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Thermal property of regioregular poly(3-hexylthiophene)/nanotube composites using modified single-walled carbon nanotubes via ion irradiation

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Thermal property of regioregular poly(3-hexylthiophene)/nanotube composites using modified single-walled carbon nanotubes via ion irradiation
  This content has been downloaded from IOPscience. Please scroll down to see the full text.Download details:IP Address: content was downloaded on 27/02/2014 at 21:18Please note that terms and conditions apply. Thermal property of regioregular poly(3-hexylthiophene)/nanotube composites using modifiedsingle-walled carbon nanotubes via ion irradiation View the table of contents for this issue, or go to the  journal homepage for more 2006 Nanotechnology 17 5947( usMy IOPscience  I NSTITUTE OF  P HYSICS  P UBLISHING  N ANOTECHNOLOGY Nanotechnology  17  (2006) 5947–5953 doi:10.1088/0957-4484/17/24/008 Thermal property of regioregularpoly(3-hexylthiophene)/nanotubecomposites using modified single-walledcarbon nanotubes via ion irradiation A R Adhikari 1 , M Huang 1 , H Bakhru 1 , M Chipara 2 , C Y Ryu 3 andP M Ajayan 4 1 College of Nanoscale Science and Engineering, State University of New York, Albany,NY 12203, USA 2 Department of Physics and Geology, University of Texas Pan American, Edinburg,TX 78541-2999, USA 3 Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy,NY 12180, USA 4 Department of Material Science and Engineering, Rensselaer Polytechnic Institute, Troy andRensselaer Nanotechnology Center, NY 12180, USAE-mail: Received 28 June 2006, in final form 4 October 2006Published 22 November 2006Online at Abstract The effects of radiation-induced modifications on the thermal stability andphase transition behaviour of composites made of 1% pristine or ionirradiated single-walled carbon nanotubes (SWNTs) andpoly(3-hexylthiophene) (P3HT) are reported. Thermogravimetry analysis(TGA), differential scanning calorimetry (DSC), Raman spectroscopy andelectron spin resonance (ESR) were used to investigate the radiation-inducedfunctionalization of carbon nanotubes and to assess the effect of ionizingradiation on the adhesion between macromolecular polymer and carbonnanotubes. Irradiation was used to introduce defects in a controlled waysolely within pristine nanotubes before composite synthesis. The addition of irradiated SWNTs to a polymer matrix was found to enhancethermo-oxidative stability and phase transition behaviour. Further, ESRstudies demonstrate the electronic interaction through charge transferbetween filler and matrix. These results could have immense applications innanotube composite processing. Based on the experimental data, a model forthe interaction between polymeric chains and carbon nanotubes is proposed. 1. Introduction Since the discovery of integrated circuits (ICs) in 1960 [1],portable microelectronics has revolutionized many aspects of our lives from entertainment equipment to computers. Theresearch then geared to make an innovative discovery forsmaller, lighter, and cheaper materials required for complexICs. In addition to silicon-based semiconductor technology,organic conducting and semiconducting polymers have shownpromise for future applications in organic optoelectronics aswell as flexible large area displays [2, 3]. However, various properties of conductingpolymers such as charge mobility andelectricalconductivity need to be improved in order to achievethe performance level of electronic devices based on inorganicamorphous silicon.Regioregular P3HT is one of the most studied conductingpolymers due to its high solubility (solution processability),thermal features (fusibility) and environmental stability [4]. The supramolecularstructureof P3HT includes intermolecular π – π  stacking of conjugated backbones [5, 6], which is considered as one of the reasons behind the excellentperformance of conjugated polymers (e.g. higher electrical 0957-4484/06/245947+07 $ 30.00  ©  2006 IOP Publishing Ltd Printed in the UK  5947  A R Adhikari  et al Table 1.  Ion irradiation conditions calculated by using TRIM. Note that d  E  / d  x   values correspond to the surface values.CarbonDose Energy Range  ( d  E  / d  x  ) e  ( d  E  / d  x  ) n  ( d  E  / d  x  ) n ( d  E  / d  x  ) e   vacancies (˚A / ion)Ion  ( # cm − 2 )  (MeV) ( µ m) (eV ˚A − 1 ) (10 − 3 eV ˚A − 1 ) maximum value 1 H 1 × 10 15 0.4 5.36 5.48 4.51 0.82 45 × 10 − 54 He 1 × 10 15 1.2 5.34 23.3 24.3 1.04 0.007 10 Ne 1 × 10 14 5.75 5.36 163.5 549 3.36 0.1 conductivity, excellent nonlinear optical properties, and ionsensing ability) [7–9]. Carbon nanotubes (CNTs) haveextraordinary electrical, optical, thermal and mechanicalproperties [10]. These novel features stimulated a broad interest in using CNTs as reinforcement, conductingfiller and thermal management for different types of polymers [11–13]. The incorporation of CNTs in polymerhas demonstrated enhanced transport properties required forelectronic device applications such as organic light-emittingdiodes (OLEDs) [14, 15]. The most sensible issue for obtaining composite materials for such applications is theproper dispersion of CNTs and the control over the interfacialinteraction between CNTs and polymer [16, 17]. Understanding the interactions between polymers andnanotube is still a crucial issue. A better understanding of these interactions will allow the optimization of polymer–nanotubes composites and will result in composite materialswith improved performances. Up to now, most studieson the polymer–nanotube interface have been focused onthe chemical grafting of organic or polymeric moietiesonto CNTs [18]. This was used to enhance the dispersion of nanotubes in polymer matrices and to promoteenhanced polymer–nanotube adhesion. The ion irradiationtechnique offers an alternative strategy for modifying CNTs.Many studies on radiation-induced modifications in polymercomposites have been reported [19–21]. The energy depositedby the incident particleinduces chemical and physical changesalong the incident particle’s trajectory in the target [22]. This scenario is even worse in the case of polymers, as theirradiation affects the average molecular mass of the pristinepolymer and finally shifts its glass, melting and crystallizationtemperatures [23]. We have reported [24] that ion implantation enhanced the thermal stability of SWNT thin films (by about 30 ◦ C afterhydrogen ion implantation and about 17 ◦ C after helium ionimplantation). This behaviour showed that ion irradiationcould be a possible route to introduce defects so as totailor the properties of nanotubes. Besides, the interactionof macromolecular chains with CNTs and the dispersionof CNTs within polymers depend on the nature of thenanotubes’ outermost surface. Therefore, an ion irradiationtechnique was used in the present study to introduce defectson nanotube surfaces before composite formation. Unlikechemical modification using ‘wet chemistry’, the dry processof ion irradiation allows the modification of pre-dispersedCNTs on dielectric surfaces prior to solution casting or inkjetprinting of semiconducting polymers.This paper focuses on the investigation of thermalstability and phase transition behaviour of composites withimplanted and non-implanted SWNTs using TGA and DSCin a non-isothermal mode. Further, the evolution of defectswas monitored using Raman spectroscopy and electron spinresonance (ESR). 2. Experimental section SWNTs were purchased from Carbon NanotechnologyIncorporated (CNI) and utilized as received without furtherpurification to avoid introducing additionaldefects. Accordingto the manufacturer the purity of the as-received SWNTswas about 90%. However, our TGA measurements showeda purity of about 81%. Three sets of nanotube thin films( ∼ 5 . 5 µ m) were prepared and irradiated with hydrogen,helium or neon ions. Table 1 shows details of the ion implantation conditions employed. The conducting polymer,P3HT, waspurchasedfromAlfaAesar. Itischaracterizedbyanaverage molecular weight of about 70000. P3HT was purifiedby extracting low molecular weight impurities using acetonewith a Soxhlet [25]. The composite was prepared by mixingappropriate quantities of pristine or ion implanted SWNTs andP3HT in chloroform, at room temperature. The solution wasextensively ultrasonicated using a Branson sonicator 2510 forabout an hour after each addition. Solid composites containing1% SWNTs by weight were obtained after precipitation inmethanol. The residue was assigned to SWNTs coated withP3HT. Then, these composites were vacuumed and treated for24 h at room temperatureto remove the solvent. The followingabbreviations will be used for composites in the rest of thediscussion: XNT-P3HT for composite of SWNTs and P3HTwhere XNT represents pristine (P) and hydrogen (H), helium(He) or neon (Ne) irradiated SWNTs, respectively.The thermal stability of these composites in air wasanalyzed by TGA (Mettler-Toledo TGA/SDTA851e) inthe temperature range 50–800 ◦ C, with a heating rate of 10 ◦ C min − 1 . The oven was purged with constant air-flow (50 ml min − 1 ) during these experiments. Compositeswere also investigated using DSC, TA Instruments DSC-2920. The samples were preheated from  − 20 to 260 ◦ C ata rate of 10 ◦ C min − 1 and cooled at the same rate. Prior tocrystallization, the samples were kept at 260 ◦ C for 10 minto erase the previous thermal effects. All reported DSC datawere the second heating and second cooling cycle. Samplesof about 5 mg were used in all measurements. Ramanscattering measurements were performed using a RenishawRaman microscope, where the polarized Ar ion laser beam hasan excitation wavelength of 514.5 nm and 24 mW power. Theback-scatteredradiationwas then collectedand analyzedusinga double-grating monochromator. To further understand thelattice defects and radiation damage, ESR measurements wereperformed on pristine polymer and its composites by using aBruker ESR-300 spectrometer, operating in the X band (8–10 GHz), equipped with NMR magnetic field controller andvariable temperature accessory.5948  Thermal property of regioregular poly(3-hexylthiophene)/nanotube composites using modified SWNTs via ion irradiation 0100200300400500600    P  e  r  c  e  n   t  a  g  e  o   f   i  n   i   t   i  a   l  m  a  s  s 020406080100120    d  m   /   d   T AB 12345    T    A    (   o    C   ) 450460470480490    T    B    (   o    C   ) 5405505605705801 - (P3HT)2 - (PNT-P3HT)3 - (HNT-P3HT)4 - (HeNT-P3HT)5 - (NeNT-P3HT) Temperature ( ° C) Figure 1.  TGA profile of P3HT and its first derivative plot, showingtwo stage decompositions A and B. The inset shows thecorresponding peak shift with the morphology of SWNTs in thecomposites. 3. Results and discussion Figure 1 shows the typical TGA thermogram of P3HT asmeasured in air. It is observed that the mass loss takes placein a two-step mechanism. The first step of the mass loss beganat about 350 ◦ C and the second step began at about 510 ◦ C.Such an oxidationat high temperatureobviouslycannot be dueto physisorbed species. We suggest that the beginning of themass loss is due to the loss of an alkyl side group attachedto the aromatic thiophene backbone (here a hexyl group).As the temperature increases above 500 ◦ C, the oxidationis accelerated and the pyrolysis of the aromatic backboneof polymer chains is ignited. TGA analysis monitored thepeak positions (A and B—see figure 1) for all samples asshown in the inset of figure 1. The position of A remained fairly unchanged for all sample although a weak trend of decreasewas observedfor samplescontainingheliumandneonimplanted SWNTs. Peak B shifted to a higher temperature(by about 10 ◦ C) upon the addition of pristine SWNTs. Thisenhancementofthethermalstabilityofcompositesisattributedto the adsorption of P3HT on thermally stable SWNTs. Thismay suggest that the aromatic thiophene backbones are tightlyadsorbed, more than the side alkyl group [26]. This peak is further shifted to higher temperature (by about 10 ◦ C) forhydrogen irradiated SWNTs, and began to decrease for othercomposites containing helium and neon implanted SWNTs.This experiment further showed that both the pristine and theirradiated filler interact stronger with the polymer backbonethan with the side group of P3HT.Irradiation generates defects within CNTs, representedpredominantly by localized and delocalized uncoupledelectrons. This could enhance the interaction with the  π electrons of the organic polymer and even the bonding (if theuncoupledelectronsbelongingto CNTs arecloseenoughtothe π  electronsof P3HT). Theincreaseofthetemperatureatwhichthe mass loss rate is maximum after dispersion of 1% CNTswithin P3HT indicates an enhanced thermo-oxidative stability,triggered by interactions between the conducting polymer andCNTs. The thermo-oxidativestabilityis further enhanced afterpristine CNTs were replaced by H irradiated CNTs, reflectinga stronger interaction between conducting polymeric chainsand nanotubes. When the mass of the incident particle wasincreasedfromhydrogentoneon,adecreaseofthetemperatureat which the mass loss rate is maximum has been observed. Asheavierincidentparticlesdeposit more energywithin the target(CNTs), consequently, a larger number of defects is expectedin CNTs. The large numbers of defects in turn decreasesthe thermal stability of the composites. We shall discuss theprobablephenomenonresponsibleforsuchachangeofthermalstability using the model below.To further elucidate the thermal properties of these com-posites, DSC was employed to monitor the thermodynamicsof composites across melting and crystallization phase transi-tions. Figure 2 shows the thermograms for the melting (fig-ure 2(a)) and crystallization(figure 2(b)) of pristine P3HT and its composites. The melting curve (figure 2(a)) of the pris-tine P3HT showed a single peak located at 231 . 3 ◦ C. No sig-nificant shift of the peak position and enthalpy was observedfor other composites excepting for the neon implanted sam-ple (figure 2(c)). Noticeable modifications related to the crys-tallization of these composites were observed in the coolingcurve (figure 2(b)). The cooling branch of the DSC spectrumof P3HT showed two distinct crystallization peaks located at192.6 and 198 . 6 ◦ C, respectively, indicating the formation of two crystals. The addition of carbon nanotubes induced thecrystallizationof P3HT athighertemperaturesthan thepristineP3HT, suggesting strong interactions between molten P3HTchains and CNTs. This trend is preserved for hydrogen andhelium implanted nanotubes (figure 2(d)) to support that the CNTs and the defects located on H and He implanted CNTsact as nucleation centres, thus facilitating the crystallizationprocess. This showsaneasycrystallizationforcompositescon-taining hydrogen or helium irradiatednanotubes. However, forneon implantednanotubesthe crystallizationpeak positionandthe corresponding enthalpy are shifted to lower temperatures.This suggests restricted chain motions due to the interactionof macromolecularchains with CNTs. This also demonstratestheclear decrease in interaction between polymer and CNTs.The relative degree of crystallinity (  X  T  ) (see figure 3(a)) has been estimated from DSC data by using equation [27]  X  T   =    T T  0  d  H  d t   d t     T  ∞ T  0  d  H  d t   d t  (1)where  T  0  and  T  ∞  are the initial and final crystallizationtemperatures, respectively, and d  H  / d t   is the rate of heatevolution. All the curves are found to be sigmoidal inshape. In non-isothermal crystallization, the abscissa of temperature can be related to time through the relation, t   =  ( T  0  −  T  )/β , where  β  is the cooling rate and  t  is the corresponding time at the particular crystallizationtemperature  T   (figure 3(b)). From figures 3(a) and (b), it is observed that the full crystallization is achieved ina shorter time for composite compared to P3HT in theorder of P3HT  >  PNT-P3HT  >  HNT-P3HT  >  HeNT-P3HT.However, the crystallization process is delayed for compositescontaining neon implanted nanotubes. Accordingly, P3HTcrystallizesfasterforcompositecontaininghydrogenorheliumimplanted nanotubes compared to the composite containingneon implanted nanotubes. This tunable behaviour is due toirradiation-induceddefects.Raman spectroscopy was utilized to analyze the defectsinducedby irradiation. The Raman spectrumof the as-received5949  A R Adhikari  et al Temperature ( o C) 180 200 220 240 260    E  n   d  o (a) P3HTPNT-P3HTHNT-P3HTHeNT-P3HTNeNT-P3HT Temperature ( o C) 160 180 200 220    E  n   d  o P3HTPNT-P3HTHNT-P3HTHeNT-P3HTNeNT-P3HT (b) Sample # 1 2 3 4 5    T  m   (   o    C   ) 224226228230232234236       ∆    H  m   (   J   /  g   ) 1112131415161718 1 - (P3HT)2 - (PNT-P3HT)3 - (HNT-P3HT)4 - (HeNT-P3HT)5 - (NeNT-P3HT) (c)Sample # 1 2 3 4 5    T  c   (   o    C   ) 186192198204       ∆    H  c   (   J   /  g   ) 8101214161820 1 - (P3HT)2 - (PNT-P3HT)3 - (HNT-P3HT)4 - (HeNT-P3HT)5 - (NeNT-P3HT) (d) Figure 2.  Dynamic DSC thermograms at a heating rate of 10 ◦ C min − 1 of various samples (a) and the corresponding cooling curves at thesame cooling rate (b). (c) and (d) demonstrate the effects of filler morphology on the enthalpy and peak temperature for the melting andcrystallization, respectively. Temperature ( o C) 160 170 180 190 200 210 220    X   T (P3HT)(PNT-P3HT)(HNT-P3HT)(HeNT-P3HT) (NeNT-P3HT) Time (min) 1.0 1.5 2.0 2.5 3.0 3.5 4.0    X   T  (P3HT)(PNT-P3HT)(HNT-P3HT)(HeNT-P3HT) (NeNT-P3HT) (a) (b) Figure 3.  Plots of the relative degree of crystallinity (  X  T  ) as a function of temperature (a) and time (b) for pristine P3HT and their composites.(This figure is in colour only in the electronic version) Raman shift (cm -1 )0 500 1000 1500 2000    I  n   t  e  n  s   i   t  y   (  a .  u .   )   I  n   t  e  n  s   i   t  y   (  a .  u .   ) RBMD-bandG-band    I   D   /   I   G HNT HeNT NeNT Raman shift (cm -1 ) 100 150 200 250 300 35002x10 3 4x10 3 6x10 3 8x10 3 PNTHNTHeNTNeNT (a) (b) Figure 4.  (a) Raman spectra of as-received SWNTs with inset figure for schematic variation of   I  D /  I  G  and (b) the plot of RBM mode of thecorresponding pristine and ion irradiated nanotubes. nanotube film is shown in figure 4(a). The characteristics Gand D peaks are indicated. The G peak is due to the in-planevibrational movement of carbon atoms for both bending andstretching mode of the C–C bonds [28, 29]. D band is highly dispersive [30, 31] and activated by the presence of defects such as vacancies, hetero-atoms, or any other defects that5950
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