Study On Preparation And Structure-properties Relationship Of Carborane-containing Imide Oligomers

High-temperature thermosetting polyimides possess excellent thermal resistant and mechanical properties.Among them,the phenylethynyl terminated polyimide,which exhibits many advantages such as processing performance and high temperature resistance and so on,could satisfy the performance requirements of aerospace and electronics fields on advanced resin matrix composite,and became an attractive resin matrix for study.Although polyimides can provide service performance approaching 320 ℃,the future development of material requires improvement in processability,mechanical properties and durability and higher use temperatures for the composite materials at the same time.How to improve the thermal oxidation stability,durability,process performance,and not reduce the mechanical properties of materials meanwhile,is an urgent problem.It is unlikely,based on the fundamental limitations of purely organic polymer chemistry,that organic polyimides can meet these industrial applications.The previous work demonstrates that making the polymer structure organic/inorganic hybrid could effectively address the issue.Carboranes have great thermal and chemical stability due to their stable cage structures and rich boron contents.The thermal stability of polymers could be greatly enhanced by the incorporation of carboranes into the matrices.Particular attention has been paid to the development of carborane-containing polymers with high thermal stability.Based on the properties advantage of carborane and the requirements on high temperature material.In this paper,several carborane amines were copolymerized with 4,4’-(hexafluoroisopropylidene)diphthalic anhydride(6FDA),and end-up with phenylethynylphthalic anhydride(PEPA)to form a series of carborane-containing imide oligomers.The cure kinetics and degradation mechanism of imide oligomers were investigated detailed.The main research conclusions are shown as follows:The carborane amine was prepered by p-nitrophenyl acetylene and B10H14 as raw material.The o-carborane diamine was prepared by phenyl acetylene and B10H14.The m-carborane diamine was successfully obtained by 4-nitrobenzyl bromide and m-carborane as raw material.The chemical strucutre of target compoud was charactered by FTIR,1H-NMR and 13C-NMR.A novel carborane-containing phenylethynyl terminated imide model compound Carb-PEPA was synthesized based on 1-(4-aminopheyl)-2-dicarbodeaborane and characterized by FTIR and ’H-NMR,and the thermal stability was characterized by DSC and TGA.The results showed that the char yield of Carb-PEPA was above 85%at 800℃ under nitrogen,indicating the compounds possess excellent thermal stability.It could be attributed to the presence of a passivation layer with oxidized carborane formed on sample’s surface to prevent further erosion.The resin systems consisting of Carb-PEPA and AFR-PEPA were prepared and cured to obtain carborane-containing polyimide,the effect of Carb-PEPA on the properties of imide system was investigated.The results show that Tg of cured polyimides increases with the increasing of Carb-PEPA contents in the blends,the Tg of blends increased from 374.2 to 404.4℃ by adding 20 wt%Carb-PEPA into AFR-PEPA.The mixed diamine composed of o-carborane diamine and p-PDA(content of carborane diamine:25wt%,50wt%,75wt%),was copolymerized with 6FDA,and end-up with PEPA to prepare carborane-containing phenylethynyl terminated imide oligomers 1-25、1-50 and 1-75.The mixed diamine composed of m-carborane diamine and p-PDA(content of carborane diamine:25wt%,50wt%,75wt%),was copolymerized with 6FDA,and end-up with PEPA to prepare carborane-containing phenylethynyl terminated imide oligomers 2-25、2-50 and 2-75.The structure of the products was confirmed by FTIR spectroscopy,and the thermal stability was characterized by DSC and TGA.It was found that the T5%and T10%for carborane-containing polyimide slightly decreased with the incorporation of carborane compared to AFR-PEPA,and the peak temperature of weight loss is decreased with the addition of carborane,but the thermal oxidative stability was increaesd.The char yield of imide oligomers increased with the addition of carborane.Besides,the char yield of carborane-containing polyimide based on m-carborane is higher than the polyimide based on o-carborane.By way of example,the char yield of 1-75 and 2-75 was 69.3%and 73.6%under nitrogen,respectively;the char yield of 1-75 and 2-75 was 60.2%and 75.3%under air,respectively.The cured polyimide possess the same tendency with above mentioned.The cure kinetics of the carborane-containing imide model compound Carb-PEPA、the imide blends system of Carb-PEPA/AFR-PEPA and carborane diamine-containing imide oligomers were studied by non-isothermal DSC method.The kinetics parameters were calculated with Kissinger and Crane equation,and the dynamic equation is established based on the results.The curing process temperature of resin system was obtained by extrapolation method.The kinetic data indicate that the activation energy E and frequency A of the AFR-PEPA/Carb-PEPA imide blends were higher than that of AFR-PEPA.The imide Carb-PEPA was added into imide oligomers of AFR-PEPA and BPDA-PEPA,respectively to prepare resin system,which was used to study the cure kinetics.The results show that the addition of Carb-PEPA into BPDA-PEPA results a decrease in activation energy E and frequency factor A compared to the addition into AFR-PEPA.This difference might be caused by low molecular weight and rigid strucutre of BPDA-PEPA.The apparent activation energy E of AFR-PEPA decreased with the addition of carborane diamine.In addition,the activation energy E of imide oligomers based on m-carborane is lower than that of based on o-carborane.The thermal degradation of Carb-PEPA and carborane diamine-containing imide oligomers 1-75 and 2-75 were studied by thermogravimetric analyzer coupled with Fourier transform infrared analysis(TG-FTIR),in situ FTIR and pyrolysis-gas chromatography-mass spectrometry(Py-GC/MS).The Carb-PEPA experienced the crosslinking process to form the polyimide during the heating from 330℃ to 400℃,following by the thermal degradation at higher temperatures(over 400℃).The protection layer was then generated from the oxidation of B-H(around 500℃)to effectively prevent the further degradation of Carb-PEPA.This degradation mechanism is suitable for both air and nitrogen atmosphere,but the char yield of Carb-PEPA in air was relatively higher compared with that in nitrogen due to the easy generation of B-O in air.The degradation mechanism of carborane-containing diamine imide oligomers 1-75 and 2-75 was similar to Carb-PEPA,the degradation could be prevented due to the formation of a protection layer by carborane oxidation at higher temperature.The results of the thermal degradation kinetics of carborane-containing imide oligomers shows that the thermal degradation activation energy of imide oligomer decreased with the addition of carborane diamine.In addition,the activation energy of imide oligomers based on m-carborane is lower than that of based on o-carborane.