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Gary S Grest - One of the best experts on this subject based on the ideXlab platform.
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rouse mode analysis of chain relaxation in polymer nanocomposites
Soft Matter, 2015Co-Authors: Sanat K Kumar, Jagannathan T Kalathi, Michael Rubinstein, Gary S GrestAbstract:Large-scale molecular dynamics simulations are used to study the internal relaxations of chains in nanoparticle (NP)/polymer composites. We examine the Rouse modes of the chains, a quantity that is closest in spirit to the self-intermediate scattering function, typically determined in an (incoherent) inelastic neutron scattering experiment. Our simulations show that for weakly interacting mixtures of NPs and polymers, the effective monomeric relaxation rates are faster than in a neat melt when the NPs are smaller than the Entanglement mesh size. In this case, the NPs serve to reduce both the monomeric friction and the Entanglements in the polymer melt, as in the case of a polymer–solvent system. However, for NPs larger than half the Entanglement mesh size, the effective monomer relaxation is essentially unaffected for low NP concentrations. Even in this case, we observe a strong reduction in chain Entanglements for larger NP loadings. Thus, the role of NPs is to always reduce the number of Entanglements, with this effect only becoming pronounced for small NPs or for high concentrations of large NPs. Our studies of the relaxation of single chains resonate with recent neutron spin echo (NSE) experiments, which deduce a similar Entanglement dilution effect.
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tensile fracture of welded polymer interfaces miscibility Entanglements and crazing
Macromolecules, 2014Co-Authors: Ting Ge, Gary S Grest, Mark O. RobbinsAbstract:Large-scale molecular simulations are performed to investigate tensile failure of polymer interfaces as a function of welding time t. Changes in the tensile stress, mode of failure and interfacial fracture energy GI are correlated to changes in the interfacial Entanglements as determined from primitive path analysis. Bulk polymers fail through craze formation, followed by craze breakdown through chain scission. At small t welded interfaces are not strong enough to support craze formation and fail at small strains through chain pullout at the interface. Once chains have formed an average of about one Entanglement across the interface, a stable craze is formed throughout the sample. The failure stress of the craze rises with welding time and the mode of craze breakdown changes from chain pullout to chain scission as the interface approaches bulk strength. The interfacial fracture energy GI is calculated by coupling the simulation results to a continuum fracture mechanics model. As in experiment, GI increase...
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rouse mode analysis of chain relaxation in homopolymer melts
Macromolecules, 2014Co-Authors: Jagannathan T Kalathi, Sanat K Kumar, Michael Rubinstein, Gary S GrestAbstract:We use molecular dynamics simulations of the Kremer–Grest (KG) bead–spring model of polymer chains of length between 10 and 500, and a closely related analogue that allows for chain crossing, to clearly delineate the effects of Entanglements on the length-scale-dependent chain relaxation in polymer melts. We analyze the resulting trajectories using the Rouse modes of the chains and find that Entanglements strongly affect these modes. The relaxation rates of the chains show two limiting effective monomeric frictions, with the local modes experiencing much lower effective friction than the longer modes. The monomeric relaxation rates of longer modes vary approximately inversely with chain length due to kinetic confinement effects. The time-dependent relaxation of Rouse modes has a stretched exponential character with a minimum of stretching exponent in the vicinity of the Entanglement chain length. None of these trends are found in models that allow for chain crossing. These facts, in combination, argue for...
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Healing of polymer interfaces: Interfacial dynamics, Entanglements, and strength.
Physical Review E, 2014Co-Authors: Ting Ge, Mark O. Robbins, Dvora Perahia, Gary S GrestAbstract:Self-healing of polymer films often takes place as the molecules diffuse across a damaged region, above their melting temperature. Using molecular dynamics simulations we probe the healing of polymer films and compare the results with those obtained for thermal welding of homopolymer slabs. These two processes differ from each other in their interfacial structure since damage leads to increased polydispersity and more short chains. A polymer sample was cut into two separate films that were then held together in the melt state. The recovery of the damaged film was followed as time elapsed and polymer molecules diffused across the interface. The mass uptake and formation of Entanglements, as obtained from primitive path analysis, are extracted and correlated with the interfacial strength obtained from shear simulations. We find that the diffusion across the interface is signifcantly faster in the damaged film compared to welding because of the presence of short chains. Though interfacial Entanglements increase more rapidly for the damaged films, a large fraction of these Entanglements are near chain ends. As a result, the interfacial strength of the healing film increases more slowly than for welding. For both healing and welding, the interfacial strength saturates as the bulk Entanglement densitymore » is recovered across the interface. However, the saturation strength of the damaged film is below the bulk strength for the polymer sample. At saturation, cut chains remain near the healing interface. They are less entangled and as a result they mechanically weaken the interface. When the strength of the interface saturates, the number of interfacial Entanglements scales with the corresponding bulk Entanglement density. Chain stiffness increases the density of Entanglements, which increases the strength of the interface. Our results show that a few Entanglements across the interface are sufficient to resist interfacial chain pullout and enhance the mechanical strength.« less
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end grafted polymer nanoparticles in a polymeric matrix effect of coverage and curvature
Soft Matter, 2011Co-Authors: Joshua Kalb, Sanat K Kumar, Douglas Dukes, Gary S GrestAbstract:It has recently been proposed that the miscibility of nanoparticles with a polymer matrix can be controlled by grafting polymer chains to the nanoparticle surface. We examine this hypothesis using molecular dynamics simulations on a single nanoparticle of radius R (4σ ≤ R ≤ 16σ, where σ is the diameter of a polymer monomer) grafted with chains of length 500 in a polymer melt of chains of length 1000. The grafting density Σ is varied between 0.04–0.32 chains/σ2. To facilitate equilibration a Monte Carlo double-bridging algorithm is applied - new bonds are formed across a pair of chains, creating two new chains each substantially different from the original. For the long brush chains studied here, the structure of the brush assumes its large particle limit even for R as small as 8σ, which is consistent with recent experimental findings and the small chain lattice simulations of Klos and Pakula. We study autophobic dewetting of the melt from the brush as a function of increasing Σ. Even these long brush and matrix chains of lengths 6 and 12 Ne, respectively, (the Entanglement length is Ne ∼ 85) give somewhat ambiguous results for the interfacial width, showing that studies of two or more nanoparticles are necessary to properly understand these miscibility issues. Entanglement between the brush and melt chains is identified using path analysis. We find that the number of Entanglements between the brush and melt chains scale simply with the product of the local monomer densities of brush and melt chains.
Jagannathan T Kalathi - One of the best experts on this subject based on the ideXlab platform.
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rouse mode analysis of chain relaxation in polymer nanocomposites
Soft Matter, 2015Co-Authors: Sanat K Kumar, Jagannathan T Kalathi, Michael Rubinstein, Gary S GrestAbstract:Large-scale molecular dynamics simulations are used to study the internal relaxations of chains in nanoparticle (NP)/polymer composites. We examine the Rouse modes of the chains, a quantity that is closest in spirit to the self-intermediate scattering function, typically determined in an (incoherent) inelastic neutron scattering experiment. Our simulations show that for weakly interacting mixtures of NPs and polymers, the effective monomeric relaxation rates are faster than in a neat melt when the NPs are smaller than the Entanglement mesh size. In this case, the NPs serve to reduce both the monomeric friction and the Entanglements in the polymer melt, as in the case of a polymer–solvent system. However, for NPs larger than half the Entanglement mesh size, the effective monomer relaxation is essentially unaffected for low NP concentrations. Even in this case, we observe a strong reduction in chain Entanglements for larger NP loadings. Thus, the role of NPs is to always reduce the number of Entanglements, with this effect only becoming pronounced for small NPs or for high concentrations of large NPs. Our studies of the relaxation of single chains resonate with recent neutron spin echo (NSE) experiments, which deduce a similar Entanglement dilution effect.
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rouse mode analysis of chain relaxation in homopolymer melts
Macromolecules, 2014Co-Authors: Jagannathan T Kalathi, Sanat K Kumar, Michael Rubinstein, Gary S GrestAbstract:We use molecular dynamics simulations of the Kremer–Grest (KG) bead–spring model of polymer chains of length between 10 and 500, and a closely related analogue that allows for chain crossing, to clearly delineate the effects of Entanglements on the length-scale-dependent chain relaxation in polymer melts. We analyze the resulting trajectories using the Rouse modes of the chains and find that Entanglements strongly affect these modes. The relaxation rates of the chains show two limiting effective monomeric frictions, with the local modes experiencing much lower effective friction than the longer modes. The monomeric relaxation rates of longer modes vary approximately inversely with chain length due to kinetic confinement effects. The time-dependent relaxation of Rouse modes has a stretched exponential character with a minimum of stretching exponent in the vicinity of the Entanglement chain length. None of these trends are found in models that allow for chain crossing. These facts, in combination, argue for...
Sanat K Kumar - One of the best experts on this subject based on the ideXlab platform.
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rouse mode analysis of chain relaxation in polymer nanocomposites
Soft Matter, 2015Co-Authors: Sanat K Kumar, Jagannathan T Kalathi, Michael Rubinstein, Gary S GrestAbstract:Large-scale molecular dynamics simulations are used to study the internal relaxations of chains in nanoparticle (NP)/polymer composites. We examine the Rouse modes of the chains, a quantity that is closest in spirit to the self-intermediate scattering function, typically determined in an (incoherent) inelastic neutron scattering experiment. Our simulations show that for weakly interacting mixtures of NPs and polymers, the effective monomeric relaxation rates are faster than in a neat melt when the NPs are smaller than the Entanglement mesh size. In this case, the NPs serve to reduce both the monomeric friction and the Entanglements in the polymer melt, as in the case of a polymer–solvent system. However, for NPs larger than half the Entanglement mesh size, the effective monomer relaxation is essentially unaffected for low NP concentrations. Even in this case, we observe a strong reduction in chain Entanglements for larger NP loadings. Thus, the role of NPs is to always reduce the number of Entanglements, with this effect only becoming pronounced for small NPs or for high concentrations of large NPs. Our studies of the relaxation of single chains resonate with recent neutron spin echo (NSE) experiments, which deduce a similar Entanglement dilution effect.
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rouse mode analysis of chain relaxation in homopolymer melts
Macromolecules, 2014Co-Authors: Jagannathan T Kalathi, Sanat K Kumar, Michael Rubinstein, Gary S GrestAbstract:We use molecular dynamics simulations of the Kremer–Grest (KG) bead–spring model of polymer chains of length between 10 and 500, and a closely related analogue that allows for chain crossing, to clearly delineate the effects of Entanglements on the length-scale-dependent chain relaxation in polymer melts. We analyze the resulting trajectories using the Rouse modes of the chains and find that Entanglements strongly affect these modes. The relaxation rates of the chains show two limiting effective monomeric frictions, with the local modes experiencing much lower effective friction than the longer modes. The monomeric relaxation rates of longer modes vary approximately inversely with chain length due to kinetic confinement effects. The time-dependent relaxation of Rouse modes has a stretched exponential character with a minimum of stretching exponent in the vicinity of the Entanglement chain length. None of these trends are found in models that allow for chain crossing. These facts, in combination, argue for...
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end grafted polymer nanoparticles in a polymeric matrix effect of coverage and curvature
Soft Matter, 2011Co-Authors: Joshua Kalb, Sanat K Kumar, Douglas Dukes, Gary S GrestAbstract:It has recently been proposed that the miscibility of nanoparticles with a polymer matrix can be controlled by grafting polymer chains to the nanoparticle surface. We examine this hypothesis using molecular dynamics simulations on a single nanoparticle of radius R (4σ ≤ R ≤ 16σ, where σ is the diameter of a polymer monomer) grafted with chains of length 500 in a polymer melt of chains of length 1000. The grafting density Σ is varied between 0.04–0.32 chains/σ2. To facilitate equilibration a Monte Carlo double-bridging algorithm is applied - new bonds are formed across a pair of chains, creating two new chains each substantially different from the original. For the long brush chains studied here, the structure of the brush assumes its large particle limit even for R as small as 8σ, which is consistent with recent experimental findings and the small chain lattice simulations of Klos and Pakula. We study autophobic dewetting of the melt from the brush as a function of increasing Σ. Even these long brush and matrix chains of lengths 6 and 12 Ne, respectively, (the Entanglement length is Ne ∼ 85) give somewhat ambiguous results for the interfacial width, showing that studies of two or more nanoparticles are necessary to properly understand these miscibility issues. Entanglement between the brush and melt chains is identified using path analysis. We find that the number of Entanglements between the brush and melt chains scale simply with the product of the local monomer densities of brush and melt chains.
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end grafted polymer nanoparticles in a polymeric matrix effect of coverage and curvature
arXiv: Soft Condensed Matter, 2010Co-Authors: Joshua Kalb, Sanat K Kumar, Douglas Dukes, Gary S GrestAbstract:It has recently been proposed that the miscibility of nanoparticles with a polymer matrix can be controlled by grafting polymer chains to the nanoparticle surface. As a first step to study this situation, we have used molecular dynamics simulations on a single nanoparticle of radius R ($4\sigma \le$R$\le 16\sigma$, where $\sigma$ is the diameter of a polymer monomer) grafted with chains of length 500 in a polymer melt of chains of length 1000. The grafting density $\Sigma$ was varied between $0.04$-$0.32$ chains/$\sigma^2$. To facilitate equilibration a Monte Carlo double-bridging algorithm is applied - new bonds are formed across a pair of chains, creating two new chains each substantially different from the original. For the long brush chains studied here, the structure of the brush assumes its large particle limit even for $R$ as small as 8$\sigma$, which is consistent with recent experimental findings. We study autophobic dewetting of the melt from the brush as a function of increasing $\Sigma$. Even these long brush and matrix chains of length $6$ and $12$ $N_e$, respectively, (the Entanglement length is $N_e \sim 85$) give somewhat ambiguous results for the interfacial width, showing that studies of two or more nanoparticles are necessary to properly understand these miscibility issues. Entanglement between the brush and melt chains were identified using the primitive path analysis. We find that the number of Entanglements between the brush and melt chains scale simply with the product of the local monomer densities of brush and melt chains.
Michael Rubinstein - One of the best experts on this subject based on the ideXlab platform.
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rouse mode analysis of chain relaxation in polymer nanocomposites
Soft Matter, 2015Co-Authors: Sanat K Kumar, Jagannathan T Kalathi, Michael Rubinstein, Gary S GrestAbstract:Large-scale molecular dynamics simulations are used to study the internal relaxations of chains in nanoparticle (NP)/polymer composites. We examine the Rouse modes of the chains, a quantity that is closest in spirit to the self-intermediate scattering function, typically determined in an (incoherent) inelastic neutron scattering experiment. Our simulations show that for weakly interacting mixtures of NPs and polymers, the effective monomeric relaxation rates are faster than in a neat melt when the NPs are smaller than the Entanglement mesh size. In this case, the NPs serve to reduce both the monomeric friction and the Entanglements in the polymer melt, as in the case of a polymer–solvent system. However, for NPs larger than half the Entanglement mesh size, the effective monomer relaxation is essentially unaffected for low NP concentrations. Even in this case, we observe a strong reduction in chain Entanglements for larger NP loadings. Thus, the role of NPs is to always reduce the number of Entanglements, with this effect only becoming pronounced for small NPs or for high concentrations of large NPs. Our studies of the relaxation of single chains resonate with recent neutron spin echo (NSE) experiments, which deduce a similar Entanglement dilution effect.
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rouse mode analysis of chain relaxation in homopolymer melts
Macromolecules, 2014Co-Authors: Jagannathan T Kalathi, Sanat K Kumar, Michael Rubinstein, Gary S GrestAbstract:We use molecular dynamics simulations of the Kremer–Grest (KG) bead–spring model of polymer chains of length between 10 and 500, and a closely related analogue that allows for chain crossing, to clearly delineate the effects of Entanglements on the length-scale-dependent chain relaxation in polymer melts. We analyze the resulting trajectories using the Rouse modes of the chains and find that Entanglements strongly affect these modes. The relaxation rates of the chains show two limiting effective monomeric frictions, with the local modes experiencing much lower effective friction than the longer modes. The monomeric relaxation rates of longer modes vary approximately inversely with chain length due to kinetic confinement effects. The time-dependent relaxation of Rouse modes has a stretched exponential character with a minimum of stretching exponent in the vicinity of the Entanglement chain length. None of these trends are found in models that allow for chain crossing. These facts, in combination, argue for...
Christos Tzoumanekas - One of the best experts on this subject based on the ideXlab platform.
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Entanglements in glassy polymer crazing cross links or tubes
Macromolecules, 2017Co-Authors: Ting Ge, Christos Tzoumanekas, Stefanos D Anogiannakis, Mark O. RobbinsAbstract:Models of the mechanical response of glassy polymers commonly assume that Entanglements inherited from the melt act like chemical cross-links. The predictions from these network models and the physical picture they are based on are tested by following the evolution of topological constraints in simulations of model polymer glasses. The same behavior is observed for polymers with Entanglement lengths Ne that vary by a factor of 3. A prediction for the craze extension ratio Λ based on the network model describes trends with Ne, but polymers do not have the taut configurations it assumes. There is also no evidence of the predicted geometrically necessary Entanglement loss. While the number of Entanglements remains constant, the identity of the chains forming constraints changes. The same relation between the amount of Entanglement exchange and nonaffine displacement of monomers is found for crazing and thermal diffusion in end-constrained melts. In both cases, about 1/3 of the constraints change when monomer...
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influence of molecular architecture on the Entanglement network topological analysis of linear long and short chain branched polyethylene melts via monte carlo simulations
Soft Matter, 2016Co-Authors: Seung Heum Jeong, Christos Tzoumanekas, Jeongha Yoon, Martin Kroger, Chunggi BaigAbstract:We present detailed results on the effect of chain branching on the topological properties of entangled polymer melts via an advanced connectivity-altering Monte Carlo (MC) algorithm. Eleven representative model linear, short-chain branched (SCB), and long-chain branched (LCB) polyethylene (PE) melts were employed, based on the total chain length and/or the longest linear chain dimension. Directly analyzing the Entanglement [or the primitive path (PP)] network of the system via the Z-code, we quantified several important topological measures: (a) the PP contour length Lpp, (b) the number of Entanglements Zes per chain, (c) the end-to-end length of an Entanglement strand des, (d) the number of carbon atoms per Entanglement strand Nes, and (e) the probability distribution for each of these quantities. The results show that the SCB polymer melts have significantly more compact overall chain conformations compared to the linear polymers, exhibiting, relative to the corresponding linear analogues, (a) ∼20% smaller values of 〈Lpp〉 (the statistical average of Lpp), (b) ∼30% smaller values of 〈Zes〉, (c) ∼20% larger values of 〈des〉, and (d) ∼50% larger values of 〈Nes〉. In contrast, despite the intrinsically smaller overall chain dimensions than those of the linear analogues, the LCB (H-shaped and A3AA3 multiarm) PE melts exhibit relatively (a) 7–8% larger values of 〈Lpp〉, (b) 6–11% larger values of 〈Zes〉 for the H-shaped melt and ∼2% smaller values of 〈Zes〉 for the A3AA3 multiarm, (c) 2–5% smaller values of 〈des〉, and (d) 7–11% smaller values of 〈Nes〉. Several interesting features were also found in the results of the probability distribution functions P for each topological measure.
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coarse grained and reverse mapped united atom simulations of long chain atactic polystyrene melts structure thermodynamic properties chain conformation and Entanglements
Macromolecules, 2007Co-Authors: Theodora Spyriouni, Christos Tzoumanekas, Doros N Theodorou, Florian Mullerplathe, Giuseppe MilanoAbstract:A coarse-grained model of atactic polystyrene, in which meso and racemo diads are represented as single “superatoms,” parametrized using Iterative Boltzmann Inversion, has been subjected to connectivity-altering Monte Carlo simulations in order to simulate monodisperse atactic polystyrene melts of molar mass up to 210000 g mol-1 at 500 or 413 K and 1 bar. Analysis of the Monte Carlo results reveals excellent equilibration of chain conformations at all length scales. Chain dimensions, as determined from the mean square end-to-end distance, the mean square radius of gyration, and simulated Kratky plots of the single-chain scattering function, are in excellent agreement with experiment. The equilibrated long-chain configurations are reduced to Entanglement networks via topological analysis with the CReTA algorithm. The resulting Kuhn length of primitive paths provides an excellent estimate of the molar mass between Entanglements and of the Entanglement tube diameter extracted from plateau modulus measurement...
