Polybenzoxazole Research Articles

Computer-aided design of thermosetting benzoxazoles containing bis-endoalkynyl groups: Low melting points and high thermal stability

High-temperature-resistant polybenzoxazoles (PBOs) have recently become a prominent research area due to their potential applications in aviation and aerospace. However, achieving a balance between thermal stability and processability remains a significant challenge. In this study, a computer-aided method to develop PBOs with high thermal stability and processability is explored. First, thermoset benzoxazoles (CPBOs) are designed using a material genomic approach. Subsequently, their zero shear viscosity, temperature at 50 % thermal weight loss, dielectric constant and dielectric loss are predicted using a computer-aided method. Finally, two screened thermosetting benzoxazoles, CPBO-1 and CPBO-6, are synthesized and experimentally validated. The experiments indicate that their melting points are below 100 °C, with the lowest melt viscosities being 0.5 Pa•s and 1.5 Pa•s, respectively. The corresponding polymers, pCPBO-1 and pCPBO-6, feature high thermal stability. The 5 % weight loss temperature of pCPBO-1 in N2 is 618.9 °C, while the dielectric constant and dielectric loss are 3.1 and 0.0063, respectively. These are excellent values for thermosetting resins. This computer-aided screening method is more efficient and cost-effective compared to conventional trial-and-error methods.

Thermal aging of high‐performance fabrics used in the outer shell of firefighters' protective clothing

AbstractHigh‐performance fibers are known for their exceptional specific strength and resistance to various hazardous conditions, including fire. However, the long‐term performance of these fibers when exposed to convective heat has rarely been reported. This study investigated the accelerated thermal aging behavior of three high‐performance fabrics of different blends of inherently flame‐resistant high‐performance fibers: copolymer of aramids (Technora®)/polybenzoxazole (PBO); para‐aramid/meta‐aramid; and para‐aramid/polybenzimidazole (PBI). Fabric specimens were thermally aged for up to 1200 h at temperatures ranging from 90 to 320°C. While all three fabrics experienced losses in the breaking force, the Technora®/PBO fabric displayed the best strength retention, despite the complete disappearance of the Technora® fiber crystallinity after aging at 320°C for 1200 h. The para‐aramid/PBI blended fabric showed signs of competing aging processes at high temperatures. An increase in the fabrics' crystallinity and evidence of chain scission were observed after thermal aging. Additionally, degradation in the fabric's water‐repellent finish was observed. The findings of this study will contribute to the development of more durable and safer protective gear, particularly for high‐risk activities like firefighting.

Pseudo-ductile behaviour of fibrous composite Z-pins

This paper describes the development and characterisation of a novel type of composite Z-pin able to accommodate large deformation without exhibiting fibre failure. Three types of pseudo-ductile Z-pins are fabricated by means of micro-pultrusion of polybenzoxazole (PBO) fibres. Namely, unidirectional PBO fibre (uPBO) Z-pins in combination with a ductile (uPBO/DCT) and a brittle (uPBO/BT) matrix system are developed, together with a twisted PBO fibre Z-pin in combination with a ductile matrix (tPBO/DCT). Single pin bridging tests are carried out across the full mode mixity range from mode I (Φ = 0) to mode II (Φ = 1). The tests reveal that uPBO-based pins are able to pull out throughout the full mode mixity range, regardless of the ductility of the pin matrix. uPBO/DCT pins exhibit a 20-fold increase in energy dissipation per pin than traditional carbon fibre/bismaleimide (CF/BMI) pins at load mode mixities higher than Φ = 0.9. The mode I behaviour of all pins considered is comparable. All PBO pins exhibit an apparent delamination toughness enhancement superior to CF/BMI at mode mixities above Φ = 0.2, with a ductile matrix increasing the average energy dissipated per pin by a further 2–8 %.

Behaviour of Full-scale Shear Critical RC Beams Strengthened with Textile Reinforced Mortar

Textile reinforced mortars (TRM) have recently been used for strengthening of Reinforced Concrete (RC) structural members by researchers. Carbon, glass, aramid and Polyphenylene Benzobisoxazole (PBO) fibres have extensively caught the eyes of the scientists and research enthusiasts, basalt fibre has been mostly explored for strengthening of masonry. For beams, a narrow band of data is available TRM using basalt fibres as strengthening material, most of which comprises of small-scale tests. An experimental investigation of full-scale shear critical RC beams with varying shear span-to-depth (a/d) ratios between 1 to 2.5 is presented in this paper. Six beams were tested in which three beams served as control beams for respective a/d ratios, while the remaining three beams were strengthened with basalt fibres based TRM. TRM was provided at the bottom of the RC beams in the shape of U-shaped wraps for corresponding a/d ratios. The basalt fibres based TRM was found to be effective in improving the performance of RC beams in terms of serviceability. Both cracking and deflections were controlled. The use of TRM also improved load carrying capacities of TRM strengthened beams in flexure. Appearance of first cracks was delayed, post cracking stiffness of the flexural members was enhanced and showed better performance in terms of ductility.

Breaking the Permeability-Selectivity Trade-Off: Advanced carbon molecular sieve membranes derived from thermally rearranged Mixed-Matrix membrane precursors

A crucial problem in membrane-based gas separation is to fabricate membranes with excellent permeability, selectivity, and stability. Here, a high-performance carbon molecular sieves membrane of TR-CMS/SSZ-13 was successfully produced by pyrolyzing a thermally rearranged (TR) mixed matrix membrane (MMM) that obtained from a copolyimide precursor (6FDA/ODA/6FAP(3:2)) and SSZ-13 zeolite as filler at 450 °C. The effect of SSZ-13 zeolite content and structures of precursors on the transport behavior of derived CMS membranes was thoroughly studied. After TR process, the as formed polybenzoxazole (PBO) chain resulted in an enhanced rigidity, higher fractional free volume (0.15 vs 0.17), and a significantly increased (1898 times) BET surface area. Further enhancing the treatment temperature to 650 °C, the formed TR-CMS membranes exhibited a huge improved BET surface area from 228.2 to 639.3 m2/g, delivering a remarkable gas separation performance with H2 permeability of 7089 Barrer and H2/CH4 selectivity of 136. When doping with SSZ-13 till 15 wt%, the TR-CMS/SSZ-13–15 % showed an even higher H2 (9726 Barrer) and CO2 permeability (6932 Barrer), as well as H2/CH4 (150 vs 136) and CO2/CH4 (107 vs 57) selectivity than the pure TR-CMS, which surpassed the newest 2019 Robeson Upper bound. This can associate with the strong interaction between the modified SSZ-13 and the polymer matrix that prevent the collapse of the aromatic strand during the pyrolysis process, thereby create more ultra-micropore volume and concentration. Conclusively, pyrolyzing TR-MMM precursor is an effective way to form high-performance membranes with good potential for CO2 separation and H2 purification.

Tailoring thermosetting benzoxazoles for optimal performance: Balancing thermal stability, dielectric properties, and processability

Polybenzoxazole (PBO) has excellent heat resistance and dielectric properties. However, it lacks a distinct melting point and is only soluble in sulfuric and methyl sulfonic acids. This limited solubility significantly hinders its practical application due to poor processability. In order to improve the processability of PBO while preserving its thermal stability and dielectric properties, this paper investigates the impact of introducing bridging structures between benzoxazole groups and employing cross-linking groups at the chain ends. The study primarily focuses on assessing the effects of these modifications on the thermosetting PBO properties, including heat resistance, dielectric properties, and processability. The results indicate that the cured polymer exhibits high thermal stability when a biphenyl group is used as bridging structure and a cyano group as the end group. Among these thermosetting monomers, p-LARBCN has the best overall performance. The 5 % thermal decomposition temperature (Td5) of cured p-LARBCN is 612 °C, which is very similar to PBO. The dielectric constant of the cured p-LARBCN is below 2.7, and the dielectric loss is below 0.02. The dielectric performance is close to that of PBO. In addition, p-LARBCN shows good solubility in solvents like DMSO, DMF, and DMAc, which greatly improves its processability compared to PBO. This study presents a modified thermosetting PBO resin that combines high thermal stability with improved processability. This resin holds promise for applications in the aviation and aerospace fields, particularly as a high-temperature-resistant material.

Electrospinning of High-Performance Nanofibres: State of the Art and Insights into the Path Forward

Nanofibrous membranes have gained interest for their small pore size, light weight, and excellent filtration. When produced from high-performance polymers, nanofibrous membranes also benefit from excellent mechanical properties, thermal resistance, and chemical resistance. Electrospinning is a common method of producing high-performance nanofibres. However, there are still major challenges with the dissolution and electrospinning of these polymers, as well as in the performance of the resulting nanofibres, which is often less than what would be expected from a conventional high-performance fibre. This review assesses the state of progress in the electrospinning of five high-performance fibres: meta-aramid (m-aramid), para-aramid (p-aramid), polyamide-imide (PAI), polybenzoxazole (PBO), and polybenzimidazole (PBI). Polymers that can be readily dissolved in organic solvents, such as m-aramid, PAI, and PBI, have been more widely researched for electrospinning compared to those that can only be spun from precursors or dissolved in non-volatile solvents. Major focuses within the literature include optimizing the electrospinning process and improving the mechanical performance of the nanofibres. This review demonstrates a clear need for more standardized characterization methods and consideration for the longevity of the nanofibrous membranes. Future research should also focus on scale-up methods of electrospinning so that the benefits of nanofibres made from high-performance polymers can be leveraged by the industry.

Blended crosslinked polybenzoxazole (PBO) membranes derived from phenolphthalein-based polyamide and polyimide for CO2/CH4 gas separation

Designing membrane materials with excellent gas separation performance is always a goal of membrane researchers. However, to be able to deliver stable performance and maintain membrane integrity under actual working conditions, a membrane material must be mechanically strong. In the last two decades, thermally rearranged polybenzoxazole (TR-PBO) polymers exhibit out-standing gas transport properties and great anti-plasticization behavior. But the TR reaction often lead to catastrophic deterioration in roughness of the polymer. To overcome this problem, we developed a polymer blending system consisting of a phenolphthalein-based polyamide (PHA, 6FC-DAP) and polyimide (API, OH-6FDA-DAP). Because of their similar chemical structures and the formation of inter-molecular hydrogen bonds, homogenous polymer blended membranes were obtained at molar ratios from 1:3 to 3:1 of PHA and API. Importantly, the B11 (PHA to API molar ratio of 1:1) derived PBO-based membranes exhibited superior mechanical and gas separation performance compared to the B31 (PHA: API=3:1) and B13 (PHA: API=3:1) derived PBO-based membranes. Note that, when the blended membrane B11 was fully converted into PBO, its tensile strength and elongation at break are 5.89 times and 3.27 times higher than those of the API derived TR-PBO. In addition, the B11–350 and B11–400 (thermal treatment at 350 °C and 400 °C in air atmosphere for 2 h, respectively) were not plasticized at a CO2 pressure of 30 atm. The excellent mechanical properties with appealing gas separation properties of these TR-PBO polymers have potential for CO2 relative gas separation applications.

Sub-Tg cross-linked thermally rearranged polybenzoxazole derived from phenolphthalein diamine for natural gas purification

Thermally rearranged (TR) polymers have shown a great potential for gas separation. However, the temperature of TR reaction often goes beyond the glass transition temperature (Tg) of polymer precursor and results in pore collapsing and low gas permeance to asymmetric membranes. To solve this problem, we prepared a series of crosslinked thermally rearranged (XTR) polybenzoxazoles (PBO) at sub-Tg. We synthesized crosslinkable phenolphthalein-based polyimides via chemical imidization (EPPI) and azeotropic distillation (HPPI). The two polyimides were crosslinked at 300–335 °C to increase their Tg so that the subsequent TR reaction were accomplished at sub-Tg. The HPPBO-425 exhibited the best separation performance with a CO2 permeability of 122 barrer and a CO2/CH4 selectivity of 40.4 for pure gases. Moreover, the XTR PBO was not plasticized at a CO2 pressure of 30 atm. After aged for 60 days, the CO2 permeability decreased to 100 barrer and the CO2/CH4 selectivity increased to 44. When separating CO2/CH4 (1:1) mixed gases, the CO2 permeability gradually decreased from 120 to 74 barrer with a decrement in the CO2/CH4 selectivity from 47 to 26 because of the progressively saturated Langmuir sorption sites and the competitive sorption by CH4 decreasing the solubility of CO2. The excellent and stable CO2/CH4 separation property makes the sub-Tg XTR PBO an ideal material for fabricating hollow fiber gas separation membranes.

Prediction of the additional compressive strength due to PBO-FRCM-confinement of brick-masonry depending on the stiffening effect caused by different discontinuous wrappings: new design-oriented perspective

Fabric Reinforced Cementitious Mortar (FRCM) is an innovative composite system able to strength or rehabilitate masonry members with an efficacy, which is affected by the mechanical and geometrical properties of its constituents: matrix and fabric. Since one of the most appreciated benefits of FRCM is the breathability, the discontinuous application is commonly not considered in the retrofitting. On the other side, the thickness of the matrix may be considerable (ranging between 10 and 30 mm) implying an over-weight and over-stiffness which may influence the seismic response of the structure in a detrimental manner; especially when dealing with column confinement. Consequently, the experimental evidence and the proper design for the partially wrapping with FRCMs is currently lacking.For this reason, the present research reports on an experimental and analytical investigation about PBO (polybenzoxazoles) FRCM-confinement of clay-brick based masonry columns by varying the confinement scheme. The aim is to evaluate the effectiveness of the confinement in terms of axial bearing load capacity and ductility and, at the same time, focusing on the parameters related to the confining scheme showing how these affect the results. Two columns were tested unconfined as lower-bound reference; six columns (i.e. three couples) were partially confined respectively with 3, 4 and 5 wraps having the same width and lastly, a further couple was fully-jacketed as upper-bound reference. The findings evidenced that an appreciable gain in axial strength and deformability was ensured. On the other side, the axial stiffness increased proportionally to the number of confining wraps.Thus, a new analytical model was herein proposed considering the geometry of the confining scheme, as well as the stiffening effect related to it. In particular, a novel effectiveness coefficient (namely KE) is evaluated affecting the theoretical confining pressure based on the axial stiffness of the FRCM confined masonry. The comparison with the own and other available experimental data confirmed the reliability of the proposal which wants to offer a new perspective for the design of the FRCM partial confinement of masonry that needs further investigations to be confirmed by including a variety of masonry and FRCM typologies.

Structural evolution and gas separation properties of thermally rearranged polybenzoxazole (TR-PBO), polymer-carbon transition (PCT) and early-stage carbon (ESC) membranes derived from a 6FDA-hydroxyl-functionalized Tröger's base polyimide

The changes in physical, chemical, and gas permeation properties of a heat-treated hydroxyl-functionalized Tröger's base-derived intrinsically microporous polyimide, 6FDA-HTB, were systematically studied. Polyimide samples were heat-treated for 30 min at a fixed temperature to form (i) polybenzoxazole (PBO) by thermal rearrangement (TR) from ∼420 to ∼440 °C, (ii) polymer-carbon transition (PCT) from ∼460 to ∼500 °C, and (iii) early-stage carbon (ESC) membranes from >500 to 600 °C. As frequently observed in previous studies, TR-derived PBO membranes showed an increase in gas permeability with a concomitant decrease in gas-pair selectivity. The intermediate PCT region represents a transition state from a PBO to a partially carbonized polymer with superior properties by showing a simultaneous boost in gas permeability and gas-pair selectivity. The ESC membranes formed between 550 and 600 °C possessed further enhanced selectivity due to tightening of the amorphous CMS structure. A 135-day aged 6FDA-HTB-600 membrane showed very high hydrogen permeability of 3544 barrer coupled with an outstanding H2/CH4 selectivity of 939. The mixed-gas separation properties of selected aged samples, pristine 6FDA-HTB, PBO (6FDA-HTB-420), intermediate PCT (6FDA-HTB-480), and early stage CMS analogs (6FDA-HTB-550 and 600), were investigated using a 1:1 CO2/CH4 feed up to 30 bar. In this series, early-stage CMS membranes displayed the best mixed-gas CO2/CH4 separation properties with performance significantly surpassing the 2018 polymer mixed-gas upper bound.

Heat-Resistant Polymers with Intense, Visible Photoluminescence Functionality and Fluorescence Probing Application

Heat-resistant polymers with an intense, visible photoluminescence (PL) functionality are presented. A polybenzoxazole (PBO) containing hexafluoroisopropylidene (HFIP) side groups exhibited an intense purple PL with a quantum yield, ΦPL, of 0.22 (22%), owing to the effectively disturbed concentration quenching (CQ) in the fluorophore units by the bulky HFIP side groups. The chain ends of a wholly cycloaliphatic polyimide (PI), derived from 1,2,3,4-cyclobutanetetracarboxylic dianhydride (CBDA) and 4,4′-methylenebis(cyclohexylamine) (MBCHA), were modified with conjugated monoamines. The PI derived from 2,3,6,7-naphthalenetetracarboxylic dianhydride (2,3,6,7-NTDA) and MBCHA exhibited a very high glass transition temperature (Tg = 376 °C) and purple fluorescence from the S1(π,π*) state. However, its ΦPL value was lower than expected. A pronounced effect of fluorophore dilution using CBDA on the PL enhancement was observed. This is closely related to the planar structure of the 2,3,6,7-NTDA-based diimide units. By contrast, the counterpart using an 2,3,6,7-NTDA isomer, 1,4,5,8-NTDA, was virtually non-fluorescent, despite its sufficient dilution using CBDA. The PI film obtained using 3,3″,4,4″-p-terphenyltetracarboxylic dianhydride (TPDA) with a non-coplanar structure and MBCHA exhibited an intense blue fluorescence spectrum (ΦPL = 0.26) peaking at 434 nm. The dilution approach using CBDA enhanced its fluorescence up to a high ΦPL value of 0.41. Even when TPDA was combined with an aromatic diamine, 2,2′-bis(trifluoromethyl)benzidine (TFMB), the intense blue fluorescence was observed without charge-transfer fluorescence. A semi-cycloaliphatic PI derived from TFMB and a novel cycloaliphatic tetracarboxylic dianhydride, which was obtained from a hydrogenated trimellitic anhydride derivative and 4,4′-biphenol, was used as another host polymer for 9,10-bis(4-aminophenyl)anthracene (BAPA). The BAPA-incorporating PI film resulted in a significant PL enhancement with a considerably high ΦPL of 0.48. This PI film also had a relatively high Tg (265 °C). A reactive dye, N,N′-bis[4-(4-amino-3-methylbenzyl)-2-methylphenyl]-3,4,9,10-perylenetetracarboxydiimide, was harnessed as a fluorescence probe to explore transamidation between polyimide precursors in solution.

Preparation and Properties of PBO Nanofiber Membranes via Electrospinning and Thermal Conversion

In this study, Polybenzoxazole (PBO) nanofiber membranes are prepared by applying a three-step process, including synthesis of the electrospinning precursor with ortho-hydroxyl, electrospinning, and thermal conversion. Electrospinning is conducted at 10 kV with a distance of 10 cm and the solution concentration is 17.8% (w/v). Different flow rates are adopted to prepare polymer nanofiber membranes and test their properties. At the rate of 0.5 μL/min and 1 μL/min, uniform and compact films with nanofibers are obtained. The high-temperature transition from the precursor to PBO is then investigated systematically. Thermal treatment at 420°C is needed for complete cyclodecarboxylation conversion from the precursor to PBO. Diameters of the fibers in the as-prepared PBO nanofiber membrane are within the range of 120-130 nm. The membrane shows high thermal stability with the decomposition temperature of 619.8°C in N2 and 513.6°C in air respectively, presenting a promising application prospect in high-temperature areas.

Synthesis and lithographic properties of positive-tone poly(benzoxazole)s by the effect of amine end-cappers

Positive-tone photosensitive poly(o-hydroxyamide) (PHA) precursors were synthesized from the polycondensation reaction of 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (Bis-AP-AF), 4,4’-oxybisbenzoyl chloride (OBC) and three mono-functional amines as end-cappers. The synthesized PHA precursors were characterized and converted to polybenzoxazoles (PBOs) by thermal cyclization reaction. The photoresist formulations composed of the prepared PHA precursor and diazonaphthoquinone (DNQ) photoactive compound were prepared, and the photolithographic properties of the photoresist formulations were investigated by UV light irradiation, followed by a 2.38 wt.% tetramethylammonium hydroxide (TMAH) solution treatment in terms of the chemical structure of end-cappers. The dissolution rate at 2.38 wt.% TMAH solution by various UV light exposure doses and the shrinkage ratio by thermal cyclization of micro-patterns by UV light irradiation and developing process from the photoresist formulations are affected by the chemical structure of end-cappers.

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.

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).

The translaminar fracture toughness of high-performance polymer fibre composites and their carbon fibre hybrids

The translaminar fracture toughness is a key property that governs the damage tolerance and notch sensitivity of fibre-reinforced composites. Compact tension tests were performed to investigate the translaminar fracture toughness of composites reinforced with three types of high-performance polymer fibres: polyarylate (PAR), polybenzobisoxazole (PBO) and aramid fibre. A carbon fibre composite was used as the reference system. The propagation translaminar fracture toughnesses of the PAR and PBO fibre composites were 492 kJ/m2 and 547 kJ/m2, respectively. These are among the highest translaminar fracture toughness values recorded in the literature. It was hypothesized that the fibrillation of the fibres upon failure was an important energy dissipating mechanism alongside pull-outs that were much longer than for carbon fibre. Replacing a small strip of a carbon fibre ply by a strip of PAR or PBO fibres successfully reinforced the material by locally arresting crack growth. By contrast, the performance of the aramid fibre composites and their hybrids with carbon fibre was lacklustre. The results presented in this work can be used to further improve the safety of composite parts by optimising the design for damage tolerance while also reducing the weight of the parts.

Cu2O nanoparticle-catalyzed tandem reactions for the synthesis of robust polybenzoxazole.

We report the synthesis of Cu2O nanoparticles (NPs) by controlled oxidation of Cu NPs and the study of these NPs as a robust catalyst for ammonia borane dehydrogenation, nitroarene hydrogenation, and amine/aldehyde condensation into Schiff-base compounds. Upon investigation of the size-dependent catalysis for ammonia borane dehydrogenation and nitroarene hydrogenation using 8-18 nm Cu2O NPs, we found 13 nm Cu2O NPs to be especially active with quantitative conversion of nitro groups to amines. The 13 nm Cu2O NPs also efficiently catalyze tandem reactions of ammonia borane, diisopropoxy-dinitrobenzene, and terephthalaldehyde, leading to a controlled polymerization and the facile synthesis of polybenzoxazole (PBO). The highly pure PBO (Mw = 19 kDa) shows much enhanced chemical stability than the commercial PBO against hydrolysis in boiling water or simulated seawater, demonstrating a great potential of using noble metal-free catalysts for green chemistry synthesis of PBO as a robust lightweight structural material for thermally and mechanically demanding applications.

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.

Boosted electrochemical properties of polyimide-based carbon nanofibers containing micro/mesopore for high-performance supercapacitors by thermal rearrangement

In this work, the hydroxyl-containing polyimide(HPI) nanofibers were fabricated by the electrospinning of poly(amic acid)(PAA) derived from pyromellitic anhydride(PMDA) and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane(6FAP), followed by thermal imidization. After further thermal rearrangement from HPI to polybenzoxazole(PBO) and pyrolysis, the HPI-based carbon nanofibers(HPICNFs) were obtained with abundant nitrogen and oxygen doping content. The effects of PAA concentration, thermally rearranged time and carbonizing temperature on the electrochemical performance of HPICNFs were evaluated. When the PAA concentration was 20%, the thermal rearrangement time was 2 h and the carbonization temperature was 800 °C, the specific surface area and pore volume of the HPICNFs reached 1107.6 m2 g−1 and 0.6204 cm3 g−1. Due to the abundant micro- and mesoporous structures, the HPICNFs exhibited a high specific capacitance of 263.9 F g−1 and excellent capacity retention rate about 75% from 0.5 to 10 A g−1. After 10,000 charging and discharging cycles, the capacitance retention of the symmetrical coin cell was maintained at about 99.82%, indicating superior long-term charging-discharging cycle stability. Additionally, a light-emitting diode was successfully lighted up by the three coin cells. Thus, this work provides a facile way to prepare the HPICNFs by thermal rearrangement and carbonization, which are expected to be used as high-performance electrode materials for supercapacitors.