ВЫСОКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ, Серия А, 2012, том 54, № 1, с. 44-51
МОДЕЛИРОВАНИЕ
УДК 541(64+183):539.199
ADSORPTION PROPERTIES OF COMB-LIKE POLYMER ON NANOTUBE SURFACE1
© 2012 г. Z. Zhang, C. C. Zuo, Q. Q. Cao, L. J. Li, and M. F. Gao
College of Mechanical Science and Engineering, Jilin University, Changchun 130022, the People's Republic of China
e-mail: qqcao07@mails.jlu.edu.cn Received May 22, 2011 Revised Manuscript Received August 18, 2011
Abstract—The adsorption and wrapping process of a single flexible comb-like polymer to a single wall nan-otube was studied by Molecular Dynamics simulation of a coarse-grained model. We varied the grafting density and length of the side chains, the radius of the nanotube and strength of interaction between the monomers of nanotube and side chains of polymer brush. We investigated the structural and dynamical characters of interactions of the nanotube-polymer composite, such as the effect of Lennard-Jones energy parameter sLJ and the nanotube radius on the adsorption behavior and how the wrapping conformation is affected by the structure of the polymer brush. The simulation results indicate that single comb-like polymer with flexible backbone tends to adsorb and wrap around the nanotube, when the interaction energy exceeds a critical value. The monomer adsorption ratio, interaction energy profiles and moment of inertia are obtained. The helical wrapping only occurs when the interaction energy is large enough. Also, the influence of the polymer structure on the conformational behavior is analyzed. This work underscores design elements important for engineering well-defined nanotube-polymer nanocomposite.
INTRODUCTION
Recently, there has been a lot of interest in macro-molecules with comb-like architecture, where linear or branched side chains are grafted regularly or randomly to a backbone chain molecule that may either be intrinsically rigid or flexible [1—5]. These so-called "comb-like polymers" are of immense technological importance, both for the rheology control and for modification of surface properties [6]. Consequently, adsorbed or grafted comb-like polymer layers have received significant attention in recent years, the fundamental and applied research results have shown that adsorption of comb-like polymers with hydrophilic non-ionic side chains leads to low non-specific protein adsorption [7, 8], strongly repulsive steric interactions [9, 10], and favourable lubrication properties
[11], with friction coefficients as low as achievable with efficient biochemical lubricants such as mucin
[12]. The noncovalent "wrapping" of polymer chains around a nanotube is a fascinating application, and this wrapping behavior can be utilized to solubilize nanotube, tune the dispersity, drive assembly mechanisms and alter the functionalization, but preserve key optical, electrical, and structural properties of nanotube [13, 14]. Consequently, these nanotube-polymer nanocomposites are expected to have more useful and enhanced electrical [15] and optical [16] properties, thermal conductivity [17], and superior mechanical strength [18].
A prerequisite for the successful use of physically sorbed comb-like polymers in applications is a good
1 Статья печатается в представленном авторами виде.
understanding of their adsorption properties, which can be affected by the nature of the surface, the polymer architecture, solution composition, and by interactions with surfactants and other polymers present in solution [6, 9, 19—21]. However, not all aspect will be followed up here; rather we focus our interest on the theoretical understanding of the relationships of the polymer structure, interaction strength with nanotube and adsorption property exploring the molecular mechanics of a coarse-gained model by computer simulation methods.
For the wrapping behavior of polymer around nanotube, to achieve good steric stabilization, the surface should be fully coated. Further, the polymer-nanotube affinity should be sufficiently high to allow for adsorption and the wrapping conformation of large loops and tails [22]. However, clarifying the influence of comblike polymer architecture on adsorption and wrapping behavior is difficult due to the multitude of length scales characterizing the polymer structure [1, 23, 24] and the intricate interplay between steric repulsion among the monomers of the grafted side chains and effective attractive interactions of polymer-nanotube [25]. The flexibility of the comb-like polymers, which has a great impact on the conformational behavior, depends not only on the chemical character of the backbone [25], but also on the chain length and grafting density of the side chains. With increasing chain length and grafting density stiffening of intrinsically flexible backbones is induced [2, 3, 26] due to the large intermolecular excluded volume interactions. On the other hand, the good warping conformation also depends on the efficiency of interactions between the nanotube and polymer chains, as a strong interfacial
Fig. 1. Schematic structure of a single comb-like polymer. The main chain and the side chains are presented in black and silver.
binding guarantees an efficient load transfer from polymer to the nanotube [27]. Thus, the structure of polymer brush and the interaction between the nanotube and polymer play a major role in determining the mechanical performance and structural integrity of such nanotube-polymer nanocomposites.
However since it is difficult to study the nanotube-polymer interface by experimental methods [27], molecular mechanics and MD simulations can be used effectively in the investigation of reinforcement mechanisms in nanotube-polymer nanocomposites, as they present a clear view of the interface between the polymer and nanotube. These computer simulation methods have been proven to be very useful to validate approximations needed to interpret experimental data [24, 28—30]. Up to now, a few MD simulations have been performed by several research groups [4, 25, 27, 31—34]. Here we extend this work to consider a single flexible comb-like polymer adsorbed onto a single wall nanotube; varying both the grafting density and length of side chains. We focus on the adsorption and wrapping process, analyze the interaction of these nano-tube-polymer composites under different structure and strength of affinity. Then, we will discuss how do these factors affect the conformation behavior.
In the next section we will give the model system and the details of the molecular dynamics simulations. Then we will discuss our numerical results. Finally, the last section presents the conclusions of this work.
THE MODEL AND SYSTEM
The model considered in our study consists of a single comb-like polymer and a nanotube. Each polymer brush includes one flexible backbone of Nb monomers and N side chains of length Ns. The value of N depends on the grafting density of side chain, Den = N/Nb, and the side chains were grafted evenly on the backbone (Fig. 1). The single wall nanotubes with different radius (rnanotube = 4, 6, 8) were chosen for this study with the same monomer density, the length of the nanotube L is set to 0.96Nnp, where Nnp is the monomers number of the nanotube in axis direction, in present work Nnp = 168 is used. Each nanotube consists of Nc x Nnp monomers, where Nc is the monomers number in circle and Nc = 26, 39, 52 for different radius, and the monomers of the nanotube are fixed and have no interaction with each other. The simulation box of
size L x L x L is periodic in the direction of nanotube axis.
We assume that the short-range interaction between any two particles separated by a distance r is the 12-6 Lennard-Jones (LJ) potential,
v (r) = j4ex/[(^/r)12 - (^A)6], r < rc (1) L/ |0, r > rc, where a and sLJ are the LJ potential parameters. The cut-off radius rc is set to 2.5a for the particle pair of nanotube with side chains and 21/6a for other particle pairs. This choice corresponds with that only the side chains have strong interaction with nanotube. In this paper, a, suand m are taken as the length, energy and mass units, respectively. Other units are derived from these basic units, such as time unit t = (ma2/sL/)1/2, temperature unit sLJ/kB (where kB is Boltzmann constant). The single comb-like polymer is modeled using a widely utilized, coarse-grained bead-spring model. The beads are coupled by a finitely extendable nonlinear elastic (FENE) potential [35],
Ubond(r) = -(kR0/2)ln(1 - r2/R2), (2) where the maximum bond length is R = 1.5a and the spring constant is given by k = 30sLJ/a2. This choice of parameters gives an average bond length a = 0.98a. The combination of LJ and FENE potentials ensure that the constituent of chains cannot pass through one another.
During the simulation, the system temperature is controlled by the Langevin thermostat based on the dissipation theorem [36]. The damping rate v and the desired temperature T are set to 0.5t-1 and 1.2sLJ/kB. The positions and velocities of the monomers are calculated using the velocity Verlet algorithm. All simulations are conducted with a time step At = 0.005t. First, we run the simulation for 1 x 107 time steps to obtain an equilibrious conformation of the single comb-like polymer, and then the macromolecule was located next to the nanotube with a suitable distance in order to ensure that the interaction between the nanotube and macromolecule can play a role. The exact value depends on the radius of the nanotube and the polymer structure, and it was obtained by repeated adjustment during several MD runs. The initial speeds of monomers were set randomly.
We performed several MD runs for each considered case and also we varied the starting coordinates of
Na/Ntotal 1.0h
2.0 гы
Fig. 2. Monomer adsorption ratio NA/Ntotai profiles versus zLj for different radius rm„otube = 4, 6 and 8.
Monomer of side chain Monomer of main chain
Fig. 3. Schematic force analysis for the mo
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.