Several researchers have also attempted to improve the properties of chitosan fibers by blending or modifying chitosan. N-acyl chitosan fibers were prepared by posttreating chitosan fibers with a series of carboxylic anhydrides (N-acetyl, N-propionyl, N-butyryl, N-hexanoyl). Increasing the length of the acyl chain increased the elongation but decreased the strength of the fibers due to the destruc­tion of the hydrogen bonding. However, N-hexanoyl chitosan fibers had higher strength than the N-acyl chitosan fibers due to the higher hydrophobicity of the fibers [07Cho]. Before fiber production, chitosan was mixed with vanillin, and the N-(4′-hydroxy-3′-methoxybenzylidene) product was collected. Later, the powder was extruded into a coagulation bath containing various chemicals. Fibers obtained were drawn 1.2-1.4 times in 2 % aqueous NaOH-ethylene glycol solution, and the filaments obtained were later cut into staple fibers. Further treatment of the fibers was done using NaOH and methanol to obtain cotton-like chitosan fibers [99Hir1]. Properties of the fibers produced when different coagulation baths were used are given in Table 25.3. As a general trend, it was found that increasing the degree of substitution or treating chitosan with vanillin decreased the strength and elongation of the fibers. N-acyl chitosan fibers were produced by wet spinning using aqueous solution of sodium N-acyl and N-propionylchitosan salts in aqueous 14 % NaOH. Fibers with N-acyl chitosan % ranging from 28 to 95 % were prepared with tenacity from 0.5 to 0.9 g/den and elongation from 19 to 30 %. This method of preparing chitosan fibers would enable mixing the chitosan solution with cellulose xanthate solutions to produce N-acyl chitosan-cellulose fibers [98Hir]. Similarly, chitosan butyrate was blended with cellulose acetate and made into fibers with tensile strength between 0.7 and 0.9 g/den and elongation between 4 and 11 % [07El].

O-Hydroxyethyl chitosan xanthate prepared by esterification was added into cellulose xanthate to produce blend fibers with 3.1, 4.5, and 6.2 wt% of chitosan [10Xu]. Properties of the blend fibers in comparison to pure viscose rayon are given in Table 25.4. As seen from the table, blending did not significantly modify the dry or wet strength, but the elongation of the fibers increased. Thermal decomposition temperature increased, and the rate of decomposition decreased by adding chitosan. In a similar approach, N, O-carboxymethylated chitosan and chitosan emulsion were blended with viscose rayon and wet spun into fibers [02Li]. Addition of chitosan was found to decrease the tensile properties but improved the antibacterial properties.

Chitosan fibers were wet spun using acetic acid as the solvent, and the fibers were later acetylated using acetic anhydride [93Eas]. It was reported that acetylation improved the thermal stability and tensile properties. Fibers with tenacities ranging from 1.8 to 2.0 g/den and elongation varying from 4.9 to 10 % were obtained. Similarly, carboxymethylation of chitosan fibers was done using chloroacetic acid to improve chelating properties and the absorption of Cu(II) ions [06Qin]. Fibers were carboxymethylated up to 41 %. Cu(II) removal ranged from 51.7 to 99.3 % with absorption capacity from 16 to 148 mg Cu(II)/g fiber depending on the degree of carboxymethylation. Absorption was considered to be rapid, and the process could occur at room temperature over a wide range of acid and alkali conditions.

Microcrystalline chitosan was blended with cellulose xanthate alkaline solution, and the effect of aqueous microcrystalline chitosan cellulose gel concentration and additives such as sodium alginate on the spinnability and properties of the fibers was studied by Nousiainen et al. [00Nou]. Fibers obtained appeared normal but had slightly lower tenacity and increased water retention, fineness, and elongation compared to standard viscose fibers. Fineness of the fibers produced was between 3.0 and 5.2 dtex, tenacity was between 1.4 and 1.5 g/den, and elongation was between 15 and 19 %. Silk fibroin and cellulose xanthate were combined with chitosan and extruded into fibers using acyl chitosan in aqueous NaOH [02Hir]. Fibers (4.9-9.9 den) containing less than 10 % fibroin had tensile strength between 1.08 and 1.2 g/den and elongation between 30 and 35 %. Combination of chitosan — fibroin and cellulose acetate with 43 % fibroin produced 3.9-5.0 den fibers with

tenacity between 0.7 and 0.9 g/den and elongation between 21 and 29 %. Morpho­logically, the surface of the fibers was extensively striated due to the coagulation process.

A wet spinning approach was adopted to develop poly(e-caprolacton) (PCL)/ chitosan blend fibers with various diameters. The blend polymers were dissolved using 70/30 formic acid/acetone mixture and extruded into a methanol coagulation bath [10Mal]. Fiber diameters in the dry state varied between 112 and 139 qm and between 135 and 372 qm in the wet state depending on the ratio of chitosan and PCL in the blend. It was suggested that a phase separation between the polymers occurred only at the microlevel (<10 qm), and the fibers had relatively low modulus between 7.7 and 23 g/den. Surface roughness of the blend fibers was considered to be suitable for tissue engineering.

Poly(vinyl alcohol) was blended with chitosan with an aim to improve the wet stability of the fibers, and the fibers were produced by extruding into a NaOH and ethanol bath [01Zhe]. PVA contents in the fibers varied from 10 to 50 %, and fibers were stretched to 29 % at 35 °C and air dried to obtain white fibers which were later cross-linked with glutaraldehyde. Some of the properties of the pure chitosan and chitosan-PVA blend fibers are listed in Table 25.5. As seen from the table, the addition of PVA into chitosan marginally increased strength and elongation but water retention more than doubled due to the hydrophilicity of the PVA. It should be noted that the wet strength of the fibers was about 50 % of their dry strength, whereas wet elongation was approximately twice that of the dry elongation. Cross­linking with glutaraldehyde increased the dry strength of the fibers to 2.6 g/den, elongation decreased to about 15 %, and wet strength nearly doubled to about

1.6 g/den without major change in the elongation.

A blend of chitosan and collagen fibers were developed by Hirano et al., and the fibers were N-modified using carboxylic anhydrides and aldehydes [99Hir2]. To prepare the fibers, tropocollagen or lyophilized collagen was dissolved using 2 % acetic acid solution, and powdered chitosan was added into the solution and allowed to age overnight at room temperature. Solution was extruded into a bath containing 5 % aqueous ammonia solution and 40-43 % ammonium sulfate. Later, the fibers were stretched 1.2-1.3 times in an ethylene glycol solution containing 2 % NaOH. To N-modify the fibers, the fibers were suspended in methanol to which acetic anhydride, n-propionic anhydride, n-butyric anhydride, n-hexanoic anhy­dride, succinic anhydride, benzaldehyde, and vanillin were added. Compatibility of the fibers with blood was also evaluated. Some of the properties of the fibers

obtained are listed in Tables 25.6 and 25.7. It was found that tropocollagen-coated chitosan fibers had no blood coagulation on the surface, whereas some coagulation was seen on the N-acyl chitosan and chitosan-tropocollagen. As can be seen from Table 25.6, fiber fineness and tensile properties significantly varied with change in fiber preparation conditions. No major differences were observed in the strength and elongation of the chitosan fibers coated with collagen or tropocollagen. How­ever, N-substituted fibers show considerable variations in strength and elongation depending on the type of anhydride or aldehyde used as seen in Table 25.7.