Prediction and Evaluation of Styrenic Block Copolymers as Surface

Ind. Eng. Chem. Res. 2010, 49, 11393–11401

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Prediction and Evaluation of Styrenic Block Copolymers as Surface Modifiers for Multiwalled Carbon Nanotubes in r-Terpineol-Based Pastes Sung Chul Hong,*,† Jeong Eun Shin,† Hee Jung Choi,† Hee Hyun Gong,† Kyungkon Kim,‡ and Nam-Gyu Park§ Department of Nano Science and Technology, Sejong UniVersity, 98 Gunja-dong, Gwanjin-gu, Seoul 143-747, South Korea, Solar Cell Research Center, Korea Institute of Science and Technology, Seoul 136-791, South Korea, and School of Chemical Engineering, Sungkyunkwan UniVersity, Suwon, Gyeonggi-do 440-746, South Korea

A well-defined styrenic block copolymer was prepared through controlled/“living” radical polymerization technique and evaluated as a polymeric surface modifier for multiwalled carbon nanotube (MWCNT) in R-terpineol-based paste. First, poly(maleic anhydride-co-p-acetoxystyrene)-block-poly(p-acetoxystyrene) copolymer was prepared through a nitroxide-mediated polymerization (NMP) technique in an efficient “onepot” reaction. The copolymer was then functionalized with pyrene through an imidization reaction (SPM). Finally, p-acetoxystyrene units were converted to p-hydroxystyrene units through hydrolysis, affording pyrenefunctionalized poly(maleic acid-co-p-hydroxystyrene)-block-poly(p-hydroxystyrene) (HSPM). Pyrene units in one block afforded efficient attachment points to the surface of MWCNT through π-π interaction, while poly(p-hydroxystyrene) or poly(p-acetoxystyrene) tails afforded enhanced affinities with R-terpineol, as predicted by Hansen solubility parameter theory. Fabrications of electrodes through screen printing procedures employing MWCNT/HSPM or MWCNT/SPM pastes were facilitated through the surface modification of MWCNTs with the block copolymers, as evidenced by low viscosity, more homogeneous and smooth pastes, homogeneous/uniform MWCNT coatings, and low sheet resistance of the electrode. 1. Introduction Carbon nanotubes (CNTs) have been attractive for electronic applications such as flexible electronics, photovoltaics, field emission displays, sensors, and transistors owing to unique hexagonal arrangements of carbons that form themselves into tubes, affording extraordinary electronic properties.1,2 Such high aspect ratio facilitates CNT to form a “network-like” structure in the matrix, even at a low concentration, often termed as “percolation”. These have made CNT one of the most attractive materials for the fabrication of electrodes, where an efficient electron conducting network is especially important.3 Usually inks or pastes of CNTs are applied to substrates such as films and transparent conductive oxide (TCO) glasses through spraying, screen printing, or doctor blading technique to afford CNTbased electrodes. However, the applications of CNTs have been often impeded by their agglomerations and entanglements, which result in poor dispersion, high viscosity, and low stability of CNT inks or pastes.4 These originated from the high aspect ratio and strong intertubular van der Waals attractions of CNTs. Therefore, pretreatment of CNTs to afford better and stable dispersion is a prerequisite for successful applications.5,6 A collection of literature proved the positive effects of various pretreatments on electrical, thermal, and structural properties of CNTs compared with those of pristine CNT.5 The strategy to manipulate the surface energy of CNT can be categorized into chemical method (covalent functionalization) and physical method (noncovalent functionalization). Although it is evident that covalent modifications of CNT have been much emphasized * To whom correspondence should be addressed. Tel: +82-2-34083750. Fax: +82-2-3408-3664. E-mail: [email protected]. † Sejong University. ‡ Korea Institute of Science and Technology. § Sungkyunkwan University.

as those enable efficient tailoring of CNT surface properties, noncovalent treatments may minimize perturbation on the extended π-conjugation system of CNT and preserve the intrinsic electronic characteristics of the tubes. These obviously can be beneficial for CNT-based electrodes, where electronic properties of the CNTs are especially important. Noncovalent treatments can be performed by the adsorption of carefully selected hierarchical structures on the surfaces of CNTs through specific interactions. Aromatic and diene compounds such as pyrene can be representative functional groups,7-12 which accomplish π-π stacking interaction with the graphitic structure of CNTs. Different from low molecular weight surface modifiers,10,11 segmented polymeric architectures such as block copolymer can be beneficial, since multiple numbers of, but spatially localized, functional groups can minimize single chain/multiple CNT bonding or inter-CNT cross-linking. Specific interactions of functional groups in one segment provide multiple attachment points and the formation of stable “polymer-bound layers”. The other segments of the block polymers, which have no affinity for the surface of CNT but may stretch into the solutions or matrixes, can be designed to form not only a chemically but also a sterically stabilizing layer. The segmented polymers also afford an additional tool to finely tune the dispersion properties of CNTs through control of molecular weight and functionality of each segment. We have demonstrated in our previous report that a segmented surface modifier employing a polystyrene tail significantly improved the dispersion property of CNTs in a polystyrene matrix.7 However, examples of such polymeric modifiers are generally limited due to tedious synthetic procedures.8,9,12 Especially, considering the wide use of alcohols in such important processes as pastes for screen printings, methodology of dispersing CNTs into alcohols, using either covalent functionalization or a dispersant agent, has been surprisingly limited.13

10.1021/ie1013822  2010 American Chemical Society Published on Web 10/21/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 22, 2010

The development of controlled/“living” polymerization (CRP) techniques has provided a revolutionary broadening of the spectrum of the materials capable of being prepared, where accurate design and control over polymer architectures through simple preparation processes become possible.14,15 Among the CRPs, nitroxide-mediated polymerization method (NMP) has been attractive for practical success, since the simple addition of a nitroxide agent in radical polymerization system endows control over the polymerization.16 The NMP method is especially useful for the control over the polymerization of styrenic monomers. In this study, a well-defined functional block copolymer containing multiple numbers of pyrene units in one block and a functional styrenic polymer chain in another block was prepared through the NMP technique. The block copolymer was evaluated as a polymeric surface modifier for multiwalled carbon nanotubes (MWCNT) in R-terpineol-based paste. Multiple numbers of pyrene units were targeted to afford strong enough interaction with MWCNTs.8,9,12 For the preparation of R-terpineol-based paste, hydroxystyrene or acetoxystyrene functionality in the tail of the block copolymer was targeted for improved affinity with R-terpineol matrix. The surfaces of MWCNTs were modified through a noncovalent functionalization method employing the block copolymers and the dispersion properties of the modified MWCNT were examined. Fabrications of electrodes through screen printing procedures employing the pastes were then attempted. 2. Experimental Section 2.1. Materials. p-Acetoxystyrene (96%, Aldrich) was purified by passing through a column filled with neutral alumina and stored in a freezer under nitrogen. Maleic anhydride (MA, 99%, Aldrich), 2,2′-azobisisobutyronitrile (AIBN, 98%, Samchun Chemicals), 2,2,6,6-tetramethyl-1-peperidine 1-oxyl (TEMPO, 98%, Aldrich), anisole (99%, Aldrich), 1-pyrenemethylamine hydrochloride (95%, Aldrich), tetrahydrofuran (THF, 99.5%, Samchun Chemicals), methanol (99.5%, Samchun Chemicals), ethanol (95.0%, Samchun Chemicals), toluene (99.5%, Samchun Chemicals), N,N-dimethylforamide (DMF, 99.5%, Samchun Chemicals), hexane (95.0%, Samchun Chemicals), dimethyl sulfoxide (DMSO, 99.8%, Samchun Chemicals), R-terpineol (95%, Kanto), and ethyl cellulose (EC, ethoxy content 48%, Samchun) were used without further purification. All other chemicals were purchased from Aldrich and also used without purification. MWCNTs were donated by JEIO and used without purification. The MWCNTs were prepared by CVD and have a diameter of 10-30 nm. Polystyrene homopolymer (PS, Mn ) 12 100 g/mol, Mw/Mn ) 1.06) was synthesized separately through the NMP technique. F-doped SnO (fluorine-doped tin oxide, FTO, Tec8) was purchased from Pilkinton. 2.2. Preparation of Poly(maleic anhydride-co-p-acetoxystyrene)-block-poly(p-acetoxystyrene) (PAS). MA (1.176 g, 1.199 × 10-2 mol), p-acetoxystyrene (15 mL, 9.80 × 10-2 mol), AIBN (0.07 g, 4.26 × 10-4 mol), TEMPO (0.165 g, 1.056 × 10-3 mol), and anisole (1.5 mL, added as a standard for GC monitoring of the progress of polymerization reaction) were placed in a 100 mL Schlenk flask followed by degassing through three freeze-pump-thaw cycles ([p-acetoxystyrene]:[MA]: [AIBN]:[TEMPO] ) 230:28:1:2.4). The reactor was immersed in an oil bath that was preset to a specific reaction temperature (135 °C). Samples were taken out from the flask via syringe at timed intervals to allow kinetic data to be determined. The samples were diluted with THF for analyses, such as gas chromatography (GC) and gel permeation chromatography

(GPC). After a certain polymerization time (2 h), the reactor was removed from the oil bath and cooled to room temperature. The viscous solution was diluted with THF and the polymer was recovered through the precipitation under methanol followed by drying under vacuum at 60 °C for 2 h. The number average molecular weight (Mn) and molecular weight distribution (Mw/ Mn) of PAS were 14 600 g/mol and 1.23, respectively. Chemical structures of polymers prepared in this study are summarized in Figure 1. 2.3. Preparation of Poly(maleic acid-co-p-hydroxystyrene)-block-poly(p-hydroxystyrene) (HPAS). PAS (7 g), 1,4dioxane (44 mL), water (3.5 mL), and sulfuric acid (7 drops) were placed in a 100 mL round-bottom flask. The reactor was immersed in an oil bath that was preset to a specific reaction temperature (90 °C). After a reaction time of 24 h, the reactor was removed from the oil bath and cooled to room temperature. The product was recovered by precipitation under deionized water followed by drying under vacuum at 80 °C for 24 h. 2.4. Preparation of Pyrene-Functionalized Poly(maleic anhydride-co-p-acetoxystyrene)-block-poly(p-acetoxystyrene) (SPM). PAS (0.5 g), DMSO (15 mL), 1-pyrenemethylamine hydrochloride (0.127 g, 4.74 × 10-4 mol), and NaOH (0.0198 g, 4.95 × 10-4 mol) that was presolubilized in small amount of ethanol were placed in a 100 mL Schlenk flask. The reactor was immersed in an oil bath that was preset to a specific reaction temperature (100 °C) for 7 h. The polymer was recovered through precipitation under ethanol followed by repeated washing procedures using fresh ethanol. The solid product was then dried under vacuum at 60 °C for 24 h. 2.5. Preparation of Pyrene-Functionalized Poly(maleic acidco-p-hydroxystyrene)-block-poly(p-hydroxystyrene) (HSPM). SPM (0.3 g), 1,4-dioxane (19 mL), water (1.5 mL), and sulfuric acid (1 drop) were placed in a 100 mL round-bottom flask. The reactor was immersed in an oil bath that was preset to a specific reaction temperature (90 °C). After a reaction time of 24 h, the reactor was removed from the oil bath and cooled to room temperature. The product was recovered by precipitation under deionized water followed by washing with fresh hexane and drying under vacuum at 80 °C for 24 h. 2.6. Fabrication of Pastes and Electrodes. Pastes were prepared through three-roll milling (Exak, 22851 Norderstedt, Germany) of components after sonication in a bath type sonicator for 10 min. The compositions and designations of the pastes are described in Table 1. Electrodes were fabricated on FTO glass through a screen-printing method by using ST 325 mesh screen employing the pastes. Electrodes were thermally treated at 350 °C for 10 min followed by 420 °C for 20 min to remove organic components. 2.7. Characterization. Conversions of monomers to polymers were determined by using a Shimadzu GC 2010AF gas chromatograph equipped with a FID detector using a ValcoBond VB-wax 30 m column. The molecular weights and molecular weight distributions of polymers were determined by a Shimadzu LC-20A GPC using PSS columns (styrogel HR 2, 4, 5) equipped with a Shimadzu RID-10A refractive index detector and a Shimadzu SPD-20AV UV detector. THF was used as an eluent at the flow rate 1 mL/min. Linear polystyrene standards (641 to 1.35 × 106 g/mol) were used for calibration. The theoretical molecular weight of the PAS was calculated using eq 1. Mn,th ) [([MA]0 /2[initiator]0) × conversion(MA) × 98.06] + [([p-acetoxystyrene]0 /2[initiator]0) × conversion(p-acetoxystyrene) × 162.19] (1)


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Figure 1. Chemical structures of polymers prepared in this study. Table 1. Compositions and Properties of Pastes Based on Multiwalled Carbon Nanotubes with and without Modifier for the Fabrication of Electrodesa

c

designation

MWCNTb (wt %)

HSPMc (wt %)

ethyl cellulose (wt %)

R-terpineol (wt %)

ethanol (wt %)

viscosity (cP)

conductivity (S/m)

sheet resistance (Ω/sq)

C1/E1 C1/H2/E1 C1/E3 C3/E1 C4/E1 C4/H1/E1 C4/H2/E1 C4/E3 C4/PS2/E1 C4/H8/E1

1 1 1 3 4 4 4 4 4 4

2 1 2 2d 8

1 1 3 1 1 1 1 3 1 1

93 91 91 91 90 89 88 88 88 82

5 5 5 5 5 5 5 5 5 5

4 850 1 970 1 580 12 780 41 410 22 270 19 590 14 710 14 350 11 240

94.48 94.78 87.47 95.88 96.61 85.45 98.90 92.31 92.63 100.00

7.63 7.61 8.25 7.52 7.47 8.44 7.29 7.81 7.79 7.21

a The electrodes from the pastes were thermally treated at 350 °C for 10 min followed by 420 °C for 20 min. b Multiwalled carbon nanotubes. Pyrene-functionalized poly(maleic acid-co-p-hydroxystyrene)-block-poly(p-hydroxystyrene). d Polystyrene was added instead of HSPM.

Proton nuclear magnetic resonance (1H NMR) measurement was performed using a Bruker 500 MHz instrument with CDCl3 or deuterated dimethyl sulfoxide (DMSO-d6) as solvents. Fourier transform infrared (FT-IR) spectroscopy spectra were recorded with Nicolet 380 spectrometer. Morphology of electrodes was observed using a field emission scanning electron microscope (FE-SEM, Hitachi S-4700). Dispersion properties of MWCNTs in various solvents were tested by employing deionized water, ethanol, DMSO, DMF, THF, R-terpineol, toluene, and hexane. Samples (0.5 mg of MWCNT or 0.5 mg of MWCNT + 0.25 mg of polymers) were dispersed in 10 mL of solvents through sonication for 10 min at 30 °C. Viscosities of the pastes were measured by using Brookfield DV3 viscometer at room temperature. Surface resistance of electrodes was measured by using a four-point probe tester (6220 Kelthley). 3. Results and Discussion Preparation of a functional block copolymer precursor through a one-step process of CRP represents a significant advantage of this study. Chains in CRP are initiated simultaneously and

grow at the same rate, experiencing the same changes in monomer feed ratios and offering uniform polymer chains. In this study, a functional block copolymer containing multiple numbers of pyrene units in one block was targeted. The binding energy of pyrene to CNTs through π-stack can be up to 50 kJ/ mol,17 which is strong enough to form irreversible adsorption of polymer chains.12 These afford a versatile and nondestructive strategy for the noncovalent functionalization of MWCNTs. While pyrene functional groups afford binding sites to the surface of MWCNTs, the dispersion properties of MWCNTs can be modulated through controlling chemical nature of the other segment in the block copolymer. A theoretical initial approach based on Hansen solubility parameter theory was done in order to establish candidates for polymer tail in the block copolymer for better dispersion of MWCNTs in R-terpineol. Hansen solubility parameters of various solvents and polymers are summarized in Table 2.18,19 In Table 2, solvents and polymers are listed in the order of decreasing overall solubility parameter.

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Table 2. Hansen Solubility Parameters of Solvents and Polymer Segments Used in This Studya designation c

water (1) ethanol (2) dimethyl sulfoxide (3) N,N-dimethylformamide (4) poly(p-hydroxystyrene) polystyrene poly(p-acetoxystyrene) tetrahydrofuran (5) R-terpineol (6)19 toluene (7) hexane (8)

δb

δd

δp

δh

47.9 26.6 26.6 24.8 24.6 22.5 21.7 19.4 19.0 18.2 14.9

15.5 15.8 18.4 17.4 17.6 21.8 17.8 16.8 13.9 18.0 14.9

16.0 8.8 16.4 13.7 10.0 5.8 9.0 5.7 8.0 1.4 0

42.4 19.4 10.2 11.3 13.7 4.3 8.4 8.0 10.2 2.0 0

a All values are in MPa1/2 and from ref 18. Values of R-terpineol are from ref 19. b δ is calculated by taking the square root of the sum of the squares of the Hansen solubility parameters δd, δp, and δh, where δd, δp, δh represent dispersive, dipole-dipole, and hydrogen-bonding contributions, respectively. c Numbers in parentheses indicate the numbers of solvents in Figure 7.

For R-terpineol, the contribution of hydrogen bonding (δh) forces may be important due to the presence of hydroxyl group (OH), where contribution from hydrogen bonding (δh) force is extraordinarily high (10.2). On the basis of the fact that R-terpineol is a nonsolvent for polystyrene, these authors propose two styrenic derivatives, p-acetoxystyrene and phydroxystyrene. p-Acetoxy and p-hydroxy functionalities have improved hydrogen bonding (δh) forces [4.3 for polystyrene versus 8.4 for poly(p-acetoxystyrene) and 13.7 for poly(phydroxystyrene)]. In addition, although accurate solubility parameter values of the polymers are controversial,20 the solubility parameter value of poly(p-acetoxystyrene) (δ ) 21.7) is closer to that of R-terpineol (δ ) 19.0) than that of polystyrene (δ ) 22.5),18 which would be beneficial for improved affinity of the polymer chains with R-terpineol. Figure 2 represents the synthetic strategy for the preparation of the block copolymers. Maleic anhydride (MA) was employed in this study to afford anchoring sites for pyrene units. MA tends to form charge transfer complex with styrene, which leads to almost alternating copolymerization between styrenic monomer and MA. This alternating nature of the polymerization between MA and styrenic monomer led to a preferential consumption of MA at the beginning of polymerization, forming a MAfunctionalized polystyrenic block. Since an excess amount of styrenic monomer was employed, the remaining styrenic monomer continued additional CRP to form the second polystyrenic block. This procedure led to a very convenient onestep preparation of functionalized block copolymers. Gas chromatography (GC) results for the copolymerization between p-acetoxystyrene and MA exhibited almost complete conversion of MA to polymer at 40 min, where the conversion of p-acetoxystyrene to polymer was approximately 34% (Figure 3a). The mole ratio of MA to p-acetoxystyrene calculated from the monomer conversion during this period was approximately 1 to 2.8. Theoretically, this first block contains approximately 14 units of MA and 39 units of p-acetoxystyrene, assuming that the initiation efficiency of AIBN was 1. Continuous CRP of the remaining p-acetoxystyrene afforded the second block, forming PAS (Mn ) 14 600 g/mol, Mw/Mn ) 1.23). Relatively good agreements between theoretical and experimental molecular weights and a continuous increase of molecular weight with conversion support a successful CRP (Figure 3b). Fourier transform infrared (FT-IR) spectrum of PAS (Figure 4a) exhibited the presence of carbonyl peaks from MA and p-acetoxystyrene (∼1780 cm-1). Proton nuclear magnetic resonance (1H NMR) spectroscopy spectra of PAS (Figure 5)

Figure 2. Schematic representation for the preparation of pyrene-functionalized poly(maleic acid-co-p-hydroxystyrene)-block-poly(p-hydroxystyrene) (HSPM): the number of each repeating unit was approximated by GC, GPC, and 1H NMR.

showed aromatic and aliphatic proton peaks originating from p-acetoxystyrene around 6.3-7.0 and 1.0-3.0 ppm, respectively. Broad peaks around 3.0-4.0 ppm represented protons originated from maleic anhydride, although determination of the amount of MA in the polymer was not successful due to the low signal intensity. Pyrene groups were incorporated into the copolymer through imidization reaction between MA in PAS and 1-pyrenemethylamine, affording the block copolymer composed of pyrene groups in one block and p-acetoxystyrene groups in the other block (SPM, Figure 2, second step). Successful incorporation of pyrene groups was confirmed through 1H NMR, as evidenced by aromatic proton peaks originated from pyrene around 7.5-8.5 ppm (Figure 5). From the 1H NMR and molecular weight data, the number of pyrene group in each polymer chain was determined to be three. The incorporation of pyrene group in SPM copolymer was also confirmed through gel permeation chromatography (GPC) analysis employing UV and RI detectors (Figure 6). The detection wavelength for UV detector was set to be the characteristic UV absorption wavelength of pyrene. As can be seen in Figure 6, the GPC diagrams from RI and UV detectors were almost identical, indicating that most polymer chains were successfully decorated with pyrene units.


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Figure 3. Kinetic plots (a) and evolution of Mn and Mw/Mn versus conversion of p-acetoxystyrene (b) for the polymerization of p-acetoxystyrene and maleic anhydride using TEMPO. Polymerization conditions: [p-acetoxystyrene]:[MA]:[AIBN]:[TEMPO] ) 230:28:1:2.4, [styrene]0 ) 5.94 mol/L, temperature ) 135 °C.

Figure 4. Fourier transform infrared (FT-IR) spectra of PAS (a), SPM (b), and HSPM (c).

Block copolymer composed of p-hydroxystyrene groups in the tail was prepared through the hydrolysis reaction of SPM in 1,4-dioxane in the presence of strong acid as a catalyst (HSPM, Figure 2, third step). After the conversion of SPM to HSPM, HSPM was no longer precipitated in ethanol that was

Figure 6. GPC curves of SPM employing RI and UV detectors.

used as a nonsolvent for SPM. Instead, the solubility of HSPM in ethanol increased dramatically, as can be speculated from similar solubility parameters [24.6 for poly(p-hydroxystyrene) versus 26.6 for ethanol, Table 2]. These suggest successful

Figure 5. Proton nuclear magnetic resonance (1H NMR) spectra of PAS (a), SPM (b), and HSPM (c).

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Figure 7. Photographs of solutions containing multiwalled carbon nanotubes with or without HSPM taken immediately after preparation and after 72 h. Sonication was performed at 30 °C for 10 min; composition ) 5 mg of MWCNT + 0.25 mg of additive in 10 mL of solvents: 1 ) deionized water, 2 ) ethanol, 3 ) dimethyl sulfoxide, 4 ) N,N-dimethylformamide, 5 ) tetrahydrofuran, 6 ) R-terpineol, 7 ) toluene, 8 ) hexane.

conversion of maleic anhydride and p-acetoxy group to maleic acid and p-hydroxy groups. The successful hydrolysis reaction was also confirmed through FT-IR (Figure 4c) and 1H NMR (Figure 5) analysis. O-H stretching vibrations from carboxylic acids of maleic acid and hydroxyl group of p-hydroxystyrene were clearly observed around 3100-3600 cm-1. In addition, the intensity of carbonyl stretching peak around 1750 cm-1, decreased significantly after the hydrolysis reaction, suggesting successful conversion of acetoxy groups to hydroxy groups. The position of the remaining carbonyl stretching peak originating from maleic acid group was around ∼1710 cm-1, which was significantly lower than that for maleic anhydride (∼1780 cm-1). Considering the fact that carbonyl stretching peaks for carboxylic acids are generally observed at lower frequency than that from anhydrides,21 these results also support the successful formation of maleic acids. The 1H NMR spectrum (Figure 5) also clearly exhibited the hydroxy proton peak around 9.0 ppm. From the area ratio between aromatic and hydroxy proton peaks, the quantitative conversion of 4-acetoxy to hydroxy group was confirmed. DMSO-d6 (H2O was an impurity in DMSO-d6) was employed as a solvent for NMR analysis of HSPM, since HSPM was no longer soluble in CDCl3. The effects of surface modification on the dispersion behaviors of MWCNTs in various solvents were examined and are shown in Figure 7. Nonmodified MWCNT exhibited immediate sedimentation after sonication in all solvents except N,Ndimethylforamide (DMF), indicating that the affinities of MWCNTs with most of the solvents are poor. The addition of PAS or HPAS (MWCNT/PAS or MWCNT/HPAS) did not exhibit any improvements on the dispersion of MWCNTs, probably because of the lack of the anchoring bridge (e.g., pyrene) between MWCNT and the block copolymers. The addition of SPM resulted in much improved dispersion of MWCNT, especially in R-terpineol, as shown in MWCNT/ SPM in Figure 7. The solution was stable even after 72 h,

Figure 8. Photographs of R-terpineol solutions (10 mL) containing multiwalled carbon nanotubes (0.5 mg) with different additives before sonication (a) and 12 h after sonication (b): 1 ) 1-pyrenemethylamine (0.5 mg), 2 ) PAS (0.25 mg) + 1-pyrenemethylamine (0.25 mg), 3 ) SPM (0.5 mg).

representing effective surface modification of MWCNT with SPM. Pyrene groups probably established a stable adsorption of SPM on MWCNT, while the adsorbed p-acetoxystyrene polymer tail modified the surface properties of MWCNT. These assertions were also confirmed through the comparison of the dispersion behavior of MWCNT/PAS and MWCNT/SPM. PAS, which lacks pyrene but is composed of p-acetoxystyrene units, did not exhibit any improvements of the dispersion properties of MWCNTs in R-terpineol (MWCNT/PAS in Figure 7). The incorporation of pyrene groups in one block, SPM, significantly improved the dispersion properties of MWCNTs in R-terpineol (MWCNT/SPM in Figure 7). The cooperative activity of pyrene and p-acetoxystyrene units in SPM chain was also confirmed by the dispersion behaviors of MWCNT, as shown in Figure 8. R-Terpineol solution did not exhibit any stable dispersion of MWCNT in the presence of low molecular weight 1-pyrenemethylamine (Figure 8, solution 1). R-Terpineol solution along with the physical mixture of 1-pyrenemethylamine and PAS did not exhibit any stable dispersion of MWCNT, either (Figure 8, solution 2). Only SPM, where pyrene and p-acetoxystyrene units are linked together in a blocky polymer chain, dramatically improved the dispersion


of MWCNT in R-terpineol (Figure 8, solution 3). These results clearly confirm the cooperative surface modification activity of SPM. Strangely enough, although the poly(p-acetoxystyrene) tail in SPM has a similar solubility parameter to that of tetrahydrofuran (THF), the dispersion properties of MWCNT/SPM in THF was not improved. This behavior suggests that individual contributions, such as hydrogen bonding (δh) contributions, may play an important role in the dispersion behaviors of MWCNT in R-terpineol, probably because of the high δh values of R-terpineol. HSPM, which contains a p-hydroxy group and exhibits very high δh (13.7), also significantly improved the dispersion of MWCNT in R-terpineol (MWCNT/HSPM in Figure 7). In addition, MWCNT/HSPM exhibited much improved dispersion also in ethanol and dimethyl sulfoxide, for which SPM did not afford any improved dispersion of MWCNT. These results clearly suggest successful surface treatment of MWCNT with HSPM, where the affinity of HSPM [poly(p-hydroxystyrene) tail, δ ) 24.6] with ethanol (δ ) 26.6) and dimethyl sulfoxide (δ ) 26.6) increased the solubility of MWCNT in the solvents. R-Terpineol-based pastes containing MWCNT and HSPM were prepared for the fabrication of electrode. The electrodes were prepared through a screen printing method, followed by thermal treatment at 350 °C for 10 min and 420 °C for 20 min to remove organic compounds. The compositions and viscosities of the pastes and the electrical resistances of the electrodes are summarized in Table 1. Ethyl cellulose was incorporated into the pastes as a general viscosity modifier. On the basis of the fact that ethanol is a good solvent for MWCNT/HSPM, as shown in Figure 7, a little ethanol (5 wt %) was employed for homogeneous mixing of the components. As can be seen in Table 1, lower viscosities were observed for pastes containing more ethyl cellulose (4850 cP for C1/E1 versus 1580 cP for C1/E3; 41 410 cP for C4/E1 versus 14 710 cP for C4/E3). However, slightly higher sheet resistances were observed for the electrodes prepared from the pastes containing more ethyl cellulose. For example, electrode prepared from C1/ E1 exhibited a sheet resistance of 7.63 Ω/sq, while C1/E3 exhibited slightly higher sheet resistance of 8.25 Ω/sq. Electrode prepared from C4/E1 exhibited a sheet resistance of 7.47 Ω/sq, while C4/E3 exhibited 7.81 Ω/sq. These results probably originated from the limited damage of carbon structures of MWCNT during thermal treatment procedures in the presence of ethyl cellulose, where the thermal decomposition of the organic compounds may induce thermal damage of MWCNT. An increased amount of MWCNT of course compensated the effect, as can be seen from the decreased sheet resistance values in the order of C1/E1 (7.63 Ω/sq), C3/E1 (7.52 Ω/sq), and C4/ E1 (7.47 Ω/sq). However, simple addition of MWCNT resulted in much higher viscosity of the pastes. The viscosity of C1/E1 was 4850 cP, which increased to 12 780 and 41 410 cP in the presence of more amount of MWCNT (C3/E1 and C4/E1). These are quite understandable because a greater amount of MWCNT indicates less R-terpineol and also probably results in more agglomeration of MWCNTs, affording higher viscosity. As can be seen from Figure 9a, C4/E1 paste is quite dry and crumbly, which is hard to process during screen printing procedures. These results suggest that simple addition of ethyl cellulose or MWCNT cannot optimize both the processablilty of the paste and the electrical conductivity of electrode at the same time. This is because more ethyl cellulose may induce thermal damage of MWCNT, while more MWCNT may result in high viscosity of the paste and inferior processability.

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Figure 9. Photographs of pastes and R-terpineol solutions containing multiwalled carbon nanotubes with or without HSPM. C4/E1 paste (a), C4/ H2/E1 paste (b), R-terpineol solutions before sonication (c), and R-terpineol solutions after sonication at 30 °C for 10 min (d): 1 ) 0.5 mg of MWCNT + 0.25 mg of polystyrene, 2 ) 0.5 mg of MWCNT + 0.25 mg of HSPM, 3 ) 0.5 mg of MWCNT + 0.25 mg of ethyl cellulose. The sample designations are defined in Table 1.

However, HSPM exhibited different behaviors. The electrodes prepared from the pastes containing 1 wt % of HSPM exhibited slightly higher sheet resistance (7.47 Ω/sq for C4/E1 versus 8.44 Ω/sq for C4/H1/E1), probably because of the same reason as ethyl cellulose. That is, HSPM may induce damage on MWCNT during thermal treatment procedures. However, further incorporation of HSPM resulted in lower sheet resistance (7.29 Ω/sq for C4/H2/E1 and 7.21 Ω/sq for C4/H8/E1). Lower sheet resistance was also observed for C1/H2/E1 compared to C1/E1 (7.63 Ω/sq for C1/E1 versus 7.61 Ω/sq for C1/H2/E1). The addition of surface modifier (HSPM) also continuously decreased the viscosity (4850 cP for C1/E1 versus 1970 cP for C1/H2/E1; 41 410 cP for C4/E1 versus 19 590 cP for C4/H2/ E1 and 11 240 cP for C4/H8/E1), affording a homogeneous and smooth paste (Figure 9b). These results indicate that the processability of the paste during screen printing procedures was improved through the incorporation of HSPM. The different behavior of pastes containing ethyl cellulose and HSPM probably originated from the tailored surface modification effect of HSPM. That is, HSPM effectively enhanced the dispersion property of MWCNT through surface modification of MWCNT, which resulted in both lower viscosity of the paste and lower sheet resistance of the electrodes (7.29 Ω/sq for C4/H2/E1 versus 7.81 Ω/sq for C1/E3). This can be supported through the experiment incorporating an analogue of HSPM, polystyrene (PS), into the paste. PS is similar to HSPM but lacks functional groups for the surface modification and dispersion of MWCNT in R-terpineol. The incorporation of PS resulted in relatively low viscosity (14 350 cP for C4/PS2/E1) like ethyl cellulose, but higher sheet resistance (7.79 Ω/sq for C4/PS2/E1) than that of C4/H2/E1 (7.29 Ω/sq for C4/PS2/E1), indicating inferior surface modification effect and thermal damage on MWCNT, as can be speculated in ethyl cellulose. The enhanced dispersion of MWCNT in the presence of HSPM was also confirmed through the observation on dispersion behaviors of the MWCNTs in diluted a-terpineol solutions (Figure 9c,d). After sonication, MWCNT in R-terpineol exhib-

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Figure 10. Scanning electron microscope (SEM) images of electrodes prepared from the pastes containing nonmodified multiwalled carbon nanotubes (C4/E1, a), multiwalled carbon nanotubes modified with pyrene functionalized poly(maleic acid-co-p-hydroxystyrene)-block-poly(p-hydroxystyrene) (C4/ H2/E1, b), multiwalled carbon nanotubes modified with additional amount of ethyl cellulose (C4/E3, c), and multiwalled carbon nanotubes modified with polystyrene (C4/PS2/E1, d). Samples were thermally treated at 350 °C for 10 min followed by 420 °C for 20 min. Left images, side view; right images, top view.

ited agglomerations and precipitations in the presence of polystyrene (1 in Figure 9d) and ethyl cellulose (3 in Figure 9d), while a quite stable dispersion was observed in the presence of HSPM (2 in Figure 9d). The better surface modification and improved dispersion of MWCNT in C4/H2/E1 also resulted in more homogeneous and uniform MWCNT coating in electrode (Figure 10). While electrode from C4/E1 exhibited rough surfaces (Figure 10a), the electrode from C4/H2/E1 afforded smooth and homogeneous surfaces (Figure 10b). This indicates a good dispersion property of MWCNT and a superior processability of the paste. Although the viscosity values of the pastes containing the corresponding amount of ethyl cellulose (C4/E3) and PS (C4/PS2/E1) were also relatively low (Table 1), the electrodes from C4/E3 (Figure 10c) and C4/PS2/E1 (Figure 10d) exhibited rough surfaces, suggesting insufficient surface modification and dispersion of MWCNT. 4. Conclusions A well-defined functional block copolymer containing multiple numbers of pyrene units in one block and functional styrenic polymer chain in another block was prepared through nitroxide mediated controlled/“living” polymerization techniques (NMP technique) as a polymeric surface modifier for MWCNT in R-terpineol. Effective “one-pot” NMP procedures between maleic anhydride and p-acetoxystyrene afforded a precursor for the functional block copolymer, poly(maleic anhydride-co-p-acetoxystyrene)-block-poly(p-acetoxystyrene). Successive funtionalization of the precursor with 1-pyrenemethylamine [pyrene-functionalized poly(maleic anhydride-co-p-acetoxystyrene)-block-poly(p-acetoxystyrene)] and hydrolysis reaction successfully afforded pyrenefunctionalized poly(maleic acid-co-p-hydroxystyrene)-blockpoly(p-hydroxystyrene), as evidenced by FT-IR, 1H NMR, and GPC analyses. Multiple numbers of pyrene units in the block copolymer afforded strong enough adsorption of the block copolymers to the surface of MWCNTs through nondestructive π-π interaction. p-Hydroxystyrene or pacetoxystyrene functionality in the tail of the block copolymer significantly improved the dispersion properties of MWCNTs in R-terpineol, as predicted by Hansen solubility parameter

theory. Fabrications of electrodes through a screen printing procedure employing the R-terpineol-based MWCNT pastes were facilitated through the surface modification of MWCNTs with HSPM, as evidenced by lower viscosity, more homogeneous/smooth pastes, more homogeneous/uniform MWCNT coating, and lower sheet resistance of the electrodes. Acknowledgment This research was supported by a grant from KIST institutional program. Literature Cited (1) Cao, Q.; Rogers, J. A. Ultrathin films of single-walled carbon nanotubes for electronics and sensors: A review of fundamental and applied aspects. AdV. Mater. 2009, 21, 29. (2) Ajayan, P. M. Nanotubes from carbon. Chem. ReV. 1999, 99, 1787. (3) Gruner, G. Carbon nanotube films for transparent and plastic electronics. J. Mater. Chem. 2006, 16, 3533. (4) Xiea, X.-L.; Mai, Y.-W.; Zhou, X.-P. Dispersion and alignment of carbon nanotubes in polymer matrix: A review. Mater. Sci. Eng. R-Rep. 2005, 49, 89. (5) Bose, S.; Khare, R. A.; Moldenaers, P. Assessing the strengths and weaknesses of various types of pre-treatments of carbon nanotubes on the properties of polymer/carbon nanotubes composites: A critical review. Polymer 2010, 51, 975. (6) Nakashima, N. Soluble carbon nanotubes: Fundamentals and applications. Int. J. Nanosci. 2005, 4, 119. (7) Choi, I. H.; Park, M.; Lee, S.-S.; Hong, S. C. Pyrene-containing polystyrene segmented copolymer from nitroxide mediated polymerization and its application for the noncovalent functionalization of as-prepared multiwalled carbon nanotubes. Eur. Polym. J. 2008, 44, 3087. (8) Bahun, G. J.; Adronov, A. Interactions of carbon nanotubes with pyrene-functionalized linear-dendritic hybrid polymers. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 1016. (9) Lou, S.; Daussin, R.; Cuenot, S.; Duwez, A.-S.; Pagnoulle, C.; Detrembleur, C.; Bailly, C.; Jerome, R. Synthesis of pyrene-containing polymers and noncovalent sidewall functionalization of multiwalled carbon nanotubes. Chem. Mater. 2004, 16, 4005. (10) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H. Noncovalent sidewall functionalization of single-walled carbon nanotubes for protein immobilization. J. Am. Chem. Soc. 2001, 123, 3838. (11) Nakashima, N.; Tomonari, Y.; Murakami, H. Water-soluble singlewalled carbon nanotubes via noncovalent sidewall-functionalization with a pyrene-carrying ammonium ion. Chem. Lett. 2002, 638.

Ind. Eng. Chem. Res., Vol. 49, No. 22, 2010 (12) Petrov, P.; Stassin, F.; Pagnoulle, C.; Je´rome, R. Noncovalent functionalization of multi-walled carbon nanotubes by pyrene containing polymers. Chem. Commun. 2003, 2904. (13) Rouse, J. H. Polymer-assisted dispersion of single-walled carbon nanotubes in alcohols and applicability toward carbon nanotube/sol-gel composite formation. Langmuir 2005, 21, 1055. (14) Matyjaszewski, K. Macromolecular engineering: From rational design through precise macromolecular synthesis and processing to targeted macroscopic material properties. Prog. Polym. Sci. 2005, 30, 858. (15) Braunecker, W. A.; Matyjaszewski, K. Controlled/living radical polymerization: Features, developments, and perspectives. Prog. Polym. Sci. 2007, 32, 93. (16) Hawker, C. J.; Bosman, A. W.; Harth, E. New polymer synthesis by nitroxide mediated living radical polymerizations. Chem. ReV. 2001, 101, 3661. (17) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. About supramolecular assemblies of π-conjugated systems. Chem. ReV. 2005, 105, 1491.

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(18) Grulke, E. A. Solubility Parameter Values. In Polymer Handbook; Brandrup, J., Immergut, E. H., Grulke, E. A., Eds.; John Wiley and Sons: New York, 1999; Vol. 2, p VII675. (19) Garcia, M. T.; Gracia, I.; Duque, G.; Lucas, A. d.; Rodriguez, J. F. Study of the solubility and stability of polystyrene wastes in a dissolution recycling process. Waste Manage. 2009, 29, 1814. (20) Arichi, S.; Himuro, S. Solubility parameters of poly(4-acetoxystyrene) and poly(4-hydroxystyrene). Polymer 1989, 30, 686. (21) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C., Spectrometric Indentification of Organic Compounds, 4th ed.; John Wiley & Sons: New York, 1981.

ReceiVed for reView June 29, 2010 ReVised manuscript receiVed October 4, 2010 Accepted October 10, 2010 IE1013822

Prediction and Evaluation of Styrenic Block Copolymers as Surface

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