Synthetic Metals156(2006)
117–123
Blends of polyimide and dodecylbenzene sulfonic acid-doped polyaniline:Effects of polyimide structure on electrical
conductivity and its thermal degradation
Xuehong Lu a,∗,Jianwei Xu b,Limin Wong a
a School of Materials Science and Engineering,Nanyang Technological University,Nanyang Avenue,Singapore639798,Singapore
b Institute of Materials Rearch and Engineering,3Rearch Link,Singapore117602,Singapore
Received4May2005;received in revid form14September2005;accepted11October2005
Available online28December2005
Abstract
严嵩简介
This paper describes the effects of polyimide(PI)structure on electrical conductivity of blends of dodecylbenzene sulfonic acid-doped polyaniline (PANI-DBSA)and PI,as well as its thermal degradati
on behavior.Four types of PIs with different molecular architecture were synthesized and subquently solution blended with PANI-DBSA.Of the four types of PIs,4-4 -diaminodiphenyl sulfone(DADPS)-bad PI provides the highest conductivity to the blends.It is attributed to the rigid nature of DADPS,which may induce more extended conformation of PANI chains,and hence result in a more ordered structure.The conductivity of the blends has significant higher thermal stability than that of PANI-DBSA.The thermal stability is,however,independent on the polyimide structure.TGA studies show that the PI matrix may have hindered the thermo-oxidative degradation and evaporation of the dopants and thus slowed down the process of thermal degradation of the conductivity.
©2005Elvier B.V.All rights rerved.
管理学基础知识点整理Keywords:Polyaniline;Polyimide;Blend;Electrical conductivity;Thermal degradation
1.Introduction
Intrinsic conductive polymers have gained great attention since their discovery.The polymers offer the promi of achieving materials with electrical properties of metals or mi-conductors while retaining the attractive mechanical properties and processing advantages of polymers.Polyaniline(P
ANI)is one of the most intensively studied conductive polymers.It has demonstrated significant potential for technological applications due to its simple and economical production routes,and rela-tively high environment stability.The great potential of PANI is,however,masked by its rious disadvantages such as insolu-bility,infusibility and hence poor processibility.Attempts have been made to improve its processibility,of which the most widely adopted strategy is to dope PANI with organic acids con-taining long alkyl chains such as dodecylbenzene sulfonic acid (DBSA)and camphorsulfonic acid(CSA)[1–6].The conduc-∗Corresponding author.Tel.:+6567904585;fax:+6567909081.
E-mail address:asxhlu@ntu.edu.sg(X.Lu).tive complexes are,however,susceptible to heat treatment during polymer compounding process,particularly when the process-ing temperature is higher than180◦C.Our previous study has indicated that the thermal degradation of the conductivity of PANI-DBSA can be attributed to gregation of the dopants from the polymer backbones,cross-linking of the polymer backbones and degradation and evaporation of the dopants[7].
To improve thermal stability,as well as mechanical properties and chemical resistance,of the organic acid-doped PANI,they have been blended with a number of high-performance insulat-ing polymers to form conductive blends[8–11].Polyimide(PI) is a polymer with outstanding thermal and thermo-oxidat
ive stability,and excellent mechanical properties.Hence attention has been directed to formation of PANI/PI blends[10–14]. Moon and Seung prepared PANI-DBSA/polyamic acid(PAA) blends via blending PANI-DBSA with pyromellitic dianhy-dride(PMDA)and4-4 -oxydianiline(ODA)in a co-solvent and subquently obtained PANI-DBSA/PI blends through thermal imidization[11–14].Their studies showed that the PANI-DBSA/PAA blends exhibited relatively low percolation threshold value and high electrical conductivity compared
0379-6779/$–e front matter©2005Elvier B.V.All rights rerved. doi:10.1016/j.synthmet.2005.10.022
118X.Lu et al./Synthetic Metals 156(2006)117–123
to their PANI-CSA/PAA counterparts [11].The conductivity of the PANI-DBSA/PI blends also possd better thermal stability than the PANI-CSA/PI blends [12].The effects of the counter-ion and the underlying mechanisms were studied using X-ray photoelectron spectroscopy [12],small-angle X-ray diffraction [13]and electron spin resonance [14].
Among various types of PIs,hexafluorotetracarboxylic dian-hydride (6FDA)bad PIs showed outstanding properties.The fluorine atoms provide fluorinated PIs many attractive properties such as
excellent thermal stability,low water uptake,resis-tance to wear and tear,and chemical stability.6FDA-bad PIs have thus been widely ud in microelectronic industry.In this work,PANI-DBSA was blended with 6FDA-bad PIs to form PANI/PI conductive blends.Differing from the previous studies,the focus of this work is to investigate the effects of polyimide backbone,rather than counter-ion,on the electrical conductiv-ity of the blends and its thermal degradation behavior.Four types of diamines with different molecular architecture were ud as monomers for PI synthesis.The structural and morpho-logical differences between the blends induced by the variation of polyimide backbone structure and their impact on electrical conductivity behavior of the blends are reported in this paper.2.Experimental
2.1.Synthesis of DBSA-PANI
DBSA-PANI was prepared by oxidative emulsion polymer-ization of aniline in the prence of DBSA with ammonium
棉质面料peroxydisulfate as catalyst and xylene as solvent according to the procedure reported by Kim et al.[15].Aniline and ammonium peroxydisulfate were supplied by Lancaster.DBSA,xylene and acetone were from Fluka,Fisher Scientific and Aldrich,respec-tively.All chemicals were ud as received.PANI-
DBSA syn-thesized was dried in a vacuum oven at room temperature for 24h prior to further u.2.2.Synthesis of polyamic acids
Four types of polyamic acids were prepared by solution poly-merization of 6FDA (Aldrich)with 1,3-bis(4-aminophenoxy)benzene (1,3-BAPB;Tokyo Kai Kogyo),1,4-bis(4-aminophenoxy)benzene (1,4-BAPB;Tokyo Kai Kogyo),4-4 -oxydianiline (ODA;Aldrich)and 4-4 -diaminodiphenyl sulfone (DADPS;Aldrich),respectively,according to the procedure reported by Moon and Seung [11].The chemical structures of the monomers are given in Table 1.The solvent ud was N -methyl-2-pyrrolidone (NMP)supplied by Ana-lytical Lab-scan Sciences.The polyamic acid/NMP solutions obtained were stored in a refrigerator for further u.2.3.Blending and imidization
炫酷的图片The PANI-DBSA powder synthesized was dissolved in NMP at PANI-DBSA concentration of 2wt.%.The PANI-DBSA/NMP solution was then mixed with each of the four polyamic acid/NMP solutions,respectively,at an appropriate PANI-DBSA concentration and stirred for 5h to form homoge-
Table 1
Chemical structures of the monomers ud in preparation of polyimides Monomer
Abbreviation
Chemical structure
1,3-Bis(4-aminophenoxy)benzene
1,3-BAPB
1,4-Bis(4-aminophenoxy)benzene 1,4-BAPB
4-4
全国扶贫开发信息系统业务管理子系统
-Oxydianiline狮子王简笔画
ODA测光模式
4-4
-Diaminodiphenyl sulfone
DADPS
藿香正气胶囊
X.Lu et al./Synthetic Metals156(2006)117–123119
nous blend solutions.They were then poured into an aluminum mold and dried in vacuum at40◦C for approximately2–3days until all NMP were evaporated.The dried blends were then heated from40to150◦C at the speed of4◦C/min.Finally ther-mal imidization was conducted for3h in the temperature range of150–170◦C under high vacuum environment.The PANI/PI blends obtained were cooled in a desicator to room tempera-ture.After removing from the aluminum mold the blends were grinded into powder form.For each of the four types of the blends,the PANI-DBSA concentration was20,40and60wt.%, respectively.For simplicity,in this paper all the blend samples are named bad on the diamine ud for PI synthesis and their PANI-DBSA concentration.For example,the blends of PANI-DBSA and ODA-bad PI with PANI-DBSA concentration of 20,40and60wt.%are named as ODA/PANI20,ODA/PANI40 and ODA/PANI60,respectively.As-synthesized PANI-DBSA powder was also thermally treated under the imidization condi-tion to t a reference.All powder samples were compresd into discs of diameter of13mm and thickness of about1mm under a pressure of12tonnes at room temperature using a Graby Specac hydraulic press for subquent characterizations.
2.4.Thermal treatment
The discs were heat treated in air.The heat treatment consists of quickly raising the temperature of t
he samples in an oven and holding the samples at200◦C in air for0.5,1.0,1.5and 2.0h,respectively,and then cooling the samples back to room temperature in a desicator.
2.5.Conductivity measurement
A four-point probe system(Signatone SP4-62.5-85-TC)with a Keithley220programmable current source and Keithley2000 digital multimeter was ud to measure the electrical conduc-tivity of the discs according to the procedure reported in Ref.
[7].
2.6.Wide angle X-ray scattering(WAXS)
W AXS measurements on fraction of the discs were carried out using a Bruker AXS X-ray diffraction system with Cu K␣radiation.All samples were scanned from10◦to40◦2θ.
2.7.Scanning electronic microscopy(SEM)
The discs were manually fractured at its centre(where the conductivity measurements have been taken)and coated with gold.A scanning electron microscope(JEOL JSM5600)was ud to examine the fractured surfaces of the discs.The magni-fication ud was1000×.
2.8.Thermogravimetric analysis(TGA)
The TGA experiments were carried out on fraction of the discs using a TGA2850(TA instrument)thermogravimetric analyr.Temperature scans were conducted from50to800◦C at a heating rate of20◦C/min.Air and nitrogen were ud as purge gas,respectively,in the experiments.The samples were also heated at a ramping temperature of20◦C/min from room temperature to200◦C and then kept at200◦C over a period of 2h in air.
3.Results and discussion
3.1.Effect of polyimide structure on conductivity
The conductivity of as-synthesized PANI-DBSA was 2.0×10−2S/cm.It was decread to6.1×10−3S/cm after heat-treatment under the imidization condition.Fig.1shows that the conductivities of the PI/PANI blends containing60wt.% PANI-DBSA are all lower than the heat-treated PANI-DBSA reference.Of the four blends,DADPS/PANI60has the highest conductivity,followed by1,3-BAPB/PANI60,ODA/PANI60 andfinally1,4-BAPB/PANI60.The trend ems to coincide with the order of the rigidity of the diamines.DADPS has much straighter conformation than the other three diamines as the sulfone group is much more rigid than the ether li
nkage. The straight conformation of DADPS-bad PI is possible to induce more extended conformation of PANI chains,which would result in a more ordered structure and hence higher conductivity[16].Although1,3-BAPB,ODA and1,4-BAPB all have ether linkage(s),the steric hindrance caud by the meta substitution in1,3-BAPB may render it higher dynamic rigidity than ODA and1,4-BAPB.Both ODA and1,4-BAPB contain only para-substituted benzenes.1,4-BAPB has,however,two ether linkages,which is obviously moreflexible than ODA. The lowest conductivity exhibited by1,4-BAPB/PANI60may, therefore,be attributed to the greatestflexibility of1,4-BAPB. Molecular weight of the polyamic acids may also influence the conductivity of the blends.Although electron-withdrawing effect of thefluorine atoms renders6FDA relatively high reac-tivity(high electrophilicity)there are some differences between the four systems due to different dianhydride structures.The two aniline groups of DADPS are connected by a sulfone linkage,which is an electron-withdrawing group.The reactivity (nucleophilicity)of DADPS is,therefore,not as good as the other three diamines with ether linkages[17].The
DADPS-Fig.1.Electrical conductivity of the PI/PANI-DBSA blends containing60wt.% PANI-DBSA in comparison with that of the PANI-DBSA reference.
120X.Lu et al./Synthetic Metals156(2006)117–123
bad polyamic acid might,thus,have a lower molecular weight.The effects of the molecular weight on the conductivity are,however,fairly complicated.A lower molecular weight of the polyamic acid may enhance the conductivity of the blend due to the reduced entanglement and higher crystallinity,while it may have a negative impact on the conductivity due to the better miscibility between the polyamic acid and DBSA-PANI. The complicated effects will be addresd in future studies.
Fig.2shows X-ray diffraction patterns of the heat-treated PANI-DBSA and the four blends with60wt.%PANI-DBSA.In the X-ray diffraction pattern of the PANI-DBSA reference,there are three broad peaks at ca.2θ∼15◦,20◦and25◦,which are very similar to the ones obrved from polyacrylic acid-doped polyaniline[18].The structure may somewhat remble that of EBI-ESI reported by Pouget et al.[19].When PANI-DBSA is blended with PI two new peak appear at around22.4◦and 29.4◦2θ,respectively,which can be attributed to inter-chain d-spacing of PI crystallites[11].The two peaks are at higher angle than that obrved from unfluorinated PMDA–ODA polyimide, which indicates a more compact structure of the6FDA-bad PI.Among the four blends,DADPS/PANI60displayed slightly stronger PANI-DBSA diffraction he peaks at ca. 2θ∼15◦,20◦and25◦,than the other three blends,which verified its most ordered structure induced by the rigid DADPS gments. In contrast,in the diffraction pattern of1,4-BAPB/PANI60
the Fig.2.W AXD patterns of the PI/PANI-DBSA blends containing60wt.%PANI-DBSA in comparison with that of the PANI-DBSA reference.
peaks at ca.2θ∼20◦and25◦almost disappeared,which verified its least ordered structure and hence provided a strong justifica-tion for its lowest conductivity induced by the greatestflexibility of1,4-BAPB.
Fig.3shows SEM micrographs of the four PI/PANI60blends. It is striking to e that the morphology of DADPS/PANI60is very different from that of the other three blends.The
fractured Fig.3.SEM micrographs showing morphology of the fractured surfaces of(a)1,3-BAPB/PANI60;(b)1,4-BAPB/PANI60;(c)ODA/PANI60;(d)DADPS/PANI60.
X.Lu et al./Synthetic Metals156(2006)117–123
121
Fig.4.Electrical conductivity of DADPS-bad PI/PANI-DBSA blends as a function of PANI-DBSA concentration.
surface of DADPS/PANI60exhibits very coar morphology, as shown in Fig.3(d),while the other three blends show rela-tively smooth fractured surfaces,as shown in Fig.3(a)–(c).The rough and heterogeneous fractured surface of DADPS/PANI60 implies that pha paration occurred and both phas devel-oped relatively high crystallinity,which is the conquence of the high rigidity procesd by DADPS gments.In contrast, 1,3-BAPB/PANI60,1,4-BAPB/PANI60and ODA/PANI60may posss significantly lower crystallinity and less extent of pha paration.
3.2.Dependence of conductivity on PANI-DBSA concentration
Fig.4shows the dependence of the conductivity of the DADPS blend system on PANI-DBSA concentration.At 20wt.%PANI-DBSA the electrical conductivity of the blend could not be detected with the instrument ud.The perco-lation threshold,therefore,lies between20and40wt.%of PANI-DBSA.The relatively high percolation threshold value is likely to be caud by molecular level mixing of PANI-DBSA and6FDA-bad PI at low PANI-DBSA concentrations,which destructed the crystalline
structure of the conductive pha[13]. SEM micrographs of fractured surfaces of DADPS/PANI20, DADPS/PANI40and DADPS/PANI60(Fig.5)provided strong support to this claim.The fractured surface of
DADPS/PANI20Fig.6.Electrical conductivity of the PI/PANI-DBSA blends as a function of annealing time in air. , ,᭹, and reprent the PANI-DBSA reference, 1,3-BAPB/PANI60,1,4-BAPB/PANI60,ODA/PANI60and DADPS/PANI60, respectively.
is smooth and homogeneous showing no sign of pha para-tion.As the concentration of PANI-DBSA increas,the surface becomes more rough and heterogeneous.
3.3.Effect of annealing on conductivity
The effect of blending on thermal stability of the conduc-tivity can be en clearly by plotting log of conductivity as a function of annealing time,as shown in Fig.6.Before anneal-ing,the conductivity of the PANI-DBSA reference is higher than that of the blends.After annealing at200◦C for0.5h, the conductivity of the PANI-DBSA reference is reduced to an undetectable level while the conductivities of the four blends are only reduced slightly.The trend of very slow degradation of the conductivity for the blends was obrved until1.5h of annealing.After annealing for2h,the conductivity of all blends drastically dropped to an undetectable level.The results clearly demonstrated that blending of PI with PANI-DBSA can help to improve the thermal stability of the conductivity pha.There is,however,no clear trend showing that the thermal stability is dependent on the PI backbone structure.The slight fa
ster degra-dation rate of the conductivity of1,4-BAPB/PANI60is likely to be caud by its smaller initial size of conductive islands[18]. The conductivity of the other three blends showed nearly
the Fig.5.SEM micrographs showing morphology of the fractured surfaces of(a)DADPS/PANI20;(b)DADPS/PANI40;(c)DADPS/PANI60.
