Hybrid polymer/nanoparticle systems are a relatively new class of materials that has attracted growing scientific and technological interest [1-6]. In particular the study of the self-assembly and the dynamics of mesoscopic polymer/nanoparticle systems is an intense research area. The goal of the present work is to predict the properties of hybrid polymer/gold systems at the molecular level through molecular simulations and compared to the behavior of the bulk polymer system. Here, we study polyethylene (PE)/gold nanoparticle (Au NP) nanostructured systems. In more detail, the properties of polyethylene chains around Au NPs and functionalized (core/shell) Au NPs are investigated using atomistic molecular dynamics (MD) simulations.

Several gold nanoparticles with Wulff construction were studied, with diameter ranging from around 2.5–5 nm (concentration 6 - 40 wt%) [4]. The functionalized (core/shell) Au NPs, made from polyethylene branches (hairs) consisting of 20 to 62 monomers [3]. The free Polyethylene chains consist of 22 monomers [2]. Follows a snapshot from our MD simulation with the polyethylene/nanoparticle system.

       Snapshot from MD simulation of hybrid polyethylene/gold nanoparticle at 450K. Au nanoparticle (diameter of 5.0 nm) and polyethylene (5040 chains, 22-mers per chain) are shown. With yellow is the Au and with grey are the edges of Au nanoparticle. With blue are the CH2 and with green the CH3 monomers. (Left) PE/Au NP system; (Right) PE/Au Grafted NP system. The PE branches of the functionalized Au NP are with red color.   

We performed MD simulations of hybrid polymer/nanoparticle systems in the NPT ensemble with constant temperature at 450 K (Nosé Hoover thermostat) and pressure at 1 atm (Nosé Hoover barostat). Periodic boundary conditions were used in all three dimensions. The simulation method was carried out by following a hierarchical modeling strategy consisting of: (a) generation of initial structure; (b) equilibration of the hybrid system for long time; (c) execution of long MD simulations (production runs) for times up to 50 ns; and (d) a detailed analysis of the atomistic configurations gathered in part (c). Here such a detailed analysis was proposed based on averaging over atoms (or chains) within radial layers equidistant from the center of the Au NP [1].

The analysis that we have performed allows us to examine the way spatial heterogeneities are related to structural and dynamical features of the hybrid systems as a function of distance from the polymer/Au NP interface. Results can be summarized as follows:

(a) Local structural and conformational features were analyzed at the level of both individual segments (atoms or bonds) and entire chains. The local monomer PE mass density near the nanoparticle exhibits a maximum due to the intermolecular PE/Gold NP (adhesive) interaction. Chain segments show a tendency for an almost parallel to the nanoparticle orientation at short distances which is gradually randomized as one moves away from the interface.

(b) Orientational relaxation of PE chains in the hybrid PE/Au NP systems at the segmental level was quantified through the time autocorrelation function of the second Legendre polynomial. PE chains closest to the to the Au NP show much slower segmental dynamics compared to the bulk one. Faster P2(t) decorrelation is observed moving away from the interface up to a specific distance. In addition, broader distribution of the polymer terminal dynamics, compared to the bulk one was found (smaller β-exponent values).

(c) Translational segmental dynamics of PE chains was examined through the calculation of the average segmental mean-square displacement. PE chains closest to the Gold NP are slower, compared to the bulk one, for all hybrid systems. In addition, equilibrium desorption kinetics of polymer atoms and chains’ center of masses that are initial close to the nanoparticle, was also found to follow a rather broad distribution of characteristic times, which is an additional indication of the strong dynamic heterogeneities induced in the hybrid systems due to the polymer/Au NP interface.

 

References:

  1. Rissanou, A., A. Power, and V. Harmandaris, Structural and Dynamical Properties of Polyethylene/Graphene Nanocomposites through Molecular Dynamics Simulations. Polymers, 2015. 7(3): p. 390-417.
  2. Rissanou, A. and V. Harmandaris, Soft Matter, 2014. 10: p. 2876-2888.
  3. Johnson, K. and V. Harmandaris, Soft Matter, 2012. 8: p. 6320-6332
  4. Barmparis, G.D. and I.N. Remediakis, Phys. Rev. B, 2012. 86(085457).
  5. Breen, T.L., et al., Science, 1999. 284(948).
  6. Harmandaris, V.A., V.G. Mavrantzas, and D.N. Theodorou, Atomistic Molecular Dynamics Simulation of Polydisperse Linear Polyethylene Melts. Macromolecules, 1998. 31(22): p. 7934-7943.