Study On Synthesis Of Polyaryletherketone Bearing Pendant Carboxyl Group And Its Modification On Epoxy Resin As Macromolecular Curing Agent

Epoxy resins are one of the most important classes of thermosetting polymers,and widely used in many applications as resin matrixs due to their outstanding advantages on mechanical properties,electrical insulation property,chemical and heat resistance as well as shrinkage on curing and processibility,and so on.However,the highly crosslinked structure makes these materials suffer from brittleness,poor crack resistance and low fracture toughness,which limited them applications in many high-tech fields.Therefore,it is very important to investigate the toughening of epoxy resins for their theoretical and practical researches.In this dissertation,a newly synthesized polyaryletherketone bearing pendant carboxyl group was used as macromolecular curing agent to modify the diglycidyl ether of bisphenol-A epoxy resin(DGEBA).Main contents and conclusions of this dissertation are presented as follows:(1) Firstly,the new polyaryletherketone(PEK-L) was synthesized via solution nucleophilic polycondensation reaction from 4,4'-difluorobenzophenone(DFBP) and phenolphthalin(PPL),and the structure and physical properties were characterized by nuclear magnetic resonance proton spectra(¹HNMR),FTIR,X-ray diffraction(XRD),gel permeation chromatography(GPC),differential scanning calorimetry(DSC) and thermogravimetric analysis(TGA) methods.The results revealed that PEK-L was an amorphous polymer with the number-average molecular weight(M_n) of 10095,the weight-average molecular weight (M_w) of 14060 and the polydispersity index of 1.39.Furthermore,PEK-L possesses excellent thermal properties with a high glass transition temperature(T_g) of 228℃,5%thermal weight loss of 420℃and char yield at 700℃of 58%.(2) The DSC results and the FTIR results demonstrated the feasibility of using PEK-L as a curing agent for epoxy resin,and the curing process of the modified epoxy system was determined to be 130℃/2h + 150℃/2h + 180℃/2h.A more effective catalyst and an optimum reaction ratio were ascertained after investigating the effect of different catalysts and reaction ratios on the curing reaction of the modified epoxy system.Furthermore,the curing reaction mechanism of the modified epoxy system was discussed.Finally,the curing kinetics of the modified epoxy resin was studied by Kissinger method,Ozawa method and isoconversional method,respectively,and the activation energies calculated by the three methods were then compared.(3) The mechanical properties,thermal properties and morphology of the epoxy systems modified with PEK-L were examined and then were compared with those of the neat epoxy resin cured with a conventional curing agent of 4-methyl-hexahydrophthalic anhydride (MeHHPA).It was found that the flexural modulus decreased slightly with the addition of PEK-L to epoxy resin,whereas the flexural strength of the modified epoxy systems increased, and the maximum increase was up to 20%when compared with the neat epoxy resin.The flexural strain at break of the modified epoxy system also increased significantly,and the maximum value was 10.03%,which was enhanced by 283%when compared with the neat epoxy resin.When the molar ratio of carboxyl group to epoxy group in the modified epoxy system was 1:1,the fracture toughness(K_IC) reached the maximum value of 2.53 MN/m^3/2 which was increased by 70%relative to the neat epoxy resin,and the fracture energy(G_IC) also reached the maximum value of 1.789 KJ/m² which is 2.93 times of that of the neat epoxy resin.Furthermore,the DSC results and the TGA results showed that PEK-L dramatically enhanced the heat resistance and thermal stability of epoxy resin.Both the glass transition temperature and the thermal degradation temperature increased with increasing the content of PEK-L in the modified epoxy systems.At last,the fracture surfaces of neat epoxy resin and modified epoxy blends were subjected to scanning electron microscopy(SEM) analysis and it was found that all the systems were homogeneous structure.It was clearly observed that the fracture surface of neat epoxy resin was smooth,glassy,sharp and without any plastic deformation.It was typical characteristics of brittle fracture,which accounted for its poor fracture toughness,as there was no energy dissipation mechanism operating here.On the contrary,the fracture surfaces of the modified epoxy blends were rough and ridge patterns and river marks as well as highly deformed nature could be seen on the fracture surface,indicating the nature of ductile fracture,which was responsible for their significantly enhanced fracture toughness.