مطالعه شبیه سازی دینامیک مولکولی در اثر گنجاندن اندازه در نانوکامپوزیتهای پلیمری

The elastic behavior of polymeric nanocomposites is investigated with molecular dynamics simulations. Inclusions are modeled as spherical nanoparticles. It is demonstrated with numerically performed tensile tests that Young’s moduli of nanocomposites are strongly affected by the size of the nanoparticle as well as by the interaction strength between polymer chains and the nanoparticle. The Young’s modulus of the nanocomposite is enhanced as the size of the nanoparticle decreases as long as the strength of polymer–nanoparticle interaction is stronger than or equal to that of the polymer–polymer interaction. The analysis of stresses on polymer monomers shows that the composite modulus enhancement by smaller nanoparticles is attributed to the stiffer layer of polymer of higher densities around the nanoparticle.

Filled polymeric composites are widely used in aerospace, automobile and marine applications due to their excellent mechanical properties. In recent years, various nanomaterials with unique properties have been produced. Due to their much greater reactive surface area per unit volume compared to that of larger particles [1], nanoparticles together with polymers have been explored to form nanocomposites. As a result, mechanical and physical properties of nanocomposites have gained great interest for their structural applications. The molecular simulation has been employed as an important tool for analyzing and understanding material behavior at nanoscale. From studies of molecular simulations, interfacial conditions between polymer chains and nanoparticles and size of nanoparticles have been considered to have significant influence on the mechanical properties of nanocomposites. Smith et al. [2] showed with a model polymer nanocomposite in melt condition that the composite shear modulus and viscosity dramatically increase with attractive nanoparticle–polymer interactions relative to the pure polymer melt, exhibit little change for neutral systems, and decrease for repulsive systems. Brown et al. [3] investigated polymer structures and bulk modulus around a SiO2 nanoparticle of about 4.4 nm in diameter with a simplified linear polyethylene. In their simulations, polymer–polymer and polymer–nanoparticle interactions were all repulsive, while the attractive interactions were compensated by applied pressures. It was shown that a layer of oriented polymer chains is formed around the nanoparticle; the bulk modulus of the polymer layer is lower than that of the pure polymer; and the elastic modulus of their nanocomposite is lower than that of the pure polymer matrix [3]. Odegard et al. [4] attempted to model the mechanical properties of SiO2/polyimide nanocomposite based on the Mori–Tanaka micromechanical in conjunction with a molecular simulation of the interface between the nanoparticle and the polymer matrix. Various interfacial chemical treatments and different sizes of the nanoparticle were considered. It was demonstrated in [4] that the predicted Young’s modulus increases as the nanoparticle size increases and asymptotically approaches to the result of the Mori–Tanaka model without incorporating molecular simulations. Recently, we investigated experimentally and numerically the particle size effect on the elastic properties of particulate polymeric composites with particle size ranging from 15 nm to 500 μm [5]. From experimental results, it was observed that the Young’s modulus of the composite increases as particle size decreases in nanoscale, while for composites with micron size particles the Young’s modulus show little dependence on particle size [5]. The trend found in [5] is opposite to that found in [3] and [4]. A possible explanation of this phenomenon is that nanoparticles may have more constraints on the surrounding polymer chains than micron size particles. The purpose of this study is to employ molecular dynamic (MD) simulations to investigate the particle size effect on the behavior of polymer surrounding the particle. Representative nanocomposites are studied using the MD simulation, in order to further our understanding of the effect of particle-size as well as polymer–nanoparticle interaction on the elastic behavior of nanocomposites. Model nanocomposites consisting of amorphous linear glassy polymer chains and rigid nanoparticles are considered. Polymer chains are modeled as coarse-grained polymers [6], and nanoparticles of different sizes are constructed to be roughly spherical. Various levels of interfacial strength between the polymer chain and the nanoparticle are assumed by varying the energy unit (well depth) of the Lennard–Jones potential [2] and [7]. These interfacial strengths are chosen to represent stronger, neutral, and weaker interactions, respectively, relative to the interaction strength among polymer chains themselves. Numerical tensile loadings are simulated on these nanocomposite models. Young’s moduli are calculated for the nanocomposites with different particle sizes, particle volume fractions, and interfacial strengths. The density distribution of polymer atoms and stress distributions around the nanoparticle are examined to understand the mechanism that effects the increase of the composite Young’s modulus with a reduction in the size of nanoparticles.

The effect of nanoparticle size on the Young’s modulus of nanocomposites was investigated with molecular dynamics simulations in consideration of various polymer–nanoparticle interaction strengths and nanoparticle volume fractions. From the results, the following conclusions were obtained: (1) Average stress–strain curves obtained based on small volume molecular simulations can be used to characterize the mechanical behavior of polymeric nanocomposites. (2) The Young’s modulus of nanocomposites is enhanced with smaller nanoparticle sizes. However, in order to obtain the reinforcement effect, the polymer–nanoparticle interaction strength must be greater or comparable to that of the polymer–polymer interaction. (3) Density of polymer chains near the nanoparticle is dependent on the polymer–nanoparticle interaction strength and the nanoparticle size. A higher density results from the stronger polymer–nanoparticle interaction and the smaller size of the nanoparticle. (4) The higher density layer of polymer chains carries more stresses and, thus, behaves as a stiff layer surrounding the nanoparticle producing a higher elastic modulus of the nanocomposite.