A multiscale simulation model for poly(ethylene oxide)

An overview of the recent development of a multiscale simulation of amorphous polymeric materials at the bulk density is presented. Poly(ethylene oxide), (PEO), (CH₃O-[CH₂-CH₂-O]nCH₃) was selected to illustrate the method. The model starts from an ab initio quantum chemistry to obtain the statistical weights of polymer conformation based on the rotational isomeric state (RIS) theory. PEO chains were then mapped to a coarse-grained model using the modified RIS model onto the second nearest neighbor diamond (2nnd) lattice. The average non-bonded interactions were treated by the discretized Lennard-Jones (LJ) potential. Bulk PEO melts with molecular weight up to 8000 g/mol was generated and equilibrated. The on-lattice properties such as molecular size and conformational statistics agree well with the theory. Fully atomistic amorphous PEO models can be obtained by the reverse-mapping procedure to recover the missing atoms. After an energy minimization step, properties including torsional angle distribution, solubility parameter and static neutron scattering structure factor are in good agreement with experimental results.     

Monte Carlo simulation study of the effect of chain tacticity on demixing of polyethylene/polypropylene blends

The molecular origin of the demixing behavior for 50: 50 (wt/wt) polyethylene/polypropylene (PE/PP) with different tacticity of PP at the melts (473 K) was investigated by Monte Carlo simulation of coarse-grained polymer model. Isotactic (iPP), atactic (aPP) and syndiotactic (sPP) polypropylenes were used for blending with PE. Coarse-graining polymer chains were represented by 50 beads, corresponding to C₁₀₀H₂₀₂ and C₁₅₀H₃₀₂ for PE and PP, respectively. The simulation was performed on a high coordination lattice incorporating short-range intramolecular interactions from the Rotational Isomeric State (RIS) model and long-range intermolecular interactions Lennard-Jones (LJ) potential function of ethane and propane units. Chain dimensions, the characteristic ratio (C _n ) and self-diffusion coefficient (D) of PE in the blends are sensitive to the stereochemistry of PP chains. Compared with neat PE melts, PE dimension was relatively unchanged in PE/iPP and PE/aPP blends but slightly decreased in PE/sPP blends. PP dimension was increased in PE/iPP and PE/aPP mixture but decreased in PE/sPP blend in comparison with neat PP melts. In addition, diffusion of PE and PP chains in PE/PP mixture was decreased and increased, respectively, compared to the pure melts. Interchain pair correlation functions were used to detect the immiscibility of the blends. The tendency of demixing of PE/aPP and PE/iPP blends were weaker than that of PE/sPP blend.     

Topological effects on static and dynamic properties in an amorphous nanofiber composed of cyclic polymers

The static and dynamic properties of an amorphous polymer nanofiber are investigated via Monte Carlo simulation. Each nanofiber is composed of rings represented by 50 beads. The results are compared with recent simulation, by a similar method, of a nanofiber composed of linear chains with the same number of beads. The coarse‐grained model is one that permits reverse‐mapping of the macromolecules to C₁₀₀H₂₀₂ or C₁₀₀H₂₀₀. The radial density profiles can be fitted to a hyperbolic tangent function. The orientation of the chains is similar in the two fibers. The cyclic chains move slower than the linear chains, both on the scale of individual beads and on the scale of entire molecules. Both fibers experience an increase in mobility in the surface region, with this increase being larger in the fibers composed of linear chains.     

Dynamics of polypropylene chains in their binary blends of different stereochemical sequences studied by Monte Carlo simulations

The conformational and dynamic properties of polypropylene （PP） for both pure melts and blends with different chain tacticity were investigated by Monte Carlo simulation of isotactic （iPP）, atactic （aPP） and syndiotactic （sPP） polypropylenes. The simulation of coarse-grained PP models was performed on a high coordination lattice incorporating short- and long-range intramolecular interactions from the rotational isomeric state （RIS） model and Lennard-Jones （LJ） potential function of propane pairs, respectively. The dynamics of chains in binary PP/PP mixture were investigated with the composition of C150H302 with different chain taciticity. The diffusion rates of PP with different stereochemistry are generally in the order as： iPP 〉 aPP 〉〉 sPP. For PP/PP blends with 50：50 wt% binary mixtures, immiscibility was observed when sPP was introduced into the mixtures. The diffusion rate of iPP and aPP became slower after mixing, while sPP diffuses significantly faster in the binary mixtures. The mobility of PP chains depends on both intramolecular （molecular size and chain stiffness） and intermolecular （chain packing） interactions. The effect of intramolecular contribution is greater than that of intermolecular contribution for iPP and aPP chains in binary mixtures. For sPP chain, intermolecular interaction has greater influence on the dynamics than intramolecular contribution.     

Monte Carlo simulation of the structure of mono- and bidisperse polyethylene nanocomposites

The structure of bidisperse polyethylene(PE) nanocomposite mixtures of 50:50(by mole) of long and short chains of C160H322/C80H162 and C160H322/C40H82 filled with spherical nanoparticles were investigated by a coarse-grained, on lattice Monte Carlo method using rotational isomeric state theory for short-range and Lennard-Jones for long-range energetic interactions. Simulations were performed to evaluate the effect of wall-to-wall distance between fillers(D), polymer-filler interaction(w) and polydispersity(number of short chains in the mixture) on the behavior of the long PE chains. The results indicate that long chain conformation statistics remain Gaussian regardless of the effects of confinement, interaction strength and polydispersity. The various long PE subchain structures(bridges, dangling ends, trains, and loops) are influenced strongly by confinement whereas monomer-filler interaction and polydispersity did not have any impact. In addition, the average number of subchain segments per filler in bidisperse PE nanocomposites decreased by about 50% compared to the nanocomposite system with monodisperse PE chains. The presence of short PE chains in the polymer matrix leads to a reduction of the repeat unit density of long PE chains at the interface suggesting that the interface is preferentially populated by short chains.     

Similarities and Differences in the Rapid Crystallization Induced in n‐Tetracontane by an Instantaneous Deep Quench of the Free‐Standing Nanofiber and Free‐Standing Thin Film

The crystallization of a confined short polyethylene chain, n‐tetracontane, quenched from the melt has been investigated by a dynamic Monte Carlo method on a high coordination lattice. The periodic boundary conditions force the formation of a free‐standing nanofiber exposed to a vacuum. After the deep quench, the nanofiber adopts a configuration dominated by extended chains aligned parallel to the fiber axis. The vicinity of the fiber axis is less dense, and less well ordered, than portions of the fiber located further from the fiber axis. Annealing finds that this low‐density region inside the fiber is not as easily removed as is the grain boundary that usually develops inside a free‐standing thin film upon rapid crystallization.     

Structure Formation in the Crystallization and Annealing of Tetracontane Nanoparticles

Summary: Crystallization, melting and annealing of nanoparticles of tetracontane were simulated via a Monte Carlo method on the second nearest neighbor diamond (2nnd) lattice by including short‐ and long‐range interactions. Nanoparticles can be obtained from an equilibrated tetracontane melt by increasing three periodic lengths to values that are effectively infinite. Nanoparticles, which contain 155 chains of C₄₀H₈₂, have been produced. After a deep quench from 473 K to 298 K, the crystallization process was investigated by the evolution of the density profile, fraction of bonds in the trans state, and the orientational order parameter. The vicinity of the center is less dense and less well ordered than portions of the nanoparticle located further from the center. The crystals form first in the region close to the surface. Each nanoparticle usually contains multiple crystalline domains. A melting phenomenon was observed at a temperature about 365 K when the nanoparticle crystal was heated. Annealing of the multiple domain crystal at 360 K can transform the structure to a more regular one without a grain boundary.

Snapshot of the final structure containing a single domain crystal after 20 million MCS.     

Molecular modeling simulation and experimental measurements to characterize chitosan and poly(vinyl pyrrolidone) blend interactions

Blends of chitosan and poly(vinyl pyrrolidone) (PVP) have a high potential for use in various biomedical applications and in advanced drug‐delivery systems. Recently, the physical and chemical properties of these blends have been extensively characterized. However, the molecular interaction between these two polymers is not fully understood. In this study, the intermolecular interaction between chitosan and PVP was experimentally investigated using ¹³C cross‐polarization magic angle‐spinning nuclear magnetic resonance (¹³C CP/MAS NMR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). According to these experimental results, the interaction between the polymers takes place through the carbonyl group of PVP and either the OH? C₆, OH? C₃, or NH? C₂ of chitosan. In an attempt to identify the interacting groups of these polymers, molecular modeling simulation was performed. Molecular simulation was able to clarify that the hydrogen atom of OH? C₆ of chitosan was the most favorable site to form hydrogen bonding with the oxygen atom of C?O of PVP, followed by that of OH? C₃, whereas that of NH? C₂ was the weakest proton donor group. The nitrogen atom of PVP was not involved in the intermolecular interaction between these polymers. Furthermore, the interactions between these polymers are higher when PVP concentrations are lower, and interactions decrease with increasing amounts of PVP. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1258–1264, 2008     

Monte Carlo Simulation of the Structures and Dynamics of Amorphous Polyethylene Nanoparticles

Monte Carlo simulations of amorphous polyethylene (PE) nanoparticle have been performed on the second nearest neighbor diamond lattice by including short and long‐range interactions. A droplet can be either obtained from equilibrated PE bulk or from nanofiber snapshots by modifying periodic boundary conditions. The presence of attractive long‐range interactions gives cohesion to the nanoparticle. PE nanoparticles, which contain up to 72 chains of C₁₀₀ and have the radius ˜5.0 nm, have been produced and equilibrated on the 2nnd lattice. In these droplets, the density profiles are hyperbolic, with end beads being more abundant than middle beads at the surface. There are orientational preferences at the surface on the scale of individual bonds and whole chains. Comparison of nanoparticles with different sizes, which contain different numbers of chains, does not indicate any significant differences in local and global equilibrium properties – for thickness in the range 5.8 to 7.4 nm. Surface energies are calculated directly from the on‐lattice energetics. The mobility of the chain in the droplet at the level of individual chords or an entire chain is greater than in the bulk, and the mobility increases as the size of the droplet decreases.