6FDA-based Polyimides Research Articles

Effects of Supercritical CO2Conditioning on Un-Cross-Linked Polyimide Membranes for Natural Gas Purification

The effects of supercritical CO2 (scCO2) conditioning are examined through changes in gas permeability and solubility for 6FDA-based polyimide membranes. At pressures above the supercritical point, both CO2 permeability and sorption unexpectedly decline, suggesting a structural rearrangement of the polymer facilitated by the presence of a highly soluble species. High-pressure permeability isotherms for He and N2 do not exhibit a similar decline. Upon depressurization, a typical hysteresis is not observed; rather, CO2 permeability remains at reduced levels compared to the original pressurization. Permeability isotherms of the conditioned polymer show a reduction in transport compared to unconditioned samples; however, sorption isotherms show an increase in penetrant solubility following scCO2 conditioning. Pre- and post-CO2/CH4 mixed gas permeation testing at 35 °C shows a reduction in membrane permeability with a corresponding increase in the separation factor after scCO2 conditioning.

Relationship between gas transport properties and fractional free volume determined from dielectric constant in polyimide films containing the hexafluoroisopropylidene group

AbstractThe dielectric constant and gas transport properties (i.e., permeability, diffusivity, and solubility) in 2,2′‐bis(3,4‐dicarboxyphenyl)hexafluoropropane dianhydride (6FDA)‐based polyimides were systematically investigated in terms of their polymer fractional free volumes (FFVs) at 30°C. The permeability and diffusion coefficients of the 6FDA‐based polyimide films to hydrogen, oxygen, nitrogen, methane, and carbon dioxide were correlated with their FFVs estimated using van Krevelen's group contribution method. There appeared, however, small linear correlation coefficients. Linear correlations were also observed between the gas transport properties and dielectric constant of these polyimides. This study described FFVas a function of the dielectric constant based on the Clausius‐Mossotti equation. It was found that the gas permeability and diffusion coefficients of these 6FDA‐based polyimide films increased as their dielectric constant‐based FFV increased. A better linear relationship was observed between the gas transport properties and the FFV determined from the polymer dielectric constant in comparison to that estimated using the group contribution method. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008

Effects of CO 2 exposure and physical aging on the gas permeability of thin 6FDA-based polyimide membranes : Part 2. with crosslinking

The changes in permeability of N 2, O 2, He, CH 4 and CO 2 of thin films (∼300 nm) of crosslinked and uncrosslinked 6FDA-based polyimides with DABA units in the chain were monitored as a function of aging time at 35 °C. The crosslinked polyimides have significantly lower permeability than the corresponding uncrosslinked material. Over about 2000 h of aging, the oxygen permeability decreased by two- to three-fold depending on the polymer structure. The changes in density for the crosslinked and uncrosslinked polyimides with aging time were calculated from refractive index measurements made by ellipsometry using the Lorentz–Lorenz equation. An excellent correlation between the volumetric aging rate and the permeability reduction rate is observed for these polyimides and other glassy polymers. On a relative basis, it seems that the crosslinked polyimide films show slightly higher or about the same aging rates than uncrosslinked polyimide films. Periodic exposure to CO 2 has a significant effect on the permeability and selectivity and how they change with physical aging for the uncrosslinked polyimides but the effects for crosslinked films are much less. Thus, crosslinking appears to be a viable strategy for mitigating the plasticization or conditioning effects of CO 2 exposure.

Effects of CO 2 exposure and physical aging on the gas permeability of thin 6FDA-based polyimide membranes : Part 1. Without crosslinking

The changes in permeability of He, O 2, N 2, CH 4 and CO 2 through thin films (∼350 nm) of 6FDA-based polyimides with and without DABA units in the chain were monitored as a function of aging time at 35 °C using different methodologies that varied the exposure of these films to CO 2. A pulse of CO 2 exposure after aging causes the permeability of each gas to increase; the change is greater the larger the probe gas molecule. Continuous exposure to CO 2 over 100 h leads to a larger increase in permeability than periodic CO 2 exposure over the same period, but the trends are basically similar for the two cases. After a CO 2 pulse, the permeability relaxes and approaches the aging response established prior to CO 2 exposure. The change in refractive index for CO 2 exposed films is consistent with the change in gas permeability in most regards; however, the recovery of refractive index following CO 2 exposure is somewhat less than observed by permeability. Periodic exposure to CO 2 during aging seems to retard the decline in gas permeability with the effect being greater for larger probe molecules. An increase in CO 2/CH 4 and O 2/N 2 selectivities with aging was observed for thin films that were never exposed to CO 2 while these selectivities decrease with aging when periodic exposure to CO 2 was imposed.

Physical aging of thin 6FDA-based polyimide membranes containing carboxyl acid groups. Part II. Optical properties

The change in refractive index with time for thin films (∼350 nm) formed from glassy 6FDA-based polyimides was monitored by ellipsometry to quantitatively track the physical aging process. The refractive index increased linearly, attributed to the densification of the glassy polyimide, with respect to aging time, on a logarithmic scale; this result is consistent with the decrease in gas permeability during physical aging reported in part I of this series. An excellent correlation was formed between the volumetric aging rate r, computed from the refractive index change by the Lorentz–Lorenz equation, and the permeability reduction rate, − ( ∂ log P / ∂ log t ) ; this relationship depends on the type of gas but appears to be the same for all polymer structures examined. The change in fractional free volume was examined from the refractive index data using parameters determined by group contribution methods. The free volume versus aging time results are well-described by the self-retarding relaxation model of Struik; however, this model does not explain the strong effect of thickness on aging rate. The change in free volume correlates well with the change in gas permeability of these thin films.

Physical aging of thin 6FDA-based polyimide membranes containing carboxyl acid groups. Part I. Transport properties

The effect of molecular structure on the kinetics of physical aging of thin films (∼350 nm) formed from glassy 6FDA-based polyimides was investigated by tracking the changes in gas permeability (He, O 2 and N 2) at 35 °C for more than 2000 h. The structures studied included homopolymers of 6FDA with 6FpDA and with DAM plus copolymers where a portion of the latter two monomers was replaced with DABA to introduce carboxyl units into the structure for subsequent cross-linking studies. Over this period of aging, the oxygen permeability decreased by a factor of two for the polyimide containing the diamine 6FpDA and by a factor of five for the polyimide containing the diamine DAM. Introduction of DABA units accelerated the aging in the case of 6FpDA and slowed the aging in the case of DAM. Aging rate seems to correlate with the level of free volume of the polymer; the higher the free volume, the faster is the aging. Selectivity for all gas pairs increased upon aging but the rate is not simply explained by free volume alone because the difference in size of the two gas molecules is also reflected in the response to physical aging observed.

Formation of a 6FDA-based ring polyimide with nanoscale cavity evaluated by DFT calculations

The computer-aided molecular design of a rigid ring molecule has been performed. As a candidate molecule, the polyimide derived from 2,2-bis(3,4-carboxylphenyl) hexafluoropropane dianhydride (6FDA) with m-phenylenediamine (MDA) has been used. The optimized structures of the 6FDA–MDA model compounds including a precursor type amic acid model were investigated using the density functional theory (DFT) at the B3LYP/6-311G(d,p) level. Using the optimized structures of the model compounds, the probable combinations to form a flat ring polyimide are considered by taking the spatial angles between the respective aromatic groups into consideration. We selected several combinations with different conformations and the number of monomer units. We showed that the dimer, trimer and tetramer of not only the 6FDA-based ring imide but also the corresponding ring amic acid can have a stable geometry. Each of them contains a cavity of sub-nanometer size and characteristic shape. Among them, the interaction energy with some guest molecules are evaluated for the smallest ring imide constructed from two units of 6FDA–MDA using the DFT calculations.

Gas separation properties of 6FDA-based polyimide membranes with a polar group

6FDA-based polyimides were prepared from the thermal imidization reaction of 6FDA with diamines of BAPAF, DAP, and DABA having a polar group of hydroxyl or carboxyl. Properties of the dense polyimide membranes were characterized and their gas permeation properties for H2, CO2, O2, N2, and CH4 were investigated. Permeabilities, diffusion coefficients and diffusivity selectivities of polar group-containing polyimide membranes including 6FDA-BAPAF, 6FDA-DAP, and 6FDA-DABA polymer for the gases did not change largely. The separation properties of 6FDA-TrMPD polyimide membrane used as a reference polymer were compared with those of the polyimide membranes mentioned above. It was found that the polyimides of 6FDA-BAPAF, 6FDA-DAP, and 6FDA-DABA, which were soluble in alcohol or/and 2-methoxyethanol, could be applicable to the preparation of a dense composite membrane by dip-coating method.

Solid-State Covalent Cross-Linking of Polyimide Membranes for Carbon Dioxide Plasticization Reduction

Solid-state covalent cross-linking of 6FDA-based polyimides with esterification reactions is shown to be effective in stabilizing membranes against CO2 plasticization up to 40 atm feed pressure. The selection of cross-linking agent has a major impact on the cross-linking degree and the gas transport properties. A generalized cross-linking strategy is presented that enables analysis of the effects of the cross-linking agent structure and thermal treatment on pure gas permeation and sorption for CO2 and CH4 at 35 °C. The polyimide 6FDA-DAM:DABA 2:1 was cross-linked with ethylene glycol, 1,4-butylene glycol, 1,4-cyclohexanedimethanol, and 1,4-benzenedimethanol to illustrate these effects. The cross-linking degree is evaluated by a combination of solution 1H NMR and solid-state IR spectroscopy. The annealing temperature is a significant factor in determining the membrane transport properties, since it affects the polymer chain rigidity and free volume distribution. It is believed that this cross-linking appro...

CO 2 separation performances of composite membranes of 6FDA-based polyimides with a polar group

2,2-bis(3,4-Dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)–based polyimides with a polar group of hydroxyl or carboxyl such as 6FDA–BAPAF, 6FDA–DAP, and 6FDA–DABA were synthesized by the thermal imidization method. The corresponding composite membranes were then prepared by the dip-coating technique using a poly (ether sulfone) (PES) membrane as a supporting layer. Some alcohols and glycol ethers were used as coating solvents during the membrane preparation. The solubility of the polyimides synthesized in this study in these solvents depended on the nature of polymers and solvents. CO 2 permeances for these composite membranes were measured in comparison with those for other gases such as H 2, O 2, N 2, and CH 4. The membrane performances were affected considerably by the preparation conditions such as the kinds of diamine moiety, coating solvent, and coating polymer concentration. It was expected that these composite membranes could be applied usefully to the CO 2 separation, considering the CO 2 permeances in the range of 20–38 gas permeation unit (GPU) and the selectivities for CO 2/N 2 and CO 2/CH 4 equal to or even higher than those of other dense or asymmetric membranes of 6FDA-based polyimides reported in the literature.

Membrane-Based Gas Separation of Ethylene/Ethylene Oxide Mixtures for Product Enrichment in Microreactor Technology

For the off-gas product separation in an ethylene oxide microreactor, new concepts need to be developed. Instead of using the technical process of low-temperature distillation, the performance of gas separation through a dense polymeric membrane was investigated. Different noncrosslinked and crosslinked 6FDA-based polyimides and copolyimides (6FDA=4,4'-hexafluoroisopropylidene diphthalic anhydride) were screened as potential membrane materials. The polymer structure of the membrane material was found to influence the solubility of ethylene oxide and ethylene. The introduction of polar substituents in the polymer backbone increases the solubility selectivity drastically. For permeation experiments in the same pressure range, a novel setup was developed using microstructured membrane supports. With this device, different polymer membranes were tested with respect to the pure gas permeability of ethylene oxide and ethylene as a function of the feed pressure, so that plasticisation effects could be discussed. For different polyimides and copolyimides, ideal ethylene oxide/ethylene selectivities between 3 and 4.5 were found combined with permeabilities between 1.8 and 80.8 barrer.

Interpretation of d-spacing determined by wide angle X-ray scattering in 6FDA-based polyimide by molecular modeling

Interpretation of d-spacing determined by wide angle X-ray scattering (WAXS) is of great interest in the study of gas diffusivity or permeability in glassy polymers. Studies of WAXS analysis and of molecular modeling were carried out as a new approach in which to interpret the d-spacings in polyimide synthesized from 2,2-bis (3,4-carboxyphenyl) hexafluoropropane dianhydride (6FDA). The tetramer conformation and the amorphous polymer structure at 25°C were modeled using the semi-empirical molecular orbital method and the molecular dynamics method. The corresponding theoretical X-ray scattering functions were calculated from the atomic coordinates using Debye’s equation and then compared with the experimental data. The amorphous polymer model was found to be superior to the tetramer conformation model in the predicting experimental X-ray scattering function. It was found that the d-spacing for 6FDA-BAAF polyimide was clearly affected by intramolecular distances containing F atoms.

Relationships between the chemical structures and the solubility, diffusivity, and permselectivity of propylene and propane in 6FDA-based polyimides

The solubility, diffusivity, and permselectivity of propylene and propane in 40 different polyimides synthesized from 2,2-bis(3,4-decarboxyphenyl)hexafluoropropane dianhydride (6FDA) were determined at 298 K. The influence of the chemical structures on the physical and gas permeation properties of the 6FDA-based polyimides was studied. The solubility of propylene in an unrelaxed volume of a polymer matrix mainly contributes to the total solubility of propylene for various 6FDA-based polyimides. The diffusivity, the permeability of propylene, and the permselectivity in the propylene/propane mixed-gas system depend on the solubility of propylene. This is thought to be associated with the penetrant-induced plasticization effect. 6FDA-based polyimides, which have a high glass-transition temperature and a large fractional free volume, exhibit a high permeability with a relatively low permselectivity. Changing the number of-CH3 substituents in the phenylene linkage and changing the connectivity in the main chain are good ways of controlling the solubility of propylene and the corresponding permselectivity in the propylene/propane mixed-gas system. Some 6FDA-based polyimides restrict the solubility of propylene through the introduction of a -CONH- linkage between the phenylene linkage; the -Cl substituent in the phenylene linkage at the diamine moiety exhibits a high separation performance in the mixed-gas system. The polyimides are potentially useful membrane materials for the separation of propylene and propane in the petrochemical industry.

Relationships between chemical structures and solubility, diffusivity, and permselectivity of 1,3‐butadiene and n‐butane in 6FDA‐based polyimides

The solubility, diffusivity, and permselectivity of 1,3-butadiene and n-butane in seven different polyimides synthesized from 2,2-bis (3,4-carboxyphenyl) hexafluoropropane dianhydride (6FDA) were determined at 298 K. The influence of chemical structures on physical and gas permeation properties of 6FDA-based polyimides was studied. Solubility of 1,3-butadiene in 6FDA-based polyimides can be described by a dual-mode sorption model. 1,3-Butadiene-induced plasticization is considered to be associated with the increasing permeabilities of 1,3-butadiene and n-butane and the decreasing permselectivity of 1,3-butadiene vs. n-butane in the mixed gas system containing a high concentration of 1,3-butadiene. It was found that controlling the solubility of 1,3-butadiene in an unrelaxed volume in 6FDA-based polyimides is very important to maintain the high permselectivity of 1,3-butadiene vs. n-butane in the mixed gas system. Changing the C(CF3)2 linkage to a CH2, O linkage, removing methyl substituents at the ortho position of the imide linkage, and changing the p-phenylene linkage to an m-phenylene linkage in the main chains in some 6FDA-based polyimides are effective to decrease fractional free volume and restrict the solubility of 1,3-butadiene in the unrelaxed volume of a polymer matrix. The 6FDA-based polyimides restricting the solubility of 1,3-butadiene in an unrelaxed volume exhibit high separation performance in the 1,3-butadiene/n-butane mixed gas system compared with conventional glassy polymers and, therefore, are potentially useful membrane materials for the separation of 1,3-butadiene and n-butane in the petrochemical industry. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2941–2949, 1999

Relationships between chemical structures and solubility, diffusivity, and permselectivity of 1,3-butadiene andn-butane in 6FDA-based polyimides

The solubility, diffusivity, and permselectivity of 1,3-butadiene and n-butane in seven different polyimides synthesized from 2,2-bis (3,4-carboxyphenyl) hexafluoropropane dianhydride (6FDA) were determined at 298 K. The influence of chemical structures on physical and gas permeation properties of 6FDA-based polyimides was studied. Solubility of 1,3-butadiene in 6FDA-based polyimides can be described by a dual-mode sorption model. 1,3-Butadiene-induced plasticization is considered to be associated with the increasing permeabilities of 1,3-butadiene and n-butane and the decreasing permselectivity of 1,3-butadiene vs. n-butane in the mixed gas system containing a high concentration of 1,3-butadiene. It was found that controlling the solubility of 1,3-butadiene in an unrelaxed volume in 6FDA-based polyimides is very important to maintain the high permselectivity of 1,3-butadiene vs. n-butane in the mixed gas system. Changing the-C(CF 3 ) 2 - linkage to a -CH 2 -, -O- linkage, removing methyl substituents at the ortho position of the imide linkage, and changing the p-phenylene linkage to an m-phenylene linkage in the main chains in some 6FDA-based polyimides are effective to decrease fractional free volume and restrict the solubility of 1,3-butadiene in the unrelaxed volume of a polymer matrix. The 6FDA-based polyimides restricting the solubility of 1,3-butadiene in an unrelaxed volume exhibit high separation performance in the 1,3-butadiene/n-butane mixed gas system compared with conventional glassy polymers and, therefore, are potentially useful membrane materials for the separation of 1,3-butadiene and n-butane in the petrochemical industry.

Synthesis and Properties of Processable Polyimides Containing Diacetylene Groups

Diacetylene-containing polyimides have been prepared by the oxidative coupling of new diethynyl diimide monomers. Condensation of various ethynyl-terminated amines with either 4,4‘-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) or pyromellitic dianhydride (PMDA) provided the monomers. Processability was enhanced by increasing the number of consecutive methylenes in the backbone, by incorporating aliphatic side chains, and for the 6FDA-based polyimides in general. The 6FDA-based diacetylene-containing polyimides are soluble in many common solvents including acetone. By controlling polymerization time, either oligomers or polyimides can be produced in high yield. High-resolution 1H NMR and IR spectroscopy were used to monitor the polymerization. With the exception of the polyimides containing aliphatic side groups, thermal analysis reveals a diacetylene reaction exotherm that occurs without evolution of volatile byproducts; upon thermal annealing, the exotherm disappears and these polyimides become insoluble in all common solvents (i.e., cross-linked). Gas-transport properties are reported for one of the cross-linked 6FDA-based materials.

Gas transport properties of soluble poly(phenylene sulfone imide)s

Gas transport of helium, hydrogen, carbon dioxide, oxygen, argon, nitrogen, and methane in three soluble poly(phenylene sulfone imide)s based on 2,2-bis(3,4- decarboxyphenyl) hexafluoropropane dianhydride (6FDA) has been investigated. The effects of increasing length of well-defined oligo(phenylene sulfone) units on the gas permeabilities and diffusivities were determined and correlated with chain packing of the polymers. Activation energies of diffusion and permeation were calculated from temperature-dependent time-lag measurements. The influences of the central group in the diamine moiety of 6FDA-based polyimides on physical and gas transport properties are discussed. The incorporation of a long oligo(phenylene sulfone) segment in the polymer backbone decreases gas permeability and permselectivity simultaneously. The decreases in permeability coefficients can be mainly related to decreases in diffusion coefficients. Changing the central group of diamine moiety from {S{ to {SO2{ leads to a 45-50% decrease in CO2 and O2 permeabilities without appreciable increase in the selectivities. This is considered to be due to the formation of charge transfer complexes. q 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1855-1868, 1997

Preparation of 6FDA-based polyimide composite membrane by CVDP process

Preparation of polyimide composite membranes by a chemical vapor deposition and polymerization process (CVDP) was studied for the development of pervaporation or gas separation membranes. Hexafluoroisopropylidene-2,2-bis [phethalic acid anhydride] (6F-dianhydride) (6FDA) and 4,4′-diaminodiphenyl ether (ODA) were used as monomers and were simultaneously deposited on an asymmetric polyimide membrane in vacuum of 10 −6 Torr. The deposited layer of the monomers was converted to polyimide by heating at 250°C at 3 h in an ordinary vacuum oven, and its separation performance for water-ethanol and CO 2−N 2 systems was characterized.

Permeability and permselectivity of gases in fluorinated and non-fluorinated polyimides

Solubility and permeability coefficients for gases in a series of fluorinated and non-fluorinated polyimides were measured to investigate the effects of chemical structure on permeability and permselectivity. Polyimides with higher concentration of fluorine had lower chain packing density and higher permeability. Polyimides with C(CF 3) 2 linkages had higher permeability with relatively higher permselectivity. 2,2-Bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride/2,2-bis(4-aminophenyl)hexafluoropropane (6FDA-BAHF) polyimide with two C(CF 3) 2 linkages in the polymer repeat unit displayed excellent performance for CO 2 CH 4 and O 2 N 2 separations; its permeabilities for O 2 and CO 2 were 3.7 and 3.1 times, respectively, larger than those of 6FDA- pp′ODA (4,4′-oxydianiline), a conventional 6FDA-based polyimide, while the permselectivities for O 2 N 2 and CO 2 CH 4 were 13% and 22%, respectively, smaller. On the other hand, polyimides having two C(CF 3) 2 linkages together with two ether ones in the repeat unit had lower permeability with a similar level of permselectivity as compared with 6FDA-BAHF polyimide. For H 2 CH 4 and H 2 CO separations, polyimides prepared from 6FDA and meta-oriented non-fluorinated diamines were preferable. These separation properties were discussed in connection with packing density and local segmental mobility.