Micro fibrils mechanism

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... 

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. 

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]. 

Various reports are present in the literature on the usage of cellulose fibers in the preparation of composites. Cellulose fibers like banana, sisal, oil palm, jute, pine apple leaf fiber were found to have a very good reinforcing effect on polymer matrices [38 2]. The mechanical properties and water absorption were found to be dependent on the amount of micro-fibrils. 

As will be discussed later (see p. 332), these data provide strong support for the argument that the microfibrils are produced by on-the-site synthesis and orientation (apposition) of the cellulosic microfibrils under the guiding influence of the living cell, rather than by a mechanism that proposes synthesis of the micro fibrils within the cell and subsequent translocation and crystallization (deposition) of microfibrils on the cell wall by exocellular factors. Further factors relevant to these opposing theories emerge from study of the fine structure of the cellulosic microfibrils, as discussed in the following Section. 

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. 

As previously mentioned, natural fibres present a multi-level organization and consist of several cells formed out of semi-crystalline oriented cellulose micro fibrils. Each microfibril can be considered as a string of cellulose crystallites, linked along the chain axis by amorphous domains (Fig. 19.10) and having a modulus close to the theoretical limit for cellulose. They are biosynthesized by enzymes and deposited in a continuous fashion. A similar structure is reported for chitin, as discussed in Chapter 25. Nanoscale dimensions and impressive mechanical properties make polysaccharide nanocrystals, particularly when occurring as high aspect ratio rod-like nanoparticles, ideal candidates to improve the mechanical properties of the host material. These properties are profitably exploited by Mother Nature. 

Lei and Wu [40] prepared wood plastic composites based on in-sifw-formed polyethylene terephthalate (PET) sub-micro-fibril (less than 500 nm in diameter) reinforced high-density polyethylene (HDPE) matrices through strand die extrusion and hot strand stretching. The PET fibrils obviously increased mechanical properties of the... 

The dominating S2-layer contains the cellulose micro fibrils orientated under an acute angle toward the fiber axis. Such orientation of the microfibrils imparts to natural cellulose fibers increased mechanical properties. The microfibrils of the cell wall consist of elementary nanofibrils, and each such fibril is built of ordered nanocrystallites and low ordered non-crystalline (amorphous] domains statistically alternated along the fibril. The early investigations supposed the presence in various cellulose samples of elementary nanofibrils having a constant lateral size of 3.5 nm (Manley, 1964 Muhlenthaler, 1969]. However, recent... 

With increasing petroleum cost and environmental concerns, the composite industry is in need of an alternative source of fibers and polymers. Cellulose is the most abundant natural polymer on the planet. Thus, harnessing cellulose fibers from the biomass for composite fabrication seems like a logical step [1,2]. Even though cellulose fibers, especially cellulose nano and micro fibrils, have excellent mechanical characteristics [3,4], these fibers are yet to reach widespread recognition in the composite industry. Other biomass natural polymers, such as hemicellulose, lignin, and various pectins, could also be utilized in composite fabrication. 

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