Fluorine-containing Polymer Synthesis In Supercritical Fluids

Supercritical carbon dioxide(scCO₂) is a green and effective alterative to the organic solvent of chlorofluorocarbons(CFCs) in the synthesis of fluorinated polymers.Studying on the polymerization of fluorine-containing monomers will attribute great to upgrade the quality of products,widen their application,lower contaminant to environment and the consumption of energy, which shows great influence on the perspective of fluorinated polymers.It was proved that copolymerizaiton and composition are valid and effective methods to enhance the properties of polymeric materials.By the way of copolymerization,the functional groups can be introduced into polymer chains and the crystallinity of polymer decreases,so as to make the polymeric materials easier to process and better for application.By the way of composition,the inorganic components can be introduced into polymeric materials,which can strengthen the mechanical properties and provide new properties to the resulting products.In this research,the traditional polymerization process in supercritical fluids(SCFs) was improved through modifying the polymerization apparatus, designing,and using sampling-tubes.Based on this improvement,homopolymerization and copolymerization of fluorine-containing monomer and synthesis of hybrid materials were studied. The research mainly includes three parts as follows.(1) New procedure and apparatus were designed to improve the traditional radical polymerization in SCFs.The sample-in tubes were designed,and used to add monomer and initiator at different reaction stages.The volatile reactant(s) was charged after purging the reactor and initiator was introduced after the reaction system reached the desired temperature.Then the pressure was adjusted to start the polymerization.The new process allows to carry out on-line sampling and off-line analyzing the products at different reaction stages,and tracking the monomer conversion as well as distribution of molecular weight.The improved polymerization procedure was illustrated with the homopolymerization of 2,2,2-trifluoroethyl methacrylate(TFEMA) in scCO₂.The synthesized polymers were characterized by FT-IR,¹H NMR,¹³C NMR,GPC,TGA and DSC, respectively.Our newly developed methods showed a lower PDI together with accurately measured monomer conversion and polymeric product yield.In this process,volatile reactant was charged after purging the reactor,avoiding the losing of monomers and minimizing the pollution to the environment.The results make it possible for quantitative analysis and reaction time optimization in SCFs,attributing great to better design of polymerization systems,and more effectively control of the polymerization procedure. (2) The polymerization kinetics of TFEMA in scCO₂ was investigated by on-line sampling from the reactor and off-line measurement by ¹H NMR.It was indicated that the initiation process is affected by monomer and the initiation rate is related to the monomer content,leading to a higher reaction order(up to 1.5) of monomer,TFEMA.The polymeric chain radical is easily diffused in scCO₂,resulting in a lower molecular weight distribution.The active energy of polymerization(Ea) is mainly contributed by that of decomposition of initiator,AIBN.The rate equation of polymerization is given by Rp=k[TFEMA]^1.47[AIBN]^0.45,and the apparent active energy is 59.74 kJ·mol^-1.In this part,orthogonal measurements were designed to investigate the effects of temperature,pressure and initiator content on the conversion of monomer and the molecular weight of polymer,and also to optimize the reaction conditions.The experimental results show that increasing pressure not only attributes to accelerate reaction rate and monomer conversion,but also promote molecular weight and degree of polymerization.The reaction rate and monomer conversion can be promoted by increasing the content of radical initiator and the polymerization temperature, but molecular weight will decrease at the same time.At relatively low temperatur,the effect of initiator content on polymerization is weak and pressure can be mainly used to control the molecular weight and polymerization degree,however,at higher temperature,the effect of initiator content on polymerization is sensitive,so,to some extent,the molecular weight and polymerization degree can be contronled by the initiator content.To synthesize PTFEMA in scCO₂,the polymer with high conversion rate and high molecular weight can be expected by increasing pressure,decreasing temperature and increasing reaction time.When homopolymerization of TFEMA was performed at 55℃,32.5 MPa,the mole ratio of TFEMA to AIBN of 100:1,and reaction time of 24 h,the monomer conversion was 88.3%,M_n was 40353,and molecular weight distribution was 1.33.(3) The copolymerization of TFEMA in scCO₂ was also studied by using high pressure sample-in tubes to add TFEMA and its comonomer(s) at different reaction stages.It was indicated that random compolymer(s) is obtained when the monomers mixture of TFEMA and NVP,TFEMA and MAPTMS,or TFEMA and St was added to the(purged) reactor;while diblock copolymer could be obtained when the second monomer(MMA) was added before the exhausting of the first monomer(TFEMA) during the process of polymerization.TFEMA can easily copolymerize with monomer(s) mentioned above in scCO₂,and fluorinated polymers with different structures and applications can be obtained by using different monomer(s),different adding sequence of reactants and different polymerization methods.MAPTMS was introduced to modify the surface of nano-SiO₂ to get a kind of macromonomer, SiO₂-MAPTMS and PTFEMA/SiO₂ composite materials were obtained when the SiO₂-MAPTMS macromonomer copolymerizes with the fluorinated monomer,TFEMA.It was clearly indicated that reaction medium impacts not only on the surface modification of nano-SiO₂,but also on the copolymerization of the fluorinated monomer and the macromonomer.Comparing with the traditional organic solvents,scCO₂ is a green and perfect reaction medium,both for the surface modification of nano-SiO₂ and for the copolymerization of TFEMA and SiO₂-MAPTMS,and can be used in the synthesis of fluorinated polymer composite materials.It also indicates that the thermal stability of PTFEMA/SiO₂ composite materials from in-situ chemical synthesis is better than that of PTFEMA/SiO₂ composite materials from in-situ physical synthesis.The result of copolymerization in chemical synthesis is greatly affected by the result of the surface slianization modification.With some C=C group on its surface after slianization by MAPTMS,nano-SiO₂ particles can act as the chemical cross-linking points in PTFEMA/SiO₂ composite materials,which can increase the thermal stability,and can be reasonably expected to improve the mechanical properties of composite materials.