Subsequent Thermal Imidization Research Articles

Mechanically strong, transparent polyimide composite thin films with a low dielectric constant

Aromatic polyimide is widely used in microelectronics as dielectric packaging layers. There is a high demand to reduce its dielectric constant to meet the fast development of communication technology. Here we report a simple and eco-friendly approach to preparing polyimide films with a lowered dielectric constant by adding silica hollow nanospheres with an average size of 100 nm in an aqueous solution of poly(amic acid) salt, and subsequent film casting and thermal imidization. The silica hollow nanospheres present a uniform distribution in polyimide matrix after surface modification with 3-aminopropyltriethoxysilane. The dielectric constant of the resulting composite films decreases linearly with particle loading till 15 wt% and reaches the lowest value of 2.5. The surface hardness and elastic modulus of the films improve by adding hollow nanospheres, while maintaining high optical transparency, high flexibility and a low coefficient of thermal expansion.

Frequency-Dependent Dielectric Permittivity and Water Permeability in Ordered Mesoporous Silica-Grafted Fluorinated Polyimides.

Polymers with a low dielectric constant (Dk) are promising materials for high-speed communication networks, which demand exceptional thermal stability, ultralow Dk and dissipation factor, and minimum moisture absorption. In this paper, we prepared a series of novel low-Dk polyimide films containing an MCM-41-type amino-functionalized mesoporous silica (AMS) via in situ polymerization and subsequent thermal imidization and investigated their morphologies, thermal properties, frequency-dependent dielectric behaviors, and water permeabilities. Incorporating 6 wt.% AMS reduced the Dk at 1 MHz from 2.91 of the pristine fluorinated polyimide (FPI) to 2.67 of the AMS-grafted FPI (FPI-g-AMS), attributed to the free volume and low polarizability of fluorine moieties in the backbone and the incorporation of air voids within the mesoporous AMS particles. The FPI-g-AMS films presented a stable dissipation factor across a wide frequency range. Introducing a silane coupling agent increased the hydrophobicity of AMS surfaces, which inhibited the approaching of the water molecules, avoiding the hydrolysis of Si-O-Si bonds of the AMS pore walls. The increased tortuosity caused by the AMS particles also reduced water permeability. All the FPI-g-AMS films displayed excellent thermooxidative/thermomechanical stability, including a high 5% weight loss temperature (>531 °C), char residue at 800 °C (>51%), and glass transition temperature (>300 °C).

Regulating porous microstructure of polyimide aerogels toward efficient shape memory performance

The development of lightweight shape memory materials with exceptional thermal insulation properties is highly important in aerospace applications. Herein, shape memory polyimide (PI) aerogels was fabricated by molecularly designed PI molecular chains through freeze gelation and subsequent thermal imidization. The freezing conditions were controlled to regulate the porous microstructures of the aerogels. The interactions between PI chains formed physically crosslinked netpoints, which could overcome the expansive force and capillary force, thereby creating a stable porous structure. Because shape memory was thermally activated, the shape memory mechanisms associated with the thermal conductivity of the aerogels were comprehensively explored. Upon decreasing the pore size, dense frameworks were formed, contributing to the improved thermo-mechanical and compressive properties. Moreover, the heat transfer efficiency in the aerogel skeleton was enhanced, resulting in a rapid shape recovery response. Benefiting from the physically crosslinked structures, including entangled chains and π-π interactions, the fabricated PI aerogels demonstrated exceptional shape fixation and recovery capabilities. These findings may provide guidelines for designing smart aerogels for aerospace applications.

Shape memory polyimide/carbon nanotube composite aerogels with physical and chemical crosslinking architectures for thermal insulating applications

The development of lightweight smart materials with exceptional shape memory and thermal insulation properties is highly important in the aerospace industry. The reported shape memory polymer aerogels (SMPAs) are restricted to harsh environments because of their poor high-temperature resistance and large volume shrinkage at elevated temperatures. Herein, shape memory polyimide (PI) composite aerogels with superior thermal insulation were fabricated by combining molecularly designed PI chains with amino-functionalized carbon nanotubes (NH2-CNTs) through freezing gelation and subsequent thermal imidization. Because shape memory behavior was thermally triggered, the thermal conductivity associated with the shape memory mechanisms of the aerogels was comprehensively explored. The presence of CNTs promoted heat transfer in the aerogel skeleton, facilitating the shape recovery response. Benefiting from chemical-crosslinked and physical-crosslinked structures, the fabricated PI composite aerogels demonstrated desirable thermo-mechanical properties, exceptional shape memory and superior thermal insulation performance. This study may provide guidelines for designing shape memory PI composite aerogels for harsh environment applications in aerospace engineering.

Polyimide/titanium dioxide self-cleaning nano-hybrid films with high photocatalytic properties

Self-cleaning films are considered an emerging paradigm of environmentally functional materials. Enhancing the photocatalytic activities of self-cleaning film is a viable strategy for improving its antifouling characteristics. In this work, a polyimide/titanium dioxide (TiO2) film (PI/TiO2) film was developed by casting a mixture of polyamic acid solution and TiO2 hydrosol with a subsequent thermal imidization. TiO2 with a concentration of 1, 3, and 5 ​wt% was successfully incorporated into the PI matrix and was equally distributed in nanometer size in the PI/TiO2 nano-hybrid films. The addition of TiO2 into the film significantly improved its catalytic performance as indicated by the photocatalytic degradation of methylene blue and resazurin solution, suggesting a good self-cleaning property for antifouling applications. In addition, the thermal and mechanical stability of the PI film were improved with the addition of TiO2. After eight cycles, the PI-TiO2 films also demonstrated excellent cycling stability. This study introduces a novel self-cleaning film family composed of PI/TiO2 nano-hybrid film and emphasizes the engineering potential of photocatalytic materials in the context of self-cleaning film.

Autocatalytic one-step high-temperature synthesis of carboxylated polyimides for in-situ high performance applications

Carboxylated polyimides (CxPIs) represent a great potential for gas separation membranes, highly thermostable coatings and fibers, ion-conducting materials, aerogels, (nano)composites, etc. However a prolonged and expensive two-stage method proceeding through the formation of an unstable poly(amic acid) and subsequent thermal or chemical imidization is traditionally used for the synthesis of CxPIs. Herein a new strategy consisting in one-step high-temperature synthesis of CxPIs based on 3,5-diaminobenzoic acid (DABA), the carboxyl groups of which provide an autocatalytic effect, in N-methyl-2-pyrrolidone (NMP) is presented. The effect of DABA content on the formation and properties of CxPIs is studied. It is established that high-molecular-weight CxPIs (Mw = 158–356 kDa) with five- and more stable six-membered imide rings from various dianhydrides of aromatic tetracarboxylic acids and diamines as comonomers are synthesized within 1–4 h of the reaction even with 25 mol% of DABA content. Additional catalyst-free synthesis of CxPIs in NMP allows to use the solutions obtained in situ for the production of various materials. In particular, CxPI-varnishes form the primary protective coating of the optical fiber superior to the commercial analogue in terms of its manufacturing technology, thermal and hydrolytic stability, which provides more reliable and long-term operation.

Synthesis and characterization of nanocomposites based on polyimide bearing benzimidazole side groups filled with titania nanoparticles

ABSTRACT This study reports on the fabrication and property investigation of nanocomposites based on polyimide (PI) containing benzimidazole pendant groups reinforced with titania nanoparticles (TiO2 NPs) via the in-situ thermal imidization approach. This method involves the formation of poly(amic acid) (PAA), as the precursor of PI, by polycondensation of an equimolar amount of dianhydride, pyromellitic dianhydride (PMDA), with a benzimidazole-containing diamine monomer, 5-(2-benzimidazole)-1,3-bis(4-aminophenoxy)benzene (BIDA), in dimethylacetamide (DMAc) solvent, followed by embedding various weight percentages (3, 5, and 8 wt%) of amine-functionalized titania nanoparticles (NH2-TiO2 NPs) in PAA media, and subsequent thermal imidization to be converted to PI/TiO2 nanocomposite films. The structural, morphological, and thermal properties of the obtained neat PI and its nanocomposite films were studied by various methods, such as FT-IR spectroscopy, X-ray diffraction (XRD) analysis, thermal gravimetric analysis (TGA), and scanning electron microscopy (SEM). The data resulted from TGA analysis indicated that the incorporation of NH2-TiO2 NPs into the PI matrix show higher thermal stability when compared to the neat PI. The SEM images revealed that the dispersion of titania NPs was uniformly done in PI matrix, with a particle size of around 42 nm.

Investigation of the structure–dielectric relationship of polyimides with ultralow dielectric constant and dissipation factors using density functional theory

Polyimides (PIs) possessing both low dielectric constant (Dk) and dissipation factor (Df) remain an intriguing study for the ever-increasing demand for the high frequency transmission of signals. In this study, the relationship between the chemical structures and the dielectric properties of PIs were comprehensively investigated. 35 kinds of PIs with different types of functional groups such as ester, fluorine and ether were prepared via the polyaddition and the subsequent thermal imidization. Their chemical structures and physical properties including the mechanical and dielectric properties were systematically studied. We found that the additive group contribution method failed to provide an effective correlation on the PIs with low Dk and Df values, and it is incapable of distinguishing geometry isomers of PIs. Herein, by utilizing the density functional theory simulation, we successfully remedied those issues. The Dk and Df values of PIs provided strong correlations with volumetric polarizability and volumetric dipole moment. The results elaborated the effect of the interchain behavior to the Dk values and the impact of the chain rigidity on the Df values of PIs. Combining with the molar volume derived from the film density, and a correction factor expressing the polymer rigidity, we eventually established a confident correlation of Dk and Df values with the structure parameters of PIs, respectively. In the meantime, this study interprets the guidelines for the design on PIs molecular structure which sheds light on the development of the insulating materials at high frequencies.

An ultrapermeable thin film composite membrane supported by “green” nanofibrous polyimide substrate for polar aprotic organic solvent recovery

The preparation of polymeric-based thin film composite (TFC) membrane with high permeance and superior solvent resistance in organic solvent nanofiltration (OSN) applications always involves the consumption of toxic chemicals or high energy, which does not meet the requirements of green chemistry. In this work, intrinsic insoluble polyimide (PI) nanofiber substrates were obtained by a two-step method including electrospinning of poly (amic acid) synthesized by polycondensation reaction between various monomers and subsequent thermal imidization, and the optimized one with low thermal imidization temperature was employed as substrate for TFC membrane in strong polar aprotic solvents recovery, therefore achieving the green process. The effects of different dianhydride and diamine monomers and different thermal imidization temperatures on the solvent resistance and microscopic morphology of nanofiber substrates are also studied. The optimized monomers in this work contribute to a low-temperature imidization of PI nanofiber substrate. The solvent permeance of corresponding TFC OSN membrane with the insoluble PI nanofiber substrate reaches up to 11.2 L m −2 h −1 bar −1 with 99.7% rejection in rose bengal/DMF system, and good long-term stability. This work provides potential guidelines for developing next-generation OSN membranes under a green process. • Insoluble polyimide (PI) nanofibrous substrate was prepared facilely with low-temperature thermal imidization. • A green modification without toxic chemical solvent involved is applied. • PI nanofibrous substrate can withstand strong polar aprotic solvents for more than three months. • The high porosity and interconnected pores endow substrate superior solvent permeance. • TFC membrane exhibits outstanding OSN performance and long-term stability.

Syntheses of Soluble Biopolyimides Using 4-Aminophenylalanine

4-Aminophenylalanine (4APhe), an exotic amino acid which is obtained as a microorganism metabolite of glucose, is polycondensed with various tetracarboxylic dianhydrides as a diamine monomer to obtain poly(amic acid)s. Subsequent thermal imidization of poly(amic acid)s is made at 220 °C with stepwise heating from 100 °C. Some of the obtained polyimides (PIs) exhibited good solubility in organic solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, and more. The progress of imidization was observed by proton nuclear magnetic resonance and infrared spectroscopy to confirm that the imidization ratio was up to 98%. Carboxylate group of the side-chains of PIs affected their solubilities despite the high imidization ratio, and the solubility was lost for any organic solvents by decarboxylation at 280 °C, confirmed from mass-loss of thermogravimetric analysis. Thus, a new series of PIs were obtained with abilities of solvent-molding in PI state and thermal resistivity enhancement by further heating after molding.

Synthesis and characterization of novel polyimides based on 10,10-bis[4-(4-aminophenoxy)-3-methylphenyl]-9(10H)-anthrone

10,10-Bis[4-(4-aminophenoxy)-3-methylphenyl]-9(10H)-anthrone, as a novel aromatic diamine, was synthesized from anthrone by three-step process. A series of polyimides containing methyl substituents and 9(10H)-anthrone pendant groups was prepared from this diamine with various aromatic dianhydrides via the poly(amic acid) (PAA) precursors and subsequent thermal or chemical imidization procedure. Inherent viscosities of PAAs are between 0.87 and 1.58 dL/g, and the corresponding polyimides have the weight-average molecular weight and number-average molecular weight in the range of 49,400–64,200 and 28,700–37,700, respectively. Most of the polyimides obtained through the chemical cyclodehydration method are soluble in NMP, DMF, DMAc, Py and m-cresol at room temperature. These polyimides have glass transition temperatures of 289–313 °C, 10% weight loss temperatures of 484–524 °C, char yields of 55–63% at 800 °C, and temperature at the maximum-rate degradation of 560–584 °C in nitrogen, respectively. The polyimide films obtained through the thermal imidization have tensile strength of 76.43–119.74 MPa, elongation at break of 6.93–12.33%, and tensile modulus of 1.91–2.38 GPa.

Heat Resistance and Dynamic Mechanical and Rheological Properties of a Blend of Crystallizing Polymers, Polyimide and Copoly(urethane—imide), at Identical Chemical Structure of the Imide Blocks in the Initial Polymers

A blend of crystallizing polyimide and newly synthesized partially crystalline multiblock (segmented) copoly(urethane—imide), consisting of 75 wt % copoly(urethane—imide) and 25 wt % polyimide, was prepared and studied by thermal gravimetric analysis, differential scanning calorimetry, dynamic mechanical analysis, and melt rheology. Blend samples were prepared by mixing copoly(urethane—imide) and polyimide prepolymers in an N-methyl-2-pyrrolidone solution, in which the subsequent thermal or chemical imidization of the system components was performed. The properties of the initial polyimide, copoly(urethane—imide), and the blend were compared. The results of studies by differential scanning calorimetry and dynamic mechanical analysis demonstrate the presence of a crystalline soft polyester phase in the copoly(urethane—imide) structure and of the crystalline hard imide phase in the blend structure, which suggests the crystallization suppression in samples of the blend. The dynamic viscosity of the melt of chemically imidized copoly(urethane—imide) and blend samples is lower than that of their thermally imidized analogs. Dynamic mechanical analysis shows that the initial copoly(urethane—imide) behaves as a thermoelastoplastic. The blends consisting of copoly(urethane—imide) and thermoplastic polyimide are similar in properties to polyimide thermoplastics for structural purposes.

Polyimide-Based PolyHIPEs Prepared via Pickering High Internal Phase Emulsions.

Pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA) oligoimide particles and PMDA-ODA poly(amic acid) salt (PAAS) were synthesized and used as stabilizers to prepare oil-in-water Pickering high internal phase emulsions (HIPEs). The stability of the Pickering HIPEs was investigated by dispersion stability analysis. Polyimide-based polyHIPEs could be prepared through freeze-drying and subsequent thermal imidization of the Pickering HIPEs. The characteristics of the polyHIPEs, including their morphology, porosity, thermal decomposition temperature, and compression modulus, were investigated. The thermal decomposition temperature (T10) of the polyHIPEs was very high (>530 °C), and their porosity was as high as 92%. The polyimide-based polyHIPEs have the potential to be used in high-temperature environments.

Preparation, structure, and pervaporation performance of poly(amide–imide)‐sulfonated polyimide composites

ABSTRACTPoly{[4,4′‐bisdiphenyl‐2,2′‐disulfonic acid]imide‐1,3‐bis(3,4‐dicarboxyphenoxy)benzene} was synthesized via polycondensation and subsequent thermal imidization in m‐cresol:triethylamine. The obtained polymer and its composites with aromatic poly(amide–imide) obtained through polycondensation of 4‐chloroformyl‐(N‐p‐chloroformylphenyl)phthalimide and 4,4′‐diaminodiphenyl ether were investigated by use of nuclear magnetic resonance, thermogravimetric analysis, dynamic mechanical analysis, differential scanning calorimetry, Fourier transform infrared spectroscopy, scanning electron microscopy, and X‐ray diffraction analysis. Transport properties of composite films were studied during pervaporation of water–alcohol mixtures. On the basis of the data obtained, suggestions on the structure of materials and its influence on transport and other physical properties were made. The addition of 30 wt % of sulfonated polyimide to the poly(amide–imide) matrix enhances its separation factor in the process of separation of an azeotrope water/ethanol mixture by four times while permeability remains unchanged. The methanol permeability increases with an increase of the mass fraction of sulfonated polyimide in the composite film. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 48197.

Constructing polymeric interlayer with dual effects toward high dielectric constant and low dielectric loss

Polymeric dielectric materials recently attract much attention due to their good comprehensive performances. However, it is still challenging to prepare polymeric dielectric materials with high dielectric constant, low dielectric loss and high breakdown strength. In this work, insulated polyimide (PI) interlayer with nanoscale thickness was coated on the surface of barium titanate (BaTiO3, BT) through the in-situ polymerization and subsequent thermal imidization treatment. The hybrid nanoparticles exhibited the typical ‘core-shell’-like structure with strong interfacial interaction between BT and PI. Different from the serious aggregation of BT nanoparticles, the PI@BT nanoparticles exhibited good dispersion in the poly(vinylidene fluoride) (PVDF) composite. There was strong interfacial interaction between PI and PVDF, which resulted in the decrease of crystallinity of the PVDF matrix at relatively high filler content. The flexible PVDF/PI@BT composites exhibited largely enhanced dielectric constant, suppressed dielectric loss and enhanced breakdown strength compared with the common PVDF/BT composites. It was suggested that the intense interfacial polarization at the interface between PI and BT contributed to the largely enhanced dielectric constant while the reduced interfacial polarization at the interface between PI and PVDF contributed to the suppressed dielectric loss. This work demonstrates that the nanoengineering of the interface between BT and PVDF using PI is an alternative way to fabricate the dielectric materials with excellent comprehensive performances.

Organosoluble and transparent cardo polyimides with high Tg derived from 9,9-bis(4-aminophenyl)xanthene

9,9-Bis(4-aminophenyl)xanthene (BAPX) was prepared simply and effectively via one-pot, two-step procedure using xanthenone and aniline as main substrates. The monomer BAPX was reacted with six aromatic dianhydrides in N, N-dimethylacetamide (DMAc) to yield the corresponding polyimides (PIs) via the poly(amic acid) precursors and subsequent thermal or chemical imidization. The resulting PIs exhibited good thermal stability with glass transition temperatures of 308–348°C, initial decomposition temperatures of 470–510°C, 10% weight loss temperatures of 540–565°C, and char yields of 55–59% at 800°C in nitrogen, respectively. All polymers were amorphous and readily soluble in organic solvents such as N-methyl-2-pyrrolidone and DMAc. The PI films had tensile strengths of 71–92 MPa, tensile moduli of 1.91–2.35 GPa, and elongations at break of 5–13%. Meanwhile, these polymer films also had high optical transparency with a cutoff wavelength in the range of 367–415 nm, lower dielectric constants (3.02–3.34 at 10 MHz), and low water uptake of 0.30–0.52%.

Negatively charged polyimide nanofiltration membranes with high selectivity and performance stability by optimization of synergistic imidization

A simple strategy is proposed for high performance polyimide nanofiltration (PI-NF) membranes by optimization of a synergistic imidization reaction. Soluble poly(amic acid) polymers with partial imidization was successfully controlled and fabricated by chemical imidization. With the product as a matrix, NF membranes with an integrally skinned asymmetric architecture were obtained by phase inversion and subsequent thermal imidization. The brittle and ruptured defects caused by directly high temperature conversion for fabricating PI-NF membranes were effectively avoided. The prepared membrane showed high rejections for dye, PEG and typical salts. The fabricated PI-NF membranes were endowed with a high permeance of isopropyl alcohol [approximately 1 L/(m2 h bar)] and a high rejection of rose Bengal (greater than 92%). The molecular weight cut-off of PI-NF membrane is below 800 Da. The pure water flux of PI-NF membrane prepared under optimized conditions was approximately 20 L/(m2 h) and the rejection for Na2SO4 was 93.91% at 5 bar. The rejection for different salts followed the order: Na2SO4 > MgSO4 > NaCl > MgCl2, revealed that Donnan exclusion theory and steric hindrance effect dominated the rejection mechanism. Furthermore, the NF membranes showed excellent stability over a long-term filtration process.

Simple and easy recycling of poly(amic acid) gels through microwave irradiation

ABSTRACTPoly(amic acid)s (PAAs), which are precursors of polyimides, often undergo gel formation during their synthesis or storage, and these insoluble gels have been discarded. In this work, we discovered that the gels could be converted to homogeneous PAA solutions by fast and simple microwave (MW) irradiation. The PAA gels were placed inside a domestic MW oven, and MW irradiation was carried out with 240 W for 2 min. The recycled PAA solutions afforded polyimide films, coatings, and powders. The polyimides prepared from the recycled PAA solutions exhibited higher glass transition temperatures (Tgs), decomposition temperatures, and char yields than comparison polyimides obtained from ordinary PAA solutions. Flexible free‐standing polyimide films were obtained by drop‐casting of the MW‐treated solutions and subsequent thermal imidization. Mechanical properties and dielectric constants were measured for the polyimide films and coatings, respectively. This new method has significant advantages for the environment, economy, and industry. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 981–987

In situ solution polymerization for preparation of MDI-modified graphene/hyperbranched poly(ether imide) nanocomposites and their properties

Two kinds of (GE-MDI/HBPEI) nanocomposites with highly enhanced thermal, mechanical and gas barrier properties, were preparedvia in situsolution polymerization, as well as subsequent synchronous thermal imidization and reduction.

Calcium-activated gene transfection from DNA/poly(amic acid-co-imide) complexes

In this study, we synthesized a water-soluble poly(amic acid-co-imide) (PA-I) from ethylenediaminetetraacetic dianhydride (EDTA) and 2,2′-(ethylenedioxy)bis(ethylamine) that possesses comparable transfection efficiency to that of polyethylenimine (PEI), when prepared in combination with divalent calcium cations. The polycondensation of monomers afforded poly(amic acid) (PA) precursors, and subsequent thermal imidization resulted in the formation of PA-I. At a polymer/DNA ratio (indicated by the molar ratio of nitrogen in the polymer to phosphate in DNA) of 40, complete retardation of the DNA band was observed by gel electrophoresis, indicating the strong association of DNA with PA-I. A zeta potential of −22 mV was recorded for the PA-I polymer solution, and no apparent cytotoxicity was observed at concentrations up to 500 μg·mL−1. In the presence of divalent Ca2+, the transfection efficiency of PA-I was higher than that of PA, due to the formation of a copolymer/Ca2+/DNA polyplex and the reduction in negative charge due to thermal cyclization. Interestingly, a synergistic effect of Ca2+ and the synthesized copolymer on DNA transfection was observed. The use of Ca2+ or copolymer alone resulted in unsatisfactory delivery, whereas the formation of three-component polyplexes synergistically increased DNA transfection. Our findings demonstrated that a PA-I/Ca2+/DNA polyplex could serve as a promising candidate for gene delivery.