Composites: Part B 56 (2014) 365–371

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Composites: Part B

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Synthesis and properties of iodo functionalized grapheneoxide/polyimide nanocomposites

1359-8368/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesb.2013.08.065

⇑ Corresponding authors. Tel.: +82 63 219 8132; fax: +82 63 219 8247.E-mail addresses: cnt@kist.re.kr (B.-C. Ku), jhl@jbnu.ac.kr (J.H. Lee).

Ok-Kyung Park a,b, Seon-Guk Kim a,b, Nam-Ho You a, Bon-Cheol Ku a,⇑, David Hui c, Joong Hee Lee b,⇑a Carbon Convergence Materials Research Center, Institute of Advanced Composites Materials, Korea Institute of Science and Technology (KIST), Eunha-ri san 101, Bondong-eup,Wanju-gun, Jeollabuk-do 565-905, Republic of Koreab Advanced Wind Turbine Research Center, Department of BIN Fusion Technology, Chonbuk National University, Duckjin-dong 1Ga, 64-14, Jeonju, Jeollabuk-do 561-756,Republic of Koreac Department of Mechanical Engineering, University of New Orleans, New Orleans, LA 70148, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 5 June 2013Accepted 3 August 2013Available online 16 August 2013

Keywords:A. Polymer matrix composites (PMCs)A. Thin filmsB. Electrical propertiesB. Mechanical propertiesA. Nano-structures

We report an effective method to fabricate the graphene-based high performance polyimide (PI) nano-composites via the in situ polymerization with iodo functionalized graphene oxide (I-Ph-GO). The elec-trical conductivity of the reduced iodo functionalized graphene oxide (R-I-Ph-GO)/PI (1/99 w/w)nanocomposites was 5.2 � 10�2 S/m, which is about 107 times higher than that of the reduced grapheneoxide (R-GO)/PI (1/99 w/w) nanocomposites. The tensile modulus of R-I-Ph-GO/PI (0.5/99.5 w/w) nano-composites was increased from 2.5 GPa to 6.8 GPa, and the tensile strength was increased from 75 MPa to123 MPa, which were approximately 170% and 64% enhancement compared to those of pure PI, respec-tively. In addition, the water transmission rate of an I-Ph-GO/PI (0.2/99.8 w/w) nanocomposites film waslower than that of R-GO/PI nanocomposites and was reduced by about 67% compared to that of pure PI.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Graphene based polymer nanocomposites have attracted wide-spread industrial interest because graphene has great potential forimproving the electrical conductivity, mechanical properties andgas barrier properties of the nanocomposites due to their uniqueproperties [1–12].

Graphene is a carbon allotrope with a 2-dimensional honey-comb lattice structure which shows an excellent electron chargemobility of about 200,000 cm2/V at room temperature [13], andelectrical conductivity (�10�6 X cm resistivity) [14], providing apercolated pathway for electron transfer and making the graphenebased nanocomposites electrically conductive [15,16].

Polyimide (PI) is a strong candidate for a variety of applicationssuch as electronics, aerospace, vehicles, and dielectric materials,due to its good mechanical properties, flexibility, high glass transi-tion temperature, excellent thermal stability, and radiation resis-tance. However, pure PI has limitations in a few applicationsbecause of its insulating nature of polymer. The exceptionally highelectrical conductivity of graphene makes it an ideal candidate forthe preparation of conducting PI nanocomposites. Therefore,graphene based PI nanocomposites have been developed to im-prove the electrical conductivity [17,18] and are widely used in

electronic device applications, such as anti-static agents (1 kX/sq), EMI/RFI shielding materials (100 X/sq), transistors andsensors.

Improving the electrical conductivity, mechanical and thermalproperties of graphene/polymer nanocomposites is an importantissue for the development of high performance polymer nanocom-posites. However, most of the studies of graphene oxide (GO)/PInanocomposites merely report an improvement in either themechanical properties or electrical conductivity of PI nanocompos-ites through the surface functionalization of the graphene [6,19–24]. Such a phenomenon is considered to be due to the inhomoge-neous dispersion of the graphene oxide in the polymer matrix andthe poor reduction efficiency of the GO. Therefore, to improve themechanical properties, electrical conductivity and thermal proper-ties of PI nanocomposites, it is necessary to develop surface func-tionalization methods for GO with good dispersibility andreduction efficiency by heat at relatively low temperatures suchas 300 �C to avoid thermal degradation of polyimide.

Our previous research indicated that the iodo groups acted as acatalyst to enhance the electrical conductivity of reduced grapheneoxide (R-GO) [25]. Although the electrical conductivity of reducediodo functionalized graphene oxide (R-I-Ph-GO) was 42,000 S/mwhich was significantly higher than R-GO, electrical conductivityof R-I-Ph-GO/PI nanocomposites was �1 S/m at 10 wt% of R-I-Ph-GO in composites due to the defects and aggregation of R-GO.The high content of GO in polymer nanocomposites has been re-

366 O.-K. Park et al. / Composites: Part B 56 (2014) 365–371

ported to significantly enhance the electrical conductivity of nano-composites, but to have a trade-off effect on the mechanicalproperties.

Herein, we propose an effective method of fabricating graph-ene/PI nanocomposites to improve both electrical conductivityand mechanical properties. The highly exfoliated GO was preparedby homogenizing GO flakes and in situ polymerization method wasused to disperse functionalized GO in polymer matrix. The electri-cal conductivity of up to 5.2 � 10�2 S/m and tensile modulus of upto 6.8 GPa were achieved for composites with less than 1 wt% ofthe iodo functionalized graphene oxide (I-Ph-GO).

2. Experimental methods

2.1. Materials

Natural graphite flake of conducting grade (�325 mesh) waspurchased from Alfa Aesar, USA. Potassium permanganate(KMnO4), 4-Iodoaniline and sodium nitrite (NaNO2) were pur-chased from Sigma–Aldrich, USA. Sulfuric acid (H2SO4) and hydro-gen peroxide (H2O2) were purchased from PFP MatunoenChemicals Ltd., Japan. Poly(amic acid) (PAA) was prepared viain situ polymerization. Pyromellitic dianhydride (PMDA) and 4,40-oxidianiline (ODA) were used as precursors of poly(amic acid).PMDA, ODA, and 1-methyl-2-pyrolidinone (NMP) were purchasedfrom Sigma–Aldrich, USA.

2.2. Preparation of graphite oxide

The oxidation of graphite was carried out following a modifiedhummer’s method. In a typical procedure, graphite (1 g) was addedinto 50 mL sulfuric acid (H2SO4) under stirring. After 10 min, 3 g ofKMnO4 was added slowly. The mixture was then heated up to 35 �Cand stirred for 6 h. Subsequently, 80 mL of water was added undervigorous stirring, resulting in a quick rise in the temperature to�80 �C. The slurry was further stirred at the same temperaturefor another 30 min.

Afterwards, 200 mL of DI-water and 6 mL of H2O2 solution wereadded in sequence to dissolve the insoluble manganese species.The resulting graphite oxide suspension was washed repeatedlyby a large amount of DI-water until the solution pH reached a con-stant value at �5, and finally the suspension was diluted with1000 mL of DI-water. Finally, the graphite oxide suspension wascentrifuged (1000 rpm for 1 h), and a setting product was dialyzedfor 1 week. The product was dried for 24 h in a freezing drier.

2.3. Preparation of iodo phenyl and phenyl functionalized graphiteoxide (I-Ph-GO and Ph-GO)

0.05 g of GO was immersed in 100 mL of H2SO4 solution at roomtemperature and sonicated for 30 min. The mixture was thenpoured into a flask. After that, 1.8 g of 4-iodoaniline (or aniline)and 0.69 g of sodium nitrite (NaNO2) were quickly added via a syr-inge. The mixture was vigorously magnetically stirred at 60 �C for1 h. After cooling to room temperature, the suspension was dilutedand washed with DMF until colorless to remove any unreacted dia-zonium salt from the mixture. Finally, the product was washedwith ethanol and dried at 60 �C for 24 h in a vacuum oven.

2.4. Preparation of functionalized graphene oxide (GO and I-Ph-GO)reinforced polyimide (PI) nanocomposites

Surface functionalized graphite oxide (GO or I-Ph-GO) was im-mersed in a NMP solution and homogenized for 1 h at room tem-perature. After that, ODA was added to the mixture. The surface

functionalized GO and diamine mixture continued to be stirredfor 1 h before adding the PMDA to completely dissolve the ODA.Once the ODA was dissolved in NMP followed by the addition ofPMDA, the mixture was stirred overnight at room temperature.The prepared surface functionalized GO incorporated poly(amicacid) (PAA) was casted on the polyimide film using a doctor blade.The cast films were dried at 80 �C for 4 h in a vacuum oven to re-move the residual solvent. R-GO/PI and R-I-Ph-GO compositesfilms were prepared with the thermal curing of PAA. The GO/PAAand I-Ph-GO/PAA composites were thermally imidized via the fol-lowing procedure: (1) heating them up to 100 �C at a rate of 10 �C/min and then annealing them for 2 h, (2) heating them up to 200 �Cat a rate of 10 �C/min and annealing them for 15 min, (3) heatingthem up to 300 �C at a rate of 5 �C/min and annealing them for6 h, and (4) heating them up to 430 �C at a heating rate of 2 �C/min and then annealing them for 30 min.

2.5. Characterization

The dispersibility of functionalized graphene oxide (GO, Ph-GOand I-Ph-GO) in DI-water was measured by UV–vis spectroscopy(V-670, JASCO, USA). The spectra were measured in a 300–900 nm wavelength range. By measuring the absorbance of func-tionalized graphene dispersion at 660 nm, the concentration func-tionalized graphene solution was calculated based on Lambert–Beer law.

The thickness of the I-Ph-GO was measured by atomic forcemicroscopy (AFM) (NanoscopeIIIa-Multimode AFM, Veeco-Digitalinstrument, USA). The electrical conductivities of the R-GO/PI andR-I-Ph-GO/PI nanocomposite films were measured using a 4-pointprobe (FPP-RS8, Dasol Eng, Korea).

The tensile strength and modulus of the R-GO/PI and R-I-Ph-GO/PI nanocomposite films were measured by a universal testingmachine (UTM, 5567A, Instron. USA). Twenty specimens withlength of 25 mm and width of 5 mm were prepared for each sam-ple. The speed of the crosshead was 1 mm/min. The resultingcross-section formed after the rupture of the tensile specimenwas analyzed by field emission scanning electron microscopy(FE-SEM: S-4700, Hitachi, Japan).

The water vapor permeability of the R-GO/PI and R-I-Ph-GO/PIcomposite films was measured using a water vapor transmissionanalyzer (AQUATRAN Model 1, Mocon, USA).

Thermogravimetric analysis (TGA, Q5000, TA Instruments, USA)was performed to determine the thermal stability of the R-GO/PIand R-I-Ph-GO/PI nanocomposite films. The TGA was performedin the temperature range from room temperature to 750 �C at aheating rate of 5 �C/min in an air atmosphere.

3. Results and discussion

3.1. Characterization of functionalized graphene oxide (GO and I-Ph-GO)

These dispersions were then characterized by UV/vis spectros-copy, with the absorption coefficient plotted versus wavelength.The absorption coefficient, a, which is related to the absorbance(a) per unit path length (l), A/l, through the Lambert–Beer law A/l = aC, is an important parameter in characterizing any dispersion.In order to accurately ascertain the graphene concentration, theabsorption coefficient, a, must be determined experimentally. Sowe prepared a large volume of graphene dispersion in DI-water.Each of the five dispersions was diluted a number of times andthe absorption spectra were recorded. The absorbance (660 nm) di-vided by cell length was plotted versus concentration (Fig. 1). Themaximum solubility for I-Ph-GO in DI-water was determined to be

Table 1Comparison of dispersibility of pure GO and functionalized GO in DI-water.

Sample Dispersibility (mg/mL) R2

GO 4.2 0.96Ph-GO 2.5 0.97I-Ph-GO 3.8 0.98

Fig. 1. Linear relationship between absorbance and concentration of graphene inDI-water.

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3.8 mg/mL, which was higher than that of Ph-GO (2.5 mg/mL),implying that introduced iodo (I) groups on the graphene surfaceimproved the interaction between the graphene and DI-water(Table 1).

I-Ph-GO was exfoliated in an NMP solution by a strong shearforce achieved using a homogenizer instead of ultrasonic treat-ment. This method enables the facile and straightforward produc-tion of large area GO sheets with less structural defects [22]. Thesuccessful preparation of GO via homogenization in the NMP sol-vent was confirmed by AFM analysis. The thickness changes of I-Ph-GO with increasing homogenization time were investigatedand the results are shown Fig.2.

Fig. 2 shows typical AFM images of I-Ph-GO at different homog-enization time and the thickness decreases with increasing homog-enization time. The AFM images of I-Ph-GO showed the presence ofa few layers of GO sheets after 2 h of homogenization. The thick-ness of the I-Ph-GO was shown to be less than 1.1 nm, whichmeans that it might consist of two layers, as compared to the ideal

Fig. 2. AFM images of I-Ph-GO: (a) before homogenization, (b) after 1 h and (c) after 2 h htime.

monolayer GO sheet (�0.7 nm). This might be due to the functionalgroups on the graphene plane during the surface functionalizationprocess, and the sp3-hydridized carbon atoms generated on the ori-ginal graphene oxide plane should increase the thickness [27–29].

3.2. Electrical conductivity of functionalized graphene oxideincorporated PI nanocomposites

The effect of the functionalized GO (I-Ph-GO) on the electricalconductivity of the PI nanocomposites was investigated. Duringthe imidization process, the I-Ph-GO would be thermally reducedand the resulting R-I-Ph-GO/PI nanocomposites would be electri-cally conductive [25]. The electrical conductivity of the PI nano-composites with different contents of R-GO and R-I-Ph-GO issummarized in Fig. 3 and Table 2.

Fig. 3 shows that there is a significant improvement in the elec-trical conductivity of the thermochemically reduced GO or I-Ph-GOincorporated PI (0.5/99.5 w/w) nanocomposites compared to thatof pure PI. The R-I-Ph-GO/PI nanocomposite films have an electricalpercolation threshold at about 0.5 wt%. It is interesting to note thatthe electrical conductivity of the in situ polymerized R-I-Ph-GO/PInanocomposites is much higher than that of the nanocomposites ofour previous report [25]. A sharp improvement in the electricalconductivity was observed when the R-I-Ph-GO/PI nanocompositeswas made by homogenizing of I-Ph-GO followed by in situ poly-merization, as compared to that of the PI nanocomposites materialmanufactured via the direct mixing method. This is not only be-cause of the improvement in the electrical conductivity of func-tionalized GO due to the decreased structural defects on thesurface of functionalized GO during the exfoliation process [26],but also due to the formation of an electrical network in the PI ma-trix caused by the improved dispersion.

The electrical conductivity of the 1 wt% R-I-Ph-GO filled PInanocomposites (R-I-Ph-GO/PI) was 7 orders of magnitude higherthan that of the R-GO/PI nanocomposites. This is considered tobe due to the effective recovery of the sp2-hybrid carbon networkof graphene oxide though thermochemical reduction [25]. Theelectrical conductivity of graphene is critically dependent on thesp2-hybrid carbon network of the graphene lattice. The iodo groupof I-Ph-GO is considered to induce deoxygenation during the imi-dization process, playing the role of a catalyst which improvesthe reduction efficiency of I-Ph-GO in the PAA matrix comparedto that of GO [25].

The graphitization resulting from the deoxygenation of GO isconsidered to enhance the electrical conductivity of the nanocom-posites. Hence, the R-I-Ph-GO/PI nanocomposites showed higher

omogenization and (d) thickness change of I-Ph-GO with increasing homogenization

Fig.3. Electrical conductivity of functionalized GO filled PI nanocomposites withvarious contents of I-Ph-GO and GO.

Table 2Electrical conductivity of pure PI and functionalized GO (I-Ph-GO and GO) filled PInanocomposites.

Samplea Electrical conductivity (S/m)

Pure PI 2.4 � 10–13

R-GO-0.2/PI 2.1 � 10�11

R-GO-0.5/PI 9.8 � 10�10

R-GO-1/PI 1.2 � 10�9

R-GO-2/PI 8.5 � 10�9

R-I-Ph-GO-0.2/PI 1.4 � 10�10

R-I-Ph-GO-0.5/PI 7.5 � 10�3

R-I-Ph-GO-1/PI 5.2 � 10�2

R-I-Ph-GO-2/PI 9.2 � 10�2

a GO or I-Ph-GO filler content: wt%.

368 O.-K. Park et al. / Composites: Part B 56 (2014) 365–371

electrical conductivity due to the high reduction efficiency of I-Ph-GO in the PI matrix compared to the R-GO/PI nanocomposites.

3.3. Mechanical properties of functionalized graphene oxideincorporated PI nanocomposites

Incorporating functionalized GO into the polymer matrix im-proves the mechanical properties of polymer nanocomposites[30,31]. The effects of GO or I-Ph-GO on the mechanical propertiesof the PI nanocomposites were investigated. The mechanical prop-erties of pure PI and R-I-Ph-GO/PI nanocomposites with variousloadings of R-GO and R-I-Ph-GO were measured, and the typicalstress–strain curves were shown in Fig.4.

Fig. 4. Strain–Stress curves of functionalized GO filled PI nanocomposites withvarious contents of I-Ph-GO and GO.

The tensile strength, young’s modulus and strain of the pure PIand R-I-Ph-GO/PI nanocomposites are summarized in Fig.5 and Ta-ble 3, which shows that there is an increase in the mechanicalproperties after incorporating R-GO and R-I-Ph-GO in the PI. Fur-thermore, Fig. 5 clearly shows that the mechanical properties ofthe R-I-Ph-GO/PI nanocomposites were greatly improved fromthose of the R-GO/PI nanocomposites. The modulus of the R-I-Ph-GO/PI nanocomposites containing only 0.5 wt% of R-I-Ph-GO wasincreased from 2.5 to 6.8 GPa, an enhancement of�170% comparedto that of pure PI. The tensile strength was also increased from 75to 123 MPa, an enhancement of �64% compared to that of pure PI.

Improvements in the mechanical properties were also observedfor the R-GO/PI nanocomposites. The incorporation of 0.5 wt% R-GO increased the tensile strength and modulus to 91 MPa and4.5 GPa, respectively. However, these values are much lower thanthose of the R-I-Ph-GO/PI nanocomposites. These results suggest

Fig. 5. Mechanical properties of PI nanocomposites with various contents of I-Ph-GO and GO.

Table 3Mechanical properties of pure PI, R-GO/PI and R-I-Ph-GO/PI nanocomposites.

Samplea Tensile strength (MPa) Modulus (GPa) Elongation (%)

Pure PI 75.7 ± 7.2 2.5 ± 1.1 12 ± 2.6R-GO-0.5/PI 91 ± 5.4 4.5 ± 1.2 9.9 ± 0.9R-GO-1/PI 80.2 ± 6.9 4.8 ± 0.7 4.3 ± 1.2R-GO-2/PI 67 ± 3.4 5.6 ± 0.9 3.2 ± 1.3R-I-Ph-GO-0.5/PI 123 ± 6.5 6.8 ± 1.2 6.5 ± 1.2R-I-Ph-GO-1/PI 111 ± 4.6 7.9 ± 1.1 4.8 ± 1.4R-I-Ph-GO-2/PI 98 ± 3.7 9.6 ± 0.9 4.0 ± 1.8

a GO or I-Ph-GO filler content: wt%.

Fig. 6. SEM images of fracture surface of R-GO/PI and R-I-Ph-GO/PI nanocomposites; (a) Pure PI, (b) R-GO/PI(0.5:99.5 w/w), (c) R-GO/PI (1:99 w/w), (d) R-I-Ph-GO/PI(0.5:99.5 w/w) and (e) R-I-Ph-GO/PI (1:99 w/w).

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that the reinforcement efficiency of I-Ph-GO is higher than that ofGO in the PI matrix.

The enhancement in both the tensile strength and modulus ofthe R-I-Ph-GO/PI nanocomposites was attributed to the morehomogeneous dispersion of I-Ph-GO in the PI matrix and improvedinterfacial interaction between I-Ph-GO and the PI matrix. Thestrong interfacial interaction efficiently transfers stress from thePI matrix to the I-Ph-GO, thereby enhancing the mechanical prop-erties of the PI nanocomposites.

The modulus of the R-I-Ph-GO/PI nanocomposites increasedwith increasing I-Ph-GO content. The tensile strength of the nano-composites was found to decrease with the addition of over0.5 wt% of I-Ph-GO. This is due to the increase in the aggregationtendency of I-Ph-GO. When the filler content reaches a critical le-vel, the distance between the individual graphene sheets is sosmall that the van der waals forces become significant and thesheets agglomerate. This kind of agglomeration forms some defectsin the nanostructure of the nanocomposites, causing a reducedreinforcing effect of graphene [20,21]. In addition, the I-Ph-GOmay possess a strong interfacial interaction with the polymer, soas to restrict the movement of the polymer chains effectively,resulting in increased brittleness of the nanocomposites [19]. Thus,the elongation at break of the nanocomposites film decreasedgradually as the I-Ph-GO content increased.

To better understand the tensile properties of the nanocompos-ites, the fractured surfaces upon tensile testing were observed bySEM, as shown in Fig.6. It can be seen that, at low magnification,the fractured surface of pure PI was smooth. For the PI nanocom-posites with 1 wt% R-GO and R-I-Ph-GO, the fractured surfacewas relatively rough compared with that of the pure PI. On theother hand, the R-I-Ph-GO was well-dispersed in the PI matrixand formed rougher fractured surfaces. The homogeneous disper-sion of R-I-Ph-GO in the PI matrix is one of the most important fac-tors for improving the mechanical properties of the PInanocomposites.

3.4. Thermal properties of functionalized graphene oxide incorporatedPI nanocomposites

The thermal stability is one of the most important properties forPI nanocomposites, as they are potentially used as materials forrelatively high temperature applications. The thermal stability ofPI is usually improved by the addition of carbon based additives[19]. In order to study the effect of functionalized GO incorporationon the thermal stability of the PI nanocomposites, thermogravi-metric analysis (TGA) was carried out in an air atmosphere in thetemperature range of 50–750 �C. The TGA curves of the pure PIand R-I-Ph-GO/PI nanocomposites are illustrated in Fig. 7. It can

Fig. 7. TGA curves of pure PI and 1 wt% of surface functionalized GO incorporated PInanocomposites.

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be seen that the thermal stability of the R-I-Ph-GO/PI nanocompos-ites is superior to those of the pure PI. This result suggests that theI-Ph-GO is a more effective filler for the improvement of the ther-mal stability of PI nanocomposites than GO.

The decomposition of the pure PI and R-GO/PI nanocompositesis completed at �720 �C. On the other hand, the R-I-Ph-GO/PInanocomposites showed a residual weight of �18 wt% at 720 �C,indicating that the I-Ph-GO can prevent the thermal degradationof the PI matrix. This may be caused by the better reduction abilityof I-Ph-GO during the imidization process compared to that of GO[25]. During the imidization, the sp2-crystallinity of I-Ph-GO is in-creased [25], which might contribute to the enhanced thermal sta-bility of the R-I-Ph-GO/PI nanocomposites.

3.5. Water vapor permeability of functionalized graphene oxideincorporated PI nanocomposites

Graphene acts as a barrier against gas passing though the poly-mer matrix when it is uniformly dispersed in it, thereby improvingthe gas barrier property of composites [26,27]. Hence, combininggraphene with the polymer matrix makes it possible to developbarrier polymer composite films.

The water vapor permeability is strongly dependent on thehydrophobicity of the polymer nanocomposites. Therefore, highlyreduced GO incorporated polymer nanocomposites should exhibitlow water vapor transmittance due to the increase in the hydro-phobicity of the polymer nanocomposites [32]. Herein, we investi-gated the water vapor permeability of the pure PI and R-I-Ph-GO/PI

Fig. 8. Water vapor permeability of I-Ph-GO and GO filled PI nanocomposites withincreasing the contents.

nanocomposites for the purpose of confirming the reductionefficiency of I-Ph-GO in the PI matrix and gas barrier properties.The water vapor permeability of the pure PI and R-I-Ph-GO/PInanocomposites at 25 �C is summarized in Fig. 8. It shows thatthe water vapor permeability of the thermochemically reducedGO (1 wt%) (R-GO and R-I-Ph-GO) incorporated PI nanocompositesare remarkably reduced compared to that of pure PI. The water va-por transmission rate of both samples was reduced by about 67%compared to that of pure PI. When 1 wt% of GO or I-Ph-GO wasincorporated, the type of surface functionalization did not have asignificant influence on the water vapor transmission rate. Such aphenomenon probably occurs because the barrier effect of GO itselfhas a greater influence on the water vapor transmission. It is sug-gested that the R-I-Ph-GO incorporated in the PI matrix can effec-tively extend the path of the water vapor passing though the PInanocomposite films and thus significantly improve the water va-por barrier properties. However, the low content such as 0.2 wt% ofR-I-Ph-GO in the PI nanocomposites shows that the type of surfacefunctionalization strongly influences the water vapor permeability.The water vapor permeability of the R-GO/PI nanocomposites wasonly 20% less than that of PI. However, R-I-Ph-GO showed a signif-icant decrease in its water vapor permeability, from 150 to 40 g/m2�day, which is almost the same as that of the 1 wt% I-Ph-GOincorporated PI nanocomposites. The decrease in the water vaporpermeability of the R-I-Ph-GO/PI nanocomposites occurs becauseof the better sp2-crystallinity, the reduction efficacy afforded bythe imidization process, and the improved dispersion of R-I-Ph-GO in PI compared to the case where GO is used [25].

4. Conclusions

In this study, PI nanocomposites were manufactured usingin situ polymerization with iodo functionalized GO (I-Ph-GO) toimprove the electrical conductivity, mechanical properties, ther-mal properties and water vapor barrier properties. In-situ poly-merization provides good compatibility and homogeneousdispersion of graphene oxide in a polymer matrix, and is an effec-tive approach to maximize the polymer reinforcing efficiency of I-Ph-GO so as to provide superior performance of the nanocompos-ites compared to those obtained by the direct mixing method.

I-Ph-GO showed a better reinforcing effect in the PI matrix,thereby enhancing the mechanical properties, electrical conductiv-ity, thermal properties and water barrier properties compared tothat of GO. An electrical conductivity of 5.2 � 10�2 S/m was shownafter loading 1 wt% of R-I-Ph-GO, which is about 107 times higherthan that of the PI composite loaded with an identical amount of R-GO. Moreover, the modulus was increased from 4.5 to 6.8 GPa andthe tensile strength was increased from 75 to 123 MPa with theaddition of 0.5 wt% of I-Ph-GO. A significant decrease in the watervapor permeability from 150 to 40 g/m2�day was also observed.

All of these findings suggest that I-Ph-GO can act as effective fil-ler for the improvement of the electrical conductivity, mechanicalproperties, thermal properties and gas barrier properties of PInanocomposites compared to GO. Consequently, the incorporationof I-Ph-GO in the PI matrix provides an effective method of devel-oping multi-functional high performance nanocomposites materi-als with a wide range of applications.

Acknowledgements

This work was supported by a grant from Korea Institute of Sci-ence and Technology (KIST) Institutional program and by the Con-verging Research Center Program (2013K000404) through theMinistry of Science, ICT & Future Planning. The financial supportfrom the Production Technology of Large Tow Carbon Fiber and

O.-K. Park et al. / Composites: Part B 56 (2014) 365–371 371

Development of Intermediate Materials (2M32050) funded by theMinistry of Trade, Industry and Energy, Republic of Korea is alsogratefully acknowledged.

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