Short chain branched (SCB) polyolefins as a model of metallocene ethylene/R-olefin copolymers were simulated by Monte Carlo (MC) and molecular dynamics (MD) methods. Melt density, which was evaluated by MD in the isothermal-isobaric ensemble (NPT-MD), slightly increases with the SCB content. A mix of different MC moves was adopted and connectivity-altering moves, such as end-bridging, were modified in order to incorporate the branches into the simulation. This MC simulation strategy performed very well in equilibrating molten SCB copolymers at all length scales. The chain size and local packing in the melt, as obtained from the MC simulations, are discussed. At given backbone length, chain size, as quantified by the radius of gyration, decreases with the number of branches. On the other hand, the presence of short branches leads to a less effective intermolecular local packing in the melt. Rheological properties of the copolymers are discussed based on a mapping of the Monte Carlo atomistic simulations on the packing length model, and compared with experimental results. In general, good agreement with experimental results is found.

No abstract

The microscopic structure, thermodynamic properties, local segmental dynamics, and self-diffusion coefficients of three ionic liquids (ILs) with a common anion, namely, the bis(trifluoromethylsulfonyl) imide ([Tf2N-]), and imidazolium-based cations that differ in the alkyl tail length, namely, the 1-butyl-3-methylimidazolium ([C4mim+]), the 1-hexyl-3-methylimidazolium ([C6mim+]), and the 1-octyl-3-methylimidazolium ([C8mim+]), are calculated over the temperature range of 298.15-333.15 K and pressure range of 0.1-60 MPa. Quantum calculations based on density functional theory are performed on isolated ion pairs, and minimum energy conformers are identified. Electronic density results are used to estimate the electrostatic potential of a molecular force field that is used subsequently for long molecular dynamics (MD) simulations of bulk ILs. Thermodynamic properties calculated from MD are shown to be in excellent agreement for the bulk density and good agreement for derivative properties when compared to experimental data. The new force field is an improvement over earlier ones for the same ILs. The microscopic structure as expressed through the radial distribution function is thoroughly calculated, and it is shown that the bulk structure characteristics are very similar to those obtained from the quantum calculations on isolated ion pairs. The segmental dynamics expressed in terms of bond and torsion angle decorrelation is shown to assume a broad range of characteristic times. Molecular segments in the alkyl tail of the cations are significantly faster than segments in the vicinity of the imidazolium ring. Finally, the new force field predicts accurately the self-diffusion coefficients of the cations and the anions over the entire temperature range examined, thus confirming its validity for a broad range of physical properties.

Long molecular dynamics simulations of the melt dynamics, glass transition and nonisothermal crystallization of a C 192 polyethylene model have been carried out. In this model, the molecules are sufficiently long to form entanglements in the melt and folds in the crystalline state. On the other hand, the molecules are short enough to enable the use of atomistic simulations on a large scale of time. Two force fields, widely used for polyethylene, are taken into account comparing the simulation results with a broad set of literature experimental data. Although both force fields are able to capture the general physics of the system, TraPPe-UA is in a better quantitative agreement with the experimental data. According with the simulation results some fundamental aspects of polyethylene physical parameters are discussed such as the characteristic ratio (C n = 8.2 and 7.6 at 500 K, for TraPPe-UA and PYS force fields, respectively), the isothermal compressibility (α = 8.57 × 10 −4 K −1 ), the static structure factor and the melt dynamics regimes corresponding to an entangled polymer. Furthermore, the simulated T g (187.0 K) obtained for linear PE is in a very good agreement with the extrapolated T g values (185−195 K) using the Gordon−Taylor equation. Finally, the simulation of the nonisothermal crystallization process supports the view of a mixed state of adjacent and nonadjacent re-entry model. The simulated two phase model reproduces very well the initial fold length expected for high supercoolings and the segregation of the system in ordered and disordered layers. The paper highlights the importance of combining simulation techniques with experimental data as a powerful means to explain the polymer physics.

In the present study, the thermophysical properties of the tetracyanoborate-based ionic liquids (ILs) 1-ethyl-3-methylimidazolium tetracyanoborate ([EMIM][B(CN)4]) and 1-hexyl-3-methylimidazolium tetracyanoborate ([HMIM][B(CN)4]) obtained by both experimental methods and molecular dynamics (MD) simulations are presented. Conventional experimental techniques were applied for the determination of refractive index, density, interfacial tension, and self-diffusion coefficients for [HMIM][B(CN)4] at atmospheric pressure in the temperature range from 283.15 to 363.15 K. In addition, surface light scattering (SLS) experiments provided accurate viscosity and interfacial tension data. As no complete molecular parametrization was available for the MD simulations of [HMIM][B(CN)4], our recently developed united-atom force field for [EMIM][B(CN)4] was partially transferred to the homologous IL [HMIM][B(CN)4]. Deviations between our simulated and experimental data for the equilibrium properties are less than ±0.3% in the case of density and less than ±8% in the case of interfacial tension for both ILs. Furthermore, the calculated and measured data for the transport properties viscosity and self-diffusion coefficient are in good agreement, with deviations of less than ±30% over the whole temperature range. In addition to a comparison with the literature, the influence of varying cation chain length on thermophysical properties of [EMIM][B(CN)4] and [HMIM][B(CN)4] is discussed.

A combined computer simulation and experimental study describing the viscoelastic properties of linear polyethylene is presented. For the simulation, a set of C1000 polyethylene models were equilibrated using advanced Monte Carlo moves. Then, MD trajectories were calculated. From these simulations the entanglement molecular weight, M e, and the entanglement relaxation time, τe, were directly obtained. By introducing the experimental value for the plateau modulus and the simulated values for M e and τe into the reptation model, one finds that the derived curves of the relaxation shear modulus nicely coincide with the experimental ones.

This feature article reviews several aspects of computational approaches to polyethylene melt and solid state properties in relation to existing experimental results. Based on 40 years of experience in the field, we offer a personal view of how computer simulations are helping to understand the physics of polyethylene as a model polymer. The first issue discussed is the molten state of polyethylene, including static and dynamic properties and entanglement features along with their impacts on rheological behaviour. We then examine the glass transition, crystallization process and solid state structure, including the interlamellar region. This is followed by brief descriptions of the latest advances in simulating mechanical properties and of the various methodologies used to simulate the physics of polyethylene. Throughout the manuscript, references are made to our own work and also to studies by many other authors that have nicely contributed to developments in simulating the physics of polyethylene in close agreement with experimental results.

2-Iminopyridine N-oxides (PymNox) constitute a promising class of ligands that can be considered as the neutral counterparts of the well-known salicylaldiminate system. A series of nickel PymNox complexes displaying different substitution patterns in the ligand have been synthesized, and their activity as ethylene polymerization catalysts has been studied. While an electron-donor group (OMe) at the remote position 4 of the pyridine ring causes a moderate decrease of the catalytic activity and increase of the polyethylene molecular weight, a strongly electron-withdrawing group (NO2) in this position shifts the catalyst selectivity from ethylene polymerization to oligomerization. The introduction of a phenyl substituent next to the pyridine nitrogen (position 6) causes a significant increase of the catalytic activity and the polymer molecular weight. Although aldimino PymNox catalysts are inactive in ethylene−methyl acrylate copolymerization, we observed that acetaldimino (displaying the Me-CNAr group) catalysts display a small but significant activity on this account, giving rise to copolymers incorporating ca. 1% methyl acrylate in their structure. The trends observed in the PymNox catalytic system are strongly reminiscent of those of nickel salicylaldiminates (although the former are considerably more active), demonstrating for the first time that substitution of the widely used phenoxo anionic group by the neutral pyridine N-oxide fragment is a useful possibility in the design of late transition metal catalysts for olefin polymerization.