Study On Synthesis And Curing Behavior Of Hydroxy-terminated Polyfluoropolyether

Energetic binders are the basic raw materials and important components of explosives and propellants.As the skeleton of energetic materials,it is necessary to improve the mechanical properties of energetic materials and provide a part of energy.In this topic,through the design of molecular structure,enhancing the flexibility of the main chain to provide a basis for excellent mechanical properties,the introduction of energy-containing groups to increase energy,the introduction of fluorine elements to improve density and oxygen balance,synthesis of four hydroxy-terminated polyfluoropolyether energetic prepolymer,and its structure and physical and chemical properties were characterized;in addition,the curing behavior of the binder is the basis of its application research,detailed kinetic parameters can provide guidance for practical applications.So in this article,the curing kinetics of the synthesized four hydroxy-terminated polyfluoropolyether energetic prepolymer and the mechanical properties,glass transition temperature,and thermal stability of the cured products were studied.The main research work is as follows:（1）Taking 2,2,3,3,3-pentafluoropropanol,2,2,3,3,4,4,4-heptafluorobutanol,epichlorohydrin and nitric acid as raw materials through substitution reaction,nitration reaction,etc,three kinds of energetic monomers were synthesized-2-（（2,2,3,3,3-pentafluoropropoxy）methyl）ethylene oxide（PFEE）,2-（（2,2,3,3,4,4,4,4-heptafluorobutoxy）methyl）ethylene oxide（HFEE）and glycidyl ether nitrate（GN）.Then,using 1,4-butanediol as an initiator and BF₃·Et₂O as a catalyst,the above monomers were polymerized to obtain three hydroxyl-terminated polyfluoropolyether,polypentafluoropropoxyl-methyl-oxirane（poly（PFEE））,polyheptafluorobutoxy-methyl-oxirane（poly（HFEE））,pentafluoropropoxy-methyl-oxirane and glycidyl ether nitrate copolymer(poly（PFEE-co-GN）;in addition,PFEE and epoxy chloride propane is a monomer,copolymerized first and then azidated to obtain pentafluoropropoxy-methyl-ethylene oxide and azide glycidyl ether copolymer（poly（PFEE-co-GA））.Modern tests such as nuclear magnetic resonance hydrogen spectroscopy（¹H NMR）,nuclear magnetic resonance carbon spectroscopy(¹³C NMR),nuclear magnetic resonance fluorometry(¹⁹F NMR),infrared spectroscopy,differential scanning calorimeter（DSC）,gel chromatography（GPC）.The structure,molecular weight,polydispersity index,glass transition temperature,hydroxyl value,etc.of the four polyfluoro-terminated hydroxyl polyethers were tested and characterized.（2）Using the curing of poly（PFEE）as a model reaction,the effect of different curing temperatures,different functional group ratios,different chain extenders and different catalysts on its curing behavior was studied using a microcalorimetry,and the reaction kinetic model was calculated as n Kinetic model,and calculated a series of kinetic and thermodynamic constants-apparent rate constant,apparent activation energy,activation enthalpy change,activation entropy change and activation Gibbs free energy change,etc.,to determine the appropriate curing process conditions;Then,four hydroxyl-terminated polyfluoropolyether energetic prepolymers were cured according to the appropriate process conditions obtained by the study,and prepared into standard splines,and the mechanical properties of them were tested with a universal testing machine.It was found that the mechanical properties of the four binders after curing were different.Among them,the cured poly（PFEE）performance was the most excellent.The maximum elongation at break was 231%and increasing the content of PAPI can effectively increase the fracture strength,the maximum breaking strength is 3.04MPa.While for poly（HFEE）and poly（PFEE-co-GA）,the former is due to the longer side chains and the latter is due to the increase in the polarity of the side chains,and the mechanical properties after curing are slightly reduced by comparison.Poly（PFEE-co-GN）contains a large number of nitrate groups,the molecular chain polarity is greater,the force between the molecular chains is enhanced,the cured product has a stress yield phenomenon during stretching,and the tensile properties are obvious under the same conditions get worse.（3）DSC was used to test the glass transition temperature of the four cured products under different conditions.The results show that the glass transition temperature changes due to different curing processes,because different curing processes cause the degree of microphase separation of binder and chain extender and the structure of the hydrogen bonding network is different,which in turn causes mechanical properties to be affected,and the low microphase separation degree is greater in fracture strength.The study found that the compatibility of poly（PFEE）,poly（HFEE）and PAPI is very poor,but after the curing reaction,the compatibility of the two phases has been improved.Introducing azide groups or nitrate groups into the polymer,the compatibility after curing has been further improved.Different curing processes have basically no effect on the degree of micro-phase separation of poly（PFEE-co-GA）.The introduction of nitrate groups creates a stronger hydrogen bond with the hard segment,and the glass transition temperature is still affected by the curing process.（4）Using thermogravimetric analysis and isothermal pyrolysis test,the thermal stability of the four adhesives after curing was studied.It was found that the thermal stability of the four fluorinated binders was good,the initial decomposition temperature was more than 180℃,and the compatibility with the two common explosives CL-20 and HMX was good,which had potential for practical application.