Removable Nonconjugated Polymers To Debundle and Disperse

May 29, 2019 - In this study, we explore design rules for block copolymer (BCP)-based dispersants for carbon nanotubes (CNTs). We demonstrate the ...
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Removable Nonconjugated Polymers To Debundle and Disperse Carbon Nanotubes Catherine Kanimozhi, Matthew J. Shea, Jaehyoung Ko, Wei Wei, Peishen Huang, Michael S. Arnold, and Padma Gopalan* Department of Materials Science and Engineering, University of Wisconsin, Madison, Wisconsin 53706, United States

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ABSTRACT: In this study, we explore design rules for block copolymer (BCP)-based dispersants for carbon nanotubes (CNTs). We demonstrate the influence of polymer architecture on the dispersion, debundling, and stability of single-walled CNTs. These polymer dispersants based on pyrenefunctionalized BCPs are tailored to perform multiple functions, namely, to (a) solubilize CNTs, (b) debundle CNTs, and (c) lift off CNTs following processing. BCPs were synthesized through an efficient ring-opening reaction of a poly 2-vinyl-4,4-dimethylazlactone (PVDMA) block. This chemistry provides greater flexibility to alter the polymer architecture, solubility, and degradability as well as to achieve a higher degree of incorporation of pyrene side groups. UV−vis−NIR absorption and photoluminescence emission studies indicate that a blockbrush architecture consisting of polystyrene (PS) as the first block and mixed side chains of pyrene/PS or pyrene/ polymethylmethacrylate grafted to the second PVDMA block gave the most stable CNT dispersions with high yields. Our studies show that a large number of pyrene side groups, as well as a large molecular weight solubilizing block, are required to disperse CNTs with a wide range of diameters from 0.7 to 1.7 nm. We further show that our design allows for thiol−thioester exchange chemistry to release the polymer wrappers from the CNT surface in an acid-free organic solvent medium. We envision this method to be generalizable for the dispersion of CNTs from small to large diameters.

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INTRODUCTION Dispersion and debundling of carbon nanotubes (CNTs) are an essential step to the integration of CNTs into (opto)electronic devices because of CNT’s limited intrinsic solubility.1 Though covalent functionalization can result in stable dispersions of CNTs, it comes at the cost of electrical conductivity and shortening in the length of CNTs because of the harsh reaction conditions.2,3 Dispersants that function by noncovalent interactions with CNTs include a number of small molecules and macromolecules, which preserve the intrinsic properties of CNT. However, the dispersion stability of these noncovalently functionalized CNTs can vary widely even under ambient conditions.4 These dispersants include smallmolecule surfactants5 and aromatic compounds,6 conjugated polymers,7,8 DNA,9,10 and polyelectrolytes.11 A number of secondary interactions, such as π−π, CH−π, hydrophobic, electrostatic, and sterics, are all known to contribute to dispersion. Of the macromolecular dispersants, polymers with conjugated backbones are widely studied as they can wrap around or strongly adsorb on CNTs to create very stable dispersion in organic solvents.12−14 Nonconjugated polymers include cationic polymers,15 block copolymers (BCPs),16 and polymers with side or chain-end pyrene functionalization.17−19 However, the design rules for nonconjugated dispersants are poorly understood, mainly due to the variability in the type and source of the starting CNT, as well as the solvent used for dispersion. For example, multiwalled CNTs20−22 can be more easily dispersed by most nonconjugated polymers because of © XXXX American Chemical Society

inherent defects in the structure compared to single-walled CNTs. Similarly, highly processed short single-walled CNTs23−25 can be more easily solubilized than longer singlewalled CNTs. Predominantly even with pyrene-functionalized polymers, water26−28 has been used as the solvent, where the driving forces are quite different from that of an organic solvent. Here, we present the rational design of a new polymer wrapper that is based on nonconjugated polymers that can disperse single-walled CNTs and easily lift off the CNTs using mild chemistry. These dispersants are based on pyrenefunctionalized BCPs. Pyrene-functionalized BCPs themselves are not new. Earlier studies have shown the synthesis of BCPs with one of the blocks containing pyrene side groups.22,25,29 Certain compositions of these BCPs showed good dispersion of acid-treated or shortened single-walled CNTs,23−25 while others were used to disperse multiwalled CNTs.20−22 To solubilize CNTs with minimal chemical processing, the pyrene-containing polymer wrapper should ideally meet several criteria: (a) it should be amenable to a synthetic route that leads to a high degree of pyrene incorporation, (b) it should exhibit good solubility in organic solvents such as tetrahydrofuran (THF), toluene, or chloroform, which are typically used for device fabrication, and (d) it should be removable after device fabrication using a mild process. The removal of Received: February 18, 2019 Revised: May 15, 2019

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DOI: 10.1021/acs.macromol.9b00352 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules the dispersants from the final device is important,13,14,30 as they are typically treated as contaminants for most device applications. Here, we report a BCP consisting of polystyrene (PS) as the first block and polyvinyl-4,4-dimethylazlactone (PVDMA) as the second block synthesized by reversible addition fragmentation transfer (RAFT) polymerization. The choice of the second block is to exploit the efficient ring opening of the azlactone in VDMA using nucleophiles to quantitatively introduce pyrene side groups. This chemistry overcomes many of the challenges with past methods such as limited pyrene incorporation and provides greater flexibility to alter the polymer architecture, solubility, and degradability. We explore three block-brush BCPs starting from the PS-blockPVDMA (PS-b-PVDMA)-based BCP and test their effectiveness in not only solubilizing CNTs but in minimizing the bundling, resulting in a stable solution. These architectures differ in how the second block is functionalized, namely, if the PVDMA block is functionalized with (a) only pyrene, (b) a mixture of pyrene and PS, and (c) a mixture of pyrene and poly(methyl methacrylate) (PMMA) side groups. These polymers are denoted as P1, P2, and P3. UV−vis−NIR absorption and photoluminescence (PL) emission studies indicate that a block-brush architecture with mixed pyrene/PS or pyrene/PMMA side chains grafted to the second block (P2 and P3) gave the most stable CNT dispersions with highest yields, likely due to the steric barrier posed by the polymer side-chain grafts. The molecular weight of the first PS block and the number of pyrene side groups in the second block were both found to be important to solubilize CNTs in organic solvents. In addition, we show that our design allows for thiol− thioester exchange chemistry to release the polymer dispersants from the CNT surface in an acid-free organic solvent medium. The mild polymer removal process minimizes any unintentional doping or introduction of defects in the CNT structure. We envision this method to be generalizable for the dispersion of single-walled CNTs from small to large diameters.

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spectrophotometer and home-built setup, in which an input monochromator was used to produce a beam of wavelength (10 nm resolution) and was used to scan over the range of 300−1500 nm. Synthesis. Synthesis of 4-Pyrene-1-yl-butyl Methanesulfonate (2). A solution of 4-pyrene-1-yl-butanol (1, 0.76 g, 2.77 mmol) in dichloromethane (DCM, 15 mL) was prepared under dry argon. N,NDiisopropyl ethylamine (0.72 g, 5.54 mmol) was added to the solution at room temperature with stirring. This mixture was then cooled to −5 °C. In an adding funnel, the diluted methanesulfonyl chloride (0.48 g, 4.16 mmol) solution in DCM (5 mL) was added slowly, and the reaction was stirred at −5 °C for 3 h. Ice water (50 mL) was used to quench the reaction. The mixture was extracted with DCM (50 mL) thrice. The organic layers were combined, dried with anhydrous magnesium sulfate, and concentrated by a rotary evaporator. The crude product obtained as a pale yellow oil was purified by column chromatography (hexane/ethyl acetate, 1:1) to yield a white solid (2, 0.93 g, 96%). 1H NMR (500 MHz, CDCl3, TMS): δ (ppm) 1.87 (m, 2H), 1.94 (m, 2H), 2.90 (s, 3H), 3.34 (t, 2H), 4.21 (t, 2H), 7.80 (d, 1H), 7.97 (m, 1H), 8.00 (s, 2H), 8.07 (s, 1H), 8.09 (d, 1H), 8.14 (m, 2H), 8.20 (d, 1H). 13C NMR (126 MHz, CDCl3, TMS): δ (ppm) 135.70, 131.36, 130.81, 129.92, 128.55, 127.44, 127.38, 127.19, 126.71, 125.85, 125.05, 124.95, 124.93, 124.78, 124.76, 123.10, 69.76, 37.27, 32.70, 28.97, 27.46. Synthesis of Thioacetic Acid S-(4-Pyren-1-yl-butyl) Ester (3). The purified 4-pyrene-1-yl-butyl methanesulfonate (2, 1.36 g, 3.86 mmol) and potassium thioacetate (1.10 g, 9.65 mmol) were mixed in DMF (20 mL) under ambient conditions. The solution was stirred at 30 °C for 4 h. After 4 h, water (50 mL) and diethyl ether (50 mL) was added to the reaction mixture. The aqueous phase was extracted with diethyl ether (50 mL) twice. The organic layers were combined, dried with anhydrous magnesium sulfate, and concentrated by a rotary evaporator. The crude product obtained as a yellow oil was purified by column chromatography (hexane/ethyl acetate, 6:1). The yellow solid collected can be recrystallized in diethyl ether to yield a white solid (3, 0.87 g, 94%) as a pure product. 1H NMR (500 MHz, CDCl3, TMS): δ (ppm) 1.76 (m, 2H), 1.94 (m, 2H), 2.32 (s, 3H), 2.95 (t, 2H), 3.36 (t, 2H), 7.85 (d, 1H), 7.99 (m, 1H), 8.03 (m, 2H), 8.10 (s, 1H), 8.12 (s, 1H), 8.17 (m, 2H), 8.26 (d, 1H). 13C NMR (126 MHz, CDCl3, TMS): δ (ppm) 195.95, 136.35, 131.42, 130.89, 129.84, 128.58, 127.50, 127.26, 127.24, 126.61, 125.80, 125.08, 125.02, 124.87, 124.80, 124.70, 123.32, 33.02, 30.87, 30.66, 29.60, 28.96. Synthesis of 4-Pyren-1-yl-butyl-1-thiol. Thioacetic acid S-(4pyren-1-yl-butyl) ester (3, 0.87 mg, 2.59 mmol) was dissolved in 1,4dioxane (40 mL) and bubbled with nitrogen for 30 min before adding degassed potassium hydroxide (1.45 g, 25.9 mmol) solution in water (5 mL). The mixture was heated at 100 °C and stirred for 4 h. The cooled reaction mixture was diluted with deionized water (3 × 50 mL) and extracted with chloroform. The organic phases were combined, dried over anhydrous magnesium sulfate, and evaporated by a rotary evaporator. The crude yellow solid was purified by column chromatography using a hexane and chloroform mixture (6:1 to 2:1 in gradient). The solid collected can be recrystallized in diethyl ether to yield light yellow crystals (Py−SH, 0.73 g, 96%) as a pure product. The thiol was kept in ambient environment for 4 weeks without detectable formation of disulfide. 1H NMR (500 MHz, CDCl3, TMS): δ (ppm) 1.33 (t, 1H) 1.77 (m, 2H), 1.94 (m, 2H), 2.58 (q, 2H), 3.33 (t, 2H), 7.84 (d, 1H), 7.97 (m, 1H), 8.01 (d, 2H), 8.08 (d, 1H), 8.10 (d, 1H), 8.15 (m, 2H), 8.24 (d, 1H). 13C NMR (126 MHz, CDCl3, TMS): δ (ppm) 136.39, 131.40, 130.86, 129.81, 128.56, 127.47, 127.24, 127.19, 126.59, 125.79, 125.06, 124.99, 124.86, 124.77, 124.68, 123.28, 33.96, 32.96, 30.46, 24.53. MS (C20H18S) m/ z: [M]+• calcd, 290.11; found, 290.1124. Synthesis of 4-Pyrene-1-yl-butanamide (5). A suspension of 1pyrenebutyric acid (1 g, 3.47 mmol) in chloroform (20 mL) and DMF (0.1 mL) was cooled in an ice bath. Oxalyl chloride (0.5 g, 3.94 mmol) was diluted in chloroform (5 mL) and then added slowly into the suspension. The reaction was completed within 30 min, indicating the formation of a dark purple solution. This solution was bubbled with dry ammonia gas for 30 min at 0 °C and then kept at room temperature for 2 h with vigorous stirring. Dry ammonia gas was

EXPERIMENTAL SECTION

Materials and Methods. All solvents and reagents were purchased from Sigma-Aldrich or other commercial sources and used without further purification unless otherwise noted. Styrene was stirred over calcium hydride, distilled under high vacuum, and stored at 0 °C. 2-Vinyl-4,4-dimethylazlactone (VDMA) was synthesized according to the literature procedure,31 distilled under vacuum at 35 °C in the presence of 2,6-di-t-butyl-4-methylphenol (50 mg), and stored under argon at 0 °C. 2,2′-Azobis (2-methylpropionitrile) (AIBN, 98% purity) was recrystallized from acetone and dried under vacuum prior to use. S-1-Dodecyl-S′-(α,α′-dimethyl-α″-acetic acid)trithiocarbonate, compounds 2−5, 4-pyrene-1-yl-butyl-1-amine, and 4-pyrene-1-yl-butyl-1-thiol were synthesized according to the reported literature procedure32,33 and stored at 0 °C. Small-diameter CoMoCAT CNTs with a diameter of 0.7−0.9 nm were purchased from Sigma-Aldrich (773 735), and large-diameter, arc-discharge CNT powder was also purchased from Sigma-Aldrich (750 514). High-pressure carbon monoxide (HiPCO) CNT powder (Raw NanoIntegris R1-831) was purchased from NanoIntegris. Characterization. 1H NMR and 13C NMR spectra were recorded using a Bruker AVANCE-400 spectrometer in CDCl 3 with trimethylsilane (0.1 vol %) as an internal standard. Gel permeation chromatography (GPC) was performed with a Viscotek 2210 system equipped with three Waters columns (HR 4, HR 4E, HR 3) and a 1 mL/min flow rate of THF as an eluent at 30 °C. UV−vis measurements were performed using a Shimadzu PC-2401 B

DOI: 10.1021/acs.macromol.9b00352 Macromolecules XXXX, XXX, XXX−XXX

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(2 mL) and reprecipitated into hexane, and the final polymer was dried under vacuum and stored at 0 °C. 1H NMR: (400 MHz, CDCl3, TMS): δ (ppm) 8.25−7.34 (pyrene aromatic C−H), 7.23−6.23 (m, styrene aromatic C−H), 3.37−0.88 (backbone C−H2). Nucleophilic Ring Opening of Azlactone by 1-Pyrene Butyl Amine. PVDMA (1 mmol) was dissolved in 1.5 mL of anhydrous THF in a 6 mL glass vial. To this solution, 1-pyrene butyl amine (40 mmol) was added. The vial was sealed with a Teflon cap and placed in a 50 °C oil bath overnight. The solution was concentrated using a rotary evaporator and precipitated into ∼8 mL of hexane. The resulting white powder was redissolved in ∼0.5 mL of DCM and reprecipitated once more in hexane, and the solid was dried under high vacuum and stored at 0 °C. 1H NMR: (400 MHz, CDCl3, TMS): δ (ppm) 1.62−3.45 (aliphatic backbone protons); 6.35−7.25 (styrene aromatic protons); 7.45−8.25 (pyrene aromatic protons). Dispersion of CoMoCAT CNTs, Arc-D, and HiPCo CNTs Using P1−P3. In a typical procedure for dispersion of the CNTs with the polymers, a horn tip sonicator (Thermo Fisher Scientific, Sonic Dismembrator 500) was used at 64 W power, with a 1:10 weight ratio of 1 mg/mL of the CNT and 10 mg/mL of P1−P3 in 10 mL of toluene. The sonication time of the initial dispersion was 10 min. Following the initial dispersion, the CNT solution was centrifuged (Thermo Fisher Scientific, Sorvall WX, swing bucket rotor, TH-641) at 41 krpm for 10 min to remove undispersed materials. The supernatant was collected and, optionally, toluene was distilled, rendering a gel-like polymer−CNT mixture. The solution was then washed with toluene and chloroform and centrifuged to rinse off as much excess polymer as possible. Polymer Exchange Reaction by the Thiol−Thioester Exchange Reaction. To a stirred solution of P2 in 10 mL of toluene was added 10-fold excess of n-butane thiol at room temperature. To this reaction mixture, excess of DBU was added under the inert atmosphere, and the solution was stirred at the same temperature for 20 min. After 20 min of reaction, the solvent was reduced to half, and the residual viscous solution was precipitated in methanol. Precipitates were collected and redissolved in 5 mL of chloroform and reprecipitated in 20 mL of methanol. The resulting white solid was confirmed as the new exchanged polymer P2* in the absence of pyrene peaks in the 1H NMR spectra, and the supernatant contained the pyrene small molecules. Removal of P2 from the CNT Surface in Solution. To a stirred solution of CNT@P2 in toluene, excess of n-butane thiol was added at room temperature. To this reaction mixture, excess of DBU was added, and the solution was stirred at room temperature for 10 min. Precipitates were observed after the addition of DBU. The stirring was continued for 10 more min, and the mixture was centrifuged. The supernatant (70%) was removed, and the precipitates were washed with toluene and chloroform to rinse off polymer P2*. The polymer removal was monitored by UV−vis absorption spectroscopy. The absence of absorption peaks of pyrene at ∼300 and 320 nm confirmed the complete removal of the polymer. Removal of P2 from the CNT Surface in Thin Films. CNT@P2 was spin-coated on a HMDS-treated Si substrate, and the excess polymer was removed by rinsing with hot toluene. After rinsing, the substrate was immersed in a flask containing 20 mL of toluene and excess of n-butane thiol (2 mL). To this solution, excess of DBU was added, and the solution was stirred at a temperature of 50 °C for 2 h. The Si substrate was washed with hot toluene, THF, and acetone to remove the exchanged polymer (P2*) and other byproducts. Polymer removal was confirmed by XPS and AFM images taken before and after polymer removal from CNT@P2 (Figures S7 and 2e).

generated by adding concentrated aqueous ammonia to sodium hydroxide. The reaction mixture was washed with 10% NaOH aqueous solution and water (50 mL) thrice, dried with anhydrous sodium sulfate, and concentrated by a rotary evaporator. The crude product 4-pyrene-1-yl-butanamide (5, 0.94 g, 93%) was obtained as a light brown solid (5, 0.94 g, 93%) and used without further purification. Synthesis of 4-Pyrene-1-yl-butyl-1-amine. 4-Pyrene-1-yl-butanamide (5, 0.94 g, 3.27 mmol) was dissolved in anhydrous THF (50 mL) under nitrogen. Lithium aluminum hydride (LiAlH4, 0.38 g, 10 mmol) was added slowly to the stirred solution at room temperature under nitrogen. A cloudy mixture was observed, which was stirred for 3 h at room temperature and then quenched by deionized water (1 mL). NaOH aqueous solution (2 mL, 10%) and water (5 mL) were added to precipitate a white solid. The precipitate was filtered and washed with water to yield a white solid (Py-NH2, 0.7 g, 78%) as a pure product. 1H NMR (400 MHz, CDCl3, TMS): δ (ppm) 1.62 (m, 2H), 1.88 (m, 2H), 2.75 (t, 2H), 3.36 (t, 2H), 7.86 (d, 1H), 7.98 (m, 1H), 8.01 (d, 2H), 8.10 (d1, 1H), 8.10 (d, 2H), 8.14 (d, 2H), 8.16 (dd, 2H), 8.27 (d, 1H). 13C NMR (126 MHz, CDCl3): δ (ppm) 136.85, 131.44, 130.91, 129.78, 128.61, 127.52, 127.27, 127.20, 126.56, 125.80, 125.10, 125.05, 124.84, 124.79, 124.67, 123.41, 42.21, 33.93, 33.40, 29.19. Synthesis of Poly(2-Vinyl-4,4-dimethylazlactone). VDMA (7.5 g, 0.054 mol), S-1-dodecyl-S′-(α,α′-dimethyl-α″-acetic acid)trithiocarbonate (0.122 g, 3.37 × 10−4 mol), and AIBN (9.0 mg) were added to a dried 25 mL Schlenk tube. To this mixture, anhydrous toluene (8 mL) was added, and the solution was degassed via three freeze−pump−thaw cycles. The solution was placed in a 60 °C oil bath and stirred for 4 h. After 4 h of reaction, the reaction mixture was precipitated in hexane, and the precipitate was collected, redissolved in THF, and reprecipitated in hexane. 1H NMR (400 MHz, CDCl3, TMS): δ (ppm) 2.80−2.57 (broad singlet, backbone C−H), 2.16−1.57 (m, backbone C−H2), 1.37 (s, azlactone C−H3). Synthesis of PS Macroinitiator. Freshly purified styrene (10 mL, 0.096 mol) and S-1-dodecyl-S′-(α,α′-dimethyl-α″-acetic acid)trithiocarbonate (0.157 mg, 4.32 × 10−4 mol) were added to a dried 100 mL Schlenk tube and degassed thrice via freeze−pump− thaw cycles. The solution was placed in an oil bath preheated to 120 °C and stirred for 13 h. The resulting polymer was precipitated in cold methanol, and the precipitate was collected. The precipitate was redissolved in THF and reprecipitated in cold methanol. This process was repeated twice, and the solid was dried under high vacuum and stored at 0 °C. 1H NMR (400 MHz, CDCl3, TMS): δ (ppm) 7.25− 6.30 (m, aromatic C−H), 2.20−1.26 (m, backbone C−H2). General Synthesis of PS-b-PVDMA. PS macroinitiator (PSMI) (1.0159 g, 5 × 10−5 mol), VDMA (1.5 mL, 0.01 mol), and AIBN (1.4 mg) were added to a dried 25 mL Schlenk tube. Anhydrous toluene (8 mL) was added under nitrogen, and the reaction mixture was degassed three times via freeze−pump−thaw cycles. The Schlenk tube was placed in an oil bath preheated at 60 °C and stirred for 4 h. The reaction mixture was precipitated in hexane, redissolved in THF, and reprecipitated in hexane, and the solid was dried under vacuum and stored at 0 °C. 1H NMR spectra confirm the formation of PSMI and chain extension with VDMA to form PS-b-PVDMA. 1H NMR (400 MHz, CDCl3, TMS): δ (ppm) 7.22−6.30 (m, styrene aromatic C− H), 2.81−2.57 (bs, azlactone C−H), 2.20−1.54 (m, backbone C− H2), 1.36 (bs, azlactone C−H3). Nucleophilic Ring Opening of Azlactone by 1-Pyrene Butane Thiol. PS-b-PVDMA (1 mmol) and Py-SH (20, 45, and 75 mmol for the synthesis of P1A, P1B, and P1D, respectively) were added into a dried 25 mL Schlenk tube. To this mixture, 2 mL of anhydrous THF was added under the inert atmosphere, and the mixture was degassed via three freeze−pump−thaw cycles to remove any dissolved oxygen. To this deoxygenated reaction mixture, excess of 1,8diazabicyclo[5.4.0]undec-7-ene (DBU) was added immediately, and the mixture was stirred at room temperature for 2 h. After 2 h, the solvent was removed from the reaction mixture, and the residual solid was dissolved with a small amount of DCM (3 mL) and precipitated in hexane (8 mL). The resultant white solid was redissolved in DCM

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RESULTS AND DISCUSSION Design and Synthesis of BCPs. The design of the BCP dispersant was aimed at maximizing the dispersion and minimizing the bundling of CNTs in organic solvents. In addition, our goal was to build in degradability in the polymer side chains for easy removal post-processing. To incorporate all three functionalities in the polymer structure, we choose a C

DOI: 10.1021/acs.macromol.9b00352 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. (a) Synthesis Scheme for the Functional Groups Present in VDMA and Its RAFT Polymerization (1), Nucleophilic Addition (2), and the Thiol−Thioester Exchange Reaction Forming a New Thioester Bond from a Second Thiol (3); (b) Synthesis of PSMI by RAFT Polymerization, Followed by Chain Extension Using VDMA for the Synthesis of PS-b-PVDMA; (c) Postfunctionalization of PVDMA Block by Nucleophilic Ring Opening of Azlactone Ring for the Synthesis of P1 (R = PySH), P2 (R = PySH, R′ = PS−SH), and P3 (R = PySH, R′ = PS−SH)

Figure 1. Chemical structure and schematic representation of pyrene-containing BCPs (a) P1, (b) P2, and (c) P3.

block-brush architecture. The first block is a PS block to mainly impart solubility of the functionalized polymers in common organic solvents. The second block has a high density of pyrene side chains. The pyrene groups34−36 are known to preferentially interact with the CNT surface (e.g., bind to CNT or graphene surface stronger than other aromatic planar units such as benzene, naphthalene, and anthracene) and hence solubilize them in organic solvents. To achieve high grafting density, we choose PVDMA as the second block. The fivemembered heterocyclic VDMA ring is an excellent candidate

for the second block, as it has a polymerizable vinyl group (Scheme 1a, Step1) that can be readily polymerized by RAFT. Furthermore, quantitative functionalization of PVDMA can be achieved by ring-opening the azlactone (Scheme 1a, Step 2) by nucleophiles such as amine and thiol (−NH2 and −SH).37 The resulting functional groups such as amide (RC(O)NHR′) or thioester (RSC(O)R′) are chemically reversible (Scheme 1a, Step3), though amide degradation requires much harsher conditions. Typically, in the literature, azlactone ring opening with a thiolate anion38 (RS−) is the least explored when D

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Figure 2. (a) Absorbance spectra of CNT@P1-toluene (black), CNT@P2-toluene (blue), CNT@PFO−BPy-toluene (green), and CNT@P3-THF (red), (b) pictures of CNT@P1-toluene, CNT@P2-toluene, and CNT@P3-THF taken after 20 h of standing, (c) PL emission spectra (excited the S22 transition of (6, 5) CNTs at 580 nm) for CNT@P1-toluene (black), CNT@P2-toluene (blue), CNT@PFO−BPy-toluene (green), and CNT@P3-THF (red). (d) Photoluminescence excitation (PLE) map of CNT@P2-toluene. (e,f) AFM image and height profile of CNT@P2toluene after polymer removal.

40 K while keeping the PVDMA block length (∼10 K) similar. The best dispersion was obtained with 20 and 40 K PS block lengths. On the basis of these results, we fixed the PS block length to 20 K and designed the PVDMA block to incorporate at least 60 pyrene units for the rest of the studies. Our second goal was to study if, in addition to the pyrene side groups that are necessary to latch onto and solubilize CNTs, we can achieve dispersed tubes that are also isolated and not bundled. In principle, each pyrene unit on a given polymer can bind to a different CNT, leading to bundling. In order to maximize dispersion and minimize bundling, an additional polymeric side chain was explored. Specifically, PS and PMMA chains were grafted onto PVDMA to both aid in solubilizing CNTs in the organic solvent of choice and add steric barriers to forming bundles. To incorporate these side groups along with 60 pyrene units in the second block, we synthesized PS-bPVDMA [20 K (192)−26.4 K (208)] (Table S2). This parent BCP was functionalized with (a) 1-pyrene butane thiol, resulting in P1, (b) a mixture of 1-pyrene butane thiol and thiol-terminated PS (Mn = 5 kg/mol), resulting in P2, and (c) a mixture of 1-pyrene butane thiol and thiol-terminated PMMA (Mn = 5 kg/mol), resulting in P3. Following reaction and purification, the final copolymer composition determined by 1H NMR confirmed the incorporation of ∼65 units of pyrene in P1, 64 pyrene units + 60 PS units in P2, and 70 pyrene units +50 PMMA units in P3. Further studies on the dispersion of CNTs were carried out using these optimized compositions of P1, P2, and P3 (Figure 1). The neat polymers P1 and P2 exhibit good solubility of >50 mg/mL in both THF and toluene. However, P3 was less soluble in toluene (95%) was quantified by 1 H NMR spectroscopy by integrating the peaks for the aromatic protons of the pyrene ring (7.54−8.35 ppm) (Figure S6). To optimize the composition of the BCP, we first synthesized three BCPs with P1 architecture (Figure 1a). The molecular weight (Table S1) of the first PS block was fixed at 20 K and that of the PVDMA block was varied from 2.8 K (P1A), 5.6 K (P1B) to 9.1 K (P1C). Quantitative functionalization of P1A, P1B, and P1C with 1-pyrene butane thiol resulted in 20, 40, and 64 units of pyrene, respectively. All three BCPs were used to disperse CNTs in toluene. The P1C composition (Figure S6) with 64 units of pyrene resulted in a stable, concentrated CNT dispersion in toluene compared to P1A and P1B. However, the optical density (OD) at 998 nm was low (0.033) even with the P1C dispersion of CNTs in toluene. We also conducted control experiments where the homopolymers of PVDMA (20, 40, 60 K) were quantitatively functionalized with 1-pyrene butane thiol. These PVDMA homopolymers with pendant pyrenes solubilized multiwalled CNTs but not single-walled CNTs. We also varied the length of the first PS block in the BCP dispersant from 10, 12, 20 to E

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Figure 3. Emission spectra of the P1, P2, and P3 polymers in (a) toluene and (b) THF at 2 μM concentrationwithout nanotubes. The excitation wavelength was 340 nm, and the emission was collected from 370 to 491 nm. All spectra were normalized against the excimer peak at 490 nm.

solvent-specific manner. This conclusion is substantiated by the observation that stable CNT@P2-toluene and CNT@P3THF dispersions with high dispersion yields were obtained (Table S2). The PL intensities further support this conclusion (Figure 2c). The PL intensity typically indicates the abundance of individualized CNTs and CNTs in small bundles. Both metallic CNTs in large bundles and CNT aggregates quench the PL.41 CNT@P2-toluene shows the highest PL intensity from (6, 5) CNT and (7, 5) CNT (Figure 2c) and a broad distribution of intensities from other semiconducting CNTs. However, the PL intensity from CNT@P1-toluene and CNT@P3-toluene is quite low, most likely due to the presence of larger bundles of CNTs. AFM imaging (Figure 2e,f) of tubes from the CNT@P2-toluene dispersion shows mostly isolated CNTs. Though these studies were carried out in CoMoCAT tubes, P2 works equally well to disperse large-diameter tubes with diameters between 0.7 and 1.7 nm. For example, arcdischarge CNTs with a diameter of 1.2−1.7 nm and HiPCOgrown CNTs with a diameter of 0.9−1.2 nm were also dispersed using P2 (Figure S9). The disruption of π−π interactions between the pyrene units due to sterics imposed by the PS or PMMA side chains can be indirectly quantified by monitoring the emission peak from the pyrene units. The emission spectra were measured as a function of concentration (2, 1, 0.8, 0.6, 0.4, and 0.2 μM) for all three polymers in toluene and THF. All spectra show an excimer peak at 490 nm, which is a high-intensity peak and two weak emission peaks at ∼375 and 396 nm corresponding to emission from pyrene in its unassociated or monomeric state (i.e., free pendant pyrene units). Irrespective of the solvent and the concentration, the pyrene monomer emission peaks (at 375 and 396 nm) follow the order: P3 > P2 > P1. Though we cannot fully explain the difference between P3 and P2, their higher monomer emission intensities compared to P1 clearly show that pyrene units exist more in the monomeric state and hence are more available for binding with CNTs in P2 and P3 (Figure 3). Polymer Removal by Exchange Reaction. Thioesters are an important class of functional groups that can undergo reversible trans thio-esterification process42−44 with competing thiols. The attachment chemistry for incorporating pyrene side groups that we designed has this exchangeable thioester bond. To confirm the effectiveness of this exchange reaction, P2 was dissolved in toluene and reacted with an excess of n-butane thiol (n-C4H9SH) in the presence of DBU. DBU generates the more nucleophilic thiolate anion (n−C4H9S−). The presence of large excess of n−C4H9S− shifts the equilibrium to the right as shown in Figure 4a. This reaction effectively replaces the

bundles. In toluene, while P1 and P2 resulted in dark solutions of dispersed CNTs, P3 showed no dispersion. We denote these dispersions as CNT@P1-toluene, CNT@P2-toluene, and CNT@P3-toluene. For comparison, we dispersed CoMoCAT CNTs with the commonly used π-conjugated commercial polymer PFO−BPy14 using an identical procedure. When the dispersion solvent is switched to a more polar THF, while all the three polymers P1, P2, and P3 dispersed CNTs in THF, there is a clear difference in their stability. In THF, CNT@P2THF and CNT@P3-THF were stable for months at room temperature, while the CNT@P1-THF dispersion precipitates or agglomerates within 5 min of dispersion. Dispersion yields were quantified by the UV−vis−NIR absorption spectrum (Figure 2a). The absorption spectrum for all three dispersions consisted of two sets of peaks, from 400 to 550 nm, corresponding to M-CNTs (M11) and a second set of peaks at higher wavelengths (580−800, 800−1300 nm) corresponding to S22 and S11 peaks from S-CNTs. This is expected as the polymers are designed to only disperse and not sort the semiconducting CNTs from metallic tubes. The OD of the dispersion is roughly proportional to the concentration of the CNTs. The ODs observed at 998 nm for CNT@P2-toluene, CNT@P1-toluene, and CNT@PFO−BPy-toluene were 4.20, 0.015, and 2.3, respectively. The dispersion yield calculated from the S22 peaks of (6, 5) CNTs also follows the same trend (Table S2). It is notable that CNT@P2-toluene gives a higher dispersion yield than the standard CNT@PFO−BPy-toluene. In THF, the highest OD of 0.5 was obtained for CNT@P3THF, followed by CNT@P2-THF with an OD of 0.15. These values are overall much lower than the highest OD of 4.2 achieved for CNT@P2-toluene. From these studies, in THF, only the P3 polymer with a mixed PMMA and pyrene side chain seems to be slightly effective. These are interesting results as all three pyrene-containing polymers have identical PS block length and ∼60 pyrene side groups in the second block. The main difference lies in the introduction of additional PS or PMMA side chains. Mixed side chains in the second block were introduced to disrupt the π−π interactions between the pyrene units, promote solubility in the dispersion solvent, and add a steric barrier to bundle formation. Of these polymers, P3 with PMMA side chains has a low solubility in toluene to start with; hence, it is not surprising that CNT@P3-toluene solution has low dispersion yields. In the case of CNT@P1-toluene, though the solubility is good in toluene, the π−π interactions between the pyrene units likely inhibit the efficient dispersion of CNTs, resulting in low yields. However, by tailoring the side chain, the solubility, stability, and dispersion yields can all be controlled in a F

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Figure 4. (a) Thiol−thioester exchange reaction between P2 and P2* in the presence of excess n-butane thiol, (b) absorbance spectra of the starting CNT@P2-toluene (black) solution, CNT aggregate (blue), and supernatant solution (red) after the thiol exchange reaction. Inset of CNT@P2-toluene and CNT + P2*-toluene after thiol exchange reaction showing CNT aggregation in toluene.

Figure 5. 1H NMR spectra of P2 (black) and post-wash solution of extracted P2* from CNT@P2-toluene in CDCl3 after the thiol exchange reaction (maroon) (highlighted are the peaks of pyrene aromatic protons from 7.54 to 8.35 ppm).

respectively. The 1H NMR spectrum of the post-wash solution from CNT@P2-toluene after the exchange reaction shows complete removal of the peaks from pyrene (from 7.54 to 8.35 ppm) (Figure 5b, maroon), confirming quantitative replacement of the pyrene butyl thioester and PS side chains with nbutane thiol. This quantitative exchange is due to shift in the equilibrium of the reaction because of excess butane thiol, as well as the low solubility of the resulting product, namely, pyrene thiol in toluene, which drives the reaction forward. As the new polymer P2* has no pyrene side chains, it is unable to bind to CNTs and is solubilized in the solvent. In fact, this solution process can be effectively used for polymer removal from the spin-cast film of CNT@P2 on HMDS-treated silicon substrates (Figures 2e and S7). The mild chemistry to remove the polymer from the CNTs is a highly desirable aspect of this BCP dispersant for reducing organic contaminants in a final device structure. In future, we envision that the polymerwrapped CNT dispersions in organic solvents can be potentially subjected to shear or evaporative self-assembly processes45,46 to fabricate aligned CNT arrays and subsequently subjected to the thio-ester exchange reaction to lift off the polymer from the device structure.

pyrene and styrene side group with butane side chains via the trans-thio-esterification process, resulting in a new BCP P2*. This exchange reaction was implemented on the CNT@P2toluene dispersion, resulting in complete release of the CNTs from the P2 polymer. Evidence for this exchange was gathered by monitoring the characteristic peaks from the pyrene groups (in the P2 polymer) and the CNT peaks in the UV−vis absorption studies. The starting CNT@P2-toluene (Figure 4b, black) dispersion shows a signature for pyrene from P2 at 300 nm and for (6, 5) S-CNTs at S22 (∼580 nm) and S11 (∼1000 nm) peaks. After the addition of DBU, bare CNTs crash out of the solution (Figure 4 inset), likely due to polymer removal. This aggregate was centrifuged and washed with toluene and chloroform to remove any adsorbed polymer residues. The UV−vis spectrum from the washing solution shows an absorption peak corresponding to pyrene/PS groups from 200 to 350 nm region (Figure 4b, red). The absorption spectra of the residue redispersed in o-chlorobenzene showed peaks corresponding only to the CNTs (Figure 4b, blue). The chemical identity of the exchanged polymer was confirmed by 1H NMR. The starting P2 (Figure 5a, black) polymer shows a broad peak from 7.54 to 8.35 ppm, 7.20 to 6.30, and 0.50 to 2.35 ppm corresponding to protons from pendant pyrene units, PS chains, and aliphatic protons, G

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CONCLUSIONS In summary, we have designed and synthesized a nonconjugated block-brush copolymer as a dispersant for CNTs in organic solvents. This BCP can disperse, debundle, and subsequently release CNTs in toluene. Our experiments show that these BCPs should have at least a 20 K PS as the first block and a functionalizable PVDMA as the second block. The use of PVDMA as the second block allows tuning of the solubility of the BCPs in toluene or THF and hence maximizing the CNT dispersion. The tunability comes from the introduction of pyrene and PMMA/PS side chains to the PVDMA block, using an efficient ring-opening reaction of the azlactone ring with thiol-functionalized nucleophiles. The optimized compositions show that at least 60 pyrene units are necessary in the second block to solubilize these CNTs. However, to improve the stability of the dispersion and yield of dispersion and to achieve isolated CNTs, a mixed side chain of pyrene with PS/PMMA is necessary. Of these compositions, the BCP with a close to 1:1 molar ratio of PS and pyrene side chains in the PVDMA block resulted in the highest yields and most stable CNT dispersion. If the PS side chain is substituted with PMMA, we can effectively switch the CNT-dispersing solvent from toluene to THF. Our studies highlight that while designing BCP dispersants for CNTs, in addition to the length of the solubilizing block and the number of pyrene pendant groups in the second block, it is necessary to disrupt the strong interactions between the pyrene groups to create stable dispersions. Our design does the latter effectively by introducing mixed pyrene and PS/PMMA pendant groups. The attachment chemistry for these side chains allows the application of thiol−thioester exchange reaction to completely remove the polymer wrapper in an acid-free organic medium to obtain pristine CNTs. In future, these custom-designed polymers can be potentially used for the self-assembly, precise positioning, and packing of CNTs in the device architecture in CNT-based semiconductor electronics.

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ACKNOWLEDGMENTS We acknowledge funding from the National Science Foundation grant # CMMI-1462771 and NSF-1727523.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00352. Synthetic scheme and NMR, GPC, UV−vis, and AFM characterization of the polymers and polymer-wrapped CNTs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 608-265-4258. ORCID

Michael S. Arnold: 0000-0002-2946-5480 Padma Gopalan: 0000-0002-1955-640X Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. H

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