Synthesis And Characteration Of Soluble And Transparent Polyimides Containing Tert-butyl With High Glass Transition Temperature

As a kind of high-performance polymers,polyimides possess excellent themal stability,mechanical properties,insulated properties and have been widely applied in many high-tech fields,including space flight,optoelectronic technology,composite materials and so on.However the applications of most traditional polyimides were limited because of suffering from their inherent defects,such as poor solubility(rigid backbone),deep color and low optical transparency(Charge-transfer complex,CTC)etc.Thus various approaches have been adopted to modify the chemical structrue of polyimides and improve their solubility and transparency.But some of them(for example,introduction of flexible linkages,long alkyl chains etc.into polyimides)would damage their thermal stability and decrease their glass transition temperatures.As an effective strategy,introduction of bulky side groups into polyimides can resolve these problems to a certain extent.As a typical bulky side group,the introduction of tert-butyl has significant and distinctive effects on the physical properties of polyimides,including improving their solubility and optical properties without sacrificing the inherent thermal properties.In this dissertation,we synthesized a series of linear and hyperbranched polyimides containing tert-butyl side groups,and the effects of tert-butyl on the synthesis and properties of polyimides were systematically studied.The main contents and conclusions are shown as follows:1.Study on the effects of the tert-butyl number on the properties of polyimidesTwo novel tert-butylated diamines,3-tert-butyl-4,4’-diaminodiphenyl ether(1)and 3,3’-di(tert-butyl)-4,4’-diaminodiphenyl ether(2),with the same molecular backbone and different numbers of tert-butyl side groups were prepared with the Ullmann coupling reaction.They were polymerized with some commercial aromatic dianhydrides to afford two series of polyimides with different numbers of tert-butyl per repeating unit.We characterized these polyimides by NMR,GPC,DSC,TGA,XRD,FTIR and UV-Vis spectra etc to study the effects of the tert-butyl number on the properties of polyimides.From the results we could conclude that,the solubility and transparency of polyimides increased with the number of tert-butyls,while the increase of the number of tert-butyls would slightly decrease the thermal stability of polyimides.It seemed that the number of tert-butyls had little effect on the mechanical properties of polyimides.In addition,the glass transition temperature of the two series of polyimides also increased with the number of tert-butyls.2.Synthesis and characteration of soluble polyimides with high glass transition temperature from benzidine containing tert-butyl groupsA novel diamine monomer 3 was synthesized by introducing tert-butyl into benzidine,and several commercial aromatic dianhydrides were polymerized with 3 to afford a series of linear polyimides.The number-average molecular weights of these polyimides were in the range from 4.21 × 104 to 1.25 × 105 with the polydispersity indexes(PDIs)from 2.34 to 3.04 by means of gel permeation chromatography(GPC)relative to a polystyrene standard.All these polyimides could be dissolved in common organic solvents and the corresponding flexible and tough films could be obtained by the solution-casting.The XRD results indicated that they were amorphous materials.All these polyimides possessed high glass transition temperature above 330 oC except the 3-BPADA,and especially the Tg of 3-BPDA could reach up to 375 oC.All of them were stable up to 450 oC under a nitrogen atmosphere,which indicated that the introduction of tert-butyl could improve their solubility,but not obviously decrease their thermal stability.3.Synthesis and characteration of soluble and transparent polyimides from a novel diamine containing tert-butyl and fluorene unitsA series of polyimides containing tert-butyl and fluorene units were prepared through the polymerization of several commercial aromatic dianhydrides and a novel diamine monomer 4,which was synthesized through the condensation reaction of 9-fluorenone and 2-tert-butylaniline.The number-average molecular weights of these polyimides ranged from 4.54 × 104 to 8.82 × 104 with the polydispersity indexes between 2.51 and 4.33 by means of GPC relative to a polystyrene standard.All the polyimides displayed high solubility in common organic solvents and tough films could be prepared by solution-casting.These films(about 15 μm thickness)exhibited high optical transparency above 90% in a wavelength range of 400-700 nm by UV-Vis measurement,which were far higher than the traditional aromatic polyimide PMDA-ODA.And the colors of them were also relatively lower than that of the PMDA-ODA.In conclusion,the introduction of the side groups tert-butyl and fluorene unit into polyimides could enhance the optical transparency of polyimide films,which guarantee them as potential candidates for optical applications.4.Application of stereo-hindrance effect of tert-butyl in the synthesis of hyperbranched polyimidesA novel B’B2-type triamine,N1,N1-bis(4-aminophenyl)-4-(tert-butyl)benzene-1,3-diamine(5)was synthesized through the introduction of tert-butyl into the ortho positon of 3-amino of N1,N1-bis(4-aminophenyl)benzene-1,3-diamine.Due to the stereo-hindrance effect,the reactivity of 3-amino group in triamine 5 with the ortho-tert-butyl group is much lower than those of 4’/4’’-amino groups at 20 oC.Therefore,when 5(B’B2)was polymerized with dianhydrides(A2)with the molar ratio of 5:dianhydrides 1:2 at 20 oC,the dianhydrides would react with the 4’/4’’-amino selectively to form the AB2-type intermediate in situ.And with increasing the reaction temperature,3-amino group with ortho-tert-butyl group is activated and polycondensed with anhydride groups in AB2-type intermediates rapidly to afford the hyperbranched polyimide(HBPI)with terminal anhydrides and high DB.As expected,no gelation is formed in the polymerization process.The number-average molecular weights of these HBPIs ranged from 2.89 to 3.91 ×104 with PDIs from 2.10 to 2.27 and DBs from 0.39 to 0.45.All the HBPIs displayed excellent solubility and thermal properties.