Fibrillar hierarchy

Fig. 5.118 STM images of Kevlar most clearly reveal the nature of the fibrillar hierarchy at the macromolecular level. A bundle of uniform microfibrils about 10 nm wide is observed (A) within a larger aggregated fibrillar structure unit in images taken at successively higher magnifications. Finer fibrils are also clearly seen (B). (From Sawyer et al [435] reproduced with permission.)...

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. 

Figure 1 Structu ral hierarchy of the hierarchical gecko fibrillar adhesive. From Hansen, W. R. Autumn, K. Proc. Natl. Acad. Sci. U.S.A. 2005,102,385. Copyright 2005 National Academy of Sciences, U.S.A.

A general structure model developed for highly oriented liquid crystalline fibers [429, 430] is shown schematically in Fig. 5.111. The model was initially defined for the developmental Vectran LCP fibers, but it has been extended by study of the aromatic polyamides and the major features appear to be general in nature. The model extends the structure hierarchy proposed by Dobb, Johnson and Saville [475] for the aramids. Three fibrillar elements have been noted microfibrils < 50 nm in size ... 

Wess, T. J. (2008). Collagen Fibrillar Structure and Hierarchies. In Collagen Structure and Mechanics. P. Eratzl (Ed.), Springer... 

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. 

Fibrillar samples consist of oriented crystallites arranged into needlelike structures of various sizes. There is little consensus of opinion in the literature regarding the nomenclature or sizes of the features that comprise the fibrillar morphology (see, e.g.. Refs. 8-11). However, it is clear that a hierarchy of sizes exists macrofibrils consist of bundles of microfibrils, which in turn are composed of bundles of nanofibrils. Typical macrofibrils (which are visible to the naked eye) may be up to several hundred micrometers thick with a length... 

Much has been written about fibrillar morphology, and several morphological models have been advanced to explain their observed features and physical properties. The vast majority of this research has centered on high density polyethylene, often drawn at temperatiu es substantially above ambient that favor high draw ratios. The various morphologies proposed may be rationalized into the hierarchy of stractures illustrated in Figure 3, which incorporates the most important features reported by different authors [9,10,12-17]. 

Figure 3 Hierarchy of structures comprising a fibrillar morphology, (a) Macrofibrils (b) microfibrils and (c) nanofibrils.

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