Effect of nano inclusions on the structural and physical properties of polyethylene polymer matrix

Abstract

The reinforcement mechanism of nano inclusions on the mechanical properties of nanocomposites is still a controversial issue. In this work, the polyethylene (PE) polymer nanocomposites were simulated by infusing different kinds of nano inclusions (bucky-ball, graphene, single-walled-carbon-nanotube (SWNT), X-shaped SWNT junction and Y-shaped SWNT junction) into PE matrices. The effects of these nano inclusions on structural and physical properties of PE matrices were considered through molecular dynamics (MD) simulations. The packaging densities of PE matrices were highly different from each other. The graphene inclusion had larger interaction with the PE matrix than the others. However, Y-shaped SWNT junction applied the strongest restriction on diffusion behaviors of PE matrices. Therefore, mean squared displacement of the PE matrix infused with Y-shaped SWNT junction is much smaller than that of pure PE. The effect of nano inclusions on primitive chain network of PE matrices was also considered. However, these nano inclusions did not significantly alter the underlying mesh of primitive chain network of PE matrices. The change of mechanical properties associated with these nano inclusions should be directly attributed to the interaction between polymer matrices and nano inclusions.

Introduction

The motion behavior of polymer chains has a great impact on the mechanical properties of polymers. The dynamics of polymer chains are mostly determined by topological constrains in the high molecular weight polymers arising from the chain connectivity, chain uncrossability and obstacles. From motion behavior of a single polymer chain, Doi and Edwards [1] have derived an analytical solution for the viscoelastic property of polymers based on an effective tube model according to the constrained effect induced by its surrounding part. In the tube model [1], the motion of a single polymer chain is constrained in a tube-like region and the polymer chain can only move back and forth, i.e. reptates, along the shortest path (also primitive path) connecting its two ends. Then, the corresponding viscoelastic property of polymer could be evaluated in a mean-field approach [1]. Along this direction, Doi and Edwards [2], [3], [4] and de Gennes [5] have done the pioneering works on melts of entangled polymers and polymers in concentrated solutions.

Although theoretical works [1], [2], [3], [4], [5] have successfully applied the tube model to explain motion behaviors of polymer chains, quite a few experimental works have been done on this subject. Perkins et al. [6] have observed the tube-like motion of a labeled single DNA molecule in an entangled solution of unlabeled DNA molecules by fluorescence microscopy. Besides, the computer simulations have successfully captured the tube-like motion behaviors of polymer chains. Kroger [7] and Tzoumanekas et al. [8] have developed the computer code, Z code (also Z1 code as an update version) and CReTA code, to calculate the tube diameter, primitive chain length as well as entanglement information for a polymer system, respectively. Both Z1 and CReTA codes have given the same results on the same polymer systems [9]. The detail implementation information for Z1 code on the structural analysis of a polymer has also been discussed in Ref.[10]. Then, the Z1 code has been utilized to analyze primitive path of entanglements over a wide range of chain lengths for PE polymer melts [11]. As a powerful tool, the Z1 code also has been used for structural analysis on long liner PE chains and scaling laws between the molecular weight and primitive chain length, tube diameter as well as entanglement statistic have been obtained [12], which agree reasonable well with experimental results. The shear strain rate effect is also considered on PE melts under a steady shear condition [13]. The chain orientation and stretching, chain rotation and tumbling as well as chain disentanglement of PE have been well understood by Z1 code calculation [13]. Combining Z1 code calculation [7] with the tube model [1], the survival probability function of polymer chains is obtained by Kröger and co-workers [14], [15], which could be directly used to obtain linear viscoelastic properties of polymers. Based on this method, the linear viscoelastic properties of PE and polybutadiene have been calculated by a very simple MD simulation [14], [15]. The obtained results are also in good agreement with experimental results, which also verified that the tube model could well describe motion behaviors of polymer chains [14], [15].

Although plenty of works have been done on structural analysis and motion behaviors of polymer chains, quite a few attentions have been paid on structures of polymer nanocomposites. It will be very helpful for us to know how a nano inclusion changes structure of its polymer matrix, which can be applied to explain underlying mechanisms of reinforcement effect. Riggleman et al. [16] have done the simulation on spherical inclusions reinforced polymers with the CReTA algorithm. The entanglement of polymer matrix has not been changed by spherical inclusions, except, additional topological constrains were induced by these particles in deformation process [16]. More recently, Toepperwein et al. [17] have analyzed the effect of nanorod inclusions on structure, primitive chain network of polymer matrices both at equilibrium state and deformation state. They also found that nanorod inclusions did not significantly change primitive chain network of polymer matrices instead of applying more topological constrains [17]. The effect of nano particle shape on rheology and tensile strength of polymer nanocomposites has been studied by Knauert et al. [18]. The enhancement of viscosity for polymer nanocomposites is the largest for the rod-like nano particles and the least for sheet-like nano particles [18]. However, the polymer chain length in their work is well below the entanglement length and they only considered three kinds of shape, icosahedron, rod and sheet [18]. Grest and co-workers [19], [20] studied the effect of particle shape on bulk rheology of nano particle suspensions. They have found that spheres, plates and rods of the same mass have approximately the same influence on viscosity and only extended nano inclusions like a jack can significantly increase the viscosity of suspensions [19], [20]. Besides, Mansfield and Douglas [21] have developed an improved path integration method, which can be applied for theoretically estimating the intrinsic viscosity of arbitrarily shaped particles.

In this work, we will study the effect of nano inclusions with different shapes on structure behaviors of PE matrices. Firstly, PE nanocomposites models are built with a pure PE polymer matrix infused with five different kinds of nano inclusions. Secondly, all the nanocomposites are equilibrated for an enough long time to obtain realistic molecular structures for these nanocomposites. Thirdly, how the PE polymer chains are packaged around different nano inclusions is explored by radial distribution function. Finally, effects of these nano inclusions on structural and physical properties of PE polymer matrices are discussed.