. Gassan et al. have performed calculations on the elastic properties of natural fibers [30]. 

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