Thermally Rearranged Polybenzoxazole Research Articles

Molecular design of free volume structure in thermally rearranged polybenzoxazole membranes for gas separation studied by positron annihilation

AbstractIn this work, 2,2‐bis(3‐amino‐4‐hydroxyphenyl) hexafluoropropane (APAF) was used as the fixed diamine monomer, and 4,4′‐(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 4,4′‐oxydiphthalic anhydride (ODPA), 3,3′,4,4′‐biphenyltetracarboxylic diandhydride (BPDA) and pyromellitic dianhydride (PMDA) were selected as four different dianhydride monomers, respectively, to prepare the thermally rearranged polybenzoxazole (TR‐PBO) gas separation membranes with different structures via a thermal rearrangement reaction at 450°C. The microstructure of TR‐PBO membranes was investigated using X‐ray diffraction and positron annihilation spectroscopy. All the TR‐PBO membranes exhibit bimodal free volume structure, containing ultramicropores (<0.7 nm) and micropores (<2 nm) with size of 3.6–4.6 and 7.6–8.4 Å, respectively. As the steric hindrance of the bridging groups in the dianhydride monomers increases, the radius and number of pores in the membranes increases, leading to an increase in fractional free volume (FFV). The pure gas permeation test shows a positive correlation between gas permeability and FFV in the TR‐PBO membranes, indicating that an increase in FFV is favorable for the diffusion of gas molecules. Particularly, the ultramicropores play a decisive role on the gas selectivity. All the TR‐PBO membranes exhibit excellent gas permeaselectivity. Among them, the H2/CH4 separation performance of APAF‐6FDA and APAF‐ODPA membranes approaches the 2008 Robeson upper bound, which is due to the high number of both micropores and ultramicropores. Our results demonstrate that the free volume structure of polymer membranes can be designed by selecting monomers with different structures and utilizing thermal rearrangement reactions, allowing for the preparation of gas separation membranes with high permeability and selectivity.

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.

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.

Highly Permeable Mixed Matrix Membranes of Thermally Rearranged Polymers and Porous Polymer Networks for Gas Separations

Mixed matrix membranes (MMMs) have been obtained by blending an aromatic ortho-hydroxypolyimide (PIOH) or an ortho-acetylpolyimide (PIOAc) with different loading amounts (15 and 30 wt %) of a microporous polymer network (PPN), which was obtained from the reaction of triptycene and isatin. The excellent thermal resistance of the PPN (above 500 °C) allowed it to be used as a filler to successfully prepare thermally rearranged polybenzoxazole (TR-PBO)-MMMs obtained from the thermal treatment of the above MMMs. Moreover, PPN showed relatively good compatibility with the polyimide matrix, which improved the TR-PBO formation. The gas separation performances of these MMMs before and after the thermal process were studied for five representative gases (He, O2, N2, CO2, and CH4). For the MMMs derived from ortho-functional polyimides, the gas permeability considerably increased for all of the gases, whereas the selectivity for gas pairs, such as CO2/N2 and CO2/CH4, remained similar. Thus, the selectivity-permeability performance of PIOH- and PIOAc-MMMs containing 30 wt % of PPN (PIOH30 and PIOAc30) surpassed the 1991 Robeson limit for the CO2/CH4 gas pair. In the case of TR-PBO-MMMs (TROH and TROAc-MMMs), the thermal rearrangement process led to an increase in the gas permeability, showing values much better than those observed for the TR-PBO material employed as a MMM matrix. The selectivity values were a little bit lower than the pristine TR-PBO membranes. The CO2 permeability of TROAc30 was 1036 barrer with a CO2/CH4 selectivity of 28, surpassing the 2008 Robeson limit.

Gas transport properties of thermally rearranged (TR) polybenzoxazole –silica hybrid membranes

Novel thermally rearranged polybenzoxazole (TR-PBO) -based silica hybrid membranes were prepared via thermal rearrangement and sol – gel processes, and their gas transport properties were investigated. The pristine TR-PBOs rearranged at different temperatures showed higher fractional free volume (FFV) and d-spacing values than the poly(ortho-hydroxy imide) as a precursor polymer. Gas permeability of TR-PBO – silica hybrids increased with increasing silica content. The increased gas permeability was mainly attributed to increased gas diffusivity, suggesting the formation of free volume holes at polymer/silica interfacial region. The TR-PBO – silica hybrids possessed an outstanding CO2/CH4 selectivity, exceeding the upper bound trade-off line for CO2/CH4 separation. The notable CO2/CH4 selectivity was achieved by synergistic effects of (1) enlarged FFV and d-spacing by the thermal rearrangement towards PBO and (2) the additional formation of characteristic free volume holes with a size-selective CO2/CH4 separation ability at polymer/silica interfacial region by the hybridization with silica.

Cardo-type porous organic nanospheres: Tailoring interfacial compatibility in thermally rearranged mixed matrix membranes for improved hydrogen purification

Mixed-matrix membrane (MMM) is an effective way to overcome trade-off limitations of conventional polymeric membranes. However, the existence of defect voids at the polymer/filler interface often limits their performance improvement. Similar issues are also present in thermally rearranged polybenzoxazole (TR-PBO)-derived MMMs. To address this challenge, the selection of fillers is of great importance. Herein, a novel organic porous nanosphere (TC-cPSB), which is prepared by the polycondensation of 9,9-bis(4-aminophenyl) fluorene (BAFL) and terephthalaldehyde (TPAL) followed by thermal crosslinking, is chosen to engineer the polymer/filler interface. Benefiting from strong intermolecular interaction (π-π stacking and hydrogen bonding), the TC-cPSB nanosphere can well disperse in TR-PBO matrix with a defect-free interface. With an increase in TC-cPSB loading, well-designed MMMs exhibit a significant “anti-trade-off” phenomenon whereby gas permeability and selectivity increase simultaneously, following the trend predicted by the Maxwell model. Compared with TR-PBO membrane, the MMM containing 15 wt% of nanosphere shows an increase of 282% and 217.6% in H2/CO2 selectivity and H2 permeability, respectively, which is far beyond 2008 Robeson upper bound.

Effect of hexafluoroisopropylidene group contents and treatment temperature on the performance of thermally rearranged poly(hydroxyamide)s membranes

Soluble aromatic polyamide films were prepared with different molar ratios of 2,2′-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (APAF) and 3,3′-dihydroxy-4,4′-diaminobiphenyl (HAB), and then in-situ treated at different temperature to obtain thermally rearranged polybenzoxazole (TR-PBO) membranes with ultra-high gas permeability. Compared to polyamide precursors, the obtained TR-PBO membranes exhibited bimodal cavity distribution. The size of smaller cavities, ranging from around 3.3 Å to around 4.1 Å, increased with APAF content. And the larger cavities of 5.6–6.0 Å were almost constant in size, irrespective of thermal treatment temperature or precursor structure. It was found that increasing thermal treatment temperature could prompt smaller cavities to coalesce into bigger ones, thus resulting in better gas permeability. Higher APAF content results in higher gas permeability but inferior resistance to CO2 plasticization, however, too low APAF content is also not beneficial to plasticization resistance due to the lower thermal conversion ratio caused by close-packed array of HAB-TPC. It was found that with APAF/HAB molar ratio of 5/5, the prepared TR-PBO membranes exhibited better resistance to CO2 plasticization as well as comparable mechanical properties and stable permeation performance.

Highly Selective and Permeable Microporous Polymer Membranes for Hydrogen Purification and CO2 Removal from Natural Gas

This paper reports a new macromolecular design that incorporates hierarchical triptycene unit into thermally rearranged polybenzoxazole (TR-PBO) structures for highly selective and permeable gas se...

An atomistic insight on CO2 plasticization resistance of thermally rearranged 6FDA-bisAPAF

An emerging class of thermally rearranged (TR) polymer has been of great interest for its extraordinary transport properties. More importantly, harnessing the full potential of TR polymer as gas separation membrane for CO2/CH4 separation is through its ability to resist plasticization at high pressures. Accordingly, we report here on the effect of CO2-induced plasticization on polyimide precursor (6FDA-bis-APAF: 4,4-hexafluoro isopropylidene- diphthalic anhydride – 2,2′-bis(3-amino-4-hydroxyphenyl) – hexafluoropropane) and on the resulting thermally rearranged polybenzoxazole (TR-PBO) polymer membranes as investigated through the radial distribution function and accessible free volume analyses. Using molecular simulation techniques, structural properties such as d-spacing, glass transition temperature, fractional free volume, etc. were estimated in agreement with wide range of experimental observations, which are published within the last decade. Results showed that, TR polymer displayed restricted % FFV increase up to 40 bar due to its limited chain mobility as indicated by the dihedral distribution, and sorption sites on its backbone with lower affinity to CO2 as shown by the RDF analyses. Additionally, analysis of free volume elements suggests that the ability of TR polymers to maintain their interconnected microstructure and resistance to CO2-induced plasticization at high pressures leads also to higher diffusion and hence permeation performances and as a result, make them promising materials in gas separation applications.

Wet CO2/N2 permeation through a crosslinked thermally rearranged poly(benzoxazole-co-imide) (XTR-PBOI) hollow fiber membrane module for CO2 capture

Recently developed crosslinked-thermally rearranged (XTR) polymeric membranes, which show enhanced gas separation, are applicable for CO2 separation from post-combustion gases. Moreover, the resulting thermally rearranged polybenzoxazole (TR-PBO) structure that is thermally induced from hydroxyl-polyimides (HPI) provides high thermal and chemical resistance, which is favorable for gas separation applications containing condensable gases such as water vapor. Herein, the influence of water vapor on the CO2 capture efficiency was evaluated using XTR poly(benzoxazole-co-imide) (XTR-PBOI) hollow fiber membrane modules which were compared with crosslinked HPI (XHPI) in mixture gas and single gases for CO2 and N2. The results revealed that the permeate CO2 flow rate in the hydrophobic XTR-PBOI module showed enhanced separation performance, whereas the flow rate severely decreased in the relatively hydrophilic XHPI module, reflecting more significant capillary condensation effect in the XHPI membranes. Both membrane modules showed excellent plasticization resistance, and the XTR-PBOI hollow fiber membrane module presented reasonable long-term stability over 240h.

Sorption and Diffusion of CO2/N2 in gas mixture in thermally-rearranged polymeric membranes: A molecular investigation

In this work, we study the adsorption and diffusion of nitrogen and carbon dioxide through an atomistically detailed model of a thermally rearranged polybenzoxazole (TR-PBO) polymer membranes, via equilibrium molecular dynamics (MD) simulations. This work represents a first explicit molecular modelling of the behavior of CO2/N2 binary mixture in TR-PBO and demonstrates how diffusivity and solubility in mixtures can be coherently obtained. In particular, the number of molecules present in the polymer matrix is estimated using the Gran Canonical Monte Carlo approach. As for the sorption in mixture conditions, MD simulations are used in a synergistic pairing with GCMC and Ideal Adsorption Solution Theory (IAST). For this purpose, the single-gas isotherms calculated from GCMC simulations are fitted with Langmuir and Dual-Langmuir adsorption models to obtain the parameters needed for the IAST simulations.As for diffusion, single-gas and mixture (Maxwell-Stefan) diffusion coefficients are performed by MD simulations. As main results, it is observed that the evaluated diffusion coefficients of CO2 and N2 are in a satisfactory agreement with the values estimated using the available experimental permeability data. More specifically, the CO2 diffusivity in mixture conditions is found to be the same as that in the single-gas one, whereas the N2 diffusivity is slightly higher. These differences are explained in terms of the effect of both the mutual gas diffusion and the competing occupancy of the available free space preferentially occupied by the CO2 molecules in mixture.

Electrochemical performance of a thermally rearranged polybenzoxazole nanocomposite membrane as a separator for lithium-ion batteries at elevated temperature

Shape-tunable hydroxyl copolyimide (HPI) nanoparticles are fabricated by a re-precipitation method and are coated onto electrospun HPI membranes, followed by heat treatment to prepare thermally rearranged polybenzoxazole (TR-PBO) composite membranes. The morphology of HPI nanoparticles consisted of sphere and sea-squirt structures, which is controlled by changing the concentration of the stabilizer. The morphological characteristics of TR-PBO nanoparticles convert from HPI nanoparticles by heat treatment and their composite membranes is confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), infrared spectroscopy (ATR-IR), thermogravimetric analysis (TGA) analysis, and contact angle measurements. TGA and DSC measurements confirm the excellent thermal stability compared to Celgard, a commercial PP separator for lithium-ion batteries (LIBs). Further, TR-PBO nano-composite membranes used in coin-cell type LIBs as a separator show excellent high power density performance as compared to Celgard. This is due to the fact that sea-squirt structured nanoparticles have better electrochemical properties than sphere structured nanoparticles at high temperature.

Simulation and feasibility study of using thermally rearranged polymeric hollow fiber membranes for various industrial gas separation applications

Due to the unprecedented permeation properties of the recently developed thermally rearranged (TR) polymers, previous models developed for conventional less permeable membranes are no longer suitable to predict the separation performance of TR membranes. In this regard, a new mathematical model was developed taking the often-neglected shell side pressure drop and non-ideal gas behavior into account to result in a more accurate simulation for TR membrane. The model demonstrates the feasibility of using industrial size thermally-rearranged polybenzoxazole (TR-PBO) hollow fiber membrane modules in various gas separation applications. The optimal operating conditions for each application in a single-stage configuration were identified, and flue gas carbon capture was used as an example to demonstrate the substantially higher process capacity which the TR-PBO hollow fiber membranes can offer over conventional polymeric membranes.

Fabrication of thermally rearranged (TR) polybenzoxazole hollow fiber membranes with superior CO2/N2 separation performance

Thermally rearranged polybenzoxazole (TR-PBO) hollow fiber membranes were fabricated from a poly(amic acid) (HPAAc) precursor through a non-solvent induced phase separation technique (NIPS). All the major fabrication conditions (e.g. dope composition, the use of additional inorganic salt, dope and bore flow rates, and coagulation bath temperature) were systematically evaluated and optimized, in order to produce defect-free hollow fiber membranes with an ultra-thin skin layer. The hollow fiber membranes fabricated with the optimized spinning conditions exhibited superior pure gas permeation behavior (CO2 permeance of 2500GPU and CO2/N2 ideal selectivity of 16). Slow beam positron annihilation lifetime spectroscopy (slow beam PALs) measurements revealed that such an exceptional separation performance was mainly attributed to the ideal cavity radius (3.584Å) and ultra-thin skin layer thickness (193nm) obtained using the optimal fabrication conditions. In addition, mixed-gas permeation tests were also performed to demonstrate the feasibility of using such membranes for post-combustion CO2 capture.

Cross-Linked Thermally Rearranged Poly(benzoxazole-co-imide) Membranes for Gas Separation

A novel strategy to tune the cavity size and free volume of thermally rearranged polybenzoxazole (TR-PBO) copolymer membranes by transesterification cross-linking reaction of o-hydroxy polyimide precursors with 1,4-butylene glycol in the solid state is demonstrated in this study. During the thermal rearrangement (TR) process at high temperatures, loose diester interchain cross-linkers are prone to degrade while formation of a much more rigid cross-linked structure occurs following or alongside the imide-to-benzoxazole rearrangement. As a result, a synergistic effect of high permeability and high selectivity appeared to be created in one step, and the newly synthesized cross-linked TR-PBO membranes exhibited outstanding gas separation performance, surpassing the so-called 2008 upper bound for CO2/CH4 separation.

Highly proton-conductive thermally rearranged polybenzoxazole for medium-temperature and low-humidity polymer electrolyte fuel cells

Here a new membrane design concept to improve proton conductivity and to maintain a constant acid doping level for an extended period of time for medium-temperature and low-humidity polymer electrolyte fuel cells (MT/LH-PEFC) is presented. A polymer electrolyte membrane is prepared via thermal rearrangement of hydroxyl-containing polyimide (HPI) precursor membranes over 350 °C, followed by subsequent acid-impregnation. The thermal treatment for 1 h converts HPI into a thermally rearranged polybenzoxazole (TR-PBO) membrane with high surface area, similar to zeolites. The microporous structure and the basic sites (–CN–) in benzoxazole moieties contribute to the stable impregnation of small acidic molecules (e.g., HCl, HNO3, H3PO4, and HPF6) in large quantities within the polymer matrix. The acid-doping level decreases with the increasing size of acidic dopants. TR-PBO impregnated using HCl with the smallest radius has a much higher doping level and excellent proton conductivity (1.60 × 10−1 S cm−1 at 130 °C and RH 28%) when compared with PBI (6.59 × 10−2 S cm−1 at the same conditions). Different from a common acid-doped PBI system, the acid-doped TR-PBO membranes do not exhibit a severe acid leaching even in repeated heating and cooling cycles between 90 and 130 °C.

Sorption and transport of small gas molecules in thermally rearranged (TR) polybenzoxazole membranes based on 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (bisAPAF) and 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA)

The gas solubility of thermally rearranged polybenzoxazole (TR-PBO) membranes and precursor polymer membranes was determined for five representative small gas molecules, H2, N2, O2, CH4, and CO2, at 35°C and pressures up to 23atm. Precursor membranes that thermally rearranged to TR-PBO were prepared from 2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane (bisAPAF) and 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA) using three different imidization methods. Sorption isotherms of TR-PBO followed the dual-mode sorption model, which is regarded as a typical model for glassy polymers. The Henry's law coefficient (kD), Langmuir affinity parameter (b), and Langmuir capacity parameter (C′H) were determined using the dual-mode sorption equation. During the thermal rearrangement process, excess free volume in the polymer membrane matrix increased and improved molecular transport was observed. A similar trend of increasing solubility was observed during the thermal rearrangement process. The gas permeability and diffusivity of TR-PBO membranes were also studied using the solution–diffusion model.

Thermal Treatment Effect on the Structure and Property Change between Hydroxy-Containing Polyimides (HPIs) and Thermally Rearranged Polybenzoxazole (TR-PBO)

In this study, we report on the effect of thermal treatment on polyimide precursors (HPIs) and on the resulting thermally rearranged polybenzoxazole (TR-PBO) polymer membranes as investigated through the use of molecular dynamics (MD) simulations. For this purpose, we have first analyzed the structures of hydroxy-containing polyimides before thermal treatment and those of the thermally rearranged polybenzoxazoles after the thermal treatment, according to their temperature conditions. As expected, HPIs and TR-PBOs always show very limited motion of their polymer chains, indicated by the radius of gyration, due to their well-known thermal stability. In particular, the very rigid and stiff PBO linkages did not undergo significant change in their torsional angle distribution. On the other hand, in regards to intrachain movement, HPI chains were significantly affected by temperature. Their conformational changes were notably observed, which affected the distances between possible reaction sites, oxygen atoms in hydroxyl groups, and carbon atoms in the imido-ring. The free volume analysis, performed on both polymers and during thermal treatment, indicates that HPIs have a unimodal distribution of free volume areas, which partially coalesce in larger areas having, however, a relatively narrow size. Further, TR-PBO shows a bimodal cavity distribution, and after thermal treatment and TR reaction, the free volume structures in TR-PBO are maintained. The cavity size distributions determined by simulation were also consistent with free volume distributions determined by positron annihilation lifetime spectroscopy.

Thermally rearranged (TR) polybenzoxazole hollow fiber membranes for CO2 capture

Abstract Thermally rearranged polybenzoxazole (TR-PBO) hollow fiber membranes were prepared using a non-solvent induced phase separation method intended to apply for CO2 separation from post-combustion flue gas. Asymmetric hollow fiber membranes were spun from a hydroxyl poly(amic acid) (HPAAc) precursor and subsequently converted to TR-PBO hollow fiber membranes after thermal treatment above 400 °C. The skin structure and porous substructure in TR-PBO hollow fiber membranes were maintained even after thermal treatment at above glass transition temperature (Tg) of precursor polymers. To fabricate defect-free hollow fiber membranes, the effects of dope composition and thermal rearrangement conditions were investigated. Gas permeation properties of TR-PBO hollow fiber membranes measured by constant pressure method with small laboratory scale test module revealed a high CO2 permeance (1940 GPU) with a CO2/N2 selectivity of 13.