Physicochemical Characterization of Polyaniline Bull. Korean Chem. Soc. 1999, Vol. 20, No. 3 333
Physicochemical and Electrical Characterization of Polyaniline Induced by Crosslinking, Stretching, and Doping
Kwang Sun Ryu,* SoonHoChang,Seong-GuKang,Eung Ju OhJ andChulHyunYo*
Electronics andTelecommunicationsResearch Institute, Taejon 305-600, Korea
* Department ofChemistry, Myong Ji University, Kyonggi 449-728, Korea
^Department of Chemistry, Yonsei University, Seoul 120-749, Korea Received November 10, 1998
The polyaniline films with variousinsoluble partsarefabricated. Theoxidation state(1-y) of these polyaniline is 0.53 and 0.54, respectively. To controlthe interchain and intrachaininteraction of the polymer, thepolya nilinefilmsarestretched with appropriateratio. The insolublepartof polyaniline synthesized at room temper
ature (lowmolecularweight) is 12%-76% andthat ofpolyaniline synthesized at 0 oC (intermediatemolecular weight) is 65%-89%.Thelowmolecular weight polyaniline films with various drawing ratios have amorphous structure.Inthe intermediate weight polyaniline films,thecrystallinity of films increaseswithdrawing ratio as well as insoluble part. The difference of the insoluble part affects electrical conductivity which is increased dra matically with draw ratio. Inparticular,the higher insolublepartcaused greaterincrease in electricalconduc tivity.
Introduction
The conducting polymer is unique type polymer, which has n-conjugatedelectrons spreadalong the polymer back
bone and has delocalized electron structure after doping.
Polyanilineisone of the most stable and popularconducting polymers.1 Polyaniline canbe classifiedas three kinds (leu- coemeraldine base, emeraldine base, and pernigraniline base) by its oxidation state.2,3 Since Angelopoulos et al.4 have found NMP(N-methyl-2-pyrrolidinone) as a goodsol vent, for emeraldinebase, many researchers make an effort to verify the intrinsic property of conducting polymers in processing. For instance, polyaniline film, fiber, elastomer and blendwere produced from the NMP solution, and the crystallinity, orientation ofpolymer chain, morphology, and conduction characteristics of these materials were investi gated.
The molecular weight and its distribution of polymer chains which canbecontrolledbysynthetic condition affect the mechanical properties of polymer.According to the pre
viousreports,theweight-average molecular weight (Mw) of polyaniline is about70,000whenit was synthesized at 0 oC5 and about 430,000 at -35 oC,6 respectively. The polymer solutionis prepared by dissolvingpolyaniline with various molecular weights in NMPand the filmscan be fabricated from polymer solutions. In the literature, whenpolyaniline films were stretched in equal ratio, the high molecular weight polyanilinefilm showed higher crystallinity and elec tricconductivity.7
MacDiarmid et al.2 systematically investigated thechange of electricconductivity of polyanilines by protonic acid dop ing. They presentedthe variation of electrical conductivity for emeraldinehydrochloride synthesized with chemical and electrochemicalmethods as a function of pH(concentration of HCl solution). The electricalconductivity of these poly
mers are about 1 S/cm at pH= -1-+1, and 10-10 S/cm at pH=5-6, respectively.Recently, it hasbeen reported7 that the electrical conductivity of polyaniline is largely affected by polymer chainalignment and the crystallinity formedbythe alignment ofpolymer stretching.The electricalconductivity ofpolyanilinefilms whichare stretched to fourfold increases from 5 S/cmto 170 S/cmafterdoping with 1 M HCl.
Inthe conductingpolymer,the fieldsof study and practi
cal applicationswere selected accordingtotheelectric con
ductivityof polymer.Therefore, inthisresearch,polyaniline powder with low and intermediate molecular weight are manufactured and filmswhich have different insolublepart were cast from the NMP solution of each sample. These films were stretched in appropriate ratio to transform their interchain or intrachain interaction. In order to findout the factors that affect electrical properties ofpolyaniline film, thechanges of thecrystallinity and electricalconductivity of polyaniline film were investigated. This investigation can contribute to developing polyaniline with higher electrical conductivity and processibility.
Experiment지
The detailed synthetic procedure to prepare polyaniline was followed the method which is reportedbyMacDiarmid et al. We synthesized low molecularweight polyaniline (LMW-PANI) powderusing moreoxidant thanstoichiomet ric amount atroomtemperature (30 oC). And the synthesis ofintermediate molecular weight polyaniline (IMW-PANI) powder was proceeded at 0 oC. The rest of the steps are same.
We prepared several films with various insolubleparts by dissolving polyaniline powder in NMP, pouring it on the glasssubstrate (14cmx14cmx0.5 cm) slowly, and dryingit in theovenat 75 oC. Forchanging thealignmentof polymer
334 Bull. Korean Chem. Soc. 1999, Vol. 20, No. 3 Kwang-Sun Ryu et al.
chain in the film, the cast film was stretched to fourfold at 100-150 oC. The stretched filmsweresoaked into HCl solu tion for 48 hours to accomplish theprotonic acid doping.
The oxidation state of polyaniline can be determined by relativeconcentrationsof benzenoidringandquinoidringin the polymer chain. For obtaining the concentrations of two basic units, the polyaniline powder sample and excess amount of the Ti3+ solution (TiCl3, the concentration is known) whichconverts quinoid ring into benzenoid ringin thepolyanilinearemixedinaceticacidsolvent. Thereaction whichisreducingqunoidring to benzenoidring is occurred.
After the reaction,theextra Ti3+solutioncanbeback-titrated with Fe3+ solution(FeNH4(SO4)2 -12H2O, the concentration isknown). In here, Fe3+ which is changed to Fe2+ converts Ti3+ into Ti4+). When Ti3+ are all consumedinthistitration, the extra Fe3+ does not react further but displays colorby indicator(5%NaSCN).From this,wecouldfind theamount of Ti3+ solutionconsumedinthe reaction. Then,wecalculate the amount ofquinoidring and verify the oxidation state of synthesizedpolyaniline.
In order to analyze the insoluble part of the films from solution casting methods, reference sample and measuring sample are prepared. We measured the initialweight ofthe sample which is corrected by solvent content of reference sample.Theinsoluble part of thefilm was determined asfol lows; i) The measuring sample film putintoNMP. ii) The solublepart of filmisdissolvedslowly inNMP. iii) The dis solutionof the film is proceeded until the color of NMP is not changed. If somesoluble part ofthe film remained,the color ofsolution is blue. iv) After dissolution of film, the sample film was taken out from NMP and rinsed with CH3OH. v) Afterthe filmwasvacuum dried for 12 hours, wemeasuredthefinalmassofthesample which iscorrected by solventcontentofthis film. The solventcontentof films aredetermined byTGA at room temperature ~300 oC with scanrate 5 oC/mininN2 atmosphere.Finally, we can calcu latethe insolublepartfromtheratioofinitialandfinalmass ofsample.
To examine the degree of alignment and crystallizing of polymerchains, we used Philips PW 1825/00 X-raydiffrac tometerwith Ni-filtered Cu Ka radiation (冗=1.5418A) by 0.05o/sec scan in 5o < 20 < 35o. The electrical conductivity was examinedbyfour-probemethod. The potentialand cur
rent were measured independently in orderto remove the contactresistance between platinumelectrodeandsample.
ResultsandDiscussion
Polyaniline can be classified asfollowby its oxidationstate.
Where 1-y=0.0 is leucoemeraldine base, 1-y=0.5 is emeral- dinebase, and 1-y=1.0 is pernigranilinebase. The analysis ofoxidationstate(1-y)for polyaniline, which wascalculated bythe amount ofquinoid ring,showed that oxidationstate of low molecularweight polyaniline is 0.53 and thatof inter
mediatemolecularweightis0.54. In general,thepolyaniline synthesizedatN2 atmospherehasoxidation state around 0.4 and that synthesizedatair has almost 0.6. Here, the reason for 1-y having bigger value than 0.5 wouldbethe fact that it wasmadeinair.
To calculate the insoluble part for the films which were casted with NMP solutions of various weight percentage, thisequationisused.
Percentageofinsolublepart (%) (Wfinal-W(samp.)solution)
=---X 100 (W1n1t1al-W(ref.)solut1on)
WhereWinitial isinitialweightof the samplefilmandWfinal
is final mass ofthe sample film afterdissolving thesoluble part. W(ref.)solution is the solvent content of reference films which aredeterminedbyTGAandW(samp.)solution isthesol vent content of sample films after dissolving the soluble part.The results ofthis analysis are shownin Table 1. From this result,the insoluble part increases with weightpercent- age(wt%) of emeraldine base dissolved in NMP solution. If theweightpercentageoffilmis same, lowmolecularweight polyaniline has lowerpercentageof insoluble part than the intermediatemolecular weight one. The lowmolecularweight polyaniline has less opportunity to form insoluble part bet
ween thechainsbecauseit has the shortpolymerchain.
We investigated the correlationfor the insolublepart, crys
tallinity,andmolecular weight usingX-raydiffractionanaly sis. Figure 1-(a) and (b) showthe X-ray diffractionpatterns of the films which areprepared with low and intermediate molecular weight polyaniline with different percentage of insoluble part.Both low and intermediate molecularweight polyaniline have amorphous structure. Only the area of broadpeakdiffersfromeach other. Figure2-(a)showsX-ray diffractionpatterns ofthelowmolecularweightpolyaniline filmpreparedbyusing 11 wt%EB-NMP solutionwithvari ous drawingratio.Although thedraw ratioof thepolyaniline films gets larger, the amorphous structure of the film main
tained,butthe area ofthepeaksischanged.Figure2-(b)and (c) show the X-ray diffraction patterns of the intermediate molecularweight polyanilinefilmsprepared with 3 wt% and 11 wt% EB-NMPsolution. In these films, the crystallinity increases with the draw ratio. As mentioned above, all the low molecular weight polyaniline films have amorphous structure in spite ofincreasingthe draw ratiooffilms. How
ever, the intermediate molecular weight polyaniline films show large increase of crystallinity as the drawratio gets larger with increasing percentage of insoluble part.In addi-
Table 1. The percentage of insoluble part of various polyaniline films with different molecular weight
Sample Low molecular weight Intermediate molecular weight EB/NMP(wt%)a 3wt% 5wt% 11wt% 3wt% 5wt% 11wt%
Percentage of
insoluble part(%) 12.59" 55.29 76.34 65.38 80.30 88.91 aEB is emealdine base andNMP is Mmethyl-2-pyrrolidinone. "±5%
deviation.
Physicochem ical Characterization of Polyaniline Bull. Korean Chem. Soc. 1999, Vol. 20, No. 3 335
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5 10 15 20 25 30 35
5 10 15 20 25 30 35
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Figure 1. X-ray diffraction patterns of polyaniline films prepared from various weight percentage of NMP solution. (a) low molecular weight, (b) intermediate molecular weight.
tion, we find that the films with large insoluble part also have large increasingeffectof crystallinity. Fromthese results, the low molecular weight polyaniline film is expected to have little change ofphysical properties during the stretch ingoffilm.
In general, stretching of polymer film orients the polymer chain in drawing direction. This means the change of its array from three-dimensional structure (coil-like structure) to one-dimensional structure (rod-like structure). The one dimensional array of polymerchain bystretching has more regulation in its structure. Thisregulationcausesmicrocrys tallinedomains9), which makes partialcrystallinity in X-ray diffractionpattern. If theinsoluble partcan becontrolled, the film has better crystallinitywith increasingthe draw ratio.
However, the low molecular weight film maintains amor phouspattern. This implies thatthe degree of crystallinity is related to the insoluble part and its molecular weight. In intermediate molecular weight polyaniline with long poly
mer chain, the insolublepartacts as anuclearformation site which increasesthecrystallinity of the filmthrough stretch ing. However, in low molecular weight polyaniline with short polymerchain, the connection between the chains is noteffective to maintain the amorphous pattern.
To investigate the relation of the insoluble part and the electrical properties of films with various draw ratios, we
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Figure 2. X-ray diffraction patterns of polyaniline films with various draw ratios. Films were cast from (a) 11 wt% low molecular weight, (b) 3 wt% and (c) 11 wt% intermediate molecular weight poly
aniline in NMP.
Table 2. The electrical conductivity of polyaniline films with several draw ratios
Sample EB/NMP wt % L/Lo o (S/cm) at 298K
3 wt%a 1 16.5
1 16.8
Low 5 wt% 2 20.2
Molecular 3 31.8
Weight 1 17.8
11 wt% 2 30.5
3 33.4
1 17.1
3 wt% 2 40.2
3 73.9
4 114.6
Intermediate
5 wt%
1 2
17.7
Molecular 3 42.194.4
Weight 4 123.4
1 19.5
11 wt% 2 44.6
3 105.1
4 150.9
aThe low molecularweight 3wt% film is not elongated.
336 Bull. Korean Chem. Soc. 1999, Vol. 20, No. 3 Kwang-Sun Ryu et al.
Figure 3. The relationship among the anisotropy of electrical conductivity, weight percentage, and draw ratio in the films. (a) low molecular weight, (b) intermediate molecular weight.
molecular weight polyaniline maintains the amorphous structure notwithstanding the small electrical conductivity
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change.From these results, the site wheretheinsolublepart is formed in polymer acts as the nuclear formation site. It increases the crystallinityupon stretching, and the polymer arranges as interlocking knuckle in small crystallinity domain. Thus the electrons as charge carriers, can be hop pingbetween them.
In order to investigate the correlationamong the anisot ropyofelectrical conductivity, the insolublepartinthe film, and the film draw ratio, we measuredthe roomtemperature electrical conductivity of polyaniline film inparallel direc tion (5) (direction in which the film was stretched) and per
pendicular direction (o
丄
)(direction perpendicular tothe one in which the filmwas stretched). Theresults are shown in Figure 3-(a) and (b). While o丄
increases little, 이 increases dramatically as the drawratio increases. The measurement of the electrical conductivity in this direction gives great electrical conductivity value due to the improvement of mobilityof charge carrierin the intrachain.Meanwhile, the measurement of electrical conductivity in perpendicular direction gives almost same value as initial state although the distance of interchainis closed.Becausethe charge carrier mobility of paralleldirection in the polymer chains is main factorin increase of electric conductivity. In addition, intermediatemolecularweight polyanilinefilm was found to havelargerconductivityratio (이/o
丄
) than thelow molecular weightone.References measured D.C. electrical conductivity. Table 2 shows the
electrical conductivityatroomtemperature forlow molecu
lar weight and intermediate molecular weight polyaniline films dopedwith 1 M HCl, which havedifferentpercentage ofinsolublepart. When thefilmis not stretched, the electri cal conductivity is slightly increased as the insoluble part increases. In case of stretching the film, the electrical con
ductivityincreasesdrastically with the draw ratioand insolu
ble part. Therefore, the insoluble partitself does not affect theelectricalconductivityofunstretched film, but it playsan important rolewhen the film is stretched to orient the poly merchain. In addition,theincrease of electricalconductivity implies that the changes in the polymer chain structure can varytheelectricalproperties through thetransform process
ing.
The X-raydiffraction analysis and conductivity measure
ment indicate that the stretching of polymer film causes to orient the polymer chain in draw direction, which enlarges theregulation ofstructure and makes partial crystallinityin thepolymer film. However, the filmwhich has thelow per
centage of insoluble part or which was prepared from low
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