Conjugated Polymer Assemblies on Carbon Nanotubes

Jan 8, 2014 - Graphoepitaxial assembly of carbon nanotubes on polysynthetic twins assisted by synergistic secondary interactions. Bei-Kai Yang , Yi-Ka...
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Conjugated Polymer Assemblies on Carbon Nanotubes Jianhua Liu,†,‡ Joseph Moo-Young,§ Matthew McInnis,† Melissa A. Pasquinelli,§ and Lei Zhai†,* †

NanoScience Technology Center and Department of Chemistry, University of Central Florida, 12424 Research Parkway Suite 400, Orlando, Florida 32826, United States § Department of Textile Engineering, Chemistry, and Science, North Carolina State University, 2401 Research Drive, Raleigh, North Carolina 27695, United States S Supporting Information *

ABSTRACT: The assembling behavior of four thiophenecontaining conjugated polymers, regioregular poly(3-hexythiophene) (rr-P3HT), poly(3,3-didodecylquaterthiophene) (PQT-12), poly(2,5-bis(3- tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-14), and poly(2,5-bis(3-tetradecylthiophen-2-yl)thiophen-2-yl)thiophen-2-ylthiazolo[5,4-d]thiazole) (PTzQT-14), on carbon nanotubes was investigated through microscopic studies of nanowire formation and theoretical simulation. It is found that polymer backbone rigidity and shape influence the attachment mode on carbon nanotubes. rr-P3HT and PQT-12 have a zigzag backbone structure that allows a thermodynamically stable coaxial attachment on CNTs, providing an ordered growth front for the nanowire formation. In contrast, fused rings in PTzQT-14 and PBTTT-14 create a stair-step like backbone structure that causes a kinetically controlled wrapping conformation on CNTs, generating a twisted growth front that hinders the nanowire formation. In addition, the rigidity of polymer backbone influences the wrapping mode. Polymers with more flexible backbones (i.e., PBTTT-14) would take a dense irregular wrapping mode on CNTs. The CNT diameter plays an important role in the nanowire formation when CPs attach to the CNT in the wrapping mode. Larger nanotubes with smaller surface curvature provides a less twisted polymer growth front, allowing the formation of CPNWs.

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INTRODUCTION Conjugated polymers (CPs) and carbon nanotubes (CNTs) have been widely used as key active materials in various applications such as field effect transistors, organic photovoltaic cells, supercapacitors and functional composites, owing to the combination of their unique electrical, optoelectrical and mechanical properties.1−10 To improve the materials’ performance in these applications, well-assembled CPs and CNTs, with enhanced order and orientation at the nanoscale are expected to generate synergistic effects during the device operations.6,10−15 Due to the relatively strong π−π intermolecular interactions of CPs, crystal formation is predominately onedimensional, yielding wire-like assemblies.12 Such π−π interaction can also occur between CPs and CNTs because of the π conjugated surface of CNTs. Therefore, compared with traditional polymers, CPs have greatly enhanced interactions with CNTs, as demonstrated by their ability to efficiently debundle CNT aggregates, improving their solubility in most common organic solvents.16,17 Furthermore, CNTs can be utilized as nucleation sites of CP crystallization due to these enhanced π−π interactions. Recently, we have found that carbon nanotubes can induce the regioregular poly(3-hexylthiophene) (rr-P3HT) nanowire formation, generating a centipede-like supramolecular CP/ CNT assemblies. This bottom-up approach offers an effective method to construct intriguing CP/CNT hierarchical assemblies for organic electronic devices.18 For example, organic field-effect transistor (OFET) devices were fabricated by © 2014 American Chemical Society

directly growing crystalline P3HT nanowires on single-walled carbon nanotube (SWCNT) electrodes and showed much improved performance including higher mobility and higher current on−off ratio, compared to the OFETs with metal electrodes. Such remarkable improvement of the device performance is attributed to the improved interfacial contact via strong π−π interactions between SWCNT electrodes and the crystalline P3HT nanowires as well as the improved morphology of P3HT due to one-dimensional crystalline structure. These results clearly suggest the advantage of using carbon nanomaterial guided assembly of CP in organic electronic devices.10 Expanding the investigation of the P3HT/CNT assemblies to other CPs is essential to understand the formation mechanism of this type CP/CNT hierarchical assemblies, establishing the relationships between polymer molecular structures and the assembled structures, and further explore the potential applications. The formation of conjugated polymer nanowires (CPNWs) on SWCNTs is similar to that of hybrid shish-kebabs prepared by the SWCNT induced growth of polyethylenene (PE) single crystals, where SWCNT are the shish and PE single crystals are kebabs.19,20 Due to the van der Waals interaction between SWCNT and PE polymer chains and constraint effects derived from the high curvature of SWCNTs (inhibiting the polymer Received: July 31, 2013 Revised: December 12, 2013 Published: January 8, 2014 705

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PTzQT-14.21 Interestingly, PBTTT-14 has a greater tendency to fold than PQT-12 despite its rigid fused ring structures, and PTzQT-14 demonstrates very high rigidity due to the combination of fused ring structure and high inter-ring σ bond rotation energy barrier. Such different backbone rigidity leads to chain-folding (for rrP3HT and PBTTT-14) and chainextending (for PQT-12 and PTzQT-14) structures in CP crystals in solutions. Using these CPs to study the assembly behavior and the formation of CPNWs on CNTs will grant useful information about how the polymers’ backbone rigidity and shape determine their assembly behavior. Here we report an investigation of the effect of polymer structures and CNT diameter on the thiophene-containing CP crystallization behavior on CNTs by studying the formation of centipede-like CNT/CP supramolecular structures from CPs with different chemical structures (Figure 1). The effect of CNT diameter on the nanowire formation was studied by comparing the formation of CNT/CP supramolecular structures on SWCNTs (0.9−1.2 nm) and two types of MWCNTs with different diameters (10−20 and 110−180 nm). Transmission electron microscopy (TEM) and in situ UV−vis spectroscopy were used to monitor the growth of CPNWs. The ability of each CP to form CPNWs on CNTs was compared along with molecular dynamics (MD) simulations to understand the crystallization of the CPs on CNTs. The combination of experimental and theoretical studies suggests that the polymer structures including shape (i.e., zigzag or stair-step) and backbone rigidity have significant effect on their crystallization on CNTs.

wrapping on SWCNTs), PE polymer chains attach on the SWCNT surface along its axis and then initiate the “soft” epitaxial growth of PE single crystals. The formation process of CNT/CPNW supramolecular structures involves two primary steps: CP absorption on SWCNTs and then a further crystallization of polymers on the preabsorbed polymer to form nanowires. The orientation and conformation of the adsorbing CP (i.e., wrapping or coaxial attachment) determines the growth front structure for further CP crystallization. A welldefined growth front produced by coaxial attachment will initiate the crystallization of CPs and generate CPNWs from CNTs. On the contrary, ill-defined growth fronts generated by wrapping of SWCNTs will prevent the formation of CPNWs. Therefore, the assembling behavior of CPs on CNTs can be investigated by observing the formation of CPNWs from CNTs. The impact of polymer backbone rigidity of rr-P3HT and other thiophene-containing CPs including poly(3,3-didodecylquaterthiophene) (PQT-12), poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-14), and poly(2,5-bis(3-tetradecylthiophen-2-yl)thiophen-2-yl)thiophen-2ylthiazolo[5,4-d]thiazole) (PTzQT-14) (Figure 1) on their

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RESULTS AND DISCUSSION An approach similar to growing rr-P3HT nanowires on CNTs was applied to prepare CNT/CP supramolecular structures of PQT-12, PBTTT-14, and PTzQT-14 (Supporting Information). An appropriate cooling rate in the nanowire growing process is very important to generate ordered structures. A small cooling rate (∼20 °C/h) usually leads to more ordered structures, especially for PTzQT-14 and PQT-12. SWCNTs usually offer more uniform and homogeneous CNT/CPNW supramolecular structures than MWCNTs due to their uniform diameter (0.9−1.2 nm) and straight shape.18 Therefore, the investigation of the effect of polymer molecular structures on CNT/CPNW supramolecular structure was focused on the SWCNT system. The effect of CNT diameter on the CPNW formation was examined by comparing SWCNT and MWCNT systems. SWCNT/CPNW supramolecular structures prepared from different CPs were examined by TEM (Figure 2 and Figure S1, Supporting Information). As shown in these TEM images, all four polymers form nanowire crystals with different widths because of their different crystallization behavior.8 rr-P3HT and PQT-12 nanowires perpendicularly attached on the SWCNTs, forming the centipede-like supramolecular structures. In contrast, PBTTT-14 and PTzQT-12 nanowires do not have any orientations or ordered connections with SWCNTs, indicating the absence of SWCNT/CPNW supramolecular structures. The obtained SWCNT/rr-P3HT nanowire supramolecular structures have the same morphology as those reported previously using rr-P3HT of 14.5 kDa,18 although a much higher Mn (35 kDa) of rr-P3HT was applied here. All rrP3HT nanowires growing on SWCNTs have a width around 15 nm because of the chain folding of rr-P3HT inside its nanowires.7 The PQT-12 used to grow nanowires did not

Figure 1. (A) Chemical structures of four thiophene-containing CPs and (B) backbone structures and the corresponding simplified AB curves illustrating the backbone conformation of rr-P3HT (zigzag), PQT-12 (zigzag), PBTTT-14 (stair-step), and PTzQT-14 (stair-step). Two repeat units presented for each CP, the arrows indicating the direction of A and B parts in AB curves (inside AB curves) and the polymer backbone extending direction (under the AB curves).

crystallization behavior in solution has been investigated in our group through structure characterization, theoretical simulation, and single molecule fluorescence spectroscopy (SMS).21,22 Our studies suggest that the inter-ring σ bond rotation energy barrier and backbone conformational transformability of CPs determine the polymer chain flexibility which gives a chainfolding tendency of rrP3HT > PBTTT-14 > PQT-12 > 706

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Figure 3. TEM images of CP nanowires (A, rr-P3HT; B, PQT-12; C, PBTTT-14; D, PTzQT-14) induced by MWCNTs (d = 10−20 nm). Arrows in the images indicating the MWCNTs; scale bar 100 nm.

Figure 2. TEM images of CP nanowires (A, rr-P3HT; B, PQT-12; C, PBTTT-14; D, PTzQT-14) induced to by SWCNTs to form SWCNT/CPNW supramolecular structures. Arrows in the images indicating the SWCNTs, scale bar: 100 nm.

than one hour). The evolution of UV−vis absorption of CPs with SWCNTs and MWCNTs during the crystallization process is compared with that of CP alone in their marginal solvents (Figure S4−S7). For these polymers, the absorption at short wavelength around 450 nm is attributed to individual or isolated polymer chain in solution, while the low-energy absorption bands around 600 nm appear as polymer nanowire assemblies. The absorbance in these fine structural peaks can be employed as a quantitative method to evaluate the amount of the assembled CPNWs on CNTs in the dispersion. Therefore, the process could be investigated by plotting the change of the UV−vis absorbance at long wavelength versus time. Figure 4 shows the crystallization of four CPs with and without CNTs. The crystallization of rr-P3HT and PQT-12 is enhanced by the presence of both SWCNTs and MWCNTs while the crystallization of PTzQT-14 is only enhanced by the presence of MWCNTs. In contrast, the crystallization of PBTTT-14 is neither enhanced by SWCNTs or MWCNTs. Both the TEM and UV−vis studies suggest that the polymer structure affects the formation of CPNWs from CNTs. Although the four CPs we studied here can form nanowires without carbon nanotubes, only rr-P3HT and PQT-12 can grow nanowires from SWCNTs to form centipede-like supramolecular structures. As to CPNW/MWCNT supramolecular assemblies, PTzQT-14, similar to rr-P3HT and PQT-12, is found to be able to form the supramolecular structures. Such results indicate that both CP structures and carbon nanotube diameter affect the supramolecular structure formation. Understanding this effect is essential to reveal the interaction mechanism between CNTs and CPs, and grant the guidelines to control the CNT/CPNW supramolecular structures. Similar to CNT/PE shish-kebabs that the polymer chains in kebab and shish have the same orientation along the long axis of CNTs, the CP chains absorbed on CNTs and inside nanowires also have the same orientation. Given the perpendicular growth of nanowires from the SWCNTs and the small size (∼1 nm) of SWCNTs, the chain inside nanowires should be parallel the SWCNT long axis. Therefore, the CPs in the initial absorption layers have an orientation along the SWCNTs long axis (coaxial attachment), providing a crystalline

undergo fractionation and had a large molecular weight distribution. Since the PQT-12 backbone fully extends in nanowire formation,21 PQT-12 nanowires have a width proportional to the molecular weights and range from 10−40 nm. However, the PQT-12 nanowires in SWCNT/PQT-12 supramolecular structures (Figures 2B and S1B) usually have a larger width (30−40 nm), showing a trend that PQT-12 with higher molecular weight form nanowires on SWCNT more easily. Because higher Mn CPs have lower solubility and a stronger per-chain interaction with CNTs, they tend to precipitate out from the solution first and generate nanowires on CNTs. Although PBTTT-14 and PTzQT-14 cannot form centipede-like supramolecular structures on CNTs, their nanowire crystals in solution are similar to those prepared without SWCNTs.21 MWCNTs with a diameter of about 10−20 nm were used to induce the formation of CP nanowires to investigate the effect of CNT diameter. TEM images in Figure 3 and Figure S2 show the obtained results of four CPs. Similar to the results observed in the SWCNT system, rr-P3HT and PQT-12 can grow nanowires on MWCNTs (Figures 3A and 4B) to form MWCNT/CPNW supramolecular structures. PQT-12 nanowires with larger widths (30−40 nm) also show a higher tendency to grow on MWCNTs. However, PBTTT-14 and PTzQT-14 that cannot grow nanowires on SWCNTs show interesting and different results in the MWCNT system. PTzQT-14 can grow nanowires from MWCNT surface while PBTTT-14 still cannot. However, PBTTT-14 can form nanowires from CNTs as the diameter of CNTs increased to 100 nm (Figure S3). The formation of nanowires on CNTs from rr-P3HT, PQT12, PBTTT-14, and PTzQT-14 has also been monitored using in situ UV−vis spectroscopy. Since PQT-12 and PTzQT-14 have low solubility in their marginal solvents (anisole and chloroform mixture and dichlorobenzene, respectively.), the crystallization temperature for these two polymers was set to 50 and 60 °C. The crystallization process of these polymers on CNTs can be considered quasi-isothermal since the quenching of the hot solution to room temperature takes much less time (one minute) than the completion of the crystallization (more 707

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Figure 4. UV−vis absorbance changes around 600 nm for four CPs when they crystallize in their marginal solvents with and without CNTs.

Table 1. Snapshots from the Final Timestep of Each MD Simulation of the (9,9) Armchair SWCNT with Each CPa

a

The starting configuration of the polymer was parallel (left) and perpendicular (right) to the SWCNT axis.

(1) Polymers with flexible backbones such as poly(phenylacetylene) (PPA)28 have more of a tendency to wrap CNTs than rigid polymers. (2) In organic solvents, CPs with linear rigid backbones including poly(aryleneethynylene)30,31 and poly(fluorene)32,35 tend to attach to CNT surfaces along nanotube axes. In contrast, the polymers with stair-step structures such as poly(p-phenylenevinylene) (PmPV)23 and poly(phenylenevinylene) (PPV)26 wrap CNTs. (3) Charged side chains on CPs make the polymer soluble in water; the rigid polymer tends to wrap CNTs probably due to the strong interaction with water and weaker interaction with CNTs.25 In the coaxial attachment mode, CPs are absorbed along the axis of SWCNTs. The ordered conformation along the SWCNT axis can support further nanowire growth. In contrast, polymer chains in the wrapping mode are bound to SWCNT surfaces through a helical wrapping. The conformation of CP is more disordered, which impedes the nucleation and further growth of

growth front with ordered polymer conformations to initiate the nanowire growth. The observation of nanowire growth from SWCNTs for rr-P3HT and PQT-12 indicates a coaxial attachment on CNTs and the absence of nanowires on SWCNTs (PBTTT-14 and PTzQT-14) suggests that the polymers wrap the nanotubes. Typically, CPs can adopt two attachment modes, coaxial attaching and wrapping, on SWCNT surfaces depending on the relative degrees of polymer−polymer, solvent−polymer and CNT-polymer interaction, as well as the polymer rigidity and linearity. Polymers may wrap or orient coaxially along CNTs due to chemical interactions between the polymer functional groups or backbone and the CNT π-system through π-stacking, electrostatic interactions, hydrogen bonding with oxidized sites on CNTs, or others.23−36 On the basis of previous results from different polymer systems, the impact of polymer structures on the CP crystallization on CNTs can be summarized as follows: 708

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Table 2. Average Energy Terms That Are Calculated from the MD Simulation Trajectory over the Final 50 000 Steps: The Interaction Energy (Einter) between the CNT and the Polymer, Conformational Energy (Econf), Coulombic Energy (Ecoul), and van der Waals Energy (Evdw) Einter (kcal/mol) P3HT PQT12 PBTTT14 PTzQT14

∥ ⊥ ∥ ⊥ ∥ ⊥ ∥ ⊥

−341.6 −260.0 −377.5 −305.2 −380.0 −329.3 −380.7 −418.3

± ± ± ± ± ± ± ±

4.4 5.3 4.8 4.5 5.6 5.3 5.4 5.8

Econf (kcal/mol) 149,507 150,082 149,891 149,904 150,065 150,053 149,191 149,164

± ± ± ± ± ± ± ±

21 19 21 22 24 51 27 26

Ecoul (kcal/mol) 65 65 46 52 53 49 151 138

± ± ± ± ± ± ± ±

6 8 7 9 9 9 13 12

Evdw (kcal/mol) −1973 −1676 −1965 −1921 −2075 −2052 −1943 −1959

± ± ± ± ± ± ± ±

8 8 8 9 9 11 8 8

energy (Econf), Coulombic (electrostatics) energy (Ecoul), and van der Waals energy (Evdw). The latter three terms are for the entire system, thus the Ecoul and Evdw terms include the energy due to interaction between the two components and the intramolecular nonbonding contributions, whereas Econf represents all of the ″bonded″ contributions, such as energies due to bonds, angles, and torsions. As shown in Table 2, P3HT and PQT-12 have higher Einter when the polymer attachment starts with a position parallel to CNT axis, leading to a preferred coaxial attachment. In contrast, Einter of PTzQT-14 is higher when the polymer attachment starts with a position perpendicular to CNT axis, suggesting a preferred wrapping mode. The Ecoul of PTzQT-14 is much higher than those of other polymers due to the presence of the nitrogen atoms along the backbone. The higher nonbonded energy leads to stronger interaction between PTzQT-14 and CNTs. It is interesting to notice that Einter of PBTTT-14 is higher when the polymer attachment starts with a position parallel to CNT axis, although it is observed that PBTTT-14 does not coaxially attach to CNT. Such phenomenon is due to the significant number of backbone atoms that are not close to the CNT surface, which will be discussed in next paragraph. The Einter differences due to the different starting conformations and positions of the aliphatic groups highlight how the alignment (and thus molecular wire formation) can also be dictated by the polymer’s local environment, including choice of solvent, and the processing conditions. The distance between the polymer chains and CNT surfaces also provides information about the configuration of conjugated polymers on CNTs. A few observations can be made from the plots of the interfacial distance of each sulfur atom relative to the CNT surface for each CP, which is given in Figure S9. In general, the backbones of the polymers are relatively close to the CNT surface (within 3−4 Å), and this observation is more true for the polymers that have a parallel initial configuration as compared to the perpendicular one for all four polymers. The most variability in the interfacial distance was observed for both the parallel and perpendicular configurations of PBTTT-14. The system energies in Table 2 indicate that the van der Waals energies are greatest for PBTTT-14 which is attributed to its long alkyl chains. The microscopic observation and theoretical simulation suggest that PBTTT-14 has the tendency to attach CNTs in an irregular wrapping which is probably due to the folding of the polymer backbone and the interaction of the long aliphatic side chains with CNTs (refer to the MD simulation snapshots in Table 1). Such irregular structure prevents the polymer crystallization from CNTs. The different interaction modes with SWCNTs (wrapping or coaxial attachment) are attributed to the shape and rigidity of

nanowires. Theoretically speaking, a coaxial attaching mode leads to the maximum π−π interaction and less conformational torsion energy, making it thermodynamically more stable than the wrapping mode.27,33 However, the wrapping mode of CPs on SWCNTs is often observed due to a kinetic trap. It is noteworthy that wrapping or coaxial attaching of a CP chain on SWCNTs is still controversial due to the lack of effective characterization methods and the complicated kinetically controlled interaction process. Our studies on the nanowire formation on pristine SWCNTs imply that rr-P3HT and PQT12 attach along the SWCNT axis while PTzQT-14 and PBTTT-14 wrap SWCNTs. To confirm this deduction, molecular dynamics (MD) simulations of the interfacial binding as well as the distance between the single-walled carbon nanotubes (SWCNTs) and different CPs are investigated. Table 1 summarizes the final conformations of each polymer relative to the SWCNT axis from two different starting geometries, where the polymer is either initially parallel or perpendicular to the longitudinal axis of the SWCNT. These results suggest that all four polymers have the potential to align coaxially on the surface. Our results agree with other theoretical investigations which also indicate the coaxial attachment of rr-P3HT on SWCNTs.34,37 However, it is interesting to note that the coaxial alignment for both P3HT and PQT-12 is independent of the starting conformation, whereas there is a degree of variability and some degree of wrapping for PBTTT-14 and PTzQT-14. Thus, this variability alludes to a degree of disorder that could inhibit the formation of supramolecular structures. Another interesting observation from the simulation is the different wrapping behavior between PBTTT-14 and PTzQT-14 at starting configuration perpendicular to SWCNT axis. PBTTT-14 takes a compact wrapping mode perpendicular to the SWCNT axis with sharply folded polymer backbones while PTzQT-14 takes a loosely wrapping mode along the SWCNT axis. Such different wrapping behavior is probably attributed to the fact that PTzQT-14 has a much more rigid backbone than PBTTT-14.21,22 Studying the interaction energy between CPs and CNTs can also shed light on the crystallization of CPs on CNTs. The interaction energy (Einter) is calculated by separating the conformation of the polymer from the conformation of the CNT, performing a single point energy calculation on each isolated component, and subtracting those energies from the total potential energy of the combined system. Thus, since the conformational energy will be the same for the combined system and the sum of the individual systems, Einter only takes into account the Coulombic (electrostatic) and van der Waals energies that result from the interaction between each CP and the CNT. Table 2 lists the interaction energy, conformational 709

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and PBTTT-14 nanowires can only grow from MWCNTs with suitable diameters. Thiophene rings in rr-P3HT and PQT-12 provide a zigzag backbone structure that allows a thermodynamic stable coaxial attachment on CNTs, providing an ordered growth front for the nanowire formation. In contrast, fused rings in PTzQT-14 and PBTTT-14 create a stair-step like backbone structure that causes a kinetically controlled wrapping conformation on CNTs, generating a twisted growth front that hinders the nanowire formation. In addition, the rigidity of polymer backbone influences the wrapping mode. Polymers with more flexible backbones and long alkyl side chains (i.e., PBTTT-14) would take a denser irregular wrapping mode on CNTs. The CNT diameter plays an important role in the nanowire formation when CPs attach the CNT in the wrapping mode. Larger nanotubes with smaller surface curvature will create less twisted polymer growth front, allowing the formation of CPNWs. The understanding of the impact of polymer structures and CNT size on CPNW formation provides important information to design hierarchical CNT/ CPNW supramolecular structures.

CPs. Usually, a CP is composed of a conjugated backbone and substituted alkyl side chains where the conjugated backbone interacts with SWCNT through π−π interaction (major) and alkyl side chains interact with SWCNT through van der Waals interaction (minor). Simplified AB curves in Figure 1B show that rr-P3HT and PQT-12 have the symmetric AB curve (A = B), generating a zigzag backbone structure. On the other hand, PBTTT-14 and PTzQT-14 have asymmetric AB curves (A > B) due to the incorporated fused thiophene or thiazole rings, demonstrating a stair-step backbone structure. For the rr-P3HT and PQT-12, the short and identical conjugated length of portion A and portion B leads to weak π−π interactions with SWCNTs, allowing the polymer to arrange the entire chain along the SWCNT axis easily. For PBTTT-14 and PTzQT-14, the portion A has much longer conjugated length than B and has stronger interaction with SWCNTs. Therefore, portion A tends to dominate the initial interaction with CNTs by attaching along the SWCNT axis, and consequently trap polymer conformation, causing the polymer chain to wrap SWCNTs. In summary, rr-P3HT and PQT-12 having the same zigzag backbones but side chains with different in length and density interact with SWCNTs following the coaxial attachment mode. PBTTT-14, having similar side chain length and density but different backbone structure (i.e., stair-step) from PQT-12, interacts with SWCNTs through the wrapping mode. Such observation suggests that the π−π interactions between conjugated backbone and SWCNT are much stronger than the interaction between alkyl chain and SWCNT and the CP backbone structure plays a major role in determining the interaction modes. PBTTT-14 and PTzQT-14 have stair-step like backbones and wrap SWCNTs. However, the wrapping mode of PBTTT-14 is different from that of PTzQT-14 due different chain rigidity. Compared with SWCNTs, MWCNTs have a much larger diameter and less surface curvature. The polymer chains wrapping MWCNTs are therefore less twisted than those wrapping SWCNTs (Figure S8), which might provide the growth front required for the nanowire formation. The observed PTzQT-14 nanowires growing on MWCNTs (10− 20 nm) confirmed such diameter effect induced by SWCNTs and MWCNTs. Moreover, it is interesting to observe that PBTTT-14 nanowires cannot grow on the MWCNTs (10−20 nm), which indicates that the conformation of PBTTT-14 absorbed on the MWCNTs is still not flat enough for the nanowire growth due to the compact wrapping discussed previously. In contrast, PTzQT-14 loosely wraps the nanotube along the curve with a lower curvature because of its rigid backbone, diminishing the impact of nanotube diameter on the growth front. Therefore, PTzQT-14 can grow nanowire on MWCNTs with a diameter about 10−20 nm, while PBTTT-14 cannot. On the other hand, PBTTT-14 nanowires will grow from the CNTs of a larger diameter, as shown in Figure S3. PBTTT-14 grows nanowires from the MWCNT surface with a diameter of 110−180 nm, forming the centipede-like supramolecular structures.

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EXPERIMENTAL SECTION

Chemicals and Materials. Poly(3,3‴-didodecyl quarter thiophene) (PQT-12; Mn = 21 kDa; PDI = 2.2) was purchased from American Dye Source, Inc. Poly(2,5-bis(3-tetradecylthiophen-2yl)thieno[3,2-b]thiophene) (PBTTT-C14; Mn = 28 kDa; PDI = 2.1) (lisicon SP210) and regioregular poly(3-hexylthiophene) (P3HT; Mn = 35 kDa, PDI = 1.6) were donated by Merck Chemicals, Ltd. Poly(2,5-bis(3-tetradecylthiophen-2-yl)thiophen-2-yl) thiophen-2ylthiazolo[5,4-d]thiazole) (PTzQT-14; PDI = 18 kDa, PDI = 1.9) was synthesized according to the reported procedure.38 HiPco singlewalled carbon nanotubes (SWCNTs) (diameter, 0.9−1.2 nm; length, 100−1000 nm) was purchased from Carbon Nanotechnologies with a purity above 65%. Multiwalled carbon nanotubes (MWCNTs; diameter, 10−20 nm) were purchased from Nanolab with a purity above 95%. MWCNTs (diameter: 110−180 nm) were purchased from Sigma Aldrich. These CNTs were used directly in the experiments without any further purification or chemical modification. Chloroform, tetrahydrofuran (THF), anisole, and 1,2-dichlorobenzene (DCB) were purchased from Acros Organics (New Jersey, USA) and used as received. Characterization. The average molecular weight (Mn) and polydispersity index (PDI) were determined by GPC (JASCO LC2000) equipped with a diode-array UV−vis detector, and a refractive-index detector, using polystyrene as standards and THF as an eluent. TEM images were obtained on JEOL 1011 electron microscope at 100 kV. Preparation of CNT Dispersion. CNT dispersion was prepared by sonicating CNTs and a conjugated polymer in chloroform. The mass ratios of conjugated polymer to SWCNTs, MWCNTs (d = 10− 20 nm), and MWCNTs (d = 110−118 nm) are 2:1, 1:1, and 1:3, respectively. A typical experimental procedure is following: 4.0 mg MWCNTs (d = 10−20 nm) and 4.0 mg conjugated polymer (PQT12) were mixed in 8.0 mL chloroform and ultrasonically agitated for 1 h in an ice−water bath using a horn ultrasonicator. The concentration of CNT dispersions (counting on CNTs) prepared from rr-P3HT, PQT-12, and PBTTT-14 is 0.5 mg/mL. The concentration of CNT dispersions prepared from PTzQT-14 is 0.25 mg/mL due to the low solubility of PTzQT-14. The obtained CNT dispersion was then treated as nucleation seeds to induce the crystallization of the conjugated polymer that was applied to disperse CNTs in the corresponding marginal solvent. Preparation of CNT/CPNW Supramolecular Structures. The CNT/CPNW supramolecular structures were prepared by adding the CNT dispersion to induce the CPNW formation in a marginal solvent. The conjugated polymer concentration is usually 0.1 mg/mL, and the mass ratio of conjugated polymer to CNTs was controlled by the

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CONCLUSION In summary, the CP assemblies on CNTs forming centipedelike CNT/CPNW supramolecular structures were investigated. Four thiophene-containing CPs with different molecular structures were applied to grow nanowire crystals on both SWCNTs and MWCNTs. rr-P3HT and PQT-12 nanowires can grow from both SWCNTs and MWCNTs, while PTzQT-14 710

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added amount of CNT dispersion. The marginal solvents for rr-P3HT, PQT-12, PBTTT-14, and PTzQT-14 are anisole, anisole and chloroform mixture (v/v: 3:1), dichlorobenzene, and dichlorobenzene, respectively.8 In a typical procedure, 0.2 mg PQT-12 was first dissolved into 2.0 mL anisole/chloroform mixture (v/v = 3:1) at 90 °C. After adding 40.0 μL of the MWCNT dispersion ([MWCNT] = 0.5 mg/mL), the obtained mixture solution (PQT-12/MWCNTs mass ratio =10) was cooled to room temperature with a low cooling rate, and kept at room temperature for 12 h. MD Simulation Details. Accelrys’ Materials Studio version 5.5 was used to build each molecule, using the polymer consistent force field (PCFF). The polymers were constructed with the polymer building utility to have a final length of approximately 100 Å. A monomer unit was first constructed and then geometry optimized with the Forcite module. The polymer was built and geometry optimized with random torsion angles; for PTzQT-14, it was necessary set them to 90° in order to maintain linearity of the polymer. In addition, (15,0) and (9,9) SWCNTs were constructed using the nanostructure builder utility to be approximately 200 Å in length and 12 Å in diameter, and then geometry optimized. For each polymer-SWCNT configuration, the polymer chain was placed at a distance of 8 to 16 Å from the SWCNT surface where the polymer is initially either parallel or perpendicular to the SWCNT axis. The orthogonal distance was adjusted such that the polymer and SWCNT were as close together as possible without any physical contact. LAMMPS2005 was used to perform the MD simulations. The number of molecules, volume, and temperature (NVT) were held constant, and periodic boundary conditions were not included, thus the system contains infinite volume. The time step was set to 1 fs, with a minimization time of 0.02 ns and a total time of 0.5 ns; the total potential energy of the system was monitored to indicate that equilibration was achieved. The interfacial distances between each sulfur atom in the polymer and the CNT surface were determined as follows using TCL scripting within the Visual Molecular Dynamics (VMD) software.39 At each time step, the script looped through every sulfur atom within the polymer. Since the CNT is aligned in the transverse direction along the Z-axis, the nearby CNT surface atoms can be defined by finding the set of CNT atoms that are within ±2.5 Å of the sulfur atoms. The center of mass of this set was then obtained, and the distance between the sulfur atom and the center of mass of the nearby CNT fragment was calculated. Since the center of mass of the CNT fragment will be on the interior of the CNT rather than on the surface, the diameter of the CNT (6.1 Å) was then subtracted from this distance value.

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ACKNOWLEDGMENTS Support from the National Science Foundation (DMR 0746499 and ECCS 1102228) and the Scialog Program from the Research Corporation for Science Advancement are gratefully acknowledged.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

TEM images of various CP nanowires induced by both SWCNTs and MWCNTs at the low magnification, conjugated polymer nanowire formation on CNTs monitored by in situ UV−vis spectroscopy, schematic illustration of a conjugated polymer backbone wrapping carbon nanotubes, and a plot of interfacial distance between the each sulfur atom within the polymer and the CNT surface. 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: (L.Z.) [email protected]. Present Address ‡

NuSil Technology LLC., 1050 Cindy Lane, Carpinteria, CA 93013 Notes

The authors declare no competing financial interest. 711

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