The Effect Of Intermolecular Forces On Diffusion In Polymers

Author : Abolfazl Noorjahan
Publisher :
ISBN 13 :
Total Pages : 153 pages
Book Rating : 4.:/5 (918 download)

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Download or read book On the Atomistic Simulation Approach Towards the Estimation of the Polymer/solvent Mutual Diffusion Coefficient written by Abolfazl Noorjahan and published by . This book was released on 2014 with total page 153 pages. Available in PDF, EPUB and Kindle. Book excerpt: With increasing computational power, the multi scale simulation of materials is getting more possible. To show the ability of this method to address macro scale problems, in this work, the problem of polymer/solvent mutual diffusion was selected based on its scientific and industrial significance. Poly(vinyl alcohol) (PVA) was selected as the polymeric medium and water, ethanol and benzene as typical solvents. After careful formulation of the problem of mutual diffusion, the target variables were reduced to self diffusion coefficient of the solvent and Flory-Huggins interaction parameter of the binary system. Then, using extensive molecular dynamics simulations, the well-known OPLS-AA force field was validated for the simulation of PVA by benchmarking its predictions against a few of interesting parameters of pure PVA, namely the specific volume, glass transition temperature, Hildebrand solubility parameter and heat capacity. 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 with different tacticities over a wide range of temperatures (200-550 K). For the heat capacities, 300% overestimations were obtained. Such overestimations were reduced significantly to about 30% by applying the quantum correction method. It is well-known that properties of PVA in the pure and solution states depend largely on the hydrogen bonding networks formed. In the context of molecular simulation, such networks are handled through the Coulombic interactions. Therefore, a good set of partial atom charges (PACs) for simulations involving PVA is highly desirable. As the original PACs in OPLS-AA have been parameterized for small molecular species (ethanol), they need to be systematically validated for PVA macromolecule. Accordingly, the PACs for PVA were calculated using a few commonly used population analysis schemes with a hope to identify an accurate set of PACs for PVA monomers. To evaluate the quality of the calculated parameters, we have benchmarked their predictions for free energy of solvation (FES) in selected solvents by molecular dynamics simulations against the ab initio calculated values. Selected solvents were water, ethanol and benzene as they covered a range of size and polarity. The best candidate for PVA/solvent simulation was found to be PACs by Merz-Singh-Kollman which will serve future simulations for the prediction of the PVA/solvent Flory-Huggins interaction parameter. In an attempt to provide more insight into the diffusion mechanism of solvents in PVA matrix, molecular dynamics simulation was used to study the diffusion of two selected penetrants, water and benzene, in PVA over a wide range of temperatures and concentrations. To help understand the effect of free volume on the diffusion behavior of water and benzene, we used the technique of Voronoi tessellation to determine key characteristics of free volume redistribution. In the case of water, we showed that it was the free volume redistribution frequency that led to an observation, previously reported in the literature, that the self-diffusion coefficient of water increases with increasing water concentration despite the fact that increase in water concentration decreases the mean fractional free volume in PVA. In the case of benzene (non-polar) diffusing in PVA (polar), we demonstrated that the failure of the Macki-Meares model was not entirely due to the dissimilarity between the intermolecular interactions. Rather, one of the reasons was the inability of the benzene molecules to break the hydrogen bonds between the PVA chains which are essential to increase the polymer segments mobility. Based on the mechanism found, the free volume model was selected as the best candidate for describing the diffusion of small molecules in polymers. However, the experimental procedure for parameterizing the model is not trivial and in fact extremely time consuming. Accordingly, following the best practice in the literature, we carried out isobaric-isothermal molecular dynamics simulations to generate thermodynamic data of a few selected polymer/solvent systems and used the data to parameterize the corresponding Sanchez-Lacombe equation of state (SLEOS). The characteristic parameters of the SLEOS were then used in the parameterization of lattice free volume (LFV) model so that diffusivity of the solvents in the polymers were then calculated. Owing to its consideration of glass transition temperature effects, LFV model was more successful in correlation of the self diffusion data the Mackie-Meares model.