Structure and dynamics of polymer melt confined between two solid surfaces: A molecular dynamics study (original) (raw)
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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.
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.
Structure, conformation and dynamics of polymer chains at solid melt interfaces
Makromolekulare Chemie. Macromolecular Symposia, 1993
We have performed molecular dynamics, and lattice Monte Car10 simulations of polymeric melts in the vicinity of solid surfaces. The structural features of the solid-melt interface were very simple. The interfacial width was comparable to the segment size. Inside this narrow interface the segment density profile was oscillatory. The density oscillations were much less pronounced than those present at solid-atomic liquid interfaces. On a scale much larger than the segment size, chain conformations were found to be identical with those of ideal chains next to a reflective barrier. In particular, the number of surface-segment contacts scaled like the square root of the molecular weight. Extensive molecular dynamics simulations showed that chain desorption times increase with molecular weight but at a rate much slower than the longest relaxation time of Rouse chains. Therefore, sufficiently long chains desorbed almost freely from the surface despite the presence of attractive surface-segment interactions. A study of chain relaxation dynamics c o n f i i e d that the Rouse modes constitute an appropriate set of normal coordinates for chains in the melt interacting with a solid surface. The effect of the surface on mode relaxation was significant. All relaxation processes of chains located within a couple of radii of gyration from the surface were slowed down considerably. This effect, however was approximately the same for fast and slow modes and independent of molecular weight for sufficiently long chains.
Chain Dynamics in Polymer Melts at Flat Surfaces
Macromolecules
We investigate, by extensive molecular dynamics simulations as well as a simplified single-chain model, the influence of steric hindrance on the dynamic properties of nonentangled chains in polymer melt due to confining surfaces. We extend the Rouse model to also include wall effects, using an additional potential that results from the assumption that chain conformations have reflected random-walk statistics, as first advocated by Silberberg. Results for end-to-end vector and Rouse mode correlation functions of chains end-tethered to the surface compare well with those obtained from molecular dynamics simulations of a multi-chain system using the Kremer-Grest beadspring model (KG MD). Even though the additional single-chain potential is parameterfree, we show that the accuracy of the model for surface chains is comparable to that of the Rouse model for bulk chains. An analytic dumbbell model accurately describes the longest Rouse mode correlation function of surface-tethered 'mushroom' chains immersed in a polymer melt at low grafting density. In addition, we find that a perfectly smooth surface enhances the influence of hydrodynamic visco-elastic coupling on the centre of mass motion near the surface.
The Journal of Chemical Physics, 2016
Using molecular dynamics simulations, we study and compare the pressure, P, and the surface tension, γ, of linear chains and of ring polymers at the hard walls confining both melts into a slit. We examine the dependence of P and γ on the length (i.e., molecular weight) N of the macromolecules. For linear chains, we find that both pressure and surface tension are inversely proportional to the chain length, P(N)−P(N→∞)∝N−1,γ(N)−γ(N→∞)∝N−1, irrespective of whether the confining planes attract or repel the monomers. In contrast, for melts comprised of cyclic (ring) polymers, neither the pressure nor the surface tension is found to depend on molecular weight N for both kinds of wall-monomer interactions. While other structural properties as, e.g., the probability distributions of trains and loops at impenetrable walls appear quantitatively indistinguishable, we observe an amazing dissimilarity in the probability to find a chain end or a tagged monomer of a ring at a given distance from t...
Structure of an Associating Polymer Melt in a Narrow Slit by Molecular Dynamics Simulation
The Journal of Physical Chemistry B, 2005
Molecular dynamics simulation has been used to study the equilibrium properties of a generic coarse-grained polymer melt with associating terminal groups, confined in a narrow slit by two atomically smooth walls. Simulations were carried out as a function of wall separation and attracting strength as well as polymer end-end interaction strength. We find that confinement has an important effect on the melt properties. In particular, strongly attracting walls can produce radical changes in chain conformation, the nature of the transient network, and the structure of the aggregates formed by the associating terminals.
Revealed Architectures of Adsorbed Polymer Chains at Solid-Polymer Melt Interfaces
Physical Review Letters, 2012
We report the chain conformations of polymer molecules accommodated at the solid-polymer melt interfaces in equilibrium. Polystyrene ''Guiselin'' brushes (adsorbed layers) with different molecular weights were prepared on Si substrates and characterized by using x-ray and neutron reflectivity. The results are intriguing to show that the adsorbed layers are composed of the two different nanoarchitectures: flattened chains that constitute the inner higher density region of the adsorbed layers and loosely adsorbed polymer chains that form the outer bulklike density region. In addition, we found that the lone flattened chains, which are uncovered by the additional prolonged solvent leaching ($ 120 days), are reversibly densified with increasing temperature up to 150 C. By generalizing the chain conformations of bulks, we postulate that the change in probabilities of the local chain conformations (i.e., trans and gauche states) of polymer molecules is the origin of this densification process.
A lattice Monte Carlo study of long chain conformations at solid–polymer melt interfaces
The Journal of Chemical Physics, 1993
In this paper we present a comprehensive lattice Monte Carlo study of long chain conformations at solid-polymer melt interfaces. Segmental scale interfacial features, like the bond orientational distribution were found to be independent of surface-segment energetics, and statistically identical with Helfand's predictions for the full-occupancy, infinite chain length limit. Conformational statistics of chains longer than 5-6 statistical segments followed the predictions of the Scheutjens-Fleer theory and the same power laws as a single ideal chain at the critical value of the surface-segment adsorption free-energy. Our simulations tested the predictions of random walk next to a "reflective" surface statistics for the spatial variations of chain dimensions and chain center of mass density. It was found that these statistics furnish the correct long chain limit, independently of surfac+segment energetics. The random walk next to a "reflective" boundary predictions for the "adsorbed" amount and the distributions of tail, loop, and train number, as well as tail and loop size were in quantitative agreement with the simulation data. The correspondence between random walks and real (or simulated) chains required the knowledge of a single, microscopic parameter, the number of chemical segments per statistical segment, a, . This quantity was very close to the average length of adsorbed sequences (trains), in the long chain limit. Our simulations tested thoroughly and established firmly the validity of "reflective" boundary statistics in the melt. The inevitability of these statistics has broad implications on "desorption" kinetics, chain mobility, and chain relaxation, which are currently under study. 3100