Polyimide Precursor Research Articles

Gas sorption, diffusion, and permeation in thermally rearranged poly(benzoxazole-co-imide) membranes

Thermally rearranged polymer membranes exhibit extraordinary gas permeability based on a rigid polymer structure with a high free volume element. In this study, TR poly(benzoxazole-co-imide) membranes from 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA), 3,3′-dihydroxyl-4-4′-diamino-biphenyl (HAB), 2,2′-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (bisAPAF), and 2,4,6-trimethyl-m-phenylenediamine (DAM) were prepared to improve their gas separation properties. Copolymer membranes of polyimides and TR polybenzoxazoles may be desirable to generate efficient gas transport properties as well as to process polymers into fiber or film forms. Gas permeability, diffusivity, and solubility of the precursor polyimide and TR poly(benzoxazole-co-imide) membranes were investigated to characterize gas transport properties for small gas molecules including H2, O2, N2, CH4, and CO2. Thermal rearrangement process resulted in an increase in polymer free volume, which improved the diffusion and sorption coefficients.

Cross-Linked Thermally Rearranged Poly(benzoxazole-co-imide) Membranes for Gas Separation

A novel strategy to tune the cavity size and free volume of thermally rearranged polybenzoxazole (TR-PBO) copolymer membranes by transesterification cross-linking reaction of o-hydroxy polyimide precursors with 1,4-butylene glycol in the solid state is demonstrated in this study. During the thermal rearrangement (TR) process at high temperatures, loose diester interchain cross-linkers are prone to degrade while formation of a much more rigid cross-linked structure occurs following or alongside the imide-to-benzoxazole rearrangement. As a result, a synergistic effect of high permeability and high selectivity appeared to be created in one step, and the newly synthesized cross-linked TR-PBO membranes exhibited outstanding gas separation performance, surpassing the so-called 2008 upper bound for CO2/CH4 separation.

Preparation of highly uniform self‐standing submicrometer polyimide films and an investigation of their antibulging capabilities

ABSTRACTAiming for X‐ray astronomy applications, we prepared large‐area submicrometer polyimide (PI) films [diameter (Φ) = 8 cm] with great thickness uniformity via the spin‐coating technique by using a PI precursor, poly(amic acid) (PAA) derived from 3,3′,4,4′‐biphenyltetracarboxylic dianhydride, and p‐phenylenediamine as the starting materials. The effects of the spinning speed, apparent viscosity of the PAA solution (η), and spinning time on the PI film thickness and its uniformity, as characterized by the measurement of the film thickness every 0.5 cm along the diameter direction, were investigated. By optimizing the spin‐coating conditions, we prepared final submicrometer PI films with average thicknesses in the range of 200–850 nm and with film thickness fluctuations of less than 1.3%. The pressure bulge test results indicate that at a thickness of 805 nm and an inside test aperture diameter of 2.64 cm, the prepared PI films reached a final burst pressure of 20.2 KPa; this suggested excellent mechanical performances in the self‐standing submicrometer PI film. This study makes a contribution by providing a typical example and opening the way for the preparation of robust self‐standing submicrometer PI films with great thickness uniformities for X‐ray astronomy applications. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014, 131, 39977.

Thermal rearranged poly(benzoxazole-co-imide) membranes for CO2 separation

The thermal rearrangement of α-hydroxyl-polyimide membranes improves gas permselectivity properties compared to the polyimide precursor. By introducing segments within the polymer that do not undergo thermal rearrangement, the gas separation properties of the thermally rearranged membrane can be modified. In particular, polyimides are a class of polymer that have wide gas separation performance, which can be incorporated into thermal rearranged membranes through random copolymers and enable tailoring of the gas separation properties of the resulting membrane, without the need to alter the thermal rearranged moieties. A range of polyimide copolymers based on 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA) and diamines 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) with 2,3,5,6-tetramethyl-1,4-phenylenediamine (4MPD) or 9,9′-bis (4-aminophenyl) fluorene (FDA) were synthesised, and thermally rearranged into poly(benzoxazole-co-imide) membranes. The diamine moiety HAB undergoes thermal rearrangement to form the benzoxazole segments of the copolymer, while 4MPD or FDA based segments remain as imide segments in the copolymer. These copolymers, in both polyimide and poly(benzoxazole-co-imide) form, were tested as gas separation membranes for CO2 separation from N2 and CH4. Thermal rearrangement of the homo-polymer HAB–6FDA resulted in increased gas permeability compared to the polyimide precursor. With the addition of the 4MPD moiety in the copolymer, it was discovered that the gas permeability, solubility and selectivity of membrane were similar to the homo-polymer HAB–6FDA. This result was attributed to the 4MPD–6FDA segments having similar gas separation properties as the benzoxazole segments. The presence of FDA in the copolymer resulted in a reduction in gas permeability as the FDA to PBO segmental ratio increased. However, the gas selectivity of this poly(benzoxazole-co-imide) was independent of FDA amount. Hence, through the addition of diamine moieties in poly(benzoxazole-co-imide), the gas separation properties of the thermal rearranged membrane can be varied.

Highly proton-conductive thermally rearranged polybenzoxazole for medium-temperature and low-humidity polymer electrolyte fuel cells

Here a new membrane design concept to improve proton conductivity and to maintain a constant acid doping level for an extended period of time for medium-temperature and low-humidity polymer electrolyte fuel cells (MT/LH-PEFC) is presented. A polymer electrolyte membrane is prepared via thermal rearrangement of hydroxyl-containing polyimide (HPI) precursor membranes over 350 °C, followed by subsequent acid-impregnation. The thermal treatment for 1 h converts HPI into a thermally rearranged polybenzoxazole (TR-PBO) membrane with high surface area, similar to zeolites. The microporous structure and the basic sites (–CN–) in benzoxazole moieties contribute to the stable impregnation of small acidic molecules (e.g., HCl, HNO3, H3PO4, and HPF6) in large quantities within the polymer matrix. The acid-doping level decreases with the increasing size of acidic dopants. TR-PBO impregnated using HCl with the smallest radius has a much higher doping level and excellent proton conductivity (1.60 × 10−1 S cm−1 at 130 °C and RH 28%) when compared with PBI (6.59 × 10−2 S cm−1 at the same conditions). Different from a common acid-doped PBI system, the acid-doped TR-PBO membranes do not exhibit a severe acid leaching even in repeated heating and cooling cycles between 90 and 130 °C.

Growth and Characterization of Vapor Deposited Polyimide Thin Films

In this paper, we report on the preparation and characterization of polyimide (PI) thin films through vapor deposition polymerization (VDP). PI thin films were formed by vapour co-deposition of the precursor monomers pyromellitic dianhydride (PMDA) and 4,4-oxydianiline (ODA) following a thermal treatment at a temperature of over 300°C. The effects of the polymerization temperature on the characterization of PI films were investigated using scanning electron microscopy and Fourier transform infrared spectroscopy. The results showed that the deposited polyamic acid (PAA, a precursor of polyimide) with imidization in the temperature interval of 300-400°C are smooth, uniformly thick and form without shrinkage.

Effect of Carbon Black on the Properties of Polyimide Foams

Polyimide foams (PIF) contained carbon black were prepared from polyimide precursors by inflation process. The structure, compressive strength and heat-resistance of the PIF were studied. The FT-IR results confirmed the successful synthesis of PIF.Tgvalue of PIF with different content carbon black was over 255°C.With the increasing of carbon black content,Tgof PIF decreased. PIF decomposition temperature (Td5%) was higher than 540°C,while the compression strength of PIF about 0.12MPa with low content carbon black.

Colorless polyimides with low coefficient of thermal expansion derived from alkyl-substituted cyclobutanetetracarboxylic dianhydrides

Alkyl-substituted cyclobutanetetracarboxylic dianhydrides (CBDAs) were synthesized by photo-dimerization of alkyl-substituted maleic anhydrides to obtain novel colorless polyimides (PIs). Dimethyl-substituted CBDA (DM-CBDA) showed much higher polymerizability with various diamines than conventional cycloaliphatic tetracarboxylic dianhydrides and led to high molecular weights of PI precursors. Polyaddition of non-substituted CBDA and trans-1,4-cyclohexanediamine (t-CHDA) was completely inhibited by salt formation in the initial reaction stage. The use of DM-CBDA allowed the formation of a homogeneous/viscous PI precursor solution by overcoming the salt formation problem. The prominent substituent effect probably reflects how the methyl substituents of DM-CBDA contributed to increasing the salt solubility. Some of the thermally imidized DM-CBDA-based systems simultaneously possessed non-coloration, low coefficient of thermal expansion (CTE), very high Tg exceeding 300 °C and very low dielectric constant. Copolymerization was very effective for improving the solubility of DM-CBDA-based PIs. The copolyimide cast films prepared via chemical imidization displayed a further decreased CTE without sacrificing other target properties, suggesting that the present materials can be useful as plastic substrates in display devices. The mechanism of self-chain orientation behavior during solution casting is also discussed. A potential application of the copolyimide systems as optical compensation film materials in liquid crystal displays is proposed. © 2013 Society of Chemical Industry.

Inverse Gas Chromatographic Study of Sorption Thermodynamics in Thermally Rearranged Polymer Based on 2,2-Bis(3-amino-4-hydroxyphenyl)-hexafluoropropane and 4,4′-Hexafluoroisopropylidene Diphthalic Anhydride

This work reports the first investigation using the inverse gas chromatography method of a polyimide precursor and the product of its thermal rearrangement (TR polymer), extensively studied earlier. Sorption of gases (CO2, C2H6, C3H8) was studied at the finite dilution regime, while vapors (n-alkanes C7, C8, C10, C14, C16) were investigated at infinite dilution. It was demonstrated that thermal treatment at 450 °C results in a significant increase in the solubility coefficients S for large gas molecules. The absolute values observed for the TR polymer solubility coefficients of solutes are significant and comparable with those for the polymer of intrinsic microporosity (PIM-1), the polymer known by the greatest S values among all of the polymers studied. Sorption thermodynamics in the TR polymer is distinguished by very large and negative mixing parameters: enthalpy h̅1E,∞ and entropy s̅1E,∞.

Hydrogen sorption in polymers for membrane applications

Hydrogen sorption between −20 °C and 70 °C and at pressures up to 60 bara was determined for a polyimide and corresponding thermally rearranged (TR) polymers prepared from 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). Hydrogen sorption increased by a factor of approximately 2.6 between the polyimide precursor and the most highly converted TR polymer. This relative increase in sorption was similar to that observed for other non-polar light gases such as N2, O2, and CH4, but less than that observed for CO2. Additionally, H2 sorption was measured for other polymers commonly studied in the membrane literature, including AF 2400, Matrimid®, polysulfone, and poly(dimethylsiloxane). Among the glassy polymers tested, polysulfone had the lowest H2 sorption, and the HAB-6FDA TR polymer had the highest H2 sorption. A slight dual-mode curvature was observed for H2 sorption in several of the glassy materials, and it was most pronounced at low temperatures and for the TR polymers. Enthalpies of sorption were also determined. The most exothermic enthalpy of sorption occurred in Matrimid®, and a slightly endothermic enthalpy of sorption was observed in rubbery poly(dimethylsiloxane). Comparisons between gravimetric and volumetric sorption showed similar results.

Rapid Production of Ultralow Dielectric Constant Porous Polyimide Films via CO2-tert-Amine Zwitterion-Induced Phase Separation and Subsequent Photopolymerization

Porous polymeric films are promising materials for the production of ultralow-dielectric constant materials. A high porosity polyimide thin film was prepared via the phase separation of a polyimide precursor in an N,N-dimethylacetamide (solvent)/2-(diethylamino)ethyl methacrylate/photoinitiator system. A novel technique involving high-pressure CO2 (5 MPa) gas injection was used to form CO2-2-(diethylamino)ethyl methacrylate zwitterion salt and induce the immediate phase separation and solvent droplet formation (within 60 s) of a wet precursor film on a metal substrate. The film was exposed to UV light through quartz windows for 30 s to polymerize the 2-(diethylamino)ethyl methacrylate while maintaining a constant CO2 pressure. The cured thin film containing numerous pores with an average diameter of approximately 1 μm ± 1.0 μm was treated at 320 °C for 1 h under a continuous flow of nitrogen. The obtained film was 30 μm thick and exhibited pores with an average diameter of approximately 1 μm ± 0.9 μm. The...

Supramolecular structure formation of Langmuir-Blodgett films of comblike precursor and polyimide

The surface structure of Langmuir-Blodgett films of a comblike polyimide precursor—a rigid-chain polyamic acid alkylamine salt bearing multichains of tertiary amine—and films of the corresponding polyimide were studied by atomic force microscopy (AFM). An analysis of the images of the surface of three-layer films revealed a domain structure. It was found that the Langmuir-Blodgett film formation of the precursor occurs as a result of the layer-by-layer deposition of two-dimensional domains (composed of polyamic acid salt molecules on the water surface) onto a substrate. The formation of domains in a monolayer is associated with the chemical structure of the precursor, to be more precise, with the rigidity of the main chain and the presence of closely spaced aliphatic side chains in the polymer chain, whose total cross-section area is close to the surface area of the projection onto the plane of the repeating unit of the main chain. Polyimide films inherit the domain structure of the precursor films; the inhomogeneity of the film thickness substantially decreases, whereas the domain size and character of their distribution in the film remain unchanged.

Structural and surface heterogeneity of phosphorus-containing polyimide-derived carbons: effect of heat treatment temperature

Phosphorus-containing carbons have been obtained by carbonization of porous copolymer of 4,4′-bis(maleimidodiphenyl)methane (50 mol%) and divinylbenzene (50 mol%) in presence of phosphoric acid at temperatures 400–1000 °C. Porous structure was analyzed by nitrogen adsorption isotherms while surface chemistry was investigated by potentiometric titration method. It has been shown that carbons obtained at 500–1000 °C are micro-mesoporous with pore sizes of 1–1.1, 2–3 and 5.4 nm. The most developed porosity was achieved at 600 °C reaching BET surface area 890 m2/g and total pore volume 0.45 cm3/g. Carbons obtained by carbonization of polyimide precursor in presence of phosphoric acid showed acidic character with 30–40 % of phosphate surface groups. Maximum total amount of acidic surface groups was achieved at 800 °C reaching 3.2 mmol/g. Assignment of strongly acidic surface groups to phosphates was corroborated by pK value, phosphorus content and thermal gravimetric analysis.

Synthesis and characterization of Thermally Rearranged (TR) polymers: influence of ortho-positioned functional groups of polyimide precursors on TR process and gas transport properties

Aromatic polyimides bearing various ortho-functional groups (i.e., acetate group and pivalic acetate group) were prepared via acetylation of a poly(hydroxyimide) containing ortho-positioned hydroxy groups using acetic anhydride or pivalic anhydride. The completeness of acetylation was confirmed by 1H NMR and FTIR. Chemically derivatized polyimides were used as precursors for an imide-to-benzoxazole thermal rearrangement (TR) process. The influence of various ortho-functionalities on the TR process and gas transport properties of the resulting TR polymers was studied. Differing from the –OH groups in a poly(hydroxyimide), the acetate groups of acetylated polyimide precursors degrade at elevated temperatures, and the degradation process interplays with imide-to-benzoxazole conversion. The acidic degradation product, as detected by 1H NMR, is suspected to have some catalytic effect on the TR process, which along with the protecting function of the acetate groups, resulted in a lower onset TR conversion temperature, the ability to conduct the TR process in air, and a higher TR conversion level. Gas permeation properties greatly depend on the ortho-functionality of polyimide precursors as well. The precursor films containing larger functional groups are much more permeable with comparable gas selectivities. Similarly, the resulting TR polymers formed from polyimides with larger leaving groups also showed much higher gas permeabilities despite similar degrees of TR conversion. The incorporation of bulkier functional groups in the TR precursors provides an effective way to significantly improve the gas transport performance, particularly the gas permeabilities of both the polyimide precursors and the resulting TR polymers.

Fabrication and Properties of Flexible OLEDs on Polyimide-Graphene Composite Film Substrate

In this work a clear polyimide (PI) film was synthesized by using selected monomers containing multiple CF3 groups. The polyimide-graphene composite films were made by film coating of mixture of polyamic acid (precursor of polyimide) solution with reduced grapheme oxide. The flexible OLEDs were fabricated by using the polyimide-graphene composite films on which the ITO/Ag/ITO thin films were deposited by roll-to-roll sputter instead of usual ITO-glass and the properties of flexible OLEDs were examined from the viewpoint of flexible substrates.

Low-dielectric, nanoporous polyimide thin films prepared from block copolymer templating

In this paper, a new method to the preparation of low-dielectric nanoporous polyimide (PI) films was addressed, based on the self-assembly structures of PS-b-P4VP/poly(amic acid) (PAA, precursor of PI) blends. It is found the microphase-separation structure of PS-b-P4VP/PAA is a precondition of the formation of nanoporous structures, which could be achieved by solvent annealing. Nanoporous PI films with spherical pore size of ~11 nm were obtained by thermal imidization followed by the removal of the PS-b-P4VP block copolymer. The porosity of the nanoporous PI films could be controlled by the weight fraction of the PS-b-P4VP block copolymer. The dielectric properties of the nanoporous PI films were studied, and it was found that the introduction of nanopores could effectively reduce the dielectric constant from 3.60 of dense PI films to 2.41 of nanoporous PI films with a porosity of 26%, making it promising in microelectronic devices. The fabrication method described here could be extended to other polymer systems.

Surface modification of polyimide gate insulators for solution-processed 2,7-didecyl[1]benzothieno[3,2-b][1]benzothiophene (C10-BTBT) thin-film transistors

The surface property of a polyimide gate insulator was successfully modified with an n-octadecyl side-chain. Alkyl chain-grafted poly(amic acid), the polyimide precursor, was synthesized using the diamine comonomer with an alkyl side-chain. By adding a base catalyst to the poly(amic acid) coating solution, the imidization temperature of the spin-coated film could be reduced to 200 °C. The 350 nm-thick polyimide film had a dielectric constant of 3.3 at 10 kHz and a leakage current density of less than 8.7 × 10(-10) A cm(-2), while biased from 0 to 100 V. To investigate the potential of the alkyl chain-grafted polyimide film as a gate insulator for solution-processed organic thin-film transistors (TFTs), we fabricated C(10)-BTBT TFTs. C(10)-BTBT was deposited on the alkyl chain-grafted polyimide gate insulator by spin-coating, forming a well-ordered crystal structure. The field-effect mobility and the on/off current ratio of the TFT device were measured to be 0.20-0.56 cm(2) V(-1) s(-1) and >10(5), respectively.

Synthesis and properties of multi-block semi-alicyclic polyimides for thermally stable transparent and low CTE film

Highly transparent polyimide (PI) with a low coefficient of thermal expansion (CTE), comparable to that of copper foil (17 ppm/K), has been developed in conjunction with a relatively rigid and symmetrical monomer, trans-cyclohexyl diamine (CHDA), and in so doing we clarified the effectiveness of a multi-block copolymer system in terms of maximizing the degree of CTE reduction as well as mechanical strength without sacrificing other PI film properties such as high thermal stability (Tg > 280 °C) or high transparency. Multi-block co-PIs were synthesized through the formation of end-functional oligomers, anhydride-terminated oligo-imides (OIs), and amine-terminated oligo(amic acid)s (OAAs). Although there is a large polarity difference between OI and OAA, the multi-block copolymerization still took place well enough to produce a highly transparent PI film as well as a clear PI precursor solution. For comparison, random co-PIs and homo-PIs derived from the same monomer species were synthesized. We found that the difference in the polymer sequence – block vs. random – influenced the film properties so that even the respective PIs had the same monomer composition. Multi-block co-PIs effectively decreased CTE with less CHDA content, and marked the highest tensile modulus (Et: 5.1 GPa) and tensile strength (σt: 208 MPa) which surpassed that of homo-PIs. This can probably be attributed to the micro-aggregation for the multi-block system. It is also suggested that they have a close relationship between the micro-aggregation and the cut-off wavelength of the film while keeping a high light transmittance in the visible region.

Dynamic relaxation characteristics of thermally rearranged aromatic polyimides

The glass–rubber and sub-glass relaxation characteristics of ortho-functionalized aromatic polyimides and thermally rearranged polymers were investigated by dynamic mechanical and dielectric methods. Soluble polyimides (HAB–6FDA; APAF–ODPA) were synthesized by chemical and thermal imidization and subject to thermal rearrangement at elevated temperature. For the thermal exposure histories investigated, mass loss studies indicated partial conversion of the polyimide precursor, suggesting the formation of TR copolymers containing both benzoxazole units and residual imide segments. Measurement of storage modulus and loss tangent was used to follow the thermal rearrangement process in-situ as reflected in the suppression of the polyimide glass transition as a function of precursor structure, the nature of the ortho functional groups and prior thermal exposure. In addition, changes in the position and intensity of local relaxations detected across the sub-glass temperature range were correlated with the degree of thermal rearrangement in these polymers.

Solution‐processable colorless polyimides derived from hydrogenated pyromellitic dianhydride with controlled steric structure

AbstractThis work presents novel colorless polyimides (PIs) derived from 1R,2S,4S,5R‐cyclohexanetetracarboxylic dianhydride (H″‐PMDA). Isomer effects were also discussed by comparing with PI systems derived from conventional hydrogenated pyromellitic dianhydride, that is, 1S,2R,4S,5R‐cyclohexanetetracarboxylic dianhydride (H‐PMDA). H″‐PMDA was much more reactive with various diamines than H‐PMDA, and the former led to PI precursors with much higher molecular weights. The results can be explained from the quite different steric structures of these isomers. The thermally imidized H″‐PMDA‐based films were colorless regardless of diamines because of inhibited charge‐transfer interaction. In particular, the H″‐PMDA/4,4′‐oxydianiline system simultaneously achieved a very high Tg exceeding 300 °C, high toughness (elongation at break > 70%), and good solution processability. In contrast, the H‐PMDA‐based counterparts were essentially insoluble. The outstanding solubility of the former probably results from disturbed chain stacking by its nonplanar steric structure. An advantage of chemical imidization process is also proposed. In some cases, a copolymerization approach with an aromatic tetracarboxylic dianhydride was effective to improve the thermal expansion property. The results suggest that the H″‐PMDA‐based PI systems can be promising candidates for novel high‐temperature plastic substrate materials in electronic paper displays. A potential application as optical compensation film materials in liquid crystal displays (LCD) is also proposed in this work. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013