Joel R. Fried

The molecular basis of CO2 interaction with polymers containing fluorinated groups: computational chemistry of model compounds and molecular simulation of poly[bis(2,2,2-trifluoroethoxy)phosphazene]

Abstract Ab initio molecular orbital calculations of CO2 and model compounds have been used to identify the nature of specific interactions between CO2 and the fluorinated substituent groups of polymers such as poly(trifluoropropyl methyl siloxane) and poly[bis(2,2,2-trifluoroethoxy)phosphazene] (PTFEP) that exhibits high CO2 permeability and permselectivity. Second-order Moller–Plesset (MP2) perturbation calculations (6-311++G∗∗ basis set) were used to obtain energies, dipole and quadrupole moments, and polarizabilities of CO2, three alkanes (CH4, CH3CH3, and CH3CH2CH3), and three fluoroalkanes (CF4, CH3CF3, and CH3CH2CF3). Results of energy calculations of three CO2–alkane and three CO2–fluoroalkane dimers indicate that CO2 forms a favorable quadrupole–dipole interaction with the fluoroalkyl groups. The maximum quadrupole–dipole interaction energy obtained was −11.5 kJ mol−1 for CO2–CF3CH2CH3. This value is less than interaction energies typical for hydrogen bonding but greater than the London dispersion values reported for the interaction of CO2 with the carbonyl group of poly(methyl methacrylate) (PMMA). Electrostatic potential distributions indicate a small redistribution of electron density to the fluorine atoms of the trifluoroalkanes and to the oxygen atoms of CO2 in the CO2–CF3CH3 and CO2–CF3CH2CH3 dimers. Sorption and molecular association of CO2 with PTFEP has been investigated by molecular simulation of an amorphous cell using the COMPASS molecular mechanics force field. CO2 sorption isotherms obtained by Grand Canonical Monte Carlo (GCMC) simulation indicate an upward deviation from the linear relationship between log S and the Lennard–Jones potential well depth parameter, e/k, in agreement with reported permeability data. Pair-correlation analysis obtained from molecular dynamics simulation show strong correlation of CO2 with the trifluoromethyl group of PTFEP in agreement with the MP2 results showing an association of CO2 with both CH3CF3 and CH3CH2CF3.

Pervaporation of methanol from a triglyme solution using a Nafion membrane: 1. Transport studies

Abstract The feasibility of using pervaporation as a means to recover low concentrations of methanol produced from the catalytic reaction of syngas in triglyme (triethylene glycol dimethyl ether) has been investigated. In this study, Nafion (a perfluorosulfonate ionomer) has been found to be a highly permeable and permselective membrane for methanol separation. Both sorption and diffusion favor methanol, resulting in high permeability and selectivity. A mathematical model for methanol pervaporation through Nafion has been validated by which flux is related to membrane thickness, the equilibrium permeant concentration in the membrane, a diffusion coefficient, and a plasticization term. Using this model to analyze the pervaporization data, the diffusion coefficient for methanol flux through Nafion was determined to be 6.8 × 10 −8 cm 2 s −1 at 30°C while the Arrhenius activation energy for methanol flux was estimated to be a low 3 kcal mol −1 . By contrast, the diffusion coefficient and activation energy for triglyme are estimated as 1.0 × 10 −8 cm 2 s −1 and 20 kcal mol −1 , respectively. These values are consistent with the cluster-network model for Nafion whereby methanol would be expected to easily pass through open ionic channels while the larger, less polar triglyme molecule diffuses primarily through the amorphous regions surrounding the clusters.

Secondary relaxation processes in bisphenol-A polysulphone

Abstract Dynamic mechanical spectra of bisphenol-A polysulphone in torsional deformation have been obtained as a function of frequency and thermal history. Results indicate a major low-temperature γ peak at 166 K and a broad, low-intensity β peak near 352 K at 10 rad s −1 for quenched samples. Activation energies have been estimated to be 10.7 and 67.3 kcal mol −1 , respectively. Annealing eliminates the β peak and intensifies the shoulder of the γ peak. Geometry-optimized CNDO/2 (complete neglect of differential overlap) molecular-orbital calculations for the diphenyl ether and isopropylidene units of polysulphone and use of molecular-mechanical calculations for diphenylsulphone indicate intramolecular conformational energies in the range from 5 to 10.6 kcal mol −1 . Comparison of these calculated energy barriers to torsional motions with experimentally determined activation energies from dynamic mechanical measurements suggests that phenyl and methyl group rotations in the isopropylidene unit, possibly phenyl ring rotation in the diphenyl sulphone unit, and phenyl group rotations in the diphenyl ether unit may all contribute to the γ transition. The higher-energy β transition is most probably due to intra- or interchain cooperative motions involving motions of several groups along the chain.

Preparation of a Nafion composite membrane using a porous teflon support

This article describes several approaches that can be used to prepare a composite membrane having a very thin Nafion film supported on a porous Teflon substructure.

Pervaporation of methanol from a triglyme solution using a Nafion membrane: 2. Concentration polarization

Abstract In the case of the pervaporation of methanol and triglyme (triethylene glycol dimethyl ether) through a Nafion membrane, it is shown that concentration polarization can be significant for even nondilute solutions (e.g. 2–4 wt% methanol). Concentration polarization was found to be particularly severe for thin composite membranes prepared by casting a modified Nafion solution onto a porous Teflon support. The Grober equation for the liquid-phase mass-transfer coefficient ( k l ) and a resistance-in-series model for the overall mass-transfer coefficient were used to model the boundary-layer effect. The value of k l was found to be related to the Reynolds number as 0.145 N Re 0.5 (units of m h −1 for the methanol/triglyme feed stream while the intrinsic membrane permeability was determined to be 2.66 × 10 −6 m 2 h −1 . Using these parameters, the model yielded a reasonable prediction of methanol flux in the feed concentration range of interest.

A method for the determination of the pore size distribution of molecular sieve materials and its application to the characterization of partially pyrolyzed polysilastyrene/porous glass composite membranes

Abstract A technique for the determination of the pore size distribution of molecular sieve materials is described. This method is based on the determination of the microporous volume accessible to molecular probes of different sizes. The method is used to follow the molecular sieve structure which develops during the pyrolysis of polysilastyrene (dimethylsilane-methylphenylsilane copolymer). A molecular sieve membrane is prepared by partially pyrolyzing a polysilastyrene solution-coated porous Vycor glass tube.

The water-channel forming ability of heptapeptide-based anion channels: insights from molecular dynamics simulations

Molecular dynamics simulations were performed to gain insight into the channel-forming capabilities of the SCMTR class of anion channels. These results support pore formation by the experimentally predicted single-surface dimeric SCMTR configuration. Simulated currents of 11.1 pA and 3.7 pA were reported for simulations involving a charge imbalance on either side of the bilayer and simulations where an electric field is applied, respectively. Stable water-channels were formed; these began from the SCMTR and extended to the opposing face. Removal of the driving force within the charge separation simulation was found to close the pore within a 10 ns simulation. As predicted, opposing face lipid head-group rearrangement was found to assist in the stabilization of the water-channel. Furthermore, these results suggest that the SCMTR molecules may help thin the bilayer by moving deeper into its surface and, thereby, helping to stabilize the water-channel. These results confirm that the proposed dimeric insertion model is sufficient to stabilize a channel, while providing atomistic insight into the channels function.

A hierarchical approach for predicting the transport properties of the gramicidin A channel

A hierarchical computational approach combining results of molecular dynamics (MD) simulations with continuous Poisson-Nernst-Planck (PNP) theory was used to investigate ion transport in a gramicidin A (gA) channel embedded within a 1,2-dimyristoylphosphatidylcholine (DMPC) bilayer. Molecular dynamics (MD) employing the CHARMM force field was used to investigate the diffusion of Na and K at different locations along the gA channel in both singly- and doubly-occupied states. Self-diffusion coefficients for single Na and K cations were determined to be 4.7 × 10 cm s and 6.2 × 10 cm s, respectively. Using these values, maximum ionic conductivities calculated from the Nernst-Einstein equation were 37 pS and 49 pS for Na and K, respectively, in the singly-occupied gA channel. These values agree with experimental data within an order of magnitude. Conductance of the gA channel was calculated from simulation results using the three-dimensional Poisson-Nernst-Planck (3D-PNP) model. Partial charge distributions for gA and for DMPC were assigned using the Poisson-Boltzmann module available in CHARMM. Diffusion coefficients were those obtained from the MD simulation. Results confirm that DMPC electrostatics have significant influence on channel conductivity. At low electrolyte concentrations, the channel cannot be occupied by more than one monovalent cation. Using ion diffusion coefficients obtained at different locations along the channel, current-voltage values obtained using 3D-PNP predictions for a channel immersed in an aqueous NaCl solution show good agreement with experimental values.

An Analysis of Secondary Relaxations in Bisphenol-A Polysulfone

The dynamic mechanical2–12 and dielectric7 behavior of bisphenol-A polysulfone (PSF) has been studied by several investigators over a wide range of frequency. These measurements suggest the presence of a low temperature (gamma) transition in the range of 163–197 K (corrected to a frequency of 11 Hz). Baccaredda et al.2 have associated this relaxation with rotational motion of water molecules bound to polar groups along the chain backbone thereby offering an explanation as to why the gamma peak is enhanced by the absorption of water. Kurz et al.6 have noted the persistence of this transition when samples have been dried. Kurz et al.6 suggest that the gamma process may be associated with motions involving the sulfone group which are enhanced through association with water.