Modifications of nanocellulose

Esterification, etherification, silylation and pofymer grafting are regarded as methods of covalent modification used for nanocellulose [29, 30]. 

Degree of acetylation plays an important role in defining material properties. Acetylated nanocellulose in composites is believed to enhance transparency, mechanical properties, reduce refractive and thermal expansion indexes and decrease hygroscopicity [29-31]. 

By reaction with hydroxyl groups of the fibers theses silanols are able to form well chemisorbed onto the fiber surface polysiloxanes (Fig. 21.17). Hydrocarbon chains of silanecoupling agent, due to covalent conding between the matrix and the fiber, create a crosslinked network that prevents the fiber from swelling. It has been demonstrated that cellulose whiskers produced by the acid hydrolysis of tunicate, partially silylated, so that the carbon backbone of the alkyl 

Advantages Polymers can be fully characterized before grafting 

Oxidation of cellulose with 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO) in presence of NaOCl and NaBr is used to convert the hydroxymethyl groups of MFCs to their carboxylic form. This reaction is regarded as green, simple to implement and, what is even more important, very highly selective for primary alcohols, since secondary hydroxyls remain unaffected [30], 

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Zoppe J, Habibi, Y, Efimenko K et al (2009) ATRP modification of nanocellulose substrates, Abstr Pap ACS Natl Meet 237 Cell46... 

The mechanical properties of PLA/nanocellulose biocomposites are also strongly affected by the processing strategy and surface chemical modification of nanocellulose. Table 9.2 provides an overview of modulus, tensile... 

Thus suitable chemical modification of nanocellulosic fillers and their incorporation in a synthetic matrix like PMMA by in-situ polymerization proved to be an effective method to make them more environmentally friendly and partly degradable [69]. 

While techniques for preparation of nanocellulose-reinforced nanocomposite are different in complexity, they typically involve physically mixing and dispersing the nanocellulose and resin in a solvent system. In many cases, solvent exchange techniques are used, often along with surface modification of nanocellulose to make it compatible with organic solvents and/or the resin system. In this context, nanocomposite films from nanocellulose generally are prepared through three various techniques as below ... 

Bodin, A, Ahrenstedt, L., Fink, H., Brumer, H., Risberg, B., and Gatenholm, P. (2007]. Modification of nanocellulose with a xyloglucan-RGD conjugate enhances adhesion and proliferation of endothelial cells Implications for tissue engineering, Biomacromolecules, 8,3697-3704. 

There have been numerous investigations into the subsequent modification of bacterial and wood nanocelluloses. The additives range from other polysaccharides, albuminoids such as gelatine, different types of monomers and synthetic polymers, to metals, metal oxides, and inorganic fibers. On the... 

Due to the amorphous structure, ANP acquires such specific features as increased content of functional groups, high accessibility, and sorption ability (loelovich, 2013a, 2014a). Freeze-dried amorphous nanocellulose has an enhanced wetting enthalpy (—125 to — 130J/g), absorbs up to 35—40% water vapor, and completely decomposes under action of cellulolytic enzymes. Moreover, the chemical modification of ANP is carried out faster and deeper than other kinds of nanocellulose. 

This chapter aims to introduce the concept of nanocellulosic-fibril and precipitated calcium carbonate (PCC) composites. The effect of cogrinding of lime and pulp fibres and different precipitation conditions on the production of PCC and modification of its morphologies are analysed in this chapter. 

Figure 9.2 Surface modification chemistries of nanocellulose for PLA/nanocellulose biocomposites, a, Acetylation b, Esterification with various organic acids c, d, e, Grafting of PCL, PLA, P(CL-fi-LA) f, Silanization g, Silylation h, Carbojymethylation combined with hexanoation i, PEG grafting j, Modified with polyhedral oligomeric silsesquioxane (POSS).

A summary of the different chemical modification techniques used to alter the surface characteristics of nanocellulose can be found in reference [47]. 

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... 

Elowever, cellulose nanofillers have some disadvantages such as moisture absorption and poor compatibility with the hydrophobic polymer matrices [27, 28]. The use of nanocellulosic fillers is mostly limited to hydrosoluble polymers, latexes, or DMF-soluble polymers [12, 29, 30]. To overcome this problem, several methods were proposed recently, which involved the chemical modification of the cellulosic fillers [12, 30] with various chemical reagents such as acetic anhydride [31], alkenyl succinic anhydrides [32], chlorosilanes [33,34], or hexamethyldisilazane [35]. 

The adoption of ternary polymeric systems including cellulosic nanoreinforcements and other nanoparticles leads to considerable assets in terms of multifunctionality, but also of tailored modification of the properties of the selected polymer matrix. A number of issues remain nonetheless open for discussion, particularly linked to the polysaccharides source from which nanocellulose is obtained, the process used to fabricate it and the strength of its interface with the polymer matrix and with the particles. This involves other aspects, such as the degree of hydrophilicity and the improvement of... 

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