Responses of 6FDA-based polyimide thin membranes to CO2 exposure and physical aging as monitored by gas permeability

Membranes made from glassy polymers have been of great interest in the past decade for CO₂ removal from natural gas streams; however, strongly soluble gases, such as CO₂, can cause “plasticization” of polymer membranes, which greatly reduces the separation efficiency. This work examines the response of several 6FDA-based polyimides thin film membranes with thicknesses around 200 nm to CO₂ exposure and physical aging. DABA units are incorporated to create crosslinkable sites for such materials. Introducing DABA units to the 6FDA-DAM and 6FDA-mPDA polymers seems to result in materials even more prone to CO₂ plasticization. A unique thermal annealing approach is used to crosslink the polyimides via decarboxylation of the DABA units; the resulting crosslinked polymers appear to be much more plasticization resistant at high CO₂ pressures compared to their DABA containing counterparts prior to crosslinking. Prior thermal history plays a significant role in both the physical aging of the thin film membranes and their CO₂ plasticization resistance particularly for chemical structures that tend to lead to high free volume and permeability.     

Physical aging of thin glassy polymer films monitored by gas permeability

The physical aging at 35 °C of three glassy polymers, polysulfone, a polyimide and poly(2,6-dimethyl-1,4-phenylene oxide), has been tracked by measurement of the permeation of three gases, O₂, N₂, and CH₄, for over 200 days. Several techniques were used to accurately determine the thickness of films (∼400 nm-62 μm) in order to obtain absolute permeability coefficients and to study the effects of film thickness on the rate of physical aging. Each film was heated above the polymer T_g to set the aging clock to time zero; ellipsometry revealed that this procedure leads to isotropic films having initial characteristics independent of film thickness. A substantial pronounced aging response, attributed to a decrease in polymer free volume, was observed at temperatures more than 150 °C below T_g for thin films of each polymer compared to what is observed for the bulk polymers. The films with thicknesses of approximately 400 nm of the three polymers exhibit an oxygen permeability decrease by as much as two-fold or more and about 14-15% increase in O₂/N₂ selectivity at an aging time of 1000 h. The results obtained in this study were compared with prior work on thickness dependent aging. The effects of crystallinity on physical aging were examined briefly.     

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, O₂ and N₂) 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.     

Physical aging of ultrathin glassy polymer films tracked by gas permeability

Membrane-based separations play a key role in energy conservation and reducing greenhouse gas emissions by providing low energy routes for a wide variety of industrially-important separations. For reasons not completely understood, membrane permeability changes with time, due to physical aging, and the rate of permeability change can become orders of magnitude faster in films thinner than one micron. The gas transport properties and physical aging behavior of free-standing glassy polysulfone and Matrimid^® films as thin as 18 nm are presented. Physical aging persists in glassy films approaching the length scale of individual polymer coils. The films studied ranged from 18–550 nm thick. They exhibited reductions in gas permeability, some more than 50%, after 1000 h of aging at 35 °C, and increases in selectivity. The properties of these ultrathin films deviate dramatically from bulk behavior, and the nature of these deviations is consistent with enhanced mobility and reduced T_g in ultrathin films. The Struik physical aging model was extended to account for the influence of film thickness on aging rate, and it was shown to adequately describe the aging data.     

Physical aging of layered glassy polymer films via gas permeability tracking

The physical aging of polymers in confined environments has been an area of intensive study in recent times. The rate of physical aging in thin films of many polymers used in gas separation membranes is dependent on film thickness and accelerated relative to bulk. In this study, the physical aging of polymer films with alternating glassy polysulfone and rubbery polyolefin layers was monitored by measuring the gas permeability of O₂ and N₂ as a function of aging time at 35 °C. The alternating layer structures were formed by a melt co-extrusion process. The polysulfone layers have thicknesses ranging from 185 to 400 nm, and the overall thicknesses of the films are on the order of 80-120 μm. The aging of freestanding thin films of polysulfone is rapid and exhibits clear thickness dependence, whereas the aging of multilayered films was observed to be similar to bulk and showed no dependence on layer thickness. At 1000 h of aging time, a 400 nm freestanding PSF film decreased in O₂ permeability by 35%, whereas on average the bulk and multilayered films only experienced a decline of 10-15%. A slight increase in O₂/N₂ selectivity for the multilayered films was observed over the course of aging.     

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, ; 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.     

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 CO₂−N₂ systems was characterized.     

Carbon dioxide plasticization of thin glassy polymer films

We have demonstrated in previous studies that thin glassy polymer films exhibit complex responses to highly sorbing penetrants, such as CO₂, relative to their thick film counterparts. In this paper, we apply similar experiments to two new polymers, including a polysulfone made from bisphenol A (PSF), and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), and compare their responses to Matrimid^® to understand better CO₂ plasticization behavior of these materials when in thin film form. As expected, the extent of plasticization response tracks with CO₂ solubility; CO₂ diffusivity may also be an important factor at shorter exposure times. Experiments at longer CO₂ exposure times revealed that each polymer experiences the permeability maximum observed in our previous work as well. However, polymers that are not as highly sorbing to CO₂, like polysulfone, may not at some conditions exhibit a distinct permeability maximum but will still decrease in permeability after a long period of CO₂ exposure owing to physical aging.     

Influence of previous history on physical aging in thin glassy polymer films as gas separation membranes

The physical aging behavior of thin glassy polysulfone (PSF) films (∼125 nm) with different previous histories was tracked using gas permeability measurements. The initial states of these materials were modulated by thermal annealing at fixed temperatures below the glass transition or by exposure to high pressure (800 psig (56.2 bara)) CO₂ for various times. Regardless of the previous history, the nature of the aging response in these samples was consistent with the aging behavior of an untreated film that was freshly quenched from above T_g, i.e., permeability decreased and pure gas selectivity increased with aging time. However, the extent of aging-induced changes in transport properties of these materials depended strongly on previous history. The aging behavior was described using Struik’s aging model by allowing the initial conditions to depend on each sample’s previous history.     

Carbon dioxide plasticization and conditioning effects in thick vs. thin glassy polymer films

Recent studies have shown that thin glassy polymer films undergo physical aging more rapidly than thick films. This suggests that thickness may also play a role in the plasticization and conditioning responses of thin glassy films in the presence of highly-sorbing penetrants such as CO₂. In this paper, a carefully designed systematic study explores the effect of thickness on the CO₂ plasticization and conditioning phenomena in Matrimid^®, a polyimide commonly used in commercial gas separation membranes. Thin films are found to be more sensitive than thick films to CO₂ exposure, undergoing more extensive and rapid plasticization at any pressure. The response of glassy polymers films to CO₂ is not only dependent on thickness, but also on aging time, CO₂ pressure, exposure time, and prior history. Finally, thin films experiencing constant CO₂ exposure for longer periods of time exhibit an initial large increase in CO₂ permeability, which eventually reaches a maximum, followed by a significant decrease in permeability for the duration of the experiment. Thick films, in contrast, do not seem to exhibit this trend for the range of conditions explored.     

Intrinsic permeability of refractories from gas permeability measurements: Comparison of results

The gas permeability of porous materials largely depends on pressure, with the intrinsic permeability typically being determined using Klinkenberg's model, which is valid when the gas flows under viscous conditions. However, measurements performed on refractories with the outlet at atmospheric pressure often reveal inertial effects. Therefore, gas permeability is assumed to be flow-rate dependent; an alternative approach is proposed to determine the intrinsic properties using the Forchheimer number. A gas permeameter has been developed to improve measurement accuracy. It allows for the conduction of a test in viscous flow conditions or with inertial effects. The improved gas permeameter was used to compare the different approaches to the evaluation of the intrinsic permeability of four refractory materials with permeabilities ranging from 0.03 to 6 darcies. Combining a modified pressure drop method with Klinkenberg's model proved to be a reliable method for consistently evaluating refractory material permeability.     

Physical aging of polystyrene films tracked by gas permeability

Most studies using gas permeation to characterize physical aging in thin polymer films have focused on polymers of interest as membrane materials, such as polysulfone (PSF) and Matrimid. Many other physical aging studies, using techniques other than gas permeation, focus on polystyrene (PS). In this work, physical aging in bulk PS films and PDMS-coated thin PS films was studied using well-established gas permeation techniques. The ～400 nm PS films aged slightly faster than bulk PS. However, the difference between rates of aging in thin and thick films was much less than that reported in PSF and Matrimid films of similar thicknesses. The ～800 nm films aged in a manner generally similar to bulk PS. Comparison of the normalized oxygen permeability of ～400 nm films of PS, PSF, and Matrimid revealed that a ～400 nm PS film experiences a slower decline in relative permeability than a PSF or Matrimid film does. Unlike what has been observed previously in studies of PSF and Matrimid films, PS films do not appear to show aging behavior that is strongly dependent on film thickness or highly accelerated relative to bulk. Because it would be difficult to use the results of PS aging studies to predict the aging behavior of typical gas separation polymers, we suggest that PS is not a good model for the aging behavior of commercially useful gas separation membrane materials.     

Molecular design of high-performance polyimide membranes for gas separations

The main objective of this study is to acquire a better understanding of the relationships between the chemical structure of polymers and their permeability to different gases. This information is required for the development of new membrane processes for the separation of industrial gases. Based on our previous investigation of structure/permeability relationships, this study provides additional information on gas diffusivity and solubility data in different polyimides. The control factors of this relationship are identified as the structures of diamine and dianhydride, the degree of curing, and the chain morphology and mobilities.This study confirms that both the gas selectivity and permeability of polyimides can be enhanced simultaneously. The desired membrane materials can be achieved by synthesizing polyimides with over 60% aromatic rings and containing bulky functional groups, such as(CF₃)₂, which impede chain rotation and act as spacers to increase intersegmental distances. In addition, the diamine moieties must be short and rigid to produce a high gas selectivity. Polyimide membranes, then, behave as polymeric molecular sieves. This is demonstrated by the fact that the systematic changes in the structure of the polyimides affect mainly the diffusivity rather than thesolubility of the penetrant gases.     

Effect of ultraviolet light irradiation on gas permeability in polyimide membranes. II. Irradiation of membranes with high-pressure mercury lamp

A photocrosslinkable polyimide membrane was prepared and investigated with regard to the effect of ultraviolet light irradiation (UV-irradiation) using a high-pressure mercury lamp on their gas permeabilities and permselectivities. Permeability and diffusion coefficients for O₂, N₂, H₂, and CO₂ were determined using the vacuum-pressure and time-lag methods. Sorption properties for carbon dioxide were determined to evaluate the changes in the free volume in the membranes by the irradiation. The apparent gas permeabilities decreased and permselectivity, particularly for H₂ over N₂, increased with increasing UV-irradiation time without a significant decrease in the flux of H₂. They depended on the membrane thickness, suggesting asymmetrical changes in the membrane due to UV-irradiation. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 49–60, 1998     

Novel poly(diphenylacetylene)s with both alkyl and silyl groups as gas permeable membranes: Synthesis, desilylation, and gas permeability

Diphenylacetylenes having a dimethyloctylsilyl group and an alkyl group at para positions [Me₂n-C₈H₁₇SiC₆H₄CCC₆H₄R; R = H (1a), i-Pr (1b), t-Bu (1c), n-Bu (1d)] and having only an alkyl group [PhCCC₆H₄R; R = i-Pr (1B), t-Bu (1C)] were synthesized and then polymerized with TaCl₅/n-Bu₄Sn catalyst to provide the corresponding poly(diphenylacetylene)s (2a, 2b, 2c, 2d, 2B, and 2C). The formed polymers afforded tough free-standing membranes by casting from toluene solutions. Desilylation reaction of Si-containing membranes (2a-d) was carried out with trifluoroacetic acid to give the desilylated membranes (3a-d). The permeability of these membranes to O₂, N₂, and CO₂ were determined. All the Si-containing membranes exhibited almost the same gas permeability. The desilylation of Si-containing membranes of 2a-c resulted in large increase of gas permeability. No apparent increasing of gas permeability was observed in the desilylation of 2d. To clarify the effects of desilylation, CO₂ diffusivity (D(CO₂)), CO₂ solubility (S(CO₂)), and fractional free volume (FFV) of the polymer membranes were investigated. The S(CO₂) values of desilylated membranes were much larger than that of Si-containing counterparts. The D(CO₂) and FFV of membranes of 2a-c increased through desilylation. The desilylated membrane of 3d had small D(CO₂) value and almost the same FFV compared with 2d. Further, the comparison of the permeability between three types of membranes with the same chemical structure revealed that the microvoids were not generated by the desilylation of membranes of poly(diphenylacetylene)s containing alkyl groups.     

Low temperature TiO2 based gas sensors for CO2

Monitoring the level of CO₂, especially in closed spaces, is more and more required in technological applications, or in human activities. Since most of the literature data reveal CO₂ detection materials with high sensitivities over 300 °C, here we have concentrated on the gas sensing abilities of Cr doped TiO₂ thin films in front of CO₂, close to the room temperature and at atmospheric pressure. The films were obtained by RF reactive sputtering. The undoped films contain a mixture of anatase and rutile phases. With the increase of Cr content, the crystallites size decreases, and the films become pure rutile for a 4 at% Cr concentration. We found out that these material based sensors are more sensitive to CO₂ for higher Cr concentration, the optimum operating temperature approaching to the room temperature, determining in fact low energy consumption. The explanation is related to the observed increase of oxygen vacancies number (which we have evidenced and clarified), and also to the presence of the rutile phase, whose higher dielectric constant (compared to anatase), and its finer crystallites, determine a better gas sensing. More, the surface active area in front of CO₂ increases, as the films become rougher for higher Cr contents. The increase of Cr³⁺ percentage enhance the power of interaction with the adsorbed species (O₂ and/or CO₂). A grain boundary model was proposed for the thermal activation of the electrical conductivity. The energy barrier height at the grain boundary, the impurities concentration (characteristic parameters of this model) were calculated and found to agree well with the data in the literature.     

Synthesis, characterization, and high gas permeability of poly(diarylacetylene)s having fluorenyl groups

Diarylacetylenes having fluorenyl groups and other substituents (trimethylsilyl, t-butyl, bromine, fluorine) (1a-1) were polymerized with TaCl₅-n-Bu₄Sn. Monomers 1a-l produced high molecular weight polymers 2a-l (M_w 5.1 × 10⁵-1.3 × 10⁶) in 12-59% yields. All of the polymers were soluble in common organic solvents, and gave tough free-standing membranes by the solution casting method. The onset temperatures of weight loss of polymers 2a-l in air were over 400 °C, indicating considerably high thermal stability. All the polymer membranes showed high gas permeability; e.g., the oxygen permeability coefficient (PO₂) of 2a was as large as 4800 barrers. Membrane 2d possessing two fluorine atoms at meta and para positions of the phenyl ring showed the highest oxygen permeability (PO₂ = 6600 barrers) among the present polymers.     

Effect of pore structure on gas permeability constants of porous alumina

Porous alumina was fabricated using different particle size, sintering temperature, and particle size and content of poly (methyl-methacrylate) (PMMA) as pore former. The Forchheimer equation was used to investigate the relationship between porosity and average pore size, and obtain the permeability constants k₁ and k₂ (the viscous effect and the inertial effect, respectively). Compared to Darcy's law, the Forchheimer equation established a more realistic and reliable relationship between fluid pressure and fluid velocity. k₁ and k₂ were found to be more sensitive to the average pore size than to the porosity of alumina. Moreover, reliable relationships were confirmed between the average pore size and k₁, k₂, and their ratio (k₁/k₂).     

Gas separation performance of 6FDA-based polyimides with different chemical structures

This work reports the gas separation performance of several 6FDA-based polyimides with different chemical structures, to correlate chemical structure with gas transport properties with a special focus on CO₂ and CH₄ transport and plasticization stability of the polyimides membranes relevant to natural gas purification. The consideration of the other gases (He, O₂ and N₂) provided additional insights regarding effects of backbone structure on detailed penetrant properties. The polyimides studied include 6FDA-DAM, 6FDA-mPDA, 6FDA-DABA, 6FDA-DAM:DABA (3:2), 6FDA-DAM:mPDA (3:2) and 6FDA-mPDA:DABA (3:2). Both pure and binary gas permeation were investigated. The packing density, which is tunable by adjusting monomer type and composition of the various samples, correlated with transport permeability and selectivity. The separation performance of the polyimides for various gas pairs were also plotted for comparison to the upper bound curves, and it was found that this family of materials shows attractive performance. The CO₂ plasticization responses for the un-cross-linked polyimides showed good plasticization resistance to CO₂/CH₄ mixed gas with 10% CO₂; however, only the cross-linked polyimides showed good plasticization resistance under aggressive gas feed conditions (CO₂/CH₄ mixed gas with 50% CO₂ or pure CO₂). For future work, asymmetric hollow fibers and carbon molecular sieve membranes based on the most attractive members of the family will be considered.