Rohitkumar H. Vora

The effect of shear rates on gas separation performance of 6FDA-durene polyimide hollow fibers

Abstract We have determined the effect of shear rates within a spinneret on gas separation performance of asymmetric 6FDA-durene polyimide hollow fiber membranes. We purposely wet-spun the hollow fibers at different dope flow rates without drawing and with a constant ratio of bore fluid flow rate to dope flow rate in order to study the effect of shear within a spinneret during hollow fiber spinning on fiber morphology and gas separation performance. For the first time, we found there is a V (down and up) pattern for permeance versus shear rate relationship, while a Λ (up and down) pattern for selectivity versus shear rate relationship. At low shear rates, the permeances of non-polar molecules such as H 2 , O 2 , N 2 , and CH 4 decrease, while their relative selectivities increase with an increase in shear rates. Once a certain shear rate is reached, all permeances increase, while their selectivities decrease with an increase in shear rates. In low shear rate regions, the decrease in permeance or increase in selectivity with increasing shear rates arises from the better molecular orientation and chain packing induced by shear. With increasing shear in high shear rate regions, the increase in permeance or decrease in selectivity is mainly attributed to relatively porous skin structures induced by the low viscosity nature of a power-law spinning fluid at high shear rates, fracture, and modified thermodynamics and kinetics of phase inversion process. This work suggests there may exist an optimum shear rate to yield optimal membrane morphology for gas separation. To our surprise, an increase in CO 2 permeance with increasing shear rates are possibly due to enhanced coupling effect between CO 2 and the highly oriented and closely packed fluoropolyimide molecular chains induced by shear.

Development of a defect-free 6FDA-durene asymmetric hollow fiber and its composite hollow fibers☆

Abstract We have developed 6FDA-durene polyimide and poly(4-vinylpyridine) (PVP)/6FDA-durene composite hollow fibers. The 6FDA-durene fiber was defect-free and had a selectivity of 4.3–5.1 for O 2 /N 2 . It had an average permeance for O 2 of ≈ 240×10 −6 cm 3 (STP)/cm 2 cmHg and its dense layer thickness was ≈ 2000–3000 μm. The dense layer location was at the outer surface. This fiber was wet-spun directly from an in-situ imidization dope consisting of 14.94 wt% 6FDA-durene in a (70.24/21.5/7.51/0.75) N -methyl-2-pyrrolidone (NMP)/propionic acid (PA)/β-picoline/propionic anhydride solvent mixture. The fiber diameter was ≈500 μm with a wall thickness ≈50–70 μm. The PVP/6FDA-durene composite fiber had a selectivity of 6.9 for O 2 /N 2 and a permeance for O 2 of 14.5×10 ∂ cm 3 (STP)/cm 2 s cmHg. The newly developed technology suggests, for the first time, that a defect-free hollow fiber can be produced from a modified Lewis acid:base complex dope. Compared to most other conventional hollow fiber fabrication processes, our approach has many advantages because that (1) it is simpler and more economical, (2) spins fibers at low temperatures, (3) employs a dope directly from polymerization, and (4) requires no secondary operations, such as silicone coating and curing.

Synthesis and Properties of Designed Low-k Fluoro-Copolyetherimides. Part 1

Polyetherimides are versatile melt-processable engineering resins. A series of fluoro-polyetherimides compositions based on diether-containing diamines were designed on paper and their dielectric constants (ϵ′) estimated before their synthesis. Experimentally determined ϵ′ values were compared with the estimated values. The amorphous polyetherimides containing bis-trifluoromethyl groups exhibited not only low dielectric constants—lower than those of the commercially available polyetherimide ULTEM 1000 and polyimide Kapton H at 1 kHz—but also excellent long-term thermo-oxidative stability and reduced water absorption relative to non-fluorinated polyimides.

Ultra-low-κ materials based on nanoporous fluorinated polyimide with well-defined pores via the RAFT-moderated graft polymerization process

Thermally-initiated living radical graft polymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA) with ozone-pretreated fluorinated polyimide (FPI) via the reversible addition–fragmentation chain transfer (RAFT)–moderated process was carried out. The chemical composition and structure of the copolymers were characterized by nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), and molecular weight measurements. The “living” character of the grafted PEGMA side chains was ascertained in the subsequent extension of the PEGMA side chains. Nanoporous ultra-low dielectric constant (κ) fluorinated polyimide films were prepared by solution casting of the graft copolymers, followed by thermal decomposition of the labile PEGMA side chains in air. The nanoporous FPI films obtained from the RAFT-moderated graft copolymers had well-preserved FPI backbones, porosity in the range of 2–10% and pore size in the range of 20–50 nm. These films also had more well-defined pores, more uniform pore size distribution, and better-retained mechanical properties than those of the corresponding nanoporous FPI films obtained from the graft copolymers prepared by the conventional free-radical process. Dielectric constants approaching 2.0 were obtained for the nanoporous FPI films prepared from the RAFT-moderated graft copolymers.

Effect of diamine composition on the gas transport properties in 6FDA-durene/3,3′-diaminodiphenyl sulfone copolyimides

Abstract The gas permeation behavior of hexafluoro-dianhydride (6FDA)-durene/3,3′-diaminodiphenyl sulfone (DDS) polyimides was investigated by systematically varying the diamine ratios. Generally, the gas permeability, diffusion and solubility coefficients were found to decrease with increasing 3,3′-DDS diamine content. The permeability coefficients of H 2 , O 2 , N 2 and CO 2 decreased with the increasing order of kinetic diameters of the penetrant gases. However, the decrease in diffusion coefficients of O 2 , N 2 and CO 2 was in accordance with the effective diameters of the gas molecules. The permselectivity of gas pairs such as CO 2 /N 2 , O 2 /N 2 and H 2 /N 2 was enhanced with the incorporation of 3,3′-DDS moiety. The gas transport coefficients of the copolyimides predicted from the addition rule were compared with the experimental results. It was seen that gas permeation coefficients for the copolyimides slightly deviated from the predicted values. In contrast, the logarithm of gas permeation coefficients and the reciprocal of fractional free volume (1/FFV) exhibited a better correlation.

Experimental Investigation and Monte Carlo Simulation of Glass Transition in Polymer Nanocomposites

The glass transition temperature (Tg) of Fluoro-Poly(ether-imide) and Fluoro-Poly(either-Imide)/MMY clay Nanocomposites has been investigated by both experiments and Monte Carlo Simulation. It was observed that the (Tg) values of the nanocomposites increases with increase in clay contents.

Investigation into the Effects of Transformer Oil on Fluoro Poly(ether imide)s and their Nanocomposites Films

In the present work, the fluoro polyimide films were aged in the transformer oil and the characteristic changes in the film and the transformer oil are analyzed through physico-chemical diagnostic studies. The WAXD and FTIR studies showed that no characteristic change in the material due to thermal aging of the material. The FTIR spectra of the corona aged specimen indicate the formation of new functional groups. The fluorescence spectra of the aged transformer oil suggest dissolvement of heavy ions in the clay material.

Synthesis, Fabrication, Characterization, Properties and Thermo-Oxidative Stability Study of Poly(ether imide)/MMT Clay Nanocomposites

A series of partially fluorinated poly(ether imide) (6F-PEI) and (6F-PEI)/Organosoluble MMT clay nanocomposite were synthesized. XRD data, indirectly confirmed the exfoliation of diamine modified MMT clay at molecular level in the nanocomposite. Thermal properties, and longterm thermo-oxidative stability, thermal degradation kinetics, moisture absorption, chemical resistance of (6F-PEI)s, and nanocomposite films were studied and reported in this paper.

Recent Developments In Fluoro-containing Polyamideimides

The synthesis, characterization and development of a new generation of fluoro-containing polyamideimide (PAI) polymers are described in this paper. The polymers are generally prepared by forming the polycondensation product of an aromatic diamine, a trifunctional aromatic anhydride acid chloride and an aromatic dianhydride containing trifluoromethyl moieties. These new materials possess high glass transition temperatures, useful mechanical properties and outstanding thermoplastic flow behavior which render them readily melt processable into fibers, films, sheets and other molded articles. The as-precipitated fluoro-containing PAI materials are soluble in many organic solvents and are thus amenable to solution casting techniques. They also show excellent resistance towards thermooxidative degradation at temperatures to 450°F and have low moisture uptake. In addition, compatible blends with polybenzimidazole (PBI) resin have yield synergistic effects on the mechanical properties of the resulting polymer.