Amic Acid Research Articles

Freezing‐Assisted Direct Ink Writing of Customized Polyimide Aerogels with Controllable Micro‐ and Macro‐ Structures for Thermal Insulation

AbstractPolyimide (PI) aerogels have attracted attention in thermal management due to their low thermal conductivity, high porosity, and robust mechanical properties. However, building PI aerogels with elaborately tailored micro‐ and macro‐ structures for optimized thermal insulation performance in specific scenarios remains a challenge. Here, a scalable approach is reported to prepare PI aerogels with controllable micro‐ and macro‐ structures by freezing‐assisted direct ink writing (FADIW) of poly(amic acid) (PAA) inks. This FADIW strategy can enable continuous ink deposition and transient low‐temperature‐induced gelation, contributing to the facile construction of spatially free geometries with structural complexity and shape fidelity. Importantly, controllable construction of microstructures of the aerogels is achieved by controlling the extrusion substrate temperature. The resulting customized PI aerogels show good thermal insulation owing to the porous microstructure and customized macrostructure, enabling good thermal management for portable electronics. In addition, this FADIW strategy is universal and allows for multi‐material integration, broadening the functionality of PI aerogels. Significantly, such FADIW approach can be promisingly applied to realize on‐demand design of PI aerogels and other polymer aerogels with targeted micro‐ and macro‐structures, offering an alternative avenue for the manufacture of customized aerogels toward widespread applications.

Mechanical, thermal and gas separation properties of copolyimides and their thermal rearrangement copolymer membranes with spirobisindane structures

In this work, two kinds of dimaine monomers with spirocyclic structures, including 3,3,3′,3′-tetramethyl-6,6′-bis[3-amino-4-hydroxyphenoxy]-1,1′-spirobisindane (TBAHS) and 3,3,3′,3′-tetramethyl-bis(5-amino-6-hydroxyphenyl)-1,1′-spirobisindane (TBHAS) are firstly synthesized. Then, the two diamines TBAHS and TBHAS are copolymerized in different molar ratios with equimolar 4,4′-(hexafluoroisopropenyl)diphthalic anhydride (6FDA) to synthesize poly(amic acid) (PAA) solution, followed by thermal imidization to obtain a series of copolyimide (coPI) membranes. Next, these coPI(TBAHS-TBHAS) specimens are subjected to thermal rearrangement (TR) treatment at 450°C to obtain the corresponding coTR(TBAHS-TBHAS) copolymer membranes. The influences of the molar ratio of TBAHS to TBHAS, thermal treatment temperature and time on the microstructures and properties of the PI and TR copolymers are investigated. With the increase of rigid TBHAS molar ratio, the glass transition temperature ( Tg) and d-spacing value of the coPI(TBAHS-TBHAS) samples are significantly enhanced. Furthermore, as the thermal treatment temperature increases, the mechanical properties of the copolymer membranes display a sharply decline trend, but the d-spacing values are obviously increased. When the molar ratio of TBAHS to TBHAS is 7:3, the gas permeabilities of the coTR membrane reach 1140.5, 1180.6, 306.6, and 44.9 Barrer for H2, CO2, O2, and N2, respectively. Meanwhile, the ideal selectivity of O2/ N2 is 6.82, suggesting a superior separation performance close to the 2015 upper limit. Furthermore, with the increase of thermal treatment temperature and time, the gas permeabilities are also improved.

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.

Porous polyimide composite films containing mesoporous hollow silica nanospheres with ultralow dielectric constant and dissipation loss

Porous polyimide hybrid films with ultralow dielectric constant and dissipation loss were synthesized during chemical imidization and rapid evaporation of polymer solutions. Polyimide (PI) of bis(4-aminophenyl)-1,4-diisopropylbenzene (BISP) and 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) were synthesized in N-methylpyrrolidinone (NMP) to yield the corresponding poly(amic acid)s (PAA)s, which were then chemically imidized. To reduce the dielectric properties of the porous PI films, amine-functionalized, surface-modified hollow silica nanoparticles (AHNS) with various sizes from 100 nm to 1 μm, prepared via sol-gel process, were added with PAA at 3, 5, or 10 % weight content, followed by chemical imidization to produce the porous PI/AHNS films. To avoid the high-volume shrinkage of wet gels and maintain the high porosity of the PI with low polarizability, AHNS nanoparticles were added to the porous structure of the PI xerogels as a physical crosslinker. The polyimide hybrid film with 10 nm AHNS nanoparticles at 10 % weight content showed a dielectric constant of 1.151 and a dissipation rate of 0.001 at a frequency of 10 GHz.

3D printing carbon from aqueous all aromatic polyimides

All-aromatic polyimides serve as versatile precursors for carbon formation due to their high aromatic content and repeating unit planarity, which synergistically enables pyrolysis to ordered carbon. Recent publications disclosed facile synthetic methods for additive manufacturing (3D printing) of all-aromatic polyimides using a pendant salt approach for ultraviolet-assisted direct ink write (DIW) additive manufacturing. This manuscript describes the introduction of pendant sodium sulfonate functionality to the poly(amic acids), and water-soluble all-aromatic polyimide precursors displayed suitable rheological properties for DIW at room temperature and vat photopolymerization (VP) at 50 °C. These aqueous solutions enabled the formation of microporous polyimide structures with a direct freeze-casting method and dense structures with gradual heating under inert atmosphere to 200 °C and subsequent thermal post-processing to 400 °C. Furthermore, polyimide 3D structures function as efficient carbon precursors when pyrolyzed at 2000 °C under inert atmosphere. Raman spectroscopy revealed carbonized 3D structures with characteristic graphitic and disordered carbon bands, which was indicative of polycrystalline, disordered glassy carbon. Furthermore, electrical conductivity measurements suggest opportunities for the formation of 3D conductive structures.

Small-angle X-ray scattering analysis of poly(amic acid) dispersed in a liquid matrix to understand the size control of polyimide nanoparticles.

Poly(amic ‍acid) ‍nanoparticles ‍prepared ‍by ‍‍precipitation ‍polymerization with a dispersant were evaluated by small-angle X-ray scattering (SAXS) and field-emission scanning electron microscopy (FE-SEM). The particle size evaluation of poly(amic acid) nanoparticles in the liquid phase by SAXS was performed to gain insight into the size control of poly(amic acid) nanoparticles, and showed good agreement with visual observation by FE-SEM, explaining the effect of the dispersant in obtaining polyimide nanoparticles with small particle size. This indicates that the particle size is maintained without change during the solvent evaporation process. The polyamide nanoparticles controlled by the dispersant effect maintained their size after imidization, and polyimide nanoparticles with a minimum radius of about 60 nm were prepared.

Construction of magnetic Fe2O3-Decorated electroactive Poly(amic acid) composites as recyclable Catalysts: Increased loading of gold Nanoparticles, Controlled Size, and Accelerated catalytic reduction of 4-Nitrophenol

Nanocomposites are widely used in heterogeneous catalysts in the field of water treatment, which exhibit controllable structural advantages and synergistic effects between their components further enhancing catalytic activity. In the current research, we developed the Au nanoparticles immobilized magnetic γ-Fe2O3/Electroactive Poly (amic acid) composite (6Au/EPAA-Fe2O3) prepared by using the in-situ redox reaction strategy. Sundry techniques such as FT-IR, XRD, TGA, SEM/HR-TEM, CV, and XPS analysis have been applied to explain the chemical structure and morphology of as-synthesized composites. The catalytic performance of the catalyst during the reduction of 4-nitrophenol using NaBH4 as a hydrogen donor was assessed in an aqueous medium at room temperature. Attractively, 6Au/EPAA-Fe2O3 composite exhibits superior catalytic performance, which completes the reduction of 4-NP within the 50 s. The pseudo-first-order kinetic rate constants were estimated as 3.0 × 10-2 s−1 for the reduction of 4-NP. Furthermore, the 6Au/EPAA-Fe2O3 composite has excellent stability and reproducibility of 10 consecutive runs by using an external magnet without loss of catalytic activity.

Heteroatomic-scale insight into the extraction selectivity of amic acid ligands and gallium and indium recovery from spent solar panels

The rapid growth of traditional electronic and emerging renewable energy devices has compelled an increasing global demand for critical metals, such as gallium (Ga) and indium (In). Extractants capable of capturing Ga and In from complex sources, especially the generated spent devices, are thus needed for the critical metals sustainability and the spent devices recycling. However, developing an environmentally benign extractant with high selectivity remains a challenge. Here, a method for regulating the extraction selectivity of “green” amic acid ligands toward Ga3+ and In3+ by heteroatom (S, O and N) substitution was proposed and verified by experimental and theoretical calculations. The experimental results indicated that heteroatom substitution contributed to a great increase of Ga3+ and In3+ extraction capacities, and the hard O-substituted amic acid ligand (AA-O) exhibited outstanding selectivity in separating Ga3+ and In3+ from the competing metal ions of Al3+, Cu2+, Cu2+ and Zn2+. The theoretical calculations showed that heteroatoms made the dominant contribution to the extraction selectivity over amide and carboxyl coordination sites in amic acid ligands. Based on these findings, the AA-O ligand was employed as the optimal extractant to recover Ga3+ and In3+ from the leachate of spent solar cells via a single-step extraction process, without the use of additional extractants. The successful separation of Ga3+ and In3+, with purity improvement from 8.73 and 21.39 wt% to 78.31 and 97.05 wt%, respectively, suggested that AA-O ligand is a promising extractant and supported the feasibility of selectivity regulation for amic acid ligands via heteroatom substitution.

Emulsion-Based Multiscale Structural Design Realizes Lightweight and Superelastic Graphene Aerogels for Electromagnetic Interference Shielding.

Ultralight graphene aerogels with high electrical conductivity and superelasticity are demanded yet difficult to produce. A versatile emulsion-based approach is demonstrate to optimize multiscale structure of lightweight, elastic, and conductive graphene aerogels. By constructing Pickering emulsion using graphene oxide (GO), poly (amic acid) (PAA), and octadeyl amine (ODA), micron-level close-pore structure is realized while thermal shrinkage mismatch between GO and PAA creates numerous nanowrinkles during thermal annealing. GO nanosheets are bridged by PAA-derived carbon, enhancing the structural integrity at molecular level. These multiscale structural features facilitate rapid electron transport and efficient load transfer, conferring graphene aerogels with intriguing mechanical and electromagnetic interference (EMI) shielding properties. The emulsion-based graphene aerogel with an ultralow density of ≈3.0mg cm-3 integrates outstanding electrical conductivity, air-caliber thermal insulation, high EMI shielding effectiveness of 75.0dB, and 90% strain compressibility with superb fatigue resistance. Intriguingly, thanks to the gel-like rheological behavior of the emulsion, ultralight graphene scaffolds with programmable geometries are obtained by 3D printing. This work provides a general approach for the preparation of ultralight and superelastic graphene aerogels with excellent EMI shielding properties, showing broad application prospects in various fields.

Facile preparation of PtNi/Cs derived from the precipitation polymerization of a Poly(amic acid) for efficient hydrogen evolution reaction catalysis

Hydrogen evolution reaction (HER) of water is currently one of the most potential ‘green hydrogen’ production technology to alleviate the energy scarcity and environmental contamination. In this study, ultrafine platinum/nickel bimetallic nanoparticles loaded porous carbon nanocomposite, noted as PtNi/Cs, is facilely prepared by employing the precipitation polymerization of a poly (amic acid) (PAA), which shows superior catalytic performance toward HER. The synthesis of PAA is achieved via a stepwise polymerization of 4,6-diaminino-2-mercaptopyrimidine and pyromellitic dianhydride at room temperature, and the porous precursor is formed during the polymerization. Upon pyrolysis of the precursor in an inert atmosphere, carbon nanoaggregates with inherited porosity are fabricated. Ultrafine PtNi bimetallic nanoparticles with size of 3.3 ± 0.7 nm can be efficiently deposited, exhibiting an overpotential of 26.4 mV at 10 mA cm−2 and a mass activity of 2.92 A mgPt−1, superior than commercial Pt/C of 36.8 mV. The excellent HER performance can be attributed to the formation of ultrafine PtNi bimetallic nanoparticles and the electron transfer from PtNi to the nitrogen atoms on the carbon support.

Crystalline Nanoflowers Derived from the Intramolecular Cyclization-Induced Self-Assembly of an Amorphous Poly(amic acid) at High Solid Content.

The investigation of the amorphous to crystalline transformation and the corresponding influence on the self-assembly behavior of amphiphilic polymers are of significant interest in this field. Herein, we propose the concept of intramolecular cyclization-induced self-assembly (ICISA) to prepare crystalline nanoflowers at a high solid content of 15% on the basis of the amorphous to crystalline transformation of poly(amic acid) (PAA). Taking advantage of the reactive property of the PAA, rigid and crystalline polyimide (PI) segments are introduced to the backbone of the PAA to give P(AA-stat-I) induced by the intramolecular cyclization reaction upon thermal treatment, leading to the in situ formation of crystalline nanoflowers. Revealing the formation mechanism of the nanoflowers, we found that the nanosheets are formed at the early stage and then stacked to form the nanoflowers at high concentrations. The relationship between the degree of imidization and incubation temperature is quantitatively analyzed, and the effects of temperature on the morphology, degree of imidization, and crystallinity of the assemblies are also investigated. Furthermore, computer simulations demonstrate the optimized temperature of ICISA of 160 °C, which ensures the match between the intramolecular cyclization reaction rate, the self-assembly process, and the lowest energy state of the self-assembly system, resulting in the formation of nanoflowers with high crystallinity. Overall, a facile one-step strategy is proposed to prepare crystalline nanoflowers based on the in situ thermally triggered intramolecular cyclization reaction of a PAA, which may bring fresh insights into the dynamic amorphous to the crystalline transformation of polymers.

Synthesis and characterization of a new multifunctional aliphatic poly(amic acid): an efficient functional polymeric catalyst

Synthesis and characterization of a new multifunctional aliphatic poly(amic acid): an efficient functional polymeric catalyst

Water Soluble Poly(amic acid) Binder for Anode Coating and High Temperature Processing

The advent of lithium batteries has been a cornerstone in the evolution of electric vehicle technology, offering a sustainable alternative to fossil fuels and transforming the transportation landscape. The integration of water-soluble binders in the scalable production of anodes significantly enhances the efficiency and sustainability of lithium battery manufacturing. Further advances in battery manufacturing require enhanced tolerance to high temperatures and increased mechanical robustness, especially for high-throughput roll-to-roll production process. In this study, we present a water-soluble poly(amic acid)-based binder, uniquely designed to facilitate aqueous coating processes and withstand high processing temperatures for anode manufacturing. This innovative binder demonstrates remarkable efficiency in securely binding active materials, such as graphite or silicon, ensuring their integrity and performance throughout the battery cycling process.

Development of a Novel, Easy-to-Prepare, and Potentially Valuable Peptide Coupling Technology Utilizing Amide Acid as a Linker.

The process of synthesizing radionuclide-coupled drugs, especially shutdown technology that links bipotent chelators with biomolecules, utilizes traditional coupling reactions, including emerging click chemistry; these reactions involve different drawbacks, such as complex and cumbersome reaction steps, long reaction times, and the use of catalysts at various pH values, which can negatively impact the effects of the chelating agent. To address the above problems in this study, This research designed a novel bipotent chelator coupled with peptides. In the present study, dichloromethane was used as a solvent, and the reaction was conducted at room temperature for 12 h. A one-step ring-opening method was employed to introduce the coupling functional group of tridentate amide acid. The coupling materials consisted of the amino active site of the peptide and diethylene glycol anhydride. In this paper, this study explored the reactions between different equivalents of acid anhydride coupled to the peptide (peptide sequence: HLRKLRKR) and determined that the maximum conversion of the peptide feedstock was 87%. To determine the selectivity of the reaction sites in this polypeptide, This study identified the peptide sequence at the reaction site using nuclear magnetic resonance (NMR) and liquid chromatography-mass spectrometry (LC-MS). For the selected peptide, the first reactive site was on the terminal amino group, followed by the amino group on the tetra- and hepta-lysine side chains. The tridentate amic acid framework functions as a chelating agent, capable of binding a range of lanthanide ions. This significantly reduces and optimizes the time and cost associated with synthesizing radionuclide-coupled drugs.

Synthesis and characterization of a new multifunctional aliphatic poly(amic acid): An efficient polymeric adsorbent for removing the heavy metal ions

The heavy-metal ions in wastewater have attracted intense attention due to environmental issues, including their toxicity, bioaccumulation tendency, and persistency in nature. Despite other strategies for heavy metal removal from wastewater, adsorption is one of the most promising methods, owing to its simplicity and efficiency. To conserve the carboxyl and amine functional groups and prevent amidation reaction, a new aliphatic tertiary poly(amic acid) with linear and cyclic spacers was synthesized via the catalyst-free process. The ring opening of ethylenediaminetetraacetic acid dianhydride (EDTADA) by piperazine, a bifunctional cyclic secondary diamine, was conducted in a polar solvent (DMF) in a nitrogen atmosphere for 3 h. The characterization was conducted by FTIR and NMR, and their morphology and elemental composition were investigated by field emission scanning electron microscopy and energy dispersive X-ray (FESEM-EDX). As-synthesized aliphatic tertiary poly(amic acid) could be separated as a white powder with an O/N ratio of 0.93 ± 0.03 and morphology of woven fiber. The white powder showed an average molecular weight (M̅w) of ∼12,000 and a polydispersity index (PDI) of 1.52 ± 0.05. In addition, the poly(amic acid) TGA/DTA analysis showed a high thermal stability with stepwise degradation with the onset of 346 °C. The poly(amic acid) DSC profile displayed a high glass transition at 124 °C. The Brunauer–Emmett–Teller (BET) surface area and pore volume were determined to be 146.05 m2/g and 0.78 cm3/g. The poly(amic acid) contains the mesopores and macropores based on its pore size distribution curve. Regarding an acidic pH and abundance of functional groups, as-prepared tertiary poly(amic acid) was employed as a heterogeneous adsorbent of different types of heavy-metal ions, including Ag (I), Cr (III), and Cu (II). A removal efficiency > 90 % after 10 min for three heavy metal ions demonstrates a strong chelating capacity of as-prepared tertiary poly(amic acid). An adsorption capacity of > 50 mg per gram of poly(amice acid) was recorded for three heavy-metal ions with a pH of 3 or higher than 3. Poly(amic acid) exhibited good recyclability. This work exhibited the applicability of novel poly(amic acid) as an efficient adsorbent for the heavy metal ions removal from wastewater.

Highly porous layered carbon nanocomposites from the self-assembly of a poly(amic acid) for efficient oxygen reduction reaction catalysis

Porous layered carbon nanomaterials play important role in the efficient loading of electroactive substances for high performance energy conversion techniques. Herein, an intramolecular cyclization induced self-assembly (ICISA) strategy is proposed to prepare three-dimensional highly porous layered carbon nanostructures (LCs) for efficient loading of various transition metal nanoparticles for oxygen reduction reaction (ORR) catalysis. The amphiphilic alternating copolymer poly(amic acid) (PAA) is synthesized by the stepwise polymerization of hydrazine hydrate and pyromellitic dianhydride. Upon heating above 160 °C, the intramolecular cyclization reaction occurs, leading to the formation of rigid, crystalline and non-soluble polyimide segments in the backbone, which induces the self-assembly of the polymer to form dumbbell-shaped assemblies with a high solid content of 15%. After pyrolysis in a nitrogen atmosphere, LCs are obtained. Cobalt (Co), iron (Fe) and FeCo nanoparticles can be efficiently loaded, noted as Co@LCs, Fe@LCs and FeCo@LCs, respectively. The ORR catalytic performance of the catalysts is evaluated to give half wave potentials of 0.791, 0.787 and 0.812 V, and limit current densities of 4.92, 4.73 and 4.71 mA cm−2 for Co@LCs, Fe@LCs and FeCo@LCs, respectively. Overall, we propose an ICISA strategy to facilely prepare three-dimensional carbon nanomaterials with highly porous layered structure for efficient ORR catalysis.

A novel approach for environmentally benign synthesis and solid-state polymerization of fluorinated polyimides in aqueous co-solvent

This study presents a novel approach that enables an environmentally benign water-based co-solvent synthesis of fluorinated polyimides (FPIs) using conventional fluorine-based monomers such as 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 2,2′-bis(trifluoromethyl)benzidine (TFMB). The high surface tension inherent in fluorine-based materials, which inhibits water solubility, can be overcome by mixing water with an alcohol-based solvent. The surface tension of the co-solvent decreases proportionally with the alcohol content, facilitating synthesis from fluorine-based monomers that were previously inaccessible in water. Oligomeric fluorinated poly(amic acid) salt (FPAAS) can be rapidly synthesized in aqueous solutions containing alcohol, particularly 1-propanol (PrOH). Subsequent coating and thermal treatment processes allow high molecular weight FPI films to be produced by solid-state polymerization. A comprehensive analysis of the resulting FPI films shows excellent physical properties comparable to conventionally synthesized FPI films. The 6FDA-TFMB-based FPI exhibits remarkable thermal stability up to 570 °C (Td,5%), low yellow index of 1.37, high transparency of 92.9 % and tensile strength of 116.5 MPa. The presented synthetic strategy allows the adoption of monomers with different fluorine-based substituents, thus providing insights for future aqueous synthesis. The thermal, mechanical, and optical qualities of FPIs can be utilized to create optically clear heating devices. The transparent heater, fabricated on the FPI film utilizing a laser patterning technique to create electrodes from silver nanowires, was capable of increasing the temperature up to 100 °C under a 9 V voltage application.

Fabrication of hybrid polyimide composites containing Ladder-like poly(phenyl-co-glycidoxypropyl)silsesquioxane)

Ladder-like structured poly(phenyl-co-glycidoxypropyl) silsesquioxanes with different ratios of phenyl to glycidyl group (LPS64 and LPS 82) were synthesized using simple hydrolysis reaction followed by condensation polymerization of phenyltrimethoxysilane (PTMS) and (3-glycidyloxypropyl)trimethoxysilane (GPTMS). Two series of hybrid polyimide composites (PI/LPS64 and PI/LPS82) with different contents of LPS (10, 20, and 30 wt%) were prepared by the chemical reaction of epoxy groups in LPS64 or LPS82 and the terminal amine groups in poly(amic acid). The dielectric constant (k) of hybrid PI composites (k = 1.3) was lower than that of pure polyimide (k = 2.9) and the degree of roughness was increased according to the increase of LPS contents. The dispersion of LPS in the PI matrix was improved due to the chemical bonding between LPS and PI. The thermal stability of PI/LPS82 was higher than that of PI/LPS64 due to the enhanced π-π stacking between PI matrix and LPS82 with more phenyl groups as compared to LPS64.

Effect of polymer concentration and cross-linking density on the microstructure and properties of polyimide aerogels

Polyimide aerogels display excellent mechanical strength, high thermal stability, low thermal conductivity, and outstanding dielectric properties. Typically, the synthesis of polyimide aerogels involves the polycondensation of dianhydride and diamine into poly(amic acid) (PAA) oligomers, which are then cross-linked and chemically imidized into polyimide. The stoichiometry of dianhydride and diamine determines the number of repeat units and length of the PAA oligomers, which in turn determines the cross-linking density. Despite the critical role of polymer concentration and number of repeating units in determining the microstructure and properties of polyimide aerogels, few detailed studies exist on these two parameters. Here, we synthesized and characterized 16 polyimide aerogel formulations from the common monomers biphenyl-3,3′,4,4′-tetracarboxylic dianhydride (BPDA) and 4,4′-oxydianiline (ODA), with different repeat units (n = 5, 15, 30, 45) and total polymer concentrations (4, 7, 10, 13 wt%). An increased polymer concentration accelerated gelation and enhanced the mechanical performance of aerogels, but surprisingly, it also led to higher volumetric shrinkage during aging, solvent exchange, and supercritical drying (SCD). Specific surface areas (SSAs) reached a maximum at intermediate polymer concentrations. A shorter oligomer chain length, i.e., a higher cross-linking density, led to moderately higher SSAs (between 320 and 400 m2/g) and reduced shrinkage, resulting in lower densities for a given polymer concentration. The density dependence of the thermal conductivity exhibits a pronounced U-shaped curve with a minimum in thermal conductivity of 21–23 mW/(m·K) between 0.080 and 0.120 g/cm3, with somewhat lower values for more highly cross-linked aerogels. This systematic study of polyimide aerogels forms the basis for designing polyimide aerogels with tailored properties for targeted applications.Graphical

Colorless and transparent polyimide nanocomposites using organically modified montmorillonite and mica

In this study, we introduce a method for replacing the glass used in existing display electronic materials, lighting, and solar cells by synthesizing a colorless and transparent polyimide (CPI) film with excellent mechanical properties and thermal stability using a combination of new monomers. Poly(amic acid) (PAA) was synthesized using dianhydride 4,4′-biphthalic anhydride (BPA) and diamine 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (AHP). Various contents of organically modified montmorillonite (MMT) and mica were dispersed in PAA solution through solution intercalation, and then CPI hybrid films were prepared through multi-step thermal imidization. The organoclays synthesized to prepare CPI hybrid films were Cloisite 93A (CS-MMT) and hexadimethrine-mica (HM-Mica) based on MMT and mica, respectively. In particular, the diamine monomer AHP containing a –OH group was selected to increase the dispersibility and compatibility between the hydrophilic clays and the CPI matrix. To demonstrate the characteristics of CPI, the overall polymer structure was bent and a strong electron withdrawing –CF3 group was used as a substituent. The thermomechanical properties, morphology of clay dispersion, and optical transparency of the CPI hybrid films were investigated and compared according to the type and content of organoclays. Two types of organoclays, CS-MMT and HM-Mica, were dispersed in a CPI matrix at 1 to 7 wt%, respectively. In electron microscopy, most of the clays were uniformly dispersed in a plate-like shape of less than 20 nm at a certain critical content of the two types of organoclays, but agglomeration of the clays was observed when the content was higher than the critical content. Hybrids using HM-Mica had better thermomechanical properties and hybrids containing CS-MMT had better optical transparency.