Colloid Polym Sci DOI 10.1007/s00396-015-3539-2

ORIGINAL CONTRIBUTION

Easy preparation of graphene-based conducting polymer composite via organogel Rie Yamazaki Kuwahara & Takuya Oi & Kumi Hashimoto & Shingo Tamesue & Takeshi Yamauchi & Norio Tsubokawa

Received: 11 November 2014 / Revised: 24 January 2015 / Accepted: 10 February 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract A highly electrically conducting graphene (reduced graphene oxide, rGO)-based polymer composite was successfully prepared via organogels using an organically dispersible polyaniline (PANI) and a low molecular-weight organogelator consisting of cholesterol derivatives. The plain gel was prepared by heat treatment of a mixture of toluene and the organogelator. PANI-rGO organogel was prepared by adding PANI and rGO to the plain gel and mixing them. Microscopic studies established that organogelators formed threedimensional networks and that the PANI aggregates and rGO particles were dispersed within the organogels. Viscoelastic measurements demonstrated that the PANI-rGO organogel showed rapid and repeatedly thixotropic behavior. BDried^ PANI-rGO composites showed high electrical conductivity. A ballpoint pen was filled with the PANI-rGO organogel and used to produce lines similar to the ones produced by commercially available ballpoint pen. The line showed electrical conductivity. The novel PANI-rGO composite shows high electrical conductivity through easy preparation, one which is just like mixing.

Keywords Reduced graphene oxide . Conducting polymer . Organogel . Composite material . Electrical conductivity . Organogelator

R. Y. Kuwahara : T. Oi : S. Tamesue : T. Yamauchi Graduate School of Science and Technology, Niigata University, 8050, Ikarashi 2-no-cho, Nishiku, Niigata 950-2181, Japan K. Hashimoto : S. Tamesue : T. Yamauchi (*) : N. Tsubokawa Department of Science and Technology, Faculty of Engineering, Niigata University, 8050, Ikarashi 2-no-cho, Nishiku, Niigata 950-2181, Japan e-mail: [email protected]

Introduction Graphene oxide (GO), a two dimensional carbon substance currently, is one of the most attractive materials due to its thinness, mechanical strength, electrical properties, optical absorption properties, and energy storage capacity [1–7]. GO has characteristics of aromatic rings and some oxygen-containing functional groups such as hydroxyl, carboxyl, and epoxy groups on its edges and surface. Reduced GO (rGO) is prepared by means of a reductive reaction. rGO shows higher electrical conductivity, because of the π-conjugated systems restored, which makes it attractive for the studies of organic electric materials [8, 9]. GO, rGO, and their polymer composites have been widely studied [10–14]. Graphene-based polymer composites demonstrate exceptional mechanical, thermal, electrical, gas barrier, and flame-resistant properties compared to the neat polymers. Recently, graphenebased conducting polymer composite materials have been attracting much attention because of the high performance of graphene as a nanofiller, giving these materials enhanced mechanical and electrical properties [15–19]. These composites can be expected to be useful in sensors, supercapacitors, solar cells, and other devices. Some graphene-based conducting polymer composites have been reported, including polyaniline (PANI), polypyrrole (PPy), poly(phenylenediamine) (PPD), and poly(3,4-ethylenedioxythiophene) (PEDOT). Nevertheless, owing to the easily aggregating nature of graphene, great efforts have been made to prepare graphene-dispersed composites in which graphene is homogeneously dispersed into polymer matrices. To this end, great efforts and several approaches have been developed to prepare graphene-based conducting polymer composites. One of those was preparation through

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covalent grafting of a conducting polymer on graphene [20, 21]. Another composite was prepared using in situ polymerization of conducting polymer in a graphene dispersion [19, 22–24]. However, the experimental handling and the chemical or electrochemical techniques used in these references are quite difficult to perform. A more simplified preparation method is required for practical application and industrialization of graphenebased conducting polymer composites. In this study, an easy way to prepare graphene-based conducting polymer composite materials via an organogel which has thermal reversibility and thixotropic property is proposed. The used organogel is composed of a low molecular-weight gelator (LMWG) and an organic solvent. In recent decades, supramolecular gels or molecular gels using LMWGs have been studied as an excellent strategy for developing many kinds of functional materials with tunable properties [25–27]. Lately, various research applying features of LMWGs has been performed in the field of soft matter with electronic properties [28, 29]. Most recently, it has been demonstrated that PANI-toluene organogel using organically well-dispersed PANI and LMWGs consisting of a cholesterol derivative can be easily prepared, and the resulting organogel showed thermal reversibility and unique thixotropic property [30]. Accordingly, this study proposes an easy preparation method of graphene-based conducting polymer composite materials using characteristics of LMWG’s organogels sufficiently. Here, we used the complex salt type organogelator that consisted of cholesterol derivatives and normal aliphatic diamine. We previously reported its ability to gelate various organic solvents [31]. The organogel in this study showed a thermal reversibility and unique rheological properties. We tried to provide an easy preparation method of graphene-based conducting polymer composite material via organogels by exploiting the rheological properties of organogels and the cholesterol analog’s ability to disperse nano-carbons. We used an organic solvent-well-dispersed PANI. It is known that an organic solvent-soluble PANI in a highly electrically conductive state is easily and inexpensively prepared using some kind of functional dopant [32, 33]. We observed stability, thermal reversibility, and thixotropic sol-gel phase transition behavior of the resulting organogels by naked eye in detail, in addition to dynamic viscoelastic measurements and digital microscopic observations of the gel specimens. We considered a conceivable mechanism of the thixotropic behavior of PANI-rGO organogels. Furthermore, we have tried to prepare a Bdried^ PANI-rGO composite and investigated electrical conductivities of the composites. Finally, we presented a simple application as Bgel ink^ with

electronic properties using the composite organogel with good thixotropic properties.

Materials and methods Materials All organic fluids were commercially available as reagent grade or better unless otherwise stated. Cholesteryl hydrogen succinate (CHS), 1,10-diaminodecane (C10-diamine), Nhydroxysuccinimide, and N,N’-dicyclohexylcarbodiimide were purchased from Tokyo Kasei Co., Ltd. (Japan). CHS was recrystallized two times from ethanol. HCl, aniline (monomer), di-2-ethylhexylsulfosuccinate sodium salt (as dopant, see Scheme 1), ammonium peroxodisulphate (APS, oxidizing agent for aniline polymerization), sulfuric acid, sodium nitrate, potassium permanganate, and hydrogen peroxide were obtained from Kanto Kagaku (Japan). Natural graphite powder and hydrazine monohydrate were purchased from Wako Pure Chemical Industries, Ltd. (Japan). These materials were used without further purification. Preparation of CHS C10-diamine complex salt (organogelator) Twin head shaped CHS/C10-diamine complex salts (Scheme 1) were prepared according to following procedures [31]: CHS and C10-diamine were dissolved in benzene, respectively. The resulting solutions were mixed in a 2:1 molar ratio and stirred at RT for 1 h. Then, the solvent was evaporated from the mixture. The complex salts were obtained in white powder form. They were analyzed using Fourier transform Infrared (FT-IR). Preparation of PANI Aniline (2 mmol) and di-2-ethylhexylsulfosuccinate sodium salt (2 mmol) were dissolved in 80 mL of reaction solvent (60 mL of ion-exchanged water and 20 mL of 0.1 mol/L HCl aq.). APS solution (2 mmol APS in 20 mL of ion-exchanged water (IEW)) was added to the reactant mixture, and then the mixture was stirred (200 rpm) for 24 h at 3 °C. The mixture turned green gradually. Then, the mixture was centrifuged (15, 000 rpm, 1 h) with the solvent exchanged two times. The precipitated PANI was then dried (Scheme 1). This procedure was established referring to method of Kuramoto [33]. Preparation of reduced graphene oxide Graphene oxide (GO) was prepared by Hummers’ method from graphite (grain diameter: ca.45 μm) [34]. Reduced GO (rGO) was prepared according to a previously reported procedure [8, 9]. Dried GO (0.1 g) was dispersed in 50 mL of IEW.

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The hydrazine (200 μL) was added to the GO dispersion as a reducing agent, and then the reactant mixture was stirred for 1 h at 95 °C. The precipitated rGO was separated from the reaction mixture by suction filtration, and then the rGO was washed with IEW and dried under vacuum (Scheme 2). The obtained rGO was observed using SEM (see Fig. 1). Preparation of gels PANI, rGO, and PANI-rGO organogels were prepared according to the following (Scheme 3). The organogelator (2 %, w/ w) and toluene were put in a glass sample bottle and heated for 5 min at ca.80 °C. Then, the solution was cooled to room temperature. As shown in Table 1, given weight concentrations of PANI and/or rGO were added to those of the plain gel, and the mixture was agitated using a stirrer. The total amount of these organogels was 500 mg. Gelation was considered to be successful if the mixture formed a colored (derived from additive) mass that could be inverted without apparent flow (tube inversion method) [35]. In order to investigate the thermal reversibility of these organogels, the organogel specimens were heated at ca.80 °C in a water bath for 5 min and then cooled at room temperature. Microscopic observations The gel and the solid state morphologies were observed with a KEYENCE VHX-900 digital microscope. The gel samples were prepared in a septum-capped sample tube. To observe morphologies in the gel, a small amount of the gel was scooped out of the sample tube. The gel and a few drops of solvent were put on a glass slide and then coated with a cover glass, serving for observation. JEOL JSM-6510 scanning electron microscope (SEM) was used for taking images of gold-coated dry rGO. The accelerating voltage of the SEM was 5 kV.

Table 1

Results of gelation tests for PANI-rGO system

rGO % (w/w)

0 1 2 3 4 5 10 15 20

PANI/% (w/w) 0

2

4

6

8

10

G G G G G G V S S

G G G G G G – – –

wG G G G G G – – –

wG wG G G G G – – –

V V V V V V – – –

V V V V V V – – –

The gelator concentration was maintained at 2 % (w/w), solvent: toluene G stable gel, wG weak gel, V viscous, S solution

Viscoelastic measurements For viscoelastic measurements, an Anton Paar rheometer equipped with a cone-plate (25 mm diameter) was used. Temperature was controlled at 10 °C. The gap was adjusted to 0.105 mm. The measurement techniques used in this paper are closely related to those in a previous work [30]. Briefly, oscillatory mode rheological measurements (Fig. 4) at frequency 1 Hz (strain: 100 %) were started with a creep segment. The applied stress was steadily increased up to slightly beyond the yield stress and maintained at this value for ca.30 s. Then, G’ and G^ were recorded during a controlled deformation (strain, 0.1 %) step imposed for 40 s. The sequence of the two segments was repeated for five cycles to analyze the deformation and recovery processes.

Measurement of electrical conductivity Linear sweep voltammetry measurements were carried out with a HZ-5000 Potentiostat/Galvanostat (HOKUTO DENKO Corp.). Electrical conductivites of PANI-rGO composites were calculated from the measured resistance and the known thickness and area of the specimen.

Results and discussion Preparation of composite organogel

Fig. 1 SEM image of rGO

The composite organogels were prepared by addition of PANI and rGO to the plain gel and mixing them at room temperature according to the procedures described above (Scheme 3). The plain gel, made according to Scheme 3a, was known to be a phase transition gel shown in Fig. 2. Therefore, we have

Colloid Polym Sci Scheme 1 Structures of organogelator and PANI

-

A :

di-2-ethylhexylsulfosuccinate anion decided to adopt simple procedures as shown in Scheme 3b–d for preparation of the composite organogels. The results of the gelation tests for the PANI-rGO system are shown in Table 1. First, for rGO systems in the range of 1– 5 % (w/w) rGO, the rGO organogels were stable and blackcolored. When the ratio of the rGO exceeded 10 % (w/w), the specimens could not form stable gels. The resulting organogels showed thermal reversibility and thixotropic behavior. On the other hand, in case of PANI systems, only at 2 % (w/w) was PANI organogel stable. In concentrations of 4 and 6 % (w/w), PANI organogels were weak and fragile, and 8 and 10 % (w/w) PANI systems were viscous and fluid specimens. rGO organogels were more stable that PANI’s. This is because rGO is more hydrophobic than PANI, and therefore, the rGO has a good affinity to the plain gel consisting of toluene and cholesterol-based organogelators. Also, the cholesterol-based organogelater seems to perform a dispersing role in the rGO organogels, since the cholesterol derivatives

are polycyclic compounds that are nano-carbon dispersing agents. The PANI organogel had a dark-greenish color derived from PANI (emeraldine-green) and showed thixotropic behavior. PANI maintained emeraldine-green color before and after gelation, which suggests that PANI maintained its electrical conductivity. However, heat-treated PANI organogels turned blue (images not shown). The blue color indicates that the conductivity is lost. In the case of PANI-rGO systems, the 8 and 10 % (w/w) PANI samples were viscous and fluid. Only 6 % (w/w) PANI– 1 % (w/w) rGO organogel was a weak and fragile gel; the other organogels were stable. The coexistence of PANI with rGO in the PANI-rGO organogels increased the range of PANI concentrations in stable gel specimen compared with the PANI organogels. Some researchers reported that some kind of interaction between carbon nanotubes (CNTs) and conducting polymers was shown by the shifts in FT-IR spectra

Hummers’s method

graphite Scheme 2 Preparation procedure of rGO

reduction

GO

rGO

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a) 80 oC, 5 min → r. t.

toluene

+ organogelator (2 wt%)

b)

plain gel

c)

PANI

+ PANI (0-10 wt%)

d)

plain gel

plain gel

plain gel

PANI organogel

PANI

+ PANI (0-10 wt%)

rGO

+ rGO (0-20 wt%)

rGO organogel

rGO

+ rGO (0-5 wt%)

PANI-rGO organogel

Scheme 3 Preparation procedures of a plain gel, b PANI organogel, c rGO organogel, and d PANI-rGO organogel

[36–41], while no interaction between CNT and PANI was found by others [42]. Recently, H. Wang et al. reported that there were hydrogen bonding, electrostatic interactions, and π-π stacking in the GO/PANI composite [43]. In the case of this PANI-rGO organogel, it is thought that there are some kinds of intermolecular interactions such as π-π stacking between PANI and rGO. The color of the PANI-rGO organogels was black, derived from rGO (Fig. 3). Since the heat-treated

agitating at r. t.

resting at r. t.

heating at 80 oC Fig. 2 Images of the plain gel and its thixotropic and thermal response. Organogelator concentration was 2 % (w/w)

PANI organogels lost their conductivity, no heat treatment was performed on PANI-rGO organogels. Compared with the plain gel, the rGO, PANI, and PANIrGO organogels seem to be slightly heterogeneous. As shown in Fig. 3, PANI-rGO organogel (6 % (w/w) PANI and 5 % (w/ w) rGO) is black-colored and stable. Digital microscopic observation was performed in order to obtain information about the morphologies within the resulting gels. In Fig. 4a, the plain gel specimen shows a number of fibrils ca.2 μm in width (arrows pointing at each other). It seems that almost all the fibrils have the same width. As previously reported, these fibrils were formed by ionic bonds between−COOH of the cholesteryl hydrogen succinate and−NH2 of the C10-diamine, and as an acted driving force for organogelation [31]. Emeraldine-greenish flocks surrounded by circle frames in Fig. 4b indicated typical PANI aggregates. The PANI organogel specimen showed a number of emeraldine-green PANI aggregates with many fibrils. The PANI aggregates maintained emeraldine-green color. The PANI aggregates were of various sizes, most of them between 20 and 30 μm. The PANI in this study was prepared by the drying process of PANI. Therefore, it is supposed that most of

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a)

Fig. 3 Image of PANI-rGO organogel; PANI, 6 % (w/w); and rGO, 5 % (w/w) in the plain gel

b)

the PANI assembles in 20–30 μm sizes by intramolecular hydrogen bonding and disperses in the organogel in this form. In Fig. 4c, fibrils, PANI aggregates, and rGO particles (arrowheads) are coexisting in the specimen. The rGO particles were of various sizes.

Viscoelastic measurements of organogels The prepared rGO, PANI, and PANI-rGO organogels showed thixotropic behavior. As mentioned above, these organogels were prepared using the thixotropic properties of the plain gel. The restoration time of mixed gel specimen was about 10 min, and interestingly, the organogels exhibited a rapid and repetitive recovery of the mechanical strength after stirring broke down. Figure 5 shows the viscoelastic properties of the plain gel and the organogels in a step strain test (frequency 1 Hz). As shown in plain gel (Fig. 5a), when large-amplitude oscillations (100 %) were applied, the G’ was reduced to ca.102 Pa and the tan δ (G^/G’) was ca.3.64. However, the G’ quickly restored close to ca.105 Pa, providing a tan δ of ca.0.16, when the amplitude of oscillations turned into a small value (0.1 %) at the same frequency. These changes of tan δ values suggested that (1) when we applied mechanical stimuli to the plain gel specimen, it showed a quasi-liquid state, and then (2) when we released it from the stress, it showed a quasi-solid state and immediately restored from its quasi-liquid state. The same measurements were also carried out for the other organogel specimen. Measured results are presented in Fig. 5b–d, and the G’, G^, and tan δ values are summarized in Table 2. As presented in Fig. 5, the PANI, rGO, and PANIrGO organogels showed quite similar rheological behavior to the plain gel, which was rapid and repeated thixotropic behavior under mechanical stress. Regardless of the addition of PANI, rGO, or both, the organogels maintained the thixotropic property found in the original plain gel. These results suggest that the thixotropic behavior would be caused by a reformation of three-dimensional networks of organogelators without being hindered by PANI and/or rGO.

c)

Fig. 4 Microscopic images of a plain gel, b PANI organogel, and c PANI-rGO organogel. Organogelator, 2 % (w/w); PANI, 6 % (w/w); and rGO, 5 % (w/w). Arrows pointing at each other: typical fiber and its width of organogelators; circle: typical PANI particle and a size of the particle; arrowhead: typical rGO particle

As compared with the G’ values in Table 2, we can see that the PANI organogel specimen is the softest among these gel specimens. Under large (100 %) amplitude oscillations, at quasi-liquid state, tan δ values indicated that the PANI organogel specimen showed a more Bliquid^ tendency. This result agreed with the result of the gelation test about the 6 % (w/w) PANI organogel as presented in Table 1. In addition, the rGO organogel specimen was slightly softer than the plain gel. However, the G’ and G^ values of the PANI-rGO organogel specimen were almost same as the values of the plain gel, when the amplitude of oscillations turned into a small value (0.1 %). This result also agreed with the result of gelation test of the 6 % (w/w) PANI–5 % (w/w) rGO organogel as presented in Table 1. We assume that maintaining both stability and hardness of the PANI-rGO

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Fig. 5 Thixotropic behavior of a plain gel, b PANI organogel, c rGO organogel, and d PANI-rGO organogel specimens. Organogelator, 2 % (w/w); PANI, 6 % (w/w); and rGO, 5 % (w/w)

Table 2 Summary of the storage moduli (G’), the loss moduli (G^), and the loss tangent (tan δ) of the organogel specimens under large- (100 %) and small- (0.1 %) amplitude oscillations (frequency 1 Hz) Specimena

Plain PANI rGO PANI-rGO

G’b/Pa

tan δb

G^b/Pa

100 %

0.1 %

100 %

0.1 %

100 % 0.1 %

5.8×102 2.1×101 1.8×102 2.6×102

1.6×105 1.0×104 6.5×104 1.1×105

2.0×103 1.1×102 5.2×102 7.2×102

2.7×104 3.2×103 1.3×104 2.8×104

3.64 5.28 2.95 3.36

0.16 0.31 0.20 0.24

a

Solvent: toluene, organogelator, 2 % (w/w); PANI, 6 % (w/w); and rGO, 5 % (w/w)

Arithmetic mean values of G’, G^, and tan δ (G^/G’), when large(100 %, 30 s) or small- (0.1 %, 40 s) amplitude oscillations (frequency 1 Hz) were applied

b

organogel specimen is caused by some kind of interaction between PANI and rGO such as π-π stacking. In spite of a low organogelator concentration (2.0 %, w/w), both the plain gel and the PANI-rGO organogel had high G’ values (ca.105 Pa) at gel state. Furthermore, these organogels showed that the recovery was fully reproducible for at least five cycles. These results would occur as a consequence of the ionic bonds among organogelator complex salts. The rheological measurement indicates that the organogels have rapid and repeated thixotropic behavior under mechanical stress. Therefore, it is suggested that these composite gels can be easily formed into any shape. A conceivable mechanism of thixotropic behavior of the PANI-rGO organogels is shown in Scheme 4 based on the results of microscopic observations (Fig. 4) and viscoelastic measurements (Fig. 5). The intermolecular interactions corresponding with ionic bonds between−COOH of cholesteryl

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shear

shear stress sol

gel standing

disruption

reassemble

: PANI aggregate : rGO

: organogelator complex salt

: solvent molecule

: 3D network consist of organogelators

Scheme 4 Schematic representation of mechanism of thixotropic behavior on the PANI-rGO organogel

hydrogen succinate and−NH2 of C10-diamine results in the formation of fibril-like assemblies that construct the threedimensional network of the gel [31]. From external stimuli, like agitating, some fibril-like assemblies are disrupted leading to the fragmentation of the gel’s three-dimensional network; as a result, the viscosity is reduced to a solution-like shear layer; when the stimulus is removed, the fibril-like assemblies are reassembled by means of ionic bond immediately with or without additives. Thus, the plain gel is able to transform into a composite gel rapidly without any processing, using heating and cooling. The viscoelasticity of the composite organogels, such as hardness of the organogels, depends on the type of additives. However, there is not much difference among thixotropic behavior of these organogels.

organogel showed electrical conductivity, in spite of the presence of a large amount of nonpolar organic solvent (see Fig. 7a). We were able to dry the Bswollen^ PANI-rGO organogel specimens to produce PANI-rGO composites using air drying at room temperature for about 10 min (Fig. 7b). The conductive properties of the PANI-rGO composites obtained via the initial organogel state were investigated. Figure 8 shows that the electrical conductivity increases with increasing concentration of PANI and rGO. Maximum values of PANI (initial

Electrical conductivity The I-V curve of the PANI-rGO organogel (PANI, 6 % (w/w); rGO, 5 % (w/w); organogelator, 2 % (w/w)) is shown in Fig. 6. Current values increased with increasing applied voltage. It appears that the I-V curve obeys Ohm’s law, which suggests that PANI and/or rGO has formed an electrical conduction pathway within the organogel specimens. According to Fig. 6, PANI-rGO organogels had small electrical conductivity value (maximum: 1.41×10-6 S cm−1). The PANI-rGO

Fig. 6 I-V curve of PANI-rGO organogel. PANI, 6 wt%; rGO, 5 wt%; organogelator, 2 wt%

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b)

a)

5 mm

5 mm

c)

50 µm

organogelator’s content of the specimens was at a low level (< ca.20 %, w/w) in the dried state, the PANI-rGO composites showed high electrical conductivity (>1 S cm−1). This suggests that PANI and rGO form Bcomplementary^ and stable conducting pathways in the PANI-rGO composites. For the purpose of comparison, a PANI/rGO mixed sample without the organogelator was prepared by casting of a 6 % (w/w) PANI/2 % (w/w) rGO toluene dispersion on a glass plate at room temperature. The electrical conductivity of the PANI/rGO mixed sample without organogelator could not be detected, because there were many gaps or voids in that specimen under a digital microscope (data not shown). In contrast, as shown in Fig. 7c, d, the PANI-rGO composite with organogelator formed a continuous material with a rather stable form.

d)

(a)

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10 µm

Fig. 7 Photo images of a PANI-rGO organogel and b PANI-rGO composite. Microscopic images of c×1000 and d×2000 of dried PANIrGO composite. Organogelator, 2 % (w/w); PANI, 6 % (w/w); and rGO, 5 % (w/w) at initial organogel state

(b) concentration of PANI, 6 % (w/w); organogelator, 2 % (w/w)), rGO (initial concentration of rGO, 5 % (w/w); organogelator, 2 % (w/w)), and PANI-rGO (initial concentration of PANI, 6 % (w/w); rGO, 5 % (w/w); organogelator, 2 % (w/w)) composites were 0.051, 0.62, and 3.21 S cm−1, respectively. PANI-rGO composite showed a higher value for the conductivity compared to dried PANI and rGO. Additionally, when

(c)

50 µm Fig. 8 Electrical conductivity of PANI-rGO composites via organogel state

Fig. 9 Photo images of a the refill of a ballpoint pen packing the PANIrGO organogel and b the line drawing by BPANI-rGO organogel ink^ and c microscopic image of the line drawing by PANI-rGO organogel ink

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Writing application As an example of application, we used the obtained PANIrGO organogel as an ink gel to draw some lines. The results are shown in Fig. 9. Prepared PANI-rGO organogel samples (organogelator concentration was 2 % (w/w), PANI concentration was 6 % (w/w), and rGO concentration was 5 % (w/w)) were filled into a ballpoint pen (line width 1.0 mm). Using this ballpoint pen, we were able to produce lines similar to the ones produced by a regular ballpoint pen. Next, we tried to measure the electrical conductivity of the PANI-rGO composite line on a paper (shown in Fig. 9b). The conductivity of the PANI-rGO composite line was extremely low, approximately 2.5×10−5 S cm−1. The PANI-rGO composite line showed higher electric conductivity than the organogel, although less than the conductivity of the PANIrGO composite from the organogel. As shown in Fig. 9c, when the PANI-rGO composite line was observed with the digital microscope, the PANI-rGO composite unevenly coated the paper surface. It is thought that a drawing procedure and/or the roughness of a paper surface could prevent the formation of stable electrical pathways. Based on the rheological and electrical properties of the PANI-rGO composite material, it can be applied to produce a novel ink for easily writable electric circuit, which can be used to fix delicate circuit problems on the spot without the need to use heat or complicated equipment.

Conclusions We were able to prepare and characterize electrically conductive PANI-rGO organogels and PANI-rGO composites synthesized via initial organogels by simple method. PANI-rGO organogels showed thixotropic behavior, same as the plain gel. These gels demonstrated a recovery of viscoelasticity following application of mechanical stress. The microscopic studies established that three-dimensional networks consisting of organogelators, PANI aggregates, and rGO particles coexisted in the gel state. It is surprising that the coexistence of PANI aggregates and rGO particles hardly affects the thixotropic behavior of the plain gel. Although the value of electrical conductivity of the PANI-rGO organogel is very low, it could be detected at 2.5 x 10−5 S cm−1. Dried PANI-rGO composites synthesized via organogels showed a conductivity of 3.2 S cm−1. It should be focused on the contribution of the organogelator to form effective and complementary conducting pathways by PANI and rGO. On one hand, utilization of organogels with thixotropic behavior allows us to make the PANI-rGO organogels by a simple method, and those show thixotropic behavior and electrical conductivity. On the other hand, the PANI-rGO organogels made

easy preparation of PANI-rGO composites with a higher electrical conductivity possible. The organogelator seems to play an important role as dispersant or connecter in the composite. Further research will be necessary to investigate a presence of any interactions among components in these composites or the organogels. We believe that such kinds of conductive polymer composite organogels with a wide range of functions are promising materials for future research and applications in electrical soft materials.

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