Preparation And Properties Of Schiff Base-and Autocatalysis-type Phthalonitrile Monomers And Copolymers

Because of its excellent heat resistance,flame retardancy,low smoke and low toxic gas release characteristics,Phthalonitrile has received widespread attention in the aerospace,marine,electronics and other fields.However,the resin also has some disadvantages such as high melting point,high curing temperature,and slow curing reaction.How to solve the above problems has always been an important scientific issue in the field.In this study,a series of new phthalonitrile monomers were synthesized through molecular structure design.The monomer structures were characterized by Fourier transform infrared spectroscopy(FT-IR)and nuclear magnetic resonance hydrogen spectroscopy(¹H NMR).Polymerization behavior,curing reaction kinetics,thermal stability and flexural properties of phthalonitrile/phenol-aniline benzoxazine(P-a)and bisphenol A-aniline group benzoxazine(BA-a)/phthalonitrile were investigated by scanning calorimeter(DSC),FT-IR,thermogravimetric analyzer(TGA)and universal material experimental machine have studied.A series of hydroxyl-containing Schiff base intermediates were synthesized via the amine-aldehyde condensation reaction of aniline(or furfurylamine)and 4-hydroxy benzaldehyde(or vanillin),respectively.Next,four Schiff base-type phthalonitrile monomers were prepared by the reaction of 4-nitrophthalonitrile via nucleophilic substitution reaction.The aforementioned Schiff base was subjected to reduction and nucleophilic substitution reaction.Finally,the self-catalyzed phthalonitrile monomers containing active hydrogen were obtained.P-a is a liquid at room temperature,which can be used as a reactive diluent and curing accelerator for phthalonitrile resin(added mass fraction is 20%,the same applies hereinafter).Therefore,the processability of phthalonitrile resin was improved.Schiff base type phthalonitrile/P-a copolymer exhibits good thermal stability and bending properties.Among them,furfurylamine-p-hydroxy benzaldehyde-based phthalonitrile(KDW)copolymer has the highest thermal stability and flexural properties.The values of 5%and 10%weight loss temperatures(T₅and T₁₀)attain390 and 458?under nitrogen atmosphere,respectively.The char yield(Y_c)at 800?is 68%.The flexural strength and modulus achieve 84 MPa and 6.3 GPa,respectively.In addition,by virtue of the high heat resistance of phthalonitrile resin,KDW is used to improve the thermal properties of BA-a resin.When KDW was added at 30%,the values of T₅,T₁₀and Y_cof BA-a/KDW copolymer increased by 41?,47?and24.4%,respectively,compared to neat polybenzoxazine(poly(BA-a)).The self-curing reaction of BD and KX can be conducted without adding curing agent,but the apparent activation energy of KX is much larger than BD.The results of mixing phthalonitrile with benzoxazine resin show that with the increase of the amount of P-a or BD,the thermal stability of BD/P-a or BA-a/BD copolymers gradually improves,but the bending performance gradually decreases.When the mass fraction of BD reached 30%,the values of T₅,T₁₀and Y_cof BA-a/BD copolymers were increased by 58°C,39°C,and 20.7%,respectively,compared to neat Poly(BA-a).