Preparation And Structural Analysis Of Surface Modification Regulated Chitin Nanofibers

Chitin is the most abundant nitrogen-containing polysaccharide-based biopolymer in nature.Nano-crystallization of polysaccharide-based biopolymers are especially useful methods combining the advantages of both nanomaterials and natural polymers.In this thesis,based on the previous research,the purified chitin materials were successfully extracted from acidic and alkaline deproteinization and decolorization and bleaching treatments using waste crab shells as raw materials.The preparation of chitin nanofibers focused on the surface carboxylation modification and the surface amination modification as to regulate the characteristic of chitin nanofibers and their nanostructure analysis,were mainly studied.In previous research,the2,2,6,6-tetramethylpiperidine-1-oxy(TEMPO)mediatedoxidationsystem(TEMPO/Na Br/Na C l O oxidation system),one of the traditional preparation methods of biopolymer nanofibers,selectively oxidizes C6 primary hydroxyl groups of chitin to carboxyl groups in aqueous condition,meanwhile,high concentration alkaline solution was applied to partial deacetylation of chitin by which C2 acetamido was converted into amino groups under high temperature condition(30-35%Na OH at 90°C for 4 h),and then modified chitins were converted into nanofibers with uniform size,formed uniform dispersions with excellent stability under neutral/weakly alkali condition or weakly acidic(p H3-4)condition by mechanical disintegration treatment.However,the traditional TEMPO/Na Br/Na Cl O oxidation system inevitably contained halogens that are harmful to the environment,such as sodium bromide(Na Br)or sodium hypochlorite(Na Cl O).The traditional deacetylation method inevitably uses alkali that is highly polluting to the environment,as well as the process involves high energy consumption,which is not conducive to the requirements of green,environmentally friendly,low-carbon and efficient development policy.Because of above shortcomings,developing and optimizing the process or finding an alternative strategy are of great significance in environmental protection and sustainable development,and has important academic value and research significance for the high-value utilization of chitin.The research focus of this thesis includes:(1)Optimization of the process of preparing chitin nanofibers by surface modification,including the preparation of chitin nanofibers using a new neutral TEMPO/Na Cl O/Na C l O₂ oxidation system with mild reaction conditions,to instead of and comparison with the TEMPO/Na Br/Na C l O oxidation system.At the same time,the environmentally friendly chitin deacetylase(CDA)was applied to partial deacetylation of chitin,replaced the traditional deacetylation method,and a partially deacetylated chitin nanofiber was successfully prepared by sonication of the CDA enzymatic treated surface deacetylated chitin under aqueous p H3-4.In addition,the application of pretreatment combined with surface oxidation or partial deacetylation strategies to prepare chitin nanofibers.(2)Structural analysis of chitin nanofibers prepared by different surface modification methods.The research focus on a novel pretreatment strategy applied to the purified chitin.Investigation of the effects of structural properties of chitin,the effects o n the TEMPO/Na Br/Na Cl O oxidation and TEMPO/Na Cl O/Na C l O₂oxidation of chitin,and the effects on the preparation of chitin nanofibers,so as to evaluate the different pretreatment methods for preparing chitin nanofibers.This unique,simple and high-efficiency pretreatment strategy offers a green and ecologically friendly alternative for the preparation of chitin nanocrystals as part of an integrated biorefinery approach.At last,chitin nanofibers and oxidized chitin nanofibers were succes sfully prepared by a combination of Aqueous Counter Collision method(ACC)and TEMPO/Na Br/Na C l O oxidation method.At the same time,further preliminary study on the potential application of chitin nanofibers in assembly nanomaterials(such as film materials or gel materials),and reflect the size effect of chitin nanofibers on macroscopic films or gel materials.Neutral TEMPO/Na C l O/Na C l O₂(TNN)oxidation,with Na C l O₂ as the primary oxidant under aqueous conditions at p H 6.8 was applied to selectively oxidize surface C6 primary hydroxyl groups of?-chitin to carboxylate groups.The TNN oxidized chitin(TNN-Ch)was mostly converted to individual nanofibrils by mechanical disintegration in water,with mostly widths of 20–24 nm and average lengths of 1?m.Compared to chitin nanofibers prepared by TEMPO/Na Br/Na C l O system(TBN-Ch Ns),with average widths of 16.67±7.9 nm and average lengths of 770±170 nm,TNN-Ch Ns were wider,longer and had a higher aspect ratio;its films and hydrogels also showed better mechanical properties,and showed better flexibility,ductility and facility to form oriented three-dimensional nanostructures.In detail,TNN-Ch Ns based films exhibited an elastic modulus of 2603.4 MPa,tensile stress of 131.73±0.96 MPa and elongation at break of 5.94±0.23%,which were greater than those of the TBN-Ch Ns based films with an elastic modulus of 1627.2 MPa,tensile stress of 111.08±0.55 MPa and elongation at break of 4.90±0.35%.Moreover,the TNN-Ch Ns based hydrogels have a storage modulus of 1648 Pa,which is greater than that of the TBN-Ch Ns based hydrogels with a storage modulus of 546 Pa.This evidence prove that the size effect of chitin nanofibers and the degree of fiber orientation are the main reasons for the three-dimensional network nanostructure and macroscopic mechanical properties of nanofiber-based films and gels.The crude chitin deacetylase(a novel bacterial CDA)with a molecular mass of approximately 31 k Da with a maximum total enzyme activity of 150.09±1.66 U/m L was produced by culturing the bacteria of Acinetobacter schindleri MCDA01.The produced C DA was directly applied to surface deacetylation of crab shell?-chitin.Increased enzyme activity was observed with EDTA and metal ions of Fe³⁺,Ba²⁺,Sr²⁺,K⁺,Na⁺and Li⁺,while enzyme activity was inhibited by Co²⁺or Cd²⁺.The degree of deacetylation(DDA)was increased from6.46%to 30.58%when the chitin was treated by 12,000 U of C DA at 30°C and p H 7.0 for 96 h.The high crystallinity of original chitin was mainta ined,with yield greater than 90%after enzymatic deacetylation,indicating that the CDA deacetylation occurred on the surface of crystalline chitin fibrils without destroying the crystal structure.The chitin nanofibers(CDA-Ch Ns)were obtained with widths mostly between 25 and 45 nm and lengths of more than several micrometers by sonication of the CDA-treated surface-deacetylated chitin(CDA-Ch)aqueous suspensions at p H 3-4.The application of bacterial CDA on chitin provides a new path to the biorefinery of chitin nanofibers,and the resulting C DA-Ch Ns might exhibit advanced performance in forming multifunctional materials because of their unique fiber size and high aspect ratio.In order to further reduce the amount of chemical oxidants used in the surface TEMPO-mediated oxidation modification,four different pretreatment processes,that is,glycerol swelling,high pressure steaming,low temperature freezing,and grinding,were proposed and combined with TEMPO-mediated oxidation in order to prepare chitin nanocrystals.Based on the obtained results,the effects of the pretreatment on the oxidation and on the subsequent nanofibrillation of chitin were investigated.The results proved that 1)in the TEMPO-mediated oxidation systems,pretreatments can s ignificantly increase the carboxylate content of the oxidized chitin at the same amount of the oxidant;in other words,pretreatment methods can improve the oxidation efficiency and reduce the chemical reagent consumption.2)Degradation of chitin caused b y Na C l O can be mitigated by decreasing the dosage of Na C l O in the TEMPO oxidation system,and at the same time,adequate carboxylate content of oxidized chitin is successfully obtained by using pretreatments and TEMPO-mediated oxidation.3)Furthermore,we can obtain oxidized chitin with high carboxylate content at an impressive yield,which is beneficial for the preparation of oxidized chitin nanocrystals.This unique,simple and high-efficiency pretreatment strategy offers a green and ecologically friendly alternative for the preparation of chitin nanocrystals as part of an integrated biorefinery approach.Grinding pretreatment has become one of the most effective methods in the pretreatment method because of the refinement of the size of the raw chitin ma terial.Therefore,the more efficient fragmentation of chitin is further achieved through the application of aqueous counter collision(ACC)method.Chitin nanofibers(ACC-Ch Ns)and oxidized chitin nanofibers were successfully prepared using a combination of aqueous counter collision(ACC)method and TEMPO/Na Br/Na C l O oxidation method.The obtained results proved that the nanofibrillation of chitin by ACC process effectively improved the oxidation efficienc y.What's more,ACC-Ch Ns retains almost the original morphology during the process because of controllable slight oxidation.This novel combination of mechanical disintegration(ACC)and chemical modification(TEMPO oxidation)strategy offers a green and ecologically friendly alternative for the preparation of functionalized chitin nanofibers as part of an integrated biorefinery approach.Furthermore,the pulverized chitin nanofibers and functionalized chitin nanofibers had the good flexibility,ductility and facility to form films.Additionally,the oxidized ACC-Ch Ns based hydrogels were successfully formed by gas phase coagulation process,which possess better mechanical properties compared to TBN-Ch Ns(chitin nanofibers prepa red by oxidation of original?-chitin without ACC pulverization and the following ultrasonication)based hydrogels,and excellent dye adsorption capacity depending on the amphoteric chitin nanofibers with both amine and carboxylate groups,which showing pos itive charges at weakly acidic condition and negative charges at neutral or weakly alkaline condition.In summary,the results showed that the surface modification,size and morphology of chitin nanofibers can be controlled by adjusting the surface modification process.The neutral TNN system oxidation treatment(surface carboxylation modification)and green C DA deacetylation treatment(surface amination modification)can obtain wider,longer and higher aspect ratio than traditional TBN oxidation system treatment and hot alkali treatment,and then obtain films and gel materials with better performance.Pretreatment can effectively improve the efficiency of TEMPO-mediated oxidation,and at the same time,reduce the amount of chemical oxidation reagents in the oxidation system.At the same time,grinding and ACC method are most efficient methods.The introduction of the pretreatment strategy can not only optimize the subsequent oxidation process,but also can regulate the nanostructure characteristics of the chitin nanofibers.Besides,the subsequent TEMPO-mediated oxidation can also realize the surface carboxylation modification and functionalization of the chitin nanofibers.The thesis provide a basic theory and method for the research on the preparation of chitin nanofibers and their high-value utilization,and provide pathway with low chemical reagent consumption,low cost and low pollution for the realization of possible industrial applications of chitin nanofibers.Providing technical support in the preparation of chitin nanofibers is of great significance in environmental protection and sustainable development of biomass.