Polyimide Precursor Research Articles

High performance carbon molecular sieve membranes derived from hyperbranched polyimide precursors for improved gas separation applications

A series of hyperbranched polyimides (HBPI) were selected as precursors for the production of carbon molecular sieve membranes (CMSMs) and the gas separation performance of the resultant CMSMs was explored. By varying the monomer compositions of HBPI precursor polymers, the influences of degree of branching on pore size and distribution of the CMSMs are investigated and examined. A CO2 permeability of 1085Barrer with CO2/CH4 selectivity of 52 is attained. Thermal analyses reveal that the chain rigidity of the precursors increases with the degree of branching, while the density of the CMSMs shows an opposite trend. In view of the unique network structure offered by the HBPI precursors, it is speculated that the rigid network of the HBPI with the highest degree of branching has facilitated the formation of ultra-micropores, giving rise to the more constrained structure and improved diffusivity selectivity. The unique hyperbranched network structure found in HBPI possesses great potential to producing CMSMs with superior performance.

Thermal Treatment Effect on the Structure and Property Change between Hydroxy-Containing Polyimides (HPIs) and Thermally Rearranged Polybenzoxazole (TR-PBO)

In this study, we report on the effect of thermal treatment on polyimide precursors (HPIs) and on the resulting thermally rearranged polybenzoxazole (TR-PBO) polymer membranes as investigated through the use of molecular dynamics (MD) simulations. For this purpose, we have first analyzed the structures of hydroxy-containing polyimides before thermal treatment and those of the thermally rearranged polybenzoxazoles after the thermal treatment, according to their temperature conditions. As expected, HPIs and TR-PBOs always show very limited motion of their polymer chains, indicated by the radius of gyration, due to their well-known thermal stability. In particular, the very rigid and stiff PBO linkages did not undergo significant change in their torsional angle distribution. On the other hand, in regards to intrachain movement, HPI chains were significantly affected by temperature. Their conformational changes were notably observed, which affected the distances between possible reaction sites, oxygen atoms in hydroxyl groups, and carbon atoms in the imido-ring. The free volume analysis, performed on both polymers and during thermal treatment, indicates that HPIs have a unimodal distribution of free volume areas, which partially coalesce in larger areas having, however, a relatively narrow size. Further, TR-PBO shows a bimodal cavity distribution, and after thermal treatment and TR reaction, the free volume structures in TR-PBO are maintained. The cavity size distributions determined by simulation were also consistent with free volume distributions determined by positron annihilation lifetime spectroscopy.

Modified Organic low-k Dielectric Layers on Fired LTCC Substrates

In this study, the reduction of permittivity of LTCC substrates by coating with a polyimide compound containing hollow glass microspheres as filler material is described. By incorporating the filler material, the dielectric constant of the substrate material is locally lowered to improve the high-frequency performance of antennas operated in the GHz range. Furthermore, by adding the filler material to the liquid polyimide precursor the layer thickness is heightened from maximum 10 μm to above 80 μm which is enough to fill cavities in LTCC substrates. Two compound materials with filler to polymer ratios 1:7.5 and 1:10 are mixed. Afterwards they are deposited by spin coating onto LTCC substrates. The film thickness depends on the rotating speed and the filler content. With the higher filler concentration and low rotating speed of 500 rpm 82 μm thick polymer films can be achieved. The high surface roughness can be reduced afterwards by adding additional pure polyimide layers on top to Ra= 3 μm. The dielectric constant of the entire substrate consisting of the LTCC and the resulting compound material is measured using a ring resonator in microstrip configuration. From the resonances occurring in the transmission S-parameter |S21| spectrum between 1 to 10 GHz, the relative dielectric constant can be determined. Using 820 μm thick LTCC substrates a relatively low reduction from εr = 7.8 to 6.6 is achieved. However, due to permittivity can be reduced with higher microsphere amounts, the dielectric constant of pure polyimide of εr= 3.3 can also be reduced. Furthermore due to the sufficiently high film thickness of the modified substrates, the compound layer can be used as single dielectric layer.

Controllable low dielectric porous polyimide films templated by silica microspheres: Microstructure, formation mechanism, and properties

In the paper, controllable low dielectric macroporous polyimide (PI) films were prepared using silica microspheres as template, based on the solvent-evaporation-assisted in situ self-assembly of polyamic acid (PAA, precursor of PI) and silica microspheres in N,N-Dimethylformamide (DMF) solution. The interior microstructure of the macroporous PI films and its relationship with mechanical and dielectric properties were investigated. It was found that the macroporous PI films had triple-layer porous structures: ordered porous surface, dense layer in the middle, and interconnected porous structure at the bottom. The thickness of the dense layer in the middle changed with increasing weight content of silica. When SiO2 wt% was above 40%, the dense layer nearly disappeared. The interconnected porous structure at the bottom was quite homogenous, and the pore size could be controlled by the diameter of silica microspheres. Based on the experimental results, possible formation mechanism of the macroporous PI films was addressed. The mechanical properties of the macroporous PI films were degraded due to the introduction of air void. It was found that the degradation of mechanical properties of macroporous PI films with porosity of 37% is more obvious than that of PI films with porosity below 31%, which could be attributed to the triple-layer microstructure of the films. The air void introduction could effectively reduce the dielectric constant from 4.11 of PI dense films to 2.57 of porous PI films with porosity of 37%. The dielectric constant was only closely related to porosity and had no obvious connection with pore size.

Hyperbranched Polyimide-Silica Hybrid Materials: Synthesis, Structure, Dynamics, and Gas Transport Properties

A series of organic–inorganic hybrid materials were prepared from a hyperbranched polyimide precursor (hyperbranched polyamic acid), tetramethoxysilane, and/or 3-glycidyloxypropyl-trimethoxysilane via a sol-gel process. The hyperbranched polyimide-silica hybrids, whose polyimide moieties were based on commercially available monomers 4,4′,4″-triaminotriphenylmethane and 4,4′-oxydiphthalic anhydride taken in molar ratio 1:1, contained from 10 to 30 wt% silica. Their morphology and dynamics were characterized by using scanning electron microscopy, differential scanning calorimetry, dynamic mechanical analysis, laser-interferometric creep rate spectroscopy, and wide-angle X-ray diffraction. Attention was also focused on the relation between morphology/dynamics and gas transport properties of these materials.

Novel Modification of a Macroporous Stainless Steel Tube by Electroless Ni plating for Use as a Substrate for Preparation of Nanoporous Carbon Membranes

Nanoporous carbon (NPC)/Ni–P composite membranes were prepared on electroless Ni-plating-modified porous stainless steel tubes (PSSTs) by carbonizing polyimide or cellulose acetate precursors at 700 °C in a nitrogen atmosphere. The performance of the membranes was evaluated by single-gas permeation and binary-gas mixture separation. The effects of the substrate with or without modification on gas permeation were studied, and the diffusion mechanism of the gases through the membranes was analyzed. The real separation factors of gas mixtures through the NPC/Ni–P composite membranes were much higher than those through the nanoporous carbon membranes (NPCMs) prepared on the unmodified substrate, especially those for H2/CH4 and H2/CO2. The weight loss rate of polymer precursors was investigated by TG/DTG. The specific surface areas of the membranes and the crystallization of the Ni–P alloy layer were characterized by means of the BET and DSC techniques, respectively, indicating that localized electroless Ni pl...

Preparation and properties of polyimide/graphene oxide nanocomposite films with Mg ion crosslinker

Polyimide(PI)/graphene oxide(GO) nanocomposite films were prepared by chemical cross-linking using small amounts of divalent Mg ions. The PI/GO nanocomposites showed enhanced tensile properties compared to pristine PI due to the presence of exfoliated GO in the PI matrix as well as crosslinking between poly(amic acid) (PAA), which is a precursor of PI, and GO by Mg ions. The hydrogen bonds between PAA and GO suppressed the phase separation between PI and GO, and small amounts of Mg ions can bond between the oxygen functional groups and carboxylate groups of GO and PAA.

Grazing-incidence wide-angle X-ray diffraction study on molecular aggregation state of imprinted polyimide film before and after hard baking

Nanoimprint lithography (NIL) was carried out on precursor of polyimide (PI), poly(amic acid) film, and then hard baking to obtain imprinted PI film. The molecular aggregation states of imprinted PI films before and after hard baking were investigated by grazing-incidence wide-angle X-ray diffraction comparing with the one of flat PI film. It was found that NIL and hard baking can strongly affect the molecular aggregation states of PI film. Before hard baking, PI chain is aligned parallel to the line direction on the line. After hard baking, the alignment in ordered domain was changed to that the PI molecule of which chain axis is perpendicular to the line direction is significantly increased, while, PI molecule of which chain axis is parallel to the line direction is decreased after hard baking. Through comparing with the flat PI, crystallinity of imprinted PI film has been significantly enhanced.

A prospective study on the application of thermally rearranged acetate-containing polyimide membranes in dehydration of biofuels via pervaporation

Dehydration of biofuels (bioalcohols) derived from fermented products via distillation process is always energy intensive and often limited by the formation of azeotropic mixtures between alcohols and water. We have explored the feasibility of thermally rearranged polybenzoxazole (PBO) membranes in dehydration of biofuels via pervaporation process. The polyimide based precursor was synthesized by polycondensation of 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 3,3′-dihydroxybenzidine diamine (HAB) and cast into dense membranes. The resultant polyimide membranes were then subjected to thermal rearrangement reaction to initiate the solid-state chemical reconstruction into PBO. The effects of heating temperature and duration on pervaporation performance were systematically investigated. In general, the thermally rearranged PBO membranes showed an impressive enhancement in permeation flux with reasonable separation performance as compared to its polyimide precursor. In addition, the pervaporation performance was found to be greatly affected by the thermal rearrangement temperature as compared to the dwell duration. Furthermore, the stability of the thermally rearranged PBO membrane has been proven by continuously monitoring the pervaporation performance at 80°C for 250h. The long-term data revealed that there was no significant changes in separation performance that signifies the feasibility of the thermally rearranged PBO membrane in the separation and purification of biofuels via pervaporation process in biorefineries.

Incorporation of Silver Nanoparticles into the Bulk of the Electrospun Ultrafine Polyimide Nanofibers via a Direct Ion Exchange Self-Metallization Process

This paper reports our works on the preparation of the silver-nanoparticle-incorporated ultrafine polyimide (PI) ultrafine fibers via a direct ion exchange self-metallization technique using silver ammonia complex cation ([Ag(NH(3))(2)](+)) as the silver precursor and pyromellitic dianhydride (PMDA)/4,4'-oxidianiline (4,4'-ODA) polyimide as the matrix. The polyimide precursor, poly(amic acid) (PAA), was synthesized and then electrospun into ultrafine fibers. By thermally treating the silver(I)-doped PAA ultrafine fibers, where the silver(I) ions were loaded through the ion exchange reactions of the carboxylic acid groups of the PAA macromolecules with the [Ag(NH(3))(2)](+) cations in an aqueous solution, ultrafine polyimide fibers embedded with silver nanoparticles with diameters less than 20 nm were successfully fabricated. The fiber-electrospinning process, the ion exchange process, and various factors influencing the hybrid ultrafine fibers preparation process such as the thermal treatment atmospheres and the thermal catalytic oxidative degradation effect of the reduced silver nanoparticles were discussed. The ultrafine fibers were characterized by attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA).

Rheological Behaviours of Polyimide Precursor in Organic Solvents for Coating on Silica Optical Fibre

The role of different organic polar solvents containing methylene group on rheological behaviour of coated polyimide precursor on silica optical fibre were investigated at different concentrations and temperatures with respect to corresponding original precursor itself. The influence of keto-methyl and N-methyl polar groups are in different chemical structure of organic solvents into the coated precursor against–solvent dilution process has been explored. The interaction in between functional groups moiety containing into precursor and polar methyl group of solvents was also analysed. A substantial knowledge of rheology into coating precursor of polyimide is an important issue to coat successfully into optical fibre.

Gas permeability, diffusivity, and free volume of thermally rearranged polymers based on 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)

Abstract HAB-6FDA polyimide was synthesized from 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) by a two-step polycondensation method with chemical imidization. This polyimide was used as the precursor to prepare thermally rearranged (TR) polymers. The rearrangement reaction was performed at temperatures from 350 to 450 °C. Based on mass loss during the TR process, TR conversion levels as high as 76% were observed. CO 2 permeability increased from 12 Barrer in the precursor polyimide (HAB-6FDA) to 410 Barrer in the TR polymer prepared at 450 °C, while pure gas CO 2 /CH 4 selectivity decreased from 42 to 24. Both diffusivity and solubility increased following the rearrangement process, but the change in gas diffusivity was the largest contribution to the increase in permeability. From the polyimide to the sample heated at 450 for 30 min, CO 2 diffusivity increased by approximately a factor of 20 while solubility increased by a factor of 1.7. Similar changes were observed for other gases. Fractional free volume increased from 15% in the polyimide precursor to 20% in the TR polymer rearranged at 450 °C. The HAB-6FDA TR polymers exhibited higher CO 2 permeability than other classes of polymers with similar free volume, suggesting a free volume distribution in these TR polymers that favors high permeability.

Thermal conductivity and morphology of silver‐filled multiwalled carbon nanotubes/polyimide nanocomposite films

AbstractA facile methodology has been developed to synthesize silver‐filled multiwalled carbon nanotubes (S‐MWNTs)‐polyimide (PI) nanocomposites with high thermal conductivity for applications in flexible printed circuits or buried film capacitors, requiring efficient heat dissipation. MWNTs functioned as modules to facilitate the distribution of Ag particles within PI matrix. The intercalation of Ag within MWNTs was performed using capillary action upon mixing AgNO3 solution with PI precursor and followed by calcinations to reduce the ionic silver to Ag. The existence of Ag in the PI nanocomposites was observed from transmission electron microscope images and verified with the energy‐dispersive X‐ray spectrometer. Homogeneous dispersion of SMWNTs in PI matrix and strong interaction between S‐MWNTs and PI were also suggested by SEM cross‐section images. The thermal conductivity of S‐MWNT/PI nanocomposite was a function of the content of S‐MWNTs in PI matrix. The PI nanocomposite containing 1.5 wt % of S‐MWNTs (S‐MWNT/PI‐1.5) exhibited the highest thermal conductivity, 0.37 W/mK. A decrease in thermal conductivity was observed while the surface roughness of the nanocomposite was higher than 1 μm owing to the high content of S‐MWNT in PI. In addition, the modified MWNTs improved flexibility of the PI matrix. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Formation of double‐surface‐silvered polyimide films via a direct ion‐exchange self‐metallization technique: The case of BPADA/ODA and [Ag(NH3)2]+

AbstractDouble‐surface‐silvered polyimide (PI) films have been successfully fabricated via a direct ion‐exchange self‐metallization method using silver ammonia complex cation ([Ag(NH3)2]+) as silver resource and bis[4‐(3,4‐dicarboxyphenoxy)phenyl]propane dianhydride/4,4′‐oxydianiline (BPADA/ODA)‐based poly(amic acid) (PAA) as the PI precursor. The alkaline characteristic of the silver precursor dramatically improves the efficiency of the ion exchange and film metallization process. By using an aqueous [Ag(NH3)2]+ solution with a concentration of only 0.01M and an ion‐exchange time of only 5 min, metallized films with desirable performance could be easily obtained by simply heating the silver(I)‐doped PAA films to 300°C. The strong hydrolysis effect of the basic [Ag(NH3)2]+ cations on the flexible and acidic BPADA/ODA PAA chains was observed during the ion exchange process by the quantitative evaluation of the mass loss of PAA matrix. Nevertheless, under the present experimental conditions, the final metallized film essentially retained the basic structural, thermal, and mechanical properties of the pristine PI, which make it a truly applicable material. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Mixed matrix membranes based on 3‐aminopropyltriethoxysilane endcapped polyimides and silicalite‐1

AbstractTo improve membrane permeability and the quality of the polymer‐zeolite interface such that the separation factor is maintained or enhanced, we adopted a novel approach to the preparation of mixed matrix membranes (MMMs). Our approach involved endcapping polyamic acid (PAA) with the coupling agent 3‐aminopropyltriethoxysilane (APTES) and then bonding the endcapped polyimide (PI) to silicalite‐1. The APTES‐endcapped polyimide precursors reacted with silanol groups on the silicalite‐1 surface and were transformed into the final polyimide (PI) matrices. Five precursors were used to create five series of MMMs; each series contained membranes with a content of silicalite‐1 ranging from 0 to 50 wt %. The helium permeability coefficient of the membranes was measured and found to be dependent on both the amount of filler and type of PI matrix. In the most permeable series, helium permeability coefficient increased from 16.3 × 10−15 mol m−1 s−1 Pa−1 for unfilled membranes to 69.1 × 10−15 mol m−1 s−1 Pa−1 for membranes containing 30 wt % silicalite‐1. The ideal helium/nitrogen separation factor ranged between 22 and 45. Thus, it seems that our novel approach improves the interfacial adhesion of MMMs. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Atomic Oxygen Effects on POSS Polyimides in Low Earth Orbit

Kapton polyimde is extensively used in solar arrays, spacecraft thermal blankets, and space inflatable structures. Upon exposure to atomic oxygen in low Earth orbit (LEO), Kapton is severely eroded. An effective approach to prevent this erosion is to incorporate polyhedral oligomeric silsesquioxane (POSS) into the polyimide matrix by copolymerizing POSS monomers with the polyimide precursor. The copolymerization of POSS provides Si and O in the polymer matrix on the nano level. During exposure of POSS polyimide to atomic oxygen, organic material is degraded, and a silica passivation layer is formed. This silica layer protects the underlying polymer from further degradation. Laboratory and space-flight experiments have shown that POSS polyimides are highly resistant to atomic-oxygen attack, with erosion yields that may be as little as 1% those of Kapton. The results of all the studies indicate that POSS polyimide would be a space-survivable replacement for Kapton on spacecraft that operate in the LEO environment.

Polyimide/Poly(ethylene terephthalate) Composite Membrane by Electrospinning for Nonwoven Separator for Lithium-Ion Battery

The polyamic acid (PAA), which is the precursor of polyimide (PI), is synthesized by pyromellitic dianhydride (PMDA) and 4, 4′-oxidianiline (ODA). The PAA/PET composite membrane is prepared with poly (ethylene terephthalate) (PET) nonwoven by electrospinning of the solution of PAA. The final PI/PET composite membrane is obtained after a thermal imidization process. The porosity, liquid electrolyte uptake and room temperature ionic conductivity of PI/PET composite membrane decreases a little comparing with PI membrane, but the tensile strength of PI/PET composite membrane increases more than PI ones. The experiments demonstrate that the diameter of the fibers decrease with the increase of the electrospinning voltage. At the same time, the decrease of the fibers diameter leads to the increase of the porosity as well as the liquid electrolyte uptake of the PI/PET composite membranes. As a result, the room temperature ionic conductivity is enhanced from 0.451 to 0.897 mS·cm−1. The PI/PET composite membrane shows only about 2% of thermal shrinkage at 180°C under air atmosphere. The cell with the PI/PET composite membrane shows stable cycling performances and better rate capabilities than the cells with the Celgard one under 10 C-rate.

Stable doped sp2 C-hybrid nanostructures by reactive ion beam irradiation

Boron doping of polyimide precursors yields films, that when ion irradiated produces high aspect ratio nanowires of 15–25 nm diameter of B substituted ∼4 nm sp2 C-clusters embedded within polyimide as shown by HRTEM, AFM and electrostatic force EFM. XPS confirmed B substitution within the sp2 structures and also showed that the presence of B during the ion induced thermal transformations enhanced N co-substitution, while the G and D Raman bands indicated a high degree of disorder within these C-nanoclusters which increased with atom substitution. Electron transport properties indicated the semiconducting behaviour of the C-nanowire arrays. Impedance spectroscopy separated electron hopping transport within these nanowires from electron tunnelling between neighbouring nanowires. Nanowires were clearly surrounded by an altered polymer interphase region of increased segmental chain mobility and higher dielectric polarisability allowing control of overall electron transport in device applications. This reactive ion beam irradiation route allows separation of chemical doping and synthesis from the nanostructure fabrication, as a new nanotechnology route.

Fluorinated polyimide–silica films with low permittivity and low dielectric loss

A sol–gel process was used to prepare polyimide–silica hybrid films from the fluorinated polyimide precursors (6FDA-ODA) and tetraethylorthosilicate (TEOS) in N,N-dimethyl acetamide. The hybrid film was then treated with hydrofluoric acid to remove the dispersed silica particles, leaving inside the film pores with diameters ranged from 80 nm to 1 μm, which depended on the size of the silica particles. The chemical structures and morphology of the hybrid and porous films were characterized by Fourier transform infrared spectroscopy and scanning electron microscopy, respectively. The synthesized porous fluorinated polyimide films show low relative dielectric permittivity of 1.9, rendering them promising for microelectronic packaging materials.

Fabrication of Silver Patterns on Polyimide Films Based on Solid-Phase Electrochemical Constructive Lithography Using Ion-Exchangeable Precursor Layers

We report a fully additive-based electrochemical approach to the site-selective deposition of silver on a polyimide substrate. Using a cathode coated with ion-doped precursor polyimide layers, patterns of metal masks used as anodes were successfully reproduced at the cathode-precursor interface through electrochemical and ion-exchange reactions, which resulted in the generation of silver patterns on the polyimide films after subsequent annealing and removal from the substrate. Excellent interfacial adhesion was achieved through metal nanostructures consisting of interconnecting silver nanoparticles at the metal-polymer interface, which are electrochemically grown "in" the precursor layer. This approach is a resist- and etch-free process and thus provides an effective methodology toward lower-cost and high-throughput microfabrication.