Two‐step thermal conversion from poly(amic acid) to polybenzoxazole via polyimide: Their thermal and mechanical properties

Poly(amic acid) was synthesized with a low‐temperature solution polymerization of 3,3′‐dihydroxybenzidine and pyromellitic dianhydride in N,N‐dimethylacetamide. The cast films were thermally treated at various temperatures. The polyimide containing the hydroxyl group was rearranged by decarboxylation, resulting in a fully aromatic polybenzoxazole at temperatures higher than 430 °C. These stepwise cyclizations were monitored with elemental analysis, Fourier transform infrared, and nuclear magnetic resonance. Microanalysis results confirmed the chemical compositions of poly(amic acid), polyimide, and polybenzoxazole, respectively. A cyclodehydration from poly(amic acid) to polyimide occurred between 150 and 250 °C in differential scanning calorimetry, and a cyclodecarboxylation to polybenzoxazole appeared at 400–500 °C. All the samples were stable up to 625 °C in nitrogen and displayed excellent thermal stability. Polybenzoxazole showed better thermal stability than polyimide, but polyimide exhibited better mechanical properties than polybenzoxazole. However, polyimide showed a crystalline pattern under a wide‐angle X‐ray, whereas polybenzoxazole was amorphous. The precursor poly(amic acid) was readily soluble in a variety of solvents, whereas the polyimide and polybenzoxazole were not soluble at all. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2537–2545, 2000     

Synthesis and properties of polyimides derived from 1,6-bis(4-aminophenoxy)naphthalene and aromatic tetracarboxylic dianhydrides

1,6-Bis(4-aminophenoxy)naphthalene ( I ) was used as a monomer with various aromatic tetracarboxylic dianhydrides to synthesize polyimides via a conventional two-stage procedure that included ring-opening polyaddition in a polar solvent such as N,N-dimethylacetamide (DMAc) to give poly(amic acid)s, followed by thermal cyclodehydration to polyimides. The diamine ( I ) was prepared through the nucleophilic displacement of 1,6-dihydroxynaphthal-ene with p-chloronitrobenzene in the presence of K₂CO₃, followed by catalytic reduction. Depending on the dianhydrides used, the poly(amic acid)s obtained had inherent viscosities of 0.73–2.31 dL/g. All the poly(amic acid)s could be solution cast and thermally converted into transparent, flexible, and tough polyimide films. The polyimide films had a tensile modulus range of 1.53–1.84 GPa, a tensile strength range of 95–126 MPa, and an elongation range at break of 9–16%. The polyimide derived from 4,4′-sulfonyldiphthalic anhydride (SDPA) had a better solubility than the other polyimides. These polyimides had glass transition temperatures between 248–286°C (DSC). Thermogravimetric analyses established that these polymers were fairly stable up to 500°C, and the 10% weight loss temperatures were recorded in the range of 549–595°C in nitrogen and 539–590°C in air atmosphere. © 1995 John Wiley & Sons, Inc.     

Synthesis and properties of new polyimides derived from 1,5-bis(4-aminophenoxy)naphthalene and aromatic tetracarboxylic dianhydrides

Novel aromatic polyimides containing bis(phenoxy)naphthalene units were synthesized from 1,5-bis(4-aminophenoxy)naphthalene (APN) and various aromatic tetracarboxylic dianhydrides by the usual two-step procedure that included ring-opening polyaddition in a polar solvent such as N,N-dimethylacetamide (DMAc) to give poly(amic acid)s, followed by cyclodehydration to polyimides. The poly(amic acid)s had inherent viscosities between 0.72 and 1.94 dL/g, depending on the tetracarboxylic dianhydrides used. Excepting the polyimide IVb obtained from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), all other polyimides formed brown, flexible, and tough films by casting from the poly(amic acid) solutions. The polyimide synthesized from BPDA was characterized as semicrystalline, whereas the other polyimides showed amorphous patterns as shown by the x-ray diffraction studies. Tensile strength, initial moduli, and elongation at break of the APN-based polyimide films ranged from 105–135 MPa, 1.92–2.50 GPa, and 6–7%, respectively. These polyimides had glass transition temperatures between 228 and 317°C. Thermal analyses indicated that these polymers were fairly stable, and the 10% weight loss temperatures by TGA were recorded in the range of 543–574°C in nitrogen and 540–566°C in air atmosphere, respectively. © 1993 John Wiley & Sons, Inc.     

Preparation and characterization of a polyimide nanofoam through grafting of labile poly(propylene glycol) oligomer

Preparation of a polyimide nanofoam (PI‐F) for microelectronic applications was carried out using a polyimide precursor synthesized from poly[(amic acid)‐co‐(amic ester)] and grafted with a labile poly(propylene glycol) (PPG) oligomer. Polyimide precursor was synthesized by partial esterification of poly(amic acid) (PAA) derived from pyromellitic dianhydride (PMDA) and 4,4′‐oxydianiline (ODA). The precursor was then grafted with bromide‐terminated poly(propylene glycol) in the presence of K₂CO₃ in hexamethylphosphoramide and N‐methylpyrrolidone, imidized at 200°C in nitrogen and the product was subsequently decomposed in air at 300°C to eliminate the labile PPG oligomer to produce PMDA/ODA polyimide nanofoam. Nuclear magnetic resonance spectroscopy (¹H‐NMR) and Fourier transform infrared spectroscopy (FT‐IR) techniques were used to characterize the formation of polyimide precursor and extent of grafting of PPG with polyimide. The results of thermogravimetric analysis (TGA) showed three step decomposition of nanofoam with the removal of PPG at 350°C and decomposition of polyimide at around 600°C. The polyimide nanofoams were also characterized by small angle X‐ray scattering (SAXS), field‐emission scanning electron microscopy (FE‐SEM) and transmission electron microscopy (TEM). The morphology showed nanophase‐separated structures with uniformly distributed and non‐interconnected pores of 20–40 nm in size. Dynamic mechanical analysis (DMA) indicated higher storage modulus for the foamed structure compared to the pure PI with reduction in loss tangent for the former system. Copyright © 2004 John Wiley & Sons, Ltd.     

Cyclopolycondensations. VI. Fully aromatic polybenzoxazinones from aromatic poly(amic acids)

New high temperature aromatic polybenzoxazinones of high molecular weight have been prepared by the cyclopolycondensation of 4,4′-diaminobiphenyl-3,3′-dicarboxylic acid (I) with aromatic dicarboxylic acid halides (II). The low temperature solution polymerization techniques afforded poly(amic acid) (III) of high molecular weight in the first step. An open-chain precursor subsequently underwent thermal cyclodehydration along the polymer chain at 200–350°C. in the second step, to give in quantitative yield a fully aromatic polybenzoxazinone (IV) of outstanding heat stability both in nitrogen and in air. The poly(amic acid) is soluble in N-methyl-2-pyrrolidone, and tough, transparent films can be cast from solution. Insoluble aromatic polybenzoxazinone films which possess excellent oxidative and thermal stability were obtained by the heat treatment of the polyamic acid. A detailed account of polymerization conditions in the low temperature solution polymerization of polybenzoxazinones is given, and the reaction mechanisms of cyclopolycondensation of poly(amic acids) and the formation of polybenzoxazinones are discussed.     

Synthesis and characterization of photo‐crosslinkable poly(amic acid ester)s with 2‐hydroxy‐4‐oxo‐hept‐5‐enyl side chain

Photosensitive poly(amic acid ester)s (PAEs) with 2‐hydroxy‐4‐oxo‐hept‐5‐enyl side group were simply synthesized from a non‐photosensitive polyamic acid (PAA), which was prepared from cyclobutane‐1,2,3,4‐tetracarboxylic dianhydride (CBDA) and 4,4′‐diaminodiphenyl ether (DDE) in N‐methyl‐2‐pyrrolidinone (NMP). 1‐oxiranyl‐pent‐3‐en‐2‐one was added to the poly(amic acid) solution to give the photosensitive PAEs by a ring opening esterification of the poly(amic acid). The esterification reaction was conducted with changing a reaction time and amounts of 1‐oxiranyl‐pent‐3‐en‐2‐one. The degree of esterification (DOE) increased with increasing esterification reaction time and amounts of 1‐oxiranyl‐pent‐3‐en‐2‐one. A photo‐lithography evaluation for the PAE‐D4 with the highest DOE was conducted in the presence of 1‐[4‐(phenylthio)phenyl]‐2‐(O‐benzoyloxime)‐1,2‐octanedione (PPBO) as a photoinitiator at a wavelength of 365 nm using a high‐pressure mercury lamp. The normalized film thicknesses for PAE‐D3 were measured with various post‐exposure baking (PEB) temperatures, which showed that the optimum PEB temperature was 120°C. The resolution of the resulting polyimide film cured at 250°C for 60 min was 25 µm. The initial decomposition temperature of the polyimide film was around 354°C and there was no weight loss at the temperature of 250–350°C. Copyright © 2009 John Wiley & Sons, Ltd.     

Novel positive‐working and aqueous‐base‐developable photosensitive poly(imide benzoxazole) precursor

A novel positive‐working and aqueous‐base‐developable photosensitive poly(imide benzoxazole) precursor based on a poly(amic acid hydroxyamide) bearing phenolic hydroxyl groups and carboxylic acid groups, a diazonaphthoquinone (DNQ) photosensitive compound, and a solvent was developed. Poly(amic acid hydroxyamide) was prepared through the polymerization of 2,2‐bis(3‐amino‐4‐hydroxyphenyl)hexafluoropropane, trimellitic anhydride chloride, and 4,4′‐oxydibenzoyl chloride. Subsequently, the thermal cyclization of the poly(amic acid hydroxyamide) precursor at 350 °C produced the corresponding poly(imide benzoxazole). The inherent viscosity of the precursor polymer was 0.17 dL/g. The cyclized poly(imide benzoxazole) showed a high glass‐transition temperature of 372 °C and 5% weight loss temperatures of 535 °C in nitrogen and 509 °C in air. The structures of the precursor polymer and the fully cyclized polymer were characterized with Fourier transform infrared and ¹H NMR. The photosensitive polyimide precursor containing 25 wt % DNQ photoactive compound showed a sensitivity of 256 mJ/cm² and a contrast of 1.14 in a 3‐μm film with a 0.6 wt % tetramethylammonium hydroxide developer. A pattern with a resolution of 5 μm was obtained from this composition. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5990–5998, 2004     

Cyclopolycondensation. XIII. New synthetic route to fully aromatic poly(heterocyclic imides) with alternating repeating units

Fully aromatic poly(heterocyclic imides) of high molecular weight were prepared by the cyclopolycondensation reactions of aromatic diamines with new monomer adducts prepared by condensing orthodisubstituted aromatic diamines with chloroformyl phthalic anhydrides. The low-temperature solution polymerization techniques yielded tractable poly(amic acid), which was converted to poly(heterocyclic imides) by heat treatment to effect cyclodehydration at 250–400°C under reduced pressure. In this way, the polyaromatic imideheterocycles such as poly(benzoxazinone imides), poly(benzoxazole imides), poly(benzimidazole imides) and poly(benzothiazole imides) were prepared, which have excellent processability and thermal stability both in nitrogen and in air. The poly(amic acids) are soluble in such organic polar solvents as N,N-dimethyl-acetamide, N-methylpyrrolidone, and dimethyl sulfoxide, and the films can be cast from the polymer solution of poly(amic acids) (η_inh = 0.8–1.8). The film is made tough by being heated in nitrogen or under reduced pressure to effect cyclodehydration at 300–400°C. The polymerization was carried out by first isolating the monomer adducts, followed by polymerization with aromatic diamines. On subsequently being heated, the open-chain precursor, poly(amic acid), undergoes cyclodehydration along the polymer chain, giving the thermally stable ordered copolymers of the corresponding heterocyclic imide structure.     

Synthesis of Polyimide Nanocomposites

A series of polyimide nanocomposite (PINC) films were prepared by using poly(amic acid) and Ba, Sr, Sn, TiO₃ nanoparticles via in-situ polymerization method. Poly(amic acid) was synthesized from benzophenone tetracarboxylic anhydride and diamino diphenyl ether by ring-opening polyaddition reaction. The PINC films were characterized by FTIR spectroscopy. The thermal properties of PINC films were investigated by using differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) methods. The prepared PINC showed major weight loss in the range of 550–600°C in nitrogen atmosphere. These had char yield in the range of 50–60% at 800°C. The morphological studies of PINC films were carried out using SEM method.     

Preparation and properties of new polyimides derived from 1,6-diaminodiamantane

New polyimides containing diamantane units were prepared by a conventional two-step method starting from 1,6-diaminodiamantane and aromatic dianhydrides. The intermediate poly(amic acid)s had inherent viscosities of 0.33–0.55 dL/g. These polyimides did not decompose below 400°C in air or nitrogen atmosphere, and the temperature at 5% weight loss were above 491°C. The glass transition temperatures of the polyimides were found to be 375–429°C by DSC. These polyimides had almost the same semicrystalline patterns and exhibited crystalline diffraction peak (2 θ) at around 15°. The polyimide Vb exhibited a melting endothermic peak at 514°C. © 1996 John Wiley & Sons, Inc.     

Synthesis and blend behavior of high performance homo- and segmented thermoplastic polyimides

New synthetic methodology was developed as part of an effort to increase the processibility of high Tg polyimide homo⁻ and copolymers, suitable as matrix resins and structural adhesives. Molecular weight and end group control together with solution imidization techniques were successfully employed to convert a variety of poly(amic acid) intermediates to fully cyclized polyimides. The solution imidization was conducted in N-methylpyrrolidone (NMP) with o-dichlorobenzene used as the azeotroping agent at 165–190°C. This technique has produced products which are more soluble than polyimides prepared previously by bulk thermal cyclization of poly(amic acids) at temperature of 300°C. They are also more stable than “chemically” imidized materials. In addition, incorporation of the monofunctional reagent phthalic anhydride provides nonreactive phthalimide end groups and controlled molecular weight. This latter feature significantly further improved the melt and solution processibility of the resulting polyimides. In this study thermoplastic, fully cyclized polyimides of 10 000, 20 000, and 30 000 M̄n were prepared which displayed glass transition temperatures ranging from 260–353°C, with the highest T_g observed with phthalimide capped polyimide systems derived from 6F-dianhydride and p-phenylene diamine. Tough, transparent films were prepared from polymers of 20 000 and 30 000 g/mol by casting from NMP solution or by compression molding at 50–70°C above the glass transition temperature. For purposes of molecular weight assessment, t-butyl phthalic anhydride was used as the end blocker. This permitted 400 M-Hertz proton NMR to be used for assessing the concentration of end groups. Comparison of the 18 aliphatic protons at the end of the chain allowed M̄n values to be determined, which agreed well with theory. A series of poly(arylene ether ketone)/aromatic polyimide blends were investigated to determine the influence of structural variation and composition on miscibility. As an extension to the PEEK^TM/Ultem^TM blend system, which has been reported to be miscible over all proportions, this study examined how structural variations in both the poly(arylene ether ether ketone) and the polyimide portions affect miscibility. In particular, replacement of the hydroquinone fraction in PEEK^TM with bisphenol A or sulfonyl diphenol produced an amorphous polymer which was no longer miscible with Ultem^TM. Polyimide structures modified by employing 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) and 4,4′-[1,4-phenylene-bis-(1-methyl ethylidene)] bisaniline (Bis P) diamine to obtain higher glass transition temperatures were also investigated. This system afforded homogeneous blends with PEEK^TM when the (Bis P) diamine was utilized in the synthesis of the polyimide. Furthermore, up to 50 mole percent of hexafluoro-bis-dianhydride (6FDA) could be substituted for BTDA without loss of miscibility. However, when the more polar 3,3′-diaminodiphenylsulfone diamine was employed, immiscible blends resulted. An additional important variant has been to incorporate polyimide siloxane segmented copolymers into the PEEK^TM blend system. The polyimide segment can be designed to be miscible whereas the siloxane portion is homogeneously dispersed into a second phase which, in fact, enriches the surface behavior quite dramatically in siloxane content. The latter could be of some importance in allowing for atomic oxygen resistance and possibly improved flame resistance behavior.     

Polyimides with alicyclic diamines. I. Syntheses and thermal properties

The polyaddition reactions of alicyclic diamines such as 1,4-diaminocyclohexane (1,4-CHDA) or 4,4′-diaminodicyclohexylmethane (DCHM) and configurational isomers of 1,4-CHDA or DCHM with tetracarboxylic aromatic anhydrides in aprotic solvents were carried out to prepare high molecular weight poly(amic acid)s. Through the thermal imidization of poly(amic acid)s, several flexible polyimide films were prepared. Because of the stiffness of the alicyclic moieties in diamines, the resulting polyimides exhibit high glass transition temperatures (220–340°C) almost similar to those for corresponding aromatic polyimides which have phenylene groups in place of cyclohexyl groups, and show good thermal stability. The partial crystallization was observed for polyimides with trans-cyclohexyl moiety during the heating in differential scanning calorimetry and ascertained by wide-angle x-ray diffraction. Thus, the inhibition of the occurrence of charge transfer in polyimides is accomplished by introducing alicyclic diamines in place of aromatic diamines without reducing their thermal stability. © 1993 John Wiley & Sons, Inc.     

Surface structure of semicrystalline polyimide films

The surface structure of thin films based on poly[4,4′-bis(4″-N-phenoxy)diphenyl]amic acid of 1,3-bis(3′,4-dicarboxyphenoxy)benzene and the product of its thermal imidization—a semicrystalline polyimide—poly[4,4′-bis(4″-N-phenoxy)diphenyl]imide of 1,3-bis(3′,4-dicarboxyphenoxy)benzene—at various stges of thermal imidization and after melting and subsequent annealing has been studied by methods of transmission, scanning electron, and atomic force microscopies. The topological structure of the film surface has been described in terms of the discrete cluster model. Under heating to 200 and 280°C, a continuous network of the infinite cluster appears; subsequent annealing leads to disintegration of the network to discrete fragments that practically correspond to clusters in the starting poly(amic acid) film. The polyimide film heated to 280°C crystallizes in the form of needle crystals stable to the argon plasma. The surface morphology of polyimide films recrystallized from melt is of the spherulite character.     

Structure and properties of thin films of binary rodlike/flexible polyimide mixtures

Binary mixtures of a rodlike poly(p-phenylene pyromellitimide) (PMDA-PDA) and a flexible 6F-BDAF polyimide synthesized from hexafluoroisopropylidene diphthalic anhydride and 2,2-bis(4-aminophenoxy-p-phenylene) hexafluoropropane were prepared by solution-blending of the meta-PMDA-PDA poly(amic ethyl ester) and 6F-BDAF poly(amic acid) precursors, followed by solvent evaporation and thermal imidization. Mixtures containing different molecular weights of 6F-BDAF poly(amic acid) were studied. The size scale of the phase separation, as measured by light scattering, is ca. 1 μm or smaller in most cases. The domain size is primarily set by the demixing of the precursor polymers during solvent evaporation, with no significant coarsening observed during the thermal imidization. The observed variation of the domain size with molecular and process parameters such as composition, molecular weight, and film thickness is discussed in terms of the miscibility of the precursor polymers, rate of solvent evaporation, and solidification. Dynamic mechanical thermal analysis and dielectric relaxation measurements indicate that the glass transition temperature of 6F-BDAF is unaffected in all of the mixtures studied, indicating complete demixing of rodlike and flexible polyimides in agreement with theory. X-ray photoelectron spectroscopy results show a strong surface segregation of 6F-BDAF in mixtures containing as low as 10% by weight of the 6F-BDAF component in the bulk. The mixtures with PMDA-PDA as the major matrix component therefore exhibit excellent mechanical toughness, dimensional stability up to 500°C, low coefficients of thermal expansion (< ca. 10 ppm/°C), and low dielectric constants (<3.0). On the other hand, the surface properties of the mixtures are dominated by the flexible 6F-BDAF, resulting in excellent polymer/polymer self-adhesion (lamination) properties between fully imidized films.     

Synthesis and evaluation of novel polyimides derived from spirobichroman diether anhydride

A spirobichroman structure-containing diether anhydride (SBCDA), 6,6′-bis(3,4-dicarboxyphenoxy)-4,4,4′,4′,7,7′-hexamethyl-2,2′-spirobichroman dianhydride, was prepared by the nucleophilic nitrodisplacement of 4-nitrophthalonitrile with the phenolate ion of 6,6′-dihydroxy-4,4,4′,4′,7,7′-hexamethyl-2,2′-spirobichroman, followed by alkaline hydrolysis of the intermediate tetranitrile and dehydration of the resulting tetraacid. A series of high molecular weight poly(ether imide)s with inherent viscosities between 0.45 and 1.28 dL/g were synthesized from SBCDA and various aromatic diamines via a conventional two-stage procedure that included ring-opening polyaddition in N,N-dimethylacetamide (DMAc) to give poly(amic acid)s, followed by thermal cyclization to poly(ether imide)s. The intermediate poly(amic acid)s had inherent viscosities of 0.70–1.50 dL/g. Except for the poly(ether imide) obtained from p-phenylenediamine, the other poly(ether imide)s were soluble in various organic solvents and could be solution-cast into transparent, flexible, and tough films. These poly(ether imide)s had glass transition temperatures in the range 175–262°C and showed no significant decomposition below 420°C, with 10% weight loss being recorded above 446°C in nitrogen or air. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 2801–2809, 1997     

Formation of new banded spherulites in polyimides

The crystalline morphology and structural development of aromatic polyimides during an optimum continuous thermal imidization procedure were examined by means of polarized optical microscopy and X‐ray diffraction. During thermal imidization, 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride/1,3‐diaminobenzene polyimide samples formed complicated spherulites, which, in addition to zigzag Maltese crosses, also showed concentric extinction rings, which are characteristic of banded spherulites. The factors affecting the formation of banded spherulites were studied. The initial imidization conditions dramatically affected the formation of the banded spherulite morphology: slow heating (0.5 °C/min) or fast heating (20 °C/min) led to relatively small polyimide spherulites and less identifiable extinction rings. The morphological features were also affected by the molecular weight of the polyimide: higher molecular weight samples showed typical banded spherulites, whereas low‐molecular‐weight samples formed degenerated banded spherulites. In all the spherulites formed in 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride/1,3‐diaminobenzene polyimides, special zigzag Maltese crosses, instead of normal Maltese crosses, were observed. The relationship between the imidization procedure and the spherulite morphology formation was also studied. X‐ray and Fourier transform infrared together revealed that after several minutes of thermal treatment, the crystallization was nearly complete, with a 42.5% degree of crystallinity; meanwhile, only some poly(amic acid) converted to the corresponding polyimide, with a 27% degree of imidization. The crystalline morphology and structure formed in the initial stage of the imidization process were maintained during the following imidization processing at an elevated temperature. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1997–2004, 2005     

Polyimide containing internal acetylene units linked para to the aromatic ring

4,4′-Diaminodiphenylacetylene (p-intA) was reacted with 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) and pyromellitic dianhydride (PMDA) in N-methyl-2-pyrrolidone (NMP) to give poly(amic acid) solution of moderate to high viscosity. Thermal imidization gave polyimide having acetylene units that are linked para to the aromatic connecting unit. Polyimide having acetylene units that are linked meta to the aromatic connecting unit also was prepared utilizing 3,3′-diaminodiphenylacetylene (m-intA) for comparison. The crosslinking behavior of the acetylene units was observed with DSC. Exotherm due to the crosslinking of the para-linked acetylene units appeared at ca. 340 to 380°C depending on the structure of polyimide, whereas meta-linked acetylene units appeared at lower temperature as 340–350°C. After thermal treatment at high temperature such as 350 or 400°C, the amount of the exotherm became smaller and finally disappeared on DSC, confirming the progress of crosslinking. Dynamic mechanical properties of the polyimide films show that glass transition temperature increased with higher heat treatment, also confirming the progress of crosslinking. Tensile properties of the polyimide films showed that rigid polyimide films consisting of p-intA with BPDA or PMDA have considerably higher modulus than those consisting of m-intA. Cold-drawing of the poly(amic acid) followed by imidization gave much higher modulus in the case of rigid polyimide. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2395–2402, 1997     

Kinetic study of low‐temperature imidization of poly(amic acid)s and preparation of colorless,transparent polyimide films

Conventional synthesis of polyimides includes high‐temperature (160–350 °C) imidization of poly(amic acid)s. In the present work, imidization has been carried out at much lower temperatures (40–160 °C). 1,2,4,5,‐cyclohexanetetracarboxylic dianhydride (HPMDA) or pyromellitic dianhydride (PMDA) was polymerized with an aromatic diamine, 4,4′‐diaminodiphenylmethane (DDPM), to give poly(amic acid)s, which were then imidized chemically. Imidization was more than 90% complete even at the very low imidization temperature of 40 °C. It was found that the imidization occurs in two steps: an initial rapid cyclization and a subsequent slower cyclization. The activation energy for the rapid process was determined to be 4.3 kJ/mol, and that of the slower process, 4.8 kJ/mol. As the imidization temperature decreases, the transmittance of the resulting polyimides tends to gradually increase, the cutoff wavelength decreases and the color becomes pale. A partially aliphatic polyimide based on HPMDA and DDPM prepared at 40 °C yielded thin films that were highly transparent and colorless, and had good flexibility, solubility and thermal stability. The polyimide films prepared in this study may be good candidates for flexible, transparent plastic substrates in the display industry. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1593–1602     

Preparation and characterization of polyimide/mesoporous silica hybrid nanocomposites based on water-soluble poly(amic acid) ammonium salt

Recently, mesoporous silica was blended with polyimide to develop low dielectric constant (k) materials with improving mechanical and thermal properties of polyimide by utilizing both the nanoporous structure and silica framework. However, even the use of mesoporous silica did not show a significant decrease of k due to the phase segregation in between polyimide and the mesoporous silica materials. In this work, we attempted to prepare polyimide/mesoporous silica hybrid nanocomposites having relatively good phase mixing behavior by utilizing polyimide synthesized from a water soluble poly(amic acid) ammonium salt, which lead to low k up to 2.45. The thermal properties of polyimide were improved by adding mesoporous silicas. For this work, we have fabricated mesoporous silicas through surfactant-templated condensation of tetraethyl orthosilicate (TEOS). Pyromellitic dianhydride (PMDA)-4,4′-oxydianiline (ODA) polyimide was prepared from poly(amic acid) ammonium salt, which had been obtained by incorporating triethylamine (TEA) into PMDA-ODA poly(amic acid) in dimethylacetamide (DMAc), followed by thermal imidization.     

Effect of carbon nanotubes and their dispersion on thermal curing of polyimide precursors

The process of thermal imidization reaction is significant for temperature and time control in the polyimide industry. Here, we report the effect of carbon nanotubes and their states of dispersion on the thermal imidization of the precursor films of polyimide (poly(amic acid)) for the first time. The curing process was followed by measuring Fourier transform-infrared (FT-IR) spectra, fluorescence spectra, thermogravimetric-differential scanning calorimeter (TG-DSC) properties and the refractive indices of films. It was found that by evenly dispersing 1 wt% of carbon nanotubes assisted by a dispersant in the poly(amic acid),the full imidization temperature of the polyimide can be reduced from 300 °C to 250 °C. Different states of distribution of CNTs were observed by light microscopy and scanning electron microscopy, and proved that a better dispersion of carbon nanotubes dramatically enhanced the speed of imidization. Moreover, the DSC results showed that lower decomposition temperature of poly(amic acid) could be obtained with more uniform distribution of carbon nanotubes, which means the process of cyclodehydration of the poly(amic acid) was accelerated.