| Abstract
| - It is important for many industrial processes to design new materials with improved selective permeabilityproperties. 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 O2, H2O (vapor), N2, andCO2 in polymeric matrices from simulation methods (Molecular Dynamics and Monte Carlo) using firstprinciple 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 tensamples compare well with experimental data. For polyethylene terephtalate, the average properties betweena small (n = 10) and a large (n = 100) set are compared. Boltzmann averages and probability densitydistributions of binding and strain energies indicate that the smaller set is biased in sampling configurationswith higher energies. However, the sample with the lowest cohesive energy density from the smaller set isrepresentative of the average of the larger set. Density-wise, low molecular weight polymers tend to have onaverage lower densities. Infinite molecular weight samples do however provide a very good representation ofthe experimental density. Solubility constants calculated with two ensembles (grand canonical and Henry'sconstant) are equivalent within 20%. For each polymer sample, the solubility constant is then calculatedusing the faster (10×) Henry's constant ensemble (HCE) from 150 ps of NPT dynamics of the polymermatrix. The influence of various factors (bad contact fraction, number of iterations) on the accuracy of Henry'sconstant is discussed. To validate the calculations against experimental results, the solubilities of nitrogenand 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 withtemperature are predicted correctly. The HCE method is used to predict the solubility constants at 298 K ofwater vapor and oxygen. The water vapor solubilities follow more closely the experimental trend ofpermeabilities, both ranging over 4 orders of magnitude. For oxygen, the calculated values do not followentirely the experimental trend of permeabilities, most probably because at this temperature some of thepolymers are in the glassy regime and thus are diffusion dominated. Our study also concludes large confidencelimits are associated with the calculated Henry's constants. By investigating several factors (terminal ends ofthe polymer chains, void distribution, etc.), we conclude that the large confidence limits are intimately relatedto the polymer's conformational changes caused by thermal fluctuations and have to be regardedat least atmicroscaleas a characteristic of each polymer and the nature of its interaction with the solute. Reducing themobility of the polymer matrix as well as controlling the distribution of the free (occupiable) volume wouldact as mechanisms toward lowering both the gas solubility and the diffusion coefficients.
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