Celluloses, acid hydrolysis polymerization

Cellulose is the main component of the wood cell wall, typically 40—50% by weight of the dry wood. Pure cellulose is a polymer of glucose residues joined by 1,4-P-glucosidic bonds. The degree of polymerization (DP) is variable and may range from 700 to 10,000 DP or more. Wood cellulose is more resistant to dilute acid hydrolysis than hemiceUulose. X-ray diffraction indicates a partial crystalline stmcture for wood cellulose. The crystalline regions are more difficult to hydrolyze than the amorphous regions because removal of the easily hydrolyzed material has Htde effect on the diffraction pattern. 

Hemicellulose (or polyose) is primarily composed of xylan, a branched polymer composed of five-carbon sugar, xylose. Typical polymerization degree of hemicellulose is 50 - 200, which is shorter than the cellulose molecules. The acid hydrolysis of hemicellulose, (C6H10O5)n, produces mainly xylose (C6H10O5), which can be converted to furfural, a chemical feedstock, or can be fermented to ethanol. 

Figure 5 Changes in degrees of polymerization and yields of higher plant cellulose during acid hydrolysis at 80-95°C.

The recent interest in using stiff nanometric particles as reinforcement materials in polymeric matrixes, composites, or nanocomposites has been increasing. Two good examples of these types of particles are carbon nanotubes and cellulose nanofibers. Cellulose nanofibers, also reported in the literature as whiskers, nanocrystals, cellulose crystallites, or crystals, are the crystalline domains of ceUulosic fibers, isolated by means of acid hydrolysis, and are called in this way due to theb physical characteristics of stiffness, thickness, and length (Souza and Borsali 2004). 

Habibi Y, Goffin AL, Schiltz N et al (2008) Bionanocomposites based (m poly(epsilgrafted cellulose nanocrystals by ring-opening polymerization. J Mater Chan 18 5002-5010 Hakansson H, Ahlgren P (2005) Acid hydrolysis of some industrials pulps of hydrolysis conditions... 

A kinetic study of the hydrolysis of woods and purified plant polysaccharides in 75 % sulphuric acid was monitored by electrical conductivity measurements. The coefficients of resistance of all polysaccharide substrates increased with increasing hydrolysis time. From these measurements, the degrees of polymerization of cellulose, xylan, and inulin were estimated to be 1900, 66, and 16 respectively. The polysaccharide components of various woods and pulps can be hydrolysed with trifluoracetic acid which, in contrast to sulphuric acid hydrolysis, does not require a neutralization step since trifluoracetic acid is volatile. The presence of lignin in the wood samples impeded the hydrolysis of the polysaccharides, requiring longer reaction times and correction factors to compensate for loss of monosaccharide by degradation reactions. 

Regarding biodegradable polyurethanes, Luo et al. [35] synthesized polymeric nanocomposites by incorporating cellulose nanowhiskers (obtained by the classic sulfuric acid hydrolysis) into shape memory polyurethane based on poly (e-caprolactone) diol (PCL, Mw = 65000), 4,4 -diphenylmethane diisocyanate (MDI)... 

Wik et al. [26] reported the reaction of a natural polyol derived from castor oil and polymeric 4,4 -diphenylmethane diisocyanate (pMDl) used to prepare solid materials reinforced with nanocellulose. The polyol was obtained by the reaction of castor oil with triethanolamine. The reaction between the castor oil-based polyol and the isocyanate started almost immediately after mixing these main components, even without the addition of a catalyst. This fact forced the use of an organic solvent to reduce the initial reaction rate, which was evaporated during the curing process. Nanocellulose obtained by acid hydrolysis of commercial microcrystalline cellulose was added to the polyol (0.5, 1, and 3 wt%) and dispersed by sonication. It was found that the addition... 

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