| Thermosetting polyimides are oligomers before curing.The oligomers have a low glass transition temperature(Tg),high solubility,low melt viscosity and are suitable for advanced processing technology with low melt viscosity.Thermosetting polyimides have high Tg values,dimensional stability,mechanical strength,thermal stability with continuous exposure to high temperature.Thermosetting polyimides have been widely applied as high-temperature structural adhesives and the matrices of carbon fiber-reinforced composites in microelectronics and aerospace industries.The fast growth of electronics and aerospace industries has lead to the development of thermosetting polyimides with enhanced properties.The curing temperature of acetylene-terminated polyimide is relatively low which can reduce the requirement of high temperature molding device.Thus the cost during processing is reduced.The narrow melting processing window and worse toughness after cured are the biggest obstacles that limit the development of acetylene-terminated polyimide.Phenylethynyl groups are excellent end-capped groups,which can provide broad processing window.Phenylethylnyl-terminated polyimides exhibit good mechanical properties and excellent thermal stability.It is of great significance to design thermosetting polyimide with different molecular structures and excellent properties.The introduction of flexible links can improve solubility,reduce the Tg and melt viscosity.The polyimide with isomeric monomers have higher solubility,lower melt viscosity,high thermal stability after crosslinking.In addition,the melting point of oligomers can be reduced before curing and the thermal stability of polyimides can be improved after cured by the addition of reactive diluents.Phenylethylnyl-terminated polyimides are the important matrix resins and adhesives in the modern aerospace field.It is essential to balance the processability and a long-term use temperature.It is difficult to improve the processability and thermal stability simultaneously.The polyimide materials with good processability and high thermal stability are the most research goal.Based on the above considerations,the main details of research are as follows:1.In order to obtain oligomers with excellent processability through the combination of flexible ether-bridge chain segment with asymmetric moieties.A series of acetylene-terminated imide oligomers based on 2,3,3’,4’-diphenyl ether tetracarboxylic acid dianhydride(a-ODPA)and 3,4’-oxydianiline(3,4’-ODA)with different molecular weights were synthesized by chemical imidization.The properties of oligomers were characterized and analyzed in detail.The mechanical properties and thermal stability of cured films were measured.The results showed that a low Tg value and minimum melt viscosity of oligomer PI-1 was 141 °C and 0.6 Pa·s,respectively.PI-1 had a wide processing temperature window and can be melt at a lower temperature.All the uncured imide oligomers showed excellent solubility(more than 30 wt%)in organic solvent such as N,N-dimethylacetamide(DMAc)and N-methyl-2-pyrrolidone(NMP).The accomplishment of thermal curing reaction need 60 min in 250 °C.The cured resins had high Tg values,good tensile properties,high toughness,and high thermogravimetric temperatures in N2 and air atmosphere.2.With the goal of improving processability of imide oligomers and achieving high toughness of thermosetting polyimides,a series of 4-phenylethynylphthalic anhydride(PEPA)-terminated imide oligomers prepared by the reaction of a-ODPA and 3,4’-ODA with different molecular weights(degree of polymerization: n=1,2,4,6,9)were formed.The resultant oligomers with different molecular weights were characterized for their chemical architecture,cure behavior,thermal properties,solubility in organic solvents and rheological characteristics.Besides,the thermal properties and tensile test of cured polyimide films were also evaluated.The adhesive properties of imide oligomers were measured.In addition,the properties of acetylene-terminated and PEPA-terminated polyimides were compared.The results showed that PEI-PEPA-1 had a certain crystalline phase and the others were non-crystallizability.All the oligomers have a low Tg and wide temperature processing window.Imide oligomers(degree of polymerization: n=2,4,6,9)showed excellent solubility(45 wt%)in NMP and DMAc at room temperature.All the imide oligomers showed a very low melt viscosity and wider processing window,resulting in distinct improvements in the processability.In addition,the minimum melt viscosity of oligomer PEI-PEPA-1 was only 0.18 Pa·s.When the PEI-PEPA-1 was heated at 240 °C for 120 min,the isothermal viscosity was only in the range of 0.32~0.67 Pa·s which can be applied to resin transfer molding(RTM).The accomplishment of thermal crosslinking need 60 min in 370 °C.The cured films had high tensile strength and toughness.The tensile strength and elongation at break of cured PEI-PEPA-6 film was 95.8 MPa and 15.6%,respectively.The cured resins had good thermal stability in N2 and air atmosphere.The lap shear strength of PEI-PEPA-4 and PEI-PEPA-6 was better.In comparison,the crosslinking temperature of acetylene-terminated imide oligomers was lower.PEPA-terminated imide oligomers had better processability,especially in wider melt processing window.Furthermore,the resins of PEPA-terminated polyimides had higher Tg,thermal stability and adhesive properties.3.In order to obtain a higher Tg values of cured resins,the rigid 2,3,3’,4’-biphenyltetracarboxylic dianhydride(a-BPDA)was introduced to a-ODPA/3,4’-ODA/PEPA system with n=4.A series of PEPA-terminated oligomers were obtained by the different mole ratios of a-ODPA/a-BPDA(3.2/0.8,2.4/1.6,1.6/2.4,0.8/3.2,0/4)and 3,4’-ODA as the monomers.The structure-property relations of these prepared polyimides with different ratios of a-ODPA/a-BPDA will be systematically investigated.The adhesive properties of imide oligomers were also measured.The results showed that the oligomers were amorphous,had good solubility in organic solvents,and showed excellent solubility(40 wt%)in NMP.The melt processing properties of oligomers and the coeffient of thermal expansion were reduced with the introduction of a-BPDA.The use temperature of thermosetting polyimides was increased.The differential scanning calorimetry(DSC)and dynamic mechanical thermal analysis(DMA)showed that the Tg values were 311 °C and 338 °C,respectively.The thermosetting polyimide had good mechanical properties and adhesive properties.The resins had excellent thermal stability in N2 and air.The thermal stability of cured resins can be improved by the introduction of a-BPDA.4.A series of PEPA-terminated imide oligomers prepared by the reaction of dianhydride(a-ODPA or a-BPDA)and 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene(6FAPB)were synthesized.The resultant oligomers with different molecular weights(degree of polymerization: n=2,4,6)were characterized for their chemical architecture,molecular weights,Tg values,solubility in organic solvents and rheological characteristics.Besides,the thermal properties and tensile test of cured polyimide films were also characterized and analyzed.The results showed that these oligomers had good solubility,low melt viscosity and wide melt processing window.The oligomers had low Tg values before curing and had high Tg values after cured.The polyimide films exhibited good mechanical properties.The elongation at break of cured 2a films was 18.7 %,the tensile strength of cured 2b films was 101.3 MPa.The cured resins had high thermal stability.The temperature of maximum rate of decomposition(Tmax)was in the range of 616~674 °C in N2.5.With the goal of improving Tg values of cured resins without sacrificing their processability,a phenylethynyl-terminated reactive diluent(Card-PEPA),which contained fluorenyl cardo structures,was successfully synthesized and used as a modifier for flexible phenylethynyl terminated imide oligomer(PEI-PEPA,n=6).The chemical structure,molecular weights,the phase behavior and melt viscosity of Card-PEPA were characterized.The imide systems with addition of 10,20,30 and 40 wt% Card-PEPA to PEI-PEPA and their cured resin systems were prepared.The thermal properties,solubility,melt viscosity,mechanical properties and thermal stability of imide systems were characterized and analyzed.The Card-PEPA 、PEI-PEPA 、 PEI-PEPA-Card-30 and PEI-PEPA-Card-40 were characterized at different heating rates with non-isothermal DSC curves.The Kissinger and Crane equations were used to calculate.The mechanism of curing was deduced.The results showed that the uncured imide systems could be melt at lower temperature.The Tg value of uncured PEI-PEPA-Card-10 was only 141.5 °C.The oligomers had good solubility(30 wt%)in NMP and DMAc.The melt viscosity of imide systems was decreased and the imide systems had good melt processability.The thermal crosslinking of the blends could be completed at lower temperature.The Tg values of the cured resins were greatly improved.The Tg value of cured PEI-PEPA-Card-40 film was 359 °C.The cured films had good mechanical properties.The 10% weight loss temperature(Td10)and char yield at 800 °C(Rw)of the cured resin systems were increased with addition of Card-PEPA.The cured resin systems PEI-PEPA-Card-40 exhibited the Rw as high as 66.5%.The thermal stability of resins was improved.The characteristic temperatures of curing with heating rate of 0 °C/min were obtained.The kinetic data indicate that activation energy and reaction order were increased with the addition of Card-PEPA. |
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