Poly Amic Acid Precursors Research Articles

Correlating the synthesis protocol of aromatic polyimide film with the properties of polyamic acid precursor

Thousands of different copolyimide combinations render it technically impossible to have a single universal synthesis method to produce aromatic polyimide film. This study aimed to outline the selection of synthesis protocol, either through the casting of chemically imidized polyimide solution or thermal imidization of polyamic acid (PAA), to produce the polyimide film. The rheological behaviour, molecular weight, and solubility of five structurally different PAA were analysed and correlated to both imidization methods. In this work, a tough polyimide film was successfully synthesized by casting the chemically imidized polyimide derived from high viscosity (> 81 cP) and high molecular weight (≥ 1.35 x 106 g/mol) PAA. On the contrary, both low viscosity (< 13 cP) and high viscosity (> 81 cP) PAA demonstrated the possibility to produce polyimide film via thermal imidization route. The longer molecular chain of ODPA-6FpDA:DABA (3:2) polyimide produced from thermal imidization had restricted the passage of CO2 across the polyimide film when it was applied in the gas separation application. The outcome from this work serves as a guideline for the selection of suitable polyimide film synthesis protocol, which will minimize the time and chemical consumption in future exploration of new polyimide structure.

High thermal resistance polyimide separators prepared via soluble precusor and non-solvent induced phase separation process for lithium ion batteries

Lithium ion battery separators with excellent thermal stability are highly desired to meet the requirements of electric vehicles. Basing on the excellent natures of PMDA-ODA polymide (PI) and processibility of the corresponding precursor solution of polyamic acid (PAA), the porous PI separators were prepared from the PAA precursor via non-solvent induced phase separation (NIPS) and thermal imidization process. By controlling the NIPS process, the ideal PI separator with high porosity and pore-connectivity was successfully obtained and employed in lithium ion batteries (LIBs). Besides the quite high thermal stability, this PI separator behaved excellent wettability in the liquid electrolyte and high liquid electrolyte uptake. By activated with liquid electrolyte, the ionic conductivity of the PI separator could reach as high as 2.15mS·cm−1. Compared with the cells with the typical PP separators, the cells assembled with the PI separators performed the lower resistance, the higher discharge capacity and the better rate capability. These results suggested that the PAA-NIPS process could be an efficient and applicable route for preparing high performance PI separators.

High-performance electrospun co-polyimide nanofibers

Co-polyimide nanofiber based on BPDA (3, 3′, 4, 4′-Biphenyltetracarboxylic dianhydride)/PDA (p-Phenylene-diamine)/ODA (4, 4′-Oxydianiline) (BPO) were produced by electrospinning followed by imidization from precursor polyamic acid (PAA) nanofiber. The aligned co-polyimide nanofiber mats possessed a modulus, strength and strain-at-break of respective 10 GPa, 1.04 GPa, and 13.5%. In comparison with previously reported co-polyimide nanofibers BPDA/BPA/ODA (BBO) with similar chemical structures, these BPO co-polyimide nanofibers can be as stiff and strong while at the same time exhibiting moderate ductility. On the other hand, the current BPO co-polyimide nanofibers exhibited a greater toughness than previously reported homo-polyimide (BPDA/PDA) nanofibers due to their relatively high strain-at-break, leading to similar levels of toughness as Kevlar fibers. A novel and efficient way to evaluate mechanical properties of aligned nanofiber bundles (∼30 nanofibers in a bundle) by virtue of a micro-tensile tester was also reported. Young's modulus of 38 ± 2 GPa and tensile strength of 1550 ± 70 MPa were found for nanofiber bundles and were significantly higher than those of aligned mats, and are among the highest reported for electrospun fibers. Further evaluation of this bundle data using Daniel's theory based on Weibull statistics resulted in a predicted tensile strength of single co-polyimide nanofibers of about 1.9 GPa.

The influence of polyamic acid molecular weight on the membrane structure and performance of polyimide solvent‐resistant nanofiltration

AbstractBACKGROUNDPolyimide (PI) solvent‐resistant nanofiltration (SRNF) membranes were prepared by a two‐step method. The polyamic acid (PAA) precursor was synthesized first, and then PAA membranes were obtained via a non‐solvent induced phase separation method. Finally, the PAA membranes were transformed into PI membranes by the thermal imidization method. The effect of molecular weight of precursor on the structure and performance of PI membranes was investigated in detail. In addition, the influence of other processing parameters, such as evaporation time and immersion time in the coagulation bath, on the performance of PI membranes prepared from PAA with different molecular weights was also investigated.RESULTSThe results showed that the structure of PI membrane varied with PAA molecular weight. The PI membranes prepared from low molecular weight PAA failed to form a defect‐free membrane with enough strength, and resulted in high flux but low rejection. Accordingly, PI membranes processed from PAA with high molecular weight retain rejection higher than 90% of FCF with decreased flux. The results also indicated that the optimized molecular weight of PAA should be between 1.23 × 105 g mol−1 and 2.44 × 105 g mol−1 to prepare defect‐free PI membrane for nanofiltration. The results also showed that the performance of the PI membranes can be controlled by adjusting the evaporation time, immersion time, and also the operating pressure.CONCLUSIONSThe PAA molecular weight has a great effect on the morphology and performance of PI membranes. A suitable PAA molecular weight is necessary for preparing defect‐free PI nanofiltration membrane. © 2015 Society of Chemical Industry

Effects of Imidization Temperature on the Structure and Properties of Electrospun Polyimide Nanofibers

A two-step process for preparing polyimide nanofiber was used in this work, and the nanofibers of precursor polyamic acid (PAA) were prepared by electrospinning. The polyimide nanofibers were obtained through thermal imidization at different thermal imidization temperatures. The influences of different imidization temperatures on morphology, chemical structure, thermal stability, and mechanical properties of the polyimide nanofibers were investigated by Fourier Transform Infrared Spectroscope (FT-IR), Scanning Electronic Microscopy (SEM), thermal gravimetric analysis (TGA) and Electromechanical Universal Testing Machine. The experimental results show that the imidization of PAA nanofibers was not complete under 290°C but complete under 350°C. With the rise of imidization temperature from 290 °C to 350°C, the polyimide nanofibers became thinner, and the thermal stability of polyimide nanofibers was also improved. The breaking strength and elongation at break of the nanofiber membranes increased at higher imidization temperature.

Molecular mechanism of temporal physico/chemical changes that take place during imidization of polyamic acid: Coupled real-time rheo-optical and IR dichroism measurements

Multiple overlapping physical and chemical changes often take place during casting/drying and imidization from PMDA-ODA polyamic acid precursors from cast solutions. To shed light into details of these complex phenomena, we designed a unique real time measurement system that combines true stress, true strain, in-plane birefringence and temperature with polarized ultra-rapid scan FT-IR spectrometry (URS-FT-IR). At the early stages of heating (21°C–130 °C), initially isotropic solution cast film was observed to develop stress and birefringence as the solvent decomplexed and evaporated without showing any imidization as it was held in uniaxially constrained state. At a temperature around 130 °C, the onset of imidization reaction was detected while the stress went through a maximum. Beyond this stage, the evaporation of bound solvent and chemical conversion was observed to take place simultaneously and this is accompanied by a steady increase in birefringence. As the majority of the bound solvent evaporated, the stress and birefringence values started leveling off at long times.

Polyimide encapsulated lithium-rich cathode material for high voltage lithium-ion battery.

Lithium-rich materials represented by xLi2MnO3·(1 - x)LiMO2 (M = Mn, Co, Ni) are attractive cathode materials for lithium-ion battery due to their high specific energy and low cost. However, some drawbacks of these materials such as poor cycle and rate capability remain to be addressed before applications. In this study, a thin polyimide (PI) layer is coated on the surface of Li1.2Ni0.13Mn0.54Co0.13O2 (LNMCO) by a polyamic acid (PAA) precursor with subsequently thermal imidization process. X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HR-TEM) results confirm the successful formation of a PI layer (∼3 nm) on the surface of LNMCO without destruction of its main structure. X-ray photoelectron spectroscopy (XPS) spectra show a slight shift of the Mn valence state from Mn(IV) to Mn(III) in the PI-LNMCO treated at 450 °C, elucidating that charge transfer takes place between the PI layer and LNMCO surface. Electrochemical performances of LNMCO including cyclic stability and rate capability are evidently improved by coating a PI nanolayer, which effectively separates the cathode material from the electrolyte and stabilizes their interface at high voltage.

Preparation of porous carbon materials by using coagulated polyamic acid precursor

In recent years, porous carbon materials have received a great deal of attention due to their many applications. Especially, activated carbon materials are potentially of great technological interest for the development of catalytic, electronic, fuel cells, and hydrogen storage systems. However, there are many complex and risky chemical activation process for improving the surface area of carbon materials. Polyimides (PIs), which are synthesized by the thermal imidization of poly(amic acid) (PAA) precursors, have been adapted as a carbonization precursors because of their high carbonization yield and homogeneously shrinking property during carbonization. From these reasons, many kinds of carbon materials as like highly oriented graphite films, macroporous carbons, and carbon thin membranes are prepared from PIs. In this study, we demonstrated a new preparation method of porous carbon from a polyamic acid precursor without any chemical activation process. We prepared porous 6FDA-HAB PAA with coagulation method (Scheme I). By virtue of the high carbonization yield and homogeneous shrinking properties of 6FDA-HAB PAA, we successfully manufactured carbon materials with a porous channel structure. The structured carbon materials showed a high surface area of 1668 m/g.

Hydroscopicity and Dielectric Properties of Polyimide Containing Biphenyl and Bis-Benzimidazole Rings

A kind of Polyimide films (PI films) containing biphenyl and bis-benzimidazole rings were prepared from the its precursor Polyamic acid (PAA) solution synthesized by aromatic diamine containing biphenyl and bis-benzimidazole rings with different commercial aromatic dianhydrides. The PI films exhibited excellent hydrophobicity with the contact angle of higher than 90o and the hydroscopicity are lower than 1%. The PI films also have great dielectric properties of the dielectric constant ranges from 2.10 to 2.94 under the voltage frequency within 1 to 106HZ.

Porous Polyimide-Silica Composite: A New Thermal Resistant Flexible Material

ABSTRACTWe developed a new highly porous polyimide (PI) -silica composite with high flexibility, mechanical strength, and heat resistance. The composite was prepared by a new process consisting of (1) phase separation of a mixture of PI precursor (polyamic acid), solvent, and silicon alkoxide, induced by high-pressure CO2 (40 °C, 20 MPa), (2) silicate formation by sol-gel reaction, and (3) supercritical CO2 extraction of the solvent. The composite had a bimodal porous structure with micropores of 10-30 μm and nanopores of ∼50 nm. In the PI matrix, silica nanoparticles (&lt; 100 nm in diameter) were highly dispersed. Porosity of the composite was 78%, which is higher than that of conventional porous PI prepared by physical foaming technique. Relative dielectric constant of the material was lower than 1.4 at 1 MHz. The porous PI-silica composite sheet was flexible enough to be folded without cracking. Notably, the Young’s modulus (0.80 GPa) and the onset decomposition temperature (600 °C) of the PI-silica composite were higher than those of conventional porous PI with similar porosity, respectively. The porous PI-silica composite is promising as a flexible thermal insulator for high-temperature use and as a thermal resistant low-k material.

Study on polybenzoxazinone membrane in pervaporation processes

ABSTRACTPoly(benz‐3,1‐oxazinone‐4) was prepared by thermal cyclization of its hydrolytically stable precursor polyamic acid. Both polymer and its precursor were investigated as membrane materials. Thermogravimetric analysis and contact angle measurements were carried out for characterization of peculiarity of membrane compositions and analysis of membrane surface. Pervaporation of water–isopropanol mixture was studied in the wide range of feed composition. To interpret the pervaporation transport properties of the membranes, swelling experiments were performed, kinetic curves of sorption and desorption were plotted, and basic sorption and diffusion parameters were analyzed. It was established that poly(benz‐3,1‐oxazinone‐4) membrane is extremely effective in dehydration of water–isopropanol mixture and shows high separation factor. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4024–4031, 2013

Microstructures of BPDA-PPD polyimide thin films with different thicknesses

The microstructures of 3,4,3′,4′-biphenyltetracarboxylic dianhydride –p-phenylene diamine (BPDA-PPD) polyimide (PI) thin films with different thicknesses were investigated by grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XRR) at the SPring-8 synchrotron radiation facility. The film density calculated from XRR profiles increased with decreasing the film thickness. Processing of PI thin films from precursor polyamic acid (PAA) solutions involves simultaneous thermal imidization, evaporation of residual solvents and crystallization. The thickness reduction during imidization was smaller for the thinner PAA film. This implies that the much more larger amount of the residual solvent in the thinner PAA film brings high molecular mobility, which results in the higher conversion to PI at any curing temperature. The thinner BPDA-PPD film was more dense than the thicker film because of better molecular packing, which is a result of much more amount of the residual solvent for the thinner film during the thermal curing.

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.

Effect of the chemical structure of aromatic polyimides on their thermal aging, relaxation behavior and mechanical properties

This article examines the effects of structural changes and thermal aging treatments on the relaxation processes and mechanical properties of three polyimides differing for their molecular structure i.e. PMDA-ODA, 6FDA-ODA, and 6FDA-6FpDA. These polyimides were obtained by thermal imidization of their polyamic acid precursors, which were synthesized from the respective dianhydrides [pyromellitic anhydride (PMDA), hexahydrofluoroisopropylidene diphthalic anhydride (6FDA)], and diamines [4,4′-diaminodiphenyl ether (ODA), 4,4′-(hexafluoroisopropylidene) dianiline (6FpDA)]. After the curing process, the polyimides were thermally aged at a fixed temperature for various times Dynamic mechanical measurements performed in a multi-frequency mode, were used to determine the glass-rubber and sub-glass transitions, as well as the activation energy of the β transition. It was found that the Tg decreased in the order PMDA-ODA > 6FDA-6FpDA > 6FDA-ODA as a result of an increased chain rigidity and molecular packing induced by charge transfer interactions during the thermal imidization process. The β sub-glass transition showed two relaxation processes identified as β′ and β′′. The β′ process was attributed to the local motion of the diamine constituents while the β′′ process was caused by the local motion of the dianhydride moiety. The cooperativity of these molecular motions was also assessed via the Starkweather method. The thermal aging enhanced the state of aggregation of polyimide chains and thus the Tg and the sub-glass transition properties. This effect was particularly marked for the PMDA-ODA polyimide. Also the mechanical properties were significantly affected by chemical structure and aging treatments. For non-aged samples the more influenced parameter was the elongation at break, which decreased in the order PMDA-ODA > 6FDA-ODA > 6FDA-6FpDA. The aging enhanced the elastic modulus and the tensile strength and reduced the elongation at break.

Avoiding fusion of electrospun 3,3′,4,4′-biphenyltetracarboxylic dianhydride-4,4′-oxydianiline copolymer nanofibers during conversion to polyimide

Man-made high-performance fibers produced a revolution in structural materials in the twentieth century. Fiber properties are known to increase with the decrease of their diameters. Ultrafine sub-micron and nanometer-sized fibers attract rapidly growing interest due to their unique advantages for diverse applications. Electrospinning is a straightforward low-cost top-down nanomanufacturing technique producing continuous nanofibers. Electrospinning allows direct fabrication of macroscopic nanofilamentary assemblies for structural and functional applications. Recently, high-performance polyimide nanofibers have been produced by electrospinning of polyamic acid precursors followed by thermal curing. However, curing resulted in fusion of nanofilaments at contacts. While fusion might be beneficial for some applications, it usually results in deterioration of the ordered molecular structure and mechanical properties of fibers in bundles and tows. In this report, we investigated the effect of residual solvent dimethylacetamide (DMAc) on the fusion of nanofibers in electrospun nanofiber bundles during thermal curing of polyimide. Feasibility of removal of the residual DMAc by aging in vacuum and by washing with other solvents was explored. The effect of an additive, dodecylethyldimethylammonium chloride (DEDAC) on fusion was investigated. It was found that the removal of residual DMAc by aging in vacuum reduced the nanofilament fusion during imidization. The presence of DEDAC in the absence of DMAc was shown to further reduce the fusion. DEDAC may represent a new class of additives with a potential to preserve nanostructure of polyamic acid precursors during thermal conversion to polyimides. The results of this report can be used in the development of ultrahigh-performance nanofilamentary fibers.

Processing of high performance composites based on peek by aqueous suspension prepregging

The use of polyamic acid (PAA) precursor as interphase in polymer composites is one of the many applications of polyimides (PIs). In this work, composites based on poly(ether-ether-ketone) (PEEK) and carbon fibers were prepared using two manufacturing techniques for thermoplastic composites: hot compression molding, and aqueous polymeric suspension prepregging using PIs as interphase. Two PAAs were synthesized and used as interphases: 3,3'-4,4'-benzophenonetetracarboxylic dianhydride/oxydianiline (BTDA/ODA) and pyromellitic dianhydride/oxydianiline (PMDA/ODA). The PAA/PI systems were analyzed by differential scanning calorimetry (DSC), thermogravimetry (TGA), Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). Results from these analyses confirmed the synthesis of these compounds. Aqueous polymeric suspension prepregging was more efficient than hot compression molding when the PMDA/ODA PAA/PI interphase was used; also, the interlaminar shear strength of composites produced using this technique was 14.5% higher than the one produced using hot compression molding.

Novel Modified Aromatic Polyamides and Polyimides Derived from a Diphenyl Ether-Based Diamine Containing Laterally-Attached Phenoxy Phenylene Groups

In this research a new structurally-modified monomer, 2,2'-bis[(p-phenoxy phenyl)]-4,4'-diaminodiphenyl ether (PPAPE) was synthesized by a three-step manner starting from 4,4'-dinitrodiphenyl ether. Novel classes of wholly aromatic polyamides and polyimides were prepared from PPAPE through the conventional polycondensation reactions including triphenyl phosphate-activated phosphorylation method for the preparation of the polyamides and cyclodehydration of in situ-formed polyamic acid precursors for the preparation of the polyimides. n and w values were determined by GPC measurements. Crystallinity, organosolubility, and qualitative film characteristics of the resulting aromatic polyamides and polyimides were investigated. A low crystallinity extent was only observed using their wide-angle X-ray diffractograms. All polymers had excellent organosolubility in common polar solvents such as DMAc, DMF, DMSO, and NMP. The resulting well-designed polymers could also be cast into transparent and flexible films.

Hierarchical structure formation induced by dewetting in an imprinting process

Creating an ordered three-dimensional hierarchical structure using nanoimprint lithography remains a challenge even though such structures with micro-/nanometer scale are important with technological advances in a number of applications. The controllable polyimide (PI) hierarchical structures could be obtained by using polyamic acid (PAA) precursor in γ-butyrolatone-N-methylpyrrolidone mixture solution of various concentrations (1, 2, 3, and 4wt%) through dewetting in an imprinting process. PI patterns without a residual layer are formed due to the dewetting phenomena caused by the surface tension, and thus formed patterns of different ambits are obtained due to the fluidity of the PAA solutions. This method is not restricted to a specific polymer and substrate and could be easily applied to pattern other organic materials.

Simulation of dry‐spinning process of polyimide fibers

AbstractAs one type of high‐performance fibers, the polyimide fibers can be prepared from the precursor polyamic acid via dry‐spinning technology. Unlike the dry‐spinning process of cellulose acetate fiber or polyurethane fiber, thermal cyclization reaction of the precursor in spinline with high temperature results in the relative complex in the dry‐spinning process. However, the spinning process is considered as a steady state due to a slight degree of the imidization reaction from polyamic acid to polyimide, and therefore a one‐dimensional model based on White‐Metzer viscoelastic constitutive equation is adopted to simulate the formation of the fibers. The changes of solvent mass fraction, temperature, axial velocity, tensile stress, imidization degree, and glass transition temperature of the filament along the spinline were predicted. The effects of spinning parameters on glass transition temperature and imidization degree were thus discussed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Synthesis and gas transport properties of ODPA–TAP–ODA hyperbranched polyimides with various comonomer ratios

A series of hyperbranched copolyimides (HBPI)s based on commercially available monomers 4,4′-oxydiphthalic anhydride (ODPA), 2,4,6-triaminopyrimidine (TAP) and 4,4′-oxydianiline (ODA) were prepared. The synthesis involved the formation of hyperbranched polyamic acid (PAA) precursors in the first step and the thermal imidization of cast thin PAA films in the second step. Two basic types of HBPIs were prepared by controlling the molar ratio of ODPA and an amine mixture of TAP and ODA. When the molar ratio was 1:1, the amine-terminated HBPIs were obtained; with the molar ratio of 2:1 anhydride-terminated HBPIs were prepared. Degree of branching was estimated by 1H and 13C NMR analysis. It was found that approximately 48% of TAP units presented in ODPA:TAP:ODA = 1:0.75:0.25 HBPI macromolecules create the branching unit. Amine-terminated HBPIs showed moderate weight-average molecular weights and these values rather higher than for the anhydride-terminated HBPIs. With increasing ODA comonomer content in amine-terminated HBPIs increased their molecular weight and thermal and mechanical stability, whereas in anhydride-terminated HBPIs these trends were opposite. Amine-terminated HBPIs generally exhibited higher thermal stability than the anhydride-terminated ones. Gas permeability coefficients of both HBPIs types increased with increasing content of ODA comonomer. Prepared membranes exhibited high separation performance and have a potential to be utilized in industrial gas separation applications.