Low Temperature Promoted The Dissolution Of Chitin In Phosphoric Acid And Controlled Construction Of High-Tensile Chitin Films

In the 21^st century,sustainable development strategies and renewable energy are the order of the day as non-renewable fossil resources become scarce and petroleum-based plastics become more prevalent,depleting oil resources and increasing environmental pollution.Human beings are turning their attentions to the untapped marine resources.As the second largest biopolymer in nature,chitin has been widely used in drug delivery,flexible electronic materials,food functional materials based on its excellent biocompatibility,environmental friendliness,biodegradability,non-toxicity and low immunogenicity.However,the abundant intra-and inter-molecular hydrogen bonds makes chitin difficult to dissolve in almost organic solvents and aqueous phase solvent systems.At present,the common solvents for chitin include ionic liquids,low eutectic solvents,strong acid systems,alkali/urea systems and so on.However,due to the disadvantages of these systems,such as biological toxicity,difficulty in recovery,high cost,high energy consumption and poor universality,the research and application of chitin materials are greatly limited.Therefore,the development of an efficient,non-toxic,green and inexpensive chitin solvent system is the key to promote the progress and development of chitin science.Compared with other chitin solvent systems,phosphoric acid system has the advantages of green,high efficiency,strong universality and so on.In recent years,our group has achieved efficient dissolution of various natural polymer materials（wood pulp cellulose,silk,chitin,etc.）based on phosphoric acid,and constructed multifunctional Pickering emulsions through Pickering emulsion templating method.However,the application of this dissolution system in the dissolution of chitin needs to be further explored,which is due to the fact that the glycosidic bonds will be broken during the dissolution process,leading to unavoidable hydrolysis and limiting the application prospects of regenerated chitin materials.Based on our previous work,this thesis proposes a low-temperature phosphoric acid system with the aid of low-temperature to dissolve chitin,focusing on exploring the dissolution behavior of chitin in the low-temperature phosphoric acid.On the premise of avoiding the hydrolysis of glycosidic bonds,high-performance regenerated chitin materials are constructed by adjusting the dissolution parameters and regeneration process.In chapter 2,we explored a method for the controlled dissolution of chitin in low temperature phosphoric acid and systematically investigated the dissolution behavior.Chitin was dissolved in 60 wt%-85 wt%aqueous phosphoric acid with the temperature range of-30°C to 25°C.while morphological changes of chitin were observed by optical microscopy and polarized light microscopy.The complete dissolution time was determined based on the disappearance of chitin crystalline birefringence.The changes in the crystalline structure of chitin before and after dissolution were characterized by Fourier infrared spectroscopy（FT-IR）and X-ray diffraction（XRD）,and the rheological properties of the solutions were analyzed by a rheometer.We found that the lower temperature promoted the dissolution of chitin in aqueous phosphoric acid solution,which showed the best dissolution ability at-18°C.At this temperature,chitin could be dissolved in 72 wt%aqueous phosphoric acid solution,and the dissolution ability increased with the increase of phosphoric acid concentration,while the maximum chitin solubility could reach 10 wt%.In addition,chitin degradation was inhibited by low temperature,so the chitin solution still had a high molecular weight after long storage at-18 C.Based on this feature,chitin degradation can be controlled by adjusting the storage temperature and time to achieve a controlled adjustment of the molecular weight of chitin samples.In chapter 3,based on the preparation of high molecular weight chitin solutions in a low temperature phosphoric acid solvent system,a method for the construction of high-tensile chitin films was developed.The effects of dissolution temperature,initial concentration of chitin,regeneration bath and regeneration conditions on the microstructure and mechanical properties of chitin films were investigated.Scanning electron microscopy（SEM）and XRD were used to analyze the changes in microstructure and crystallinity of chitin films.We found that the mechanical properties of chitin films increased significantly with decreasing dissolution temperature,and the highest tensile strength and the highest elongation at break were obtained at-18 C with 86 MPa and 5.8%,respectively.In addition,as the initial concentration of chitin increased,the prepared films exhibited a denser microstructure and showed more excellent mechanical properties.The best mechanical properties were found in the films regenerated by10 wt%Na H2PO4 at 0°C,with a tensile strength of 133 MPa.The draft orientation of the chitin gel was further carried out,and the strength of the uniaxial orientation of the chitin film was up to 235 MPa.In addition,due to the dense arrangement of chitin fibers on the surface of the film,the film exhibited excellent hydrophobicity（contact angle>100°）,which made it possible to be used in wet environments.Finally,self-adhesive chitin straws were further produced based on the self-adhesive surface of the high strength chitin films and their excellent hydrophobicity.The chitin gels were simply wound,and the self-adhesive chitin straws were obtained through natural air drying,which exhibited similar water stability properties to commercial paper straws.