Influence Of Quaternary Ammonium Salts On Acrylonitrile Polymerization Catalyzed By Trialkylphosphine

Polyacrylonitrile(PAN)based carbon fibers have attracted worldwide interest as high performance materials.Their high strength and modulus,combined with their low density,make these fibers widely used from sporting goods to aircraft structures. The quality of the ultimate carbon fibers depends largely on the quality of the precursory material.Polymerization is the original process of preparing high performance carbon fibers,which does not only influence the properties of precursors but also the qualities of carbon fibers,directly.As a result of studies on high performance carbon fibers,the PAN precursors should have some characters for preparing PAN based fibers.1)It can be prepared in moderate condition without impurities to the best,especially metal ions.2)Stereoregular PAN with a high isotacticity could decrease activity energy and defects in the polymer chains during cyclization reaction,which could accelerate transformation from linear structure to heat-resistant trapezoidal frame.3)Branched chains make for more complete cyclization reaction and dehydrogenation during different chains.In this thesis,quatemary ammonium salts with asymmetry groups are successfully synthesized.Single substituted quatemary ammonium salt(salt 1)is synthesized according to quaterization between Benzyl bromide and tropane,and double substituted quaternary ammonium salt(salt 2)is synthesized according to quaterization betweenα,α'-dibromo-o-xylene and tropane.Solution polymerization of acrylonitrile carries out in DMF under moderate condition,using tricyclohexyl phosphine(PCy₃)as an initiator.Both conversion and polymerization velocity increase in systems of adding quatemary ammonium salts selected.Polymerization velocity decreases in the order of system with salt 1,salt 2,tetrabutyl ammonium bromide and none.All the systems have a high conversion at the beginning, approximately linear relationship between Ln([M]₀/[M])and reaction time.Atactic PAN which contains a few branches can be synthesized with number average molecular weight Mn upon 2×10⁵g/mol,Mw/Mn about 2.0 in each system,and the number of branches decreases appreciably when adding different quaternary ammonium salts.Molecular weight increases with an increase in reaction time,which can be adjusted better in system with salt 2,such as Mw/Mn.The effective factors of acrylonitrile polymerization initiated by tricyclohexyl phosphine(PCy₃)are discussed simply.The experimental results indicate that,the polymerization velocity is quick and molecular weight is high in this system.The initiation and mix between monomer and solvent don't have distinct influence on polymerization velocity,and the velocity decreases as concentration of initiator decreases.The appropriate condition has been investigated,and it indicates that initiator still keeps high reactivity in a low concentration.Quaternary ammonium salts affect system distinctly with a few dosages accordingly.Polarity and solvation of mixed solvent can be changed according to accommodating ratio of DMF and toluene,which have effects on quaternary ammonium salt system.The experimental results indicate that,Polarity and solvation decrease with an increase of toluene in solvent.Polymerization velocity and molecular weight also decrease distinctly,and the larger effects appear on system of salt 2,compared with system of salt 1.PAN with increasing molecular weight precipitate from solution including toluene,and then system becomes to heterogeneous polymerization.Velocity is slow,and conversion increases appreciably with time.The values of molecular weight and Mw/Mn are stable.Changes of solvent have little influence on structures,and atactic PAN with a few branches is still obtained.Thermal stability of PAN in different systems is investigated briefly by TG,and differences in postprocessing are analyzed under different atmosphere.The diversities of systems indicate that factors such as Mn and branches have influence on postprocessing of PAN and properties of carbon fibers distinctly.The molecular weight is bigger in the system with single substituted quaternary ammonium salt,so Pre-oxidation temperature decreases,which increases themal stability after heat treatment.