Perfluoro Polymers Research Articles

Effect of Perfluoropolymers on the Anti-Wear Properties of Carbon Fiber/Polyphenylene Sulfide Composites: A Comparative Study

In this work, several perfluoropolymers (PFP), including commercial polytetrafluoroethylene (PTFE), perfluorinated ethylene-propylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), and irradiated PTFE (iPTFE) were used as additives to lubricate carbon fiber (CF)-reinforced polyphenylene sulfide (PPS) composites. The tribological properties of the yielding composites were studied and correlated with the melt processability of PFPs. Although the neat FEP and PFA have higher friction coefficients when compared with neat PTFE, the composites filled with FEP and PFA additives were found to exhibit a lower friction coefficient compared to PTFE at PFP content below 10 wt %. Moreover, the iPTFE-filled composites also showed similar results as FEP or PFA filled ones, very different from PTFE at low additions. Based on the morphological investigation, we postulate that FEP, PFA, and iPTFE are melt-kneaded with PPS due to their melt processability at processing temperature, leading to the good dispersion in composites in the form of smaller deformed spheres and/or fibril bands. The well-dispersion of PFPs in composites promotes the formation and growth of the transfer film on the counterface during sliding.

Review: membrane materials for the removal of water from industrial solvents by pervaporation and vapor permeation

Organic solvents are widely used in a variety of industrial sectors. Reclaiming and reusing the solvents may be the most economically and environmentally beneficial option for managing spent solvents. Purifying the solvents to meet reuse specifications can be challenging. For hydrophilic solvents, water must be removed prior to reuse, yet many hydrophilic solvents form hard-to-separate azeotropic mixtures with water. Such mixtures make separation processes energy intensive and cause economic challenges. The membrane processes pervaporation (PV) and vapor permeation (VP) can be less energy intensive than distillation-based processes and have proven to be very effective in removing water from azeotropic mixtures. In PV/VP, separation is based on the solution-diffusion interaction between the dense permselective layer of the membrane and the solvent/water mixture. This review provides a state-of-the-science analysis of materials used as the selective layer(s) of PV/VP membranes in removing water from organic solvents. A variety of membrane materials, such as polymeric, inorganic, mixed matrix, and hybrid, have been reported in the literature. A small subset of these are commercially available and highlighted here: poly(vinyl alcohol), polyimides, amorphous perfluoro polymers, NaA zeolites, chabazite zeolites, T-type zeolites, and hybrid silicas. The typical performance characteristics and operating limits of these membranes are discussed. Solvents targeted by the U.S. Environmental Protection Agency for reclamation are emphasized and ten common solvents are chosen for analysis: acetonitrile, 1-butanol, N,N-dimethyl formamide, ethanol, methanol, methyl isobutyl ketone, methyl tert-butyl ether, tetrahydrofuran, acetone, and 2-propanol.

New amorphous perfluoro polymers: perfluorodioxolane polymers for use as plastic optical fibers and gas separation membranes

To address the need for perfluoro polymers with higher Tg, we have prepared and characterized various perfluorodioxolane monomers via direct fluorination of the hydrocarbon precursors. These monomers were readily polymerized in bulk or in solution initiated by perfluorodibenzoyl peroxide. The polymers obtained have relatively high Tg(~160°C) and exhibited low material dispersion. These polymers are completely amorphous and soluble in fluorinated solvents. The polymers are also chemically and thermally stable (Tg > 300°C). Thus, these perfluorodioxolane polymers may be used as plastic optical fiber material where high Tg is required, such as in automobile and aircraft application. These perfluorodioxolane polymers were also investigated for use as gas separation membrane. Among these polymers, the copolymer of perfluoro (2‐methylene‐1,3‐dioxolane) and perfluoro (2‐methylene‐4,5‐dimethyl dioxolane) showed superior gas separation performance compared with the commercial perfluoro polymers for a number of gas pair, including CO2/CH4, He/CH4, H2/CH4, and N2/CH4. Copyright © 2015 John Wiley & Sons, Ltd.

Advancements in Anion Exchange Membrane Cations

Anion-exchange membrane fuel cells (AME-FCs) are of increasingly popular interest as they enable the use of non-Pt fuel cell catalysts, the primary cost limitation of proton exchange membrane fuel cells. Benzyltrimethyl ammonium (BTMA) is the standard cation that has historically been utilized as the hydroxide conductor in AEMs. Herein we approach AEMs from two directions. First and foremost we study the stability of several different cations in a hydroxide solution at elevated temperatures. We specifically targeted BTMA and methoxy and nitro substituted BTMA. We've also studied the effects of adding an akyl spacer units between the ammonium cation and the phenyl group. In the second approach we use computational studies to predict stable ammonium cations, which are then synthesized and tested for stability. Our unique method to study cation stability in caustic conditions at elevated temperatures utilizes Teflon Parr reactors suitable for use under various temperatures and cation concentrations. NMR analysis was used to determine remaining cation concentrations at specific time points with GCMS analysis verifying product distribution. We then compare the experimental results with calculated modeling stabilities. Our studies show that the electron donating methoxy groups slightly increase stability (compared to that of BTMA), while the electron withdrawing nitro groups greatly decrease stability in base. These results give insight into possible linking strategies to be employed when tethering a BTMA like ammonium cation to a polymeric backbone; thus synthesizing an anion exchange membrane. Utilizing various strategies, several different cations were tethered to perfluoro polymers to produce membranes. These membranes are then ion exchanged to obtain hydroxide conductivities. In addition to conductivities, membrane stability lifetimes were also measured using a similar technique to cation stabilities. Figure 1

Concentration of aqueous hydrogen peroxide solution by pervaporation

Pervaporative separation of corrosive liquids is always challenging in terms of finding a suitable polymeric membrane that can withstand the harsh environment especially when concentrating an aqueous solution of a highly oxidizing liquid such as hydrogen peroxide (H 2O 2). H 2O 2 is one of the most powerful oxidizers known and is a stronger oxidant than chlorine, chlorine dioxide and potassium permanganate; hydroxyl radical ( OH) generated from H 2O 2 has a very high reactivity. The choice of membrane material is potentially limited to fluoro polymers and perfluoro polymers considering membrane stability in such a harsh system. Perfluorodimethyldioxole–tetrafluoroethylene (PDD–TFE) copolymer membranes (CMS-3 and CMS-7) were used to concentrate hydrogen peroxide from its aqueous solutions. The feed solution composition was varied between 4 and ∼40 wt% H 2O 2; the process was studied at 25 °C, 30 °C and 35 °C. CMS-7 displayed a higher flux than CMS-3; a reverse trend was observed for water–H 2O 2 selectivity for the same feed concentration. The highest water–H 2O 2 selectivity of ∼12 was observed in CMS-3 membrane at an H 2O 2 concentration of 43 wt% with a total flux of 6.15 × 10 −3 g/h cm 2; for CMS-7, values of 9.2 and 9.6 × 10 −3 g/h cm 2 were found for water–H 2O 2 selectivity and total flux, respectively, at the same H 2O 2 concentration. The total permeated flux increased with temperature at the cost of selectivity for both membranes. Extended-term behaviors of CMS-3 and CMS-7 membranes studied at 35 wt% H 2O 2 concentration indicated that both are quite stable for tests carried out over periods of 162 and 145 days respectively.

Effect of passivation on the sensitivity and stability of pentacene transistor sensors in aqueous media

Charge-detecting biosensors have recently become the focal point of biosensor research, especially research onto organic thin-film transistors (OTFTs), which combine compactness, a low cost, and fast and label-free detection to realize simple and stable in vivo diagnostic systems. We fabricated organic pentacene-based bottom-contact thin-film transistors with an ultra-thin insulating layer of a cyclized perfluoro polymer called CYTOP (Asahi Glass Co., Tokyo, Japan) on SiO 2 for operation in aqueous media. The stability and sensitivity of these transistor sensors were examined in aqueous buffer media with solutions of variable pH levels after the passivation of perfluoro polymers with thicknesses ranging from 50 to 300 nm. These transistor sensors were further modified with an ultra-thin film (5 nm) functional layer for selective BSA/antiBSA detection in aqueous buffer media, demonstrating a detection capability as low as 500 nM of concentrated antiBSA. The dissociation constant from the antiBSA detection results was 2.1 × 10 −6 M. Thus, this study represents a significant step forward in the development of organic electronics for a disposable and versatile chemical and bio-sensing platform.

Synthesis of Benzimidazole Tethered Perfluoropolymer/Perfluoromacromolecules for High Temperature Fuel Cell Applications

Fuel cells running at high temperatures have the advantage of higher energy efficiency, simpler water management and enhanced CO tolerance etc. However, Nafion® loses proton conductivity at high temperatures (ca. >100oC) due to water evaporation. Heterocyclic structures such as imidazole (Im) and benzimidazole (BIm) groups can replace water as proton carriers at high temperatures. Polybenzimidazole and Im, or BIm tethered hydrocarbon polymer are promising candidates for high temperature membranes. However, most of these polymers have very rigid backbone structure, which limits the application of these materials as ionomers in the electrodes as it is very hard for the rigid polymer chains to reorganize into efficient H+/e/gas conducting active sites. In this study, we synthesized benzimidazole (BIm) tethered perfluoropolymers (PF) or perfluoromacromolecules to be used as ionomers for electrode binder. The synthesized BIm-PF and Im-PF have PF flexible chain structure of both the backbones and side chains. The solubility of these BIm-tethered perfluoromaterials is very limited.

Ethanol Dehydration Using Hydrophobic and Hydrophilic Polymer Membranes

This paper describes the development of membranes based on perfluoro polymers for the separation of aqueous ethanol mixtures in pervaporation or vapor permeation mode. Hydrophobic perfluoro polymers were selected because their chemical and thermal stability allows them to be used at temperatures up to 130 °C in hot ethanol/water vapors. The permeance and selectivity of membranes made from these polymers are quite different from the properties of the cross-linked hydrophilic membranes that are commonly used to separate water/ethanol mixtures. Perfluoro polymers absorb less than 1% liquid in mixtures ranging from pure water to pure ethanol. As a result, the water permeance and water/ethanol selectivity of the membranes are essentially independent of feed water/ethanol composition. However, the water permeances of perfluoro membranes are low for commercial applications. Multilayer composite membranes, consisting of a perfluoro protective layer and a selective hydrophilic polymer underlayer, have the stability of hydrophobic perfluoro membranes combined with the high permeances and good selectivities of hydrophilic membranes.

Comparative Study of Different Probing Techniques for the Analysis of the Free Volume Distribution in Amorphous Glassy Perfluoropolymers

A comparative study of five different experimental and computational methods is presented for the characterization of the overall free volume (FV) and the free volume element (FVE) size and shape distribution in amorphous glassy perfluoropolymers (PFPs). Experimental results from the photochromic probe (PCP) method, positron annihilation lifetime spectroscopy (PALS), and inverse gas chromatography (IGC) were confronted with literature data from 129Xe NMR spectroscopy, and the experimental data were further compared with molecular dynamics (MD) simulations and a combination of MD studies and the well-known Bondi method as well as a modified Bondi method. An evaluation of the advantages and the limits of each method is presented. This is the first reported study on a so vast number of complementary techniques applied on a single glassy polymer, in this case Hyflon AD perfluoropolymer, and is also the first successful application of the photochromic probe technique in such materials. In two different grades ...

An electron spin resonance study of the particles produced in the pyrolysis of perfluoro polymers

Electron spin resonance (ESR) analysis at room temperature of the particles produced during the aerobic pyrolysis of perfluoro polymers (PFP) shows the presence of end-chain peroxyl radicals. These radicals, which would normally have lifetimes of several seconds at most, are stabilized by being immobilized in the particles and decay at a rate of about 20%/day. Normally, radicals with this stability would not be expected to be reactive; however, these peroxyl radicals react with 3-chloropropene, with iodine in benzene, with methyl linoleate in methanol, and with aqueous liposomes from soy phosphatidylcholine. Also, stable radicals of this sort would not be expected to give spin adducts; however, when the particles are suspended in a benzene solution containing alpha-phenyl-N-tert-butylnitrone (PBN), they react to give the same series of spin adducts that are detected when the unfiltered smoke from the oxidative pyrolysis of perfluoro polymers is bubbled directly into PBN solutions. This appears to be the first report of the reaction of radicals entrapped in a solid with a spin trap. The nitroxide species produced by the PBN-particle reaction include a fluorine atom spin adduct, an oxy radical adduct, and benzoyl tert-butyl nitroxide (PBNOx), the oxidation product of the spin trap; all of these appear to arise from reaction of the particle-bound peroxyl radicals with the spin trap. Because the particles are in the highly respirable range (down to 0.01 micron), these entrapped peroxyl radicals may be carried deep within the lung when fumes from PFP pyrolysis are inhaled and would be expected to place an oxidative burden on the lung.(ABSTRACT TRUNCATED AT 250 WORDS)

The smoke produced from the oxidative pyrolysis of perfluoro polymers: an ESR spin-trapping study

Spin adducts are observed when the unfiltered smoke produced during the aerobic pyrolysis of perfluoro polymers (PFP) is bubbled through a solution of the ESR spin trap alpha-phenyl-N-tert-butylnitrone (PBN). The spin adducts include those from an oxy radical and the fluorine atom, and in addition the spin trap is oxidized to PBNOx. The spin adduct of a chlorine atom also is observed, presumably because of chlorine-containing impurities in the polymers. Tetrafluoroethylene is produced during the pyrolysis of PFP; therefore, we subjected tetrafluoroethylene to the same pyrolysis as was used for PFP, and we observe the same series of spin adducts. We suggest that the spin adducts formed from the whole, unfiltered smoke from PFP smoke may result both from free radicals that form during the pyrolysis of PFP and from the oligomerization of the tetrafluoroethylene produced during the pyrolysis. These spin-trapping results support the theory that reactive free radicals are present in PFP smoke and may contribute to PFP smoke toxicity.

The nature of the initiating centres for grafting in air-irradiated perfluoro polymers

Grafting of acrylic acid on polytetrafluoroethylene has been performed using films of the polymer gamma-irradiated in air and then treated with aqueous acrylic acid. It is concluded that the initiating centres are peroxides POOP′, where P′ is a small perfluorinated fragment, and also trapped peroxy radicals PO 2· which undergo deoxygenation. Homopolymerization is suppressed by polymerization inhibitors.

The thermal degradation of perfluoro polymers

AbstractA number of perfluoro polymers were heated at a constant rate of temperature rise in an atmosphere of nitrogen or air, and the volatile degradation products were passed into an aqueous solution, where the F− ions produced were monitored with a fluoride ion‐selective electrode. Compounds giving rise to F− ions were formed in both nitrogen and air, but in larger amounts in the latter atmosphere. For the formation of appreciable quantities of F− ions, temperatures greater than 500°C were normally necessary.