Surface orientation effect of asymmetric polyimide hollow fibers on their gas transport properties

In this study, we focused on the shear stress effects within a spinneret during hollow fiber spinning on the formation of the hollow fibers and their gas transport properties. We fabricated asymmetric polyimide hollow fibers with a completely defect-free thin skin layer using a dry/wet phase inversion process. The apparent calculated skin layer thickness of the hollow fiber was 280 nm and the O₂ permeance was 2.9×10⁻⁵ cm³ (STP)/(cm² s cmHg). Interestingly, the skin layer thickness was reduced at the high shear rate. In addition, the gas permeances and selectivities of the hollow fibers increased with the increasing shear rate. We concluded that the oriented skin layer of the hollow fiber induced by shear stress had a significant influence on the formation of the skin layer and its gas transport properties. From the ATR-IR spectra results, it was clear that the surface skin layer of the hollow fiber was parallel oriented.     

Measurement and quantitative analysis of fiber orientation distribution in long fiber reinforced part by injection molding

The fiber orientation distribution is one of the important microstructure variables for thermoplastic composites reinforced with discontinuous fibers. In this paper, the long fibers in the injection molded part are measured in detail by micro X-ray CT. A three dimensional (3D) structure of the sample is built and two dimensional images are generated for image analysis. The orientation tensor of fibers is calculated in the flow plane. It shows a symmetric distribution of fibers through the thickness direction, which consists of outer skin, transition zone and the core. The skin layer is so thin that it has only one layer of highly oriented fibers. The core layer also has highly oriented fibers but the direction of fibers is different from that in the skin layer. Nevertheless, the clustering of the fibers is characterized quantitatively in the core. The transition zone can be divided into two subzones by the principal directions of the tensor.     

FT-IR photoacoustic and x-ray diffraction study of orientation of an injection-molded liquid-crystalline poly(ester-amide)

The orientation development that takes place during injection molding of liquid-crystalline polymer (LCP) is analyzed by FT-IR photoacoustic (PA) spectroscopy and X-ray diffraction. The LCP used is the commercial Hoechst-Celanese product Vectra B 900 prepared from 6-hydroxy-2-naphthoïc acid, terephthalic acid and p-acetoxyacetanilide. The analysis reveals the so-called skin-core morphology. The highly oriented skin layer is thought to originate in the advancing melt front where the “fountain flow” dominates. A less oriented transition layer due to the shear flow is located under the skin layer. The core is related to plug flow and has relatively poor orientation.     

Simulation of nanofibrous filters produced by the electrospinning method: 2. The effect of gas slip on the pressure drop

The dependence of pressure drop in a model filter on the distance between pairs of fibers, interfiber distances in the pairs, and the orientation of the pairs of fibers relative to flow direction is calculated with allowance for the effect of gas slip along the surface of doubled nanofibers. An isolated row of doubled parallel fibers oriented normal to the flow is selected as a model filter. Flow fields in the row of the fibers and drag forces acting upon them are calculated by the Stokes equations, which are solved by the numerical method of fundamental solutions. For pairs of fibers lying in the same plane in a row, the results of the numerical calculations agree with the analytical solution.     

Birefringence of highly oriented fibers

A spectrophotometric method was developed for measuring the birefringence of highly oriented synthetic fibers. This method surmounts the low birefringence limit of the standard quartz compensator method and the difficulties in interpretation of the photographic fringe method. A highly oriented aramid fiber gave birefringence values of 0.60–0.75 by this method, compared with 0.25 for polyester and 0.06 for nylon by other conventional methods. The operating principles and excellent results of this new method provide a basis for the extension of routine birefringence characterization to highly oriented fibers.     

Effect of molecular weight on gas selectivity of oriented thin polyimide membrane

In this study, we focused on effect of the molecular weight of polyimide on the gas selectivity of the asymmetric membrane with an oriented surface skin layer prepared at different shear stresses. Asymmetric polyimide membranes, which have a defect‐free surface skin layer supported by a porous substructure, were prepared by a dry/wet phase inversion process. The structures of the asymmetric polyimides consisted of a thin skin layer and a porous substructure characterized by the presence of finger‐voids. The gas selectivities of the asymmetric polyimide membranes increased with an increase in the shear rate or a decrease in the molecular weight, indicating that the oriented polyimide structure in the surface skin layer provided a high size and shape discrimination between the gas molecules. The selectivity values of (O₂/N₂) and (CO₂/CH₄) in the asymmetric polyimide membrane prepared from the 7.2 × 10⁴ molecular weight material at 1000 sec^?1 shear rate were 12 and 143, respectively. Copyright © 2003 John Wiley & Sons, Ltd.     

Gas separation characteristics of isomeric polyimide membrane prepared under shear stress

We synthesized the isomeric polyimides, 6FDA-m-DDS and 6FDA-p-DDS, and investigated the gas selectivity of the asymmetric polyimide membranes with an oriented surface skin layer. Particularly, we focused on the effect of the chemical structure of the polyimide on the molecular orientation. The asymmetric membranes with the oriented skin layer were prepared by a dry–wet phase inversion process at different shear stresses. The gas permeances of the asymmetric polyimide membranes were measured using a high vacuum apparatus with a Baratron absolute pressure gauge at 76 cmHg. The molecular orientation in the asymmetric polyimide membranes was measured using polarized ATR–FTIR spectroscopy. The gas selectivity of the asymmetric 6FDA-m-DDS membrane increased with an increased in the shear stress and were greater than that of the dense membrane. In contrast, the gas selectivities of the asymmetric 6FDA-p-DDS membrane did not depend on the shear stress and were similar to those of the dense membrane. We clarified that a parallel oriented surface formed on the asymmetric 6FDA-m-DDS membrane caused the enhanced gas selectivity of the membrane.     

Retardation of dissolution and surface modification of high-modulus poly(ethylene) fiber by the synergetic action of solvent and stress

Retardation of dissolution of highly oriented polyethylene fibers exposed to solvent under a constant tensile force has been investigated in comparison to free conditions. Beyond a critical value of the applied force, the time for dissolution increases sharply by several orders of magnitude. This effect is significant only in fibers with high initial orientation. It is attributed to the existence of a network of oriented crystallites. We have utilized this effect for surface modification of highly oriented PE fibers, by exposure to solvent at different temperatures and applied stress. At a relatively low load the action of the solvent displays pronounced effects: roughening of the fiber surface, formation of a nonoriented crystalline phase, enhancement of adhesion to epoxy resin with some loss of strength. ©1995 John Wiley & Sons, Inc.     

Brill transition of nylon-6 in electrospun nanofibers

Electrospun nylon-6 fibers were prepared from its polyelectrolyte solution in formic acid with different concentrtaions. In situ Fourier transform infrared (FTIR), wide-angle X-ray diffraction and small-angle X-ray scattering (SAXS) were performed on the nylon-6 fibers heated to various temperatures until melting. For comparison, stepwise annealing of the solution-cast film having exclusively the α-form was also carried out to elucidate the structural evolution. Our results showed that Brill transition in the electrospun fibers occurs at a lower temperature than that in the solution-cast film due to the crystal size difference. Differential scanning calorimetry heating traces on the as-spun fibers exhibited a unique crystalline phase with a melting temperature of ～235?°C, higher than the equilibrium melting temperature of nylon-6. The content of high melting temperature (HMT) phase increased with increasing nylon-6 concentration; a maximum of 30?% of the fiber crystallinity was reached for fibers obtained from the 22?wt.% solution regardless of the heating rates used. Based on the SAXS and FTIR results, we speculated that the HMT phase is associated with thick α-form crystals developed from the highly oriented nylon-6 chains that are preserved in the skin layer of the as-spun fibers. A plausible mechanism for the formation of the skin/core fiber morphology during electrospinning was proposed.     

Multiaxial mechanical behavior of aramid fibers and identification of skin/core structure from single fiber transverse compression testing

The transverse and longitudinal mechanical properties of aramid fibers like Kevlar? 29 (K29) fibers are strongly linked to their highly oriented structure. Mechanical characterization at the single fiber scale is challenging especially when the diameter is as small as 15 µm. Longitudinal tensile tests on single K29 fibers and single fiber transverse compression test (SFTCT) have been developed. Our approach consists of coupling morphological observations and mechanical experiments with SFTCT analysis by comparing analytical solutions and finite element modeling. New insights on the analysis of the transverse direction response are highlighted. Systematic loading/unloading compression tests enable to experimentally determine a transverse elastic limit. Taking account of the strong anisotropy of the fiber, the transverse mechanical response sheds light on a skin/core architecture. More importantly, results suggest that the skin of the fiber, typically representing a shell of one micrometer in thickness, has a transverse apparent modulus of 0.2 GPa. That is around more than fifteen times lower than the transverse modulus of 3.0 GPa in the core. By comparison, the measured longitudinal modulus is about 84 GPa. The stress distribution in the fiber is explored and the critical areas for damage initiation are discussed. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 374–384     

Magnetothermal Microfluidic‐Assisted Hierarchical Microfibers for Ultrahigh‐Energy‐Density Supercapacitors

Chemical architectures with an ordered porous backbone and high charge transfer are significant for fiber‐shaped supercapacitors (FSCs). However, owing to the sluggish ion kinetic diffusion and storage in compacted fibers, achieving high energy density remains a challenge. An innovative magnetothermal microfluidic method is now proposed to design hierarchical carbon polyhedrons/holey graphene (CP/HG) core–shell microfibers. Owing to highly magnetothermal etching and microfluidic reactions, the CP/HG fibers maintain an open inner‐linked ionic pathway, large specific surface area, and moderate nitrogen active site, facilitating more rapid ionic dynamic transportation and accommodation. The CP/HG FSCs show an ultrahigh energy density (335.8 μWh cm^?2) and large areal capacitance (2760 mF cm^?2). A self‐powered endurance application with the integration of chip‐based FSCs is designed to profoundly drive the durable motions of an electric car and walking robot.     

Magnetothermal Microfluidic-Assisted Hierarchical Microfibers for Ultrahigh-Energy-Density Supercapacitors

Chemical architectures with an ordered porous backbone and high charge transfer are significant for fiber-shaped supercapacitors (FSCs). However, owing to the sluggish ion kinetic diffusion and storage in compacted fibers, achieving high energy density remains a challenge. An innovative magnetothermal microfluidic method is now proposed to design hierarchical carbon polyhedrons/holey graphene (CP/HG) core–shell microfibers. Owing to highly magnetothermal etching and microfluidic reactions, the CP/HG fibers maintain an open inner-linked ionic pathway, large specific surface area, and moderate nitrogen active site, facilitating more rapid ionic dynamic transportation and accommodation. The CP/HG FSCs show an ultrahigh energy density (335.8 μWh cm⁻²) and large areal capacitance (2760 mF cm⁻²). A self-powered endurance application with the integration of chip-based FSCs is designed to profoundly drive the durable motions of an electric car and walking robot.     

Evaluating polarizable potentials on distributed memory parallel computers: Program development and applications

The efficient evaluation of polarizable molecular mechanics potentials on distributed memory parallel computers is discussed. The program executes at 7–10 Mflops/node on a 32-node CM-5 partition and is 19 times faster than comparable code running on a single-processor HP 9000/735. On the parallel computer, matrix inversion becomes a practical alternative to the commonly used iterative method for the calculation of induced dipole moments. The former method is useful in cases such as free-energy perturbation (FEP) simulations, which require highly accurate induced dipole moments. Matrix inversion is performed 110 times faster on the CM-5 than on the HP. We show that the accuracy which is needed for FEP calculations with polarization can be obtained by either matrix inversion or by performing a large number of iteration cycles to satisfy convergence tolerances that are less than 10^?6 D. © 1995 by John Wiley & Sons, Inc.     

Measurement of A Two Layer Sample Using Pseudo Pulse Excited Photoacoustic Spectrometry

Abstract

To obtain surface layer thickness easily, a simple pseudo pulse excited photoacoustic spectrometric method was proposed. An argon ion laser was chopped mechanically to generate a pseudo pulse (pulse width; 6.6 ms, duty 1.67%), which was then led to a sample enclosed in photoacoustic cell. Two layer samples made of polymethyl methacrylate (PMMA) were used as model samples. The photoacoustic signal waveform observed showed a maximum from the negative edge of the pseudo pulse of the laser. The delay of the signal increased concomitant with the sample surface thickness. The delay time of the signal was calculated by a cross-correlation method. A linear relationship was obtained between the delay time of the photoacoustic signal from the input pseudo pulse and the surface transparent layer thickness in the range of 0–90 mm. The regression line between the film thickness x (cm) and the delay time was expressed with the thermal diffusivity of the film k, as follows; Δτ (s) = 1.16 × 10^?1 κ^?½ x + 0.006. Using this result, the method proposed was successfully applied to the measurement of the thickness for laminated polyethylene film on papers. The method proposed is simple and easy to perform without any modification of usual photoacoustic instrumentation.     

Hierarchical Fibrous Honeycomb Ceramics with High Load Capability and Low Light-Off Temperature for the Next-Generation Auto Emissions Standards

Novel and stringent automotive exhaust gas emissions standards are urgently needed to counter the problems posed by the worsening global climate and environment. However, the traditional cordierite-based honeycomb ceramics substrates with ultimate pore density have seriously restricted the establishment of new emission standards. Herein, we introduce a novel robust substrate with tailored volume-specific surface area and low heat capacity. This substrate employs the synergy of high-strength ceramic fibers and ultrathin TiO₂ nanosheets. The micro-sized fibers provide support to ensure structural strength during the catalytic reaction, while the nanosheets play the dual role of connecting the fibers and providing a high surface area for catalyst immobilization. The new three-dimensional (3D) microarchitecture exhibits a high volume-specific surface area of 3.59×10⁴ cm²/cm³, a compressive strength of 2.01 MPa, and remarkable stability after high-speed air erosion at 800 °C. The honeycomb-like structure exhibit low resistance to gas flow. Furthermore, after loading with Pt and Pd nanoparticles, the composite 3D microarchitecture delivered an excellent catalytic performance and prominent structural stability, with a super low light-off temperature of 150 °C. The outstanding mechanical and thermal stability and the high surface area and light-off temperature of the new substrate indicate its potential for use as a highly efficient catalytic carrier to meet the next-generation auto emissions standards.     

Compacted UHMWPE fiber composites: Morphology and X‐ray microdiffraction experiments

The effect of compaction conditions on UHMWPE fibers is examined by microbeam X‐ray diffraction (WAXS) and scanning electron microscopy (SEM). The morphological observations indicate that melting occurs during compaction both on the surface of the fiber as well as in its internal regions. In addition, the recrystallized phase is nucleated on the fiber surface, possibly epitaxially. The recrystallized phase that originates from the internal regions of the fiber retains the initial highly oriented structure. WAXS microbeam measurements do not show any significant core‐shell structure in compacted single fibers. Considering the overall characteristics of the melting process during compaction, we can conclude that the hexagonal phase that appears upon heating of the fibers under moderate pressure is responsible for good adhesion of the fibers to each other, even more significantly than surface melting, especially because of its ability to retain the high orientation of the chains in the fibers. This information is relevant for understanding the formation and microstructure of the matrix component in the self‐reinforced composites fabricated by compaction. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1535–1541, 2007     

Oriented growth of TS-1 zeolite ultrathin films on poly(ethylene oxide) monolayer templates

Self-assembled monolayers of poly(ethylene oxide) (PEOM) were used as templates to direct the nucleation and subsequent oriented growth of TS-1 zeolite thin films. The resulting TS-1 zeolite thin films exhibited completely oriented crystalline domains with the 010 crystallographic direction parallel to the surface normal with a little deformation in the in-plane direction. Remarkably, the correlation length of the film is as extremely large as 1700 A (1200 A) in the out-of-plane (in-plane) direction. PEOM act as two-dimensional media for the nucleation and growth of highly oriented TS-1 zeolite thin films. This study demonstrates that the highly oriented TS-1 thin films can be achieved by using the inner parts of PEOM as templates.     

Pulsed Photoacoustic Study of the Diffusion of Chromophores in Human Skin

The technique of pulsed photoacoustic spectroscopy was used to investigate the diffusion of chromophores in human skin. The kinetic of diffusion has been studied for five solutions at different concentrations in a mixture of chromophores, as used in commercial sunscreens. In addition to the classical macroscopic interpretation of the diffusion process, a new method is shown to give more detailed information on chromophore presence at different depths in skin. For the first time, results are expressed in the frequency domain by means of the Fourier transform applied to the photoacoustic signal. The spectra are discussed versus the depth in skin samples and the time of diffusion kinetics. This new method of data analysis is shown to be very useful for understanding the influence of the internal structure of a medium on the penetration rate of chromophores into skin.     

Multithreaded parallelization of the energy and analytic gradient in the fragment molecular orbital method

The fragment molecular orbital method in GAMESS is parallelized in a multithreaded OpenMP implementation combined with the MPI version of the two-level generalized distributed data interface. The energy and analytic gradient in gas phase and the polarizable continuum model of solvation are parallelized in this hybrid three-level scheme, achieving a large memory footprint reduction and a high parallel efficiency on Intel Xeon Phi processors. The parallel efficiency is demonstrated on the Stampede2 and Theta supercomputers using up to 2048 nodes (262 144 threads).     

On the origin of the “core‐free” morphology in microinjection‐molded HDPE

This study investigates the morphology of a high‐density polyethylene processed with microinjection molding. Previous work pointed out that a “core‐free” morphology exists for a micropart (150‐μm thick), contrasting with the well‐known “skin‐core” morphology of a conventional part (1.5‐mm thick). Local analyses are now conducted in every structural layer of these samples. Transmission electron microscopy observations reveal highly oriented crystalline lamellae perpendicular to the flow direction in the micropart. Image analysis also shows that lamellae are thinner. Wide‐angle X‐ray diffraction measurements using a microfocused beam highlight that highly oriented shish–kebab morphologies are found through the micropart thickness, with corresponding orientation function close to 0.8. For the macropart, quiescent crystallized morphologies are found with few oriented structures. Finally, the morphology within the micropart is more homogeneous, but the crystalline structures created are disturbed due to the combined effects of flow‐induced crystallization and thermal crystallization during processing. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1470–1478, 2011