A Study On Degradation Of Carbon Dioxide-Based Polycarbonate

Realizing sustainable development oriented toward the environment,energy,and economy is a considerable challenge for the worldwide.The application and development of petroleum-based plastics cause severe environmental pollution and significant energy consumption.Thus,replacing petroleum-based plastics with renewable and biodegradable polymer materials is urgent and inevitable for sustainable development.Polypropylene carbonate（PPC）has been widely researched and developed as a highly promising green biodegradable material and is used in various fields.However,PPC exhibits unstable thermal properties and poor mechanical properties that limit its application in processing,production,and daily life.Therefore,to effectively improve its properties and broaden its application areas,PPC is often chemically modified by introducing a third monomer in the copolymerization reaction of carbon dioxide（CO₂）and propylene oxide（PO）.Moreover,this CO₂-based polycarbonate material has unpredictable service time and degradation cycle.Therefore,issues related to the degradation mechanism of CO₂-based polycarbonate material under different environments and waste-recycling treatment must be comprehensively studied.The influence of the third monomer on the degradation performance of polycarbonates when the thermal and mechanical properties of the PPC is improved through terpolymerization must be discussed.Accordingly,this thesis aims to explore the degradation behavior of CO₂-based polycarbonates obtained by chemically modifying different types of third monomers under specific degradation conditions,compare the degradation performance of different types of CO₂-based polycarbonates,reveal the degradation behavior and mechanism of CO₂-based polycarbonates in different environments,and provide predictive role and data support for the practical application of biodegradable materials at a later stage.（1）First,the bimetallic complex catalyst salen[Co（III）TFA]₂ was prepared to catalyze the dimer copolymerization of CO₂ with PO;then,different types of third monomers,including epoxy monomer（NGDE）,lactone monomer（DL-LA）,and acid anhydride（IAn,OA）,were introduced on this basis,into CO₂-based polycarbonates PPCNG,PPCLA,PPCIAn,and PPCOA.PPCNG was characterized and analyzed via infrared spectroscopy（FT-IR）,nuclear magnetic resonance hydrogen spectroscopy,gel chromatography,thermogravimetry,and differential scanning calorimetry;its thermal stability and mechanical properties were also investigated.（2）CO₂-based polycarbonate films were prepared.Hydrolytic degradation experiments were performed on the CO₂-based polycarbonates in phosphate-buffered saline to examine the hydrolytic behavior and mechanism of CO₂-based polycarbonates;additionally,hydrolytic properties of different types of CO₂-based polycarbonates were compared and analyzed.The hydrolysis performances of these five types of CO₂-based polycarbonates are on the order PPCNG>PPCLA>PPCIAn>PPC>PPCOA.The essence of hydrolysis of a CO₂-based polycarbonate is the breaking of ester bonds;moreover,hydrolysis induces increased water absorption of the samples,decreases the p H value of the degradation solution,decreases characteristic viscosity,increases the appearance of holes on the surface of samples,and deteriorates mechanical properties.Post hydrolysis,evident mass loss was observed in the samples;mass retention rates of PPCNG,PPCLA,PPCIAn,PPC,and PPCOA were 71.7%,74.3%,79.7%,94.4%,and 95.0%,respectively.However,the degree of molecular weight change was small.Presumably,the mechanism of CO₂-based polycarbonate hydrolysis causes surface degradation.（3）Microorganisms predominantly cause degradation in the natural environment.Although not directly,they break down polymeric materials through the action of enzymes secreted during metabolic processes.Thus,to explore the biodegradation mechanism of CO₂-based polycarbonates,this study investigates the biodegradation behavior of CO₂-based polycarbonates in rhizopus oryzae lipase solution.Enzymatic conditions,including the time of enzyme solution replacement,enzyme concentration,and temperature,were evaluated.The optimum replacement time of rhizopus oryzae lipase solution was 72 h,the optimum concentration of enzyme digestion was 0.20 mg/m L,and the optimum temperature was 35℃.Characterization of the samples before and after degradation by FI-IR,contact angle,weight loss,scanning electron microscopy（SEM）,characteristic viscosity,and mechanical properties.The degradation of CO₂-based polycarbonates by lipase is a surface erosion mechanism,which by nature is enzymatic hydrolysis.Compared with hydrolysis,the degree of enzymatic degradation of polycarbonates was higher,and the mass retention rates of PPCNG,PPCLA,PPCIAn,PPC,and PPCOA samples reached 59.7%,65.3%,74.7%,92.4%,and 93.6%,respectively,after 70 d of degradation.