Cross-linkable Co-polyimides Research Articles

Advanced carbon molecular sieve membranes derived from molecularly engineered cross-linkable copolyimide for gas separations.

Carbon molecular sieve (CMS) membranes with precise molecular discrimination ability and facile scalability are attractive next-generation membranes for large-scale, energy-efficient gas separations. Here, structurally engineered CMS membranes derived from a tailor-made cross-linkable copolyimide with kinked structure are reported. We demonstrate that combining two features, kinked backbones and cross-linkable backbones, to engineer polyimide precursors while controlling pyrolysis conditions allows the creation of CMS membranes with improved gas separation performance. Our results indicate that the CMS membranes provide a versatile platform for a broad spectrum of challenging gas separations. The gas transport properties of the resulting CMS membranes are interpreted in terms of a model reflecting both molecular sieving Langmuir domains and a disordered continuous phase, thereby providing insight into structure evolution from the cross-linkable polyimide precursor to a final CMS membrane. With this understanding of CMS membrane structure and separation performance, these systems are promising for environmentally friendly gas separations.

Effect on the thermal resistance and thermal decomposition properties of thermally cross-linkable polyimide films obtained from a reactive acetylene

Homopolyimides are prepared by 4,4′-(ethyne-1,2-diyl)diphthalic anhydride as dianhydride monomer and several diamine monomers. The copolyimides are prepared with 4,4′-oxydianiline as an amine monomer， 3,3′,4,4′-oxydiphthalic anhydride as an anhydride monomer with the addition of 4,4′-(ethyne-1,2-diyl)diphthalic anhydride as another anhydride monomer. The structure of polyimides is characterized by Fourier transform infrared spectroscopy and X-ray diffraction. Thermogravimetric analysis, thermomechanical analysis and thermo-oxidative degradation kinetics analysis are used to investigate the properties of polyimides. The homopolyimide show the best thermal stability in both nitrogen and air atmosphere by 4,4′-oxydianiline and 4,4′-(ethyne-1,2-diyl)diphthalic anhydride. The results also indicate that the thermally cross-linkable copolyimide films not only enhance the thermal stability behaviors of polyimide by controlling the addition of dianhydride, but also improve substantially the upper limit temperature of thermal aging by undergoing chemical cross-linking reaction of the diphenylethynylene structures and providing multifarious naphthalene ring groups and biphenyl structures. Diphenylacetylene crosslinking merging to the long-chain of polyimide is a useful conversion for enhancing the thermal stability and the thermal degradation stability of aromatic polymers.

Systematic Analysis Reveals Thermal Separations Are Not Necessarily Most Energy Intensive

Summary At the industrial level, separation of a variety of mixtures is predominantly carried out by thermally driven processes such as distillation. Nevertheless, the current literature seems to suggest that much is to be gained by replacing these processes with alternative non-thermal methods, which will potentially require a fraction of the energy used by distillation. This is primarily due to the widespread perception that thermal separation processes, particularly those that involve vaporization, are highly energy intensive. However, this belief is not well founded, because it is based on extrapolation from comparisons in the literature, many of which make ad-hoc assumptions and result in incorrect conclusions. Our analysis shows that this confusion arises because the processes utilize different types of energy: heat and electricity. Here, we present a consistent framework that enables a fair comparison between different processes that consume different forms of energies. Using this framework, we refute the general perception that thermal separation processes are inherently the most energy intensive and conclusively show through the examples of two challenging mixtures constituting components with low relative volatilities, that distillation processes, when run properly, can consume remarkably lower fuel than non-thermal membrane alternatives, which have often been touted as more energy efficient.

Decreasing the dielectric constant and water uptake of co-polyimide films by introducing hydrophobic cross-linked networks

In the present study, a series of pendant-group cross-linkable co-polyimide (Co-PI) films were successfully prepared. Via thermal treatment, the cross-linkable tetrafluorostyrol pendant-groups could induce cross-linking without any chemical additives, resulting in the formation of robust cross-linked structures in the co-polyimide films. The effects of cross-linking and the amount of cross-linkable pendant-groups on the performances of co-polyimide films were studied and discussed in detail. After cross-linking, the obtained films showed the significantly lowered dielectric constants, which decreased gradually with increasing amounts of cross-linkable pendant-groups. The best dielectric properties were obtained for cross-linked Co-PI-5 film, which exhibits a dielectric constant as low as 2.48 at 1 MHz. Meanwhile, the cross-linked Co-PI-5 film demonstrated excellent moisture resistance with a water uptake of less than 0.31%. It also exhibited outstanding thermal and dimensional stability, superior chemical resistance, excellent mechanical property and surface planarity. The low dielectric constants, excellent anti-moisture performances and outstanding comprehensive properties make the cross-linked Co-PIs a good candidate for interlayer dielectrics in the near future.

Synthesis and properties of soluble cross-linkable fluorinated co-polyimides

A series of high molecular weight soluble fluorinated cross-linkable co-polyimides (Co-PIs) were synthesized from 1,3-bis(2-trifluoromethyl-4-aminophenoxy)-5-(2,3,4,5-tetrafluorophenoxy)benzene (6FTFPB), 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene (6FAPB), 1,4-bis(4-amino-2-trifluoromethyl-phenoxy)-2-(3′-trifluoro-methylphenyl)benzene (m-3F-6FAPB) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA). The weight-average molecular weights (Mws) and polydispersities of the co-polyimides were in the range 5.53–7.24 and 1.71–2.04, respectively. By adjusting the feed ratio of diamines, the refractive indices of Co-PIs films were exactly tuned in the range of 1.5203–1.5574 at 633nm. The co-polyimides showed outstanding solubility in organic solvents and high thermal stability (5% thermal weight-loss temperature beyond 525°C). Meanwhile, the films had excellent mechanical properties with high tensile strength of 100–108MPa, Young’s modulus of 3.3–4.2GPa and elongations at break of 8–15%. Polymers in the form of cast and spun films could be cross-linked by thermal curing at 260°C. After curing, the Co-PIs films showed excellent chemical resistance, high glass transition temperatures above 270°C, higher mechanical properties and thermal stability (5% thermal weight-loss temperature beyond 540°C) and low dielectric constant in the range of 2.37–2.70.

Natural gas purification and olefin/paraffin separation using thermal cross-linkable co-polyimide/ZIF-8 mixed matrix membranes

Using three 6FDA-based polyimides (6FDA-Durene, 6FDA-Durene/DABA (9/1), 6FDA-Durene/DABA (7/3)) and nano-size zeolitic imidazolate framework-8 (ZIF-8), we have fabricated mixed matrix membranes (MMMs) with uniform morphology comprising ZIF-8as high as 40wt% loading by directly mixing as-synthesized ZIF-8 suspension into the polymer solution. Permeability of all gases (CO2, CH4, C3H6, and C3H8) increases rapidly with an increase in ZIF-8 loading. However, the addition of ZIF-8 nano-particles into the polymer matrix increases the ideal CO2/CH4 selectivity of only 6.87%, while the ideal C3H6/C3H8 selectivity improves 134% from 11.68 to 27.38 for the MMM made of 6FDA-Durene/DABA (9/1) and 40wt% ZIF-8. Experimental data show that the plasticization resistance and gas pair selectivity of MMMs are strongly dependent on the amount of cross-linkable moiety and annealing temperature. MMMs made of 6FDA-Durene do not show considerable improvements on resistance against CO2-induced plasticization after annealing at 200–400°C, while MMMs synthesized from cross-linkable co-polyimides (6FDA-Durene/DABA (9/1) and 6FDA-Durene/DABA (7/3)) show significant enhancements in CO2/CH4 and C3H6/C3H8 selectivity as well as plasticization suppression characteristics up to a CO2 pressure of 30atm after annealing at 400°C due to the cross-linking reaction of the carboxyl acid (COOH) in the DABA moiety. The MMM made of 6FDA-Durene/DABA (9/1) and 40wt% ZIF-8 possess a notable ideal C3H6/C3H8 selectivity of 27.38 and a remarkable C3H6 permeability of 47.3 Barrer. After thermally annealed at 400°C, the MMM made of 6FDA-Durene/DABA (9/1) and 20wt% ZIF-8 shows a CO2/CH4 selectivity of 19.61 and an impressive CO2 permeability 728 Barrer in mixed gas tests. The newly developed MMMs may have great potential for industrial nature gas purification and C3H6/C3H8 separation.

Natural gas purification and olefin/paraffin separation using cross-linkable 6FDA-Durene/DABA co-polyimides grafted with α, β, and γ-cyclodextrin

Using a cross-linkable co-polyimide (6FDA-Durene/DABA (9/1)), we have developed new flexible and high-performance gas separation membranes that can enhance both membrane permeability and plasticization resistance simultaneously by grafting various sizes of cyclodextrin to the polyimide matrix and then decomposing them at elevated temperatures. The gas permeability of thermally treated pristine polyimide (referred as the original PI) and CD grafted co-polyimide (referred as PI-g-CDs for 200 and 300 °C and partially pyrolyzed membranes (PPM)-CDs for 350, 400, and 425 °C) has been determined using O 2, N 2, CO 2, CH 4, C 3H 6, and C 3H 8 at 35 °C. The permeability of all gases increases with an increase in thermal treatment temperature from 200 to 425 °C. However, permeability increases more for those grafted with bigger size CD. Permeability of the original PI thermally treated at 425 °C is about 4–6 times higher than that treated at 200 °C. The permeability increase jumps to 8–10 times for PPM-α-CD and 15–17 times for PPM-γ-CD due to CD decomposition at high temperatures and bigger CD creating bigger micro-pores. Interestingly, the permeability ratios of PPM-α-CD to PPM-γ-CD and PPM-β-CD to PPM-γ-CD at 400 and 425 °C are around 0.6 and 0.8, respectively. These numbers are almost the same as the cavity diameter ratios of α-CD to γ-CD and β-CD to γ-CD. Clearly, the bigger CD creates the bigger micro-pores. Permselectivity decreases first with an increase in thermal treatment temperature up to 350 °C and then increases. Permselectivity of thermally treated CD grafted co-polyimide membranes is also slightly higher than that of the original PI due to higher degrees of cross-linking in CD grafted co-polyimide membranes. In addition, for co-polyimide membranes grafted by CDs, the higher thermal treatment temperature results in membranes with the better plasticization resistance to CO 2 and the better separation performance for 50:50 CO 2/CH 4 mixed gases. The best result for pure gas tests is achieved for PPM-γ-CD-425. This membrane has a CO 2 permeability of 4211 Barrers with a CO 2/CH 4 ideal selectivity of 22.44 and a C 3H 6 permeability of 521 Barrers with a C 3H 6/C 3H 8 ideal selectivity of 18.09. It can also resist against CO 2 plasticization until 30 atm. The CO 2 permeability drops slightly to 3976 Barrers with almost the same CO 2/CH 4 selectivity of 22.84 in mixed gas tests.

Variation of esterfication conditions to optimize solid-state crosslinking reaction of DABA-containing copolyimide membranes for gas separations

In membrane based gas separation, crosslinking of membrane materials was shown to be an effective method to stabilize the polymer structure against undesirable plasticization effects. In this study a crosslinkable copolyimide was synthesized by using the dianhydride 6FDA (4,4′-(hexafluoroisopropylidene)diphthalic anhydride) and the diamines ODA (4,4′-oxydianiline), 4MPD (2,3,5,6-tetramethyl-1,4-phenylene diamine) and DABA (3,5-diamino benzoic acid). The last mentioned diamine contains a free carboxylic acid group where crosslinking of the polymer chains is possible by esterfication reaction with a diol. In the current work EG (ethylene glycol) was chosen as crosslinker. The esterfication conditions were varied to optimize the solid-state crosslinking reaction. To investigate the influence of crosslinking conditions on the separation properties of the membranes pure gas permeation experiments were performed with non-crosslinked and the different EG-crosslinked films at 35°C for the gases CO 2 using feed pressures up to 40 bar and CH 4 at pressures up to 10 bar.

Crosslinkable copolyimides for the membrane-based separation of p-/ o-xylene mixtures

For the membrane based separation of p-/ o-xylene mixtures, pervaporation properties of several crosslinked and non-crosslinked 6FDA (4,4′-(hexafluoro-isopropylidene)-diphthalic anhydride))-based copolyimide membranes have been investigated. The copolyimides were synthesized by polycondensation of 6FDA with different diamino monomers, such as 4MPD (2,3,5,6-tetramethyl-1,4-phenylene-diamine), 6FpDA (4,4′-(hexafluoro-isopropylidene)-dianiline)) and DABA (3,5-diaminobenzoic acid). Thereby DABA was used as a monomer containing a crosslinkable carboxylic acid group. By variation of the diamino monomer ratio, different crosslinkable copolyimides were synthesized. The crosslinking of the free carboxylic acid groups was performed by the reaction with ethylene glycol (EG) in order to form diester crosslinks, or using hexamethylene diamine (HMDA) as crosslinking agent which then leads to diamide crosslinks. All membranes used in this study showed excellent chemical and thermal stability in pervaporation experiments at 60°C using a 50:50 p-/ o-xylene feed mixture. It has been found that the normalized flux through the different membranes strongly depends on the polymer backbone structure, but it is also influenced by the crosslinking agent. For p-/ o-xylene mixtures, normalized fluxes between 0.04 kg μm m −2 h −1 and 25 kg μm m −2 h −1 were reached with separation factors between 1.15 and 1.47.

Studies on cross-linkable copolyimides

Some new flexible and cross-linkable copolyimides containing rubbery unsaturated aliphatic chain moiety (CPLA) were prepared by low temperature polycondensation betweenm-phenylene diamine,bis- 3(4 carboxy phthalimido) benzene, and liquidα,Ω bis [2-(4 piperazinyl) ethyl aminocarbonyl] poly-(butadiene-co-acrylonitrile) (ATBN) in (DMAC) solution. The effect of rubbery aliphatic moiety in these copolyimides on the polymer properties has been reported. A comparative study on the properties of these copolyimides (CPLA) with the mixture of ATBN and a low molecular-weight cross-linkable copolyimide possessing the same molecular structure as the aromatic moiety of CPLA polymers has been made.

Structure property correlation in crosslinkable copolyimides

AbstractThree copolyimides containing crosslinkable alkyne groups either in the main chain backbone or as endgroups, have been synthesized and characterized in regard to their structure, molecular weight, solubility, film‐forming properties, crystallinity, crosslinking behavior, and thermal stability. A good correlation between the structure of the polymers and their properties has been established.

Synthesis and characterization of a rubber incorporated polyamideimide

AbstractA new flexible and crosslinkable copolyimide was prepared by low temperature polycondensation between bis [4‐(4‐chloroformylphthalimido)phenyl] ether (1), bis(4‐aminophenyl) ether and liquid α,ω‐bis[2‐(4‐piperazinyl)ethylaminocarbonyl]poly(butadiene‐co‐acrylonitrile) in DMAc solution. The polymer is soluble in DMF, DMAc, NMP, m‐cresol, DMSO and concentrated sulfuric acid. On heating with or without dicumyl peroxide the polymer was crosslinked.