| Chitosan (CS) is one of polysaccharide, is prepared by N-deacetylation of chitin, which is the main structural component of cr.ab and shrimp shells. As it has many favorable properties such as biodegradability, biocompatibility, bioactivity and non-toxicity, CS has been extensively studied as a promising biomaterial. CS can only dissolve in some specific organic acids and a few inorganic solvents, but it is unstable in aqueous acid. Its hydrolysis is accompanied with the cleavage of glycosidic-bonds, which brings about the poor properties of CS products. The study includes that the mechanism of CS solution dissolved in alkali aqueous solution by freezing-blasting and preparation of CS hydrogel and fibers by this alkali solution system. Therefore, the studies are as followed:(1) Exploration on the state of CS macromolecules in alkaline solvent and acid solvent was carried out by Dynamic Light Scattering particle size analysis, proving the existence of nanoparticles. The nanoparticle size was linear with CS concentration.(2) A systematic study of effects on rheological behavior of chitosan aqueous solution by CS concentration, LiOH, urea, degree of deacetylation of Chitosan. In the case of chitosan solution prepared by acid and alkali solvent system, the flow indexes (n) were less than1, which were pseudoplastic fluid; In CS alkali solution, LiOH played a vital role in forming inclusion complexes structure with CS macromolecules and destroying hydrogen bonds between CS macromolecules. With the increase of LiOH concentration, the solution formed more inclusion complexes, and hydrogen bonds between CS macromolecules were destroyed more strongly, therefore CS were dissolved sufficiently. So the viscosity of entire solution system were reduced. At the same time, along with the decrease of the free hydrophilic groups (-NH2), hydrogen bonds between chitosan macromolecules were reduced, and the temperature of gel point increased. In short, with the increase of LiOH and urea, the temperature of gel point of CS solution increased. With the increase of the degree of deacetylation. the temperature of gel point of CS solution decreased.(3) The high strength pure chitosan (CS) hydrogel was prepared. CS hydrogels were prepared by two kinds of solvent systems (alkaline solvent and acid solvent). The results from universal testing machine indicated that the high strength CS hydrogel prepared by alkaline solvent sustains a compressive stress of1.77MPa. which was47times more than that sustained by the low strength CS hydrogel by acid solvent. The fracture compressive strain (ε) of the high strength CS hydrogel was74.1%, which is much higher than that of the low strength CS hydrogel (ε=32.3%). In addition, the tensile stress and strain of CS hydrogel prepared by alkaline solvent were2.30MPa and226%.(4) SEM of two hydrogels showed the network structure, which appeared to be highly porous with the interconnected macrodomains. The morphology comprised the CS and pores as features not only of these samples but also of all of the samples that were produced. It is obvious that the structures of two CS hydrogels are different. For the High strength CS (H-CS) hydrogels, these pores possessed honeycomb-like shapes. The pore size and morphology of the various hydrogels were quite similar. The inner surface of pore was smooth, and the pores had a mean diameter of<10μm. The sizes of pore walls were1μm at most. For the low strength CS (L-CS) hydrogels. F it was a ribbon structure. The pores between ribbons had a mean diameter in a range of200to600μm, which were more than30times that H-CS hydrogels. The sizes of pore walls were almost10μm, which were more10times than the size of pore walls of the H-CS hydrogels. The H-CS hydrogels had well-distributed pores. And above all, the results showed that the physical cross-linking points of H-CS hydrogels were more regularly distributed and20times than L-CS hydrogels. The increase of cross-linking points enhanced the cross-linked network of hydrogel, and therefore, the mechanical properties of H-CS hydrogels were improved greatly.(5) Improving wet mechanical properties of chitosan fibers by a new approach was correct. The results from Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis and universal testing machine indicated that the new approach did not change the chemical structure of chitosan. The approach performed better in forming dense structure in the cross section of chitosan fiber and improved thermal stability and mechanical properties of CS65fibers. The wet and dry tensile strength of the novel CS65fibers reached0.8cN/dtex, which were increased by125%, compared with that of CS65fibers prepared by conventional approach. |
