Motion and sorption of gas molecules in poly(octadecyl acrylate) (original) (raw)
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Journal of Computational Chemistry, 2002
A theoretical strategy has been developed to study the motion of small molecules through ordered polymeric systems. The strategy, which has been incorporated into a computer program denoted MCDP/2, is especially useful to study comblike polymers organized in biphasic arrangements. This is because it is based on a configurational bias Monte Carlo algorithm, which is more efficient than conventional methods to study dense systems. The MCDP/2 program has been used to investigate the influence of CH 4 and CO 2 gas molecules in the structure of isotactic poly(octadecyl acrylate), a typical comblike polymer. For this purpose, the pure polymer and different molecular systems constituted by several gas molecules dissolved in the polymer matrix have been simulated. Results indicated that the structural relaxation of the polymer is not coupled to the motion of gas molecules. The importance of these results in the field of molecular modeling of transport properties in comblike polymers is discussed.
Journal of Physical Chemistry B, 2007
It is important for many industrial processes to design new materials with improved selective permeability properties. Besides diffusion, the molecule's solubility contributes largely to the overall permeation process. This study presents a method to calculate solubility coefficients of gases such as O 2 , H 2 O (vapor), N 2 , and CO 2 in polymeric matrices from simulation methods (Molecular Dynamics and Monte Carlo) using first principle predictions. The generation and equilibration (annealing) of five polymer models (polypropylene, polyvinyl alcohol, polyvinyl dichloride, polyvinyl chloride-trifluoroethylene, and polyethylene terephtalate) are extensively described. For each polymer, the average density and Hansen solubilities over a set of ten samples compare well with experimental data. For polyethylene terephtalate, the average properties between a small (n ) 10) and a large (n ) 100) set are compared. Boltzmann averages and probability density distributions of binding and strain energies indicate that the smaller set is biased in sampling configurations with higher energies. However, the sample with the lowest cohesive energy density from the smaller set is representative of the average of the larger set. Density-wise, low molecular weight polymers tend to have on average lower densities. Infinite molecular weight samples do however provide a very good representation of the experimental density. Solubility constants calculated with two ensembles (grand canonical and Henry's constant) are equivalent within 20%. For each polymer sample, the solubility constant is then calculated using the faster (10×) Henry's constant ensemble (HCE) from 150 ps of NPT dynamics of the polymer matrix. The influence of various factors (bad contact fraction, number of iterations) on the accuracy of Henry's constant is discussed. To validate the calculations against experimental results, the solubilities of nitrogen and carbon dioxide in polypropylene are examined over a range of temperatures between 250 and 650 K. The magnitudes of the calculated solubilities agree well with experimental results, and the trends with temperature are predicted correctly. The HCE method is used to predict the solubility constants at 298 K of water vapor and oxygen. The water vapor solubilities follow more closely the experimental trend of permeabilities, both ranging over 4 orders of magnitude. For oxygen, the calculated values do not follow entirely the experimental trend of permeabilities, most probably because at this temperature some of the polymers are in the glassy regime and thus are diffusion dominated. Our study also concludes large confidence limits are associated with the calculated Henry's constants. By investigating several factors (terminal ends of the polymer chains, void distribution, etc.), we conclude that the large confidence limits are intimately related to the polymer's conformational changes caused by thermal fluctuations and have to be regardedsat least at microscalesas a characteristic of each polymer and the nature of its interaction with the solute. Reducing the mobility of the polymer matrix as well as controlling the distribution of the free (occupiable) volume would act as mechanisms toward lowering both the gas solubility and the diffusion coefficients.
Atomistic Simulation of the Sorption of Small Gas Molecules in Polyisobutylene
Macromolecules, 2008
Polyisobutylene (PIB), an important elastomer with a low glass transition temperature, presents markedly low permeability properties to small-molecule penetrants compared to other elastomers. In the past, computer simulation approaches to explain this behavior have led to diffusivity and solubility calculations that, unfortunately, deviated significantly from the experimental values. We present here the results of a new simulation strategy which leads to accurate predictions of the solubility of four gases (He, Ar, N 2 , and O 2 ) in PIB, thus opening up the way toward understanding the molecular origin of the superior barrier properties of PIB. A critical element in the new approach is the introduction of a reliable united-atom model for PIB that can accurately reproduce its conformational characteristics and volumetric properties over a wide range of temperature conditions, thereby providing well-equilibrated representative PIB structures for subsequent permeability studies with a more accurate force field. To this, independent PIB configurations thoroughly pre-equilibrated with the new model are converted to all-atom PIB structures, re-equilibrated using the detailed COMPASS force field, and employed in a series of sorption runs for the estimation of the infinite dilution solubility coefficient, S 0 , of the small gas molecules. Simulation results for the solubility of He, Ar, N 2 , and O 2 in PIB at room temperature are found to reproduce experimental data with very good accuracy. Additional results at progressively higher temperatures show that the solubility of O 2 and Ar is always higher than that of N 2 and He, respectively. We also find that calculations based on a united-atom representation overestimate systematically the solubility of these gases, with the exception of He.
Molecular simulations of small gas diffusion and solubility in copolymers of styrene
Polymer, 2003
The objective of this study is to investigate the relationship between gas permeability and the chemical structure and conformational properties for copolymers of styrene and its homopolymer. The diffusion and the solubility coefficients of small gas molecules (He, H 2 , Ne, O 2 , N 2 , CH 4 , Ar, CO 2 ) in amorphous structures of poly (styrene-alt-maleic anhydride) copolymer (SMA), poly (styrene-stat-butadiene) rubber (SBR), and atactic polystyrene (PS) are investigated by the transition state approach. Simulation results are found to be in good agreement with the experimentally measured values. The transport behavior of H 2 O molecules is also studied in the same bulk structures by fully atomistic molecular dynamics simulations. In general, the diffusion coefficients of the gases in these matrices decrease in the following order: SBR . PS . SMA, whereas the solubility coefficients follow the reverse order. The differences in the mobility of the matrices seem to be the dominant determining factor for diffusion. And the solubility coefficients depend on the free volume distribution of the matrices. q
Molecular Modeling Investigations of Sorption and Diffusion of Small Molecules in Glassy Polymers
Membranes, 2019
With a wide range of applications, from energy and environmental engineering, such as in gas separations and water purification, to biomedical engineering and packaging, glassy polymeric materials remain in the core of novel membrane and state-of the art barrier technologies. This review focuses on molecular simulation methodologies implemented for the study of sorption and diffusion of small molecules in dense glassy polymeric systems. Basic concepts are introduced and systematic methods for the generation of realistic polymer configurations are briefly presented. Challenges related to the long length and time scale phenomena that govern the permeation process in the glassy polymer matrix are described and molecular simulation approaches developed to address the multiscale problem at hand are discussed.
Molecular simulations of the solubility of gases in polyethylene below its melting temperature
Polymer, 2010
We have employed Monte Carlo simulations in the osmotic ensemble to study the solubility of three different gases (N 2 , CH 4 , CO 2) in polyethylene. The simulations are performed at temperatures below the polymer melting point. Although under such conditions, polyethylene is in a semicrystalline state, we have used simulation boxes containing only a purely amorphous material. We show that under such circumstances, computed solubilities are 4e5 times larger than experimental data. We therefore introduce an original use of the osmotic ensemble to implicitly account for the effects of the complex morphology of semicrystalline materials on gas solubility. We have made the assumption that i) the network formed by polymer chains trapped between different crystallites and ii) the changes in local density from crystalline regions to purely amorphous regions, may be both represented by an ad-hoc constraint exerted on the amorphous phase. A single constraint value emerges, independent of the gas nature, characteristic of the crystalline degree of the polymer. It is concluded that the role of this constraint is mostly to reproduce the effective density of the permeable phase of the real material, indirectly giving insights into the morphology of a semicrystalline polymer.
A Free Volume Distribution Model of Gas Sorption and Dilation in Glassy Polymers
Macromolecules, 1995
A new approach is employed to model the sorption of gases in glassy polymers. The dualmode concept that sorption occurs in both "equilibrium" (Henry's law) sites and "nonequilibrium" (Langmuir) sites is used to separate the two contributions. The Sanchez-Lacombe equation of state is applied to predict the sorption in the Henry's law region of the polymer matrix. Characteristic parameters for the polymer liquid are used at glassy temperatures to provide a description of sorption in a hypothetical "equilibrium" state. The additional contribution provided by sorption into the regions of excess free volume that arise from the glassy state is addressed using a modified version of an approach first proposed by Kirchheim. The excess free volume is presumed to exist in a distribution of sizes, which in turn gives rise to a distribution of sorption energies. Positron annihilation lifetime spectroscopy data are used in conjunction with PVT data to characterize the glassy polymer. This independent description of the glassy state is then used to calculate the Langmuir-type sorption. The resulting model uses only one adjustable parameter to provide excellent descriptions of the sorption of several gases in three polycarbonates, as well as the volume dilation caused by the sorbed gas.