Cross-Linked Thermally Rearranged Poly(benzoxazole-co-imide

Article pubs.acs.org/Macromolecules

Cross-Linked Thermally Rearranged Poly(benzoxazole-co-imide) Membranes Prepared from ortho-Hydroxycopolyimides Containing Pendant Carboxyl Groups and Gas Separation Properties Mariola Calle,† Hye Jin Jo,† Cara M. Doherty,§ Anita J. Hill,§ and Young Moo Lee*,† †

Department of Energy Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea Commonwealth Scientific and Industrial Research Organization (CSIRO) Manufacturing Flagship, Private Bag 10, Clayton South, Victoria 3169, Australia

§

ABSTRACT: The incorporation of small amounts of 3,5diaminobenzoic acid (DABA) comonomer (usually 5−20 mol %) into an ortho-hydroxypolyimide (HPI) precursor constitutes a new methodology to easily improve the gas transport performance of thermally rearranged (TR) polymer membranes. Thermogravimetric analysis (TGA) profiles of HPI with DABA copolyimides (HPID) suggests that there is an overlap between the temperature ranges of two processes: the degradation of DABA carboxyl groups and the actual thermal rearrangement process. During thermal treatment at 450 °C, carboxyl pendant groups in DABA degrade while a more rigid biphenyl crosslinked structure is formed either following or at the same time as the rearrangement of imide to benzoxazole. Mixed-gas CO2/ CH4 (1:1) experiments proved this new series of cross-linked TR poly(benzoxazole-co-imide) (XTR-PBOI) membranes as superior materials for CO2/CH4 separation applications, with enhanced permeability and selectivity, as well as high resistance to plasticization up to 40 bar.

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INTRODUCTION Gas separation using membranes has attracted great interest in recent years because it is considered to be an energy-efficient separation process and has many advantages over traditional separation methods.1−3 In particular, gas separation using organic polymer membranes has inspired a great deal of fundamental research. In this context, polymers of high free volume have been demonstrated to be one of the most promising candidates for separation processes, due to their enhanced diffusion as well as sorption capability for small gas molecules. Recently, we reported thermally rearranged polybenzoxazole (TR-PBO) membranes as a new class of microporous organic materials showing extraordinarily fast molecular transport, as well as a molecular sieving effect for small gas molecules.4−6 The microporous structure of these materials results from a solid-state thermal rearrangement process of HPIs into a usually more rigid PBO structure. Thus, it is established that the thermal conversion of HPIs to PBOs proceeds through a carboxybenzoxazole intermediate, followed by decarboxylation to yield the fully aromatic final benzoxazole.7−14 Typically, the evolution of CO2 arising from decarboxylation (one CO2 molecule evolves per imide group) occurs in the temperature range of 350−450 °C, as detected by thermogravimetric analysis combined with mass spectroscopy (TGA−MS).15 This release of CO2 during the thermal rearrangement process, along with the significant conformational change from polyimide to polybenzoxazole, is believed to be responsible for the very great increase in free volume and permeability observed in these materials.4,5 © 2015 American Chemical Society

In the past few years, we have done extensive research to tailor precursor polyimide chemical structures for better gas separation performance of the TR family of polymer membranes.16−22 Copolymerization was found to be a useful tool to easily tune the properties of TR membranes. Thus, we reported on the preparation of various highly permeable and selective TR poly(benzoxazole-co-imide) (TTR-PBOI) membranes. A series of copolymer membranes was prepared from the fluorinated bisaminophenol bisAPAF (2,2-bis(3-amino-4hydroxyphenyl)hexafluoropropane) and from oxidianiline as a codiamine lacking hydroxyl groups, in different ratios.16 Free volume cavities formed during the thermal conversion were controlled by the copolymer composition, decreasing as the mole fraction of nonrearrangable codiamine was increased. As a matter of fact, the gas transport of TR copolymer membranes was linearly dependent upon the ratio of PBO. In another example, the effect of the chemical structure of various thermally nonrearrangeable aromatic diamines upon the gas separation of TR-PBOI membranes was studied.21 It was confirmed that the nonpolar bulky side groups in the diamines increased the fractional free volume most effectively, resulting in increased gas permeability. Also, the gas selectivity was found to increase when the thermally nonrearrangeable diamines used were of a somewhat flat and rigid structure. More recently, we demonstrated a novel strategy to tune the cavity size and free Received: February 11, 2015 Revised: April 4, 2015 Published: April 16, 2015 2603

DOI: 10.1021/acs.macromol.5b00298 Macromolecules 2015, 48, 2603−2613

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reach room temperature, and was then left overnight; this yielded a viscous solution of polyamic acid for each formulation indicated above. o-Xylene (20 mL) as an azeotropic agent was then added to the solution, which was stirred vigorously and heated for 6 h at 180 °C to promote imidization. During this step, water released from the ringclosure reaction was removed by means of azeotropic distillation with the o-xylene. The resulting brown-colored solution was cooled to room temperature, and then the product was precipitated into distilled water, washed several times with hot water, and dried in a convection oven at 120 °C for 12 h. The DABA-containing copolyimides were designated as HPID-Y, where Y indicates the mole fraction of the DABA incorporated, ranging from 5 to 25%. Similarly, the copolyimide including MPD was named HPIMPD-5. 1H NMR analysis and FT-IR spectroscopy were used to confirm the chemical structure of HPI and HPID-Y copolyimides; the results agreed well with the previous literature.18,23 Spectroscopic data for HPIMPD-5 were as follows. 1H NMR (300 MHz, DMSO-d6, ppm): 10.40 (s, −OH), 8.13 (d, Har, J = 8.0 Hz), 7.94 (d, Har, J = 8.0 Hz), 7.73 (s, Har), 7.40 (s, Har), 7.19 (d, Har, J = 8.3 Hz), 7.07 (d, Har, J = 8.3 Hz). 6.90−7.10 (m, Har). FT-IR (film): ν(O−H) at 3400 cm−1, ν(CO) at 1786 and 1716 cm−1, Ar (C−C) at 1619, 1519 cm−1, imide ν(C−N) at 1377 cm−1, (C−F) at 1299−1135, imide (C−N−C) at 1102 and 720 cm−1. Inherent viscosities for HPI, HPIMPD-5 and HPID copolyimides all ranged between 0.50 and 0.60 dL g−1. ortho-Hydroxypolyimide Membrane Formation and Thermal Conversion to Polybenzoxazole. All copolyimides were cast from a 15 wt % filtered solution in NMP onto a clean glass plate. Cast films were placed in a vacuum oven and heated slowly to 250 °C and held for 1 h each at 100, 150, and 200 °C to evaporate the solvent under vacuum. The solid films were then separated from the glass plate, rinsed with deionized water, and dried at 120 °C until the water was removed. The defect-free membranes (samples HPI, HPID-Y and HPIMPD-5) were cut into 3 cm × 3 cm strips and placed between quartz plates to prevent film deformation during the subsequent hightemperature heating step. In a high-purity argon atmosphere in a muffle furnace, each sample was heated to 300 °C at a rate of 5 °C min−1 and held isothermally for 1 h, and then heated further to 450 °C and held for 1 h. After thermal treatment, the furnace was cooled to room temperature at a rate no greater than 10 °C min−1. This protocol was used to expose the samples to thermal histories similar to those reported in our previous studies of TR polymers.18,23 Measurements. 1H spectra were recorded on a Mercury Plus 300 MHz spectrometer (Varian, Inc. CA). Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra of samples were measured by using an infrared microspectrometer (IlluminatIR, SensIR Technologies, Danbury, CT). Inherent viscosities were measured at 25 °C with an Ubbelohde viscometer using NMP as the solvent at 0.5 g dL−1 concentration. Glass transition temperatures (Tg) of ortho-hydroxycopolyimides were measured by means of differential scanning calorimetry using a TA Instruments Q20 calorimeter. A total of two heating and cooling cycles were conducted at the heating rate of 20 °C min−1, and Tg was calculated based on data collected during the second heating cycle. During the first heating, the polyimide test samples were heated to a temperature usually below the temperature at which their conversion to PBO began; they were then quenched at room temperature. During the second scan, they were reheated up to 350 °C. TGA was performed by using a TA Q500 thermobalance (TA Instruments, DE, USA), coupled with ThermoStar GSD 301T mass spectrometry (MS) instrument (Pfeiffer Vacuum GmbH, Asslar, Germany). Dynamic ramp scans were carried out at 10 °C min−1 to determine thermal rearrangement and thermal stability in the temperature range of 60−850 °C. The purge gas used was nitrogen (60 mL min−1) and the sample mass was about 5 mg. Wide-angle Xray diffraction patterns were recorded in reflection mode at room temperature by using a Rigaku Denki D/MAX-2500 (Rigaku, Japan) diffractometer and a Cu Kα (wavelength λ = 1.54 Å) radiation source. The average d-spacing value was calculated by means of Bragg’s law in the 2θ range of 5−50°, using the scan rate of 5° min−1. Densities were measured by weighing the samples in air and then in a liquid of known density, by using a Sartorius LA 120S (Sartoriuos AG, Goettingen,

volume of TR-PBOI membranes by means of transesterification cross-linking between ortho-hydroxycopolyimide precursors and 1,4-butylene glycol in the solid state.23 During the thermal rearrangement process at high temperature, diester interchain cross-linkers degrade while a much more rigid cross-linked structure forms, subsequent to or simultaneously with the imide-to-benzoxazole rearrangement. As a result, the crosslinked TTR-PBOI membranes thus synthesized exhibited improvements in both permeability and selectivity. Nevertheless, it was found that the molecular weights of pristine hydroxypolyimides decreased due to chain scissioning during the monoesterification reaction. As a result, the resulting XTRPBO membranes were somewhat mechanically weak. Furthermore, adding the transesterification step would increase the complexity of the polymer synthesis process. The creation of microvoids in polymer systems has been noted by different methods,24−27 although methods yielding molecular-scale dimensions suitable for selective transport of gas molecules are much less common. Thus, the decomposition of pendant labile groups at relatively low pyrolysis temperatures has been shown to efficiently increase the free volume of polymers, affording interesting materials for gas separation applications.28−32 Maya et al. recently reported on the synthesis and gas separation properties of partially pyrolyzed membranes derived from copolyimides bearing carboxyl groups.30 The removal of the side carboxyl groups occurred at temperatures between 375 and 475 °C, without any significant alteration to the polyimide backbone.33,34 The cavities formed by the decomposition of carboxyl groups persisted as micropores in the polymer matrix, resulting in significant improvements to permeability. Besides, the decarboxylation induced thermal cross-linking between the polymer chains, which stiffened the polymer matrix and increased its gas selectivity.35 Taking into account all these considerations and as a continuation of the study and development of new TR membrane materials with improved gas separation properties, in the present work the thermally labile pendant carboxyl group was deliberately incorporated into the precursor orthohydroxypolyimide backbone by copolymerization with DABA, using various molar ratios. The hydroxypolyimide membranes were subjected to thermal treatment at 450 °C for 1 h to induce the thermal rearrangement reaction and the simultaneous thermal decomposition of the labile carboxyl moieties. A set of novel TR-PBOI membranes have been therefore prepared by combining the cross-linking and the cavity forming strategies. Their physical and transport properties have been examined and are reported in detail herein.

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EXPERIMENTAL SECTION

Materials. Solvents and reactants were of reagent-grade quality and were used without further purification. 4,4′-Hexafluoroisopropylidene diphthalic anhydride (6FDA) and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (bisAPAF) were purchased from Daikin Industries, Ltd. (Osaka, Japan) and from Central Glass Co. Ltd. (Tokyo, Japan), respectively, and were sublimed just before use. DABA (Aldrich Co., Milwaukee, WI) was recrystallized from oxygen-free water and sublimed before use. m-Phenylenediamine (MPD) was also purchased from Aldrich and was sublimed before use. Synthesis of ortho-Hydroxypolyimide Precursors. A mixture of bisAPAF (X mmol, where X = 10, 9.5, 9.0, 8.5, 8.0, or 7.5 mmol), and DABA (10 − X mmol) or MPD (5 mmol) dissolved in anhydrous N-methylpyrrolidone (NMP) (10 mL) was cooled to 0 °C, followed by addition of 6FDA dianhydride (10 mmol) dissolved in 10.0 mL of NMP. The reaction mixture was stirred for 15 min at 0 °C, allowed to 2604

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Figure 1. 1H NMR (DMSO-d6, 300 MHz) spectra of HPIMPD-5 and HPID-15 copolyimides. were analyzed by using either a three- or four-component fit, carried out by using LTv9 software. The first lifetime (τ1) was fixed to 0.125 ns due to p-Ps annihilation, and τ2 was fitted to free annihilation (∼0.4 ns). The orthopositronium lifetimes (τ3 and τ4) were used to calculate the free volumes within the membranes by means of the Tao−Eldrup equation:

Germany) balance equipped with a density kit. Isooctane was used as the known-density liquid because this hydrocarbon was not absorbed in the membranes, which could result in erroneous measurements. The density of the sample was calculated by using the following expression.

⎛ wair − wliquid ⎞ ρsample = ρliquid ⎜ ⎟ wair ⎝ ⎠

⎡ r3 1 ⎛ 2π(r3) ⎞⎤ + τ3−1 = 2⎢1 − sin⎜ ⎟⎥ ⎢⎣ r3 + Δr 2π ⎝ r3 + Δr ⎠⎥⎦

(1)

Positron annihilation lifetime spectroscopy (PALS) was used to determine the size distribution and relative intensity of free volume elements within the polymer membranes.36 Positrons emitted from a radioactive 22Na source thermalized within the sample. Positrons can form two bound states with electrons, known as positronium (Ps): oPs is their parallel-spin configuration and p-Ps is the antiparallel-spin configuration. Self-annihilation of p-Ps and free annihilation (positron annihilation with an electron within the sample) occur at 0.125 ns and ∼0.4 ns, respectively. Pick-off annihilation of o-Ps in a molecular system is affected by the electron density in its surroundings. That is, o-Ps can survive longer in systems or local domains with low electron density, such as pores, cavities, or free volume elements.37,38 On the basis of the assumption that the vacant volume is spherical, measurement of the o-Ps lifetime (τ, ns) and intensity (I, %) not only provide information on the average size and distribution of free volume elements in the polymer, but also shed light on the topologies in which pores of different sizes exist. PALS allows the presence of bimodal porosity to be distinguished, such as in highly microporous materials like PTMSP and PIM, by means of fitting several o-Ps components.39−42 The size and distribution of free volume elements in the polymers was determined by means of PALS, using an automated EG&G Ortec (Oak Ridge, TN) fast−fast coincidence spectrometer with the resolution function of 230 ps fwhm peak when measured with 60 Co. The samples were composed of 10 mm × 10 mm size polymer films stacked to 2 mm in thickness, with a ∼30 μCi 22NaCl sealed Mylar source placed between the polymers. A source correction for the Mylar was required (1.6 ns, 3.12%). The samples were placed in a cell and subjected to a vacuum of 5 × 10−4 Pa at room temperature. The range of the time-to-amplitude converter was extended to 200 ns to allow measurement of long lifetimes, and the coincidence unit was removed to increase the count rates. A minimum of five files was collected, with 1 × 106 integrated counts per file. The acquired data

(2)

where τ3 is lifetime [ns], r3 is cavity radius [Å], and Δr is an empirically determined parameter related to the thickness of the electron layer on the pore wall that interacts with the positronium (1.66 Å). Gas permeation properties were determined by using a time-lag method, as described in our previous studies.43 For six kinds of small gas molecules, He (2.6 Å), H2 (2.89 Å), CO2 (3.3 Å), O2 (3.46 Å), N2 (3.64 Å), and CH4 (3.8 Å), pressure increase through the membrane in a fixed downstream chamber was acquired against 760 mmHg of upstream pressure; the pressures acquired fell within the 0−10 mmHg range. On the basis of the slopes and intercepts in a steady-state region of pressure increment as a function of time, gas permeability coefficients were calculated by using the following equation:

⎛ VT l ⎞ dp 0 ⎟⎟ P = ⎜⎜ ⎝ p0 T ΔpA ⎠ dt

(3)

where P [Barrer] is the gas permeability, V [cm3] is the downstream volume, l [cm] is the membrane thickness, Δp [cmHg] is the pressure difference between the upstream and downstream regions, T [K] is the measurement temperature, A [cm2] is the effective membrane area, p0 and T0 are the standard pressure and temperature, respectively, and dp/dt is the rate of pressure rise at steady state. The ideal selectivity (α1/2) for components 1 and 2 was defined as the ratio of the gas permeability of the two components. For CO2/CH4 mixed gas permeation measurements, the permeated gas concentrations were determined by means of gas chromatography (GC) by using a 490 Micro GC instrument (Agilent Technologies, Inc. Santa Clara, CA) equipped with a thermal conductivity detector. The ratio of the permeate flow rate to the feed flow rate was set below 0.01 by controlling the sweep gas using a mass flow controller (Line 2605

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Macromolecules Tech M3030VA), with a 10 cm3 (STP) min−1 full scale. The total pressure of the feed and permeate sides was controlled by using a backpressure regulator (Tescom 44-1700). The permeability of each species and the selectivity (α*) in terms of fugacity were defined as follows:

P*1 =

N1l f1, feed − f1, permeate

α*1/2 =

y1 /y2 P*1 = P*2 f1, permeate /f2, permeate

(4)

(5)

where N1 is the steady-state flux of species 1, l is the thickness of the film, and f1,feed and f1,permeate are the fugacities of a species on the feed and permeate sides, determined by considering chemical potential differences instead of partial pressure. In this study, each fugacity value was calculated by means of the virial equation of state using the second virial coefficient;44,45 y1 and y2 are the permeate-side mole fractions of species 1 and 2, respectively, as measured by gas chromatography. In this study, CO2 and CH4 were introduced in a 1:1 flow rate ratio, and argon was used as a sweep gas. The permeate side was maintained at atmospheric pressure. The feed gas mixture was pressurized at various pressures ranging from 5 to 40 bar; membranes were exposed to each pressure for more than 3 h to stabilize the concentrations of permeate gases.

Figure 2. ATR-FTIR spectra of precursor polyimide membranes.

carboxyl and hydroxyl groups, which can form hydrogen bonds as has been previously reported for other polyimides containing carboxyl groups.30,46 The 1H NMR spectral assignments for HPIMPD-5 showed small peaks from the MPD aromatic moiety at around 6.90−7.10 ppm, further upfield relative to the protons in the DABA aromatic unit (Figure 1). This indicates a significant electron-withdrawing effect from the phenyl ring by the carboxyl group. The precursor polyimides were cast from solution in NMP onto a glass plate, by heating slowly to 250 °C with holds for 1 h at 100, 150, 200, and 250 °C to evaporate the solvent under vacuum. Afterward, each sample was held isothermally for 1 h at 300 °C in a high-purity argon atmosphere to ensure complete removal of solvent. Differential scanning calorimetry was used to determine Tg for each of this series of polyimide membranes (see Table 1 and Figure 3). Tg values for HPID-Y compositions were slightly higher than that for the HPI homopolyimide membrane (Tg = 300 °C); the various DABA mole fractions used from 5 to 25 mol % did not substantially affect the polymers’ segmental mobility. Therefore, HPID-10, HPID-15, and HPID-20 samples had similar Tg values at around 314 °C, whereas Tg of HPID-5 was found to be 305 °C. In addition, the HPID-25 membrane, which had the highest content of carboxyl pendant groups, had the Tg of 300 °C. From this result, it was difficult to identify any clear trend linking Tg to the content of carboxyl groups in orthohydroxycopolyimide membranes. In general, the presence of acid pendant groups in a polymer affects its glass transition temperature, due to strong interchain associations through hydrogen bonds and dipolar attractions that increase its rigidity.30 Nevertheless, the expected gradual increase of Tg with increasing mole fraction of carboxyl polar units would be counterbalanced by the concomitant decrease in the content of bisAPAF moieties, each of which contains two polar hydroxyl groups. Besides, the HPIMPD-5 sample, which contained 5 mol % of MPD, had the lowest Tg among the samples studied (280 °C). Similar behavior has been reported previously for copolyimides of 6FDA and oxidianiline diamine that included DABA or MPD as comonomers.30 Thermal Rearrangement of ortho-Hydroxypolyimide Membranes into Polybenzoxazole Membranes. Thermal stability of a series of HPI membranes with different DABA

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RESULTS AND DISCUSSION Synthesis and Characterization of Precursor orthoHydroxypolyimides. A set of six different ortho-hydroxypolyimides was synthesized by polycondensation of 4,4′(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with the diamines bisAPAF and DABA; the bisAPAF:DABA molar ratios of 10:0, 95:5, 90:10, 85:15, 80:20, and 75:25 were used. DABA was selected as a comonomer to allow different contents of carboxyl pendant groups to be incorporated easily into the bisAPAF-6FDA structure. The copolyimides were designated as HPID-Y, where Y refers to the mole percent of the DABA comonomer. In addition, to confirm the role of the labile carboxyl group in determining the gas permeation properties of the final TR membranes, a homologous copolyimide without pendant carboxyl groups was synthesized from bisAPAF and MPD in the bisAPAF:MPD molar ratio of 95:5 (named HPIMPD-5) and was also included in this study. The membranes’ chemical structure was confirmed by IR and 1H NMR spectroscopies. As an example, 1H NMR and IR spectra of some representative compositions are shown in Figures 1 and 2, respectively. The FT-IR spectra of these compositions showed characteristic absorption bands of polyimides: the stretching vibration bands of CO (1786 and 1716 cm−1), the asymmetric stretching vibration of C−N (1377 cm−1), transverse stretching of C−N−C groups at 1102 cm−1, and out-of-plane bending of C−N−C groups at 720 cm−1. In addition, C−F stretching was characterized by absorption peaks at around 1250−1150 cm−1, whereas the broad band in the region of 3200−3600 cm−1 was attributed to the O−H vibrations of hydroxyl groups and of carboxyl moieties in the DABA unit.18,34 The carbonyl stretching of the acid group partially overlapped the more intense absorption of the imide carbonyl. The presence of the DABA moiety in this set of copolyimides was clearly evidenced by 1H NMR spectroscopy (Figure 1); note the small peaks at 7.85 and 7.80 ppm corresponding to the protons in the DABA aromatic ring. Nevertheless, the resonance of the carboxyl proton could not be clearly identified, mostly due to the low content of carboxyl moieties, but also likely due to the interaction between the 2606

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Macromolecules Table 1. Thermal Properties of Precursor Polyimide Membranes sample

Tga [°C]

TTRb [°C]

DABA CO2 wt. lossc [%]

TR process CO2 wt. lossd [%]

total CO2 wt. loss, theoretical [%]

total CO2 wt. loss, actuale [%]

HPI HPIMPD-5 HPID-5 HPID-10 HPID-15 HPID-20 HPID-25

300 280 305 313 314 314 300

407 400 417 426 430 423 429

− − 0.39 0.78 1.18 1.57 1.96

11.36 10.79 10.79 10.22 9.66 9.09 8.52

11.36 10.79 11.18 11.00 10.84 10.66 10.48

11.25 10.78 11.12 9.06 8.96 9.90 8.80

a

Middle point of the endothermic step during the second scan of differential scanning calorimetry measurements conducted under a nitrogen atmosphere using the heating rate of 20 °C min−1. bTemperature at the maximum point of CO2 weight loss. cTheoretical carbon dioxide weight loss corresponding to the removal of the carboxlic acid group from the DABA unit. dTheoretical carbon dioxide weight loss corresponding to the rearrangement reaction. eExperimental weight loss corresponding to the first step of TGA.

during the first weight loss step (300−470 °C) was proved by mass spectroscopy (Figure 4). Thermal degradation of 6FDAbased polyimide membranes including carboxyl pendant groups has been previously reported to occur in two different steps.30,34 A first minor weight loss starting at around 400 °C was attributed to the removal of carboxyl groups, followed by major polyimide backbone degradation at about 500 °C. However, the CO2 evolved in the thermal rearrangement of HPIs into PBOs is usually characterized by TGA as a weight loss step in the overlapping range of 300−500 °C.15 Accordingly, the CO2 released during the first weight loss stage observed for HPID-Y membranes could result from both thermal processes taking place simultaneously: the rearrangement reaction yielding the PBO structure as well as the decarboxylation of the DABA units. The TGA profiles of HPID-Y precursors suggested an overlap between the degradation of pendant carboxyl groups and the actual TR process. Theoretical weight loss corresponding to these events differed for the different compositions. As listed in Table 1, theoretical CO2 weight loss associated with the thermal rearrangement process decreased as the DABA monomer ratio was increased. Thus, there was a weight decrease from 11.36 to 8.45% for homopolyimide HPI and HPID-25 membranes, respectively. Conversely, weight loss corresponding to the removal of carboxyl groups from DABA moieties increased up to 1.96% for HPID-25. Indeed, the thermal rearrangement process involves a decarboxylation reaction whereby two molecules of CO2 are released per repeat unit, whereas thermal decarboxylation of DABA units involves the evolution of only one molecule of CO2 per repeat unit. As a result, the total theoretical CO2 weight losses for HPID-Y membranes are lower in every case than those for HPI. On the other hand, the weight losses measured under the dynamic analysis conditions used (10 °C min−1) correlated fairly well with the theoretical values, for HPI and HPIMPD-5 membranes as well as for HPID-5, the composition with the lowest percentage of carboxyl groups. Nevertheless, polymers with higher DABA ratios had less experimental weight loss (Table 1). This difference could be attributed to intermolecular condensation between carboxyl acid groups to form anhydride cross-links before degradation. Thus, it has been previously observed for other polyimides containing carboxyl pendant groups that, upon heating above 400 °C, the carboxyl moieties condensed to form anhydride cross-links along the polymer chain backbone.34,47 This chemical cross-linking could be difficult to some extent the rearrangement process under the dynamic conditions of analysis, explaining the lower exper-

Figure 3. Glass transition temperatures of precursor polyimide membranes determined by DSC at 20 °C/min (second scan).

content was investigated by means of TGA−MS. As an example, a TGA−MS scan of the HPID-25 sample is shown in Figure 4; TGA and first-derivative thermogram scans for two HPI compositions including DABA and the HPIMPD-5 and HPI reference samples are shown in Figure 4. All the HPID-Y precursors had two-stage weight loss TGA profiles similar to those of HPI and HPIMPD-5 membranes. Evolution of CO2

Figure 4. TGA−MS scan of typical precursor copolyimide membrane HPID-25. 2607

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ratio of nonrearrangeable codiamine content was increased; XTR-PBOI-25 had intense imide absorptions relative to those of other compositions (Figure 6). Also, the O−H stretching vibration of the hydroxyl groups and carboxyl moieties (3200− 3600 cm−1) of the precursor polyimides disappeared. Thermal decarboxylation in polyimide membranes has been previously reported.30,33−35 Thus, the decarboxylation mechanism of aromatic acids was proved to occur by means of anhydride formation and subsequent decomposition to form aryl radicals, which may combine with each other to form a biphenyl crosslink.33−35,49 Therefore, thermal treatment of HPID-Y membranes at 450 °C for 1 h, involving the rearrangement process as well as the thermal decomposition of the carboxyl pendant groups, would lead to a biphenyl cross-linked poly(benzoxazole-co-imide) structure. A further possible point for potential cross-linking exists, by means of cleavage of trifluoromethyl groups from the 6FDA and bisAPAF moieties, though this is not believed to be as important as the sites at which biphenyl cross-linking occurs.33,34 In addition, possible esterification reactions between hydroxyl and carboxyl groups from bisAPAF and DABA units cannot be ruled out either. The proposed biphenyl cross-linking sites in XTR-PBOI-Y do not significantly alter the FTIR spectral assignments compared to those of reference aPBO and TR-PBOI-MPD5 membranes (Figure 6). Note that the chemical structure of copolyimide XTR-PBOI-5 after thermal rearrangement and removal of the labile carboxyl groups to form biphenyl cross-links should be similar to the structure of TR-PBOI-MPD5 (Scheme 1 and 2). Both membranes yielded virtually identical FT-IR spectra (Figure 6). Figure 7 shows X-ray diffraction profiles of the precursor hydroxypolyimide and of thermally rearranged polybenzoxazole membranes. All the samples showed a broad amorphous halo, indicating that all these polymers were amorphous. Precursor polyimide membranes presented a preferential intersegmental distance of around 0.53−0.57 nm. As previously observed for TR polymers,6,18 thermal rearrangement to PBO resulted in larger d-spacing. Interestingly, the increments in d-spacing were found to be much larger for TR membranes including the DABA moiety (Figure 7 and Table 2). Thus, the d-spacing values of XTR-PBOI-Y were in the range of 0.62−0.67 nm, as compared to 0.58 and 0.59 for aPBO and TR-PBOI-MPD5 membranes, respectively. Another interesting observation was the higher contribution of larger distances for XTR-PBOI-5 in comparison with TR-PBOI-MPD5, in spite of their similar chemical structure after the thermal rearrangement process (Figure 7 and Table 2). The increased intermolecular distances were correlated with reduced densities; all TR membranes’ densities were lower than that of the hydroxypolyimide precursor (Table 2). Moreover, the density of XTR-PBOI-5 was found to be lower than that of the analogous TR-PBOI-MPD5, although the reverse trend was observed for the precursor membranes before the thermal process: ρHPID‑5 > ρHPIMPD‑5. The removal of carboxyl pendant groups and eventual biphenyl cross-linking seemed to result in a lower chain packing density in the XTR-PBOI-Y membranes. PALS was carried out to quantitatively analyze the average free volume size and distribution of TR samples. In PALS, the free volume cavities are probed by the lifetime of orthopositronium (o-Ps) before its annihilation in the free volume regions of the materials.38 Hence, lifetime (τ) is directly correlated with cavity size in the material, while the intensity of the annihilation (I) is often indicative of the concentration of

imental weight losses as well as the higher temperatures of maximum amount of CO2 evolution (TTR) found for the compositions with high DABA ratios (Table 1 and Figure 5).23

Figure 5. TGA and first-derivative thermogram curves of some representative precursor membranes, collected under N2 atmosphere at a heating rate of 10 °C min−1.

The second weight loss step corresponded to the poly(benzoxazole-co-imide) chain decomposition and was observed at a similar temperature for all polymers, between 560 and 570 °C, except for HPIMPD-5, 550 °C, very probably due to its lower degree of interchain cross-linking. Characterization of Thermally Rearranged Polybenzoxazole Membranes. The structural changes of precursor hydroxypolyimide membranes during the thermal treatment were monitored by means of ATR-FTIR analysis. Defect-free pieces of HPID-Y films as well as of HPIMPD-5 and HPI samples (previously treated at 300 °C for 1 h) were ramped at 5 °C min−1 in a muffle furnace, and held at 450 °C for 1 h under a high-purity argon atmosphere. Infrared spectra of some representative membranes are shown in Figure 6. During this heating, the formation of the benzoxazole structure was easily confirmed, as unequivocal peaks were observed at wavenumbers around 1480 and 1060 cm−1, typical of benzoxazole rings.48 The percentage of conversion to PBO decreased as the

Figure 6. ATR-FTIR spectra of thermally rearranged membranes treated at 450 °C for 1 h. 2608

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Macromolecules Scheme 1. Synthesis of Cross-Linked Thermally Rearranged Poly(benzoxazole-co-imide) Membranes (XTR-PBOI-Y)

Scheme 2. Synthesis of Reference Thermally Rearranged Poly(benzoxazole-co-imide) Membrane (TR-PBO-MPD-5)

TR-PBOI-MPD5 had the smallest cavity diameter among all membranes studied. Gas Transport Behaviors of Thermally Rearranged Membranes. Pure gas permeation experiments were performed for six representative gases of interest, He, H2, O2, N2, CO2, and CH4, to investigate the gas transport properties of this new family of TR polymers and to study the influence of the DABA content in the precursor HPI. Table 4 lists the results; all XTR-PBOI-Y membranes had greater gas permeability than the reference aPBO sample, consistent with the enlarged cavity sizes of XTR-PBOI-Y. Note that XTR-PBOI-15 and XTR-PBOI-5 showed the highest permeabilities, with PCO2 around 650 and 620 Barrer, respectively, as compared to the ∼400 Barrer reported for a aPBO sample.18 In addition, smaller increments in permeability were noticed for the rest of the compositions, and XTR-PBOI-10 had the lowest permeabilities among the samples studied, with PCO2 ∼ 490 Barrer. This trend could be seen clearly in graphs of CO2 permeability and CO2/ CH4 ideal selectivity of XTR-PBOI-Y membranes versus the

the cavities. Table 3 lists the PALS results. As observed before for TR polymers,18,23 PALS data for XTR-PBOI-Y membranes revealed two o-Ps components, τ3 and τ4, indicating that two kinds of pores are present in these materials: ultrafine micropores with τ3 ∼ 1.2 ns, corresponding to a mean cavity diameter of d3 ∼ 4 Å, and micropores with τ4 ∼ 4 ns, corresponding to the cavity size d4 ∼ 8−9 Å. Note that the incorporation of DABA comonomer in the precursor HPI yielded larger cavity sizes of the resulting XTRPBOI-Y membranes, as compared to the aPBO membrane. As seen in Table 3 and Figure 8, average micropore diameters d4 for XTR-PBOI-Y samples were always larger than that of aPBO (8.37 ± 0.04 Å), and the largest d4 was found for XTR-PBOI15 (8.84 ± 0.02 Å). Increments in the smaller cavity sizes could be considered irrelevant because they lie within the range of uncertainty in most cases. On the other hand, when XTRPBOI-5 was compared with its reference membrane TR-PBOIMPD5 (Table 3), a difference in cavity size was noticed, with the reference showing a lower d4 value (8.20 ± 0.11 Å). In fact, 2609

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Figure 8. Cavity diameter [Å] of ultrafine micropores (d3) and micropores (d4) of thermally rearranged membranes versus the mole fraction of the DABA comonomer.

mole fraction of DABA codiamine (Figure 9). The same behavior was observed for all the other gases tested. Furthermore, when XTR-PBOI-5 was compared with TRPBOI-MPD5 membrane (Table 4), a great difference in permeability was observed; the permeability coefficients of the former were much higher than those of the latter. As an example, PCO2 was approximately 620 Barrer for XTR-PBOI-5, compared to approximately 360 Barrer for the TR-PBOIMPD5 sample. This result also evidenced the positive effects of biphenyl cross-linking and thermal rearrangement. In fact, XTR-PBOI-5 has the same chemical structure as its TR-PBOIMPD5 analogue, but the former is much more permeable and has larger cavities. On the other hand, as observed before for TR membranes cross-linked with butylene glycol,23 the gas selectivity did not significantly decrease with increases in permeability for the various XTR-PBOI-Y compositions, compared to the aPBO membrane (Table 4 and Figure 9). Only XTR-PBOI-25 showed diminished selectivity. The lower ratio of PBO conversion for this sample could explain this different behavior. As mentioned above, the release of CO2 during the rearrangement reaction, along with the conformational change from polyimide to rigid polybenzoxazole, were believed to be responsible for the increase in free volume and permeability of TR membranes. Freeman et al. recently reported that greater mass loss during thermal rearrangement led to the formation of larger free volume elements, which resulted in much higher gas permeabilities.49 For the XTRPBOI-Y membranes, total CO2 weight losses due to thermal rearrangement and DABA decarboxylation were lower in every case compared to that of aPBO, but their permeabilities were

Figure 7. Wide-angle X-ray diffraction patterns of some precursors and thermally rearranged membranes.

Table 2. Density and d-Spacing of Precursor and TR Analogue Membranes sample

density [g cm−3]

d-spacing [nm]

1.49 1.50 1.52 1.51 1.51 1.52 1.50 1.38 1.45 1.42 1.40 1.38 1.38 1.43

0.54 0.53 0.55 0.55 0.54 0.57 0.55 0.58 0.59 0.62 0.66 0.67 0.66 0.65

17

HPI HPIMPD-5 HPID-5 HPID-10 HPID-15 HPID-20 HPID-25 aPBO17 TR-PBOI-MPD5 XTR-PBOI-5 XTR-PBOI-10 XTR-PBOI-15 XTR-PBOI-20 XTR-PBOI-25

Table 3. PALS Characterization of Cavity Size and Annihilation Intensity in TR Membranes sample aPBO18 tPBO18 TR-PBOI-MPD5 XTR-PBOI-5 XTR-PBOI-10 XTR-PBOI-15 XTR-PBOI-20 XTR-PBOI-25

τ3 [ns] 1.06 1.24 1.12 1.10 1.07 1.19 1.16 1.21

± ± ± ± ± ± ± ±

0.09 0.13 0.14 0.11 0.05 0.05 0.11 0.12

I3 [%] 7.1 5.0 7.2 7.0 6.2 6.7 6.7 6.3

± ± ± ± ± ± ± ±

0.7 0.6 0.9 0.8 0.4 0.7 0.8 0.9

τ4 [ns] 3.90 5.26 3.75 4.03 4.15 4.33 4.20 3.99

± ± ± ± ± ± ± ±

I4 [%]

0.04 0.08 0.05 0.04 0.03 0.01 0.06 0.04 2610

12.7 6.0 13.0 12.3 10.3 11.9 10.9 11.9

± ± ± ± ± ± ± ±

0.7 0.2 0.4 0.7 0.1 0.4 0.3 0.4

cavity diameter d3 [Å] 3.51 4.04 3.69 3.63 3.55 3.91 3.80 3.94

± ± ± ± ± ± ± ±

0.28 0.35 0.86 0.68 0.33 0.30 0.62 0.67

cavity diameter d4 [Å] 8.37 9.73 8.20 8.52 8.64 8.84 8.69 8.47

± ± ± ± ± ± ± ±

0.04 0.07 0.11 0.09 0.06 0.02 0.12 0.08

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Macromolecules Table 4. Single Gas Permeation Properties of TR Membranes (30 °C, 760 mmHg) gas permeability [Barrer]a sample 18

aPBO tPBO18 TR-PBOI-MPD5 XTR-PBOI-5 XTR-PBOI-10 XTR-PBOI-15 XTR-PBOI-20 XTR-PBOI-25 a

ideal selectivityb

He

H2

CO2

O2

N2

CH4

O2/N2

CO2/N2

CO2/CH4

CO2/H2

H2/CH4

N2/CH4

356 2647 428 456 313 416 382 342

408 4194 469 578 483 553 481 446

398 4201 358 619 491 655 521 498

81 1092 83 116 90 122 97 95

19 284 21 27.8 20.2 29.2 24.0 24.7

12 151 13 18.0 13.0 19.8 15.3 17.3

4.3 3.8 3.95 4.2 4.5 4.2 4.1 3.9

21 15 17.1 22.3 24.3 22.4 21.7 20.2

34 28 27.5 34.4 37.8 33.1 34.1 28.8

1.0 1.0 0.8 1.1 1.0 1.2 1.1 1.1

35 28 36.1 32.1 37.1 27.9 31.4 29.1

1.6 1.9 1.6 1.5 1.6 1.5 1.6 1.4

1 Barrer =10−10 cm3 (STP) cm s−1 cm−2 cmHg−1. bIdeal selectivities were obtained based on the ratio of the two gases’ permeabilities.

Figure 9. CO2 permeability and CO2/CH4 ideal selectivity of XTRPBOI membranes versus the mole fraction of the DABA comonomer. Figure 10. Relationship between CO2 permeability and CO2/CH4 ideal selectivity of TR membranes, compared to polymeric upper bounds.50,51

always higher. Besides, P did not decrease as the ratio of PBO domains decreased, as observed before for other TR poly(benzoxazole-co-imide) membranes.16 These results suggest that the simultaneous decomposition of carboxyl pendant groups and eventual rigid biphenyl cross-linking retained much larger free volume elements after the thermal rearrangement process. Moreover, the gains in gas permeability were much more significant than the losses in selectivity. As a result, XTRPBOI-Y membranes exhibited improved gas performance, in particular for the gas pair CO2/CH4, surpassing in most cases the 2008 polymeric upper bound for CO2/CH4 separation (Figure 10). It is interesting to note the differences in the performance of the analogous XTR-PBOI-5 and TR-PBOIMPD5 membranes. While XTR-PBOI-5 easily surpassed the current 2008 upper bound, the TR-PBOI-MPD5 polymer remained far below it.50,51 We have previously reported on a tPBO TR membrane prepared from thermally imidized bisAPAF-6FDA precursor (tHPI, Scheme 3), which had PCO2 ∼ 4200 Barrer and αCO2/CH4 ∼ 28.5,18 The thermal imidization process in the solid state, at 300 °C, resulted in an insoluble cross-linked tHPI structure whereas linear and soluble HPI was obtained when thermal imidization was carried out in solution (aPBO).18 The extraordinary gas transport behavior of the tPBO membrane is likely to be due to the intrinsically cross-linked network of tHPI precursor and the high degree of conversion to the polybenzoxazole structure.18 The gas performance of the present series of XTR membranes was still far below these values, which we intended to mimic (Table 4 and Figure 10). The lower levels of rearrangement to PBO and the presence of polyimide domains in the final membrane structure could

Scheme 3. Synthesis of Reference Thermally Rearranged Polybenzoxazoles, aPBO from Azeotropically Imidized Polyimide HPI, and tPBO from Thermally Imidized Polyimide tHPI18

explain these significant differences. Nevertheless, we believe that the present cross-linking TR membrane strategy is very promising as a means to easily improve the transport behavior of TR membranes. Transport behaviors of typical XTR membranes were studied and compared with non-cross-linked TR membranes by using mixed gases. For mixed-gas experiments, CO2/CH4 mixtures (1:1) were also tested for three representative TR compositions: aPBO, TR-PBOI-MPD5, and XTR-PBO-15. Measure2611

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Figure 11. Mixed gas permeation and separation using a 50% CO2/CH4 mixture, for aPBO, TR-PBOI-MPD5, and XTR-PBOI-15 samples: (A) CO2 permeability, (B) CH4 permeability, and (C) CO2/CH4 mixed-gas selectivity.

ments were conducted at 35 °C at various feed pressures from 5 to 40 bar. Figure 11 shows mixed-gas permeabilities of CO2 and CH4 as well as CO2/CH4 selectivities in total feed pressure. XTR-PBO-15 membrane demonstrated superior gas permeability and selectivity. The P*CO2 and P*CH4 of XTR membranes were always greater than those of aPBO homopolymer across the pressure range studied. Interestingly, α*CO2/CH4 of XTR membrane was also found to be greater than that of aPBO (by 10−20%) despite its considerably greater permeabilities. The more condensable CO2 preferentially sorbs in micropores, hindering the transport of the less sorbing CH4. For XTR-PBO15, CO2 sorption increased due to enlarged cavities and free volume after thermal cross-linking, which would increase its hindrance to CH4 transport. Besides, the highly cross-linked structure of XTR-PBO-15 would also contribute to the increased α*CO2/CH4 compared to that of aPBO. On the other hand, XTR-PBO-15 and aPBO both resisted plasticization, with no plasticization pressure points observed for CO2 and CH4 up to 40 bar. For both polymers, as pressure increased P*CO2 decreased steadily whereas P*CH4 remained almost unchanged. In a previous study, full conversion to TRPBO as well as thermal cross-linking were found to suppress plasticization because of their influence in increasing chain stiffness and restricting chain movement.34,52 Nevertheless, the mixed-gas transport performance of TR-PBOI-MPD5 differed substantially. For this material, a plasticization effect was evidenced by a P*CH4 minimum occurring at 15 bar, followed

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CONCLUSIONS

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AUTHOR INFORMATION

ortho-Hydroxycopolyimides containing DABA in various molar ratios were synthesized via azeotropic solution imidization. DABA was selected as a comonomer to allow different contents of carboxyl pendant groups to be incorporated easily into the bisAPAF-6FDA structure. TGA profiles of HPID-Y membranes suggested an overlap between the degradation of carboxyl groups and the actual thermal rearrangement process at temperatures ranging between 325 and 450 °C. During thermal treatment at 450 °C, carboxyl pendant groups degraded while a more rigid biphenyl cross-linked structure was formed, either following or at the same time as the cyclization reaction to form the polybenzoxazole structure. The XTR-PBOI-Y membranes thus obtained showed improved gas performance due to their enlarged cavity sizes. PALS analysis revealed that the distribution of free volume in XTR-PBOI-Y was bimodal; cross-linking caused the d4 cavity sizes to increase but did not greatly affect the d3 cavity sizes. As a result, the gain in gas permeability was much more significant than the loss of selectivity of XTR-PBOI-Y. This new family of XTR-PBOI-Y membranes presented outstanding gas performance, for most formulations overcoming the 2008 polymeric upper bound for CO2/CH4 separation. Comparing the permeation properties of the cross-linked membrane XTR-PBOI-5 to its analogous copolyimide without labile groups, TR-PBOI-MPD5, showed the importance of the labile carboxyl groups in increasing the permeability of the XTR materials. Moreover, XTR-PBOI-Y membranes also demonstrated superior gas performance in mixed-gas CO2/CH4 (1:1) experiments, including improved permeability and selectivity as well as plasticization resistance up to 40 bar. These results bring to light a very promising methodology to easily improve the gas transport performance of TR polymer membranes.

by a significant drop in α*CO2/CH4 above this pressure. The CO2-induced plasticization at high partial pressures would increase the CH4 diffusivity under mixed-gas conditions, increasing its mixed-gas permeability. The larger CH 4 molecules are more sensitive to changes in free volume than CO2, explaining the absence of a plasticization point for P*CO2 data. Therefore, resistance to plasticization in TR membranes seems to be determined also by the rigidity of the final PBO structure. The small percentage of flexible MPD imide groups within the rigid PBO structure affected the polymer’s flexibility, resulting in significant plasticization at high pressures.

Corresponding Author

*(Y.M.L.) Telephone: +82-2-2220-0525. Fax: +82-2-22915982. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2612

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ACKNOWLEDGMENTS This research was supported by the Korea Carbon Capture & Sequestration R&D Center (KCRC) through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT & Future Planning (NRF2014M1A8A1049305), which we gratefully acknowledge.

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