Self-Assembly of Helical Polyacetylene Nanostructures on Carbon

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Self-Assembly of Helical Polyacetylene Nanostructures on Carbon Nanotube Meixia Shan, Qingzhong Xue, Tuo Lei, Wei Xing, and Zifeng Yan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp405155n • Publication Date (Web): 12 Jul 2013 Downloaded from http://pubs.acs.org on July 15, 2013

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The Journal of Physical Chemistry

Self-Assembly of Helical Polyacetylene Nanostructures on Carbon Nanotube Meixia Shan,†,‡ Qingzhong Xue,*,†,‡ Tuo, Lei‡, Wei, Xing,† Zifeng, Yan† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum,

Qingdao 266580, Shandong, P. R. China; ‡

College of Science, China University of Petroleum, Qingdao 266580, Shandong, P. R.

China

*Corresponding authors: E-mail: [email protected] (Prof. Q. Z. Xue)

1

ACS Paragon Plus Environment


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ABSTRACT

The

self-assembling

of

helical

poly(acetylene)

(PA)

nanostructures

on

single-walled nanotubes (SWNT) is studied using molecular dynamics (MD) simulations. The results indicate that SWNT can activate and guide the polymer chains helically wrapping onto it through van der Waals interaction and the π-π stacking interaction between the polymer chain and the outer surface of SWNT. The effects of SWNT diameter, SWNT chirality and PA chain length on the configuration of the nanostructure have been extensively examined. It is found that a DNA-like double helix of two PA chains appears when the diameter of SWNT is larger than about 13.56 Å, the SWNT chirality has a negligible effect on whether the helical process could happen, and the two PA chains can interact with each other and then influence the formation of the perfect double helix. The geometrical structures between PA and SWNT may trigger enormous interests in chemical functionalization and helical polymer synthesis, which may eventually beneficial for fabricating nanoscale devices. In addition, the self-assembly process of helical nanostructures on SWNT may also helpful for understanding biological systems at molecular level and for developing new materials.

KEYWORDS:

Polymer chain; Carbon nanotube; Dynamic simulation; Self-assembly; Helical nanostructures

2


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1. INTRODUCTION Carbon nanotubes (CNTs), composed of networks of carbon atoms in a cylindrical structure, have attracted great research interest due to their extraordinary physical, mechanical, and electrical properties. This combination of properties makes them ideal candidates for potential applications in fields like photovoltaics1, electronics2, gas storage3 and drug delivery4. Conjugated polymers have also generated great interest due to the low manufacturing cost, the improved scalability, and the potential for a multitude of applications ranging from solar cells5 to chemical sensors6 and electronic devices7. However, the poor solubility of CNTs in aqueous and organic materials has substantially limited their applications in many areas. Compounds of CNTs and polymers can successfully overcome this shortcoming and form the foundation

of

new

materials.

For

example,

the

composite

of

poly(2,2,6,6,-tetram-ethylpiperidine-1-oxy-4-yl methacrylate) (PTMA) and SWNTs shows an enhancement both in electrical conductivity and electrochemical properties (charging and discharging capacity) compared to the pristine PTMA.8 Particularly, the self-assembly of polymer on CNTs has aroused much attention since the polymer adhered to CNTs can not only aid the solubility and dispersion of CNTs without damaging its intrinsic structure and properties but also enhance the mechanical strength, electrical conduction and optical nonlinearity of the polymer matrix.9 Recently, Naito et al.10 investigated the effect of stiffness and conformations of non-aromatic polymer poly(dialkylsilane) (PSi) on wrapping behaviors around the single-walled CNT (SWNT) using combinational analyses with a differential scanning 3



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calorimeter, transmission electron microscopy, atomic force microscopy, and ordinary UV spectroscopy. Their recent study11 further showed that the PSi with linear alkyl side chains can selectively separate (7, 6) and (9, 4) SWNTs. With the help of scanning tunneling microscopy, Giulianini et al.

12

observed that regioregular

poly(3-hexyl-thiophene) (rrP3HT) can wrap on multiwall nanotubes by coiling around the main axis and further confirmed the fundamental role played by the CNT chirality in the polymer wrapping. Along with experimental investigations, computational studies have also been performed to study the interaction between polymers and SWNT. Using molecular dynamic (MD) simulations, Pasquinelli et al.13,

14

have

investigated how the chemical composition and structure affect the conformations of polymer chains on SWNT and also explored the interfacial interactions between SWNT and polymer chains with different stiffness. It is found that stiff and semi-flexible polymers chains tend to adhere into SWNT and have more distinct conformations than polymers with flexible backbones, while no regular helical patterns were observed for theses polymers. The non-covalent wrapping around the nanotube surface with biopolymers like DNAs15, 16 polysaccharides17, 18 and peptides19 has also been reported. Our group has also done a lot of work to investigate the interaction between polymer and CNTs and the results indicate that the nanotube radius and chirality, temperature, and modification of walls all have effect on the strength of interaction.20-23 Typically, non-covalent intermolecular interaction between polymers and CNTs involves π-π24, CH-π25 and van der Waals interactions.26 The strength of these 4


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interactions and the flexibility of the polymer species all have effect on the morphology of polymers adsorbed to CNTs. Different structures of polymers dictate various conformations of the polymers on nanotube surface, such as helical wrapping or linearly adsorbed, and the geometrical features of the polymer on CNTs play an important role in determining the physical properties of the composites. Recent theoretical study showed that the adsorption spectra of poly-phenylene (PPV) are strongly affected by its wrapping geometries, noticeable blue-shifting and broadening the spectra for the highly coiled PPV were observed.27 Although there have been numerous of significant research results on wrapping SWNT by polymers, the systematic study on the interaction between SWNT and poly(acetylene) (PA) is still missing. PA has a unique π-conjugated planar structure9, and when the planer PA structure are modified into a helical one some novel optical and magnetic properties might be expected.28,

29

In this study, by using MD

simulations, we have investigated how the PA chain interacts with the SWNT and what is the shape of PA chain adhering into SWNT sidewalls. In particular, we found that two PA chains can form a regular double-helix on the outer surface of SWNT. The geometrical structures between PA and SWNT may trigger enormous interests in chemical functionalization and helical polymer synthesis, which may helpful for synthesizing novel helical PA-SWNT based functional materials and eventually beneficial for fabricating nanoscale devices. Since helical conformation is a recurring ordered chiral structure and can be easily found in nature, such as DNA and polypeptides. Studying the process and elucidating the physicochemical principles of 5



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self-assembly of molecules into helical structures may also helpful for understanding biological systems at molecular level and for developing new materials. 2. MODELS AND METHODS MD simulations are implemented by the DISCOVER code embedded in Material Studio software developed by Accelrys Inc. The condensed-phase optimized molecular potential for atomistic simulation studies (COMPASS) module in the Material Studio software were used to conduct force-field computations to account for interactions between atoms and molecules.30 This is the first ab initio force field that is parametrized and validated using condensed-phase properties in addition to various ab initio and empirical data, and it has been proven to be applicable in describing the mechanical properties of CNTs.31,32 Using the MD method, we have also done a lot of researches of CNTs20-23, 33and the same method is used here. The dynamic process is conducted to allow the system to exchange heat with environment at a constant temperature. The Andersen thermostat method was employed to control the thermodynamic temperature and generate the correct statistical ensemble. As a temperature control, the thermodynamic temperature is kept constant by allowing the simulated system to exchange energy with a “heat bath”.34 The force field is expressed as a sum of valence (or bonding), cross-terms, and non-bonding interactions:

Etotal  Evalence  Ecrossterm  Enonbond

6


(1)

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[K b  b   K b  b   K b  b  ]   [ H      H      H     ]    [V [1  cos(   )]  V [1  cos(2   )]  V [1  cos(3   )]] 

Evalence 

2

0

2

0

2

3

0

3

0

3

4

0

4

b

2

0 1

1



K  x

2

3

4

0

4

0 2

2

3

0 3

(2)

 EUB

x

E crossterm   Fbb' b  b0 b'b0 ' b

b'

  F '    0  ' 0 ' 

'

b



b



b'







  Fb b  b0    0    Fb b  b0   V1 cos   V2 cos 2  V3 cos 3 

(3)

  Fb ' b'b0 'b'b0 '  F1 cos   F2 cos 2  F3 cos 3    F    0   V1 cos   V2 cos 2  V3 cos 3    K  ' cos     0   ' 0 ' 



'

A B  qq Enonbond    9ij  6ij    i j  EH bond rij  i j rij i j   rij

(4)

The valence energy, Evalence , is generally accounted for by terms including bond stretching, valence angle bending, dihedral angle torsion, and inversion. The cross-term interaction energy, Ecrossterm , accounts for factors such as bond or angle distortions caused by nearby atoms to accurately reproduce the dynamic properties of molecules. The non-bonding interaction term, Enonbond , accounts for the interactions between not bonded atoms and results mainly from van der Walls (vdW) interactions. In equations (1-4), q is the atomic charge,  is the dielectric constant, and rij is the i-j atomic separation distance. b and b' are the lengths of two adjacent bonds,  is the two-bond angle,  is the dihedral torsion angle, and  is the out of plane 7



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angle. b0 , k i (i  2  4) ,  0 , H i (i  2  4) , i0 (i  1  3) , Vi (i  1  3) , Fbb' , b0 ' ,

F ' ,  0 ' , Fb , Fb , Fb ' , Fi (i  1  3) , F , K ' , Aij , and Bij are fitted from quantum mechanics calculations and are implemented into the Discover module of Materials Studio. In this study, all the MD simulations were performed in vacuum at 300 K, using different SWNTs and polymer chains. Initially, the polymer chains were placed in the direction perpendicular to the axis of the SWNT with about 6 Å separation. The SWNTs were frozen to simplify the computational analysis of the run and the models were put into a constant-volume/constant-temperature dynamics (NVT) ensemble simulation. A fixed time step of 1 fs was used, and data was collected every 5 ps. Then the full-precision trajectory was recorded, and the results were analyzed. 3. RESULTS AND DISCUSSION 3.1 Effects of the single PA chain length on the wrapping behavior. As we know, PA is a stiff polymer with alternating single and double carbon bonds13 and the chain length have much effect on the configuration and interaction of a stiff polymer in the polymer-nanotube composites. So we first studied how the chain length influence the conformations of single PA chain wrapping on (15, 15) SWNT with a diameter of 20.34 Å and a length of 73.79 Å. Four PA chains with 25, 50, 100 and 200 repeat units are built in a head-to-tail configuration and they are put in the direction which is perpendicular to one end of SWNT. The final configurations on SWNT are presented in Figure 1. It is observed that the PA chain with 100 repeat units can helically wrapped on the SWNT surface and shows a perfect helical-like 8


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structure, while the other three chains exhibit different configurations at the tube interface. The PA chain with 25 repeat units just extends along the tube surface and the longer chain with 50 or 200 repeat units can partly wrap on the SWNT. This is because the conformation of the polymer chains is determined by the competition between the elastic energy driving toward a rigid conformation and the entropy energy favoring random chain configuration.35,36 When the chain length is shorter (25 repeat units), the stiff polymer is difficult to overcome the elastic energy to wrap onto SWNT. With increasing the chain length (50 repeat units), the deformation of the polymer becomes easier, thus we can see some fractions of the polymer chains can wrap onto SWNT. Further increasing the chain length (100 repeat units), the deformation becomes much easier, thus we observed that all the polymer chains perfectly wrapped on the SWNT in a helical shape. However, a longer chain length is not necessarily better for forming a helical PA structure. As we can see from Figure 1, the longest chain (200 repeat units) can only non-helically wrap and extend along the SWNT. This may be because the large length of the long PA chain has to overcome the high elastic energy to keep its ideal conformation and thus weaken the interaction between SWNT and polymer and hinder its wrapping on SWNT. Our results are in agreement with the findings of previous research that there exists a preferred chain length in the self-assembly of a stiff polymer at the nanotube interface and when the polymer is shorter or longer than this preferred length it can’t form a helical structure.37 3.2 Helical wrapping of two PA chains. 9



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Although the single PA chain with the suitable length can helically wrap on the SWNT, the single helix is irregular. Previously, we have found that two PA chains can form a perfect DNA-like double helix inside SWNT.38 So the next goal of our research is to study the confirmations of two PA chains wrapping onto (15, 15) SWNT with a diameter of 20.34 Å and a length of 73.79 Å. Two PA chains of equal length and atoms are built in a head-to-tail configuration, with 100 repeat units. Figure 2 shows the snapshots of the PA chains wrapping onto SWNT in the simulation from 0 to 20 ns (See Video 1 in Supporting Information for detailed self-assembly process). At beginning the two polymer chains (separated away from each other about 15 Å) were put perpendicular to the axis of the SWNT with distance of about 6 Å. As the simulation starts, the two polymer chains away from the SWNT contact each other rapidly through π-π interactions which push the head of polymer chains move forward gradually along the sidewalls of SWNT, and thus increase the contact area between polymer chains and SWNT, further increase the attraction force of the SWNT towards polymer. After 1.5 ns, the head of polymer chains reach the other end of the SWNT and begin to wrap on SWNT. As the number of simulation time steps increase, the polymer chains gradually wrap on SWNT and a perfect helical conformation appears on the surface, as shown in Figure 2 at t=1.5～10.5 ns. At t=10.5 ns, the SWNT are totally wrapped by the polymer chains, and then spirals become denser owing to the vdW binding (see 10.5 ns～15.35 ns). After 15.35 ns, the helical polymer chains slightly move left and right along the nanotube wall, while the two helical morphologies at t=15.35 ns and t=20 ns appear more or less identical. This 10


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demonstrates the relatively stable state of the composites. In addition, we found that three or four PA chains can also helically wrap on SWNT, which may reveal that the PA ribbon formation on SWNT is in helical pattern. These helical PA nanostructures are quite similar to helices found in biological systems such as DNA and polypeptides and thus may be helpful for exploring potential applications of this bio-molecule structure. In order to study the interface properties of the polymer/SWNT composites and the relevant wrapping mechanisms, we have investigated the time dependence of interaction energy between the polymer chains and SWNT, and also calculated the interaction distance between them (discussed below). Generally, the interaction energy, is estimated by the difference between the potential energy of the composites and the potential energy for the SWNT and the corresponding polymer chains as follows.39

Eint eraction  Etotal  ( ESWNT  E polymer )

(5)

Where Etotal is the energy of the system including the polymer and the SWNT,

E SWNT is the energy of the single SWNT without the polymer, and E polymer is the energy of the polymer chains without the SWNT. As presented in Figure 3, it is observed that the interaction energy shows an increasing trend as the simulation time steps increase and it can be roughly divided into three stages. Firstly, the interaction energy increased rapidly as the head of polymer chains moved forward along the sidewalls of SWNT and then gradually increased as more parts of polymer chains begin to wrap onto SWNT. Finally, the interaction energy saturates as the polymer 11



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chains completely helically-wrapped onto the SWNT (At t=10.5 ns). After the 10.5 ns, the interaction energy exhibits some minor fluctuations due to the self adjusting of the polymer chains to fully interacting with the outer surface of the tube. The information on the geometric parameters can be characterized by the concentration profile of the combination consisting of the SWNT core and the polymer helix. It is calculated for 3D periodic structures by computing the profile of the atom density profile within evenly spaced slices parallel to the bc, ca, and ab planes. In practice, this is equivalent to taking the a, b, and c components of the fractional coordinates of each atom and independently generating a plot for each component. Figure 4 shows the concentration profile of the final structure of the SWNT and polymer chains in the X direction. From the peak details marked in Figure 3, the distance between the SWNT and the helical polymer structure is about 3.5 Å, a little more than 3.4 Å which is the shortest distance of the graphite layer, and this is in accordance with the stacking distance of the offset π-π stacking interaction,40 indicating that the π-π stacking interaction plays a significant role in forming a helix. So we can conclude that the vdW interaction between the polymer chains and SWNT drives the polymer continuously moving forward along the sidewalls of SWNT and the π-π stacking interaction activates and guides the chains helically wrapping around the SWNT surface. In addition, the short layer distance (3.5 Å) and the strong interaction energy (-604.989 kcal/mol) indicate the adhesion between the helical PA chains and SWNT is so strong that they can hardly separate again. We tested the stability of the composites and found the helix-forming process is irreversible even 12


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undergoing heating or cooling process. We have also performed additional simulations to study how the initial sites and relative orientations of the two PA chains and SWNT affect the self-assembly process. Four initial conformations of the molecular systems (shown in Figure 5) with the same (15, 15) SWNT and two long PA chains (100 repeat units) but the different relative orientations between the polymer chains and SWNT were simulated. After the simulation, four similar perfectly compact double–helix structures were formed, irrespective of the initial sites and relative orientations between the two PA chains and SWNT. (the final structures are not shown here, See Video1, 2, 3, 4 in Supporting Information for detailed self-assembly process). However, for the single PA chain, helical structure was not found when the initial orientation of the PA chain is perpendicular to one end or perpendicular to the mid of SWNT. (See Figure S1). This indicates that the two PA chains interact with each other to control the perfect double helix formatting process. 3.3 Dependence of the diameter and chirality. Previous studies have found that the diameter and chirality of SWNT have an effect on the self-scrolling process of graphene(GN) sheets or nanoribbons on SWNTs.41, 42 Here, we also perform some simulations to calculate the influence of nanotube radius on the wrapping behavior of PA chains around SWNT. The same two PA chains (100 repeat unites) wrapping onto different armchair SWNTs with diameters varying from 5.42 Å to 27.12 Å (the detailed information is listed in Table 1) are simulated. Figure 6 shows the final configurations of two PA chains on SWNTs with different diameters. 13



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It can be observed that the helical wrapping of PA chains is strongly dependent on the diameter of SWNT. It is obvious that the (4, 4) SWNT cannot induce the polymer chains wrapping onto it, the two PA chains only extend along the sidewalls of SWNT. This may be because the interaction force is not strong enough to activate the PA chains to bend onto SWNT with relatively small radius. For larger diameter (6, 6) and (8, 8) nanotubes, the PA chains show a certain degree of bending, which indicates the wrapping behavior begins to appear. Further increase the nanotube diameter, we found some parts of PA chains formed a perfect helix on (10, 10) SWNT, some parts of them were still outside the nanotube, and all parts of PA chains helically wrapped on the (15, 15) and (20, 20) SWNTs. In addition, as shown in Figure 7, we tracked the difference of the interaction energy between the PA chains and SWNTs during the simulation process. When the SWNT diameters vary from 5.42 Å to 27.12 Å, the interaction energies increase as a result of the increasing of the interacting area. When the interaction energy is larger enough to induce the PA chains completely wrap onto SWNT (for (15, 15) and (20, 20) SWNTs), the contact area almost keeps constant, thus we found that the interaction energy has no significant change after 12500 ps. In addition, we have calculated the distances of the periodicities (helical pitch) in helix-formations (shown in inset of Figure 8) of different diameters of SWNT. As to be mentioned, all the selected SWNTs can induce the helical wrapping process, while the PA chains can completely wrap on the SWNTs except for (10, 10) SWNT. From Figure 8, we can observe that the helical pitch is large when the PA chains partly wrap on (10, 10) SWNT (the configuration presented in Figure 6). When the diameter of 14


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SWNT is small, the attractive interaction between the PA chains and SWNT is weak, while the strain cost for bending the PA chains to wrap in a large coiling angle (defined as in inset of Figure 8) on SWNT is high. 43 In other words, as for (10, 10) SWNT, the attractive interactions between the PA chains and SWNT is not large enough to overcome the strain cost of bending PA chains to wrap on SWNT in a large coiling angle. Therefore, the PA chains will wrap on (10, 10) SWNT in a small coiling angle, resulting in a relatively large helical pitch (about 44.25 Å). When the nanotube diameter becomes larger, all the PA chains can wrap on the SWNTs and the helical pitch has no significant change (just fluctuates around 22 Å). This is because the attractive interactions between the PA chains and SWNTs are sufficient enough to overcome the strain cost of bending PA chains to wrap on these SWNTs. The PA chains will adjust the helical pitch through the interaction between the adjacent chains until the system reach a steady state. This may lead to some fluctuations of the helical pitch. From the above discussions, we may conclude that the helical pitch is large for PA chains wrapping on SWNT with small diameters, and when the nanotube diameter is large enough to induce all the PA chains to wrap on SWNT, the helical pitch becomes smaller and fluctuates around a constant value with further increasing nanotube diameter. As we know, the chirality of SWNTs has a significant influence on their properties. A SWNT exhibits metallic properties when it is an armchair chirality, while the SWNT can be semiconducting type or semimetallic type when its chirality is zig-zag or chiral.44 Four types of SWNTs including(17, 0), (15, 4), (12, 8) and (10, 10) with 15



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similar diameters about 13.5 Å but different chiral angles θ ranging from 0 to 30ºare selected to investigate the influence of SWNT chirality on the wrapping behavior of PA chains around SWNT. The corresponding chiral angle θ and diameter of Dn of the SWNTs with (n, m) indices could be determined by using the rolling GN model.45

  arctan(

3m ); 2n  m

Dn 

3



b (n 2  m 2  nm)

(0  m  n)

(6)

Where b is the length of C-C bond (0.142 nm). The total number of atoms, diameter and length of each chiral nanotube are given in Table 1. After 20 ns simulation, all SWNTs can induce the PA chains to form perfect helical structures. To observe clearly, we only presented the final structure of two PA chains (75 repeat units) on armchair (10, 10) SWNT in Figure 9b (See Figure S2 for the other three final configurations). Figure 9a shows the interaction energy of the final structure between PA chains and different SWNTs. There is no significant difference between them, which indicates the SWNT chirality has a negligible effect on whether the helical process could happen. In addition, as marked in Figure 9b, the direction of the helix almost goes along with nanotube hexagonal cells for armchair SWNT due to the least elastic energies cost for PA chains in the self-assembling process, which is in agreement with previous research results.12, 46 From the above simulations, we may draw a conclusion that the diameter of SWNT determine whether the helical wrapping process can be happened and the chirality only slightly influence the direction of the helix. 4. CONCLUSIONS In this study, the wrapping process of two long PA chains onto SWNTs and the influences of SWNT diameter, SWNT chirality and PA chain length on the 16


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configuration of the nanostructure have been extensively investigated using MD simulations. Our simulations show that the SWNTs with small diameters cannot induce the wrapping process due to the weak attraction force and a double helix of two PA chains appears when the diameter of SWNT is larger than about 13.56 Å. The chirality of SWNT has a negligible influence on whether the helical process could happen. From the whole process, we observe that the vdW interaction between the polymer chains and SWNT drives the polymer continuously moving forward along the sidewalls of SWNT and the π-π stacking interaction activates and guides the chains to self-assemble and display a helical conformation around the SWNT surface. Our simulation results may guide the fabricating of new helical polymer-SWNTs hybrid materials and further exploring their unique properties. In addition, the self-assembly process of helical configurations may helpful for understanding the similar molecular structures in biological systems at molecular level. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (QX). Tel: 86-0532-86981169. Fax: 86-0532-86981169 ACKNOWLEDGEMENTS This work is supported by the Fundamental Research Funds for the Central Universities (13CX06004A, 14CX02018A, 14CX02025A, 13CX05009A, and 13CX05004A), and the Qingdao Science & Technology Program (12-1-4-7-(1)-jch). Supporting Information Available: 17



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Supporting information include final configurations of single PA chain on (15, 15) SWNT, two PA chains on different chiral SWNTs, and four videos of the self-assembly process of two PA chains with different initial orientations wrapping on (15, 15) SWNT. Supplementary video 1: MD simulation showing self-assembly process of two PA chains which are perpendicular to one end of (15, 15) SWNT. Supplementary video 2: MD simulation showing self-assembly process of two PA chains which are perpendicular to the mid of (15, 15) SWNT. Supplementary video 3: MD simulation showing self-assembly process of two PA chains which are parallel to one end of (15, 15) SWNT. Supplementary video 4: MD simulation showing self-assembly process of two PA chains which are parallel to each end of (15, 15) SWNT. This information is available free of charge via the Internet at http://pubs.acs.org.

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Table 1. Total number of atoms, diameter, and length of each chiral nanotube utilized in MD simulations.

C atoms

SWNT diameter (Å)

SWNT length (Å)

Chiral angle θ[deg]

(4, 4) SWNT

480

5.42

73.79

30

(6, 6) SWNT

720

27.17

73.79

30

(8, 8) SWNT

960

10.85

73.79

30

(10, 10) SWNT

1160

13.56

73.79

30

(12, 12) SWNT

1440

16.27

73.79

30

(14, 14) SWNT

1680

18.98

73.79

30

(15, 15) SWNT

1800

20.34

73.79

30

(16, 16) SWNT

1920

21.70

73.79

30

(18, 18) SWNT

2160

24.41

73.79

30

(20, 20) SWNT

2400

27.12

73.79

30

(17, 0) SWNT

1156

13.31

72.42

0

(15, 4) SWNT

1204

13.58

73.91

11.52

(12, 8) SWNT

1216

13.65

74.28

23.43

Type of SWNTs

25



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Figure Captions Figure 1. Final configurations of single PA chain with different unites on SWNT: (a) 25, (b) 50, (c) 100, and (d) 200 repeat units. Figure 2. Snapshots of the PA chains with length of 234.960 Å wrapping onto (15, 15) SWNT in the simulation from 0 to 20 ns. Figure 3. The interaction energy as a function of time in the process of helical wrapping. Figure 4. Concentration distribution profiles of PA and SWNT in the system along the X-direction. Figure 5. Initial conformations of the molecular systems: (a) perpendicular to one end of SWNT, (b) perpendicular to the mid of SWNT, (c) parallel to one end of SWNT, and (d) parallel to each end of SWNT. Figure 6. Final configurations of PA chains on SWNTs with different diameters (some parts of PA chains outside the SWNT have been cut off for clarity). Figure 7. The interaction energy evolution between two PA chains and SWNTs during the simulation process. Figure 8. The distance of the periodicity as a function of SWNT diameter. Figure 9. (a) The interaction energy of the final structure between PA chains (75 repeat units) and different chiral SWNTs. (b) Final configurations of PA chains on the armchair SWNT (some parts of PA chains outside the SWNT have been cut off for clarity).

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Figure 1.

27



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Figure 2.

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Figure 3.

29



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Figure 4.

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Figure 5.

31



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Figure 6.

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Figure 7

33



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Figure 8

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Figure 9.

35



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Table of Contents (TOC) Image:

SWNT can activate and guide the two PA chains to self-assemble into a DNA-like double helix around the side walls.

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Self-Assembly of Helical Polyacetylene Nanostructures on Carbon

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