Molecular Dynamics Insights into Behavior of Poly(ethylene succinate

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Molecular Dynamics Insights into Behavior of Poly(ethylene succinate) Single Chain on Carbon Nanotube Surface Payam Kelich, and Ahmad Asadinezhad J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07844 • Publication Date (Web): 30 Oct 2015 Downloaded from http://pubs.acs.org on October 31, 2015

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The Journal of Physical Chemistry

Molecular Dynamics Insights into Behavior of Poly(ethylene succinate) Single Chain on Carbon Nanotube Surface Payam Kelich, Ahmad Asadinezhad*

Department of Chemical Engineering, Isfahan University of Technology, 84156-83111, Esfahan, Iran

*

Corresponding author:

Email address: [email protected] Fax number: +98-31-33912677 Tel number: +98-31-33915605

ACS Paragon Plus Environment

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Abstract The conformational properties of semiflexible polymer chains at their interface with nanoscale organic fillers are classified as a challenging subject due to the vast complexities involved, apart from being the most critical phase of transcrystallization. An extensive computational effort based on atomistic molecular dynamics has been undertaken in this study in order to elucidate the behavior of a single chain of a model aliphatic polyester, poly(ethylene succinate), in the vicinity of a single-walled carbon nanotube to gain new physical insights into the conformational characteristics of this type of polymers. The core finding is that the polymer chain transits from a coiled to a folded shape in the process of adsorption on the nanotube surface being a compromise of the dominant Van der Waals interactions and the chain configurational entropy. Varying temperature, chain length, as well as the nanotube curvature has been highlighted to be influential on chain order parameter and spatial arrangement. Chain dynamics has been found to obey normal Einstein mode far from the nanotube surface while changes to Rouse type close to the interface. The folded conformation of the polymer chain in the adsorbed state explains structural ordering and growth normal to the nanotube surface.


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Introduction Aliphatic thermoplastic polyesters which possess biodegradable property have recently received significant attention due to their vast potential uses as environmental friendly materials. However, they suffer from insufficient mechanical and thermal performance. A novel effective remedy is to incorporate low concentration of nanoscale fillers into the polyester matrix so that the inherent biological feature is not adversely affected.1-4 Carbon nanotubes (CNTs) are composed of sp2 hybridized carbon atoms in single-walled or multi-walled architecture having outstanding multifunctional features. Concerning the semicrystalline nature of the prominent class of polyesters mentioned above, the crystallization kinetics, spherulite size, and crystallinity of such materials are directly influenced by the presence of CNTs.5,6 It has already been established that in comparison with the unperturbed bulk state, the conformational properties of a polymer chain remarkably change near an adsorbing surface.7 Over the last decade, several theoretical studies based on atomistic and coarse-grained molecular dynamics (MD) simulations have been carried out to gain quantitative information about synthetic polymer chains behavior on CNT surface.8-28 The limitations which exist in experimental investigations have indeed driven such theoretical attempts to find out in detail how a single polymer chain behaves at the interface with CNT. The researchers have unanimously reported that CNT retards the polymer chain mobility through slowing down the backbone atoms motion, where the extent of deceleration relies on CNT diameter and strength of interfacial interactions. However, the final equilibrium conformation of polymer chain in contact with CNT is the interplay between configurational entropy loss (geometrical confinement) and adsorption energy gain (enthalpic term) which are chiefly controlled by flexibility (structural and environmental factors) and the interfacial interactions strength (dipolar, CH-, and π stacking).It is


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understood generally from the literature that high chain rigidity makes the polymer chain extend rod-like along or adopt helical conformation around nanotube axis, while flexible backbones tend to coil and wrap around nanotube. The aliphatic thermoplastic polyesters contain semiflexible ester linkage in their backbones and despite potential complexities involved, have received very few accounts in terms of conformational properties at their interface with CNTs.27,28 Tallury et al27 did a comparative study through MD simulation of single chain conformational properties of several polymers in the vicinity of a (10,0) single-walled CNT at 300 K among them were two aliphatic thermoplastic polyesters, poly(lactide) (PLA) and poly(caprolactone) (PCL). They observed that the polymer chains were adsorbed onto CNT by wrapping entirely around CNT and also orienting partly in axial direction of nanotube without taking helical conformation. Due to the higher flexibility of PCL compared with PLA, the overall conformation of the former was found less distinct than that of the latter and also the time it took for the rotational moment of inertial around the nanotube axis to level off was shorter for PCL than PLA. The interfacial interaction energy was also computed and found stronger than that of PLA because of the greater number of aliphatic methylene groups present in PCL backbone facilitating non-covalent binding. Rouhi et al28 using MD simulation explored the poly(glycolide) chain behavior at the surface of CNT with different chirality and diameter at various temperatures and reported that the polymer chain wrapped around the nanotube regardless of the nanotube chirality. However, increasing the nanotube diameter increased the interfacial interaction energy and decreased the radius of gyration, while temperature was found to exert negligible influence on chain behavior. Poly(ethylene succinate) (PES) is an ecological polymer and a new member of semicrystalline aliphatic thermoplastic polyesters family.29 Although there is some limited experimental data on


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the influence of CNTs on PES crystallization behavior,1-3 there has yet been no publication on computational description of PES chain conformational properties in the vicinity of CNT from an atomistic viewpoint. The objective of the current effort is to simulate PES single chain as model aliphatic polyester at its interface with single-walled CNT using atomistic MD at different chain lengths, temperatures, and CNT diameters to elucidate the conformational properties of the chain. This theoretical report is intended to serve as a supplement to the existing empirical data by providing new physical insights into the behavior of single PES chain on CNT surface which underlies the early stages of structural ordering of this polymer at the interface.

Theoretical Methods Molecular Model Atomic coordinates of PES chain was obtained by a chemical drawing tool. The polymer chain terminals were end-capped with carboxyl and hydroxyl groups (Figure 1). The GROMOS 53a6 force field30 was then employed to model the polymer taking partial charges into consideration. This force field adopts united-atom approach where aliphatic hydrogen atoms are implicitly modeled. To examine the effects of chain length on conformational properties of the polymer, three different number of repeating units (x) were used, namely, x=10, 20, and 30. A single armchair CNT of 30 nm length was constructed through an appropriate nanotube builder with three different diameters, 1.3, 2.7, and 4.0 nm corresponding respectively to CNT(10,10), CNT (20,20), and CNT(30,30). This allowed for a comprehensive probe into the effect of nanotube curvature on chain conformational properties. The OPLS-AA force field31 was then utilized to model CNT. The nanotube was periodically replicated so as to represent an infinite structure to circumvent subsequent edge effects on polymer chain behavior.


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O O H

OH

O

x O

Figure 1. Chemical Structure of PES with x number of repeating units.


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Molecular Simulation The whole MD simulation runs were carried out using GROMACS package version 4.5.132 with its implemented GROMOS-96 force field33 on a supercomputer equipped with cluster technology in National High Performance Computing Center (NHPCC) of Isfahan University of Technology. The cubic simulation box dimensions were set to be 35 nm × 20 nm × 20 nm where the largest axis was x and the origin was selected at the corner of the box. The simulations were carried out under periodic boundary conditions in all three dimensions within canonical ensemble (NVT) at three temperatures 300, 350, and 400 K in vacuo to examine the impact of temperature on polymer chain behavior. The system was coupled to the velocity-rescaling thermostat34 to keep temperature fixed. Time step was 1 fs and Lennard-Jones (LJ) as well as Coulomb interactions cut-off range was set to be 1.5 nm. Initially, an unrestrained CNT was placed at the center of the box so that its longitudinal direction was along x axis of the Cartesian coordinate. Then, the relaxed PES chain was positioned in the neighborhood of the nanotube so that part of the chain was within the cut-off distance in order to reduce the computational cost. Initial configuration (t=0) was generated after performing energy minimization based on the steepest descent algorithm.32 MD production runs were then carried out for 10 ns during which the system temperature and potential energy were constantly monitored to remain equilibrated till the final configuration was obtained.


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Table 1. Specifications of all simulated systems.

System designationa

Sample group

Chain length (x)

10300S 10300M 10300L 10350S 10350M 10350L 10400S 10400M 10400L 20300S 20300M 20300L 20350S 20350M 20350L 20400S 20400M 20400L 30300S 30300M 30300L 30350S 30350M 30350L 30400S 30400M 30400L

A A A A A A A A A B B B B B B B B B C C C C C C C C C

10 10 10 10 10 10 10 10 10 20 20 20 20 20 20 20 20 20 30 30 30 30 30 30 30 30 30

Temperature (K) 300 300 300 350 350 350 400 400 400 300 300 300 350 350 350 400 400 400 300 300 300 350 350 350 400 400 400

CNT curvature

Number of CNT atoms

Number of PES carbon atoms

Number of PES oxygen atoms

(10,10) (20,20) (30,30) (10,10) (20,20) (30,30) (10,10) (20,20) (30,30) (10,10) (20,20) (30,30) (10,10) (20,20) (30,30) (10,10) (20,20) (30,30) (10,10) (20,20) (30,30) (10,10) (20,20) (30,30) (10,10) (20,20) (30,30)

4920 9840 14760 4920 9840 14760 4920 9840 14760 4920 9840 14760 4920 9840 14760 4920 9840 14760 4920 9840 14760 4920 9840 14760 4920 9840 14760

60 60 60 60 60 60 60 60 60 120 120 120 120 120 120 120 120 120 180 180 180 180 180 180 180 180 180

41 41 41 41 41 41 41 41 41 81 81 81 81 81 81 81 81 81 121 121 121 121 121 121 121 121 121

a

Each system is identified with a code of the sequence: number of repeating units – temperature – CNT size, where S refers to CNT(10,10), M denotes CNT(20,20), and L refers to CNT(30,30).


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MD trajectory output was visualized using PyMol software35 and analyzed in terms of the basic conformational properties of the single chain using GROMACS 4.5.1 options. To facilitate comparison of twenty seven simulated systems whose designations and specifications are summarized in Table 1, they were divided into three different sample groups in terms of PES chain length (x=10,20,30 corresponding respectively to sample A,B,C) where temperature and CNT diameter are systematically varied.

Results and Discussion Trajectory Visualization MD snapshots of the initial and final configurations of sample A are illustrated in Figure 2. An overall look on the initial configuration snapshots reveals that PES chain in each system resembles a statistical coil whose conformation is found to be independent of CNT curvature and temperature. Some minor geometrical distortions in CNT are evident after it interacts with PES chain which is an additive result of thermal fluctuations and energetic interactions; however its center of mass still remains at the center of the simulation box. Regarding the final configuration snapshots, one can easily see that PES chain undergoes significant changes in terms of conformation and spatial position within 10 ns. The polymer approaches and wraps around the nanotube in transverse direction while extending along CNT axis in a folded fashion so that its end conformation is significantly more organized than the starting one appearing as an adsorbed layer. As a matter of fact, the coiled PES chain begins to fold and wrap in the vicinity of the nanotube surface. This event resembles the well-known reeling in process, whereby polymer chain goes through a conformational rearrangement and transits from a melt to a crystal lamellae.36 Upon increasing CNT diameter, the conformational order of PES chain is


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considerably enhanced where a more organized layer of adsorbed chain in either direction around CNT is discernible. This is ascribed to the larger interfacial area (higher number of adsorbing sites) accessible to PES chain leading to more regular wrapping and longer fold length. The effect of temperature is not as strong as that of CNT diameter. Figure 3 presents the initial and final configurations snapshots of sample B. One can observe that the starting conformation of PES chain in this sample is different from sample A which means that it is affected by chain length, however similar to sample A, it remains independent of temperature and CNT curvature. It is demonstrated by the illustrated snapshots that PES chain goes through a transition from coiled to folded shape and wraps around CNT diameter. PES chain conformational order is increased on increasing CNT diameter while it is not obviously altered by varying temperature. Notably, part of the polymer tends to be positioned at farther radial distance rather than being in direct contact with CNT leading to less distinct conformation compared with sample A. Indeed, as PES backbone is inherently flexible, an increase in number of repeating units gives rise to lower configurational entropy loss and thus lower degree of folding. The snapshots of the initial and final configurations of sample Care given in Figure 4. Similar trends observed for previous samples also hold for sample C. PES chain reveals a multilayer adsorption behavior on smaller nanotubes, that is, part of the polymer is accommodated at farther distance in a distinct layer. However, significant changes arise as CNT diameter and temperature are enhanced, the fold length and conformational order rise. An explanation behind the influence of temperature can be based upon chain thermal energy which contributes to mobility of polymer chain and consequently increases the probability of polymernanotube interactions. This is especially required for longer polymer chains which suffer from slower dynamics. With regards to the overall influence of chain length on the resulting end


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conformation, an increase in chain length results in lower configurational entropy loss and thus lower degree of folding. To have enhanced visual insight into the conformational changes of PES single chain near CNT, the snapshots of the intermediate times corresponding to 0.01 ns, 0.1 ns, and 1 ns are given as a supplement in the Supporting Information (SI) section. Although the displayed snapshots provide qualitative insights into the arrangement of the polymer chain around the nanotube, further quantitative information is necessary for more precise descriptions. To this end, some extra analyses have been performed whose underlying mathematical definitions can be found in the referred literature and are not being explained here and only the produced output is discussed.


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Figure 2. Side- and front-view MD snapshots of sample A: a) initial configuration; b) final configuration of 10300S; c) final configuration of 10300M; d) final configuration of 10300L; e) final configuration of 10350S; f) final configuration of 10350M; g) final configuration of 103050L; h) final configuration of 10400S; i) final configuration of 10400M; j) final configuration of 10400L.


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Figure 3. Side- and front-view MD snapshots of sample B: a) initial configuration; b) final configuration of 20300S; c) final configuration of 20300M; d) final configuration of 20300L; e) final configuration of 20350S; f) final configuration of 20350M; g) final configuration of 203050L; h) final configuration of 20400S; i) final configuration of 20400M; j) final configuration of 20400L.


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Figure 4. Side- and front-view MD snapshots of sample C: a) initial configuration; b) final configuration of 30300S; c) final configuration of 30300M; d) final configuration of 30300L; e) final configuration of 30350S; f) final configuration of 30350M; g) final configuration of 303050L; h) final configuration of 30400S; i) final configuration of 30400M; j) final configuration of 30400L.


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Trajectory Analysis Total potential energy of a system is a summation of non-bonded terms (Lennard-Jones and Coulomb) together with bonded terms. To gain an insight into the level and type of dominant interactions as well as equilibration of the simulated systems, time evolution potential energy curves together with its non-bonded components, Lennard-Jones and Coulomb interactions, are taken into account. Figure 5 includes the plots on time evolution of the total potential energy (PE) as well as the respective Lennard-Jones component (ULJ) for three samples A,B,C. It is easily found out from the energy curves that all systems reach equilibrium within a fraction of the whole production run time implying that 10 ns is a period long enough for each system to become well-equilibrated. This is concluded from the curves having vanishing low-amplitude fluctuations. Temperature and CNT diameter are the most influential parameters on the level of energy without affecting the trend. In each sample, an increase in both temperature and CNT diameter raises the level of PE, where the latter parameter imposes much stronger effect as qualitatively corroborated in previous section by the given snapshots. The number of chain repeating units (PES chain length) has expectedly no discernible influence on the level of energy. An overall look on the ULJ plots reveals that not only do they show similar trends as PE curves, but also the interaction is mainly contributed by Van der Waals forces rather than Coulomb or bonded terms. This is reasonable with regards to the dominance of neutral interacting sites in PES and CNT structures, rather than charged groups. The plots on the Coulomb interaction energy for all systems are given as a supplement in the SI section. It should also be noted that since the force field employed to simulate the systems has been of united-atom basis and hydrogen atoms have thus been implicitly modeled, there is trivial portion from hydrogen bonding interactions which is hardly computable with satisfying accuracy.


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Figure 5. Potential energy and corresponding LJ components of simulated systems as a function of production time within 1 ns.


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Radial distribution function (RDF) which is a weighted sum of interatomic pair correlation functions describes the probability of finding chain atoms at a certain radial distance from nanotube surface and also a measure of local packing of chain atoms.37 The respective plots which show the variation of RDF averaged over the production (simulation) time against the radial distance are given for different samples in Figure 6. Concerning sample A, PES chain atoms are packed almost entirely into a cylindrical shell in folded conformation at CNT surface (that is the adsorbed layer) within 0.25-0.65 nm where the peak maximum occurs at 0.36 nm. This corresponds to the nonbonding internuclear separation of carbon atom which implies that the dominant interactions are of the van der Waals type. This supports the findings from the previous section. The very minor peak at 0.72 nm suggests that a trivial portion of the chain atoms is not in that region and is localized at farther radial distance. Upon increasing chain length (samples B and C), the RDF peaks are increased in strength pointing out a reduced packing order having distinctly layered structure, in particular in the case of sample C where a third peak at 1.1 nm arises. These observations are well corroborated by the snapshots previously discussed. Upon increasing the nanotube diameter, the level of local packing of PES atoms is generally enhanced inferred from an overall comparison of the intensity of the minor peaks. The magnitude of the major peak located at 0.36 nm is increased while that of the minor ones is attenuated implying that more PES atoms prefer to lie in the main shell. As temperature is raised, similar effect on the local packing can be observed in particular for longer chains where higher thermal energy encourages layering order. The results of RDF are well confirmed by the presented snapshots. It should be emphasized that none of the varying parameters (chain length, temperature, and CNT diameter) shifts the horizontal position of the peaks which implies that the dominant interactions are of the van der Waals type.


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1200 1000 800 600 400 200 0 0.0 0.2 0.4 0.6 0.8

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Figure 6. Radial distribution functions of PES chain in various samples as a function of radial distance.


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The minimum distance reached between PES and CNT molecules based on the output trajectory is around 0.25 nm, irrespective of the system, corresponding to the repulsive interaction. The adsorbed shell thickness is also constant (approximately 0.35 nm) equivalent to the carbon van der Waals diameter. This thickness defines the adsorption blob size, a critical length scale of the chain, where the conformation of an adsorbed chain should be a twodimensional array of adsorption blobs.7Such an array is in the form a folded conformation. A surface contact is maintained when a blob is within 0.6 nm from CNT surface, based on the given RDF plots. The higher the number of surface contacts, the higher the packing order will be. The number of surface contacts can also be interrelated with the interfacial area between PES and CNT molecules. An analysis on the number of such contacts counted within 0.6 nm radial distance (Nc) for different samples as a function of production run time is presented in Figure7. Too many fluctuations seen in the plots are normally due to the poor statistics. Based on this figure, Nc initially assumes a non-zero value due to this fact that part of the chain has been placed in the cut-off range before starting production run. Then, Nc reveals a sharp increase when PES chain interacts with and approaches CNT and later levels off to an asymptotic value implying an equilibrium conformation. However, Nc of sample C differs in variation as it increases progressively over a significant part of the period and reaches plateau much later than two other systems. This is expected with regards to the higher number of possible conformational states of the former system which delays an equilibrium configuration. An increase in CNT diameter affects the manner of the time variation of Nc and more importantly its asymptotic value as it is increased significantly. This is in complete agreement with previous results. On the other hand, raising temperature not only shortens the time it takes for Nc to reach plateau but also increases the asymptotic value of Nc, in particular for sample C.


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Sample C 30300S 30350S 30400S

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A characteristic size of any polymer chain is radius of gyration (Rg) which gives not only a measure of average distance of chain atoms from center of mass but also moment of inertia.38 Figure 8 presents the time evolution of Rg for all samples. It is clearly seen that the gyration radius and consequently moment of inertia increase as PES chain interacts with and approaches CNT reaching finally an almost equilibrium value. An overall increase of Rg versus time is suggestive of polymer chain expansion upon being adsorbed on CNT surface. This is a direct result of transition from a coiled conformation to a more expanded configuration. Since Rg values are far less than PES chain contour length, the polymer chain can be postulated to be in a folded conformation. Furthermore, the radial distance over which the chain is localized (that is total layer thickness, δ) is on the order of its final gyration radius, i.e. δ≈Rg which is characteristic of an adsorption of weak van der Waals type. A comparison among the Rg graphs of three samples reveals that an increase in PES chain length raises the gyration radius expectedly but not markedly due to the flexible nature of PES backbone. As for CNT curvature influence, when larger diameter is concerned, the final values of Rg are increased signifying further expansion upon interacting with CNT. On the other hand, an increase in temperature increases the final values of Rg and also the amplitude of fluctuations. This is expected since temperature elevates the level of chain thermal energy and also favors layering order as confirmed by the previous analyses. It should also be noted that Rg provides a rough insight into polymer chain shape as it is principally contributed from three Cartesian components (Rgx, Rgy, Rgz).38 The plots of each component against simulation time (which are not included herein) show that for all samples the transverse components (Rgy and Rgz) undergo more pronounced changes than the longitudinal one (Rgx) which implies that the adsorption of PES chain onto CNT induces geometrical anisotropy due to the conformational rearrangement.


21


Rg (nm)

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0.1

t (ns)

30300S 30300M 30300L 30350S 30350M 30350L 30400S 30400M 30400L

Sample C

1

0.01

0.1

t (ns)

1

10

Figure 8. Gyration radius of PES chain in various samples as a function of production time.


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To explore the orientation of PES chain with respect to CNT, the order parameter P2(θ) based on the second Legendre polynomial can be explored as a function of radial distance from nanotube longitudinal axis given in Figure 9.This key parameter is a functional form of timeaveraged cosθ where angle θ is made by chain end-to-end unit vector with x axis unit vector, in such a way that P2(θ)=1 shows parallel orientation, P2(θ)=0 signifies random orientation, and P2(θ)=-0.5 indicates perpendicular orientation. For each system, two peaks are identified. The major strong peak which emerges in the range 0.25-0.65 nm is associated with the adsorbed layer of PES chain in the vicinity of CNT whose intensity is close to 1.0. This conveys the fact that PES chain end-to-end vector becomes almost parallel with CNT main axis in final configuration which provides further support for the folded chain conformation. The weaker broader peak identified at farther radial distance for all samples is ascribed to the initial and middle PES conformations before it reaches the final equilibrium state. Therefore, one can see that the orientation of the chain end-to-end vector with respect to x axis is altered from near perpendicular (initial configuration) and random to almost parallel (final configuration).The change in overall orientation of PES chain is accompanied by conformational rearrangement from a coiled shape to a folded one. The position of the P2(θ) major peak is found independent from chain length, temperature, and CNT diameter, however the major peak magnitude is slightly affected by these variable, as it is slightly increased upon increasing temperature, CNT diameter, and chain length. An increase in temperature and chain length lends more freedom to chain ends so that they can more easily be accommodated in parallel with CNT main axis. The systems with larger CNT diameter exhibit the weaker peak at farther radial distance with stronger intensity. This implies that PES chain orientation is positively influenced at farther radial distance.


23


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Sample A 0.8

10300S 10350S 10400S

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30300M 30350M 30400M

30300L 30350L 30400L

0.4 0.2 0.0

-0.2 -0.4 0

1

2

3

4

5

6

7

8

r (nm)

Figure 9. Order parameter of PES chain end-to-end vector with respect to x axis in various samples as a function of radial distance.


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Mean-square displacement (MSD) is a measure of mobility of a molecule from which selfdiffusion coefficient (D) can also be computed based on Einstein equation.38 The results for all samples are illustrated in Figure 10 as a function of production time. MSD and D values of a polymer chain are estimated based on the translational movement of center of mass at time t with respect to reference instant (t=0). It is observed that MSD shows an increasing trend for all samples over a significant duration up to a certain time beyond which a reduction is evident. A careful examination on the graph also reveals that MSD initially exhibits a linear increase with slope around 1.0 against time which later becomes nonlinear with lower slope (0.5).This indicates that PES chain follows normal Einstein dynamics far from CNT surface while following Rouse mode close to CNT surface. As the chain length increases, the displacement expectedly decreases due to the local restriction (confinement due to the adsorption). On the other hand, an increase in temperature contributes to the mobility of PES chain by raising MSD. It is also interesting to note that CNT diameter has positive influence on MSD by facilitating interactions. The self-diffusion coefficients exhibit a linearly decreasing region followed by a broad increasing region and finally a sharp reduction. As for sample A, the first region is indeed a plateau. This region is associated with the time it takes for the chain center of mass to lie within the interaction cut-off after which PES chain interacts strongly with the nanotube which facilitates the translational diffusion. Finally, PES chain is adsorbed onto CNT resulting in a local restriction and a decreased diffusion. It should be noted that on raising PES chain length, the translational diffusion of the polymer drops. Furthermore, temperature and CNT diameter increase diffusion of the polymer chain, similar to what have been observed in the case of MSD. For almost all samples, D demonstrates a time-dependent behavior which is not unusual as reported also by other researchers before for polymer-nanofiller interacting systems.39,40


25


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Sample A

0.01

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30300S 30300M 30300L 30350S 30350M 30350L 30400S 30400M 30400L

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1E-4 1E-3

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t (ns)

t (ns)

Figure 10. Mean square displacement (left panel) along with self-diffusion coefficient (right panel) of PES chain in various samples as a function of production time.


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Conclusions It has been concluded from the results of this simulation that PES single chain having initially coil conformation is adsorbed onto CNT surface by wrapping around the nanotube and traversing longitudinally in a folded configuration. Trajectory visualization has also confirmed that an increase in CNT diameter and temperature exert favorable effects on the polymer conformational order while chain length shows negligible impact. RDF calculations have proved that the dominant interaction behind adsorption is of the van der Waals type which drives PES chain to pack shell-like around CNT. An increase in CNT diameter and temperature has contributed to the local packing order while chain length shows slightly negative effect. Gyration radius computation has revealed that PES chain expands as being adsorbed onto CNT where temperature, CNT diameter as well as chain length has increasing influence. Calculation of the order parameter based on the second Legendre polynomial has demonstrated that PES chain endto-end vector changes the orientation with respect the CNT main axis from perpendicular to parallel as it is adsorbed onto the nanotube. An increase in CNT diameter has been found to increase the order parameter while chain length and temperature exhibit negligible influence. Chain mobility studies have demonstrated a transition from normal Einstein dynamic mode to Rouse type at the vicinity of CNT surface where temperature and CNT diameter assist overall displacement while chain length shows inverse effect. An extension of the current research effort to multiple PES chains behavior which gives more realistic views into the PES pseudo-melt regime as well as crystallization phenomenon in the vicinity of CNT is underway and will appear in the forthcoming paper.


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Supporting Information Snapshots of the simulated systems at various time steps (SI-1); Coulomb energy time evolution of the simulated systems (SI-2). This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments. The authors acknowledge Dr. Sergey Larin from Institute of Macromolecular Compounds, Russian Academy of Sciences for helpful discussions on utilizing the force fields. Also, Dr. Mehdi Rahmani from NHPCC is appreciated for valuable technical assistance.

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Molecular Dynamics Insights into Behavior of Poly(ethylene succinate

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