| Pulsed discharge plasma is a new advanced oxidation technology,which has been demonstrated to have significant degradation effects on chitosan in our group’s previous studies.At the same time,the stability of chitosan molecular chain is directly related to its molecular weight,and relevant studies have shown that different molecular weights of chitosan raw materials lead to different trends in the degradation process.Therefore,it is of practical significance to study the influence of molecular weight of chitosan raw materials on the degradation process of chitosan by pulsed discharge plasma.Based on the above reasons,this thesis investigates the relationship between the molecular weight of chitasan raw materials and the degradation effect of pulsed discharge plasma by using pulsed discharge plasma as the treatment technology and three types of chitosan with viscosity-averaged molecular weight of 727.4(LMWC),2038.6(MMWC)and 4201.5(HMWC)kDa as the degradation objects.The relationship between the molecular weight of chitasan raw material and the efect of pulsed discharge plasma degradation was investigated,and the influence of molecular weight on the properties of chitosan degradation products by pulsed discharge plasma was analyzed through the physicochemical properties and structural characterization of the degradation products of chitosan with different molecular weights.The main studies are as follows.(1)The degradation process study and its kinetic analysis were carried out with the characteristic viscosity and degradation rate of chitosan as indicators,and the optimum degradation conditions were determined.The results of the degradation process showed that there was a negative correlation between the pole plate spacing and the degradation rate of chitosan raw materials.The decrease in LMWC viscosity and the increase in degradation rate were the weakest and the degradation effect was the lowest as the plate spacing decreased.In the range of 2-8 mm pole plate spacing,the degradation effect is better with the increase of molecular weight of chitasan raw material,and the increase of degradation effect is more significant and the variation of degradation effect is less.The degradation effect was found to increase significantly with increasing molecular weight of chitosan raw material,and the variation of degradation effect was less.Further kinetic analysis of the degradation effect showed that the degradation rate of different molecular weight chitosan and the pole plate spacing were negatively correlated.At the same time,the degradation rate of LMWC was higher than that of HMWC,which further confirmed that HMWC was easier to degrade.The degradation rate of LMWC was significantly higher than that of MMWC and HMWC,while the degradation rates of MMWC and HMWC were more similar.(2)The degradation products of different molecular weights of chitosan raw materials were prepared under the optimum degradation process conditions,and the effects of degradation treatment on the physicochemical properties of the degradation products were investigated by using rheological properties,antioxidant activity,thermal properties and solubility as indicators,and the effects of molecular weight on the physicochemical properties of chitosan degradation products by pulsed discharge plasma.The experimental results showed that:the degradation treatment reduced the apparent viscosity,loss modulus and thermal stability of chitosan;improved the antioxidant activity and solubility of chitosan;had no significant effect on the energy storage modulus of LMWC and MMWC,and reduced the energy storage modulus of HMWC.In addition,the effects of molecular weight of chitosan raw materials on different physicochemical properties showed variability,but generally followed the pattern of degradation effect effects.The results of the rheological properties study showed that for LMWC,the apparent viscosity at a pole plate spacing of 2 mm was lower than that at a pole plate spacing of 6 mm;MMWC and HMWC had lower apparent viscosity at a pole plate spacing of 6 mm.The molecular weight of chitosan feedstock was positively correlated WIth the apparent viscosity at 2 mm plate spacing;at 6 mm plate spacing,the trend was the same for HMWC and MMWC,but the apparent viscosity of LMWC was higher than that of MMWC,and the molecular weight of chitosan feedstock had no significant effect on the energy storage modulus and loss modulus of degradation products at the same plate spacing.For LMWC and MMWC,the loss modulus decreased more at 2 mm than at 6 mm;for HMWC,the effects were almost the same at 2 mm and 6 mm plate spacing.The results of the antioxidant activity study showed that the promotion of antioxidant activity was higher at 2 mm than at 6 mm for LMWC and MMWC,and stronger at 6 mm for HMWC;the antioxidant activity of the degradation products negatively correlated with the molecular weight of the raw material at 2 mm plate spacing,and positively correlated with the molecular weight of the raw material at 6 mm plate spacing.At a plate spacing of 2 mm,the antioxidant activity of the degradation products negatively correlated with the molecular weight of the raw material.The results of the thermal properties study showed that for LMWC and HMWC,the influence of plate spacing on the thermal stability of the degradation products was not significant;for MMWC,the thermal stability decreased more significantly at a plate spacing of 6 mm.At 2 mm plate spacing,the thermal stability of the degradation products is negatively correlated with the molecular weight of the raw material;at 6 mm plate spacing,the thermal stability of the degradation products of LMWC and MMWC is also negatively correlated,while the thermal stability of the degradation products of HMWC is higher than that of MMWC.The solubility studies showed that LMWC and MMWC were more soluble at a plate spacing of 2 mm,while HMWC was more soluble at a plate spacing of 6 mm.The solubility of the degradation products was positively correlated with the molecular weight of the raw material under the same plate spacing.(3)The degradation products with different molecular weights of chitosan raw materials were prepared under the optimum degradation process conditions,and the degradation products were subjected to FTIR,NMR,XRD,MALDI-TOF-MS and laser particle size analysis to investigate the effect of molecular weight on the structural characterization of chitosan degradation products by pulsed discharge plasma.The experimental results showed that the degradation treatment had little effect on the main molecular structure of chitosan.Meanwhile,FTIR results showed that for the characteristic absorption peaks of-NH2,-NH and amide I in the range of 1598-1640 cm-1,the influence of LMWC and MMWC degradation products became more significant wth decreasing the pole plate spacing;the influence of HMWC degradation products more significant at the pole plate spacing of 6 mm.For the-NH and amide I characteristic absorption peaks in the range of 1615-1639 cm-1 at a plate spacing of 2 mm,the effect becomes more signficant as the molecular weight of the chitosan raw material decreases.The NMR results showed that the H2 proton peak of chitosan was weakened and the H7 proton peak was enhanced by the degradation treatment.The effect of degradation treatment on the H2 proton peak and H7 proton peak of chitosan was higher at 6 mm pole plate spacing than at 2 mm pole plate spacing.XRD results showed that the degradation treatment destroyed the crystal structure of chitosan and the crystallinity of chitosan decreased.For LMWC and HMWC,the effect is more significant as the pole plate spacing decreases.Laser particle size analysis showed that the average particle size of degradation products decreased after degradation treatment.MALDI-TOF-MS results showed that the molecular weight of chitosan raw material had no significant effect on the molecular composition and overall trend of degradation products after 180 min of degradation treatment at 2 mm and 6 mm plate spacing.From the above conclusions,it can be concluded that the molecular weight of chitosan feedstock has a significant effect on the degradation by pulsed discharge plasma with certain characteristic patterns. |
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