Structure, dynamics and primitive path network of polymer nanocomposites containing spherical nanoparticles
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Structure, dynamics and primitive path network of polymer nanocomposites containing spherical nanoparticles Argyrios Karatrantos1*, Nigel Clarke1, Russel J. Composto2 and Karen I. Winey2 1
Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH, UK. Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, 19104, USA. 2
ABSTRACT We investigate the effect of nanoparticles on polymer structure, polymer dimensions and topological constraints (entanglements) in polymer melts for nanoparticle loading above percolation threshold as high as 40.9% using stochastic molecular dynamics (MD) simulations. We show unambiguously that short polymer chains are not disturbed by the presence of repulsive nanoparticles. In contrast entangled polymer chains can be perturbed by the presence of attractive nanoparticles when the polymer radius of gyration is larger than the nanoparticle radius. They can expand under the presence of attractive nanoparticles even at low nanoparticle loadings of very small nanoparticle size. We observe an increase in the number of entanglements (decrease of Ne with 40.9% volume fraction of nanoparticles dispersed in the polymer matrix) in the nanocomposites as evidenced by larger contour lengths of the primitive paths. Attraction between polymers and nanoparticles affects the entanglements in the nanocomposites and alters the primitive path. INTRODUCTION The addition of nanoparticles to a polymer matrix can improve the electrical, thermal, mechanical, rheological and tribological properties [1]. We explore how nanoparticles can affect polymer structure and dimensions when the polymer radius of gyration is larger than the nanoparticle radius (R). There is controversy as to whether the addition of nanoparticles to a polymer melt causes perturbed behaviour of polymers. In particular neutron scattering of a polystyrene (PS) chains/ (PS) nanoparticles [2] nanocomposite showed a polymer chain expansion (20% expansion at nanoparticle volume fraction φ=10% for polymer chains with radius of gyration larger than the nanoparticle radius, which is contrary to other recent studies of PS/silica [3-5], and poly(ethylene-propylene) (PEP)/silica [6] where the polymer chains were unperturbed. Moreover, in a former study of a poly(dimethylsiloxane)/polysilicate nanocomposite [7] a significant increase of polymer chain dimensions (60% expansion at nanoparticle volume fraction φ=40% was observed for Rg/R: in agreement with the observations of Mackay [2]. The quality of nanoparticle dispersion [3,4] can have an important effect on the polymer dimensions and this depends on the nanoparticle-polymer interaction, nanoparticle-polymer size ratio [8], size of nanoparticles and nanoparticle volume fraction. However, we need to note that in the experimental area of polymer nanocomposites containing spherical nanoparticles, the role of monomer-nanoparticle interactions in polymer conformations is not yet clear. The dynamics of long polymers is limited by entanglement
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