Polyimide Precursor Research Articles

Mechanically Robust and Flexible GO/PI Hybrid Aerogels as Highly Efficient Oil Absorbents.

Herein, mechanically robust and flexible graphene oxide/polyimide (GO/PI) hybrid aerogels (GIAs) were fabricated by a facile method, in which the mixed suspensions of the water-soluble polyimide precursor and graphene oxide (GO) sheets were freeze-dried, which was followed by a routine thermal imidation process. The porous GIAs obtained not only exhibit excellent elasticity and extremely low density values (from 33.3 to 38.9 mg.cm-3), but they also possess a superior compressive strength (121.7 KPa). The GIAs could support a weight of up to 31,250 times of its own weight, and such a weight-carrying capacity is much higher than that of other typical carbon-based aerogels. Having such a porous structure, and high strength and toughness properties make GIAs ideal candidates for oil spill cleanup materials. The oil/organic solvents' absorption capacity ranges from 14.6 to 85, which is higher than that of most other aerogels (sponges). With their broad temperature tolerance and acidic stability, the unique multifunctional GIAs are expected to further extend their application range into extreme environments.

Improved characterization on molecular weight of polyamic acids using gel permeation chromatography coupled with differential refractive index and multi-angle laser light scattering detectors

Polyamic acid (PAA) is the precursor of polyimide (PI) prepared by a two-step method, its molecular weight and molecular weight distribution not only play an important role during the preparation and processing, but also influence the final performance of PI. However, precise characterization on molecular weight of PAA is still a challenge because of the existence of very complicated interactions in the solution system, including the electrostatic interaction, hydrogen bond interaction, dipole-dipole interaction, etc. Thus, it is necessary to establish a suitable strategy which can completely suppress these complex effects and get reasonable data on molecular weight. Herein, the gel permeation chromatography (GPC) coupled with differential refractive index (RI) and multi-angle laser light scattering (MALLS) detectors were applied to measure the molecular weight of (6FDA-DMB) PAA using different mobile phases, LiBr/DMF, LiBr/H3PO4/THF/DMF, LiBr/HAc/THF/DMF, and LiBr/HAc/DMF, respectively. It was found that combination of LiBr with HAc can shield the above-mentioned complex interactions and is more conducive to the separation of PAA than only addition of LiBr in DMF. LiBr/HAc/DMF was employed for the first time as a mild mobile phase to effectively separate PAA and determine its molecular weight. After a series of conditional experiments, 0.02 M LiBr/0.2 M HAc/DMF was fixed as an optimized mobile phase to measure the relative and absolute molecular weights of (6FDA-DMB) PAA prepared, and the obtained Mw from GPC-MALLS and GPC-RI were 35,300 g/mol and 125,000 g/mol, respectively. Particularly, such a mobile phase is also applicable to other PAA samples with different structures, and the final results on molecular weight are also reproducible.

Polyimide-derived carbon molecular sieve membranes for high-efficient hydrogen purification: The development of a novel phthalide-containing polyimide precursor

Hydrogen-based economy has been considered as a potential solution for energy security and sustainability in the future. And the ever-increasing demand for high-purity hydrogen (H2) provides an urgent force for the H2 purification technologies. Herein, a novel polyimide (BATPPP-6FDA) with rigid and contorted structure was synthesized and used as a precursor to fabricate the carbon molecular sieve membranes (CMSMs) for H2 purification. The structural changes from precursor to CMS were detected and the H2 separation performance of the membranes was optimized by the pyrolysis temperature. The CMSM pyrolyzed at 550 °C exhibited the highest H2 permeability of 10054.1 Barrer and moderate H2 permselectivity due to its highly disordered and turbostratic carbon structure originated from the generally decomposition of thermally labile -CF3 groups and the lactone rings of phthalide structure in the early stages (<550 °C) of pyrolysis. As the pyrolysis temperature raised, the H2 permeability of the derived CMSM decreased, and the H2 permselectivity enhanced greatly. The CMSM pyrolyzed at 800 °C demonstrated a high H2 permeability of 2622.4 Barrer and superior selectivity with H2/N2 of 524.48, H2/CH4 of 1248.76 and H2/CO2 of 7.51, respectively, which was attributed to the low densification of carbon structure resulting from the rigid and contorted structure of the BATPPP-6FDA precursor. The excellent H2 separation performance showed a great potential in hydrogen purification application.

Crosslinked thermally rearranged polybenzoxazole derived from phenolphthalein-based polyimide for gas separation

We have developed a novel ortho-hydroxyl diamine 3,3-diaminophenolphthalein and formed a polyimide (Ac-6FDA-DAP) via chemical imidization between this compound and 4,4′-(hexafluoroisopropylidene) diphthalic anhydride. The acetate groups and lactone ring of the diamino moiety degraded and subsequently formed a biphenyl crosslinked polyimide, enabling subsequent conversion of the thermally crosslinked polyimide into a polybenzoxazole (PBO) with superior CO2 separation performances. Compared to the polyimide precursor, the CO2 permeability of the TR PBO, thermally rearranged at 400 °C, increased by at least 21 folds (30.1 to 613 Barrer), with 10% decrement in the CO2/CH4 selectivity (36.7 to 33.5). The CO2/CH4 separation performances of the TR PBO surpassed the “2008 Robeson Upper Bound”, and no plasticization was observed at a CO2 pressure up to 30 atm for pure CO2 and 40 atm for 50:50 CO2 and CH4 gas mixtures. The TR-PBO polymer derived from Ac- 6FDA-DAP showed a strong potential for carbon capture and natural gas treatment.

Microporous polyimide networks with tunable micropore size constructed through side-chain engineering of linear precursors

Microporous polyimides (MPIs) have been acted a significant role in porous organic polymers (POPs) for gas capture and separation. However, the MPIs are mostly constructed from monomers with rigid 3D or twisted structures or hyperbranched precursors, and their porous structure is mainly tuned by changing the length of the constructed units. Herein, the cross-linkable linear PIs with flexible skeletons are selected as precursors to construct the cross-linked microporous polyimides (C-MPIs). The method has significant comparative advantages including abundant monomers, the convenience of synthesis, cost-effectiveness, structural diversity and designability. Moreover, the micropore size of the C-MPIs (6FA-PE-CL, 6FA-PEPH-CL) can be tuned from 0.64 nm to 0.57 nm through the side-chain engineering of the cross-linkable linear PI precursors. The BET surface area of the C-MPIs are ranging from 635 to 698 m 2 g -1 and is among the best results for the reported linear MPIs. The smaller pore size (0.57 nm), higher micropore surface area (378 m 2 g -1 ) and micropore volume (0.17 cm 3 g -1 ) endow 6FA-PE-CL with better CO 2 selective uptake capacity (8.9 wt% and 58) than those of 6FA-PEPH-CL (7.3 wt% and 38). This work provides a side-chain engineering method to tune the pore structure of MPIs, which has tremendous potential for constructing high-performance polymer networks with abundant ultramicroporous structure (<0.7 nm) based on flexible linear polymers. Microporous polymer networks with tunable micropore sizes have been constructed through the side-chain engineering of cross-linkable linear polyimide precursors. • The microporous polymer networks with abundant ultramicropores were constructed from flexible linear polyimides. • The pore size can be finely tuned through the side-chain engineering method. • The microporous polyimide with a smaller pore size exhibits better CO 2 selective uptake capacity.

Structural Effect of Polyimide Precursors on Highly Thermally Conductive Graphite Films.

In this study, polyimide (PI) with high carbonization yield was used as a precursor to prepare graphite films with high thermal conductivity. The crystallinity, grain size, and thermal conductivity of the graphite films were characterized and found to vary according to the chemical structure of the PI precursor. Aromatic PIs containing ortho-substituted hydroxyl groups in the PI main chain (DHB-BPDA) were synthesized by the polycondensation reaction of 3,3′-dihydroxybenzidine (DHB) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA). The DHB-BPDA is converted to a polybenzoxazole (PBO) structure through thermolysis reaction during carbonization. The PBO containing a benzene ring and a heterocycle group can provide a strong main chain and high thermal stability due to its resonant structure. The graphite film prepared from DHB-BPDA exhibited a large grain size (63.727 nm) and a high thermal conductivity of 916 W/(mK).

Hierarchical Nitrogen‐Enriched Carbon from Rationally Designed Polyimide Precursor for Exceptional Acetone Adsorption

AbstractThe adsorption capacity of porous adsorbent to volatile organic compounds (VOCs) is restricted by pore size distribution and surface functionalities. In this study, we report an effective N‐doped porous carbon synthesis strategy using rationally designed polyimide as precursor and subsequently copper chloride (CuCl2) mild activation. Specifically, by carefully changing the thiourea/urea ratio, polyimide precursors of different configurations can be designed and synthesized, which not only facilitates the transformation to a mesoporous structure, but also slightly enriches the surface heteroatoms. These unique properties result in a series of hierarchical porous carbon with high surface area (2045 m2/g), large mesopore volume (1.32 cm3/g) and rich nitrogen content (8.95 at %). Such carbon consequently shows exceptional static acetone adsorption performance of 768 mg/g (25 °C) and 799 mg/g (15 °C) and fast dynamic transfer rate. The abundant mesoporous structure provides the necessary diffusion channels to shorten the transmission distance and transmission resistance of acetone molecules, thereby contributing to this high acetone adsorption performance. Moreover, electrostatic potential (ESP) charge analysis shows that the doping of nitrogen atoms can significantly change the charge distribution on the sample surface, and enhance the affinity with acetone molecules through electrostatic interaction and Lewis acid‐base interaction

Polyimide Hybrid Nanocomposites with Controlled Polymer Filling and Polymer-Matrix Interaction.

Polyimide hybrid nanocomposites with the polyimide confined at molecular length scales exhibit enhanced fracture resistance with excellent thermal-oxidative stability at low density. Previously, polyimide nanocomposites were fabricated by infiltration of a polyimide precursor into a nanoporous matrix followed by sequential thermally induced imidization and cross-linking of the polyimide under nanometer-scale confinement. However, byproducts formed during imidization became volatile at the cross-linking temperature, limiting the polymer fill level and degrading the nanocomposite fracture resistance. This is solved in the present work with an easier approach where the nanoporous matrix is filled with shorter preimidized polyimide chains that are cross-linked while in the pores to eliminate the need for confined imidization reactions, which produces better results compared to the previous study. In addition, we selected a preimidized polyimide that has a higher chain mobility and a stronger interaction with the matrix pore surface. Consequently, the toughness achieved with un-cross-linked preimidized polyimide chains in this work is equivalent to that achieved with the cross-linking of the previously used polyimide chains and is doubled when preimidized polyimide chains are cross-linked. The increased chain mobility enables more efficient polymer filling and higher polymer fill levels. The higher polymer-pore surface interaction increases the energy dissipation during polyimide molecular bridging, increasing the nanocomposite fracture resistance. The combination of the higher polymer fill and the stronger polymer-surface interaction is shown to provide significant improvements to the nanocomposite fracture resistance and is validated with a molecular bridging model.

Characterization of carbon fiber-reinforced thermoplastic and thermosetting polyimide matrix composites manufactured by using various synthesized PI precursor resins

Currently, it is important to manufacture thermoplastic advanced composites with high mechanical and thermal properties because of their higher impact resistance than thermoset composites. In this study, carbon fiber (CF)-reinforced thermoplastic and thermosetting polyimide (PI) matrix composites were manufactured by vacuum bagging followed by compression molding . Before the manufacturing process, prepregs were also prepared with various PI precursor resins by using two-step impregnation. These precursors were synthesized as poly (amic acid) (PAA), polyester amine salts (PEAS) and poly (amic acid) ammonium salts (PAAS). The PAAS precursor resin was applied as a novel method for manufacturing CF-reinforced PI matrix advanced composites. The advanced thermoplastic composite obtained from the PAAS precursor (PAAS-T) exhibited superior mechanical properties and enhanced thermal properties (near the thermal properties of thermoset advanced composites) in comparison to the other advanced composites in this study and those in the literature. However, the NA-terminated PAAS precursor (thermosetting PI precursor) did not exhibit a similar effect in terms of mechanical properties. On the other hand, the thermosetting composites exhibited highly enhanced storage modulus values up to 400 °C and outstanding thermal stability at TGA . The thermosetting composite that was obtained from the NA-terminated PAA precursor resin exhibited high mechanical properties besides its thermal properties.

Laser-assisted transformation of a phenol-based resin to high quality graphene-like powder for supercapacitor applications

Synthesis of high-quality graphene-like structures by means of an eco-friendly, cost effective, one-step and scalable method is highly sought after. Some of these traits have been successfully addressed in recent studies employing laser irradiation in the synthesis process, however, reports on laser-assisted high-quality graphene growth overwhelmingly rely on the rather costly polyimide precursor. In this study, we report laser-mediated synthesis of high-quality graphene powder, originating from a phenol-based resin. Raman spectroscopy and X-ray diffraction findings suggest turbostratic, i.e., rotationally faulted, stacking of graphene layers, while the porous structure of the produced material is revealed by scanning electron microscopy. X-ray photoelectron spectroscopy was employed to probe the chemical composition of the materials before and after irradiation, and revealed the high C/O ratio value (22.6) in the irradiated powder. Moreover, the potential of these graphene-like structures in energy storage applications are investigated in aqueous supercapacitors, using both three- and two- electrode configurations. The excellent conductivity is validated from both sheet resistance measurements 40Ohmsq-1 and the very low ohmic losses during galvanostatic charge discharge measurements. The two-electrode device showed a capacitance of 17.2 Fg−1 at 10 mVs−1, a fairly good rate performance 67% up to 200 mVs−1 and excellent cycling stability (87.2% after 20,000 cycles).

Carbon dioxide/methane mixed-gas adsorption, permeation and diffusion in a carbon molecular sieve film: Experimental observation and modeling

The CO 2 –CH 4 pure- and mixed-gas transport properties (at 35 °C) of a carbon molecular sieve (CMS) film obtained by pyrolysis of a 6FDA-mPDA polyimide precursor at 900 °C are reported. The competitive mixed-gas adsorption of CO 2 and CH 4 was predicted by the Ideal Adsorbed Solution Theory (IAST) model. The CO 2 /CH 4 mixed-gas solubility selectivity of the CMS film was lower than that of relevant glassy polymer films of various nature and increased with pressure. Mixed-gas adsorption data were coupled with gas permeation tests on the same film sample batch used for barometric adsorption measurements to derive concentration-averaged effective diffusion coefficients. Because of its large fraction of ultramicroporous bottlenecks, the diffusion coefficients of the CMS were of the same order of magnitude as those of glassy polymer films of low free volume (e.g., 6FDA-mPDA and CTA). In the range of pressures explored, the pure-gas and multicomponent CO 2 diffusion coefficients overlapped; most importantly, the methane diffusion coefficient was enhanced by the presence of CO 2 . This result suggests that carbon dioxide dilated the sieving domains of the CMS matrix under mixed-gas environment containing highly sorbing gases such as CO 2 . Consequently, the CMS film lost some of its size-sieving properties as indicated by a drop in mixed-gas CO 2 /CH 4 permselectivity relative to the values obtained under pure-gas conditions. • The first case of CO 2 –CH 4 pure- and mixed-gas adsorption in a CMS film. • Competitive effects are more deleterious for CH 4 than for CO 2 . • Competitive effects are beneficial for the CO 2 /CH 4 adsorption selectivity. • The IAST (Toth) model satisfactory predicts the CMS gas adsorption behavior. • CO 2 -induced alteration of the diffusion domains of the CMS film.

Preparation and characterization of low-κ polyhedral oligomeric silsesquioxane/polyimide hybrid films

A series of Polyimide (PI)/Phenyl-POSS (POSS) hybrid films with low dielectric constants and excellent comprehensive properties have been successfully prepared via physical blend methods by incorporating different proportions of POSS into the polyimide precursors (Pyromellitic dianhydride- 4, 4′-Diaminodiphenyl ether). An appropriate amount of POSS doping can effectively improve the optical and dielectric properties of hybrid films due to the contribution of POSS with hollow, cage-like molecular structure to weakening intermolecular charge transfer complexation and increasing nanopore volume fraction in hybrid films. Instead, excessive amount of POSS doping can reduce dielectric properties due to the interface polarization effect induced by POSS agglomeration. When the doping amount is 15%, the highest light transmission reaches to 84.5%. When the doping amount is 25%, the dielectric properties have been are best improved. The dielectric constant and loss at 104 Hz are reduced to 2.37 and 0.0018 respectively. However, the mechanical properties decrease in response to the increase of POSS content, because POSS molecular dispersing in the folded molecular chains hinders the interaction of molecular chains PI matrix. The correlation between the dielectric properties of PI/POSS films and their nonopore volume fraction has been explained with the help of positron lifetime spectra.

Graphene/MXene Composite Aerogels Reinforced by Polyimide for Pressure Sensing

Here, polyimide (PI) was introduced as a binder and enhancer of graphene oxide (GO) and MXene sheets to prepare robust rGO/MXene/PI (GMP) composite aerogels with good mechanical elasticity, high conductivity, excellent thermal stability, and reliable heat insulation ability. The unique physical and chemical interactions among GO, MXene nanosheets, and the PI precursor enhance the three-dimensional network framework of aerogels. As a sensing material, the GMP aerogel with a layer-strut network structure prepared by directional freezing has a low density (8.97–12.71 mg/cm3), high conductivity (3.08 S/m), significant reversible compression (90%), and excellent fatigue resistance (10,000 cycles). The piezoresistive sensor based on the GMP composite aerogel can quickly detect a tiny strain, which could be further used for pulse beating, breathing, vibration, finger bending, and other human health monitoring. With the assistance of PI, the aerogel also presents high thermal insulation performance (0.022 W/(m·k)) and excellent low-temperature tolerance (−196 °C), showing wide application potential in many fields.

Single-gas and mixed-gas permeation of N2/CH4 in thermally-rearranged TR-PBO membranes and their 6FDA-bisAPAF polyimide precursor studied by molecular dynamics simulations

High-performance polymers with polybenzoxazole (PBO) structures, formed via thermal rearrangement (TR) of aromatic polyimide precursors, have been developed for gas separation applications. The present work compares the transport of N2 and CH4 in a 6FDA-bisAPAF polyimide precursor and in its TR-PBO derivative using molecular dynamics (MD) simulations. The modelling closely mimicked the experimental approach by transforming a 6FDA-bisAPAF atomistic model into its corresponding TR-PBO structure via a specific algorithm. The densities and void spaces of both precursor and TR polymers were found to compare well to experimental data. An iterative technique was used to obtain the single-gas sorption isotherms of N2 and CH4 at 338.5 K in both polymers over a range of feed pressures up to and exceeding 65 bar. CH4 was systematically found to be more soluble than N2. Solubilities in both matrices were quite similar with those in TR-PBO being slightly higher due to its larger fraction of significant volume. Volume dilation analyses confirmed a higher resistance to plasticization for TR-PBO. Extended single-gas N2 and CH4 simulations and 2 : 1 binary CH4/N2 mixed-gas simulations were then conducted in both matrices at 338.5 K and at a pressure of ∼65 bar corresponding to natural gas processing conditions. Mixed-gas sorption was modelled using a modification of the aforementioned iterative method, which fixed the pressure and iterated to convergence the number of molecules of each type of penetrant. The gas diffusion coefficients were estimated using the Trajectory-Extending Kinetic Monte Carlo (TEKMC) procedure. As found experimentally, significantly higher diffusivities and permeabilities were observed in the TR polymer, which led to a slightly lower ideal N2/CH4 permselectivity for TR-PBO (∼2.6) when compared to its 6FDA-bisAPAF precursor (∼3.8). However, both models showed a reduced N2/CH4 separation efficiency under 2 : 1 binary CH4/N2 mixed-gas conditions bordering on the loss of selectivity. For 6FDA-bisAPAF, both permeabilities decreased in the mixed-gas case, but more for N2 than for CH4. For TR-PBO, the permeability of the faster N2 decreased while the permeability of the slower CH4 increased under mixed-gas conditions. This confirms that single-gas simulations are not sufficient for the prediction of the actual mixed-gas permselectivity behaviour in such polymers.

Closed-Cell Rigid Polyimide Foams for High-Temperature Applications: The Effect of Structure on Combined Properties.

Closed-cell rigid polyimide foams with excellent thermal stability and combined properties were prepared by thermal foaming of a reactive end-capped polyimide precursor powder in a closed mold. The precursor powder was obtained by thermal treatment of a polyester-amine salt (PEAS) solution derived from the reaction of the diethyl ester of 2,3,3′,4′-biphenyl tetracarboxylic dianhydride (α-BPDE) with an aromatic diamine mixture of p-phenylenediamine (PDA) and 2-(4-aminophenyl)-5-aminobenzimidazole (BIA) in the presence of an end-capping agent (mono-ethyl ester of nadic acid anhydride, NE) in an aliphatic alcohol. The effect of polymer mainchain structures on the foaming processability and combined properties of the closed-cell rigid polyimide foams were systematically investigated. The polyimide foams (100–300 kg/m3) with closed-cell rates of 91–95% show an outstanding thermal stability with an initial thermal decomposition temperature of ≥490 °C and a glass transition temperature of 395 °C. Polyimide foams with density of 250 kg/m3 exhibited compression creep deformation as low as 1.6% after thermal aging at 320 °C/0.4 MPa for 2 h.

In situ FTIR analysis for the determination of imidization degree of polyimide precursors

Two series of polyimide (PI) precursors, poly(amic acid) (PAA) and poly(amic acid) salt (PAAS) were prepared from biphenyltetracarboxylic dianhydride (BPDA) and 2,2′-bis(trifluoromethyl)-[1,1′-biphenyl]-4,4′-diamine (TFDB) monomers, and systematically studied for thermal imidization. Firstly, the temperature for the complete thermal imidization of two precursors was determined by the thermogravimetric analysis (TGA). Later, a new method for the determination of the imidization degree (ID) was proposed by in situ infrared spectroscopy, and it was inferred that the traditional method for ID calculation is no longer applicable to PI precursors with aromatic base. The ID and the kinetics of thermal imidization of the aforementioned two precursors were also investigated by the new method. In comparison to PAA, PAAS exhibited higher imidization rate and lower temperature for thermal imidization.

Eco-Friendly Water-Processable Polyimide Binders with High Adhesion to Silicon Anodes for Lithium-Ion Batteries.

Silicon is an attractive anode material for lithium-ion batteries (LIBs) because of its natural abundance and excellent theoretical energy density. However, Si-based electrodes are difficult to commercialize because of their significant volume changes during lithiation that can result in mechanical damage. To overcome this limitation, we synthesized an eco-friendly water-soluble polyimide (W-PI) precursor, poly(amic acid) salt (W-PAmAS), as a binder for Si anodes via a simple one-step process using water as a solvent. Using the W-PAmAS binder, a composite Si electrode was achieved by low-temperature processing at 150 °C. The adhesion between the electrode components was further enhanced by introducing 3,5-diaminobenzoic acid, which contains free carboxylic acid (–COOH) groups in the W-PAmAS backbone. The –COOH of the W-PI binder chemically interacts with the surface of Si nanoparticles (SiNPs) by forming ester bonds, which efficiently bond the SiNPs, even during severe volume changes. The Si anode with W-PI binder showed improved electrochemical performance with a high capacity of 2061 mAh g−1 and excellent cyclability of 1883 mAh g−1 after 200 cycles at 1200 mA g−1. Therefore, W-PI can be used as a highly effective polymeric binder in Si-based high-capacity LIBs.

Flexible and self-standing polyimide/lignin-derived carbon nanofibers for high-performance supercapacitor electrode material applications

For attaining flexible and self-standing electrode materials for advanced energy storage devices, we herein report the fabrication and electrochemical properties of carbon nanofibers (CNFs) derived from polyimide (PI) and lignin precursors with different mass ratios (10/0, 9/1, 8/2, and 7/3) via electrospinning and carbonization at 1000 °C. The electron microscopic and photographic images exhibit that the average diameter and flexibility of PI/lignin-derived CNFs decreases and increases, respectively, with increasing the lignin mass ratio in the precursors. The EDS, XRD and Raman analyses confirm that PI/lignin-derived CNFs possess a partially ordered graphitic structure with oxygen self-doping. The electrical conductivity of the CNFs increases from 1.5 S/cm to 2.8 S/cm with the increment of the lignin mass ratio in the precursors. Accordingly, PI/lignin (7/3)-derived CNF is found to have the most excellent electrochemical properties from the analyses using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) test, and electrochemical impedance spectroscopy (EIS). For a symmetric two-electrode supercapacitor based on PI/lignin (7/3)-derived CNFs, high specific capacitance of 83.6 F/g at 1 A/g, power density of 5000 W/kg, energy density of 13.3 Wh/kg, and capacitance retention of ∼100% after 1000 GCD cycle are attained.

Flexible Conductive Polyimide Fiber/MXene Composite Film for Electromagnetic Interference Shielding and Joule Heating with Excellent Harsh Environment Tolerance.

The development of flexible MXene-based multifunctional composites is becoming a hot research area to achieve the application of conductive MXene in wearable electric instruments. Herein, a flexible conductive polyimide fiber (PIF)/MXene composite film with densely stacked "rebar-brick-cement" lamellar structure is fabricated using the simple vacuum filtration plus thermal imidization technique. A water-soluble polyimide precursor, poly(amic acid), is applied to act as a binder and dispersant to ensure the homogeneous dispersion of MXene and its good interfacial adhesion with PIF after thermal imidization, resulting in excellent mechanical robustness and high conductivity (3787.9 S/m). Owing to the reflection on the surface, absorption through conduction loss and interfacial/dipolar polarization loss inside the material, and the lamellar structure that is beneficial for multiple reflection and scattering between adjacent layers, the resultant PIF/MXene composite film exhibits a high electromagnetic interference (EMI) shielding effectiveness of 49.9 dB in the frequency range of 8.2-12.4 GHz. More importantly, its EMI shielding capacity can be well maintained in various harsh environments (e.g., extreme high/low temperature, acid/salt solution, and long-term cyclic bending), showing excellent stability and durability. Furthermore, it also presents fast, stable, and long-term durable Joule heating performances based on its stable and excellent conductivity, demonstrating good thermal deicing effects under actual conditions. Therefore, we believe that the flexible conductive PIF/MXene composite film with excellent conductivity and harsh environment tolerance possesses promising potential for electromagnetic wave protection and personal thermal management.

Supramolecular Salts for Additive Manufacturing of Polyimides.

Recent advances in vat photopolymerization (VP) additive manufacturing of fully aromatic polyimides employed photoreactive high-molecular-weight precursors dissolved at modest loadings (<20 wt %) in organic solvent. These earlier efforts revealed high isotropic shrinkage, approaching 52% on a linear basis while converting to the desired polyimide. To increase the polyimide precursor concentration and decrease shrinkage during VP processing of high-performance polyimides, photoreactive fully aromatic polyimide and thermoplastic polyetherimide (PEI) supramolecular salt precursors now serve as versatile alternatives. Both pyromellitic dianhydride-4,4'-oxydianiline (PMDA-ODA) and 4,4'-(4,4'-isopropylidene-diphenoxy)diphthalic anhydride-meta phenylene diamine (BPADA-mPD) supramolecular dicarboxylate-diammonium salts, termed polysalts, provided prerequisite rheological performance and photoreactivity for VP. Solutions (50 wt %) of both photoactive polysalts exhibited viscosities more than two orders of magnitude lower than previously reported polyimide precursor solutions for VP. In addition, VP of 50 wt % polysalt solutions yielded high resolution, self-supporting organogel structures. During thermal postprocessing to the desired fully aromatic polyimide and PEI, photocrosslinked polysalt organogels exhibited retention of part shape in concert with linear isotropic shrinkage of only 26%, the lowest reported value using organogel strategies for VP of fully aromatic polyimides. Furthermore, the imidized structures exhibited comparable thermal and mechanical properties to analogous polyimides synthesized using classical methodologies for 2D films. The combination of facile synthesis and increased precursor concentrations designates polysalt polyimide precursors as a versatile platform for additive manufacturing of well-defined 3D polyimide structures.