Noncovalent Grafting of Carbon Nanotubes with Triblock Terpolymers

Article pubs.acs.org/Macromolecules

Noncovalent Grafting of Carbon Nanotubes with Triblock Terpolymers: Toward Patchy 1D Hybrids Thomas Gegenhuber,† André H. Gröschel,‡ Tina I. Löbling,† Markus Drechsler,§ Sascha Ehlert,§ Stephan Förster,§ and Holger Schmalz*,† †

Makromolekulare Chemie II, Universität Bayreuth, 95440 Bayreuth, Germany Molecular Materials, Department of Applied Physics, School of Science Aalto University, 00076 Aalto, Finland § Physikalische Chemie I, Universität Bayreuth, 95440 Bayreuth, Germany ‡

S Supporting Information *

ABSTRACT: The chemical structure and high aspect ratio of carbon nanotubes (CNTs) give rise to numerous exceptional physical properties but are also the origin for their intrinsic tendency to agglomerate. Since the full potential of CNTs is harnessed in homogeneous dispersions, e.g. in a polymer matrix, bundling of CNTs must be suppressed by compatibilizing unfavorable interfaces. We present a robust, noncovalent functionalization of multiwalled CNTs via physical grafting of polystyrene-block-polyethylene-block-poly(methyl methacrylate) (SEM) triblock terpolymers to the CNT surface in organic media. In an ultrasound-assisted approach at ambient temperature, the polyethylene (PE) middle block of SEM strongly adsorbs to the CNTs surface, yielding long-term stable dispersions of well-separated 1D hybrids with up to 3 wt % CNT content. Importantly, the strong affinity of PE toward CNTs prevents polymer desorption irrespective of the solvent conditions. The incompatible polystyrene (PS) and poly(methyl methacrylate) (PMMA) end blocks of SEM self-assemble into alternating PS/PMMA corona patches and provide excellent steric stabilization for the CNTs. Shorter PS and PMMA blocks give access to dispersions with higher CNT concentration and are more efficient in stabilizing longer CNTs. Unlike covalent functionalization methods, our approach preserves the conjugated sp2-structure of the CNTs and provides an efficient, simple and time-saving method for the preparation of polymer stabilized CNTs. The patchy PS/PMMA corona of the 1D hybrids is able to adapt to the surrounding environment as demonstrated on efficient high-content blending of PMMA with 5 wt % of welldispersed CNT/SEM hybrids.

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INTRODUCTION

reinforcing effect in composites and necessitate larger amounts of CNTs to reach the percolation threshold. In order to harness the full potential of CNTs in solution or a polymer matrix, intense efforts are undertaken to compatibilize CNTs with the surrounding medium mostly by addressing the CNT surface chemistry with the ultimate goal of enhancing processability.10−12 In general, CNTs can be modified by covalent or noncovalent methods. In the first case, the conjugated carbon framework is often oxidized under harsh conditions involving strong acids, oxidation agents, or highfrequency sonication.13,14 However, this results in a partial disruption of the CNTs sp2-structure, which especially for single-walled CNTs has negative effects on their properties.15,16 In order to attach polymer brushes, different approaches can be used namely grafting-from, grafting-through, and grafting-to

Carbon nanotubes (CNTs) are well-known for their excellent thermal and electric conductivity, optical properties, and extraordinary mechanical strength.1−3 CNTs hold great potential as reinforcing additives not only for composites4−6 but also in fields as diverse as biomedical engineering,2 optoelectronics,3 conductive materials,7 sensor technology,8 and lightweight constructions.9 While many appealing properties are the direct result of the CNT structure, i.e., nanoscale dimensions, high aspect ratio, and characteristic chemistry, these features come with the drawback of a limited dispersibility due to strong van der Waals interactions, π−π stacking, and, at critical length, formation of entanglements (often already during the synthesis).5 It remains one of the major challenges to efficiently disperse CNTs because they are prone to aggregate and bundle. These issues greatly reduce the feasible use of CNTs as additives in, e.g. polymer composites, because poor interfacial adhesion causes critical failure at the CNT/ polymer matrix interface.6 Large agglomerates reduce the © XXXX American Chemical Society

Received: November 19, 2014 Revised: January 19, 2015

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DOI: 10.1021/ma5023378 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules methods.4,5 In the case of the grafting-from approach, a large variety of initiating sites offers the possibility for ATRP,17,18 RAFT,19 ring-opening polymerization (ROP),20−22 and even combinations of, e.g., ATRP and ROP to prepare Janus-type CNTs,23 yet reactive moieties have to be anchored chemically onto the CNT surface.14 Employing functionalized CNTs with vinyl groups on the surface is an alternative method to covalently attach polymers via the grafting-through method.24,25 Grafting-to approaches such as the Huisgen azide−alkyne cycloaddition26 or other “click-chemistry” methods with endfunctionalized macromonomers27 have also been reported in this context. The noncovalent physical grafting of polymer brushes is much less explored due to lack of suitable groups with sufficiently strong interaction.28 Still, noncovalent functionalization is preferred as to preserve the sp2-structure of the CNTs and, hence, their physical properties. Noncovalent functionalization strategies rely on π−π stacking,29−31 van der Waals interactions,10 imidazolium containing copolymers,32 or Janus micelles.33 The CNT surface also acts as nucleation site for the crystallization of semicrystalline polymers such as polyethylene (PE), polypropylene, substituted polythiophenes, polyamides, and polyimides.34−37 Especially, the functionalization with PE has been intensively studied and is referred to as “soft epitaxy”, as a strict lattice matching is not necessary for the growth of PE crystals.38−43 For that, the CNT dispersion (produced by sonication) is mixed with a solution of PE in high boiling solvents (e.g., p-xylene) at temperatures well above the melting point of PE. Subsequent isothermal crystallization at temperatures above PE’s homogeneous nucleation temperature results in so-called “nanohybrid shish kebab” (NHSK) structures. There, the CNT forms the central stem and the PE crystals grow periodically on the CNT with the crystalline lamellae being oriented perpendicular to the CNT axis. Only a few reports describe CNT functionalization with PE containing block copolymers, like polyethylene-block-poly(ethylene oxide)44−46 or poly(vinyl cyclohexane)-block-polyethyleneblock-poly(vinyl cyclohexane),47 which indicates that epitaxial growth is not limited to crystallizable homopolymers. In that regard, semicrystalline block copolymers are of special interest because they self-assemble into well-defined superstructures.48 This crystallization-driven self-assembly was pioneered by Manners and Winnik, who showed that the self-assembly mechanism exhibits characteristics of “living” polymerization processes.49,50 The concomitant exceptional control over micellar growth can be harnessed to create complex micellar structures like block comicelles, noncentrosymmetric micelles, star-like micelles, and even colloidal hybrid structures.51−56 This concept was transferred to polystyrene-block-polyethylene-block-poly(methyl methacrylate) (SEM) triblock terpolymers, yielding spherical or wormlike crystalline-core micelles (sCCMs and wCCMs, respectively).48,57−61 Their self-assembly behavior is mainly influenced by the solvent quality for the PE block in the molten state, i.e., sCCMs form in bad solvents for the PE block (e.g., 1,4-dioxane) and wCCMs in good solvents for PE (e.g., THF, toluene). Such micelles contain a crystalline PE core and, due to a microphase separation of the incompatible polystyrene (PS) and poly(methyl methacrylate) (PMMA) blocks, a compartmentalized “patchy” corona. The wCCMs with a patchy PS/PMMA corona exhibit a pronounced interfacial activity59 comparable to that of Janus cylinders consisting of

polystyrene-block-polybutadiene-block-poly(methyl methacrylate) (SBM) triblock terpolymers.62,63 Here, we describe the ultrasound-assisted, noncovalent functionalization of multiwalled CNTs with SEM triblock terpolymers bearing a crystallizable PE middle block (Scheme 1). The PE middle block acts as a physical anchoring group that Scheme 1. Noncovalent Functionalization of Multiwalled CNTs with SEM Triblock Terpolymers

drives the adsorption on the CNT surface. The soluble PS and PMMA blocks form patch-like surface-compartmentalized brushes on top of the CNTs, providing not only sufficient steric repulsion to prevent agglomeration of the 1D hybrids but also excellent interfacial activity of neat patchy SEM micelles. We discuss the influence of the molecular characteristics of the SEM triblock terpolymers as well as the solvent on the functionalization efficiency, the morphology of the hybrids, and the stability of the obtained dispersions. In addition, we determine the effect of the CNT length on functionalization and incorporate the patchy CNT hybrids in a polymer matrix at high filler contents.

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EXPERIMENTAL SECTION

Materials. Baytubes C 150P (CNT1, 95% carbon purity, Bayer MaterialScience) and CNT2 (≥98% carbon purity, Aldrich) were used without further purification (specifications can be found in Table S1). Solvents for CNT functionalization were of p.a. grade and used as received. Poly(methyl methacrylate) (PMMA) was obtained from Aldrich (specification: Mw ≈ 120 kg/mol) and characterized by size exclusion chromatography (THF as eluent, PMMA calibration), yielding a number-average molecular weight of Mn = 103 kg/mol (PDI = 1.5). The polystyrene-block-polyethylene-block-poly(methyl methacrylate) (SEM) triblock terpolymers were synthesized by a combination of anionic polymerization and catalytic hydrogenation. In brief, polystyrene-block-poly(1,4-butadiene)-block-poly(methyl methacrylate) (SBM) triblock terpolymers were prepared first by sequential anionic polymerization of the corresponding monomers in toluene. The use of a nonpolar solvent results in a predominantly 1,4-addition of butadiene, which is indispensable to obtain a semicrystalline “pseudo-polyethylene” structure after hydrogenation. Subsequently, the polybutadiene middle block of the SBM precursor triblock terpolymers was catalytically hydrogenated using Wilkinson catalyst to yield the SEM triblock terpolymers. Further details on the polymer synthesis can be found in a previous publication.58 Functionalization of CNTs with SEM Triblock Terpolymers. Prior to functionalization, a SEM triblock terpolymer stock solution (10 g/L in the case of CNT1) in the respective solvent was prepared by heating the solution above the melting point of the PE block for at least 30 min in order to erase any thermal history. The applied temperatures for toluene, 1,4-dioxane, and chloroform were 70, 90, and 40 °C, respectively. Subsequently, the solutions were quenched to room temperature and equilibrated overnight. Then, the corresponding amount of CNT1 was added to 2 mL of the stock solution in a 5 B


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Macromolecules mL glass vial in order to reach the desired CNT1/SEM weight ratio in the final dispersion. For CNT2 functionalization the stock solutions were diluted accordingly to adjust the final overall concentration of the dispersion. Finally, an ultrasonic treatment for 5 min was applied with 10% amplitude (40 W) and a pulse sequence of 1 s pulse on/9 s pulse off, using a Branson Sonifier W-450 Digital equipped with a tapered titanium microtip with a tip diameter of 3/16 in. Transmission Electron Microscopy (TEM). TEM measurements were performed on Zeiss CEM 902 and 922 OMEGA EFTEM electron microscopes (Carl Zeiss Microscopy, Jena, Germany) operated at acceleration voltages of 80 and 200 kV, respectively. Images were recorded with a CCD camera system (Megaview III, OlympusSIS, Münster, Germany) and processed with iTEM image processing software (OlympusSIS, Münster, Germany) in the case of the CEM902 TEM. For the Zeiss 922 OMEGA EFTEM, images were registered digitally by a bottom-mounted CCD camera system (Ultrascan 1000, Gatan, Mü nchen, Germany) combined and processed with a digital image processing system (Digital Micrograph 1.9, Gatan, München, Germany). Samples for TEM measurements on the functionalized CNT dispersions were prepared by drop-casting 5 μL of the diluted CNT dispersions (c = 0.1 g/L) onto a carbon-coated copper grid. After 30 s residual solution was removed by blotting with a filter paper. Selective staining of the PS domains was achieved by exposing the sample to RuO4 vapor for 25 min. RuO4 was prepared in situ by the addition of 1 mL sodium hypochlorite solution to 13 mg of RuCl3. Afterward, the staining solution was quenched with aqueous sodium bisulfite solution. For cryo-TEM studies, a 2 μL droplet of the diluted CNT2@SEM2 dispersion in toluene (c = 0.5 g/L) was put on a lacey carbon-coated copper grid, where most of the liquid was removed with blotting paper, leaving a thin film stretched over the lace. The specimens were instantly vitrified by rapid immersion into liquid nitrogen and cooled to approximately 77 K using a temperature-controlled freezing unit (Zeiss Cryobox, Carl Zeiss Microscopy, Jena, Germany). The temperature was monitored and kept constant at approximately 105 K in the chamber during all of the sample preparation steps. After freezing, the specimen was inserted into a cryo-transfer holder (CT3500, Gatan, München, Germany) and transferred to a Zeiss 922 OMEGA EFTEM (Carl Zeiss Microscopy, Jena, Germany). Examinations were carried out at temperatures around 95 K. Zeroloss filtered micrographs (ΔE ∼ 0 eV) were recorded under reduced dose conditions (100−1000 e/nm2). For TEM investigations on the PMMA composite the solvent-cast film was cut into ultrathin sections (cut thickness: 50−60 nm) at ambient temperature using a Leica Ultracut UC7 ultramicrotome (Leica, Wetzlar, Germany). The sample was not stained prior to imaging. Micro-Differential Scanning Calorimetry (μ-DSC). The μ-DSC measurements were conducted on a Setaram micro-DSC III calorimeter using stainless-steel “batch” cells, closed by a screw-cap. Because of the used organic solvents, O-ring seals consisting of Perlast were employed. The pure solvent was used as a reference. The displayed heating traces correspond to the second heating step in order to exclude any influence of the thermal history of the sample.

Table 1. Molecular and Thermal Characteristics of SEM Triblock Terpolymers sample

polymera

Mnb [kg/mol]

ethyl branchesc

Tmd [°C]

Tcd [°C]

SEM1 SEM2 SEM3e

S340E700M360 S140E690M160 S355E340M395

91 50 86

2.9 3.4 6.5

49 44 4

17 30 −16

a

Subscripts denote the number-average degree of polymerization of the respective polymer blocks. bThe corresponding polybutadiene containing SBM triblock terpolymer precursors showed polydispersity indices of PDI < 1.05, as obtained by THF-SEC calibrated with PS standards. cAverage amount of ethyl branches per 100 main chain carbon atoms resulting from 1,2-addition in the polymerization of butadiene, determined by 1H NMR of the precursor SBM triblock terpolymer. dPeak melting (Tm) and crystallization (Tc) temperatures of the PE block were determined from μ-DSC measurements on toluene solutions (c = 10 g/L, scanning rate 0.5 K/min). eFor μ-DSC investigations on SEM3 a 30 g/L solution in toluene was used.

in agglomerates with 0.1−1 mm64 in size and a high bulk density and, thus, represents a good benchmark to test the efficiency of our process. The average outer diameter of both CNT types is in the range of d = 10−11 nm (specifications of the CNTs can be found in Table S1). We deliberately choose CNTs with a large diameter in order to facilitate a differentiation between functionalized CNTs and wCCMs formed by SEM triblock terpolymers that typically exhibit a PE core thickness of d ≈ 6 nm.65 The morphology of the obtained structures was investigated by transmission electron microscopy (TEM), and the melting/crystallization behavior of the PE block was monitored by micro-differential scanning calorimetry (μ-DSC) of the dispersions. Functionalization of CNT1 with SEM1. We start with functionalization experiments in toluene, using SEM1 (S340E700M360, subscripts denote the number-average degree of polymerization, Mn = 91 kg/mol) that features two symmetric PS and PMMA outer blocks and a semicrystalline PE middle block. A detailed description of the procedure can be found in the Experimental Section. In general, the SEM triblock terpolymer solution was added to the CNTs at room temperature followed by sonication. Here, a SEM1 concentration of c = 10 g/L was chosen, as this is an appropriate concentration to follow melting and crystallization processes by μ-DSC.58 Our experiments revealed that the overall concentration of the final dispersion can be significantly increased compared to the concentrations reported in literature (typically around c = 0.1−1 g/L). As an example, the CNT1/SEM1 ratio was set to 2/1 (w/w) at a total concentration of c = 30 g/L in toluene. After sonication for 5 min at room temperature (10% amplitude, pulse sequence: 1 s “on”/9 s “off”) a black and longterm stable dispersion was obtained, showing no noticeable remaining CNT1 agglomerates or precipitate (Figure 1A). Ultrasound is necessary to achieve an initial dispersion of the CNTs in the solvent, while the energy input is also sufficient to melt the SEM1 wCCMs (Tm = 49 °C, Table 1) producing triblock terpolymer unimers for functionalization. For TEM analysis, the dispersions were diluted, drop-cast on carboncoated copper grids, and stained with RuO4 vapor to distinguish PS (dark) from PMMA (bright) domains. The TEM images in Figure 1 show well-dispersed 1D structures (denoted in the following as CNT1@SEM1), indicating the successful functionalization of CNT1 with the SEM triblock terpolymer (an additional TEM overview image is

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RESULTS AND DISCUSSION For this study, SEM triblock terpolymers with varying PE block lengths and overall molecular weights were used for CNT functionalization (Table 1) as well as different types of multiwalled CNTs. The SEM triblock terpolymers were obtained via catalytic hydrogenation of the corresponding polystyrene-block-poly(1,4-butadiene)-block-poly(methyl methacrylate) (SBM) triblock terpolymers, synthesized by sequential anionic polymerization (details in Experimental Section).58 The multiwalled CNTs for functionalization were Baytubes C 150 P (CNT1, Bayer MaterialScience) with a specified length of l > 1 μm as well as CNTs with an average length of l = 3−6 μm (CNT2, Aldrich). The production process of Baytubes results C


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Figure 2A shows a comparison of the heating and cooling traces for the neat SEM1 and the SEM1 functionalized CNT1s

Figure 1. CNT1 functionalized with SEM1 in toluene at a CNT1/ SEM1 ratio of 2/1 (w/w) and a total concentration of c = 30 g/L. (A) Digital photograph of the dispersion. (B) TEM overview showing wellseparated CNT1@SEM1 hybrids. (C) TEM magnification of a few representative CNT1@SEM1s. (D) Zoom-in on one CNT1@SEM1 with clearly visible patchy PS/PMMA corona structure. Selective RuO4 staining renders PS dark and PMMA bright.

Figure 2. μ-DSC measurements on CNT1@SEM1 in toluene. (A) Heating and cooling traces of CNT1@SEM1 (dashed) prepared at a CNT1/SEM1 ratio of 2/1 (w/w) (overall concentration of c = 30 g/ L) and of neat SEM1 at a concentration of c = 10 g/L (solid). (B) Heating traces at varying CNT1/SEM1 ratios (as indicated).

in toluene. Whereas melting (Tm = 49 °C) and crystallization (Tc = 17 °C) transitions can be clearly detected for the neat SEM1 solution, these transitions are completely absent for CNT1@SEM1, despite the identical SEM1 content in both samples. This supports the conclusion drawn from TEM that the PE block selectively adsorbs to the CNT surface, which alters its thermal properties. Further evidence for the crucial role of PE as anchor block for CNT functionalization is provided by using S340B340M360, the precursor triblock terpolymer to SEM1 prior to hydrogenation, for CNT1 functionalization under identical conditions (CNT1/SBM = 2/1 (w/w), c = 30 g/L in toluene). In contrast to the experiment with SEM1, the functionalization with SBM was not successful and resulted in a flocculated solid sample, which could not be redispersed by addition of solvent or sonication (Figure S3). We conclude that the π−π interactions of both the PB and the PS blocks with the sp2hybridized CNT surface are too weak in toluene (good solvent for PS and PB) for an efficient adsorption of the terpolymer onto the CNT surface (poor CNT dispersion). A similar observation was reported for the modification of CNTs with SBM based Janus micelles. Here, no adsorption of the Janus micelles on the CNT surface was observed in good solvents for the adsorbing PS hemisphere.33 We aimed at finding the limiting CNT1/SEM1 ratio at which SEM1 is quantitatively grafted onto the CNT surface and successively decreased the CNT1/SEM1 ratio from 2/1 to 1/1 (w/w) while keeping the SEM1 concentration constant at c = 10 g/L. In all cases stable dispersions were obtained. However,

presented in Figure S1 of the Supporting Information). The CNTs clearly form the core (hollow interior with comparable contrast as the background) with a thickness of about d ≈ 10 nm corresponding to the diameter of the neat CNT1 (Table S1). The corona exhibits a patch-like compartmentalized structure with dark (PS) and bright (PMMA) domains similar to that found for the neat SEM1 wCCMs.58 The presence of neat SEM1 wCCMs can be excluded, as upon careful inspection of several TEM micrographs no worm-like micelles with a considerably thinner core (d ≈ 6 nm) could be detected. Thus, it can be deduced that the PE middle block of SEM1 selectively adsorbs to the CNTs’ surface to reduce unfavorable interfacial energies, and the soluble PS and PMMA blocks form the corona of the functionalized CNTs providing sufficient steric repulsion to keep the dispersion stable. Visualization of the PE layer surrounding the CNT core is challenging due to a too low electron density contrast and the fact that the PE layer is expected to be rather thin. In addition, μ-DSC was employed, which in former studies turned out to be a very efficient method to probe melting and crystallization processes of the PE block in SEM triblock terpolymers directly in solution.58 The use of closed batch cells allows measurements on sample volumes up to 1 mL and, thus, enables to detect thermal transitions even for low concentrated samples like in this study. The temperature profile for the measurements is given in Figure S2 and comprises an initial equilibration at 20 °C for 2 h followed by consecutive heating and cooling steps at 0.5 K/min. D


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Figure 3. Distinctive features of CNT1@SEM1 and neat SEM1 wCCMs. (A) TEM micrograph of the CNT1/SEM1 dispersion prepared in toluene at a ratio of 1/1 (w/w), overall concentration of c = 20 g/L (RuO4 staining). White circle indicates short wCCM fragments formed by excess, nonbound SEM1. (B) TEM magnification of CNT1@SEM1 and (C) of a neat SEM1 wCCM formed in toluene at c = 10 g/L.

Figure 4. TEM images of CNT1@SEM1 prepared in 1,4-dioxane at a CNT1/SEM1 ratio of 2/1 (w/w); overall concentration of c = 30 g/L (RuO4 staining). (A) Overview image and (B) magnification showing the patch-like structured corona.

in contrast to the sample prepared at a CNT1/SEM1 ratio of 2/1 (w/w) (Figure 1), free SEM1 wCCMs (white circle in Figure 3A) are present at a ratio of 1/1 (w/w). At this ratio the CNT surface appears to be saturated with SEM1 chains resulting in SEM1 self-nucleation. This is also supported by the appearance of a week melting endotherm in μ-DSC attributable to the free SEM1 wCCMs (Figure 2B). As the patch-like compartmentalized structures of both the functionalized CNT1 (Figure 3B) and the neat SEM1 wCCMs (Figure 3C) are very similar, characteristics for a distinction between both structures are necessary. Upon careful TEM image evaluation, the neat SEM1 wCCMs are usually considerably shorter compared to the functionalized CNTs; the PE core of the wCCMs is thinner and frequently shows a “zigzag”-like pattern. In the μ-DSC traces (Figure 2B) no clear melting transitions can be observed for CNT1/SEM1 ratios ≥1.2/1 (w/w), indicating that free SEM1 is absent or the content is below the detection limit of the μ-DSC (c(SEM1) = 0.5 g/L,

corresponding to 5 wt % of the initial SEM1 feed; for details see Figure S4). On the other hand, the respective TEM images (Figure S5) still show the presence of short SEM1 wCCMs up to a CNT1/SEM1 ratio of 1.4/1 (w/w). Consequently, the limiting CNT1/SEM1 ratio for quantitative grafting can only be estimated by TEM investigations due to the limited sensitivity of the μ-DSC and lies between 1.4/1 and 1.6/1 (w/w), where free SEM1 wCCMs are hardly detectable anymore. In view of applications, the maximum CNT content that can be dispersed by a given amount of SEM triblock terpolymer is another significant parameter. To this end, the CNT1/SEM1 weight ratio was increased gradually above 2/1 (w/w) keeping the SEM1 concentration constant at c = 10 g/L. For CNT1/ SEM1 ratios up to 2.6/1 (w/w) the dispersions were still stable and highly viscous without signs of gelation. The corresponding TEM micrographs (Figure S6) show well-dispersed functionalized CNTs up to this maximum CNT1@SEM1 concentration of c = 36 g/L (c(CNT1) = 26 g/L). At a CNT1/SEM1 ratio E


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Figure 5. TEM images of CNT1@SEM2 (A, B) and CNT1@SEM3 (C, D) prepared in toluene at a CNT1/SEM ratio of 2/1 (w/w); c = 30 g/L (RuO4 staining). Arrows indicate short wCCM fragments formed by excess, nonbound SEM2.

crystalline-core micelles (sCCMs) of SEM1 and, thus, provides unimers adsorbing on the CNT surface. In accordance with the results obtained in toluene, free SEM1 cannot be detected for the employed CNT1/SEM1 ratio of 2/1 (w/w). Here, the identification of excess, nonbound triblock terpolymer is simplified, as SEM1 sCCMs are easily distinguished from the 1D hybrids (Figure S8). TEM images of dispersions prepared at varying CNT1/SEM1 ratios and a corresponding discussion can be found in the Supporting Information (Figures S8 and S9). Influence of SEM Triblock Terpolymer Composition. We now focus on the influence of molecular weight and PE content exemplified on two further triblock terpolymers, SEM2 and SEM3 (Table 1). SEM2 (S140E690M160, Mn = 50 kg/mol) exhibits a PE block of similar length compared to SEM1, yet significantly shorter PS and PMMA blocks and lower overall molecular weight. The functionalization of CNT1 was conducted under identical conditions as before, i.e., CNT1/ SEM2 ratio of 2/1 (w/w) at an overall concentration of c = 30 g/L in toluene, and likewise we find well-dispersed functionalized CNT1@SEM2 with a patch-like corona (Figure 5A,B; TEM overview in Figure S10). However, in contrast to SEM1, we clearly identify free wCCMs formed by excess SEM2 that did not participate in grafting, despite the identical CNT1/SEM weight fraction. Since the PE block length is comparable for both triblock terpolymers and the space requirement on the CNT surface is similar, we ascribe the formation of free SEM2 wCCMs to the lower overall molecular weight (about half of that of SEM1) and, thus, the significantly higher molar amount of triblock terpolymer chains at identical weight fractions.

higher than 2.6/1 (w/w), the starting materials already formed a gel-like solid that could not be redispersed even with addition of further solvent. Here, the amount of available SEM1 triblock terpolymer chains is too low for a sufficient grafting density and, thus, an effective dispersion of the CNTs. This transition from still fluidic, highly viscous dispersions to gel-like solids was sharp and reproducible. It is worth mentioning that we occasionally observe small aggregates of still agglomerated CNT1@SEM1 in TEM micrographs taken at lower magnification (Figures S1 and S5). Since this is not the case for CNT2 (as we will see later), we attribute the presence of these aggregates to the production process of CNT1. The crude untreated product consists of agglomerates 0.1−1 mm in size,64 which are of course much harder to disperse compared to less agglomerated grades. Impact of Solvent Quality. The self-assembly and crystallization behavior of SEM triblock terpolymers strongly depend on the quality of the used solvent for the PE middle block in the molten state.58 Thus, the question arises whether bad solvents for PE such as 1,4-dioxane affect the functionalization of CNT1 with respect to stability and morphology of the formed 1D hybrids. Functionalization in 1,4-dioxane at identical conditions compared to toluene (CNT1/SEM1 = 2/1 (w/w), overall concentration of c = 30 g/L) yielded again a black, stable dispersion of hybrids with a patchy PS/PMMA corona (Figure 4, TEM overview in Figure S7). Surprisingly, the solvent quality for PE does not seem to play a major role for CNT modification with SEM triblock terpolymers. Despite the higher melting point of the PE block in 1,4-dioxane (Tm = 79 °C),58 the energy input from sonication is high enough to melt the formed spherical F


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Figure 6. TEM of CNT2@SEM1 prepared in toluene at a CNT2/SEM1 ratio of 2/1 (w/w) and overall concentration of c = 1.5 g/L (RuO4 staining). (A) Overview revealing well-separated functionalized CNTs. (B) Magnification showing the patch-like PS/PMMA corona.

Figure 7. TEM of CNT2@SEM2 prepared in toluene at a CNT2/SEM2 ratio of 2/1 (w/w) and overall concentration of c = 6 g/L (RuO4 staining). (A) TEM overview and (B) magnification. (C) Cryo-TEM of the same sample after dilution to c = 0.5 g/L in toluene.

the corresponding SBM precursor. This results in broadening of melting and crystallization transitions that shift to values below room temperature (Table 1). Hence, SEM3 is not able to crystallize and form wCCMs at ambient temperatures in toluene. For the sake of comparison, functionalization was performed at identical conditions as in the previous experiments with SEM1 and SEM2 (CNT1/SEM3 = 2/1 (w/w), c = 30 g/L in toluene). In contrast to the unsuccessful experiments with noncrystalline SBM, SEM3 yielded a stable dispersion with well-separated CNT1@SEM3 showing the characteristic patchlike compartmentalized corona (Figure 5C,D; TEM overview in Figure S12). This leads to the assumption that the ability of the PE block to crystallize in solution is not decisive for an

Upon further increasing the CNT1/SEM2 ratio, stable and comparatively low viscous dispersions were obtained even up to a maximum CNT1/SEM2 ratio of 3.2/1 (w/w) (Figure S11), corresponding to an overall concentration of c = 42 g/L (c(CNT1) = 32 g/L). The improved performance of SEM2 in CNT stabilization is attributed to the higher molar amount of SEM2 chains (at constant SEM feed) and to the shorter PS and PMMA corona chains resulting in less pronounced steric constraints. SEM3 (S355E340M395, Mn = 86 kg/mol) displays a comparable molecular weight to SEM1 but a significantly shorter PE block. In addition, the PE block in SEM3 contains about twice as many ethyl branches originating from the 1,2-butadiene units in G


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Macromolecules effective noncovalent functionalization of CNTs with PE containing block copolymers. This is in accordance with the observation that even amorphous PE homopolymers with a high content of short chain branches adsorb onto graphite.66 Influence of the CNT Type. In order to probe the influence of the CNT length on functionalization, we now turn to CNT2 (l = 3−6 μm) that display a significantly higher average length and narrower length distribution compared to CNT1 (l > 1 μm). The overall concentration had to be reduced significantly for an effective functionalization of CNT2 with SEM1 in toluene. This is ascribed to its higher length, leading to a pronounced entanglement of the polymer-grafted CNTs. Decreasing the CNT2 concentration to 1 g/L while keeping the CNT2/SEM1 ratio constant at 2/1 (w/w) (overall concentration c = 1.5 g/L) yielded in a low viscous and stable dispersion of well-separated functionalized CNT2@SEM1 (Figure 6, overview TEM image in Figure S13). The patchlike structure of the PS/PMMA corona is again clearly visible and appears to be slightly more uniform as compared to CNT1@SEM1. On the basis of the observation that dispersions with a considerably higher CNT1 concentration can be prepared with SEM2, we investigated whether this is also applicable for CNT2 and find stable dispersions of medium viscosity up to a maximum overall concentration of c = 9 g/L in toluene (CNT2/SEM2 = 2/1 (w/w), TEM overview in Figure S14). As an example, TEM images of a CNT2@SEM2 dispersion prepared at an overall concentration of c = 6 g/L are displayed in Figure 7A,B, showing the typical patch-like compartmentalized PS/PMMA corona. In addition, we have used cryogenic-TEM (cryo-TEM) analysis for this sample to verify the quality of the CNT2@ SEM2 dispersion in situ in solution and to exclude drying artifacts that may arise from sample preparation in conventional TEM. The corresponding cryo-TEM image shown in Figure 7C confirms well-separated single CNT2@SEM2 hybrids. Unfortunately, the corona is not visible in cryo-TEM due to the diminishing electron density contrast between dissolved PS and PMMA corona chains and the vitrified toluene background. PMMA Composites with CNT2@SEM2. We prepared PMMA (Mn = 103 kg/mol) composites with CNT2@SEM2 via solution casting from chloroform. Chloroform was chosen due to its high density (ρ = 1.48 g/cm3), which suppresses undesired sedimentation of CNT2@SEM2 during the casting process and, thus, favors a homogeneous distribution of the hybrids in the final composite. Functionalization in chloroform (CNT2/SEM2 = 2/1 (w/w)) yielded stable dispersions with an overall concentration of c = 6 g/L (TEM overview image in Figure S15). This further supports our assumption that the solvent plays only a minor role in CNT functionalization as long as the PS and PMMA corona blocks are soluble. Next, the CNT2@SEM2 dispersion was mixed with a PMMA solution in chloroform, resulting in a homogeneous mixture without any sign of CNT agglomeration or precipitation (5 wt % CNT2@ SEM2 with respect to PMMA, c(PMMA) = 50 g/L in the mixture). After slow evaporation of the solvent over several days the obtained film was cut into ultrathin sections for TEM investigations. The TEM images displayed in Figure 8 reveal an excellent dispersion of the functionalized CNT2 in the PMMA composite. Even at a low magnification (Figure 8A), CNT2 agglomerates are almost absent despite the high concentration of the hybrids in the composite (c = 5 wt %) and the considerable CNT2 length of l = 3−6 μm. This is attributed to

Figure 8. TEM of PMMA composites with 5 wt % CNT2@SEM2 prepared by solution casting from chloroform. (A) Overview and (B) magnification illustrating homogeneous dispersion.

the patchy PS/PMMA corona that is able to adapt to the polymer matrix, here PMMA, by expansion/collapse of the respective compatible (PMMA)/incompatible (PS) corona blocks. The compatible PMMA corona chains mix with the matrix and provide stabilization for efficient blending of the hybrids.

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CONCLUSION We describe a facile, ultrasound-assisted method for the noncovalent grafting of multiwalled CNTs with SEM triblock terpolymers in solution, yielding long-term stable dispersions of well-separated 1D hybrids. The semicrystalline PE middle block of the SEM triblock terpolymers selectively adsorbs on the CNT surface, while the soluble PS and PMMA blocks form a patchy corona. Through steric repulsion the compartmentalized corona provides excellent stabilization for the CNTs and adds further functionality due to the interfacial activity of those structures. The solvent quality for the PE block does not play a decisive role for functionalization, since the use of good (toluene) and bad (1,4-dioxane) solvents both result in quantitative functionalization of the CNTs with almost identical morphology and dispersion stability. The absence of melting and crystallization transitions in microdifferential scanning calorimetry on the dispersions further supports the selective adsorption of the PE block on the CNT’s surface. Interestingly, CNT functionalization even works with a SEM triblock terpolymer that is not able to crystallize in solution due to a too high content of ethyl branches in the PE middle block. This shows that the strong interactions between the PE block and the CNT surface are the dominant driving force for an efficient physical grafting. Compared to covalent grafting methods, our approach has the advantage that the sp2-structure of the CNTs is preserved: no chemical modifications are necessary, the functionalization is undemanding (room temperature and ultrasound), and the process can be adapted to any PE containing block copolymers. The potential of SEM functionalized CNTs for the application in polymer composites was H


Article

Macromolecules shown in a first experiment using PMMA as matrix polymer. The patchy PS/PMMA corona is able to adapt to the surrounding PMMA matrix and to provide efficient stabilization for the functionalized CNTs, which manifests in an excellent dispersion of the 1D hybrids as revealed by TEM. CNTs with a patchy PS/PMMA corona may find application as intelligent filler in polymer blends, simultaneously acting as compatibilizer and reinforcing agent.

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ASSOCIATED CONTENT

S Supporting Information *

Specifications of the carbon nanotubes, additional TEM images, and μ-DSC traces. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (H.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the German Science Foundation in the framework of the Collaborative Research Center SFB 840 (project A2). T.G. appreciates the support of the Elite Network Bavaria (ENB). A.H.G. is grateful for support from the Academy of Finland’s Centre of Excellence Program (20142019) and ERC-2011-AdG (291364-MIMEFUN). We thank M. Müller and A. Pfaffenberger for help with TEM measurements.

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Noncovalent Grafting of Carbon Nanotubes with Triblock Terpolymers

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