Synthesis Of Self-promoted And Schiff Base-based Phthalonitrile Monomers And Their Copolymerization Behavior And Properties With Benzoxazine

Phthalonitrile is a novel type of thermosetting resin.It not only inherits excellent thermal stability and thermal oxygen stability as compared to traditional thermosetting resin but also shows many advantages,such as good mechanical strength,high carbon residue rate,low water absorption,outstanding chemical stability and flame retardancy,etc.In addition,the phthalonitrile monomer has a flexible molecular design,and different functional polymer materials with modified properties can be obtained through molecular structure design.However,phthalonitrile monomer has many disadvantages,such as high melting point,low reaction activity,narrow processing window and high curing reaction temperature,which leads to its poor processing properties,which seriously limits its application in various fields.In this paper,based on the structural design,Schiff base-based phthalonitrile monomers and autocatalytic phthalonitrile monomers(PNs:SBP and AAP)were synthesized.The tetrafunctional fluorenyl benzoxazines were selected as modifiers to improve the thermal properties or reduce the dielectric constant of phthalonitrile resins while toughening them.A series of Schiff base compounds were prepared by the condensation reaction of vanillin and aliphatic diamine(1,2-ethylene diamine,1,3-propane diamine,1,6-hexane diamine),aromatic diamine(p-phenylene diamine,4,4’-diamino diphenyl methane,4,4’-diamino diphenyl ether and 3,3’-dimethyl benzidine)or siloxane diamine(1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyl disiloxane,aminopropyl terminated polydimethyl siloxane).Then,a series of aliphatic-based,aromatic-based and siloxane-based Schiff base-based PN monomers(SBP)were synthesized from Schiff base compounds and 4-nitrophthalonitrile as raw materials and anhydrous potassium carbonate as a catalyst.The results showed that the introduction of flexible alkyl chain or siloxane segment can reduce the melting temperature and curing temperature of SBP(4,4’-diamino diphenyl methane as accelerator).With the increase of alkyl chain length,the melting point and curing temperature of aliphatic-based SBP decreased gradually.For aliphatic-based SBP,the processing window was greater than 100℃,and the processability was significantly improved.The curing kinetics of SBP was studied by non-isothermal DSC method.The apparent activation energies were calculated to be 95.0-108.8 kJ/mol by Kissinger and Ozawa methods.Among them,due to the increase of the molecular chain flexibility,the aliphatic-based and siloxane-based SBP have higher reaction activity and show lower apparent activation energy.After cyano curing and crosslinking reaction,all SBP form a crosslinking network structure dominated by phthalocyanine ring and triazine ring,which gives the cured products excellent thermal properties.All the thermal curing products showed higher glass transition temperature(Tg),higher initial thermal decomposition temperature(T5 and T10)and carbon residue(Yc).Among them,3,3’-dimethyl benzidine-based SBP polymer showed higher Tg(370℃)and better thermal stability(T5:458℃,T10:513℃,Yc:74.4%).In addition,due to the introduction of alkyl chain in aliphatic-based SBP monomers,aliphatic-based SBP polymers showed relatively high bending strength of 68.6-753 MPa and bending modulus of 3.52-4.21 GPa.SBP polymers had a low dielectric constant below 4.0 at room tempperature,among which aminopropyl terminated polydimethyl siloxane-based SBP polymer had the lowest dielectric constant and remained between 2.9-3.3.A series of bisphenol compounds containing secondary amine group were synthesized by nucleophilic addition reaction with Schiff base compounds as raw material and potassium borohydride as reducing agent.Then,a series of aliphatic-based,aromatic-based,and siloxane-based autocatalytic PN monomers(AAP)were synthesized from bisphenol compounds and 4-nitrophthalonitrile as raw materials.The presence of a secondary amino group in autocatalytic phthalonitrile monomer can significantly shorten the curing induction time,catalyze cyano polymerization at a lower temperature.The non-isothermal curing kinetics showed that autocatalytic phthalonitrile monomers have an autocatalytic polymerization reaction,and their apparent activation energies were 83.7-93.9 kJ/mol,which was significantly lower than the corresponding SBP.Compared with SBP polymers,AAP polymers showed a slightly lower thermal property,which was due to the existence of Schiff base group with conjugated structure in SBP monomers.Compared with aliphatic-based and siloxane-based AAP polymers,the introduction of aromatic rings in the polymer network structure increased the molecular chain rigidity of the aromatic-based AAP polymers.Therefore,aromatic-based AAP polymers showed higher Tg and better thermal stability.The bending strength and modulus of aminopropyl terminated polydimethyl siloxane-based AAP polymers were 87.3 MPa and 3.03 GPA,respectively,showed excellent bending properties.Propylene diamine-based AAP polymers showed a relatively good toughness of 580 kJ/m3,which was 250 kJ/m3 higher than propylene diamine-based SBP polymer.The dielectric constants of all AAP polymers were kept between 2.9-3.7,which were lower than the corresponding SBP polymers,showed better dielectric properties.Benzoxazine can promote the crosslinking reaction of phthalonitrile monomer,but greatly reduce the thermal properties of the polymer.Therefore,in this paper,the synthesized phthalonitrile was modified by n-octylamine-based(t-BF-sa-o)and diethylene glycol amine-based(t-BF-sa-dca)tetrafunctional fluorenyl benzoxazines(MFFB).The cyano curing reaction of phthalonitrile was catalyzed by benzoxazine,which significantly reduced the curing temperature of the blend systems.Due to the introduction of heat-resistant fluorene ring,the copolymers showed excellent thermal properties.Among them,compared with diamino diphenyl methane-based AAP polymer,the addition of 20%t-BF-sa-dca increased the Tg of the copolymer by nearly 40℃.TGA results showed that when 20%t-BF-sa-o was added,the T5 and T10 of ethylene diamine-based SBP polymer increased by 61 and 25℃,respectively,and the Y,was 68.7%,respectively.With the increase of t-BF-sa-dca content,the bending strength and modulus of ethylene diamine SBP polymer showed a gradual increasing trend.Among them,when 20%t-BF-sa-o was added,the bending strength and modulus of ethylene diamine SBP polymer were 78.4 MPa and 4.83 GPA respectively,which were increased by 9.8 MPa and 0.62 GPa respectively.Due to the introduction of 20%t-BF-sa-dca,aminopropyl terminated polydimethyl siloxane AAP polymer showed better tensile properties,and its tensile strength,modulus,ultimate strain and toughness were 86.2 MPa,6.1 GPA,1.89%and 840 kJ/m3 respectively,which increased by 36.7%,34.4%,11.1%and 38.1%.Due to the introduction of MFFB,the dielectric constants of the copolymers were kept between 2.7-3.6.The fracture surfaces of all copolymers showed rough and dendritic morphology,and showed the characteristics of ductile fracture.