Macro fibrils

Fig. 4.32. Macro-fibril in PVC originating from coalescence of thin fibrils ...

Morphologically, the fibres are composed of the cortex and the cuticle. Each of the two components is formed of various other morphological components (Table 9.6.3). The cortex contains cortical cells and the cell membrane complex. The cortical cell is further composed of macro-fibrils and intermacro-fibrillar material. The macro-fibrils consist of micro-fibrils and intermicro-fibrillar matrix. In summary, the cortex is formed of micro-fibrils (intermediate filament, IF, or keratin proteins, KP) and keratin associated proteins (IFAP or KAP), which compose the intermicrofibrillar matrix containing cytoplasmatic and nuclear remnants. This ensemble is wrapped up in the cuticle, as an external sheath which also has its own architecture, being formed of four layers the epicuticle, the a-layer, the exocuticle and the endocuticle. 

Cortex cell Filament in matrix 5-8 macro-fibrils Intermacrofibrillar matrix... 

Macro-fibril Filament in matrix 500-800 micro-fibrils (IFs) Intermicrofibrillar matrix... 

Let us provide at least two examples of application of these mles. Sawyer and Jaffe (20) have defined the hierarchical fibrical structure of LC materials after processing. Macro fibrils, fibrils and micro fibrils they consider constitute the key entities at three different levels - as defined in Rule 3. 

Hierarchical fibrillar model [80] This model was proposed for drawn TLCP fibers. They are composed of bundles of macro fibrils (5 p,m), fibrils (0.5 (Jim), and micro fibrils (0.05 p.m) in a hierarchical order (see Figure 8.17). This type of highly oriented fibrillar morphology is also found in the inner skin region of the injection-molded parts, as illustrated in Figure 8.18. 

Figure 9.1. A. Fringe-fibril model of cellulose after Hearle [4] see also Zugenmaier [1], The right figure B. shows a schematic of a macro-fibril as existing in plant cells begin a composite of micro-fibrils. These consist of elementary fibrils which are made of 30-40 polymeric linear cellulose chains (picture based on the botany visual resource library [5]). The picture in figure A. is observed in crystalline cellulose, grown either artificially as for instance in textile fibers [1] or can be thought to mimic the structure of elementary fibrils.

Natural fibers consist of aggregated cellulose chains arranged in a hierarchical structure. These elementary fibrils are composed of cellulose chains called cellulose macro fibrils [16]. Figure 6.3 shows a fransmission elecfron microscopy (TEM) image of cellulose microfibrils, or MFC [17],... 

Figure 5.15. MFC can be obtained from incompatible polymer blends by extrusion and orientation (the fibrillization step) followed by thermal treatment at a temperature between the melting points of the two components at constant strain (the isotropization step). The block copolymers formed during the isotropization (in the case of condensation polymers) play the role of a self-compatibilizer. Prolonged annealing transforms the matrix into a block and thereafter into a random copolymer (a) an MFC on the macro level, (b) an MFC on the micro (molecular) level (Fakirov Evstatiev, 1994).

As a rule, ACF not only presents a higher adsorption capacity than conventional GAC, but the pore network is also different due to the fibril structure, which ensures a much higher adsorption kinetics. The reason is that in GAC, the adsorbate must diffuse throughout the macro and mesopores before reaching the micropore or adsorption sites, whereas the micropores are directly accessible ftom the external surface in the ACF (Fig. 23). Consequently, there is no resistance to the diffusion of adsorbates through to the adsorption pores because there is no meso/macropore network. 

In this chapter we have reviewed some of the most important characteristics of cellulose and cellulose based blends, composites and nanocomposites. The intrinsic properties of cellulose such as its remarkable mechanical properties have promoted its use as a reinforcement material for different composites. It has been showed that cellulose is a material with a defined hierarchy that tends to form fibrillar elements such as elementary fibrils, micro fibrils, and macro fibers. Physical and chemical processes allow us to obtain different scale cellulose reinforcements. Macro fibers, such as lignocellulosic fibers of sisal, jute, cabuya, etc. are used for the production of composites, whereas nano-sized fibers, such as whiskers or bacterial cellulose fibers are used to produce nanocomposites. Given that cellulose can be used to obtain macro- and nano-reinforcements, it can be used as raw material for the production of several composites and nanocomposites with many different applications. The understanding of the characteristics and properties of cellulose is important for the development of novel composites and nanocomposites with new applications. 

From Table 9.1, another important conclusion could be drawn regarding the thickness of the studied fibrils. The microfibrils have diameters around 1 pm and that of nanofibrils is between 50 and 150 nm. The specific surface is also different for the two types of fibrils. BET analysis of the scaffolds show that the nanofibril-lar material (PET-Nano) is characterized by the highest surface area, 18.8 m g (Table 9.1). The adsorption-desorption behavior of this sample corresponds to pores of meso- and micro-size (>10nm). In contrast to the PET-Nano sample, the material comprising microfibrils possesses five times less surface area, namely, of 4m g (Table 9.1, sample PET-Micro 2), and the size of the cavities formed is in the macro-range (>50 nm). The microfibrillar PGA scaffold, which possesses larger surface area (Table 9.1) and similar pore size distribution, shows comparable behavior. 

The scaffolds can be considered at different length scales the macrostructure, the microstructure and the nanostructure. On the macro- and micro-scales it has been shown that surface chemical composition and topography have strong effects on cell behaviour, but less is known about how cells react to nanoscale structures. However, cells are likely to be able to respond to nanostructures, since they live inside the extracellular matrix (ECM) containing nanoscale collagen fibrils and since their own surface is structured on the nanoscale level (receptors and filopodia). 

The application of 2-3% omf (on mass of fibre) fixed resin appears to be optimal for easy-care properties, dependant on the fabric constmction and weight. Application levels of 2% omf are needed to stop fibrillation on domestic washing. In addition to the resin, the choice of softener can have a large effect on the easy-care performance of fabrics, and it is important to consider the whole formulation and build it up to give the required performance. Silicone micro-emulsions penetrate yams more than the macro-emulsions. Polyethylene dispersions aid sewing and build the handle of the fabric, whilst some soft acrylic-based chemicals can increase the abrasion resistance. It is also worth remembering that caustic soda or liquid ammonia treatment in preparation will help to increase the easy-care rating of lyocell fabrics. 

Deformation zones (also called homogeneous crazes) possess the same orientation as fibrillated crazes, but they do not contain voids, do not show volume increase, and are the result of homogeneous deformation processes with gliding of macro-molecular segments (see Figs. 1.49 and 1.50 in PC in Part 11). 

The model extends the structural hierarchy proposed by Dobb, Johnson and Saville [374] for the aramids. Three distinct fibrillar elements have been noted microfibrils, on the order of 50 nm in size fibrils, on the order of 500 nm in size and macrofibrils, about 5 pm (5000 nm) across. The importance of this structural model is that it not only describes the structure of uniaxially oriented fibrous materials, but it also shows the fine structure of the thicker LCP forms of moldings and extrudates. In these thicker materials, process history and temperature affects macrostructures, such as skin-core, bands and layering (Fig. 5.85). The fiber structural model shows the arrangement of the fine structure within those macro units. This structural model improves the understanding of relationships between processes, structure and properties in LCPs. 

Fig. 15.1 clearly shows the hierarchical structure of the bone, from macro- to nanolevel. At the macro-level, the different structure between compact and cancellons bone is well evident. Osteons and trabeculae compose the bone at the microstrnctnral level, whereas the mineralized collagen fibrils constitute the nanostractured-bone building blocks [1,3]. 

---