The Preparation And Mechanism Of Swelling Chitosan Gel By Freeze-blasting

(1) The effect of deacetylation process conditions on the properties of the finalproduct-chitosan was investigated by orthogonal experiment,and then four chitosanwith different DD (CS65,CS70,CS75,and CS80) were prepared by the optimizedpreparing conditions according to the obtained results.The prepared chitosans canswell highly in 4.8wt% LiOH aqueous solution and form transparent gels afterfreezing-blasting treatment,with the swelling degree CS65＞CS70≈CS75＞CS80.(2) The structures of prepared chitosan were studied by FT-IR,XRD and TEM.The two-step deacetylation not only changed the DD of chitin,but also destroyed thehydrogen bonds in chitin and the crystal structure.The rigid network of chitin wasdestroyed,and numerous micro voids formed during the N-deacetylation treatment.The crystalinity of chitin first decreased and then increased with the increase of DD,and the structure of chitin grew looser at first and then became denser with theincrease of DD.(3) The swelling behaviors of four chitosans (CS65,CS70,CS75,and CS80) inalkali solvent system were systematically studied.The effects of several factors suchas the concentration of alkali solution,the kind of alkali solution,added smallmolecule reagents and freezing-blasting treatment on the swelling behavior ofchitosan as well as the mechanism of swelling were investigated.The results indicatedthat the absorption behavior of chitosan to LiOH and water was dependent on itsdeacetylation degree (DD).The swelling degree of chitosan grew at first with theincrease of concentration of alkali and then decreased.When the concentration of ureaincreased over 4 wt%,urea started to act as a solubilizer and urea had a strongersolubilization effect towards weak swelling agent.Chitosan was stable in LiOH/ureaaqueous solution,and 4.8 wt% LiOH/ 8.0 wt% urea solution may be potentialfavorable solvent for chitosan.(4)Two kinds of chitosan (CSd and CSa) were prepared from chitin or chitosan byN-deacetylation or N-acetylation,respectively,which can dissolve in both alkali andacid.Then their structures and dissolution behaviors in alkali solution were studied,as well as the mechanism of the dissolution.The mechanism of swelling and dissolution was concluded as following:The swelling of chitosan in alkali solution took place from the amorphous regionto the crystal region.Their diminished crystallinity,increased amorphous region,expanded crystal structure,destroyed inter- and intra-molecular hydrogen bonds aswell as the more disordered and much looser structures made it much easier for water,LiOH hydrate and urea hydrate to disperse and penetrate into the internal of theprepared chitosans,and therefore,the prepared chitosan can adsorb large amount offree water and small molecules.Under the freezing treatment,the free water absorbedby chitosan can freeze,with the volume of it increasing,which effectively expandedthe structure of chitosan and weakened the interaction between chitosan molecules,making it much easier for small molecules to disperse into the internal of chitosan.Moreover,[Li(H₂O)_m]⁺ and [OH(H₂O)_n]^-were more stable at low temperature,they can diffuse and penetrate into the internal of chitosan more effectively.With thecomplex reaction between Li⁺ and the acetyl group on the chitosan,and the additionreaction between Li⁺ and OH^-on the chitosan,the hydrogen the hydrogen bonds ofthe chitosan can be destroyed,making chitosan swell or dissolute eventually.The main motivation of the swelling or dissolution of chitosan is the destructionof hydrogen bonds in chitosan caused by LiOH.The freezing-blasting treatment has agreat impact on the shift of hydrogen bonds between chitosan and LiOH hydrate,contributes to the swelling of chitosan.Urea improved the stability of chitosan/LiOHsystem by forming an inclusion compound with chitosan and LiOH.