Recent development of polyimides: Synthesis, processing, and application in gas separation

Progress and Challenges in Polystyrene Recycling and Upcycling

Polystyrene is a staple plastic in the packaging and insulation market. Despite its good recyclability, the willingness of PS recycling remains low, largely due to the high recycling cost and limited profitability. This review examines the research progresses, gaps, and challenges in areas that affect the recycling costs, including but not limited to logistics, packaging design, and policymaking. We critically evaluate the recent developments in upcycling strategies, and we particularly focus on tandem and hydrogen-atom transfer (HAT) upcycling strategies. We conclude that future upcycling studies should focus on not only reaction chemistry and mechanisms but also economic viability of the processes. The goal of this review is to stimulate the development of innovative recycling strategies with low recycling costs and high economic output values. We hope to stimulate the economic and technological momentum of PS recycling towards a sustainable and circular economy.

Strategies towards Fully Recyclable Commercial Epoxy Resins: Diels-Alder Structures in Sustainable Composites

The Diels-Alder equilibrium is a widely known process in chemistry that can be used to provide a thermoset structure with recyclability and reprocessability mechanisms. In this study, a commercial epoxy resin is modified through the integration of functional groups into the network structure to provide superior performance. The present study has demonstrated that it is possible to adapt the curing process to efficiently incorporate these moieties in the final structure of commercial epoxy-based resins. It also evaluates the impact that they have on the final properties of the cured composites. In addition, different approaches have been studied for the incorporation of the functional group, adjusting and adapting the stoichiometry of the system components due to the differences in reactivity caused by the presence of the incorporated reactive groups, with the objective of maintaining comparable ratios of epoxy/amine groups in the formulation. Finally, it has been demonstrated that although the Diels-Alder equilibrium responds under external conditions, such as temperature, different sets of parameters and behaviors are to be expected as the structures are integrated into the thermoset, generating new equilibrium temperatures. In this way, the present research has explored sustainable strategies to enable the recyclability of commercial thermoset systems through crosslinking control and its modification.

Preparation and Properties of Atomic-Oxygen Resistant Polyimide Films Based on Multi-Ring Fluoro-Containing Dianhydride and Phosphorus-Containing Diamine

Colorless and transparent polyimide (CPI) films with good atomic oxygen (AO) resistance and high thermal endurance are highly required in low earth orbit (LEO) space exploration. Conventional CPI films based on fluoro-containing 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) have been widely used in space applications. However, the AO erosion yields and glass transition temperatures (Tg) of the 6FDA-based CPI films have to be modified in order to meet the severe serving environments. In the current work, novel CPI films based on a multi-ring fluoro-containing 9,9-bis(trifluoromethyl)xanthene-2,3,6,7-tetracarboxylicdianhydride (6FCDA) monomer were developed. In order to enhance the AO resistance of the derived CPI film, a phosphorus-containing aromatic diamine, 2,5-bis[(4-aminophenoxy)phenyl]diphenylphosphine oxide (BADPO) was used to polymerize with the dianhydride to create the organo-soluble resin. Then, two phosphorus-containing CPI films (PPI), including PPI-1 (6FDA-BADPO) and PPI-2 (6FCDA-BADPO) were prepared by thermally curing of the PPI solutions at elevated temperatures. The PPI films maintained good optical transparency with transmittance values over 80% at a wavelength of 450 nm. PPI-2 exhibited a Tg value of 311.0 °C by differential scanning calorimetry (DSC) measurement, which was 46.7 °C higher than that of the PPI-1 counterpart (Tg = 264.3 °C). In addition, the PPI-2 film showed a coefficient of linear thermal expansion (CTE) value of 41.7 × 10-6/K in the range of 50~250 °C, which was apparently lower than that of the PPI-1 sample (CTE = 49.2 × 10-6/K). Lastly, both of the two PPI films exhibited good AO resistance with the erosion yields (Ey) of 6.99 × 10-25 cm3/atom for PPI-1 and 7.23 × 10-25 cm3/atom for PPI-2 at an exposure flux of 5.0 × 1020 atoms/cm2. The Ey values of the current PPI films were obviously lower than that of the standard polyimide (PI) film based on pyromellitic dianhydride (PMDA) and 4,4'-oxydianiline (ODA) (Ey = 3.0 × 10-24 cm3/atom).

Effects of various types of organo-mica on the physical properties of polyimide nanocomposites

Poly(amic acid) (PAA) was synthesized using dianhydride 4,4'-oxydiphthalic anhydride and diamine 3,3'-dihydroxybenzidine, and polyimide (PI) hybrid films were synthesized by dispersing organo-mica in PAA through a solution intercalation method. Hexadimethrine-mica (HM-Mica), 1,2-dimethylhexadecylimidazolium-mica (MI-Mica), and didodecyldiphenylammonium-mica (DP-Mica), which were obtained via the organic modification of pristine mica, were used as the organo-micas for the PI hybrid films. The organo-mica content was varied from 0.5 to 3.0 wt% with respect to the PI matrix. The thermomechanical properties, morphology, and optical transparency of the resultant PI hybrid films were measured and compared. Dispersion of even small amounts of organo-mica effectively improved the physical properties of the PI hybrids, and maximum enhancements in physical properties were observed at a specific critical content. Electron microscopy of the hybrid films revealed that the organo-mica uniformly dispersed throughout the polymer matrix at the nanoscale level when added at low contents but aggregated in the matrix when added at levels above the critical content. Structural changes in the organo-mica closely influenced the changes in the physical properties of the hybrid films. All PI hybrid films with various organo-mica contents showed similar optical properties, but that prepared with MI-Mica demonstrated the best thermomechanical properties. All synthesized PI hybrid films were transparent regardless of the type and content of organo-mica used.

Fe3O4 microplate filled PEI matrix composite with remarkable nonlinear conductive characteristics, dielectric property, and low percolation threshold

As the presence of a percolating network formed by filler is indispensable for field grading composite, particulate fillers often result in high filler content that can be unfavorable in some aspects. The utilization of fillers with high aspect ratio is an effective way of reducing percolation threshold. In this work, Fe3O4 microplate (FMP) was prepared by a PVP-assisted hydrothermal method and it was adopted to fabricate composite films with different filler content by using polyetherimide (PEI) as the matrix. The composite film exhibited a percolation threshold of approximately 8 phr. The nonlinear coefficient measured 6.28 at a filler content of 10 phr. The nonlinearity in the conductive behavior of the composites was attributed to tunneling effect and Schottky emission. The filling of the FMP into PEI resulted in increase in dielectric constant and the dielectric loss maintained low. This study suggests that the FMP is a promising filler of low-filler-content field grading composite.

A Molecular Dynamics Study of Single-Gas and Mixed-Gas N2 and CH4 Transport in Triptycene-Based Polyimide Membranes

Fluorinated polyimides incorporated with triptycene units have gained growing attention over the last decade since they present potentially interesting selectivities and a higher free volume with respect to their triptycene-free counterparts. This work examines the transport of single-gas and mixed-gas N2 and CH4 in the triptycene-based 6FDA-BAPT homopolyimide and in a block 15,000 g mol-1/15,000 g mol-1 6FDA-mPDA/BAPT copolyimide by using molecular dynamics (MD) simulations. The void-space analyses reveal that, while the free volume consists of small-to-medium holes in the 6FDA-BAPT homopolyimide, there are more medium-to-large holes in the 6FDA-mPDA/BAPT copolyimide. The single-gas sorption isotherms for N2 and CH4 over the 0-70 bar range at 338.5 K show that both gases are more soluble in the block copolyimide, with a higher affinity for methane. CH4 favours sites with the most favourable energetic interactions, while N2 probes more sites in the matrices. The volume swellings remain limited since neither N2 nor CH4 plasticise penetrants. The transport of a binary-gas 2:1 CH4/N2 mixture is also examined in both polyimides under operating conditions similar to those used in current natural gas processing, i.e., at 65.5 bar and 338.5 K. In the mixed-gas simulations, the solubility selectivities in favour of CH4 are enhanced similarly in both matrices. Although diffusion is higher in 6FDA-BAPT/6FDA-mPDA, the diffusion selectivities are also close. Both triptycene-based polyimides under study favour, to a similar extent, the transport of methane over that of nitrogen under the conditions studied.

Synthesis and Characterization of Organo-Soluble Polyimides Based on Polycondensation Chemistry of Fluorene-Containing Dianhydride and Amide-Bridged Diamines with Good Optical Transparency and Glass Transition Temperatures over 400 °C

Polymeric optical films with light colors, good optical transparency and high thermal resistance have gained increasing attention in advanced optoelectronic areas in recent years. However, it is somewhat inter-conflicting for achieving the good optical properties to the conventional thermal resistant polymers, such as the standard aromatic polyimide (PI) films, which are well known for the excellent combined properties and also the deep colors. In this work, a series of wholly aromatic PI films were prepared via the polycondensation chemistry of one fluorene-containing dianhydride, 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (FDAn) and several aromatic diamines with amide linkages in the main chain, including 9,9-bis [4-(4-aminobenzamide)phenyl]fluorene (FDAADA), 2,2'-bis(trifluoromethyl)-4,4'-bis[4-(4-aminobenzamide)] biphenyl (ABTFMB), and 2,2'-bis(trifluoromethyl)-4,4'-bis[4-(4-amino-3-methyl)benzamide] biphenyl (MABTFMB). The derived FLPI-1 (FDAn-FDAADA), FLPI-2 (FDAn-ABTFMB) and FLPI-3 (FDAn-MABTFMB) resins showed good solubility in the polar aprotic solvents, such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc) and dimethyl sulfoxide (DMSO). The solution-processing FDAn-PI films exhibited good optical transmittance over 80.0% at a wavelength of 500 nm (T500), yellow indices (b*) in the range of 1.01-5.20, and haze values lower than 1.0%. In addition, the FDAn-PI films showed low optical retardance with optical retardation (Rth) values in the range of 31.7-390.6 nm. At the same time, the FDAn-PI films exhibited extremely high glass transition temperatures (Tg) over 420 °C according to dynamic mechanical analysis (DMA) tests. The FDAn-PI films showed good dimensional stability at elevated temperatures with linear coefficients of thermal expansion (CTE) in the range of (31.8-45.8) × 10-6/K.

Fluorinated Polyimide/Allomelanin Nanocomposites for UV-Shielding Applications

A series of highly fluorinated polyimide/allomelanin nanoparticles (FPI/AMNPs) films were prepared with FPI as the matrix and AMNPs as the filler. Due to the formation of hydrogen bonds, significantly reinforced mechanical and UV-shielding properties are acquired. Stress-strain curves demonstrated a maximum tensile strength of 150.59 MPa and a fracture elongation of 1.40% (0.7 wt.% AMNPs), respectively, 1.78 and 1.56× that of pure FPI. The measurements of the UV-vis spectrum, photodegradation of curcumin and repeated running tests confirmed the splendid UV-shielding capabilities of FPI/AMNPs films. The enhancement mechanisms, such as synergistic UV absorption of the charge transfer complexes in FPI and AMNPs and photothermal conversion, were the reasons for its exceptional UV shielding. The excellent comprehensive properties above enable FPI/AMNPs nanocomposites to be potential candidates in the field of UV shielding.

The Synergistic Effect of Triazine and Phosphaphenanthrene Units on the Physico-Chemical Behavior of Polyimides

With the aim to develop polymers with appealing, multifunctional characteristics, a series of polyimides were designed by anchoring 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) units on the main polymer chains containing 1,3,5-triazine and several flexible moieties, such as ether, hexafluoroisopropylidene, or isopropylidene. A detailed study was conducted to establish structure-property correlations, with a focus on the synergistic effectiveness of triazine and DOPO moieties on the overall features of polyimides. The results evidenced good solubility of the polymers in organic solvents, their amorphous nature with short-range regular-packed polymer chains, and high thermal stability with no glass transition temperature below 300 °C. Spectrophotometric measurements revealed the existence of a strong charge transfer complex in these polymers that led to a "black" appearance, which generated broad absorption bands spanning on the overall visible range. Nevertheless, these polymers displayed green light emission associated with 1,3,5-triazine emitter. The electrochemical characteristics of the polyimides in solid state demonstrated their strong n-type doping character induced by three different structural elements with electron-acceptance capability. The useful properties of these polyimides, including optical, thermal, electrochemical, aesthetics, and opaqueness, endow them with several possible applications in the microelectronic field, such as protecting layers for the inner circuits against UV light deterioration.

Teaching Old Polymers New Tricks: Improved Synthesis and Anomalous Crystallinity for a Lost Semi-Fluorinated Polyaryl Ether via Interfacial Polymerization of Hexafluoroacetone Hydrate and Diphenyl Ether

A practical and direct electrophilic polymerization of hexafluoroacetone hydrate with diphenyl ether toward the preparation of semi-fluorinated polyaryl ethers (PAE) is reported. Electrophilic aromatic substitution (EAS) polymerization under interfacial conditions with phase transfer catalyst (Aliquat 336) proceeds in trifluoromethanesulfonic anhydride by generation of trifluoromethanesulfonic acid and the protonated hexafluoroacetone (HFA) in situ affording 1,1,1,3,3,3-hexafluoroisopropylidene (6F) PAE with high regioselectivity (4,4'-DPE) and high molecular weight (≈60 kDa). Although first reported in a 1966 US Patent by DuPont using harsh conditions, improved synthetic methods or modern characterization has not been disclosed until now. Despite the presence of the 6F group, known to impart disordered morphology, this simple semi-fluorinated PAE exhibits anomalous crystallinity with polymorphic melting points (T_m ) ranging from 230-309 °C, high solubility in common organic solvents, a glass transition (T_g ) of 163 °C, and thermo-oxidative stability above 500 °C. Tough optically clear films prepared from solution give transmittance higher than 90% throughout the visible region. Synthesis, mechanistic aspects, and characterization including surface and dielectric properties are discussed.

Towards high-performance polyurethanes: a mechanism of amine catalyzed aromatic imide formation from the reaction of isocyanates with anhydrides

The mechanism of aromatic imide formation from reaction between isocyanate and anhydride using secondary amine as a pre-catalyst is driven by deprotonated urea, which is obtained from the reaction of isocyanate and amine with the help of bases.

Preparation and Performance Characterization of Exploding Foil Initiator Based on ODPA-ODA Polyimide Flyer

The exploding foil initiator (EFI) system has been extensively used in ignition and detonation sequences and proved to be of high safety and reliability. Polyimide is considered the ideal flyer material for EFI due to its excellent performance, including thermal stability, outstanding mechanical properties, high radiation resistance, and excellent dielectric properties. In this study, we prepared the EFI based on a polyimide (ODPA-ODA) flyer, which is spin-coated and solidified on patterned copper film in situ. The electric explosion test shows that the prepared EFI has good working performance, and the 4000 V working voltage drove the flyer to reach a maximum velocity of 5096 m/s. The polyimide morphology and chemical structure after the electric explosion was observed by microscope, SEM, XPS, and FTIR, which showed that the polyimide flyer underwent thermal deformation and complex chemical reactions during an electric explosion. A large number of polyimide bonds broke to form new carbonyl compounds, and the opening of aromatic rings was accompanied by the formation of aliphatic hydrocarbon chains. The morphology and chemical structure analysis after the electric explosion test will lay a foundation for us to further understand the working principle and evolution process of polyimide (ODPA-ODA) flyer.

Optical and Flame-Retardant Properties of a Series of Polyimides Containing Side Chained Bulky Phosphaphenanthrene Units

Among the multitude of polymers with carbon-based macromolecular architectures that easily ignite in certain applications where short circuits may occur, polyimide has evolved as a class of polymers with high thermal stability while exhibiting intrinsic flame retardancy at elevated temperatures via a char-forming mechanism. However, high amounts of aromatic rings in the macromolecular backbone are required for these results, which may affect other properties such as film-forming capacity or mechanical properties; thus, much work has been done to structurally derivatize or make hybrid polyimide systems. In this respect, flexible polyimide films (PI(1–4)) containing bulky 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) units have been developed starting from commercial dianhydrides and an aromatic diamine containing two side chain bulky DOPO groups. The chemical structure of PI(1–4)) was characterized by 1H NMR, 13C NMR and 31P NMR spectroscopy. The optical properties, including absorption and luminescence spectra of these polymers, were analyzed. All polyimides containing DOPO derivatives emitted blue light with an emission maxima in the range of 340–445 nm, in solvents such as N,N-dimethylformamide, N-methyl-2-pyrrolidone, chloroform, and N,N-dimethylacetamide, while green light emission (λem = 487 nm for PI-4) was evidenced in a thin-film state. The thermal decomposition mechanism and flame-retardant behavior of the resulting materials were investigated by pyrolysis-gas-chromatography spectrometry (Py-GC), scanning electron microscopy (SEM), EDX maps and FTIR spectroscopy. The residues resulting from the TGA experiments were examined by SEM microscopy images and FTIR spectra to understand the pyrolysis mechanism.