We explore the structural and dynamic properties of bulk materials composed of graphene nanosheets using coarse-grained molecular dynamics simulations. Remarkably, our results show clear evidence that bulk graphene materials exhibit a fluid-like behavior similar to linear polymer melts at elevated temperatures and that these materials transform into a glassy-like "foam" state at temperatures below the glass-transition temperature ( T) of these materials. Distinct from an isolated graphene sheet, which exhibits a relatively flat shape with fluctuations, we find that graphene sheets in a melt state structurally adopt more "crumpled" configurations and correspondingly smaller sizes, as normally found for ordinary polymers in the melt. Upon approaching the glass transition, these two-dimensional polymeric materials exhibit a dramatic slowing down of their dynamics that is likewise similar to ordinary linear polymer glass-forming liquids. Bulk graphene materials in their glassy foam state have an exceptionally large free-volume and high thermal stability due to their high T (≈ 1600 K) as compared to conventional polymer materials. Our findings show that graphene melts have interesting lubricating and "plastic" flow properties at elevated temperatures, and suggest that graphene foams are highly promising as high surface filtration materials and fire suppression additives for improving the thermal conductivities and mechanical reinforcement of polymer materials.
We perform molecular dynamics simulations on a coarse-grained polymer melt to study the dynamics of glass-formation in ring polymer melts of variable knot complexity. After generating melts of non-concatenated polymeric rings having a range of minimum crossing number values, mc, we compute the coherent intermediate scattering function, the segmental α-relaxation time, fragility, and the glass transition temperature as a function of mc. Variation of knot complexity is found to have a pronounced effect on the dynamics of polymer melts since both molecular rigidity and packing are altered, primary physical factors governing glass-formation in polymeric materials.
T he use of DNA to link nanoparticles (NPs) into complex self-assembled structures is an increasingly popular approach to the bottom-up design of nanoclusters and materials. 1À5 The selectivity and reversibility of DNA base-pair recognition, coupled with the relative stiffness of double-stranded DNA (dsDNA) 6À8 and a tunable assembly kinetics, 9 make DNA an ideal choice to create programmable interactions between NPs. This is accomplished by tethering multiple strands of DNA to a core NP (gold, silver, or CdSe core). 10À12 The outermost part of tethered strands is singlestranded DNA (ssDNA) with a specific sequence that will either link directly to another NP or connect to another NP via an additional linking strand. 13,14 The hybridization of ssDNA in linking regions directs the self-assembly of nanoparticles into largerscale structures. Their organization is controlled by the DNA sequences and linker architecture, 15À19 as well as by the NP geometry. 20 Using this approach, the precise fabrication of small nanoclusters and the formation of two-and three-dimensional superlattices has been achieved. 20À27While the approach of single-step, direct assembly is promising, many biological materials ; such as bone, hair, skin, and spider silk 28 ; take advantage of a more complex hierarchical scheme. In such a scenario, assembly at each scale results in units or structures that enable assembly at a larger scale. These processes have been evolved over aeons, so the development of synthetic multiscale self-assembly will prove challenging. In the context of DNA-tethered NPs, the most basic unit in such a hierarchical approach is a dimer of two NPs. As a first step toward a synthetic multiscale assembly, here we examine in detail the structure of dimer units. The structure of these dimers is complicated by the fact that the surface curvature of the NP is on a scale comparable to the length of the connecting DNA. In such a regime, the behavior of polymer chains (such as DNA) attached to the surface is known to deviate significantly from the free-chain behavior. 29,30 In this paper, we examine the structure of DNA-linked NP dimers by a combination of in situ dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS), complemented by molecular simulations and theory. The DLS method probes a hydrodynamic measure of an angular-averaged dimer size. The in situ SAXS experiments reveal more detailed information on the interparticle distances within the dimer. The numerical modeling provides a detailed molecular picture that helps to interpret the experimental findings and develop a theoretical description.When the NPs are linked by dsDNA with a separation less than the persistence length, the interparticle distance is a linear function of the DNA length, due to the rigidity of dsDNA. For longer ssDNA linkers, the * Address correspondence to ogang@bnl.gov, fstarr@wesleyan.edu.Received for review April 6, 2012 and accepted July 13, 2012. Published online 10.1021/nn301528hABSTRACT We construct nanoparticle dimers lin...
We computationally investigate the good solvent solution properties of knotted ring and star polymers by combining molecular dynamics (MD) simulation and path-integral calculations. We consider knotted rings having a minimal crossing number mc in the range, 0 ≤ mc ≤ 9, and star polymers having a range of f star arms, 2 ≤ f ≤ 20, attached to a common core monomer particle. After generating configurational ensembles of these polymers by MD, we use the path-integration program ZENO to calculate basic configurational properties, i.e., radius of gyration, hydrodynamic radius, intrinsic viscosity, as well as fluctuations in these properties. Our simulations indicate that the configurational properties of knotted rings and star polymers in solution show a similar decrease with increasing mc and f. Moreover, fluctuations in these properties also decrease with increasing topological complexity. Our findings should be helpful in polymer characterization and more generally for understanding the role of polymer topology in polymer material properties.
We perform coarse-grained simulations of model unentangled polymer materials to quantify the range over which interfaces alter the structure and dynamics in the vicinity of the interface. We study the interfacial zone around nanoparticles (NPs) in model polymer-NP composites with variable NP diameter, as well as the interfacial zone at the solid substrate and free surface of thin supported polymer films. These interfaces alter both the segmental packing and mobility in an interfacial zone. Variable NP size allows us to gain insight into the effect of boundary curvature, where the film is the limit of zero curvature. We find that the scale for perturbations of the density is relatively small and decreases on cooling for all cases. In other words, the interfaces become more sharply defined on cooling, as naively expected. In contrast, the interfacial mobility scale ξ for both NPs and supported films increases on cooling and is on the order of a few nanometers, regardless of the polymer-interfacial interaction strength. Additionally, the dynamical interfacial scale of the film substrate is consistent with a limiting value for polymer-NP composites as the NP size grows. These findings are based on a simple quantitative model to describe the distance dependence of relaxation that should be applicable to many interfacial polymer materials.
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