Chitosan crystallinity

Owing to the hydrophilic nature of chitosan, this compound has been used directly as compressible diluent in tablets. Chitosan has excellent properties as excipient for direct compression of tablets, where the addition of 50% chitosan results in rapid disintegration. The DD determines the extent of moisture absorption. Also, in immediate-release formulations, e.g., as a disintegrant in small amounts, where it has been found to have effects similar to or better than those of microcrystalline cellulose. Chitosan at concentrations higher than 5% is a better disintegrate than corn starch and microcrystalline cellulose, depending on chitosan crystallinity, DD, MW and particle size [61]. In addition, it has excellent tablet binder properties compared to other excipients [62]. 

Similarly, a composite of hydroxyapatite and a network formed via cross-linking of chitosan and gelatin with glutaraldehyde was developed by Yin et al. [ 169]. A porous material, with similar organic-inorganic constituents to that of natural bone, was made by the sol-gel method. The presence of hydroxyapatite did not retard the formation of the chitosan-gelatin network. On the other hand, the polymer matrix had hardly any influence on the high crystallinity of hydroxyapatite. 

Superficially phosphorylated chitosan membranes prepared from the reaction of orthophosphoric acid and urea in DMF, showed ionic conductivity about one order of magnitude larger compared to the unmodified chitosan membranes. The crystallinity of the phosphorylated chitosan membranes and the corresponding swelling indices changed pronouncedly, but these membranes did not lose either tensile strength or thermal stability [234]. 

Chitosan has found many biomedical applications, including tissue engineering approaches. Enzymes such as chitosanase and lysozyme can degrade chitosan. However, chitosan is easily soluble in the presence of acid, and generally insoluble in neutral conditions as well as in most organic solvents due to the existence of amino groups and the high crystallinity. Therefore, many derivatives have been reported to enhance the solubility and processability of this polymer. 

Important characteristics of chitosan are its MW, viscosity, DD (Bodek, 1994 Ferreira et al., 1994a,b), crystallinity index, number of monomeric units, water retention value, pKa, and energy of hydration (Kas, 1997). Chitosan has a high charge density, adheres to negatively charged surfaces, and chelates metal ions. 

Figure 4.6 shows the XRD patterns of pure clay (sodium montmoril-lonite), neat chitosan, chitosan/clay, chitosan /CNTs and chitosan/ CNT-clay composite. Neat chitosan film shows weak crystalline... 

In their further studies on chitosan for biomedical applications, Lee et al. [133] reported a procedure for preparing semi-IPN polymer network hydrogels composed of (3-chitosan and PEG diacrylate macromer, by following a similar procedure to that discussed above. The crystallinity as well as thermal and mechanical properties of gels were reported [133]. Reports on the drug release behavior of the gels are not available. 

Tablets are still considered as the dosage forms of choice for reasons such as low manufacturing cost and good stability. Many direct compression dilutents have been reported,but every dilutent has some disadvantages [295]. Crystalline cellulose (MCC) has been widely used as a tablet dilutent in Japan. Chitin and chitosan, on account of their versatility, were reported to be useful dilutents for pharmaceutical preparations [296-298].

From their investigations, it appears that the fluidity of combined powders with chitin and chitosan was greater that that of the powder with crystalline cellulose. The reported hardness of the tablets follows the order chitosan tablets >MCC> chitin. In the disintegration studies, tablets containing less than 70 % chitin or chitosan have passed the test. Moreover, the ejection force of the tablets of lactose/chitin and lactose/chitosan was significantly smaller than that of lactose/MCC tablets [301]. However, no reports are available on CDR formulations using these studies. 

Crystalline, flake chitosan from King crab exhibited patterns similar to those of chitin, but the 002 peak (b is the fiber axis) shifted from 0.962 nm for chitin to 0.857 nm for flake chitosan. The position of this peak depends on the content of water and is lowered to 0.745 nm on drying at 134°. It was postulated that water molecules that enter the lattice are loosely bound between the chains along the 001 direction. 

Generally, the sharpness of the bands is higher in the chitin samples than in their chitosan analogs due to the decrease in crystallinity. Figure 2.11 shows the XRPD patterns of (3-chitin and the corresponding hydrolysis product chitosan. The band at around 26 of 10° decreases after deacetylation and is accompanied by a significant decrease in crystallinity [22],... 

Figure 2.30A shows the X-ray powder diffraction (XRPD) patterns of chifin and chitosan with different degree of deacetylation (DD). Five crystalline index (CrI) reflections 020, 110, 120, 101, and 130 from the lower... 

Waste water treatment - Depends on pollutant and water conditiorts (pH, ionic strength) - In general, chitosan preferred over chitin - High DD, low crystallinity... 

Like cellulose, chitin occurs in more than one crystal form. The j3-chitin modification, which contains one firmly bound molecule of water of hydration per 2-acetamido-2-deoxy-D-glucose residue, is usually found in association with animal tissue of the collagen type. a-Chitin, which is more common, usually replaces tissue of the collagen type this form has been examined more thoroughly " than the p, and will be discussed in detail. A little studied derivative of chitin, called chitosan, can be obtained in crystalline, oriented form by deacetylating chitin membranes with concentrated sodium hydroxide. Naturally occurring, chitinous membranes, such as insect cuticle, show various degrees of uniplanar orientation. 

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