| With the increasingly serious problem of environment pollution,developing a sustainable resource is an important subject of scientific research at present.Natural polymeric materials,especially chitin and cellulose,have attracted tremendous attention for decades due to their special excellent performance and superiority,such as bio-renwable,biodegradable,bio-functional and non-toxicity.As the second most abundant polysaccharide in nature,chitin is widely distributed in nature,but perhaps the least exploited,due to its poor solubility characteristics.The purpose of this work is to rearch detailly of the performance of chitin and chitosan dissolve in alkaline system,and to make chitin material wider exploit and more suitable for industrial production.We study the dissolution behavior of chitin in the alkaline aqueous solutions by using NMR,solubility determination,and viscosity test.The NMR analyses indicate that the dissolution power of chitin in alkali aqueous is KOH>NaOH>LiOH,contrary to the dissolution power of cellulose in alkali aqueous as LiOH>NaOH>>KOH.As chitin and cellulose have similar structure,the contrary dissolution ability of KOH aqueous solvent for chitin and cellulose should be ascribed to the different C2 functional group on these two natural polysaccharides(i.e.,N-acetyl group and hydroxyl group,respectively).We systematically study the dissolution behavior and soluility of chitin dissolve in the KOH and KOH/urea aqueous solutions through a freeze-thawing process.The results show that chitin has good solubility(-80%)in 8.4-25 wt%KOH aqueous solution at-30°C.The degree of acetylation falls only 12.5%,and the molecular weight decreases only 25%during the dissolution and storage process at-30℃.that is to say chitin has a relatively low degradability in the KOH aqueous solution.The role of urea also has been investigated.Urea indeed enhances the solubility of chitin in KOH aqueous solutions,but the increased degree becomes unobtrusive with further decreasing temperature and increasing dissolution time.Contrasting the phase transformation of chitin in KOH and KOH/urea aqueous solution,the LCST curves of chitin-KOH and chitin-KOH/urea are nearly overlap,that is to say urea has no obvious effect on the temperature sensitivity of chitin-KOH aqueous solution.Chitosan is the most important derivative of natural chitin by deacetylation of the N-acetyl.Some exploratory research about the dissolving properties of chitoan has been done in this paper.The available experimental results show that chitosan can dissolve well in a certain concentration of several sovlents through a freeze-thawing process,such as LiOH/urea,LiOH/KX/urea(X can be OH",Cl-,Br-,I-)LiCl/KOH/urea,LiOH/KOH,etc.The dissolution power of chitosan in alkali aqueous is LiOH/urea>KOH/urea>NaOH/urea,different to the dissolution power of chitin in alkali aqueous as KOH>NaOH>LiOH.Chitosan has the similar dissolution characteristics with chitin in alkaline system.The solubility of chitosan increases as the increasing LiOH concentration,and obtain a maximum at 4.8 wt%LiOH.When the temperature below the freezing point of the solvent,temperature has no significant effect on the solubility of chitosan,but urea still plays an important role in the dissolution process.It is noteworthy that KCl,KBr,KI,NaCl in the mixed solvent had no obvious effect on the solubility of chitosan,but solubility of chitosan in LiOH NaX/urea(X for OH-,Cl-)significantly weaker than LiOH KX/urea(X for OH-,Cl-,Br-and I-).To fugure out this problem,we add the saturation NaOH solution in to chitosan-LiOH-urea aqueous solution.The results show that the motility of chitosan has confined as the saturation NaOH solution were added.We deduce that different cation may change the phase composition,or the microscopic effect between the polymer chains.The systemic study of the dissolving properties of chitosan remains to be further expanded. |
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