| In the past decades, aromatic polyimide as an important class of high-performanceengineering plastic got fast development. Due to their outstanding thermal stability, chemicalresistance, dimensional stability and mechanical properties, aromatic polyimide is widely used inmany advanced technological fields, such as aviation, spaceflight, electronics andcommunication. However, it is known that almost all of fully aromatic polyimides are insolublein most organic solvents, which is caused by rigid polymer backbones and strong inter-chaininteraction. The poor solubility of these polyimides highly impedes their application in manyfields. To improve the processability and solubility of aromatic polyimides, considerable effortshave been made focus on the modification of their chemical structure. Introducing non-coplanarmoiety, flexible links or bulky groups, has been proved to be a successful approach to improvethe solubility, but the latter two methods often bring about a loss of chain stiffness, glasstransition temperature, chemical resistance and thermal stability of polyimides. Therefore, themain direction of polyimide research is to prepare the polyimide materials possessed goodprocessability and excellent comprehensive properties in future.The crosslinking is an effective modified method towards to macromolecules. Theproperties involving thermal stability, chemical resistance and mechanical properties could beenhanced by introduction of cross-linked structure. Till now, among the cross-linked polyimides,the aryl ethynyl terminated imide oligomers had been researched most widely and deeply. In1980, Gupta et al. first reported this kind of cross-linked polyimide materials. After that, much more aryl ethynyl terminated imide oligomers had been prepared. However, during a long time,the scientists always focus on the improvement of the polyimides’ thermal and mechanicalproperties by end-capping with aryl ethynyl groups. As the understanding of the crosslinking, wethink the crosslinking also could improve many other properties besides thermal and mechanicalproperties. Therefore, we hope to explore the potential of crosslinking in modifying the structureand properties of polyimides, and expand the application scope of cross-linked polyimidesthrough further research. In this article, we will discuss the significance of crosslinking throughstudying the basic properties (thermal, mechanical properties), microstructure (morphology, porestructure) and functional properties (proton conductivity, gas adsorption) of cross-linkedpolyimide materials from different perspective.In this thesis, we hope to synthesize soluble polyimides containing cross-linkable pendantgroups through rational molecular design. Through thermal crosslinking reaction, thecross-linked polyimides will be prepared and investigated from the morphology, pore structure,adsorption behavior and proton conductivity.First of all, a novel diamine monomer1,3-bis(2-trifluoromethyl-4-aminophenoxy)-5-(2,3,4,5-tetrafluorovinylphenoxy)-benzene(6FTFPB) was designed and synthesized in the third chapter. Based on this diamine monomer, aseries of linear co-polyimides containing different content of cross-linkable pendant groups wereprepared. The crosslinking behavior of these polyimides had been investigated by DSC andFTIR. The results of characterization indicated that the crosslinking reaction had been proceededcompletely after being treated under260oC for2h. As the content of cross-linkable groupsreach30%, the glass transition temperature, gel content and the coefficient of thermal expansionof the polyimide was significantly improved to271oC,90%and60.3ppmoC-1, respectively.However, when the crosslinking density continues increasing, the comprehensive properties ofthe cross-linked polyimides did not improve any more, but the flexibility of the cross-linkedfilms decreased sharply. The results indicated that the content of cross-linkable groups of thepolyimide is proper to fixed as30%. Therefore, a series of polyimides containing30%molarratio of cross-linkable pendant groups and different molar ratio of-CF3groups were synthesized.We mainly studied the influence of cross-linked network structure on the mechanical propertiesof the polyimides. After crosslinking reaction, the cross-linked structures limited the macromolecular chain segmental motion and avoided the macromolecular in-plane orientationand chain dense packing. As a result, the cross-linked polyimides possessed higher Tg(271~283oC), better thermal stability (525~571oC at5%weight loss), mechanical properties (3.6~4.7GPa,92~113MPa,3.2~12.5%), lower CTE (60.3~76.7ppmoC-1) and dielectric constant (2.37~2.78at1MHz).Based the previous works, we want to investigate the influence more details of cross-linkednetwork on the polymer microstructure and functional properties. Firstly, a series of highlysulfonated polyimide containing different content of cross-linkable hydrophobic tetrafluorostrylpendant groups were synthesized from6FTFPB,6FAPB, ODADS and NTDA. Aftercrosslinking reaction, the proper crosslinking density was determined after investigating thethermal properties, water uptake, swelling ratio, proton conductivity and stability (chemicalresistance, oxidative stability and hydrolytic stability) of the cross-linked sulfonated polyimidemembranes. Later, a series of highly sulfonated polyimides with different degree of sulfonationwere prepared. During the thermal treatment at260°C, the cross-linking reaction took placeamong tetrafluorostyrol pendant groups without the addition of any chemical additives andelimination of sulfonic acid groups. After cross-linking reaction, hydrophobic bridge structureswere developed between the hydrophobic domains of the sulfonated polyimides main-chain. Thehydrophobic bridge structures could not only suppress excessive water uptake and swelling butalso enlarge the hydrophobic domains and facilitate phase separation between hydrophilic andhydrophobic domains forming well-connected hydrophilic channels for dramatically enhancedproton conduction. The variation of polymer membranes’ IECvwas consistent with the result ofmorphological characterization. When the sulfonated membranes were completely hydrated,excessively high water uptake diluted the concentration of sulfonic acid groups per copolymervolume. This dilution effect on IECv(wet)was more pronounced for non-cross-linked membranes.After crosslinking, the water uptake and swelling ratio of cross-linked membranes was reducedsharply compared with the corresponding non-cross-linked ones. The decrease of water uptakeled to a dramatic increase of IECv(wet)in the fully hydrated state membranes. Consequently,cross-linked membranes exhibited superior proton conductivities (103~179mS cm-1at80oC).Due to the formation of the cross-linked network, the proton conductivity of membranes in thelow RH range could be significantly improved for enhancing the water retention capacity. Moreover, the cross-linked membranes also exhibited lower swelling ratio (6.4~14.7%),methanol permeability (3.99~8.61×10-7cm2s-1) and outstanding chemical resistance, oxidativestability and mechanical properties.The improvement of properties is ascribed to the change of microstructure in thecross-linked polymers. We want to control the microstructure and endow the cross-linkedpolymers with unique morphology and functional properties through introduction of crosslinkingnetwork. In this part, a novel bis(ether anhydride)(PEPHQDA) monomer containing rigidphenylethynylphenyl pendant group was designed and synthesized. Based on PEPHQDA and1,3,5-tri(4-amino-2-trifluoromethylphenoxyl)benzene (TFAPOB), different groups (amino-andanhydride-) end-capping hyperbranched polyimides containing cross-linkablephenylethynylphenyl pendant groups were synthesized and cross-linked by thermal treatment.After crosslinking reaction, the morphology of amino groups end-capping cross-linkedhyperbranched polyimide changed dramatically. However, there was a slightly improvement inthe BET area of this hyperbranched polyimide. The result could be attributed to the collapse ofpore caused by flexible polymer skeleton and occupying of free volume by bulky–CF3groups.In order to improve the porosity, amino groups end-capping cross-linked hyperbranchedpolyimide (Cured-TAPOB-HBPI) was prepared from PEPHQDA and non-trifluoromethylsubstituted1,3,5-tri(4-aminophenoxyl)benzene (TAPOB). After cross-linking reaction, thepolymer particles became smaller and packed denser. The rigid cross-linked structures hinderedthe collapse of the polymer skeleton and prompted the formation of micropore in thehyperbranched polymimide. The Cured-TAPOB-HBPI exhibited characteristic nitrogenadsorption isotherm of permanent microporous materials and possessed comparable BET area(322m2g-1), total pore volume (0.33m3g-1) and micropore volume (0.05m3g-1) with microporouspolymers reported in literature.In order to explore the influence of cross-linked structure and polymer skeleton on theporous structure and gas adsorption, a series of amino end-capping cross-linked hyperbranchedpolyimides were prepared from PEPHQDA and multiple aromatic amine with different topology.Similar to other microporous polymers, all of the Cured-HBPIs showed the homogenousagglomerate morphology composed from denser packing nano polymer particles. Moreover, theCured-HBPIs exhibited characteristic nitrogen adsorption isotherm of permanent microporous materials and possessed comparable BET area (385~497m2g-1), total pore volume (0.19~0.25m3g-1) and micropore volume (0.089~0.115m3g-1) with microporous polymers reported inliterature. The BET areas of the Cured-HBPIs were closed, though they possessed differentpolymer skeleton. The rigid cross-linked structures could fix the three-dimension structure andhinder the collapse of the polymers skeleton. Moreover, the cross-linked networks could promptthe formation of new pore in the hyperbranched polymimides. As a result, the cross-linkedstructures could affect the pore structure and surface area of the Cured-HBPIs more significantlythan the polymer skeleton. Therefore, the Cured-HBPIs possessed closed BET surface area andpore width (0.59~0.68nm,1.27nm). The Cured-HBPIs exhibited comparable CO2uptake(7.2~9.0wt%at273K1bar) and isosteric heat of adsorption to the other microporouspolyimides.In conclusion, through studying of the morphology, pore structure, adsorption behavior andproton conductivity of polymers containing cross-linked pendant groups, it is confirmed thatcross-linked networks could not only improve the thermal properties, mechanical properties ofpolymers, but also change the microstructure and endow the polymers with unique functionalproperties. For the important significance of cross-linking towards the structure and properties ofpolymer, the intensive research of crosslinking should be carried out in future. |
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