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Fluid transport through a nanopore in a membrane was investigated by using a novel molecular dynamics approach proposed in this study. The advantages of this method, relative to dual-control-volume grand-canonical molecular dynamics method, are that it eliminates disruptions to the system dynamics that are normally created by inserting or deleting particles from control volumes, and that it functions well for dense systems due to the number of particles being fixed in the system. Using the proposed method, we examined liquid argon transport through a nanopore by performing nonequilibrium molecular dynamics (NEMD) simulations under different back pressures. Validation of the code was performed by comparing simulation results to published experimental data obtained under equilibrium conditions. NEMD results show that constant pressure difference across the membrane was readily achieved. [ABSTRACT FROM AUTHOR]

Abstract: We used molecular dynamics simulation together with the OPLS-AA force field along with a high temperature equilibration protocol proposed by Belmares et al. to calculate selected thermodynamic properties of poly(vinyl alcohol) (PVA) with different tacticities. The results showed that the OPLS-AA force field was able to reproduce specific volumes, thermal expansion coefficients, glass transition temperatures and solubility parameters of the PVAs over a wide range of temperatures (200–550 K). PVA with different tacticities in the amorphous phase showed different solubility parameters but possessed similar specific volumes, thermal expansion coefficients and glass transition temperatures. For heat capacities, 300% overestimations were obtained. Such overestimations were reduced significantly to about 30% by applying the quantum correction method of Berens et al. We have also applied the newly developed two-phase thermodynamic (2 PT) approach in an attempt to further improve the results but failed. [Copyright &y& Elsevier]

Abstract: We performed a systematic study to show that the nicely folded lamellar structure of polyethylene chains derived from previous molecular simulations is a consequence of the use of the united atom model, which treats the CH2 groups as a hard sphere. Simulations presented here with explicit hydrogen atoms show that the fold is not uniform, and that regularly aligned stems do not result. While it is known that polyethylene crystallizes with chain folded morphology, the irregular folding seen here is due to the neglect of the interactions between the hydrogen atoms attached to a carbon atom Ci and the atom Ci +2 in the united atom model. We present this paper to point out how the choice of force fields is important for polymers with a high degree of crystallinity. We hope that this paper stimulates further work on developing united atom models that take into account terms that we show to dictate the polymer conformations, such as the interactions between atoms attached to carbon atom Ci and the atom Ci +2 in polyethylene. [Copyright &y& Elsevier]

Molecular dynamics simulations were carried out on single chain models of linear low-density polyethylene in vacuum to study the effects of branch length, branch content, and branch distribution on the polymer’s crystalline structure at 300 K. The trans/gauche (t/g) ratios of the backbones of the modeled molecules were calculated and utilized to characterize their degree of crystallinity. The results show that the t/g ratio decreases with increasing branch content regardless of branch length and branch distribution, indicating that branch content is the key molecular parameter that controls the degree of crystallinity. Although t/g ratios of the models with the same branch content vary, they are of secondary importance. However, our data suggests that branch distribution (regular or random) has a significant effect on the degree of crystallinity for models containing 10 hexyl branches/1,000 backbone carbons. The fractions of branches that resided in the equilibrium crystalline structures of the models were also calculated. On average, 9.8% and 2.5% of the branches were found in the crystallites of the molecules with ethyl and hexyl branches while C[sup 13] NMR experiments showed that the respective probabilities of branch inclusion for ethyl and hexyl branches are 10% and 6% [Hosoda et al., Polymer 1990, 31, 1999–2005]. However, the degree of branch inclusion seems to be insensitive to the branch content and branch distribution. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: Molecular dynamics (MD) simulation was used to investigate the solubility of two hydrophobic drugs Cucurbitacin B (CuB) and Cucurbitacin I (CuI) in poly(ethylene oxide)-b-poly(α-benzyl carboxylate ɛ-caprolactone) (PEO-b-PBCL) block copolymers with different tacticities. In particular, di-block copolymer with three different tacticities viz. PEO-b-iPBCL, PEO-b-sPBCL, and PEO-b-aPBCL were used. The solubility was quantified by calculating the corresponding Flory–Huggins interaction parameters (χ) using random binary mixture models with 10wt% of drug. The tacticity of the di-block copolymer was found to influence significantly the solubility of two drugs in it. In particular, based on MD simulation results, only PEO-b-sPBCL exhibited solubility while the other two did not. Given the fact that the drugs were shown to be soluble in PEO-b-PBCL experimentally, it is predicted that the tacticity of the di-block copolymer synthesized in experiment is syndiotactic. This predication matches well with the dominant ring opening polymerization of cyclic lactones to syndiotactic polymers by stannous octoate as catalyst used to prepare PEO-b-PBCL block copolymers in our previous experiments. The simulation results showed that the solubility of the drugs in PEO-b-sPBCL is attributed to the favorable intra-molecular interaction of the di-block copolymer and favorable intermolecular interaction between the di-block copolymer and the drugs. Radial distribution function analysis provides useful insights into the nature and type of the intermolecular interactions. [Copyright &y& Elsevier]

Stability of water/toluene interfaces saturated with adsorbed naphthenic acids with various sizes at room temperature was studied by molecular dynamics simulation. In direct comparison with our recent results on similar systems involving n-heptane as the oil phase, it has been found that toluene significantly enhances the stability of the water/oil interface, as suggested by the considerably reduced diffusion coefficients of the interface-bound naphthenic acids. The oil phase dependence may be rationalized in terms of the oil–naphthenic acid interactions, both electronically and sterically, and the intermolecular interactions between the molecules in the oil phase. Consistent with our previous studies, mobility of naphthenic acid has been found to be dependent on its size, with naphthenic acid featuring a longer carbon chain in between the carboxylic and cycloaliphatic groups showing lower mobility, leading to a more stable interface. However, such size dependence is much more pronounced for the water/toluene than the water/n-heptane interface. This may be understood in terms of the structural influence of toluene on the compactness of the liquid crystalline mesophase formed by naphthenic acids adsorbed at the interface. [Copyright &y& Elsevier]