Effects of Copolymer Sequence on Adsorption and Dynamics Near Nanoparticle Surfaces in Simulated Polymer Nanocomposites
We study a simple nanocomposite, consisting of a single spherical nanoparticle in a dense melt of coarse-grained copolymer chains, using molecular dynamics simulations. The polymers contain equal amounts of two monomer types, which differ only in their monomer-nanoparticle interaction (adsorption) strengths, and are placed in a random sequence or in a sequence of alternating blocks with various block lengths. This model is motivated by a need to understand copolymer sequence effects relevant to designing tire tread compounds, given the synthetic ability to adjust copolymer architecture (e.g., amount of blockiness) in styrene-butadiene rubbers and the ability to choose fillers and covering agents that functionalize the filler particles to give them different affinities for the styrene and butadiene monomers (it is also possible to use coupling agents, which form covalent bonds between filler particles and polymer chains). Our simple, generalized model considers linear polymers of uniform length that are not cross-linked, and thus we do not attempt to capture the overall mechanical properties of tire tread materials. Instead, we focus our analysis on how copolymer sequence impacts polymer adsorption on the nanoparticle, known to be a significant factor in determining polymer nanocomposite properties. We find that copolymer sequence impacts both the range and magnitude of the interphase of slowed dynamics surrounding the nanoparticle, with longer block lengths yielding a greater reduction in mobility over a wider region.ABSTRACT
Representative snapshots of selected polymers from two simulated systems. Pink beads adsorb more strongly to the nanoparticle (purple) than cyan beads; the different amounts of blockiness in the monomer sequences lead to different preferred polymer conformations near the nanoparticle surface.
Schematic of some copolymer sequences used in this work. For visual clarity, each example shows half of the 100-monomer sequence.
Radial pair distribution function of monomers with respect to the nanoparticle in the homopolymer systems.
Clockwise from top left: structure of BlockSize 25, BlockSize 5, BlockSize 1, and Random systems. A monomers (ɛNA = 1) are shown in cyan, and B monomers (ɛNB = 5) are shown in pink.
Radial pair distribution of A monomers (left) and B monomers (right) with respect to the nanoparticle in the copolymer systems as labeled. With increasing block length, B monomers are increasingly found near the surface.
Radial pair distribution function of monomers with respect to the nanoparticle in the copolymer systems (equivalent to averaging gAN and gBN from Fig. 5).
End-to-end relaxation times as a function of distance from the nanoparticle surface for the homopolymer systems. Shells are centered around the indicated radius and have a width of 2σ.
End-to-end relaxation times as a function of distance from the nanoparticle surface for the four copolymer systems. Also shown for comparison are the Pure A and Pure B homopolymer systems. Shells are centered around the indicated radius and have a width of 2σ.
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