Dynamic Properties Of Polymer Chains Research Articles

Effect of threading on static and dynamic properties of polymer chains in entangled linear-ring blend systems with different stiffness

Threading is a unique topological state in linear/ring polymer blends (LRB), which is considered to be essential for evaluating both the static and dynamic properties of ring polymer (RP)-containing polymer melts. In the present work, we use molecular dynamics simulations with the Kremer-Grest model to systematically investigate the threading event and its effect on the properties of RP in linear polymer (LP) dominated LRB. The chain stiffness is also considered in the simulation to represent a wider range of polymer systems. To obtain the exact number of threads for each RP chain, the inner minimal surface (IMS) technique is applied. We find that for a wide range of chain lengths, the number of threads can be roughly estimated using the predetermined entanglement number for RP chains with different stiffness. The number of threads follows a Gaussian distribution, and the width increases monotonically for longer and stiffer chains. The effect of threading on both the size and shape of RP chains is investigated. We find that for RP chains of different chain lengths, the effect of threading on polymer size is more pronounced for more flexible chains. The effect of threading on shape of RP is also enhanced for longer chains, but the sensitivity of the shape to threading is indistinguishable for RP chains of different stiffness. The dynamics of threading is also studied in this work, and a much slower relaxation time is found for more flexible chains. In conclusion, we have developed a method that can directly detect the threading events in LRB, which allows us to quantitatively study the effect of threading. The result obtained by this method is also expected to be integrated into the viscoelastic theory to better estimate the rheological properties of LRB systems.

Interface and Interphase in Polymer Nanocomposites with Bare and Core-Shell Gold Nanoparticles.

Metal nanoparticles are used to modify/enhance the properties of a polymer matrix for a broad range of applications in bio-nanotechnology. Here, we study the properties of polymer/gold nanoparticle (NP) nanocomposites through atomistic molecular dynamics, MD, simulations. We probe the structural, conformational and dynamical properties of polymer chains at the vicinity of a gold (Au) NP and a functionalized (core/shell) Au NP, and compare them against the behavior of bulk polyethylene (PE). The bare Au NPs were constructed via a systematic methodology starting from ab-initio calculations and an atomistic Wulff construction algorithm resulting in the crystal shape with the minimum surface energy. For the functionalized NPs the interactions between gold atoms and chemically adsorbed functional groups change their shape. As a model polymer matrix we consider polyethylene of different molecular lengths, from the oligomer to unentangled Rouse like systems. The PE/Au interaction is parametrized via DFT calculations. By computing the different properties the concept of the interface, and the interphase as well, in polymer nanocomposites with metal NPs are critically examined. Results concerning polymer density profiles, bond order parameter, segmental and terminal dynamics show clearly that the size of the interface/interphase, depends on the actual property under study. In addition, the anchored polymeric chains change the behavior/properties, and especially the chain density profile and the dynamics, of the polymer chain at the vicinity of the Au NP.

The Longitudinal Superdiffusive Motion of Block Copolymer in a Tight Nanopore.

The structure and dynamic properties of polymer chains in a confined environment were studied by means of the Monte Carlo method. The studied chains were represented by coarse-grained models and embedded into a simple 3D cubic lattice. The chains stood for two-block linear copolymers of different energy of bead–bead interactions. Their behavior was studied in a nanotube formed by four impenetrable surfaces. The long-time unidirectional motion of the chain in the tight nanopore was found to be correlated with the orientation of both parts of the copolymer along the length of the nanopore. A possible mechanism of the anomalous diffusion was proposed on the basis of thermodynamics of the system, more precisely on the free energy barrier of the swapping of positions of both parts of the chain and the impulse of temporary forces induced by variation of the chain conformation. The mean bead and the mass center autocorrelation functions were examined. While the former function behaves classically, the latter indicates the period of time of superdiffusive motion similar to the ballistic motion with the autocorrelation function scaling with the exponent t5/3. A distribution of periods of time of chain diffusion between swapping events was found and discussed. The influence of the nanotube width and the chain length on the polymer diffusivity was studied.

Dynamics Transition of Polymer Films Induced by Polymer–Obstacle Entanglements on Rough Surfaces

We systematically investigate the effects of surface roughness on the static and dynamic properties of polymer chains in confined films by using molecular dynamics simulations, in which the roughne...

Structures and dynamics of polyethylene nanostructures with different free surface geometries: nanofilm, nanofiber and nanoparticle

Monte Carlo simulations of coarse-grained polyethylene (PE) models were used to compare structural and dynamic properties of polymer chains confined at the nanoscale with different free surface geometries i.e. nanofilm, nanofiber and nanoparticle. All polymer nanostructures, which contain 95 chains of C100H202 were generated by serial reduction of the periodic boundary conditions. For all these nanostructures, the density profiles are hyperbolic, with end bead segregation at the surface. Bulk densities, sizes (thickness/radius) and the interfacial widths are lower, larger and broader for nanofilm, nanofiber and nanoparticle, respectively. The local environment of bond orientation is isotropic in the middle of these nanostructures, but becomes anisotropic near the surfaces. Chain orientation at the surface is also observed and the degree of anisotropy at the whole chain is nanofilm > nanofiber > nanoparticle. Surface energies are calculated directly from the on-lattice energetics and their magnitudes are: nanofilm nanofiber > nanoparticle along the direction of periodic boundaries but this order is reversed for the diffusion along the direction normal to the free surfaces.

Theory for the Dynamics of Polymer Grafted Nanoparticle in Solution

We develop a theory for the dynamics of polymer grafted nanoparticle (PGNP) in dilute solution. A formalism of multiscale generalized Gaussian structure (mGGS) is developed where multiscale nature is described as variable size of monomers connected to each other through harmonic springs with different spring constants. This formalism allows to study various dynamic properties of polymer chains or networks grafted nanoparticles at arbitrary positions. We analyze three dynamic properties of PGNP, viz. (i) the mean displacement of a specified monomer of the polymer chain and its time evolution under external (unit step) forces, (ii) loss and storage moduli, and (iii) intrinsic viscosity. The study highlights the unfolding dynamics of a polymer chain with different sizes of nanoparticles and shows that the chain dynamics depends on the position of applied step force in PGNP. Similarly, the elastic moduli and the intrinsic viscosity reflects the viscoelastic properties of PGNP. The storage and the loss moduli ...

Heterogeneities in Polymer Structural and Dynamic Properties in Graphene and Graphene Oxide Nanocomposites: Molecular Dynamics Simulations

The effect of graphene (G) and graphene oxide (GO), used as the nanofiller in polymer nanocomposites (NC), on the structural and dynamic properties of polymer chains, has been studied by means of molecular dynamics (MD) simulations. Two polymers, i.e., poly(propylene) and poly(vinyl alcohol), are employed as matrices to cover a wider range of polymer–filler interactions. The local structural properties, e.g., density profile, average Rg, and end‐to‐end distance as well as dynamic properties, e.g., estimated translational and orientational relaxation times, of polymer chains are studied. In addition, the interaction energies are estimated between polymers and nanofillers for different hybrid systems using MD pullout simulations. Strong heterogeneities in polymer structural and dynamic properties have been observed such that chains are more oriented and exhibit slower dynamics in the vicinity of the nanofillers (G and GO) as compared to bulk. It is also found that the orientation of polymer chains at the interface is more influenced by the nanofiller in such a way that the more oriented polymer chains are observed in G‐based NC for both polymers. However, the immobilization of polymer chains at the interface proves to be very much dependent on the polymer–filler interactions. image

Hydrodynamic Interactions and Entanglements of Polymer Solutions in Many-Body Dissipative Particle Dynamics.

Using many-body dissipative particle dynamics (MDPD), polymer solutions with concentrations spanning dilute and semidilute regimes are modeled. The parameterization of MDPD interactions for systems with liquid–vapor coexistence is established by mapping to the mean-field Flory–Huggins theory. The characterization of static and dynamic properties of polymer chains is focused on the effects of hydrodynamic interactions and entanglements. The coil–globule transition of polymer chains in dilute solutions is probed by varying solvent quality and measuring the radius of gyration and end-to-end distance. Both static and dynamic scaling relations for polymer chains in poor, theta, and good solvents are in good agreement with the Zimm theory with hydrodynamic interactions considered. Semidilute solutions with polymer volume fractions up to 0.7 exhibit the screening of excluded volume interactions and subsequent shrinking of polymer coils. Furthermore, entanglements become dominant in the semidilute solutions, which inhibit diffusion and relaxation of chains. Quantitative analysis of topology violation confirms that entanglements are correctly captured in the MDPD simulations.

Edge-Functionalized Graphene as a Nanofiller: Molecular Dynamics Simulation Study

In the present simulation work we study graphene-based polymer nanocomposites composed of hydrogenated and carboxylated graphene sheets dispersed in polar and nonpolar short polymer matrices (i.e., matrices containing chains with low molecular weight). The aim of our work is to examine spatial and dynamical heterogeneities of such systems and to provide a compact picture about the effects of the edge group functionalization of graphene sheets on the properties of hybrid graphene-based materials. We perform atomistic molecular dynamics simulations of edge-functionalized graphene sheets embedded in poly(ethylene oxide) (polar matrix) and polyethylene (nonpolar matrix). We choose a low loading of the graphene nanofiller (from 1.7% to 3.6%) in agreement with experimental data. We further implement a detailed analysis of static and dynamic properties of polymer chains on the level of both the entire hybrid material and the polymer/graphene interface through a new approach that is able to distinguish between the adsorbed and edge region around the nanofiller. At the local scale, strong structural and dynamical heterogeneities are observed; i.e., the behavior of the polymer matrix appears to be highly affected by the presence of the edge-functionalized graphene. Slow dynamics was detected in the adsorbed layers in all nanocomposites. Additionaly, interaction of grafted carboxylated groups with polar matrix led to a further delay in the segmental as well as the chain relaxation close to the graphene edges. This effect seems to slow down the chain dynamics even more than the actual adsorption of the chain on the surface. Overall average orientational dynamics of polymer chains in nanocomposites as well as the collective dynamics quantified through the single chain coherent structure factor reveal slight deviations from the bulk behavior in the terminal region, presumably due to the small percentage of the slow dynamic component at the given loading of the nanofiller. Enhancement of the rheological properties is not observed within the time window of our simulations. Our results emphasize the importance of the surface/polymer interactions in the graphene-based nanocomposites and suggest that by a proper choice of edge-grafted groups we can achieve better material performance.

Multiscale Modeling of the Polymer–Silica Surface Interaction: From Atomistic to Mesoscopic Simulations

We propose here to investigate the interaction between cis-1,4 polybutadiene polymer chains and a silica surface. We opt for mesoscopic simulations in order to simulate the polymer chains onto significant length and time scales. We develop realistic coarse-grained (CG) models for this kind of interaction by using a bottom-up approach consisting of developing these effective CG models from atomistic simulations. We investigate then the structural and dynamical properties of polymer chains adsorbing onto a silica surface at different separation distances. We complete this work by studying the adsorption of free polymer chains onto the grafted silica surface as a function of the grafting density. The conformational and the dynamical properties of the free and grafted chains are then determined with respect to the surface coverage.

Equilibrium and dynamical properties of polymer chains in random medium filled with randomly distributed nano-sized fillers.

The effect of randomly distributed nano-sized fillers on the equilibrium and dynamical properties of linear polymers is studied by using off-lattice Monte Carlo simulation. Lennard-Jones interactions between polymers and fillers are considered. Results show that the statistical dimensions and dynamical diffusion of polymer are dependent on the polymer-filler interaction strength εpf. The mean square radius of gyration 〈RG(2)〉 shows a minimum at a critical polymer-filler interaction εpf*. The value of εpf* decreases with the increase in the polymer length or the concentration of fillers. The exponent ν in 〈RG(2)〉 ∼ N(2ν) is a typical value of self-avoiding walking chain at small εpf but it increases sharply to a bigger value at εpf > εpf*. The mean square displacement decreases with the increase in εpf. Moreover, the normal diffusion of the polymer at weak interactions changes to subnormal diffusion at moderate and strong attractions. We find that polymers diffuse in dilute filler regions at weak attraction and diffuse in dense filler regions at strong attraction.

Influence of surface concentration on poly(vinyl alcohol) behavior at the water-vacuum interface: a molecular dynamics simulation study.

Poly(vinyl alcohol) (PVA) is an amphiphilic macromolecule with surfactant activity. The peculiar behavior of this polymer at the water-air interface is at the basis of its use as material for hydrated microdevices, films, and nanofibers. This work aims to investigate the behavior of PVA and water within the surface domain of highly diluted aqueous solutions by means of atomistic molecular dynamics simulations. Monodisperse atactic oligomers of 30 residues were distributed within water slabs in a vacuum box and allowed to diffuse toward the surface. After equilibration, structural features and dynamical properties of polymer chains and water in the interfacial domains were analyzed as a function of PVA surface concentration at 293 K. Surface pressure values obtained from simulations are in agreement with experimental values at corresponding polymer specific surface areas. In the explored concentration range of 6-34 μmol of residues/m(2), the chains display a transition between two states. At lower surface concentrations, elongated, quite rigid structures are adsorbed on the surface, whereas partially submerged globular aggregates, locally covered by thin water layers, are formed at higher surface concentrations. At PVA concentrations higher than about 20 μmol of residues/m(2), the percolation of chain aggregates over the interface plane produces a surface-confined polymer network with stable pores filled by water molecules. A substantial slowing of polymer and water dynamics in the interfacial domains is highlighted by the mean squared displacement time behavior of terminal residues and the interaction time of PVA-water hydrogen bonding. The diffusion coefficient of water and lifetime of hydrogen bonds between solvent molecules are halved and doubled, respectively, at the interface with the highest polymer concentration. The attenuation of water and polymer mobility concur to stabilize PVA hydrated networks in contact with air.

Determining elasticity from single polymer dynamics

The ability to determine polymer elasticity and force-extension relations from polymer dynamics in flow has been challenging, mainly due to difficulties in relating equilibrium properties such as free energy to far-from-equilibrium processes. In this work, we determine polymer elasticity from the dynamic properties of polymer chains in fluid flow using recent advances in statistical mechanics. In this way, we obtain the force-extension relation for DNA from single molecule measurements of polymer dynamics in flow without the need for optical tweezers or bead tethers. We further employ simulations to demonstrate the practicality and applicability of this approach to the dynamics of complex fluids. We investigate the effects of flow type on this analysis method, and we develop scaling laws to relate the work relation to bulk polymer viscometric functions. Taken together, our results show that nonequilibrium work relations can play a key role in the analysis of soft material dynamics.

Solvent size effect on the static and dynamic properties of polymer chains in athermal solvents

The static and dynamic properties of polymer chains in athermal solvents with different sizes are studied by molecular dynamics method. With increasing solvent size, the radius of gyration and the diffusion coefficient of the polymer decay fast until a critical solvent size is reached. For the polymer diffusion coefficients, this decay only depends on the solvent size; while for the radius of gyration of polymers, this decay depends on both solvent size and the length of the polymers. The increase of solvent size also makes the polymer tend to be thicker ellipsoid until a critical solvent size is reached. The static scaling exponent of the polymer also shows the solvent size dependence. Moreover, four regions are identified where the polymers show different dynamic behaviors according to the dynamic structure factors of the polymer.

Grafted Polymer Chains Interacting with Substrates: Computer Simulations and Scaling

AbstractWe review scaling methods and computer simulations used in the study of the static and dynamic properties of polymer chains tethered to adsorbing surfaces under good solvent conditions. By varying both the grafting density and the monomer/surface interactions a variety of phases can form. In particular, for attractive interactions between the chains and the surface the classical mushroom‐brush transition known for repulsive substrates splits up into an overlap transition and a saturation transition which enclose a region of semidilute surface states. At high grafting densities oversaturation effects and a transition to a brush state can occur. We emphasize the role of the critical adsorption parameters for a correct description and understanding of such polymer adsorption phenomena.magnified image

Properties of polymer chains in confinement. Monte Carlo simulations

The main goal of this study was to investigate the dynamic properties of polymer chains located in a slit. The idealized models of polymers were built of united atoms (segments) restricted to vertices of a simple cubic lattice. The macromolecules were linear, cyclic and star-branched with f=3 and f=4 arms. The chains were at good solvent conditions and the only interaction between the segments was the excluded volume. The polymers were located in a slit formed by a pair of two parallel and impenetrable surfaces which were attractive for polymer segments. The properties of the model chains were determined by means of dynamic Monte Carlo method using a sampling algorithm based on chain’s local changes of conformation. It was shown, for all polymer architectures under consideration, that at a certain temperature and for a certain size of the slit the chains were adsorbed at one of the confining surfaces and after a certain period of time they detached from this surface and approached the opposite one. The conditions necessary for these jumps were determined. The influence of the internal chain architecture on the frequency of jumps was shown and discussed.

Influence of molecular topology on the static and dynamic properties of single polymer chain in solution

The influence of molecular topology on the structural and dynamic properties of polymer chain in solution with ring structure, three-arm branched structure, and linear structure are studied by molecular dynamics simulation. At the same degree of polymerization (N), the ring-shaped chain possesses the smallest size and largest diffusion coefficient. With increasing N, the difference of the radii of gyration between the three types of polymer chains increases, whereas the difference of the diffusion coefficients among them decreases. However, the influence of the molecular topology on the static and the dynamic scaling exponents is small. The static scaling exponents decrease slightly, and the dynamic scaling exponents increase slightly, when the topology of the polymer chain is changed from linear to ring-shaped or three-arm branched architecture. The dynamics of these three types of polymer chain in solution is Zimm-like according to the dynamic scaling exponents and the dynamic structure factors.

Hybrid method coupling fluctuating hydrodynamics and molecular dynamics for the simulation of macromolecules

We present a hybrid computational method for simulating the dynamics of macromolecules in solution which couples a mesoscale solver for the fluctuating hydrodynamics (FH) equations with molecular dynamics to describe the macromolecule. The two models interact through a dissipative Stokesian term first introduced by Ahlrichs and Dunweg [J. Chem. Phys. 111, 8225 (1999)]. We show that our method correctly captures the static and dynamical properties of polymer chains as predicted by the Zimm model. In particular, we show that the static conformations are best described when the ratio sigma/b=0.6, where sigma is the Lennard-Jones length parameter and b is the monomer bond length. We also find that the decay of the Rouse modes' autocorrelation function is better described with an analytical correction suggested by Ahlrichs and Dunweg. Our FH solver permits us to treat the fluid equation of state and transport parameters as direct simulation parameters. The expected independence of the chain dynamics on various choices of fluid equation of state and bulk viscosity is recovered, while excellent agreement is found for the temperature and shear viscosity dependence of center of mass diffusion between simulation results and predictions of the Zimm model. We find that Zimm model approximations start to fail when the Schmidt number Sc < or approximately 30. Finally, we investigate the importance of fluid fluctuations and show that using the preaveraged approximation for the hydrodynamic tensor leads to around 3% error in the diffusion coefficient for a polymer chain when the fluid discretization size is greater than 50 A.

Molecular dynamics study of the effect of solvent types on the dynamic properties of polymer chains in solution

The effect of types of solvent on the dynamic properties of atactic poly(methyl methacrylate)s (a-PMMA)s chains in solution was studied by a molecular dynamics simulation technique with the condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field. Five kinds of casting solvent molecules were randomly embedded into the a-PMMA matrix to construct various types of simulated a-PMMA solutions based upon experimentally obtained densities. The dynamic properties of polymer chains such as mean-square end-to-end distance < R 2 >, mean-square radius of gyration < S 2 >, and asphericity ratio < R 2 >/< S 2 > of polymer chains were analyzed. Simulated results revealed that the < R 2 > of the a-PMMA chains increased with the increase of the dipole moment of the surrounded solvent molecules. The asphericity ratio of polymer chains showed a high correlation to the molecular weight of solvents. Furthermore, the simulated radial distribution function of H-H atomic pairs in the system of a-PMMA solutions indicated that the expansion of a polymer chain depends mainly on the conformation of solvent molecules instead of the interaction between the polymer chain and the functional group on solvent molecules.

Polymer Melt near a Solid Surface: A Molecular Dynamics Study of Chain Conformations and Desorption Dynamics

We examine the static and dynamic properties of polymer chains in a melt in the presence of a solid surface. A molecular dynamics simulation is carried out using a coarse-grained bead−spring model of polymer chains. Both attractive and repulsive interactions between surface and monomers are examined. Static conformations are characterized in terms of trains of attached monomers and detached loops and tails. We compare these conformations to a random walk model. Chain desorption rates are measured in order to calculate characteristic desorption times. We distinguish between chains which desorb rapidly after arriving at the surface and chains which reach a relaxed state at the surface and then desorb at a constant rate. A kinetic model is developed as a means to predict desorption rates and late time chain conformations.