Molecular dynamics simulation is used to model the structure and thermodynamic properties of a novel rubbery polymer with promising membrane properties for hydrocarbon separation. A realistic united atom force field is developed based on extensive density functional theory quantum mechanics calculations for a model dimer and volumetric data at various temperatures and pressures. Both a constant bond length and a flexible bond length model are examined. Well-equilibrated structures of the polymer melt at various conditions are used to evaluate numerous thermodynamic properties, such as the isothermal compressibility, thermal expansion coefficient, and cohesive energy density, and structural properties, including intra-and intermolecular distribution functions and the static structure factor. The microscopic structure of the free volume of the polymer matrix and its evolution with time affects the diffusion of penetrant molecules considerably; they are calculated accurately using the Greenfield and Theodorou geometric analysis. The solubilities of various n-alkanes from methane to n-hexane at 300 and 400 K are calculated using the Widom test particle insertion technique. In all cases, simulation results are in good agreement with literature experimental data for the volumetric properties of the polymer melt and the solubility coefficients of n-alkanes in the polymer. In a forthcoming publication, the transport properties of these systems and the underlying molecular mechanisms will be examined.
A previously developed model of solute release from a swellable polymeric matrix induced by solvent uptake that obeys Fickian kinetics is here extended to cover the case of non‐Fickian solvent uptake caused by slow structural relaxation of the swelling polymer. For this purpose, we have adopted a model description of the absorption process (also previously developed in our laboratory), which has been shown to be capable of realistically simulating a wide range of non‐Fickian kinetics (including the well‐known two‐stage sorption and case II regimes). The aforementioned structural relaxation phenomena constitute important characteristics of glassy polymeric matrices. Accordingly, the predictions of the resulting combined model have been investigated in some detail in this series of articles. This particular article is concerned with the simplest version of the model, involving constant uptake kinetic parameters (diffusion coefficient and relaxation frequency) for both osmotically inactive and osmotically active solutes. Emphasis is put on possibilities of achieving and sustaining nearly constant release rates, and it is shown that such possibilities are considerably more numerous here than under conditions of Fickian solvent uptake kinetics. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1171–1188, 2002
Molecular dynamics trajectories of oxygen-like spherical molecules within a medium of eicosane-like chains at 303.5 K confined to a density very close to that of amorphous polyethylene are analyzed geometrically to elucidate mechanistic aspects of penetrant motion. To enable this analysis, reduced trajectories, i.e., sets of equidistant positions along the paths of penetrant molecules, are constructed with various step lengths λ. The degree and prevailing direction of local orientation in the chain liquid are quantified through diagonalization of an orientation tensor defined in the vicinity of individual nodes or steps of the reduced trajectories. As in earlier simulation work, an anomalous (subdiffusive) regime is detected, which extends up to times of ∼150 ps or penetrant displacements of ∼1.5 nm. Directional correlations between steps of the reduced trajectory reveal that the penetrant is temporarily confined within elongated “cavities”. The prevalent direction of chain orientation “seen” by the penetrant is found to persist over length scales of ∼2 nm. The degree of local anisotropy is lower along the penetrant trajectories than in the chain melt on average, because dense, strongly oriented regions can only form in the absence of penetrant. A weak but significant directional correlation is detected between penetrant displacements and the prevalent orientation of the surrounding polymer; the range of this correlation indicates that it is intimately related to anomalous diffusion. Over length scales of up to ∼1.5 nm, penetrant displacement is most facile along the backbones of chains. This is reminiscent of the idealized mechanistic model invoked in the Brandt and Pace and Datyner theories, although the degree of orientation is, of course, not nearly as large as suggested by that model.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.