Hexafluoropropane Dianhydride Research Articles

Organic Thin-Film Transistors Based on Vapor-Deposition Polymerized Gate Insulators

In this study, we demonstrated that organic thin-film transistors (OTFTs) can be fabricated by using organic gate insulators using a vapor deposition polymerization (VDP) process. We found that electrical output characteristics in our organic thin-film transistors using a staggered-inverted top-contact structure show a saturated slope in the saturation region and a subthreshold nonlinearity in the triode region. The field-effect mobility, threshold voltage, and on-off current ratio of OTFTs using 4,4'-oxydiphthalic anhydride[ODPA]-4,4'-oxydianiline[ODA] and 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride[6FDA]-[ODA] as gate insulators with a thickness of 0.45 µm were about 0.13–0.5 cm2/Vs, -7 V, and 104, respectively. To form polyimide as a gate insulator, the VDP process was also introduced instead of a spin-coating process, in which a polyimide film was codeposited by the high-vacuum thermal evaporation of ODPA and ODA, 6FDA and ODA, and cured at 150°C for 1 h followed by 200°C for 1 h after codeposition. To explain the differences in the electrical characteristics caused by the insulators, the morphology of pentacene on the polyimide from ODPA-ODA was compared with that from 6FDA-ODA, respectively.

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.

Soluble Polyimide Containing a Photoreactive 2‐Styrylpyridine Derivative as the Side Group: Synthesis and Characterization

AbstractSummary: We describe the synthesis and characterization of a soluble photoreactive polyimide. The precursor of the polyimide was prepared from 2,2′‐bis{4‐(3,4‐dicarboxyphenoxy)phenyl}hexafluoropropane dianhydride and 3,3′‐hydroxy‐4,4′‐diaminobiphenyl; the photoreactive polyimide was then prepared by the polymer reaction of the hydroxyl groups in the precursor polymer with 2‐{2‐[4‐(6‐hydroxyhexyloxy)phenyl]ethenyl}pyridine as a photoreactive 2‐styrylpyridine derivative. The photoreactive polymer and its precursor polymer were soluble in various polar organic solvents, and their thin flexible films were easily formed by solution casting. The initial decomposition temperatures of the former and latter polymers were 350 and 470 °C, respectively. The extent of the photochemical reaction of the photoreactive polymer film was measured to be 65.8% at an exposure energy of 1.5 J/cm2. The transmittance of the film was found to be approximately 92% at room temperature and approximately 85% at 200 °C. These results suggest that the polyimide is a photosensitive polymer with good photosensitivity and high optical transparency. The dichroic ratios of the film were between 0.023 and 0.025 when exposed to linearly polarized UV light (LPUVL). The liquid crystal in the film cell was perpendicularly oriented to the electric vector of LPUVL.Polar diagrams of LC cells fabricated from polyimide 6FBPA‐HAB‐SP6 film after irradiation with LPUVL.magnified imagePolar diagrams of LC cells fabricated from polyimide 6FBPA‐HAB‐SP6 film after irradiation with LPUVL.

Gas‐transport properties of indan‐containing polyimides

AbstractA series of indan‐containing polyimides were synthesized, and their gas‐permeation behavior was characterized. The four polyimides used in this study were synthesized from an indan‐containing diamine [5,7‐diamino‐1,1,4,6‐tetramethylindan (DAI)] with four dianhydrides [3,3′4,4′‐benzophenone tetracarboxylic dianhydride (BTDA), 3,3′4,4′‐oxydiphthalic dianhydride (ODPA), (3,3′4,4′‐biphenyl tetracarboxylic dianhydride (BPDA), and 2,2′‐bis(3,4′‐dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)]. The gas‐permeability coefficients of these four polyimides changed in the following order: DAI–BTDA < DAI–ODPA < DAI–BPDA < DAI–6FDA. This was consistent with the increasing order of the fraction of free volume (FFV). Moreover, the gas‐permeability coefficients were almost doubled from DAI–ODPA to DAI–BPDA and from DAI–BPDA to DAI–6FDA, although the FFV differences between the two polyimides were very small. The gas permeability and diffusivity of these indan‐containing polyimides increased with temperature, whereas the permselectivity and diffusion selectivity decreased. The activation energies for the permeation and diffusion of O2, N2, CH4, and CO2 were estimated. In comparison with the gas‐permeation behavior of other indan‐containing polymers, for these polyimides, very good gas‐permeation performance was found, that is, high gas‐permeability coefficients and reasonably high permselectivity. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2769–2779, 2004

A governing equation for physical aging of thick and thin fluoropolyimide films

AbstractAn equation based on the segmental/chain mobility in free volume theory was derived to correlate the gas permeation coefficient and the aging time. An accelerated physical aging of a fluoropolyimide was observed and employed to validate this equation. A strong thickness‐dependent aging process was found by employing pure O2 and N2 tests to monitor the change of gas permeation properties as a function of aging time. Experimental results also suggest that chain rigidity and configuration play important roles in physical aging. As a result, the thin 2,2′‐bis(3,4′‐dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) based polyimide films studied here have different permeability versus time relationship from conventional polyarylate in the early stage of aging, and the experimental data seem to fit the proposed equation well. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1758–1764, 2004

Synthesis and Properties of New Aromatic Polyimides Based on 2,6-Bis(4-aminophenoxy)naphthalene and Aromatic Tetracarboxylic Dianhydrides

A series of new aromatic polyimides containing bis(phenoxy)naphthalene units were synthesized from 2,6-bis(4-aminophenoxy)naphthalene (2,6-BAPON) and various aromatic tetracarboxylic dianhydrides by the conventional two-stage procedure with thermal imidization of poly(amic acid) films. The intermediate poly(amic acid)s obtained had inherent viscosties of 1.60–3.31 dL/g, and they could be solution cast and thermally converted into transparent, flexible, and tough polyimide films. The resulting polyimide films had tensile moduli of 1.5–2.3 GPa, tensile strengths of 105–124 MPa, and elongations at break of 7–22%. The polyimide derived from 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) was readily soluble in polar aprotic solvents. The glass-transition temperatures of these polyimides, except for that from pyromellitic dianhydride (PMDA), were recorded between 255 and 295 °C by differential scanning calorimetry (DSC). The softening temperatures of all the polyimide films stayed within 246–286 °C according to thermomechanical analysis (TMA). Thermogravimetic analyses (TGA) established that these polymers were fairly stable up to 500 °C, and the 10% weight loss temperatures were recorded in the range of 543–563 °C in nitrogen and 535–563 °C in air atmosphere.

Synthesis and properties of soluble trifluoromethyl‐substituted polyimides containing laterally attached p‐Terphenyls

AbstractA new fluorinated diamine monomer, 2′,5′‐bis(4‐amino‐2‐trifluoromethylphenoxy)‐p‐terphenyl, was synthesized from the chloro‐displacement of 2‐chloro‐5‐nitrobenzotrifluoride with the potassium phenolate of 2,5‐diphenylhydroquinone, followed by hydrazine palladium‐catalyzed reduction. A series of trifluoromethyl‐substituted polyimides containing flexible ether linkages and laterally attached side rods were synthesized from the diamine with various aromatic dianhydrides via a conventional two‐step process. The inherent viscosities of the poly(amic acid) precursors were 0.84–1.26 dL/g. All the polyimides afforded flexible and tough films. The use of 4,4′‐oxydiphthalic anhydride and 2,2′‐bis(3,4‐dicarboxyphenyl)hexafluoropropane dianhydride produced essentially colorless polyimide films. Most of the polyimides revealed an excellent solubility in many organic solvents. The glass‐transition temperatures of these polyimides were recorded between 254 and 299 °C by differential scanning calorimetry, and the softening temperatures of the polymer films stayed in the range of 253–300 °C according to thermomechanical analysis. The polyimides did not show significant decomposition before 500 °C in air or under nitrogen. These polyimides also showed low dielectric constants (2.83–3.34 at 1 MHz) and low moisture absorption (0.4–2.2%). For a comparative study, a series of analogous polyimides based on the nonfluorinated diamine 2′,5′‐bis(4‐aminophenoxy)‐p‐terphenyl were also prepared and characterized. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1255–1271, 2004

Synthesis and characterization of novel polyimides with 2,2‐bis[4(4‐aminophenoxy)phenyl]phthalein‐3′,5′‐bis(trifluoromethyl)anilide

Abstract2,2‐Bis[4(4‐aminophenoxy)phenyl]phthalein‐3′,5′‐bis(trifluoromethyl)anilide (6FADAP), containing fluorine and phthalimide moieties, was synthesized via the Williamson ether condensation reaction from 1‐chloro‐4‐nitrobenzene and phenolphthalein‐3′,5′‐bis(trifluoromethyl)anilide, which was followed by hydrogenation. Monomers such as 2,2‐bis[4(4‐aminophenoxy)phenyl]phthalein‐anilide containing phthalimide groups and 2,2‐bis[4(4‐aminophenoxy)phenyl]phthalein containing only phthalein moieties were also synthesized for comparison. The monomers were first characterized by Fourier transform infrared (FTIR), 1H NMR, 19F NMR, elemental analysis, and titration and were then used to prepare polyimides with 2,2‐bis(3,4‐dicarboxyphenyl)hexafluoropropane dianhydride. The polyimides were designed to have molecular weights of 20,000 g/mol via off‐stoichiometry and were characterized by FTIR, NMR, gel permeation chromatography (GPC), differential scanning calorimetry, and thermogravimetric analysis. Their solubility, water absorption, dielectric constant, and refractive index were also evaluated. The polyimides prepared with 6FADAP, containing fluorine and phthalimide moieties, had excellent solubility in N‐methylpyrrolidinone, N,N‐dimethylacetamide, tetrahydrofuran, CHCl3, tetrachloroethane, and acetone, and GPC analysis showed a molecular weight of 18,700 g/mol. The polyimides also exhibited a high glass‐transition temperature (290 °C), good thermal stability (∼500 °C in air), low water absorption (1.9 wt %), a low dielectric constant (2.81), a low refractive index, and low birefringence (0.0041). © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3361–3374, 2003

Preparation of Nanoporous Polyimide Films from Poly(urethane‐imide) by Thermal Treatment

AbstractNanoporous polyimide films were prepared in two steps. The first step is the preparation of poly(urethane‐imide) films by casting blend solutions containing various weight percentages of poly(amic acid) and phenol blocked polyurethane prepolymer (from 1,6‐hexamethylene diisocyanate and poly(ethylene glycol)). Three poly(amic acid)s were obtained from biphenyltetracarboxylic dianhydride (or) 2,2‐bis(3,4‐dicarboxyphenyl)hexafluoropropane dianhydride with 1,4‐phenylenediamine (or) 2,5‐dimethyl‐1,4‐phenylenediamine. Poly(urethane‐imide) films were characterized by density and surface energy measurements, AFM, DSC, TMA, mechanical properties and TGA. In the second step, these films were thermally treated above 300 °C to give nanoporous polyimide films. During thermal treatment, less thermally stable urethane domains decomposed, leaving porous polyimide films. The presence of pores was confirmed by scanning electron microscopy (SEM). The dielectric constant of the polyimide film was found to decrease with increasing amounts of urethane content.A nanoporous polyimide film.magnified imageA nanoporous polyimide film.

Synthesis and Characterization of Fluorinated Polyimide with Grafted Poly(N-isopropylacrylamide) Side Chains and the Temperature-Sensitive Microfiltration Membranes

Molecular modification of a fluorinated polyimide (FPI), 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride + 4,4‘-bis(4-aminophenoxy)diphenyl sulfone, via ozone pretreatment and thermally induced graft copolymerization with N-isopropylacrylamide (NIPAAm) in a N-methyl-2-pyrrolidone solution, was carried out. The resulting FPIs with grafted NIPAAm polymer side chains [FPI-g-P(NIPAAm) copolymers] were cast into microfiltration membranes by phase inversion in water at temperatures ranging from 4 to 55 °C. The surface composition of the membranes was characterized by X-ray photoelectron spectroscopy. The surface composition, mean pore size, and morphology of the membrane varied with the temperature of the aqueous coagulation bath. For the copolymer membrane cast below the lower critical solution temperature of the NIPAAm polymer (∼32 °C), the rate of water permeation increased substantially at a permeate temperature above 32 °C. For the flux of 2-propanol through the membrane cast above 32 °C, a rever...

Polyimide asymmetric membranes: Elaboration, morphology, and gas permeation performance

AbstractAsymmetric 2,2‐bis‐(3,4‐dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)‐2‐methyl‐1,3‐phenylenediamine (mPDA) polyimide membranes were prepared according to a phase‐inversion mechanism by using different solvent/nonsolvent blends. The membrane formation mechanism and the final performances of the asymmetric membranes have been found both nonsolvent and solvent nature dependent. From the visualization of cross sections of asymmetric membranes by scanning electron microscopy and the study of the permeation of two gases (N2, CO2) through asymmetric membranes, a relationship between elaboration conditions and asymmetric membranes characteristics could be drawn. The organization of polymer chains in solution strongly affects the final polymer arrangement and thus the final performances of the membrane. The influence of preliminary solvent evaporation before immersion has been shown to be dependent on the structure of the asymmetric membrane: finger‐like or sponge‐like structures. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1838–1848, 2003

Preparation and Characterization of 4,4′‐Bis(4‐aminophenoxy)diphenyl Sulfone Based Fluoropoly(ether‐imide)/Organo‐Modified Clay Nanocomposites

AbstractA series of fluoropoly(ether‐imide) (6F‐PEI), and [6F‐PEI/montmorillonite (MMT) clay) nanocomposites films were made by thermal curing of respective formulations containing fluoropoly(ether‐amic acid) (6F‐PEAA), synthesized from 2,2′‐bis(3,4‐dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and 4,4′‐bis(4‐aminophenoxy)diphenyl sulfone (p‐SED), and increasing concentration of p‐SED treated montmorillonite clay (modified MMT clay) at temperature from RT to 350 °C. These films showed excellent solvent resistance as well as very good thermal stability, and increased glass transition (Tg) values with increasing % clay. In addition, these trifluoromethyl groups‐containing nanocomposites films showed sharp lowering of coefficient of thermal expansion (CTE) by 22%. Furthermore, they exhibited increased long‐term thermo‐oxidative stability (TOS), with % weight retention in the range of 86 to 92% in isothermal heating at 300 °C for 300 h in air, reduced water absorption at 100 RH at 50 °C in the range of 0.5 to 1.15%. These data are still much lower than those of neat ULTEM® 1000 and Kapton® H film. The modulus of elasticity is on an average 38% higher for the nanocomposite films relative to neat fluoropoly(ether‐imide) (6FDA + p‐SED), and above non‐fluorinated polyimide films. The surface energy measurement by One‐Liquid and Two‐Liquid method showed a comparable trend of decreasing contact angle. For the nanocomposite films having 15% hydrophobic clay, the contact angle decreased by 21 and 20% for DI‐water and formamide, respectively. The surface energy increase was in the range of 8.21–8.54 mJ/m2.magnified image

Interaction between Fibroblast Cells and Fluorinated Polyimide with Nano-Modified Surface

In this study, we investigated the effect of surface nano-modification of aromatic fluorinated polyimide (6FDA-6FAP) derived from 2,2'-bis (3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and 2,2'-bis (4-aminophenyl) hexafluoropropane (6FAP) on the interaction with proteins and cells. The surface of 6FDA-6FAP was modified by surface-rubbing showed nano-ordered stripes along the rubbing direction. The rat fibroblast FR cells formed multicellular spheroids with high cell density on the rubbed surface then expressed excellent collagen production similarly in vivo, while on the non-modified surface the cells formed two-dimensional monolayers and the collagen production was negligible. The modulation of cell function by the surface nano-modification along with surface micro-modification may be one of the most important considerations during the design and manufacture of novel biochips or tissue engineering materials.

Olefin/paraffin separation performance of carbonized membranes derived from an asymmetric hollow fiber membrane of 6FDA/BPDA–DDBT copolyimide

Carbonized membranes were prepared by pyrolyzing an asymmetric hollow fiber membrane of a copolyimide from equimolar portion of 2,2-bis(3,4-dicarboxyphenyl)hexafluoro-propane dianhydride (6FDA) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) with 3,7-diamino-2,8(6)-dimethyldibenzothiophene sulfone (DDBT) at temperatures of 773–973 K under a nitrogen stream. The carbonized membranes had an asymmetric structure with a skin layer of around 200 nm in thickness and a much denser support layer compared with the precursor hollow fiber. The increase in permeance to propylene R C 3H 6 by the pyrolysis was up to 10 times. An appropriate holding time during the pyrolysis in a nitrogen stream tended to increase the olefin/paraffin selectivity. The membranes pyrolyzed at 813 K for 1 h displayed the best performance of R C 3H 6 =26 GPU and α C 3H 6/C 3H 8 =22 for mixed-component (50/50%) at 373 K and 1 atm. The effect of silicone rubber coating was also investigated.

Effect of methyl group substitution in the diamine and copolymer composition on thermal degradation of copolyimides based on 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride

Three series of linear aromatic copolyimides containing hexafluoro isopropylidine moieties (i.e., 6F) were synthesized by reacting 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) with various mole percents of (a) 1,3-phenylene diamine (mPDA) and 2,6-diamino toluene (DAT) (b) mPDA and 2,4,6-trimethyl-l,3-phenylene diamine (TMemPDA) and (c) DAT and TMemPDA in N-methyl-2-pyrrolidone (NMP). These copolyimides were characterized by inherent viscosity, GPC, DSC, XRD, and TGA. T g increased with increase in TMemPDA content in [6FDA+TMemPDA+mPDA] and [6FDA+TMemPDA+DAT] copolyimides and increase in DAT content in [6FDA+mPDA+DAT] copolyimide series. All the copolyimides were found to be amorphous. Activation energy was determined for the thermal degradation of these copolyimides using the Coats–Redfern equation and was found to follow first-order kinetics in air and nitrogen. CO 2 was the main decomposition product in air and CHF 3 in nitrogen atmosphere.

Synthesis of hexafluoroisopropylidene-containing polyimide-silver nanocomposite films evolving specularly reflective metal surfaces

Highly reflective surface-metallized flexible polyimide films have been prepared by the incorporation of the soluble silver ion complex (1,1,1-trifluoroacetylacetonato)silver(I) into dimethylacetamide solutions of the poly(amic acid) prepared from 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (4-BDAF). Thermal curing to 300 °C of solution cast silver(I)poly(amic acid) films leads to cycloimidization of the amic acid with concomitant silver(I) reduction and formation of a reflective surface-silvered film from 6–13% weight percent silver. The reflective surfaces evolve only when the cure temperature reaches 300 °C. After achieving a maximum value the reflectivity abruptly drops with further heating at 300 °C and the polyimide suffers oxidation degradation. The metallized films are thermally stable and maintain mechanical properties similar to those of the parent polyimide. TEM reveals that the interior of the composite films have ca. 5–20 nanometer-sized silver particles. SEM shows surface silver particles from ca. 50–1000 nm whose size varies with cure time at 300 °C. Neither the bulk nor the surface of the films is electrically conductive. There is strong interfacial adhesion of metal to polyimide.

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.

Facile synthesis of hyperbranched polyimides from A2 + BB′2 monomers

AbstractFacile syntheses of hyperbranched polyimides were realized by the polymerization of A2 + BB′2 monomers, 2,2‐bis(3,4‐dicarboxylphenyl) hexafluoropropane dianhydride (6FDA) + 2,4,6‐triaminopyrimidine (TAP), performed by mixing the monomers together in N‐methylpyrrolidone at 17% w/v concentrations with molar ratios of 6FDA:TAP ranging from 1:1 to 2:1. The lower reactivity of 2‐amino as compared with 4‐/6‐amino in TAP, demonstrated by 1H NMR, was probably the main reason for no gelation formed during the polymerization although monomer conversions surpassed the theoretical gel points. Fourier transform infrared spectroscopy and NMR were used to verify the structures of the obtained polymers. 1H NMR analysis indicated the degrees of branching (DB) of the polymers increased from 36 to 83% with the molar ratios of 6FDA:TAP increasing from 1:1 to 2:1. Molecular weights were determined by gel permeation chromatography, and inherent viscosities were measured. Glass‐transition temperature values, determined by differential scanning calorimetry, decreased when DB increased, and thermogravimetric analysis reflected the excellent thermal stability of the obtained hyperbranched polyimides. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4563–4569, 2002

Olefin/paraffin separation performance of asymmetric hollow fiber membrane of 6FDA/BPDA–DDBT copolyimide

Gas permeation properties of asymmetric hollow fiber membrane of copolyimide prepared from equimolar portion of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) with 3,7-diamino-2,8(6)-dimethyldibenzothiophene sulfone (DDBT) were investigated for single-component light gases, olefins and paraffins and for mixed components of C 3H 6/C 3H 8 and C 4H 6/C 4H 10. The gas permeability of the copolyimide film was close to that of 6FDA–DDBT polyimide and much larger than that of BPDA–DDBT. Gas permeance of the asymmetric copolyimide hollow fiber membrane decreased significantly in the first several months and leveled off after about 10 months of aging. The skin layer thickness calculated from the gas permeability and permeance was in the range of 0.6 (for H 2) to 1.7 μm (for C 3H 6) and about 10 times larger than the thickness estimated from the SEM observation. These results indicated that the significant densification of the skin layer was caused by physical aging. The silicone rubber coating hardly changed the selectivity for light gas pairs such as H 2/CH 4 and O 2/N 2, but enhanced that for C 3H 6/C 3H 8 and C 4H 6/C 4H 10 significantly especially at low temperatures. The evaluation of membrane quality based on the resistance model suggested that the extremely small surface porosity of defect pores significantly reduced the selectivity for the larger gas pairs. The asymmetric copolyimide hollow fiber membrane displayed better performance for C 3H 6/C 3H 8 and C 4H 6/C 4H 10: for example, permeances of C 3H 6 and C 4H 6 of 3.6 and 7.4 GPU, respectively, and separation factors of 15 and 69 for C 3H 6/C 3H 8 and C 4H 6/C 4H 10 (50/50 mol% in feed) at 373 K and 1 atm.

Thermal degradation kinetics of 6FDA/durene diamine/pPDA copolyimides

Aromatic copolyimides have been synthesised from 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) with various mol-% of 2,3,5,6-tetramethyl-1,4-phenylene diamine (durene diamine (DDA)) and 1,4-phenylene diamine (pPDA) in N -methyl-2-pyrrolidone (NMP). Copolyimides were characterised by inherent viscosity, GPC, DSC, DMA, XRD, and TGA. Inherent viscosity and molecular weight increased with increase in DDA content in copolyimides. This was attributed to higher basicity of DDA than pPDA, which resulted in higher reactivity towards dianhydride. They were found to be soluble in all common solvents except CCl4 and methanol. The glass transition temperature Tg of these amorphous copolymers increased with increase in DDA content; Tgs determined by DSC and DMA were comparable and Tg calculated using the Fox equation was compared with the experimental value. Thermal degradation of these copolyimides was found to follow second order kinetics in air. The activation energy Ea for the thermal degradation of these copolyimides in air was determined by both the Coats-Redfern and Chang methods and the result was comparable. Both Ea and ln A were found to decrease with increase in DDA content; CO2 and trace amounts of CHF3 were the decomposition products. The mechanism of thermal degradation is unique and is not affected by a change in the copolymer composition because all copolyimides display the kinetic compensation effect.