Molecular Dynamics Study of an Atactic Poly(methyl methacrylate)–Carbon Nanotube Nanocomposite

Abstract

Molecular dynamics (MD) is used to simulate a model atactic poly(methyl methacrylate) (PMMA) system in which carbon nanotubes (CNTs) have been randomly dispersed. Our purpose is to elucidate the equilibrium structure and dynamic behavior of PMMA chains at the interface with a CNT. CNTs with different diameters and at different concentrations in the host PMMA matrix are studied, and their effect on the equilibrium squared radius-of-gyration and squared end-to-end distance of PMMA chains is examined. We have analyzed PMMA density, structure, and conformation both axially and normal to the CNT surface. Our MD simulations indicate that the presence of CNTs causes a small decrease in the size of the polymer chains, which becomes more pronounced as the concentration (volume fraction) and diameter of CNTs in the nanocomposite increases. We also provide a detailed analysis of adsorbed PMMA chain conformations in terms of trains, loops, and tails, and their statistical properties. An important finding of our work is that PMMA chains tend to penetrate significantly into the CNTs through their faces; as a result of CNT filling by PMMA chains, the area near the CNT mouths is characterized by significantly higher polymer mass density (almost by 45%) than the bulk of the nanocomposite. Additional simulation results for local and terminal relaxation in the PMMA-CNT nanocomposites reveal that due to strong PMMA-CNT attractive forces, all relaxation times in the interfacial region are significantly prolonged in comparison to the bulk, and the same happens with the diffusive (translational) motion of the chains. The density profile that develops (both axially and radially) in the vicinity of CNTs appears to significantly delay PMMA dynamics at all length scales. How this affects the glass-transition temperature of the nanocomposite is also analyzed.

Individual Contributions of Adsorbed and Free Chains to Microscopic Dynamics of Unentangled poly(ethylene Glycol)/Silica Nanocomposite Melts and the Important Role of End Groups: Theory and Simulation

“…Significant insight into the dynamic properties of polymer nanocomposites has also been obtained from molecular simulations which seem to indicate that important dynamic heterogeneities emerge in polymer nanocomposites for nanoparticles of different types and for various host polymer matrices. − Atomistic-level simulations have found extensive use in the study of such systems due to the detailed and precise representation of the chemical structure of the system. − A good example is the early molecular dynamics (MD) study of Barbier et al of PEG oligomers filled with small spherical silica nanoparticles, where all components of the nanocomposite were represented in full atomistic detail. According to their work, the strong adsorption of PEG chains on the silica nanoparticles leads to the formation of a tight adsorbed layer, whose density and local dynamics are significantly altered compared to the bulk.…”

Structure and Dynamics of Highly Attractive Polymer Nanocomposites in the Semi-Dilute Regime: The Role of Interfacial Domains and Bridging Chains

“…Following previous works [21,46,56], a PEG chain is defined as adsorbed if at least one of its monomers lies inside the adsorbed layer (which here corresponds to the space that extends up to r = 3.4 Å from the surface of the nanoparticle). In complete analogy, a PEG chain is considered as non-adsorbed or free if all of its monomers lie outside the adsorbed layer (i.e., at distances beyond 3.4 Å from the surface of the nanoparticle).…”

“…In addition to PEG-silica, several other polymer nanocomposite systems have been addressed with molecular simulations [ 20 , 21 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 ]. We mention, for example, the work of Ndoro et al [ 40 , 41 ] on curvature effects in melts of poly(styrene) (PS) oligomers filled with spherical silica particles.…”

Ultrafast rotational motions of supported nanoclusters probed by electron diffraction

“…Au 923 NCs are modeled as a set of Lennard-Jones (12-6 LJ) Au atoms according to Heinz et al 28 Graphene is modeled through the all-atom DREIDING force-field, 29 already employed for the study of carbon nanotube-and graphene-based polymer nanocomposites. [30][31][32] The interactions of Au and graphene atoms are modelled with a 12-6 LJ potential according to Lewis et al 17 All MD simulations are executed with the LAMMPS software 33 (see also S3, ESI †).…”