Thermal Stability Of Complexes Of Chitosan Quaternary Ammonium Salt And Metal Ion

Chitosan(CS), the second most abundant natural polysaccharide after cellulose, is produced by the deacetylation of chitin. Chitosan-metal complexes have the characteristics both of chitosan and metal ion, and can be used as enzyme immobilization agent, separation membrane, artificial urea absorbent, chemical catalyst and plant growth regulator. However, researches about complexes of quaternized chitosan-metal are mostly applied, while there are few papers about mechanism and structure of the compounds. Recently, it is reported that the thermal stability of chitosan are declined after grafting, and there are few reports about the thermal stability of complexes of quaternary ammonium salt chitosan and metal ion. In this paper, hydroxypropyl trimethyl ammonium chloride chitosan (HTCC) was synthesized through using CS as raw material and epoxypropyl trimethyl ammonium chloride (ETA) as grafting agent. Then HTCC-Cu, HTCC-Zn, HTCC-Cd and HTCC-Mn complexes were obtained, and the effect of metal ion on the thermal stability of HTCC was discussed. These studies are to offer theoretical basis to the development and comprehensive utilization of prawns.Main contents and conclusions were shown as below:(1)Synthesis of quaternized chitosan(HTCC). CS was prepared from shell of shrimp, its average molecular weight was 4.0×10~5, and the degree of deacetylation was over 90%. HTCC was obtained after chemical modification of CS.(2) Effect of cupric ion on thermal degradation of quaternized chitosan. The thermal degradation of quaternized chitosan-cupric ion mixture was studied by thermogravimetric analysis(TG) and differential thermal analysis (DTA) in the temperature range 30-600℃. The effect of cupric ion on the thermal degradation behaviors of quaternized chitosan was discussed. The results of Fourier transform-infrared spectroscopy (FTIR) and X-ray diffractogram analysis(XRD) show that there are coordinating bonds between quaternized chitosan and cupric ion. Thermal analysis indicates that the thermal degradation of quaternary chitosan–cupric ion mixture is a two-stage reaction, in which there are the decomposition of N-C2 bond on the side-chain of quaternized chitosan and the cleavage of glycosidic linkages of chitosan. And the two reactions are exothermic. The impact of cupric ion on the thermal degradation of quaternized chitosan is significant, and the temperature and activation energy of the degradation are in certain relation with weight fraction of cupric ion.(3) Effect of cadmium ion on thermal degradation of quaternized chitosan. The kinetics of the thermal degradation of hydroxypropyl trimethyl ammonium chloride chitosan-Cd complexes(HTCC-Cd) were investigated by TG. The thermal degradation of HTCC-Cd is systematically studied by using TG, and the thermal degradation parameters are determined by using different kinetic methods(Friedman method, Flynn-Wall-Ozawa method, Coats-Redfern method and Phadnis-Deshpande method). The results showed that HTCC-Cd complexes are degraded in two stages, and the first stage is corresponding to decomposition of N-C2 bond on the side-chain of quaternized chitosan and the cleavage of glycosidic linkages of chitosan, while the second stage relates to decomposition of the residual carbon. Using different methods, the kinetic parameters of the two steps were investigated . The results show that the activation energies of the first step of degradation with Friedman and Flynn-Wall-Ozawa methods are 168.4 kJ/mol and 164.6 kJ/mol, respectively, for the second step, the corresponding activation energies are 116.5 kJ/mol and 137.3 kJ/mol, respectively. The results obtained from Coats-Redfern and Phadnis-Deshpande methods indicate that the two degradation processes are both nucleation and growth process, and obey A4 mechanism with intergral form g ( X)= [-ln(1-X)]4.(4) Effect of cupric, zinc, cadmium and manganese ion on thermal degradation of quaternized chitosan. Using HTCC and copper chloride, zinc nitrate, cadmium chloride and manganese chloride, HTCC-Cu, HTCC-Zn, HTCC-Cd and HTCC-Mn are obtained, respectively. The coordinating ability of metal ion and HTCC is determined by atomic absorption spectrum(AAS), and the results are as follows: Cu>Cd>Mn>Zn. The structure of quarternized chitosan-metal ion compounds are observed through FTIR. The results of TG showed that in thermal degradation, the characteristic temperatures of HTCC-Cu are lower compared to HTCC, while the characteristic temperatures of HTCC-Zn, HTCC-Cd and HTCC-Mn are higher. On the basis of thermogravimetric analysis and Flynn-Wall-Ozawa method, activation energy is determined and the thermal stabilities of these compounds are discussed. The results indicate that the thermal stability of HTCC-Cu are declined comparing to HTCC, while HTCC-Zn, HTCC-Cd and HTCC-Mn are enhanced. These conclusion are in agreement of TG. Through calculating other thermal degradation parameters, the degradation of these five compound are non-spontaneous.