Nanosized cellulose

The similar term nanosized cellulose is used in case of isolated crystallites and whiskers formed by acid-catalyzed degradation of cellulosics. This field and the application of that nanosized cellulose, e.g. in composites, have been intensively investigated. Typical examples have been presented in current papers [3,4] and at the 231st American Chemical Society (ACS) national meeting in Atlanta. 

To improve the thermal, mechanical, and viscoelastic properties of cellulose acetate butyrate, it was reinforced with nanocellulose crystals prepared from BC by acid hydrolysis. Using this nanosized cellulose (Sect. 1) a significant improvement in the properties of the composites was demonstrated [57]. 

Combination of hemicelluloses with nanosized cellulose creates nanocomposites that mimic natural plant cell wall structures (Table 9.4). The preparation of these nanocomposites has aimed at studying the interactions of the components or at improving the functional properties of hemicellulose-based films, such as tensile strength or water vapour barrier properties. 

There are basically two families of nanosized cellulosic particles (1) Nanofibrillar cellulose, which includes mechanically isolated microfibrils, chemically isolated microfibrils (TEMPO-oxidation), bacterial cellulose and can be considered spaghetti-like, and (2) Cellulose nanocrystals - rods of highly crystalline cellulose which are isolated by acid hydrolysis. Cellulose nanocrystals are represented in literature by synonyms like cellulose whiskers, cellulose nanowhiskers, cellulose microfibrils, micro-crystalline cellulose and nanocrystalline cellulose because they are not yet commercially available. These are needle-shaped (100 run to 200 run X 10 nm), highly crystalline, strong (E - 150 GPa) and form liquid crystal suspensions. 

The chemical modification of a natural fiber is often performed in order to enhance the properties of the interface between fiber and matrix. A more recent method of modification, involves the deposition of a coating of nanosized cellulose onto the natural fibers or dispersing a nanosized cellulose in natural fiber reinforced composites. This method has been shown to improve the fiber-matrix interface and the overall mechanical performances. Such composites have been addressed as hierarchical, multiscale, nanoengineered, or nanostructured composites. The state-of-the-art in this field has been reviewed (46). 

The previous three sections involved the treatment of natural fibres by removing substances from the natural fibres. This section, however, describes a new modification that does not involve the removal but the addition of new material onto the surface of natural fibres. This type of modification involves the deposition of nanosized cellulosic materials onto the surface of natural fibres to enhance the interfacial adhesion between the fibres and the matrix [9,10,14,104]. By doing so, a hierarchical structure can be created. These works were inspired by nature. Nature maximises the efficiency of structural materials by creating hierarchical stmctures the arrangement of the constituents at every level, from the molecular level to the macroscopic level. By applying this concept, composites that possess a hierarchical structure with improved mechanical properties can be manufactured. 

This method involves the deposition of nanosized cellulosic material onto the surface of natural fibers, controlled by bacterial action in order to enhance the interfacial adhesion between the fiber and the matrix [83]. The introduction of bacterial cellulose onto the surfaces of plant fiber and the polymer matrix is a way of controlling the... 

On the other hand, native cellulose is an abundant and inexpensive macromolecular compound that reinforces most plant cell walls. During recent years, attention has been devoted to the use of cellulose (nanocellulose or nanofibrillated cellulose), and important studies have been published [13-22]. The outstanding mechanical properties of nanocellulose or nanofibrillated cellulose, linked to its wide availability, biodegradability, and extensive number of alternatives for chemical modification, have been the driving force for its utilization as reinforcement in polymers. These cellulosic materials are composed of nanosized cellulose fibrils with a high aspect ratio (length-to-width... 

There are basically two families of nanosized cellulosic particles. The first one consists of cellulose nanocrystals and the second one is microfibrillated cellulose (MFC) [17]. However, different terminologies are used to describe these cellulose nanoparticles, leading to some misunderstanding and ambiguities. Currently, the isolation, characterization, and search for applications of novel forms of cellulose, variously termed crystallites, nanocrystals, whiskers, nanofibrils, and nanofibers, is generating... 

Preparation, Properties and Chemical Modification of Nanosized Cellulose Fibrils... 

A form of nanosized cellulose particles that has recently been quite extensively studied is cellulose whiskers or nanocrystalline cellulose [1,2]. This material, which is manufactured by acid hydrolysis of cellulose fibers (see below), will be discussed only to the extent that knowledge of its properties are of immediate relevance to MFC. 

Atomic force microscopy (AFM) has the necessary resolution to observe nanosized cellulose fibril structures without the need for metal coatings. AFM has been applied to both MFC [7, 25] and cellulose whiskers [41-44] and to generate surface profiles of films cast from cellulose nanocrystals [44—46]. AFM may, however, overestimate the width of the particles [41, 44] due to the tip-broadening effect (the shape of the tip contributes to the recorded image). One way to overcome this problem is to measure the height of the fibrils, which is not subject to tip-broadening artifacts [43]. 

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