Molecular dynamics simulation of diffusion of O 2 and CO 2 in amorphous poly (ethylene terephthalate) and related aromatic polyesters (original) (raw)
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On the diffusion of gases in partially crystalline polymers
Journal of Polymer Science Part B: Polymer Physics, 1989
The diffusion of gases through partially crystalline polymers is studied. The effective diffusion coefficient DeB is obtained as the result of the averaged superposition of two fundamental mechanisms, namely, diffusion through the crystallites is considered to be zero, and diffusion through the rubbery fraction of the polymer obeys a Fujita-like free-volume theory. The predicted Deff is compared with experimental data of Kreituss and Frisch. The behavior of the diffusion coefficient in terms of concentration and crystalline fraction is satisfactorily explained through the model.
Diffusion of small-molecule penetrants in polyethylene: free volume and morphology
Polymer, 1996
Based on desorption and permeation measurements, the diffusivity and solubility of n-hexane and oxygen have been obtained for a wide range of linear and branched polyethylenes (PEs) with crystallinities between o 3 6 l 40 and 97 Vo and mass-average molar masses between 10 and 10 g mol-. The morphology and contents of crystal core (CC), crystal-core-like lnterfaclal (Icc), liquid-like interfacial (Ih) and liquid (L) components • " 13 n were assessed by transmission electron microscopy, Raman spectroscopy, C cross-polarizatio / magic-angle spinning nuclear magnetic resonance spectroscopy, differential scanning calorimetry, density measurements and small-angle light scattering. The penetrant solubility in the non-crystalline phases increased with increasing concentration of chain ends and chain branches. This effect was masked at certain crystallinities by the constraining effect of the crystallites. The diffusivity selectivity of oxygen over n-hexane increased strongly with increasing crystallinity and decreasing non-crystalline layer thickness, demonstrating that the crystal-induced constraint on the non-crystalline chains more efficiently retards the diffusion of larger molecules. The fractional free volume of the non-crystalline components decreased strongly with increasing crystallinity in the low-crystallinity range (<60%), above which it remained practically constant. The latter is because the constraining effect of the crystals is compensated for by the plasticizing effect of the chain ends, which leads to a constant free volume in this crystallinity range. A model, based on the Cohen-Turnbull-Fujita (CTF) model, considering the polymers to consist of four components, CC, Icc, Ir and L, was applied to the diffusivity data. The branched PEs and the majority of the linear PEs could be described by the modified CTF model. However, the lowest-molar-mass linear PEs exhibited a considerably larger interfacial free volume than the other samples. Real-time Raman spectroscopy on CC14-swollen samples showed that the changes in the CC and Icc contents during sorption were only small.
Macromolecules, 2004
We present results from detailed atomistic simulations concerning the structural, conformational, dynamic, and barrier properties of the amorphous (glassy and melt) phases of two polyisomers, PET [poly(ethylene terephthalate)] and PEI [poly(ethylene isophthalate)]. First, well-relaxed atomistic configurations of the two polyesters are generated following a succession of equilibration stages consisting of energy minimizations, temperature annealings and coolings, and compressions-decompressions, as proposed by Hofmann et al. et al. Macromol. Theory Simul. 2000, 9, 293). This equilibration cycle is significantly extended here by subjecting the resulting configurations to an additional molecular dynamics (MD) simulation at a high temperature for 2 ns, affording extra relaxation at all length scales. With the statistically uncorrelated configurations generated via these extended equilibration runs, isothermal-isobaric (NPT) at P ) 1 atm as well as canonical (NVT) (with the densities set at the corresponding experimentally measured values) MD simulations are performed in the melt state at T ) 450 and 600 K to probe differences in the structure and segmental dynamics of the two polyesters. The simulations reveal significant differences in the local relaxation dynamics of the phenyl rings between the two polyisomers: In PET, these rings are found to exhibit significantly higher mobility than in PEI; this is attributed to the way in which phenyls are connected to their adjacent ester groups. Structural and conformational properties, on the other hand, are predicted to be practically identical in the two polyesters. Finally, transition-state theory (Gusev, A. A.; Suter, U. W. J. Chem. Phys. 1993, 99, 2228 is employed to calculate the rate constants of diffusive jumps between sorption sites, and hence the lowconcentration self-diffusivity, of oxygen (O 2) molecules in well-relaxed atomistic configurations of the two polymers, whose densities exactly match the experimentally measured values. Calculated O2 diffusivities are found to be in excellent qualitative and quantitative agreement with experiment: O2 diffusivity in PEI is predicted to be 1.8 times smaller than in PET, in agreement with the experimental finding that PEI is 2-2.5 times less permeable to O2 than PET in the amorphous state.
Diffusion of small-molecule penetrants in semicrystalline polymers
Progress in Polymer Science, 1996
This paper is a review of the kinetics of diffusion of small-molecule penetrants in semi-crystalline polymers. The variations in diffusion impedance resulting from variations in morphology and segmental mobility in the amorphous component are highlighted. Deviations from Fickian diffusion appearing in many systems are discussed, as well as the effect of morphology, crystal orientation and chain orientation on the diffusivity.
A molecular simulation study on gas diffusion in a dense poly(ether–ether–ketone) membrane
Polymer, 2001
Results of molecular dynamics (MD) simulations on transport and solubility of small molecules in amorphous cardo poly-ether-etherketone membranes are discussed. Atomistic simulation techniques have proven to be a useful tool for the understanding of structure-property relationships of materials. Although MD are still not an ideal tool for the quantitative prediction of gas permeation properties, this methodology can be used for a detailed description of the complex morphologies and transport mechanisms associated with rigid glassy structures.
Gas Transport in Glassy Polymers: Prediction of Diffusional Time Lag
Membranes
The transport of gases in glassy polymeric membranes has been analyzed by means of a fundamental approach based on the nonequilibrium thermodynamic model for glassy polymers (NET-GP) that considers the penetrant chemical potential gradient as the actual driving force of the diffusional process. The diffusivity of a penetrant is thus described as the product of a purely kinetic quantity, the penetrant mobility, and a thermodynamic factor, accounting for the chemical potential dependence on its concentration in the polymer. The NET-GP approach, and the nonequilibrium lattice fluid (NELF) model in particular, describes the thermodynamic behavior of penetrant/polymer mixtures in the glassy state, at each pressure or composition. Moreover, the mobility is considered to follow a simple exponential dependence on penetrant concentration, as typically observed experimentally, using only two adjustable parameters, the infinite dilution penetrant mobility L10 and the plasticization factor β, b...
Chappter 6. Diffusion of Gases in Amorphous Polymers: The Monte Carlo Void Method
We propose a method for studying diffusion in amorphous structures based on biased random walk in the free volume extracted from a polymer ("the Monte Carlo Void Method .") We analyze a number of simple free volume structures not derived from realistic polymers and show that the biased random walk method is offering intuitively realistic description of the particle motion and present a framework for computing diffusion coefficents.
Macromolecular Symposia, 2010
Summary: Diffusion of n‐hexane in poly(ethylene‐co‐1‐hexene)s with 15–75 wt.% crystallinity was studied by desorption experiments analyzing data using the Fickian equations with a concentration dependent diffusivity. The effect of the impenetrable crystalline phase on the penetrant diffusivity (D) is described by D = Da/(τβ), where Da is the diffusivity of the amorphous polymer, τ is the geometrical impedance factor and β is a factor describing the constraining effect of the crystals on the non‐crystalline phase. For a polymer with 75 wt.% crystallinity, τβ varied markedly with penetrant concentration (v1a) in the penetrable phase: 1000 (v1a = 0) and 10 (v1a = 0.15). This penetrant‐uptake had no effect on the gross crystal morphology, i.e. β must be strongly dependent on v1a. Samples saturated in n‐hexane exhibited a penetrant‐induced loosening of the interfacial structure, as revealed by an increase in crystal density that require an increased mobility in the interfacial component ...