Thermoregulated gas transport through electrospun nanofiber membranes

Thermoregulation of gas transport using electrospun fiber membranes is demonstrated experimentally for the first time. The fiber membranes comprise three layers: a middle layer of electrospun polystyrene sandwiched between two outer layers of electrospun cellulose acetate mat as supports, bonded together by hot pressing. The electrospun polystyrene layer serves as a phase change material that blocks transport of gases though the membrane when the fibers de-vitrify. The membrane exhibited a reduction in oxygen flux at temperatures in excess of 140 °C. Using a blend of polysulfone and polystyrene resulted in an upward shift of the transition temperature to 250 °C. Modeling of transport was performed to estimate the impact of the morphological properties of the membranes such as tortuosity, fiber diameter, and porosity.Philip Morris Internationa

Coarse-grained, density dependent implicit solvent model reliably reproduces behavior of a model surfactant system

Density dependent, implicit solvent (DDIS) potentials, the generation of which has been described previously [ E. C. Allen and G. C. Rutledge, J. Chem. Phys. 128, 154115 (2008) ; E. C. Allen and G. C. Rutledge, J. Chem. Phys. 130, 034904 (2009) ], are used in this work to examine the self-assembly of a model surfactant system. While the measurement of thermodynamic properties in simulations of solvated micelles requires large computational resources or specialized free energy calculations, the high degree of coarse-graining enabled by the DDIS algorithm allows for the measurement of critical micelle concentration and aggregation number distribution using single processor NVT simulations. In order to evaluate the transferability of potentials derived from the DDIS methodology, the potentials are derived from simulations of simple monomeric solutes and used in the surfactant system without modification. Despite the high degree of coarse graining and the simplicity of the fitting simulations, we demonstrate that the coarse-grained DDIS potentials generated by this method reliably reproduce key properties of the underlying surfactant system: the critical micelle concentration, and the average aggregation number. The success of the DDIS algorithm suggests its utility for more realistic surfactant models.United States. Dept. of Energy (Office of Science, Computational Science Graduate Fellowship Program)United States. Dept. of Energy (National Nuclear Security Administration, Contract No. DEFG02- 97ER25308

Molecular simulation of crystal nucleation in n-octane melts

Homogeneous nucleation of the crystal phase in n-octane melts was studied by molecular simulation with a realistic, united-atom model for n-octane. The structure of the crystal phase and the melting point of n-octane were determined through molecular dynamics simulation and found to agree with experimental results. Molecular dynamics simulations were performed to observe the nucleation events at constant pressure and constant temperature corresponding to about 20% supercooling. Umbrella sampling Monte Carlo simulations were used to calculate the nucleation free energy for three temperatures, ranging from 8% to 20% supercooling, and to reveal details of the critical nucleus for the first time. The cylindrical nucleus model was found to provide a better quantitative description of the critical nucleus than the spherical nucleus model. The interfacial free energies of the cylinder model were calculated from the simulation data. As the temperature increased, the interfacial free energy of the side surface remained relatively unchanged, at 7–8 mJ/m[superscript 2], whereas the interfacial free energy of the end surface decreased significantly from 5.4 mJ/m[superscript 2] to about 3 mJ/m[superscript 2]. These results, and the methods employed, provide valuable and quantitative information regarding the rate-limiting step during the solidification of chain molecules, with ramifications for both short alkanes and polymers.Clemson University. Center for Advanced Engineering Fibers and Films. (NSF grant)Massachusetts Institute of Technology (ERC NSFEEC 9731680)Exxon Mobil Corporatio

Liquid-vapor equilibria and interfacial properties of n-alkanes and perfluoroalkanes by molecular simulation

A molecular dynamics study is presented to assess the performance of a united-atom model in the prediction of liquid-vapor interfacial properties for short-chain perfluoroalkanes and their alkane counterparts. In particular, the ability of this model to discriminate between the surface-energy values of these two types of compounds was investigated over a wide temperature range corresponding to the liquid-vapor region. Comparisons with available experimental data and surface-tension predictions given by other force-field parameterizations, including those based on the more computationally demanding all-atom method, were performed to gauge the viability of this model. It was found that the model used in this study captures qualitatively the expected behavior of surface energy between alkanes and perfluoroalkanes and yields values that are in excellent agreement with experimental data, especially in the high-temperature limit as the critical temperature is approached

Free surface electrospinning from a wire electrode

Electrostatic jetting from a free liquid surface offers an alternative to conventional electrospinning in which jets are emitted from spinnerets. In this work we analyze a system in which a wire electrode is swept (in a rotary motion) through a bath containing a polymeric solution in contact with a high voltage, resulting in entrainment of the fluid, the formation of liquid droplets on the wire and electrostatic jetting from each liquid droplet. Solutions of polyvinylpyrrolidone in ethanol were used as test systems to evaluate each stage of the process. The volumes of individual droplets on the wire were measured by photographic methods and correlated with the viscosity, density and surface tension of the liquid, and with system parameters such as electrode rotation rate and wire diameter. The local electric field in the absence of liquid entrainment was modeled using conventional electrostatics, and jet initiation was found to occur consistently at the angular position where the electric field exceeds a critical value of 34 kV/cm, regardless of rotation rate. Two operating regimes were identified. The first is an entrainment-limited regime, in which all of the entrained liquid is jetted from the wire electrode. The second regime is field-limited, in which the residence time of the wire electrode in an electric field in excess of the critical value is too short to deplete the fluid on the wire. The productivity of the system was measured and compared to the theoretical values of liquid entrainment. As expected, highest productivity occurred at high applied potentials and high rotation rates.Novartis-MIT Center for Continuous Manufacturin

Molecular Dynamics Simulation of Surface Nucleation during Growth of an Alkane Crystal

Crystal growth from the melt of n-pentacontane (C50) was studied by molecular dynamics simulation. Quenching below the melting temperature gives rise to propagation of the crystal growth front into the C50 melt from a crystalline polyethylene surface. By tracking the location of the crystal–melt interface, crystal growth rates between 0.02 and 0.05 m/s were observed, for quench depths of 10–70 K below the melting point. These growth rates compare favorably with those from a previous study by Waheed et al. [ Polymer 2005, 46, 8689−8702]. Next, surface nucleation was identified with the formation of two-dimensional clusters of crystalline sites within layers parallel to the propagating growth front. Critical nucleus sizes, waiting times, and rates for surface nucleation were estimated by a mean first passage time analysis. A surface nucleation rate of ∼0.05 nm⁻² ns⁻¹ was observed, and it was nearly temperature-independent. Postcritical “spreading” of the surface nuclei to form a completely crystallized layer slowed with deeper supercooling.National Science Foundation (U.S.) Division of Civil, Mechanical and Manufacturing Innovation (CMMI-1235109

Mechanical and tribological properties of electrospun PA 6(3)T fiber mats

The mechanical and tribological properties of electrospun fiber mats are of paramount importance to their utility as components in a large number of applications. Although some mechanical properties of these mats have been reported previously, reports of their tribological properties are essentially nonexistent. In this work, electrospun nanofiber mats of poly(trimethyl hexamethylene terephthalamide) (PA 6(3)T) with average fiber diameter of 463 ± 64 nm are characterized mechanically and tribologically. Post-spin thermal annealing was used to modify the properties of the fiber mats. Morphological changes, in-plane tensile response, friction coefficient and wear rate were characterized as functions of the annealing temperature. The Young's moduli, yield stresses and toughnesses of the nonwoven mats improved by two- to ten-fold when annealed slightly above the glass transition temperature, but at the expense of mat porosity. The coefficient of friction and the wear rate decrease by factors of two and ten, respectively, under the same conditions. The wear rate correlates with the yield properties of the mat, in accord with a modified Ratner–Lancaster model. The variation in mechanical and tribological properties of the mats with increasing annealing temperature is consistent with the formation of fiber-to-fiber junctions and a mechanism of abrasive wear that involves the breakage of fibers between junctions.National Science Foundation (U.S.) (Grant CMMI-0700414)Masdar Institute of Science and TechnologyMassachusetts Institute of Technology. Institute for Soldier Nanotechnologies (AROW911NF-07-D-0004

Multiresolution analysis in statistical mechanics. II. The wavelet transform as a basis for Monte Carlo simulations on lattices

In this paper, we extend our analysis of lattice systems using the wavelet transform to systems for which exact enumeration is impractical. For such systems, we illustrate a wavelet-accelerated Monte Carlo (WAMC) algorithm, which hierarchically coarse-grains a lattice model by computing the probability distribution for successively larger block spins. We demonstrate that although the method perturbs the system by changing its Hamiltonian and by allowing block spins to take on values not permitted for individual spins, the results obtained agree with the analytical results in the preceding paper, and ``converge'' to exact results obtained in the absence of coarse-graining. Additionally, we show that the decorrelation time for the WAMC is no worse than that of Metropolis Monte Carlo (MMC), and that scaling laws can be constructed from data performed in several short simulations to estimate the results that would be obtained from the original simulation. Although the algorithm is not asymptotically faster than traditional MMC, because of its hierarchical design, the new algorithm executes several orders of magnitude faster than a full simulation of the original problem. Consequently, the new method allows for rapid analysis of a phase diagram, allowing computational time to be focused on regions near phase transitions.Comment: 11 pages plus 7 figures in PNG format (downloadable separately

Production of core/shell fibers by electrospinning from a free surface

Electrostatic fiber formation (“electrospinning”) is the leading technology for production of continuous fibers with submicron diameter. Applications such as drug delivery and sensors benefit from the ability to produce submicron fibers with a core/shell morphology from electrified coaxial jets of two liquids. However, low productivity of the conventional needle-based coaxial process is a barrier for commercialization. We present a novel technology that overcomes this limitation by the development of coaxial jets directly from compound droplets of immiscible liquids entrained on wires, and control of mass transfer processes to produce uniform, core/shell fibers. The enabling feature of controlled evaporation by design of solution properties is verified by a simple mass transport model. Electron micrographs confirm the formation of fibers with the desired morphology. The proposed technology creates the opportunity to produce nanofibers with core/shell morphology on an industrial scale for a wide variety of applications.Novartis-MIT Center for Continuous Manufacturin