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Hierarchical PolymerCarbon Nanotube Hybrid Mesostructures by Crystallization-Driven Self-Assembly Lin Jia,†,§ Amy Petretic,† Gregory Molev,† Gerald Guerin,*,† Ian Manners,‡ and Mitchell A. Winnik*,† †
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, ON M5S 3H6, Canada and ‡School of Chemistry, University of Bristol, Bristol, U.K. BS8 1TS §Present address: Laboratory of Polymer Chemistry, Department of Polymer Materials, College of Materials Science and Engineering, Shanghai University, Nanchen Street 333, Shanghai 200444, China.
ABSTRACT Multistep crystallization-driven self-assembly has great potential
to enable the construction of sophisticated hybrid mesostructures. During the assembly procedure, each step modifies the properties of the overall structure. Here, we demonstrate the flexibility and efficiency of this approach by preparing polymercarbon nanotube (CNT) hybrid mesostructures. We started by growing polyferrocenyldimethylsilane (PFS) homopolymer crystals onto multiwalled CNTs. This first step facilitated the redispersion of the coated CNTs in both polar (2-propanol) and nonpolar (decane) solvents. In the second step of hybrid construction, a unimer solution of a PFS block copolymer was added into the PFSCNT solution. The PFS coating on the CNT initiated the growth of elongated micelles, resulting in structures that resembled hairy caterpillars. PFS-b-P2VP (P2VP = poly-2-vinylpyridine) micelles were grown from the surface of PFS-CNT hybrids in 2-propanol, and PFS-b-PI (PI = polyisoprene) micelles were grown from these hybrids in decane. These micelles, by transmission electron microscopy were seen to have an unusual wavy kinked structure, very different from the uniform smooth structures normally formed by both block copolymers. For hybrids with PFS-b-PI micelles, cross-linking of the micelle coronas locked the whole structure in place and allowed us to use the partial oxidation of PFS components to grow metal nanoparticles in the core of these micelles. We finally investigated the influence of the corona-forming block used to grow the micelles on the wettability of films made from these mesostructures. Films formed with CNT hybrids grafted with PFS-b-PI micelles were superhydrophobic (contact angle, 152°). In contrast, the surface of the films was much more hydrophilic (contact angle, 54°) when they were prepared from CNT hybrids grafted with PFS-b-P2VP micelles. KEYWORDS: carbon nanotube . crystallization-driven self-assembly . hierarchy . hybrid mesostructures . surface modification
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arbon nanotubes (CNTs) have unique mechanical, electronic, and optical properties.1 Due to intrinsic poor dispersibility and processability of CNTs, surface functionalization of CNTs has become an essential step for most applications. Unlike pure CNTs, functionalized CNTs can be well-dispersed in various media (polymers, organic solvents, and aqueous solution), and more importantly, they demonstrate novel hybrid properties offered by the materials on their surfaces.25 Combining polymers with CNTs to generate polymerCNT composites/nanocomposites has been an attractive area of research for the development of new material systems.6 Traditional polymerCNT composites were prepared by blending CNTs into various polymer matrices with the idea of obtaining ultra-high-strength or conductive materials.79 JIA ET AL.
More recently, efforts have extended to applications of CNTpolymer assemblies in nanobiotechnology,10,11 nanomedicine,12,13 microelectronics,14,15 and single-electron devices.16 CNT functionalization techniques can be separated into chemical (covalent) and physical (noncovalent wrapping) methods. Generally, chemical functionalization involves the oxidation of CNTs under harsh conditions to obtain reactive groups (e.g., OH, COOH, CO) on their surface.2,3,17,18 These active moieties allow for covalent connection to polymers (“graft to” strategy). Alternatively, the active groups can be used to introduce various polymer initiators on the oxygenated surface of CNTs via esterification or amidation for in situ polymerizations from the functionalized CNT surfaces (“graft from” strategy).3 Both methods result VOL. XXX
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[email protected],
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often forming elongated structures of uniform and controlled length.3941 A similar process has been observed for addition of this kind of unimer solution to a suspension of PFS homopolymer crystals.42,51 To extend our understanding of PFS and its crystallization, we investigated whether we could prepare in solution hybrids consisting of PFS homopolymer crystals coated on the surface of MWCNTs. After we discovered that these types of hybrids could be formed, we then used these CNT-PFS hybrids as structural elements to construct more complex objects in which PFS block copolymer micelles were grown from the surface of the PFS homopolymer in the hybrids. We examined two different types of PFS block copolymers. The first, polyferrocenylsilane-b-polyisoprene (PFS-b-PI), formed micelles with a nonpolar PI corona. The other, polyferrocenylsilane-b-poly(2-vinylpyridine) (PFS-b-P2VP), formed micelles with a polar P2VP corona. Each of these polymers presented different challenges in the formation of hybrid structures. In this paper, we describe these challenges and how we overcame them. In addition, we prepared fiber-mat membranes of these hybrids with interesting roughness on the micro- and nanoscale. We used contact angle measurements to assess how the polarity of the corona chains affected the surface properties of these membranes.
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in covalently connected CNT-graft-polymer structures. The linkages between the polymer and CNTs are permanent and mechanically stable. Careful choice of the grafted polymers allows the hybrid structures to be further modified by depositing inorganic nanoparticles (e.g., iron oxide, gold, silver NPs) in the polymer layer of the CNT-graft-polymer structures.19,20 A large number of strategies for noncovalent attachment to CNTs have been examined. These include small molecules such as pyrene and other polycyclic aromatic species that have strong ππ interactions with the CNT surface. Polymers with conjugated backbones2123 and polymers with aromatic pendant groups (pyrene, ferrocene)24,25 also interact strongly with the CNT surface. Another approach uses polymers to generate supramolecular complexes of CNTs. Watersoluble polymers used in this way include poly(Nvinylpyrrolidone), poly(styrenesulfonate), dextran, dextran sulfate, bovine serum albumin, starch, and deoxyribonucleic acid (DNA).2629 CNTs in solution, particularly multiwalled carbon nanotubes (MWCNTs), can also nucleate the formation of homopolymer crystals.3036 The polymer crystals formed on the surface of CNTs enhance the dispersibility of these hybrids in various media. In addition, the polymer nanocrystals decorated along the CNTs lead to controlled CNT suprastructures. Several groups have studied the CNT-induced crystallization of homopolymers such as polyethylene (PE),3033 poly(3hexylthiophene) (P3HT),34 isotactic polypropylene (iPP),37 polycaprolactone (PCL),35 and poly(ethylene oxide) (PEO).36 Li and his co-workers have examined in detail the nucleation and growth of PE crystals by MWCNTs, reporting a preference for relatively evenly spaced protrusion of lamellar PE crystals from the CNT backbone.3033 They referred to these types of structures as nanohybrid shish-kebabs30 and argued that they can potentially be used toward building CNTbased nanodevices.38 We have a long-standing interest in the properties of polyferrocenyldimethylsilane (PFS) and its block copolymers.3946 PFS homopolymer forms plate-like crystals in solvents such as decane or hexane. The homopolymer dissolves when these solvents are heated and recrystallizes when these solutions cool to room temperature. PFS-based block copolymers can undergo crystallization-driven self-assembly (CDSA) in solvents selective for the corona-forming block to form low-curvature assemblies (most commonly platelet or fiber-like micelles).39,4750 A remarkable feature of PFS and its block copolymer micelles is that they can nucleate seeded growth. When a solution of a cylinderforming PFS block copolymer in a good solvent for both blocks (i.e., unimer) is added to a colloidal solution of short PFS block copolymer micelles in a selective solvent, the newly added polymer deposits on and grows epitaxially from the ends of these short micelles,
RESULTS AND DISCUSSION Our goal in this project was to construct functionalizable hierarchical hybrid structures that combine multiwalled carbon nanotubes and PFS block copolymer micelles. The first step in this process was to grow PFS homopolymer crystals on the surface of MWCNTs suspended in solution. In our experimental design, we were encouraged by a report in the literature24 demonstrating that polymers with ferrocene pendant groups bind to MWCNTs, presumably because of strong interactions between the ferrocene groups and the CNT surface. We used a sample of PFS homopolymer with a number-average degree of polymerization (DPn) of 31, synthesized by anionic polymerization. For this sample, DPn was characterized by 1 H NMR, and the dispersity was determined by gel permeation chromatography (GPC, MnGPC = 6500, Mw/Mn = 1.07). We used a commercial sample of multiwalled carbon nanotubes (Aldrich, outer diameter, d = 110170 nm, mean length, Ln MWCNT = 59 μm). This sample was used without further purification, and a transmission electron microscopy (TEM) image is presented in Figure 1a of a sample prepared from a CNT suspension in THF. The most striking feature of this CNT sample is the very broad dispersity in the tube outer diameter (Figure 1a). A variety of methods have been reported to prepare “shish-kebab” structures by cocrystallizing polymers from solution in the presence of suspended MWCNTs.3033 The choice of solvents is important. VOL. XXX
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ARTICLE Figure 1. TEM images of (a) the initial MWCNT sample, (bd) SK1 (PFS31/MWCNT = 0.25 mg/0.25 mg), (e, f) SK2 (PFS31/MWCNT = 0.5 mg/0.25 mg) from decane solution (10 mL), and (g, h) two images of homonucleated PFS31 crystals. All of the PFS31-containing samples were prepared under identical conditions by adding a 0.5 mL THF solution of PFS31 (with or without CNTs) to 20 mL of hot decane (90 °C) followed by cooling to room temperature and aging 24 h.
It must dissolve the polymer at high temperature and permit polymer crystallization at a lower temperature. In addition, it must support a dispersion of the CNTs. Sonication is normally employed to promote dispersing the CNTs. In some experiments, polymer crystallization is carried out isothermally at a temperature (Tc) where homonucleation of the polymer normally does not occur. In other experiments, the hot solutions containing the polymer and CNTs are cooled slowly to room temperature.33 Both processes lead to the formation of polymer crystals on the surface of the CNTs. Hot glycerin has been used as a solvent for preparing CNT shish-kebabs with nylon-6,6 isothermally at temperatures between 170 and 185 °C. Dichlorobenzene (DCB) and p-xylene have been used to prepare polyethyleneCNT shish-kebabs by both isothermal and nonisothermal processes. Other examples can be found in the two review articles by C. Y. Li and co-workers.4,5 In our experiments, the choice of solvents was limited by the high solubility of PFS in aromatic solvents such as DCB and p-xylene as well as in tetrahydrofuran (THF). As a consequence, we developed an alternative approach based upon hot decane. Decane is not a good solvent for dispersing CNTs, even at elevated temperatures. To proceed, we used mild sonication (30 min) at room temperature to disperse a mixture of MWCNT and PFS31 in THF. This mixture remained well dispersed without settling for over a week, likely a consequence of the strong interaction between the ferrocene groups on the polymer and the MWCNTs,24 thus imparting colloidal stability. A 0.5 mL aliquot of this CNT/PFS31 mixture in THF was added to 20 mL of JIA ET AL.
decane preheated to 90 °C, and then the mixture was allowed to cool to room temperature and age for 24 h. In this way, three types of PFS31 crystal-coated MWCNTs with different weight ratios of PFS31/MWCNT were prepared. Details are presented in Table S1 (Supporting Information, SI). We refer to the MWCNT hybrid structures formed from the weight ratios of PFS31/MWCNT of 0.1/1, 1/1, and 2/1, respectively, as SK0.1, SK1, and SK2, where the number following “SK” (“shish-kebab”) indicates the weight ratio of PFS31/CNT. TEM images of SK1 and SK2 are shown in Figure 1bf, with additional images presented in Figure S1 (SI). A TEM image of sample SK0.1 is shown in Figure 4. In these TEM images, the nanohybrid structures formed by PFS31 on MWCNTs (SK1 and SK2) share aspects in common with the nanohybrid shish-kebabs formed by PE/MWCNT3032 and nylon-6,6/MWCNT4,5 as described by Li and co-workers. For example, the main feature of the structure is the perpendicular arrangement of the homopolymer crystals with respect to the MWCNT backbone. The PFS crystals are densely packed, and the structures shown in Figure 1bf resemble the nylon-6,6/MWCNT hybrid as seen in Figure 11b of ref 4. The interaction between the CNT surface and the PFS crystals strongly affects the shape of the PFS crystal. As shown in Figure 1f, PFS crystals growing from the CNT surface appear to be curved and relatively short, while PFS crystals grown in the absence of CNTs look very different. As shown in Figure 1g,h, these homonucleated crystals grow in clusters that look like yucca plants, i.e., clusters of long flat crystallites with needle-like tips that emanate from a common stem. (Additional TEM images are shown in Figure S2.) VOL. XXX
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ARTICLE Figure 2. (a) Dark-field TEM, (b) TEM, and (c) SEM images of sample SK1-MPFSPI(8) with PFS53-b-PI637 micelles grown from the PFS homopolymer crystal-coated MWCNT. (d) Schematic representation of this structure. An aliquot of the unimer solution (200 μL, PFS53-b-PI637 = 2 mg) was added to a decane solution (10 mL) containing SK1 (PFS31/MWCNT = 0.25 mg/0.25 mg). After brief swirling, the solution was allowed to age for 24 h before aliquots were taken for analysis by electron microscopy.
The outer diameter of the CNT also appears to influence the size and thickness of the crystalline PFS layer that grows from it, as one observes in the TEM images shown in Figure 1c,d. In these images, one can see that along each individual CNT the layer of the PFS crystals is uniform. Thicker layers of longer PFS crystals are found on thick CNTs, whereas thin CNTs induced the growth of smaller and thinner layers of PFS. This difference in crystal size can be attributed to the difficulty for a PFS crystal to grow from a CNT surface of high curvature, delaying the crystal growth and, thus, the overall thickness of its layer. MWCNT-PFS Shish-Kebabs Decorated with PFS-b-PI Copolymer Micelles. PFS-b-PI block copolymers with a long PI block form corecrystalline cylindrical micelles via homonucleation when a hot solution of the polymer in decane is cooled to room temperature. By TEM one finds that these micelles are long (5 to >30 μm), smooth, and very uniform in width.52 Another way to obtain these micelles is by seeded growth. For example, if a PFS block copolymer (with a long corona-forming block) in THF is added to a hexane or decane solution containing micelle fragments, one also finds smooth cylindrical micelles uniform in width. Here, however, the length of the micelles is determined by the ratio of the amount of block copolymer added to the number of seed crystallites present.39 PFS homopolymer crystals formed by homonucleation can also seed micelle growth. The micelles grow off the edges of platelet-like crystals, and in past examples, the micelles formed were of relatively uniform length and very uniform in width.42,51,53 This is important background for the experiments described below. To examine if PFS31 homopolymer crystals bound to the surface of MWCNTs can nucleate crystallization-driven JIA ET AL.
micelle growth of this block copolymer, we added an aliquot of PFS53-b-PI637 (2 mg) in tetrahydrofuran (200 μL) to a suspension of SK1 (PFS31/MWCNT = 0.25 mg/0.25 mg) in decane (10 mL) at room temperature. After brief swirling, the mixture was set aside to age for 24 h. In this way, the micelle-decorated structure shown in Figure 2 was obtained. We refer to these micelle-decorated structures as SK1-MPFSPI(4). In this notation, M refers to micelles, the subscript refers to the block copolymer forming the micelles (here PFS53-bPI637), and (4) refers to the weight ratio (block copolymer/ CNT). A full list of samples prepared is presented in Table 1. These structures in solution are likely to be uniformly coated with grafted micelles, as indicated in the cartoon in Figure 2d, and then collapse as they dry on the TEM grid. In the TEM images, particularly the dark-field image in Figure 2a, there is a clear demarcation between the electron-dense PFS homopolymer crystals adjacent to the MWCNT core and the forest of fiber-like micelles that grew from the surface. Further confirmation of this structure can be seen in the energydispersive X-ray (EDX) image in Figure S3, where a line scan drawn perpendicularly across a micelle-decorated hybrid structure shows strong signals from iron. The signal from iron is particularly intense in the middle of the structure due to the contribution of the thick PFS31 crystals. A shoulder on the signal due to iron highlights the presence of the micelles collapsed on the sides of the structure. The number-average length of the micelles protruding from the surface of the SK1 is Ln ≈ 2 μm. This result demonstrates that the PFS31 crystallites coating the MWCNT surface are effective nucleating agents for seeded micelle growth. Micelle Structure. A closer look at TEM images of these micelle hybrids yields two unexpected results. VOL. XXX
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TABLE 1. Composition of the PFS Crystal-Coated MWCNT
Structures (SK) and the PFS-Based Micelle-Decorated MWCNT Structures (SK-M) PFS31
PFS53-b-PI637
PFS17-b-P2VP170
(mg)
(mg)
(mg)
(mg)
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
0.025 0.25 0.5 0.025 0.25 0.5 0.5 0.25 0.25
MWCNT sample
a,b
SK0.1a SK1a SK2a SK0.1-MPFSPI(4)b SK1-MPFSPI(4) SK2-MPFSPI(4) SK2-MPFSPI(8) SK1-MPFSPI(8) SK1*-MPFS‑P2VP(8)c
1 1 1 2 2 2
a
PFS31 homopolymer crystal-coated MWCNT samples. The number following SK (“shish-kebab”) indicates the weight ratio of PFS31/CNT. b Micelle (M)-decorated SK samples. The subscript refers to the block copolymer forming the micelles (PFS-b-PI or PFS-b-P2VP). In the notation SK-MPFSPI(n) n refers to the weight ratio (block copolymer/CNT). The SK-MPFSPI(n) samples were prepared in decane (10 mL). c SK1* refers to a sample of SK1 sedimented from decane and then redispersed in 2-PrOH. SK1*-MPFS‑P2VP was prepared from SK1* in 2-PrOH following the protocol described in the Experimental Section.
The first interesting observation is that alongside the hybrids are a small number of very long micelles of the sort obtained by micelle homonucleation. Examples are shown in Figure 3 and Figure S4. It is unusual to find both homonucleated micelles and micelles formed by seeded growth in the same sample.40 The most surprising result, however, is the structure of the micelles grown from the PFS crystallites on the CNT. As shown in Figure 3a, these micelles have a kinked structure with wavy features and occasional small protrusions. A remarkable feature of this image is that one can also see a portion of a long smooth homonucleated micelle alongside the wavy micelles protruding from the hybrid structure. A lower magnification image showing the extreme length of these homonucleated micelles is presented in Figure S4. Figure 3b shows another image of a long smooth micelle, accompanied by shorter fragments of kinked micelles that we believe broke off from a hybrid structure as the sample dried on the grid. It appears that the crystallization of the core-forming block is frustrated in these kinked micelles and that kinks form as a consequence of changes in the direction of crystal growth. This type of structure is unprecedented for PFS block copolymer micelles, although we have previously reported that polyferrocenylgermane-bpolyisoprene (PFG59-b-PI336) formed ill-defined kinked fiber-like micelles in n-hexane (Figure S7 of ref 53). This result indicates that the crystallization of the PFG core by self-nucleation did not allow the formation of well-defined micelles. In contrast, smoother micelles of uniform width formed when these micelles were grown heteroepitaxially from seed crystallites of JIA ET AL.
Figure 3. TEM images of (a) micelles growing from a PFS31/ MWCNT hybrid structure (sample SK1-MPFSPI(8)) alongside a section of a homonucleated micelle and (b) two sections of a (looped) homonucleated micelle in the presence of micelles broken from a sample of SK1-MPFSPI(8). A lowmagnification image showing the loop structure of the long homonucleated micelle is presented in Figure S4.
PFS76-b-PDMS456 (PDMS = polydimethylsiloxane). That example is in many ways the inverse of what we observe here. PFS53-b-PI637 forms uniform micelles when homonucleated or seeded by homonucleated PFS homopolymer crystals, but kinked wavy micelles when nucleated from the surface of PFS-CNT hybrids. What we learn from these observations is that the nucleating species plays an important role in determining the structure of the corecrystalline block copolymer micelle that propagates as the micelle grows by crystallization-driven self-assembly. It also confirms the idea that the PFS homopolymer crystals grown on the CNT surfaces are very different from those formed by homonucleation. Because the micelles grown from the CNT hybrids differ in structure from cylindrical PFS-b-PI micelles VOL. XXX
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ARTICLE Figure 4. TEM images of PFS31 crystal-coated MWCNT structures (a) SK0.1, (b) SK1, and (c) SK2 from a decane solution (10 mL). After addition of a 100 μL THF solution containing PFS53-b-PI637 (1 mg) to these decane solutions, the mixtures were briefly swirled and aged for 24 h. The corresponding PFS53-b-PI637 micelle-decorated structures were obtained: (d) SK0.1-MPFSPI(4), (e) SK1-MPFSPI(4), and (f) SK2-MPFSPI(4).
examined in the past, we explored aspects of the seeded growth process to see if this difference affected the level of control we could achieve in constructing complex hierarchal structures. Micelle Length. We used two different approaches to vary the length of the micelles grown from the shishkebab structure. In the first approach, we varied the amount of diblock copolymer unimer added to the shish-kebab suspension in decane. For example, we added two different amounts of PFS53-b-PI637 in THF (1 and 2 mg, Table 1) to identical solutions of SK1 in decane. Longer micelles were obtained when the larger amount of unimers (SK1-MPFSPI(8), Figure 2 and Figure S5d,h) was used. Attempts to characterize the micelle lengths in these images with the software program ImageJ led to a number-average length, Ln ≈ 2 μm. With the smaller amount of unimers, shorter micelles (SK1-MPFSPI(4), Ln ≈ 800 nm, Figure 4e and Figure S5b,f) were obtained. In the second approach, we added the same amount of unimer (1 mg) to solutions containing the same mass of different SK hybrid samples (with different amounts of PFS crystals on the surface). As shown in Figure 4, the shish-kebab structures with the fewest and smallest PFS31 crystals (SK0.1, Figure 4a) yielded the longest micelles (Figure 4d and Figure S5a,e, Ln ≈ 1300 nm) grown from the surface. SK1, Figure 4b, yielded the micelles of intermediate length (Figure 4e, Ln ≈ 800 nm), and SK2, Figure 4c, yielded the shortest and most polydisperse micelles (Figure 4f and Figure S5c,g, Ln ≈ 500 nm). Both sets of experiments indicate that micelle length depends upon the ratio of block copolymer JIA ET AL.
added to PFS nucleation sites on the CNTs. Only small amounts of self-nucleated micelles were found in each sample. Cross-Linking of SK-MPFSPI Structures. The SK-MPFSPI hybrid structures are colloidally stable in decane. Colloidal stability is provided by the micelle-decorated architecture, in which the micelles themselves are surrounded by PI coronas. The PI corona not only provides colloidal stability to the superstructure in solution, it also offers an opportunity to cross-link the exterior hybrid shish-kebab structures. Here we explore the consequences of cross-linking the pendant vinyl groups in the PI corona using the well-established Pt(0)-promoted hydrosilylation reaction44 with Karstedt's catalyst in the presence of 1,1,3,3-tetramethyldisiloxane (TMDS).54 A sample of SK2-MPFSPI(8) (Table 1) was diluted 10-fold with decane (Figure S6a,e), treated with Karstedt's catalyst plus TMDS, and then allowed to age 24 h at room temperature. TEM images of the structures obtained are presented in Figure 5a (and Figure S6b,f). The cartoon in Figure 5e indicates the location of the PFS crystals attached to the MWCNT surface as well as the cross-linked corona chains of the micelles. This treatment with the cross-linking agent did not lead to any obvious change in the mean diameter of the PFS31 crystal-coated structure in the center of the hybrid structure. The number-average length of the protruding micelles after cross-linking was Ln ≈ 1 μm, essentially unchanged from that of the hybrid structure prior to treatment with Karstedt's catalyst plus TMDS. We refer to this structure as SK-MPFSPIXL. VOL. XXX
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ARTICLE Figure 5. TEM images of (a) SK2-MPFSPI(8) after treatment with Karstedt's catalyst and TMDS in decane to cross-link the PI corona. We refer to this structure as SK-MPFSPIXL. (b) SK-MPFSPIXL was treated with DCM to dissolve the PFS homopolymer layer and then redispersed in decane. We refer to this structure as (SK-MPFSPIXL)DCM. (c) Sample of (SK-MPFSPIXL)DCM with Ag NPs embedded in the pendant micelles. Schematic representations of (e) SK2-MPFSPI(8), (f) (SK-MPFSPIXL)DCM, and (g) (SK-MPFSPIXL)DCM with embedded Ag NPs. These drawings correspond, respectively, to the TEM images shown in (a), (b), and (c).
One anticipates that corona cross-linking in this way should improve the robustness of the external structure and prevent dissolution of the micelles when the shish-kebab hybrids are exposed to a good solvent for PFS. To examine whether the overall structure was locked in place, we suspended a sample of the crosslinked hybrids in dichloromethane (DCM), which is a good solvent for both the PFS and PI blocks. To emphasize that these cross-linked structures have been treated with DCM, we denote them as (SK-MPFSPIXL)DCM. The sample was finally redispersed in decane with the idea that the selective solvent would lead to solidification of the PFS component of the micelles. The TEM images in Figure 5b and Figure S6c,g show the structures after treatment of SK-MPFSPIXL with DCM. It is clear not only that the micelle fibers survived but the overall morphology of the micelle-decorated CNT was also maintained. Nevertheless, a very interesting difference can be observed in the central domain of the structure, adjacent to the MWCNT, where the rough dark domain corresponding to the PFS31 crystals disappeared. The darker central core now appears much smoother. We infer that this corresponds to the MWCNT itself. Scanning electron microscopy (SEM) images of SK-MPFSPIXL structures before (Figure S7a) and after (Figure S7b) treatment with DCM revealed a similar change in structure. On the basis of the mechanism of cross-linking the PI corona via a hydrosilylation reaction, we assume that most reactions occur over very short distances between neighboring vinyl groups (“intramicellar” cross-linking). Where the cylindrical micelles are in JIA ET AL.
proximity, such as close to the SK surface, intermicellar cross-linking can occur. This is the reaction that gives mechanical integrity to the entire corona of each micelle-coated shish-kebab structure. In contrast, the PFS31 crystals coated on the MWCNTs cannot be cross-linked because the homopolymer does not have vinyl groups to react with Karstedt's catalyst. As a consequence, upon immersion in DCM, the homopolymer crystals dissolved and were washed away during the sedimentationredispersion steps. This idea is confirmed in the EDX traces presented in Figures S8 and S9. These traces show signals from the same four elements (carbon, oxygen, silicon, and iron) before and after treatment with DCM. The shape of the iron signal mapping changed significantly, from a sharp peak due to the PFS31 crystals before the treatment with DCM to a much weaker signal, reflecting the absence of PFS31 crystals and the smaller amount of iron in the block copolymer in the sample treated with DCM. The resulting structure is very interesting. It consists of a hollow nanotube surrounding but not covalently bound to the original MWCNT. On a microscale and a nanoscale, these structures resemble the sausage roll often referred to as a “pig-in-a-blanket”. The nanotube “roll” consists of solvent-swollen PFS chains covalently linked to a surrounding mesh of cross-linked PI chains. The integrity of these structures may be kinetic in origin or may be preserved through the interaction of some ferrocenes from the block copolymer with the now bare surface of the CNTs,24 but we have no evidence for this point. VOL. XXX
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layer of PFS homopolymer and disk-like plates of PFS31 homopolymer crystals. In other words, this approach was not effective at generating shish-kebab suspensions in 2-PrOH. A more effective approach was to take a sample of MWCNT-PFS31 shish-kebab hybrids generated in decane and transfer it directly into 2-PrOH. We sedimented a sample of SK1 in decane (10 mL) by centrifugation and redispersed the sample in 2-PrOH (10 mL). Then the mixture was dialyzed for 2 days against 2-PrOH to remove residual decane. In this way, we obtained the shish-kebab hybrids (SK1*) shown in the TEM images in Figure 6a and b. Comparison of the images in Figure 6a,b with that in Figure 1c shows that the shish-kebab hybrid structure was largely preserved and that PFS31 crystals did not become detached from the surface of the MWCNTs during the solvent-switch process. After transfer to 2-PrOH and 3 days of dialysis, most of the sample had settled to the bottom of the dialysis tube. Nevertheless, after transfer to a vial, the solid was easily resuspended upon mild agitation. Some of the material settled out of solution on a time scale of about a day, but could easily be redispersed by agitation. For the next step in the self-assembly process, we treated a suspension of SK1* in 2-PrOH with a solution of PFS17-b-P2VP170 in THF. By itself this block copolymer forms uniform fiber-like micelles in 2-PrOH55 with a semicrystalline PFS core and a P2VP corona. In this way, the structures shown in the TEM images in Figure 6c and d were obtained. We refer to this sample as SK1*-MPFS‑P2VP(8) (Table 1) The structures decorated with PFS17-b-P2VP170 micelles seen in Figure 6c, d resemble the structures of SK1 decorated with PFS53b-PI637 shown in Figures 2, 3, and 4. At higher magnification (Figure 6d), one can see kinked and wavy micelles, indicating frustrated growth from the CNT/ PFS surface. In the SK1*-MPFS‑P2VP(8) sample, one can also find longer and smooth micelles that appear to grow from the surface. They are significantly longer than the kinked micelles but not nearly as long as the homonucleated micelles found for PFS53-b-PI637. A lower magnification image showing the entire structure is presented in Figure S12. Further studies are needed to understand the origin of these interesting structures. The most important result, however, is that in 2-PrOH the PFS31 crystallites coating the MWCNT surface can effectively initiate the growth of PFS micelles containing a P2VP corona-forming block, demonstrating the potential of the micelle-decorated approach for surface modification of CNTs. One can imagine that other amorphous polymers (analogous to PI or P2VP), which previously were introduced into CNT composites only by chemical reaction, now can be deposited through physical functionalization. Furthermore, the surface properties of CNT composites can be VOL. XXX
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Generation of Silver NPs in Corona-Cross-Linked Hybrids after Dissolution in DCM. The structural stability of these DCMtreated structures (SK-MPFSPIXL)DCM enables them to serve as a substrate for further modification, offering an opportunity to increase the complexity of the nanohybrid structure. In previous studies involving CNTbased composites, functionalization of CNTs using inorganic nanoparticles has been of great interest in the area of nanoelectronics and single-electron devices.19,20,38 Here we show that we can functionalize the polymer/CNT structure with silver nanoparticles (Ag NPs). This reaction takes advantage of a redox reaction of solvent-swollen PFS chains in corona-crosslinked micelles to reduce Agþ ions to Ag NPs. Following the procedure originally reported by Wang et al.,44 we first treated a sample of (SK-MPFSPIXL)DCM as a colloidal suspension in DCM with the oxidizing agent tris(4-bromophenyl)aminium hexachloroantimonate (“magic blue”) to convert a fraction of the ferrocene (Fe(II)) groups in the PFS block to ferrocenium (Fe(III)) ions. This step was followed by addition of a solution of Ag[PF6] in DCM and allowed to age for 24 h. The sample was transferred to decane before TEM analysis. As one can see in the TEM images in Figure 5c and Figure S6d,h, Ag NPs (ca. 7 nm in diameter) were formed. The inset of Figure 5c shows that these Ag NPs were deposited inside individual micelles. EDX analysis of these structures (Figure S10) showed strong signals for silver. Micelle-Decorated SK Hybrids in 2-Propanol. The examples described above involved hybrid shish-kebab structures formed with MWCNT-PFS31 hybrids prepared in decane and then used to nucleate growth of PFS-b-PI micelles by CDSA in decane. To extend the range of accessible structures, it would be important to generate CNT-PFS31 hybrids in polar solvents such as ethanol or 2-propanol and use them to nucleate the growth of PFS block copolymer micelles that undergo CDSA in alcoholic media. For example, diblock copolymers of PFS with long poly(2-vinylpyridine) (P2VP) blocks form fiber-like micelles in 2-propanol (2-PrOH). To explore this possibility, we first sought ways to prepare CNT-PFS31 hybrids in 2-PrOH. In our initial experiments, we attempted to generate CNT-PFS31 shish-kebab hybrids in 2-PrOH using a protocol similar to that used to form these structures in decane: A mixture of PFS31/MWCNT (0.5 mg/0.5 mg) was dispersed in THF (0.5 mL) by sonication; then the solution was added to 2-PrOH (20 mL) preheated to 80 °C. After aging for 24 h at room temperature, we obtained a suspension that settled more quickly than the sample prepared in decane. The morphology obtained by heating the mixture of PFS31 plus MWCNT in 2-propanol was also very different from that shown in Figure 1bd. From the TEM images shown in Figure S11, we infer that the objects obtained in 2-PrOH consist of a mixture of CNTs coated with a thin
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ARTICLE Figure 6. (a, b) Sample of SK1 in decane (10 mL) sedimented and then redispersed in 2-PrOH (10 mL). We refer to SK1 transferred in this way to 2-PrOH as SK1*. (c, d) PFS-b-P2VP micelles grown from the PFS homopolymer crystals of SK1* in 2-PrOH by adding a solution in THF (200 μL) containing PFS17-b-P2VP170 (2 mg) to the 2-PrOH solution (10 mL) of SK1*. After brief swirling, the solution was allowed to age for 24 h. The inset in (d) is a lower magnification image of (d), and the scale bar is 500 nm.
modified by varying the corona-forming block of the micelles. Surface Properties of Films Formed from the MWCNT-Based Nanohybrid Composites. Since we are able to modify the polarity of the coating on the CNTs, we were interested to see how this would affect the properties of membrane films formed by the CNT hybrids. For example, Laird et al.32 have shown that one can obtain superhydrophobic coatings with films of CNTs decorated with polyethylene crystals. They used a vacuum filtering approach to prepare films of their nanohybrid shish-kebabs in which they modified porosity and surface roughness by controlling the size of the PE crystals in their shish-kebabs. Following their approach, we prepared four thin-film samples of our CNT hybrids on polytetrafluoroethylene (PTFE) membranes. These samples included (i) MWCNT itself, (ii) SK1 from decane, (iii) SK1-MPFSPI(4) (MWCNT/PFS31/ PFS53-b-PI637) from decane, and (iv) SK1*-MPFS‑P2VP(8) (MWCNT/PFS31/PFS17-b-P2VP170) from 2-PrOH. All of the films were dried under vacuum for 3 days prior to carrying out the experiments described below. We used SEM measurements to examine the surface of these films and the sessile drop technique (10 μL water droplets) in conjunction with contact angle (CA) measurements to investigate their wetting JIA ET AL.
behavior. The SEM images in Figure 7 show that all four films had rough surfaces. One can clearly see the differences in the surface morphology of the structures before and after polymer decoration. In the film formed from the MWCNTs (Figure 7a,e), the CNTs appear smooth. In the inset to Figure 7e, we show that a water droplet beads up on the MWCNT film, with a contact angle of 145.5°. The SK1 film from decane (Figure 7f) shows a rough surface of PFS31 crystalcoated structures, which corresponds to plate-like crystals on the surface of MWCNTs. In the inset to Figure 7f, we show that a water droplet has a somewhat larger contact angle, 150.4°. The presence of the PFS53-b-PI637 micelles in the SK1-MPFSPI(4) film (Figure 7c,g) appears to soften the roughness, but the contact angle measured for the droplets seen in the inset to Figure 7g is even larger, 151.7°. While these contact angle values are not very different from one another, they are very different from the film of the SK1*-MPFS‑P2VP(8) fibers, with a surface enriched in P2VP corona chains. The nanohybrid structures seen in Figure 7d look similar to those in Figure 7c; nevertheless, the contact angle measured for the SK1-MPFS‑P2VP2 film was only 54.1° (see Figure 7h, inset). In other words, the P2VP corona imparts water wettability to the CNT films. This result demonstrates that the growth of micelles VOL. XXX
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ARTICLE Figure 7. SEM images of surfaces of films prepared by a vacuum filtering approach and optical images of sessile drops of water on these films. (a, e) MWCNT film, (b, f) SK1 film (MWCNT = 0.25 mg, PFS31 = 0.25 mg), (c, g) SK1-MPFSPI(4) film, (d, h) SK1*-MPFS‑P2VP(8) film. Inset images are of a 10 μL water drop on the surface of each film, along with the corresponding contact angles of these water drops.
with a different corona is a robust approach for surface modification of CNT composites. CONCLUSIONS We demonstrate a new approach to the fabrication of hierarchical structures based on MWCNT nanohybrid shish-kebabs. In the core building block, the MWCNTs are the “shish” and PFS homopolymer crystals are the “kebab” structures. PFS crystals attached to the MWCNTs serve as nucleating sites for the crystallizationdriven self-assembly of PFS block copolymer micelles. The micelles themselves have an unusual wavy and kinked shape, suggesting frustrated micelle growth by crystallization-driven self-assembly and reflect the unique structure of the nucleating surface of the CNT-bound PFS homopolymer crystals. These hierarchical structures exhibit colloidal stability in solution, presumably as a consequence of the chains in the corona of the micelles. In spite of differences between these micelles and the smoother and more uniform PFS block copolymer micelles studied previously, the micelle-decorated hybrids are suitable substrates for further modification and transformation into increasingly complex structures. For example, the PI corona of shish-kebab nanohybrids decorated with PFS53-b-PI637 micelles could be cross-linked with Karsted's catalyst þ TMDS. In this way, we obtained robust structures that maintained
EXPERIMENTAL SECTION Materials. Multiwalled carbon nanotubes (>90% carbon basis, outer diameter length 110170 nm 59 μm, Aldrich), tetrahydrofuran (ACS reagent, g99.0%), hexane (ACS reagent, g99.8%), 2-propanol (2-PrOH, g99.5%), dichloromethane (ACS reagent, g99.0%), platinum(0) 1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex (Karstedt's catalyst) in xylenes with Pt
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their overall shape when suspended in DCM, a good solvent for PFS, yielding nanotubes surrounding but not attached to the original MWCNT. The nanotubes consist of solvent-swollen PFS chains covalently linked to a surrounding mesh of cross-linked PI chains. Partial oxidation of the ferrocene units of PFS (to ferrocenium), followed by treatment with a soluble silver salt in DCM, led to the deposition of Ag NPs along the core domain of the corona-cross-linked PFS53-b-PI637 micelles. As a result, Ag NP-embedded polymer micelle/CNT mesostructures were obtained. The fabrication protocol for the CNT/homopolymer/ micelle hierarchical structures described here should in principle apply to any block copolymer that forms elongated corecrystalline micelles by seeded growth. For example, Schmalz and co-workers recently described the formation of hybrid structures of PSPEPMMA triblock copolymers with MWCNTs.56 It would be interesting to explore what kinds of structures could be obtained if they added their triblock terpolymer to CNTs decorated with PE homopolymer crystals. Another important point to emerge from our work is that the surface properties of MWCNT membranes can be modified by the choice of the corona chains in the block copolymer micelles grown from the surface of the hybrids. This opens the door to interesting applications of MWCNT hybrid materials.
2 wt %, 1,1,3,3-tetramethyldisiloxane (97%), silver hexafluorophosphate (Ag[PF6]), and tris(4-bromophenyl)aminium hexachloroantimonate (magic blue) were purchased from SigmaAldrich. Dialysis membrane tubing (MWCO = 3500) was purchased from Spectrum Laboratories, Inc. Polytetrafluorethylene membranes (porous size: 0.2 μm, d = 13 mm) were purchased from Savillex.
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N
Ln ¼
∑ Ni Li i¼1 N
∑ Ni i¼1
(1)
where Ni is the number of micelles of length Li and N is the total number of micelles examined for each sample. The average size of PFS31 crystals coated on MWCNT structures calculated in a similar way. Preparation of PFS31 Crystal-Coated MWCNT Hybrids (SK). MWCNTPFS Shish-Kebab Structures Prepared in Decane. Three solutions of the MWCNT (1 mg) and PFS31 (0.1 mg, 1 mg, 2 mg) dispersed in THF (1 mL) were prepared. The glass vials containing the mixture of PFS31/MWCNT were sealed and sonicated at room temperature for 30 min using a 70 W ultrasonic bath. Then an aliquot of each solution (0.5 mL) was added to three preheated solutions of decane (each 20 mL, 90 °C). After brief swirling, the solutions were allowed to age for 24 h before aliquots were taken for analysis by electron microscopy. We refer to the three samples prepared (see Table 1) as SK0.1, SK1, and SK2. MWCNT-PFS Shish-Kebab Structures in 2-PrOH. In the first method, an aliquot of a THF solution (0.25 mL) containing a mixture of MWCNT (0.25 mg) and PFS31 (0.25 mg) was sonicated at room temperature (30 min) using a 70 W ultrasonic bath. The mixture was then added to preheated 2-PrOH (10 mL, 80 °C). After brief swirling, the solution was allowed to age for 24 h, and an aliquot of solution was taken for analysis by TEM. As shown in Figure S11, this method did not yield uniform hybrid structures. In the second method, a sample of SK1 (MWCNT = 0.25 mg, PFS31 = 0.25 mg) was prepared in decane (10 mL). It was sedimented by centrifugation (4000 rpm/5 min), and the
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supernatant was removed. 2-PrOH (8 mL) was then added to the vial. The sample was redispersed with mild vortexing, then placed in a dialysis membrane tube (MWCO = 3500) and dialyzed against 2-PrOH for 2 days to remove residual decane. After dialysis, the solution was transferred to a glass vial, and a volume of 2-PrOH was adjusted to 10 mL. This sample is referred to as SK1* (Table 1). Preparation of MWCNT-PFS Hybrids Decorated with PFS Block Copolymer Micelles. Two PFS-based copolymers (PFS53-b-PI637 and PFS17-b-P2VP170) were used to grow micelles from the surface of the SK hybrids. Hybrids Decorated with PFS-b-PI Micelles. A solution of PFS53-b-PI637 (c = 10 mg/mL, “unimer”) in THF was prepared. An aliquot of this unimer solution (100 or 200 μL) was added to decane suspensions (10 mL each) containing the three MWCNTPFS shish-kebab structures (SK0.1, SK1, and SK2). After brief swirling, all solutions were allowed to age at room temperature for 24 h before aliquots were taken for analysis by TEM. Our notation for the samples obtained is listed in Table 1. Hybrids Decorated with PFS-b-P2VP Micelles. A solution of PFS17-b-P2VP170 (2 mg) in THF (200 μL) was added to a suspension of SK1* hybrids in 2-PrOH (10 mL). After brief swirling, the solution was allowed to age for 24 h before aliquots were taken for analysis by TEM. We refer to this sample as SK1*-MPFS‑P2VP(8) (Table 1). Corona Cross-Linking of SK2-MPFSPI(8) to Form SK-MPFSPIXL. The reaction conditions employed here were taken from ref 44. A glass vial containing a decane solution (1 mL) of SK2-MPFSPI(8) (ca. 1 mg) was flushed with nitrogen and sealed with a septum. Then TMDS (5 μL) and Karstedt's catalyst in xylene solution (1 μL) were injected to the solution by microsyringe. The solution turned brown after swirling for a few seconds. It was allowed to age for 24 h before aliquots were taken for analysis by TEM and SEM. We refer to the PI corona cross-linked sample as SK-MPFSPIXL. Generation of Ag NPs in SK-MPFSPIXL Hybrids. The generation of Ag nanoparticles in the core region of the cross-linked PFS-b-PI micelles followed the procedure described in ref 44. A sample of SK-MPFSPIXL in decane (0.25 mL) was first collected by centrifugation (4000 rpm/5 min). After discarding the supernatant, DCM (1 mL) was added. After 30 min, the sample was subjected to two additional centrifugation/redispersion cycles into fresh DCM. After dispersing the product in DCM (0.5 mL), a solution of “magic blue” (5.8 μL,1 mmol/L in DCM, 0.125 equiv with respect to total ferrocene units) was injected into the solution by microsyringe. After ca. 10 min, a solution of AgPF6 (5.9 μL, 1 mg/mL in DCM, 0.50 equiv with respect to total ferrocene units) was injected. The solution was swirled for 10 s and allowed to age 24 h. Then the product was transferred to decane (0.5 mL), and aliquots of solution were taken for analysis by TEM and EDX. Preparation of MWCNT-Based Films and Contact Angle Measurements. Four film samples were produced following the procedure reported by Li and co-workers.32 For the MWCNT film, a sample of the original MWCNT (2 mg) was dispersed in decane (10 mL) and sonicated at room temperature for 1 h using a 70 W ultrasonic bath. The MWCNT film was formed by filtering the solution through a 0.2 μm PTFE membrane. The film was rinsed with decane (2 mL) three times and dried in a vacuum oven for 3 days before contact angle investigation. Films of SK1 and SK1-MPFSPI(8) were prepared in the same way. The SK1*-MPFS‑P2VP(8) film was prepared in an analogous fashion from a solution of the hybrid 2-PrOH (10 mL). This film was washed three times with 2-PrOH and then dried in a vacuum for 3 days. The wetting properties of the four films were investigated by drop shape analysis using a Krüss DSA110 (Krüss GmbH, Germany) contact angle device. Water drops of 10 μL were placed on the surface of each film. Determination of the CA was performed using a sessile drop fitting method of the drop shape analysis software package (DSA1 v1.9). Conflict of Interest: The authors declare no competing financial interest.
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Polyferrocenylsilane-b-polyisoprene, PFS53-b-PI637 (Mn = 56 300 g/mol, Mw/Mn = 1.01)57 and PFS17-b-P2VP170 (Mn = 19200 g/mol, Mw/Mn = 1.07)55 are the same samples reported previously. Polyferrocenylsilane was synthesized by anionic polymerization and purified as described in ref 58. The number-average degree of polymerization (DPn) was determined using 1H NMR spectroscopy via end group analysis (Varian VnmrS 400 spectrometer at 25 °C, C6D6, with chemical shifts referenced to TMS). 1H NMR (400 MHz, C6D6, 25 °C), ppm, integrated peak areas are based on a peak at 0.92 ppm (part of the n-butyl end group), which was set to 3 H: 0.31 (s, Me2Si-vinyl end group, 6 H); 0.55 (broad singlet, SiMe2 backbone, 180 H 6 H per monomer); 0.71, 0.92, 1.36 (m, n-butyl end group, 9 H); 4.10, 4.27 (broad triplets, CpFeCp backbone, 248 H 8 H per monomer); 5.74, 5.98, 6.35 (dd, vinyl end group, 3 H). The integration of the n-butyl end group from the initiator (t-BuLi) was compared with the integration of the cyclopentadienyl (Cp) peaks in the PFS backbone. The determined degree of polymerization was 31. Mn = 6500 (Mw/Mn = 1.07) was given by GPC (Viscotek SECMax size exclusion chromatography (SEC) system), which was conventionally calibrated with polystyrene standards. Instrumentation. Bright-field TEM images were taken using a Hitachi H-7000 instrument. Dark-field TEM and SEM images were taken using a FEI QUANTA model FEG 250 ESEM with a BF/DF STEM detector in the high-vacuum mode with an accelerating voltage of 10 kV. EDX analyses were taken using an EDX attachment (INCA, Oxford Instruments) on the Hitachi S-5200 SEM instrument. Composition-based line scan was carried out on the relevant structures identified on the sample grid. The accelerating voltage for EDX measurements was 30 kV, and the current was 20 μA. The data were collected over a period of 3 min. Samples were prepared by placing one drop of solution on a Formvar-carbon-coated grid, touching the edge of the droplet with filter paper to remove excess liquid and allowing the grid to dry. The size of PFS31 crystals, the micelle length, and their distributions were determined by tracing more than 100 positions, individual micelles or micelles decorated on MWCNT in the corresponding structures, using the software ImageJ (NIH, US). Taking the micelle structure as an example, the number-average lengths (Ln) of the micelles were calculated as
Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b01176. TEM and SEM images as well as EDX results (PDF)
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Acknowledgment. The authors thank the Natural Sciences Engineering Research Council of Canada for their support of this research. I.M. thanks the EU for a European Research Council (ERC) Advanced Investigator Grant. We also thank Dr. Neil Coombs and Dr. Jane Y. Howe for their help in TEM measurements.
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