Molecular Dynamics Simulation of a Polymer Melt with a Nanoscopic Particle (original) (raw)
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The Journal of Chemical Physics, 2013
In this work, we study the influence of polymer chain length (m), based on Lennard-Jones potential, and nanoparticle (NP)-polymer interaction strength (ɛnp) on aggregation and dispersion of soft repulsive spherically structured NPs in polymer melt using coarse-grain molecular dynamics simulations. A phase diagram is proposed where transitions between different structures in the NP-polymer system are shown to depend on m and ɛnp. At a very weak interaction strength ɛnp = 0.1, a transition from dispersed state to collapsed state of NPs is found with increasing m, due to the polymer's excluded volume effect. NPs are well dispersed at intermediate interaction strengths (0.5 ⩽ ɛnp ⩽ 2.0), independent of m. A transition from dispersion to agglomeration of NPs, at a moderately high NP-polymer interaction strength ɛnp = 5.0, for m = 1–30, is identified by a significant decrease in the second virial coefficient, excess entropy, and potential energy, and a sharp increase in the Kirkwood-B...
The Journal of Chemical Physics, 2014
Using large scale molecular dynamics simulations we investigate the static and dynamic properties of a linear polymer melt confined between two solid surfaces. One of the walls is repulsive and the other is attractive wall. The bottom attractive wall is characterized by different degrees of roughness which is tuned by an array of short perpendicular rigid pillars with variable grafting density. We demonstrate that the conformations of polymers at the interfaces do not depend on substrate-polymer interactions, rather they show similar conformations of a single end-grafted chain under critical adsorption condition, consistent with the Silberberg's hypothesis. This observation is found to be in a good agreement with the analysis of the size distributions of trains, loops, and tails of melt chains at the walls known from the theoretical prediction of the end-grafted single chains at critical adsorption. Furthermore, we find that the pressure of the melt P N decreases as P N − P ∞ ∝ N −1 with growing length of the chains N (where P ∞ is the extrapolated pressure for N → ∞). Moreover, the surface tension γ near both walls is found to follow γ N ∝N −2/3 . Eventually, the lateral dynamics near rough surface drops suddenly when the separation between the neighboring pillars becomes smaller than 2R g , where R g is the bulk radius of gyration. © 2014 AIP Publishing LLC. [http://dx.
Structure and dynamics of a polymer melt at an attractive surface
The European Physical Journal E, 2012
We study the structural and dynamic properties of a polymer melt in the vicinity of an adhesive solid substrate by means of Molecular Dynamics simulation at various degrees of surface adhesion. The properties of the individual polymer chains are examined as a function of the distance to the interface and found to agree favorably with theoretical predictions. Thus, the adsorbed amount at the adhesive surface is found to scale with the macromolecule length as Γ ∝ √ N , regardless of the adsorption strength. For chains within the range of adsorption we analyze in detail the probability size distributions of the various building blocks: loops, tails and trains, and find that loops and tails sizes follow power laws while train lengths decay exponentially thus confirming some recent theoretical results. The chain dynamics as well as the monomer mobility are also investigated and found to depend significantly on the proximity of a given layer to the solid adhesive surface with onset of vitrification for sufficiently strong adsorption.
Molecular Dynamic Study of the Structure and Dynamics of Polymer Melt at Solid Surfaces
Soft Materials, 2014
We investigate the dynamic and static properties of a polymer melt near solid surfaces. The melt, composed of linear chains, is confined between two solid walls with one of the walls being repulsive; whereas the opposite, attractive wall, is characterized by different degrees of roughness, caused by an array of short perpendicular pillars with variable grafting density. We demonstrate the remarkable fact that the conformations of chains in the melt at the interfaces are mostly unaffected by the strength of substrate/polymer attraction. Moreover, they practically coincide with the conformations of a single end-grafted chain at the critical point of adsorption, in agreement with Silberberg's hypothesis. This agreement is corroborated by the analysis of the size distributions of trains, loops, and tails of melt chains at the walls that are found to be perfectly described by analytical expressions pertaining to end-grafted single chains at critical adsorption. The adsorbed amount at the attractive bottom surface is found to scale with macromolecule length as ∝ √ N regardless of adsorption strength. We also find that the pressure of the melt P N decreases as P N − P ∞ ∝ N −1 (where P ∞ is the extrapolated pressure for N → ∞) with growing length N of the chains whereas the surface tension γ at both walls is found to decline as γ N ∝ N −2/3 . Eventually, a study of the polymer dynamics at the rough interface reveals that surface roughness leads to dramatic drop of the coefficient for lateral diffusion whenever the separation between obstacles (neighboring pillars) becomes less than ≈ 2R g where R g is the unperturbed radius of gyration of chains in the bulk.
Modifying Fragility and Collective Motion in Polymer Melts with Nanoparticles
Physical Review Letters, 2011
We investigate the impact of nanoparticles (NP) on the fragility and cooperative string-like motion in a model glass-forming polymer melt by molecular dynamics simulation. The NP cause significant changes to both the fragility and the average length of string-like motion, where the effect depends on the NP-polymer interaction and the NP concentration. We interpret these changes via the Adam-Gibbs (AG) theory, assuming the strings can be identified with the "cooperatively rearranging regions" of AG. Our findings indicate fragility is primarily a measure of the temperature dependence of the cooperativity of molecular motion.
On modifying properties of polymeric melts by nanoscopic particles
Journal of Polymer Science Part B: Polymer Physics, 2012
We study geometric and energetic factors that partake in modifying properties of polymeric melts via inserting well-dispersed nanoscopic particles (NP). Model systems are cis-1,4-polybutadiene melts including a single atomic clusters of size varied in the range 10-150 atoms (3-7 Å in radius; 0.1-1.5% v/v). We modify the interactions between the chains and the particle by tuning attractive van der Waals interactions. Using molecular dynamics, we study equilibrium fluctuations and dynamical properties at the interface. The NPs move in the polymer matrix in two different regimes corresponding to trapped and free diffusion, depending on the NP size. Furthermore, degree of crowding around the NP by the polymer chains is quantified. Effect of NP size and interaction strength both on volume and volumetric fluctuations is manifested in mechanical properties, quantified here by bulk modulus, K. Tuning NP size and nonbonded interactions results in $15% enhancement in K by addition of a maximum of 1.5% v/v NP.
Dynamic Arrest in Polymer Melts: Competition between Packing and Intramolecular Barriers
Physical Review Letters, 2008
We present molecular dynamics simulations of a simple model for polymer melts with intramolecular barriers. We investigate structural relaxation as a function of the barrier strength. Dynamic correlators can be consistently analyzed within the framework of the Mode Coupling Theory (MCT) of the glass transition. Control parameters are tuned in order to induce a competition between general packing effects and polymer-specific intramolecular barriers as mechanisms for dynamic arrest. This competition yields unusually large values of the so-called MCT exponent parameter and rationalize qualitatively different observations for simple bead-spring and realistic polymers. The systematic study of the effect of intramolecular barriers presented here also establishes a fundamental difference between the nature of the glass transition in polymers and in simple glass-formers.
Composites Science and Technology, 2003
Molecular dynamics (MD) simulations were performed on a model polymer-nanoparticle composite (PNPC) consisting of spherical nanoparticles in a bead-spring polymer melt. The polymer-mediated effective interaction (potential of mean force) between nanoparticles was determined as a function of polymer molecular weight and strength of the polymer-nanoparticle interaction. For all polymer-nanoparticle interactions and polymer molecular weights investigated the range of the matrix-induced interaction was greater than the direct nanoparticle-nanoparticle interaction employed in the simulations. When the polymernanoparticle interactions were relatively weak the polymer matrix promoted nanoparticle aggregation, an effect that increased with polymer molecular weight. Increasingly attractive nanoparticle-polymer interactions led to strong adsorption of the polymer chains on the surface of the nanoparticles and promoted dispersion of the nanoparticles. For PNPCs with strongly adsorbed chains the matrix-induced interaction between nanoparticles reflected the structure (layering) imposed on the melt by the nanoparticle surface and was independent of polymer molecular weight. The nanoparticle second virial coefficient obtained from the potential of mean force was utilized as an indicator of dispersion or aggregation of the particles in the PNPC, and was found to be in qualitative agreement with the aggregation properties obtained from simulations of selected PNPCs with multiple nanoparticles.
Physical Review E, 1998
We present Molecular Dynamics simulations of the thermal glass transition in a dense model polymer liquid. We performed a comparative study of both constant volume and constant pressure cooling of the polymer melt. Great emphasis was laid on a careful equilibration of the dense polymer melt at all studied temperatures. Our model introduces competing length scales in the interaction to prevent any crystallisation tendency. In this first manuscript we analyse the structural properties as a function of temperature and the long time or α-relaxation behaviour as observed in the dynamic structure factor and the self-diffusion of the polymer chains. The α-relaxation can be consistently analysed in terms of the mode coupling theory (MCT) of the glass transition. The mode coupling critical temperature, T c , and the exponent, γ, defining the power law divergence of the α-relaxation timescale both depend on the thermodynamic ensemble employed in the simulation.