Cross-Linked Thermally Rearranged Poly(benzoxazole-co

Oct 3, 2013 - Anita J. Hill,. §,# and Young Moo Lee*. ,†. †. WCU Department of Energy Engineering, College of Engineering, Hanyang University, Se...
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Cross-Linked Thermally Rearranged Poly(benzoxazole-co-imide) Membranes for Gas Separation Mariola Calle,† Cara M. Doherty,§ Anita J. Hill,§,# and Young Moo Lee*,† †

WCU Department of Energy Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea Commonwealth Scientific and Industrial Research Organization (CSIRO)Materials Science and Engineering and #CSIRO Process Science and Engineering, Private Bag 33, Clayton South, Victoria 3169, Australia

§

S Supporting Information *

ABSTRACT: 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.

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INTRODUCTION Membrane-mediated gas separation has emerged as an important and fast-growing separation technology. Compared to traditional separation processes, membrane gas separation offers many advantages, namely, low energy consumption, low operating costs and high operational flexibility.1 Specifically, gas separation using organic polymer membranes has inspired a great deal of fundamental research since the early 1980s. Conventional polymers usually show relatively low mass transport rates, due to their generally flexible nature that allows for space efficient packing with almost no micropores. High free volume polymers known as microporous organic polymers (MOPs), have shown performance commensurate with the most promising candidates for separation processes, due to their enhanced diffusion as well as sorption capability for small gas molecules. As an example, polymers with intrinsic microporosity (PIMs) based on stiff ladder-type structures with contorted sites which inhibit space efficient packing, are characterized by their relatively high gas permeability with a good permselectivity.2−6 Recently, we reported on thermally rearranged polybenzoxazole (TR-PBO) membranes, as a new class of microporous organic materials showing extraordinarily fast molecular transport, as well as molecular sieving effect for small gas molecules.7,8 The microporous structure for these materials results from a solid-state thermal rearrangement process of ortho-hydroxy-containing polyimides (HPIs) into the usually more rigid PBO structure. In our previous studies, we demonstrated that variations in the precursor polyimide © XXXX American Chemical Society

chemical structure markedly impact free volume elements size and distribution.9−11 In addition, thermal treatment protocol (e.g.: final temperature and dwell time) determined the extent of rearrangement and gas transport behavior as well.11−13 On the other hand, we also found that the type of imidization of hydroxy-polyimide resulted in significant variations in free volume elements and gas performance in TR polymer membranes. Thus, we reported recently on the preparation of TR-PBO membranes from hydroxyl-polyimide derived from 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA) and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (bisAPAF), imidized by different methods.14 TR-PBO from solution imidization, or what is known as azeotropic imidization, presented the lowest fractional free volume and gas permeation rates. Nevertheless, when the thermal imidization was carried out in the solid state, the imide-tobenzoxazole rearrangement brought about significantly larger free volume elements as well as transport rates. Thus, the CO2 permeability of the azeotropically imidized sample (aPBO) was about 400 barrer, when the TR process was carried out at 450 °C for 1 h, whereas above 4000 Barrer was measured for the thermally imidized membrane (tPBO). These important deviations in gas transport behavior of TRPBO membranes are exclusively due to the imidization method, not to the chemical structure nor to the thermal rearrangement Received: July 5, 2013 Revised: September 20, 2013

A

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was cooled to room temperature, precipitated in distilled water, washed several times with hot water and dried in a convection oven at 120 °C for 12 h. 1H NMR as well as FT-IR spectroscopic techniques were used to confirm the chemical structure of HPI polyimide, which were in good agreement with the previous literature.14 The DABAcontaining copolyimides were designated as HPIDABA-Y, where Y indicates the mole percent of the DABA diamine incorporated, ranging from 5% to 20%. General spectroscopic data for HPIDABA-Y copolymers are given in the following: 1H NMR (300 MHz, DMSO-d6, ppm): 13.50 (s, −COOH), 10.41 (s, −OH), 8.10 (d, Har, J = 8.0 Hz), 7.92 (d, Har, J = 8.0 Hz), 7.85 (s, Har), 7.80 (s, Har), 7.71 (s, Har), 7.47 (s, Har), 7.20 (d, Har, J = 8.3 Hz), 7.04 (d, Har, J = 8.3 Hz). 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 and HPIDABA polyimides ranged between 0.50 and 0.60 dL/g in every case. Monoesterification Reaction of DABA-Containing orthoHydroxy Copolyimides with 1,4-Butylene Glycol. Monoesterification reaction of the HPIDABA series of copolymers with 1,4butylene glycol was carried out using p-toluenesulfonic acid as a catalyst following the procedure reported elsewhere.26,30 Thus, HPIDABA copolyimides were dissolved in NMP in a three neck flask fitted with a condenser and a continuous nitrogen purge and a significant excess amount of butylene glycol (50 times more than stoichiometric balance to the carboxylic acid group) was added to the solution, in order to push the reaction equilibrium toward formation of an ester product. The esterification reaction was carried out by adding a catalytic amount (5 mg per gram of polymer) of p-toluensulfonic acid and heating to 140 °C under nitrogen atmosphere for around 18 h. After completion of the monoesterification reaction, the polymer solution was cooled to room temperature, and the polymer was precipitated in water, washed several times to remove unreacted butylene glycol and dried at 70 °C for 24 h under vacuum. The temperature was kept low to prevent cross-linking of the polymer particles. The modified copolyimides were designated as HPIBG-Y, where Y indicates the mol percent of DABA comonomer initially incorporated, ranging from 5% to 20%. General spectroscopic data for HPIBG-Y copolyimides are given in the following: 1H NMR (300 MHz, DMSO-d6, ppm): 10.41 (s, −OH), 8.10 (d, Har, J = 8.0 Hz), 7.92 (d, Har, J = 8.0 Hz), 7.85 (s, Har), 7.80 (s, Har), 7.71 (s, Har), 7.47 (s, Har), 7.20 (d, Har, J = 8.3 Hz), 7.04 (d, Har, J = 8.3 Hz), 4.25 (m, CH2OCO), 1.75 (m, CH2), 1.50 (m, CH2). FT-IR (film): ν(O−H) at 3400 cm−1, aliphatic (C−H) at 2980 and 2898 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 HPIBG copolymers showed values between 0.4 and 0.5 dL/g. Preparation of Cross-Linked ortho-Hydroxy Copolyimide Membranes. The casting of the copolyimides was done 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 with holds for 1 h at 100, 150, and 200 °C to evaporate the solvent under vacuum. Subsequently, the polymer films were annealed at 250 °C for 24 h also under vacuum to activate the cross-linking transesterification reaction. Thus, as it has been previously reported,26 in the transesterification step the equilibrium was pushed to the formation of cross-links by pulling vacuum and removing the volatile diol from the film. After completing the thermal treatment process, membranes were cooled naturally in the vacuum furnace to room temperature. For sample classification, the cross-linked samples were named as XHPI-Y. Thermal Conversion to Polybenzoxazole. The defect-free membranes (samples HPI and XHPI-Y) were cut into 3 cm × 3 cm size strips and placed between quartz plates to prevent film deformation at elevated temperature in a muffle furnace. Each sample was heated to 450 °C at a rate of 5 °C/min and held isothermally for 1 h in a high-purity argon atmosphere. After thermal treatment, the furnace was slowly cooled to room temperature, at a rate no greater than 10 °C/min. This protocol was used to expose the samples, XTR-

protocol. Thus, we previously found that during the thermal imidization process of o-hydroxy poly(amic acid) to HPI in the solid state, at 300 °C, thermal cross-linking occurred as proved by the insolubility of HPI in most organic solvents.14 On the contrary, linear and soluble HPI was obtained when thermal imidization was carried out in solution, by means of an azeotropic agent. The much larger gas transport rates of the tPBO membrane were very likely related to the cross-linked network structure of its thermally imidized precursor membrane. From all of these findings, we were encouraged to study the effect of chemically cross-linking the precursor hydroxypolyimide, in terms of physical properties, cavity size and transport behavior of TR-PBO membranes. Cross-linking modification of polyimides can be induced by several methods.15−20 Among all, diamine cross-linking on polyimide membranes has proved an excellent method to improve gas transport properties providing impressive separation efficency for H2/CO2 separation.21 We were interested in cross-linking at low temperatures, below the glass transition temperature of polyimides and significantly below the beginning of the TR process.22 Sub-Tg cross-linking through a transesterification reaction in polyimide membranes has been reported before, mostly for gas separation applications.23−32 Hence, a polyimide including pendant carboxylic acid groups can be covalently cross-linked in the solid state by thermally induced transesterification reactions with a diol at relatively low temperatures (200−300 °C). On the basis of this approach, and as a continuation on the study and development of new TR membrane materials with improved gas separation properties, we designed and synthesized new cross-linkable o-hydroxy polyimide precursors, based on 6FDA dianhydride and bisAPAF, by copolymerization with a diamine containing pendant carboxylic acid groups such as the 3,5-diaminobenzoic acid (DABA), in different ratios. A subsequent transesterification cross-linking reaction with 1,4-butylene glycol in the solid state was carried out by thermal annealing at 250 °C for 24 h. A set of novel cross-linked TR poly(benzoxazole-coimide) membranes have been therefore prepared by thermal rearrangement at 450 °C for 1 h. Their physical and transport properties have been examined and herein reported in detail.

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EXPERIMENTAL SECTION

Materials. Solvents and reactants were of reagent-grade quality and 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 Central Glass Co. Ltd. (Tokyo, Japan) respectively, and sublimed just before being used. 3,5-Diaminobenzoic acid (DABA) (Aldrich) was recrystallized from oxygen-free water and sublimed prior to being used. 1,4-Butylene glycol (99%) was also provided by Aldrich and used as received. Synthesis of ortho-Hydroxy Polyimide (HPI) and DABAContaining Copolyimide (HPIDABA-Y) Precursors. A mixture of bisAPAF (X mmol), where X = 10, 9.5, 9.0, 8.5, or 8.0 mmol, and DABA (10 − X mmol), dissolved in anhydrous N-methylpyrrolidone (NMP) (10 mL) was cooled to 0 °C and 6FDA dianhydride (10 mmol) was added along with another 10.0 mL of NMP. The reaction mixture was stirred for 15 min at 0 °C; then, the temperature was raised to room temperature and left overnight. A viscous solution of poly(amic acid) was obtained in every case. 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, the water released by the ring-closure reaction was separated as a xylene azeotrope. The resulting brown-colored solution B

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stacked to 1 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 (TAC) was extended to 200 ns to measure the long lifetimes, and the coincidence unit was removed to improve the count rates. A minimum of five files were collected with 1 × 106 integrated counts per file. The acquired data were analyzed using either a three- or four-component fit with the 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 using the Tao−Eldrup equation

PBOI-Y to thermal histories similar to those reported in our previous studies of TR polymers.10 Measurements. 1H spectra were recorded on a Mercury Plus 300 MHz spectrometer (Varian, Inc., Palo Alto, CA). The attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra of samples were measured using an Infrared Microspectrometer (IlluminatIR, SensIR Technologies, Danbury, CT). Inherent viscosities were measured at 25 °C with an Ubbelohde viscometer using NMP as solvent at 0.5 g/dL concentration. Glass transition temperature (Tg) of ortho-hydroxy polyimides was measured by differential scanning calorimetry (DSC) analyses on a TA Instruments Q-20 calorimeter. A total of two heating cooling cycles, at a heating rate of 20 °C/min, were conducted and Tg was obtained from the second heating cycle. Testing samples were heated to a temperature usually below the starting temperature of conversion to PBO for each polyimide during the first heating, quenched at room temperature and then reheated up to 350 °C in the second scan. Thermogravimetric analyses (TGAs) were performed on a TA Q-500 thermobalance (TA Instruments, DE, USA), coupled with mass spectroscopy (MS) ThermoStar GSD 301T (Pfeiffer Vacuum GmbH, Asslar, Germany). Dynamic ramp scans were run at 10 °C min−1, to determine thermal rearrangement and thermal stability characteristics, in the temperature range of 60−850 °C. The purge gas was nitrogen (60 mL min−1) and the sample mass was about 5 mg. Wide angle X-ray diffractometry (WAXD) were recorded in reflection mode at room temperature by using a Rigaku Denki D/ MAX-2500 (Rigaku, Japan) diffractometer. CuKα (wavelength λ = 1.54 Å) radiation was used. The average d-spacing value was determined from Bragg’s equation in the 2 theta range of 5−50° with a scan rate of 5°/min. Densities were measured on a Mettler Toledo density balance by weighting the samples in air and then in a liquid of known density. Isooctane was used in this case, since this hydrocarbon was not absorbed in the membranes that could result in erroneous measurements. The density of the sample was calculated from the expression:

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

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

(3)

where, τ3 is lifetime (ns), r3 is cavity radius (Å), and Δr is determined empirically and is related to the thickness of the electron layer on the pore wall which interacts with the positronium (1.66 Å).34 Gas permeation properties were obtained from a custom-made instrument using the time-lag method as described in our previous studies.40 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), a pressure increase through the membrane in a fixed downstream chamber was acquired from 0 to 10 mmHg against 760 mmHg of upstream pressure. From the slopes and intercepts in a steady state region of pressure increment as a function of time, gas permeability coefficients were calculated using the following equation:

⎛ V · T · l ⎞ dp 0 ⎟⎟ P = ⎜⎜ ⎝ p0 · T ·Δp · A ⎠ dt

(4) 3

where P (Barrer) is the gas permeability, V (cm ) is the downstream volume, l (cm) is the membrane thickness, Δp (cmHg) is the pressure difference between upstream and downstream, T (K) is the measurement temperature, A (cm2) is the effective membrane area, p0 and T0 are the standard pressure and temperature, and dp/dt is the rate of the pressure rise at steady state. The ideal selectivity (α1/2) for component 1 and 2 was defined as the ratio of gas permeability of the two components.

(1)

The gel content of cross-linked membranes was measured by extracting the insoluble fractions of the films immersed in THF for 24 h and drying them in vacuum at 120 °C for approximately 24 h. The weights of the polyimide films before and after extraction were measured to determine the gel content by the following equation: w gel content (%) = 2 × 100% w1 (2)

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RESULTS AND DISCUSSION Synthesis and Characterization of Precursor CrossLinkable ortho-Hydroxy Copolyimides. Four different ortho-hydroxy cross-linkable copolyimides were synthesized by polycondensation of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), and the diamines, 2,2-bis(3amino-4-hydroxyphenyl)hexafluoropropane (bisAPAF), and 3,5-diaminobenzoic acid (DABA) with molar ratios of 95:5, 90:10, 85:15, and 80:20, respectively. For comparison, ohydroxy homopolyimide from 6FDA dianhydride and bisAPAF diamine (HPI) was also prepared by solution thermal imidization method, as we reported elsewhere.14 The DABA diamine incorporates the free carboxylic acid group where cross-linking of the polymer chains is possible by an esterification reaction with a diol. Koros and his collegues studied in detail the solid-state covalent cross-linking of 6FDAbased polyimides membranes with esterification reactions for gas separation applications.23−32 Thus, the polyimide containing pendant carboxylic acid groups is first monoesterified in solution with a diol, and then thermally treated in the solid state to activate the cross-linking via transesterification reaction. The diol reactivity determines the cross-linking conversion: Glycols such as the 1,4-benzenedimethanol presented very low reactivity due to its poor nucleophilicity, whereas nearly 100% conversion in the initial monoesterification step was found for

Here w2 and w1 are the weights of the insoluble fraction and the original cross-linked film, respectively. Positron annihilation lifetime spectroscopy (PALS) was used to determine the size distribution and relative intensity of free volume elements within the polymer membranes.33 Positrons emitted from a radioactive 22Na source thermalized within the sample. The positrons can form two bound states with electrons known as positronium (Ps): o-Ps is the parallel spin combination and p-Ps has antiparallel spins. Self-annihilation of pPs 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 the surroundings. That is, the o-Ps can survive longer in a system or local domain with low electron density, such as pores, cavities, or free volume elements.34,35 On the basis of the assumption that the vacant volume has a spherical shape, the measurement of the o-Ps lifetime (τ, ns) and its intensity (I, %) not only provide the size and distribution of free volume elements in the polymer, but also shed light on the topologies in which different sized pores exist. PALS is able to distinguish the presence of bimodal porosity such as in highly microporous materials like PTMSP and PIM by fitting several o-Ps components.36−39 The size and distribution of free volume elements in the polymers was determined by PALS using an automated EG&G Ortec (Oak Ridge, TN) fast−fast coincidence spectrometer which had a resolution function of 230 ps fwhm peak when measured with 60Co. The samples were composed of 10 mm × 10 mm size polymer films C

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

the most reactive 1,4-butylene glycol.26 Accordingly, 1,4butylene glycol was chosen as the cross-linking agent in this work. The monoesterification reaction was carried out in NMP, using p-toluenesulfonic acid as a catalyst according to the procedure reported elsewhere.26,30 The modified copolyimides were designated as HPIBG-Y, where Y refers to the initial mole percent content of cross-linkable DABA comonomer incorporated. Their chemical structures are shown in Schemes 1 and 2. The degree of esterification was near 100% for all the copolyimides, as quantified by 1H NMR. As an example, the

1

H NMR spectrum of HPIBG-15 copolymer is illustrated in Figure 1. NMR spectra of copyimides HPIDABA-Y can be found in the Supporting Information (see Figures S1 and S2). The monoesterification reaction was also evidenced by FTIR spectroscopy. Low absorption bands corresponding to the aliphatic C−H stretch from the butylene moieties become perceptible in the region of 2980−2900 cm−1 for every

Scheme 2. Synthesis of Reference Thermally Rearranged Polybenzoxazole (TR-PBO)

Figure 1. Typical 1H NMR (DMSO-d6, 300 MHz) spectrum of grafted copolyimide HPIBG-15. D

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significant change in those peaks was noticeable for the XHPI-5 sample, with the lowest ratio of cross-linkable units, signifying that the extent of interchain transeterification reaction for this sample was probably low. In order to confirm these assumptions, XHPI membranes were tested for their solubility. Thus, XHPI-5 left a very small amount of gel in NMP; i.e., it almost dissolved, proving that the cross-linked fractions for this membrane were very low. Nevertheless, XHPI-10, XHPI-15 and XHPI-20 membranes were almost completely insoluble in NMP, demonstrating a high degree of cross-linking for these samples. The gel content test provided a more quantitative measurement of the efficacy of the cross-linking procedure, by measuring the fraction of undissolved polymer remaining after an extended solvent soak period. Equation 2 was used for its estimation and the results are collected in Table 1. Except for the XHPI-5 sample, high gel fraction values were found for all the XHPI membranes.

composition. The intensity of these small peaks increased as the molar ratio of esterified comonomer increases. FT-IR spectrum for HPIBG-20 copolyimide is depicted in Figure 2. Character-

Table 1. Gel Content Values for Cross-Linked XHPI Precursor Membranes Figure 2. Typical ATR-FTIR spectra of monoester copolyimide HPIBG-20 (powder) and cross-linked XHPI-20 membrane.

istic absorption bands of polyimides were also discernible; the stretching vibration bands of CO (1786 and 1716 cm−1), the asymmetric stretching vibration of C−N (1377 cm−1), the transverse stretching of C−N−C groups at 1102 cm−1 and outof-plane bending of C−N−C groups at 720 cm−1. In addition, the 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 vibration of the hydroxy groups.14,41 Besides, the carbonyl stretching of the ester group was partially overlapped with the more intense absorption of the imide carbonyl. Preparation and Characterization of Cross-Linked ortho-Hydroxy Copolyimide Membranes. We reported before that thermal rearrangement of HPIs into PBOs usually took place in the range of 300−500 °C.22 As mentioned above, we were interested in cross-linking the HPI precursor at low temperatures, prior to the rearrangement process, since this would simultaneously avoid any partial conversion to the final TR-PBO structure. Koros et al. reported that cross-linking through transesterification reactions in polyimide membranes took place at relatively low temperatures (200−300 °C).23−32 Therefore, HPIBG grafted copolyimides were cast from a NMP solution onto a clean glass plate. Cast films were then heated slowly to 250 °C with holds for 1 h at 100, 150, and 200 °C, respectively, to evaporate the solvent under vacuum, and subsequently annealed at 250 °C for 24 h also under vacuum to activate the cross-linking reaction. In the glassy solid-state, the mobility of the reactant is low and the equilibrium can only be shifted to the formation of the diester cross-links by pulling vacuum and removing the volatile diol from the film.26 For sample classification, the cross-linked samples were named as XHPI-Y, where Y refers to the initial molar ratio of DABA. The solid-state cross-linking process was monitored by IR spectroscopy.26 As an example, Figure 2 shows the chemical modifications by the monoesterification and transesterification reactions for HPIBG-20 and XHPI-20 samples. Thus, the crosslinking could be identified by the diminishing absorption peaks from the butylene moieties, at around 2980−2900 cm−1, since the density of these groups is decreased as the transesterification cross-linking reaction proceeds. Nevertheless, no

a

sample

gel fraction (%)

XHPI-5 XHPI-10 XHPI-15 XHPI-20

NDa 95.5 98.2 99.1

Not detectable due to very low gel fraction.

On the other hand, for comparison, a cast film from the reference HPI polyimide was also exposed to the same thermal protocol, that is, heated slowly to 250 °C and annealed at that temperature for 24 h. HPI film was easily dissolved in NMP at room temperature. Differential scanning calorimetry (DSC) was used to identify the glass transition temperatures of XHPI membranes (Table 2). It is well-known that cross-linking usually affects the Table 2. Thermal Properties of Cross-Linked XHPI and Reference HPI Membranes sample

Tga (°C)

TTRb (°C)

rTRc (wt %/°C)

CO2 wt loss theoreticald (%)

CO2 wt. loss founde (%)

HPI XHPI-5 XHPI-10 XHPI-15 XHPI-20

300 305 305 311 320

407 412 414 419 429

0.1920 0.1815 0.1499 0.1461 0.0985

11.36 10.79 10.22 9.66 9.09

11.25 10.94 9.94 9.42 8.62

a

Middle point of the endothermic step during the second scan of DSC measurements conducted with a heating rate of 20 °C/min under a nitrogen atmosphere. bTemperature at the maximum point of weight loss or maximum rate of conversion to PBO. cThe maximum conversion rate of the imide to benzoxazole membrane. dThe theoretical carbon dioxide weight loss (in %), corresponding to the rearrangement reaction. eThe experimental weight loss (in %), corresponding to the first step by TGA.

polymer segmental mobility. Normally, the polymer matrix is stiffened by the cross-linker and thermal motions of the polymer chain segments are restristed, resulting in an increased Tg. Nevertheless, Tg for XHPI set of membranes did not appear to be strongly affected by cross-link density. Hence, XHPI-5 and XHPI-10 samples presented a similar Tg value, around 305 °C, whereas for XHPI-15 slightly went up to 311 °C. Besides, a Tg of 320 °C was detected for XHPI-20 membrane, with the E

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highest degree of cross-linking. This phenomena is probably attributable to the packing disruption introduced by insertion of large cross-linker spacers between the polymer chains. Hence, the expected significant increase of Tg due to the high restriction of molecular mobility, would be partially counterbalanced by a plausible net increase in local free volume. Similar behavior had been previously observed for 6FDA-based polyimides, where diester cross-linked samples showed similar or even lower Tg than corresponding linear precursors.26 In addition, the presence of some unreacted monoester groups could also contribute to lowering the Tg. These Tg values are indeed quite close to the Tg reported by us for related crosslinked and linear homopolymer membranes, synthesized by thermal (tHPI) and azeotropic (aHPI) imidization methods, respectively, from 6FDA and bisAPAF, with Tg about 300 °C.14 Therefore, the molecular mobility of o-hydroxy polyimide precursor, which is a key factor in the rearrangement process,22 did not seem to be hampered to any extent with transesterification cross-linking modification. Besides, Tg for the reference homopolyimide membrane HPI thermally treated at 250 °C for 24 h, remained around 300 °C in spite of the annealing process. Thermal Rearrangement of Cross-Linked ortho-Hydroxy Copolyimide Membranes (XHPI) into Poly(benzoxazole-co-imide) Membranes (XTR-PBOI). It was previously well-demonstrated that thermal rearrangement of HPIs into PBOs proceeded through a carboxybenzoxazole intermediate, followed by decarboxylation to form the fully aromatic benzoxazole.42−44 Thermal behavior of XHPI membranes was therefore investigated by thermogravimetric analysis coupled with mass spectroscopy (TGA-MS) to elucidate thermal conversion characteristics. As an example, TGA-MS of XHPI-20 sample is shown in Figure 3. In addition,

Figure 4. TGA and DTG curves of XHPI cross-linked membranes as well as reference HPI membrane, at a heating rate of 10 °C/min under N2 atmosphere.

different compositions decreasing as the mol percent of crosslinkable (no-rearrangeable) comonomer increases (Table 2). As a result, XHPI-20 with the highest degree of cross-linking showed the lowest experimental CO2 weight loss (Table 2, Figures 3 and 4). Recently, Le et al. studied the diol cross-linking modification of DABA containing polyimides for pervaporation processes.45 TGA curves for reported membranes presented, prior to major backbone degradation at about 500 °C, a minor weight drop starting around 370 °C probably due to the onset of decomposition of diester interchain bridges. Therefore, it is expected that substantial removal of ester bridging groups could also take place simultaneously to the conversion process. Nevertheless, the very good correspondence of the measured and the theoretical CO2 weight loss show that the TR process is mainly responsible for this first mass loss step observed for XHPI membranes, under the dynamic conditions of analysis (10 °C/min). On the other hand, in previous studies we demonstrated that conversion temperature of TR-PBO membranes was influenced by chain rigidity of HPIs, and appeared to exhibit a linear relationship with Tg.22 TGA and DTG curves for XHPI membranes supported again these findings. Thus, from Figure 4, it can be clearly noticed that the rearrangement temperature for this set of XHPI polyimides is certainly affected by chain mobility, and therefore, by crosslinking density. Note that the first weight loss peak in the DTG curve significantly moved to higher temperatures, as Tg increased for the highest cross-linking percentage of XHPI20. Temperature at the maximum rate of conversion to PBO (TTR) of XHPI precursors are collected in Table 2 and evidenced this trend. Besides, the generalized decomposition of the in situ formed poly(benzoxazole-co-imide) structure takes place in the same range for all the samples, between 560 and 570 °C, irrespective of the different chemical structure. Another interesting fact is that the maximum rearrangement rates (rTR) descend with the increase in cross-linking density. For example, there is an evident decrease from 0.1920 to 0.0985 wt %/°C for the noncross-linked HPI and the XHPI-20 membranes, respectively (Table 2). Accordingly, higher degrees of cross-linking resulted in slower conversion rates to PBO for this set of XHPI samples, indicating that cross-linking partly inhibited the rearrangement reaction. In this work, we focus on the effect of diverse cross-linking ratios on the performances of TR-PBOs. Therefore, for a

Figure 3. Thermogravimetric analysis combined with mass spectroscopy (TG-MS) of cross-linked precursor copolyimide membrane XHPI-20, at a heating rate of 10 °C/min under N2 atmosphere.

TGA and first derivative thermogram (DTG) curves for every XHPI composition, as well as the reference HPI homopolymer membrane, are illustrated in Figure 4. Thus, all the thermograms presented a distinct weight loss in the range of 300−470 °C, prior to the generalized decomposition of the polymer backbone around 500−600 °C. The evolution of CO2 during this first weight loss step was evidenced by mass spectroscopy and supported the rearrangement process.14 Thereby, theoretical weight loss corresponding to this evolution differs for the F

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neighboring phenyl radicals combined together to form linkages that yielded biphenyl cross-linking structures. Accordingly, thermal treatment of XHPI membranes at 450 °C for 1 h, involving the rearrangement process as well as the thermal decomposition of the diester bridging groups, would mainly result in a biphenyl cross-linked poly(benzoxazole-co-imide) structure. Other sites of cross-linking would be also feasible, through the cleavage of −CF3 groups from 6FDA and bisAPAF moieties, although this is not believed to be as important as the site leading to the biphenyl linkage.41,49 Askari et al. observed the intensities of peaks at 1580−1600 cm−1, representing C−C stretching vibrations in the aromatic ring, increase due to the formation of the new covalent cross-linking bonds between the benzene rings once a temperature of 425 °C was attained.46 From Figure 5, an increase in the absorbance of those peaks is also observed for XTR-PBOI samples, as compared to reference TR-PBO homopolymer membrane. Nevertheless, since the chemical structure for every XTR-PBOI composition differs, it is difficult to ascribe these fluctuations in the C−C stretching vibrations in the aromatic ring specifically to the newly formed cross-linking bonds rather than to the different chemical composition. Figure 6 shows the X-ray diffraction profiles of precursor polyimide and thermally rearranged polybenzoxazole membranes. A broad amorphous halo was observed for all the samples, proving the amorphous nature of all these polymers. As shown in Figure 6, the different cross-linking degree for XHPI samples hardly changed the position of the amorphous halo maximum at around 16° (2θ), corresponding to a

comparative purpose, identical thermal rearrangement conditions were adopted for all the polymers to expose the samples to thermal histories similar to those reported in our previous studies of TR polymers, that is, 450 °C for 1 h.14 Characterization of Cross-Linked Thermally Rearranged Poly(benzoxazole-co-imide) (XTR-PBOI) Membranes. The chemical or structural changes taking place in the films during the thermal treatment were monitored using ATRFTIR analysis. Defect free and clean pieces of XHPI and HPI membranes (previously treated at 250 °C for 24 h) were ramped at 5 °C/min in a muffle furnace, and held at 450 °C for 1 h under a high-purity argon atmosphere. The infrared spectra of these samples are shown in Figure 5. The formation of the

Figure 5. ATR-FTIR spectra of thermally rearranged TR-PBO and XTR-PBOI membranes treated at 450 °C for 1h.

benzoxazole ring-structure can be easily followed by IR spectroscopy as several distinct changes occur during the thermal treatment; the characteristic O−H stretch at 3400 cm−1 for these hydroxyl-containing copolyimides (Figure 2) disappeared in the samples treated at 450 °C, and two distinct peaks at wave numbers around 1480 and 1060 cm−1 appeared typical of the benzoxazole ring.44 In addition, characteristic absorption bands of the imide group were also visible for the asprepared poly(benzoxazole-co-imide) membranes, confirming the thermal stability of the aromatic imide linkages at the conversion temperature. As expected, an increase in total imide absorptions with an increased ratio of cross-linkable (nonrearrangeable) DABA comonomer present was also noticeable; the percentage conversion to PBO decreased as the degree of cross-linking increased. Furthermore, absorption peaks from the butylene moieties, at around 2980−2900 cm−1 in XHPI precursor membranes, were absent for the thermally treated samples. Therefore, thermal decomposition of the labile diester bridging groups certainly takes place during the conversion process, as we postulated before. The partial pyrolysis of polyimide membranes containing ester or carboxylic pendant groups has been recently reported.46−48 Askari et al. studied the effects of thermal treatment temperature from 300 to 450 °C on the gas separation properties of 6FDA-based DABA containing copolyimides grafted with thermolabile cyclodextrin groups by esterification.46 They concluded that, in addition to the decomposition of the cyclodextrin structure, the thermal treatment induced cross-linking among the polymer chains. Hence, decarboxylation of the DABA moiety seemed to occur when the temperature increased over 400 °C, the resulting

Figure 6. Wide angle X-ray diffraction (WAXD) patterns of HPI and XHPI precursor membranes treated at 250 °C for 24 h, as well as thermally rearranged TR-PBO and XTR-PBOI membranes treated at 450 °C for 1 h. G

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In fact, the increased intermolecular distances coincided with reduced densities of the XTR-PBOIs. The density of all the films was measured using a density determination kit and an analytical balance, with isooctane as the auxiliary liquid. In all cases, the values for XTR-PBOI membranes were lower than that for XHPI precursors (see Table 3). Annealing at 250 °C for 24 h of linear HPI precursor and eventual physical densification resulted in an increase in density as compared to aHPI analogue thermally treated at 300 °C 1 h. Nevertheless, no differences in density were observed between TR-PBO and aPBO membranes, from HPI and aHPI respectively (Table 3).The decrease in the density of the TR-PBOs membranes during the rearrangement process is normally accompanied by an outstanding enhancement in free volume.7,8,14 Indeed, experimental density measurements allow for the estimation of the fractional free volume from the estimated occupied volume of the polymer chains by Bondi’s method.50 However, the occupied volume for these TR copoly(benzoxazole-imide) membranes XTR-PBOI could not be estimated, since their precise chemical structure is difficult to establish accurately. Therefore, positron annihilation lifetime spectroscopy (PALS) was considered to analyze quantitatively the free volume size and distribution for most representative XTR-PBOI samples. In PALS, the free volume cavities are probed by the lifetime of orthopositronium (o-Ps) before annihilation in the free volume regions of the materials.35 Hence, the 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 the cavities. The PALS results are collected in Table 4. For a comparative purpose, pore characterization for related TR polybenzoxazoles aPBO and tPBO, from azeotropic and thermally imidized HPIs respectively, have also been included.14 As observed before for TR-PBO polymers, PALS data for XTR-PBOI samples revealed two o-Ps components, τ3 and τ4, signifying that two kinds of pores are present in these materials: ultrafine micropores, with τ3 ∼ 1.2 ns that corresponds to a mean cavity diameter of d3 ∼ 4 Å, and micropores where τ4 ∼ 4 ns and cavity size d4 ∼ 8−9 Å. As seen, thermal annealing at 250 °C for 24 h of noncross-linkable linear HPI precursor altered the free volume distribution after the TR process. Thus, smaller cavity sizes and higher intensities for the larger pores were found for the TR-PBO sample in comparison to the previously reported aPBO membrane.14 Interestingly enough, this new class of XTR-PBOI materials showed a much larger cavity size than that of TR-PBO sample. Hence, average micropore diameter d4 increased from 8.22 ± 0.12 Å in TR-PBO to 8.86 ± 0.07 Å for XTR-PBOI-15 sample. This increase in cavity size with cross-linking ratio was accompanied by a decrease in the number of free volume elements. Intensity values I4 monotonically decreased from TRPBO to XTR-PBOI-15, meaning exactly the opposite trend

preferential intersegmental distance of about 0.55 nm (see Table 3), as compared to HPI uncross-linked sample, although the patterns differed slightly in both shape and width of the halo. Table 3. Density and d-Spacing of Precursor and TR Analogue Membranesa sample

apparent density (g/cm3)

HPI XHPI-5 XHPI-10 XHPI-15 XHPI-20 aHPI13 tHPI13 TR-PBO XTR-PBOI-5 XTR-PBOI-10 XTR-PBOI-15 XTR-PBOI-20 aPBO14 tPBO14

1.53 1.51 1.52 1.51 1.50 1.49 1.47 1.38 1.43 1.41 1.39 1.36 1.38 1.27

d-spacing (nm) 0.57 0.55 0.55 0.54 0.57 0.54 0.57 0.58 0.62 0.66 0.67 0.66 0.58 0.64

± ± ± ± ±

0.00632 0.00894 0.00447 0.00707 0.00447

± ± ± ± ±

0.00447 0.00447 0.00707 0.00707 0.00548

a

Note: Uncertainties were calculated as the standard deviation of at least five repeat measurements.

As observed before for TR polymers,8,14 thermal rearrangement to XTR-PBOIs resulted in larger d-spacing. Hence, by comparing the WAXD patterns of XTR-PBOIs with those of XHPIs, it can be easily appreciated that the average interchain distances for all XTR polymers had notably increased. The dspacing values of XTR-PBOIs were in the range 0.62−0.67 nm, as compared to 0.55 nm for XHPIs precursors (Table 3). On the other hand, the position and width of the halo for crosslinked XTR-PBOI membranes significantly differed from reference TR-PBO sample. Note that the maximum peak of TR-PBO film is shifted to the right, 2θ = 15ο, meaning a smaller average intersegmental distance of about 0.58 nm. Furthermore, TR-PBO showed a narrower amorphous halo, with lower contribution of larger intersegmental distances to the global scattering profile. Therefore, cross-linking brought about a lower chain packing density for these XTR-PBOI membranes. Nevertheless, as noticed from Figure 6 and Table 3, diverse cross-linking degrees did not show important differences in the average interchain distances. As reported before, changes in the d-spacing serve as an indicator of the openness of a polymer matrix or amount of room available (free volume) for penetrating small molecules to diffuse through a polymer membrane.8,14 Therefore, WAXD analysis foresees a less packed and more open structure for this set of novel XTRPBOI membranes, favorable in gas separation applications.

Table 4. Pore Size Characterization by Positron Annihilation Lifetime Spectroscopy (PALS) for TR Membranes sample TR-PBO XTR-PBOI-5 XTR-PBOI-10 XTR-PBOI-15 aPBO14 tPBO14

τ3 (ns) 1.075 1.178 1.267 1.238 1.06 1.24

± ± ± ± ± ±

0.135 0.121 0.133 0.122 0.09 0.13

I3 (%) 7.27 6.39 6.11 5.71 7.1 5.0

± ± ± ± ± ±

1.03 0.59 0.57 0.56 0.7 0.6

τ4 (ns) 3.768 3.972 4.217 4.353 3.90 5.26

± ± ± ± ± ±

I4 (%)

0.054 0.061 0.060 0.067 0.04 0.08 H

13.95 12.39 11.43 9.82 12.7 6.0

± ± ± ± ± ±

0.68 0.46 0.42 0.43 0.7 0.2

cavity diameter d3 (Å) 3.56 3.86 4.11 4.03 3.51 4.04

± ± ± ± ± ±

0.84 0.34 0.36 0.33 0.28 0.35

cavity diameter d4 (Å) 8.22 8.45 8.71 8.86 8.37 9.73

± ± ± ± ± ±

0.12 0.07 0.06 0.07 0.04 0.07

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similar for both membranes. The decreased size of the larger pores explains the diminished P coefficients, whereas the dimensions of the smaller pores determines gas selectivity.52 Moreover, XTR-PBOI membranes presented great improvements in gas transport properties and separation results, in particular for the gas pair CO2/CH4. Figure 8 exhibits the CO2

observed in d4 values. This fact suggests a possible coalescence of the smaller cavities to form larger cavities, as described previously for TR polymers.51 Besides, fluctuations in the smaller cavity sizes ranged between 3.56 ± 0.84 Å for the TRPBO sample to 4.03 ± 0.33 Å for the XTR-PBOI-15 that could be considered almost insignificant since they lie within the range of uncertainty. For a better understanding, free volume distributions of XTR-PBOI polymers are presented in Figure 7.

Figure 8. CO2 permeability and CO2/CH4 selectivity of XTR-PBOI membranes as a function of molar composition of cross-linkable comonomer incorporated. Figure 7. Free volume distribution results obtained from PALS measurements of the XTR-PBOI membranes.

permeability and CO2/CH4 selectivity of XTR-PBOI membranes as a function of mol percent of cross-linkable comonomer initially incorporated in the precursor polyimide. Gas permeation properties in glassy polymers are known to be dependent upon distribution and size of free volume elements. Accordingly, permeability coefficients for any XTR-PBOI composition proved higher than those of the reference TRPBO membrane, consistent with enlarged cavity sizes as analyzed by PALS. Hence, XTR-PBOI-10 displayed the highest P values, with PCO2 around 1000 barrer, while XTR-PBOI-20 showed the smallest P increments, with PCO2 ∼ 440 barrer as compared to 260 barrer for TR-PBO membrane. Indeed, permeability coefficients for XTR-PBOI-15 were lower than those for XTR-PBOI-10 and XTR-PBOI-5 compositions, in spite of its largest cavity size. On the basis of the PALS analysis (Table 4 and Figure 7), XTR-PBOI-15 sample has larger micropores but much less of them, thus the possibly overall decrease in free volume when compared to XTR-PBOI-10 and XTR-PBOI-5 samples. In fact, XTR-PBOI-10 membrane showed the highest diffusion coefficients (Table S1) and fractional free volume (Table S2) among the membranes

Note the bimodal characteristics of the distribution, with two peaks around 4 and 8 Å shifting toward larger cavities as a function of cross-linking percentage, which is consistent with the WAXS data. On the other hand, in comparison to reference tPBO from cross-linked homopolyimide tHPI with d4 = 9.73 ± 0.07 Å,14 XTR-PBOI series of membranes presented smaller pore sizes (Table 4). Gas Transport Behaviors of XTR-PBOI Membranes. The gas permeabilities (P) as well as ideal separation factors (α) for some interesting gas pairs of TR membranes are listed in Table 5. For comparative purposes, the permeability and ideal selectivity for related polybenzoxazoles aPBO and tPBO, from azeotropically and thermally imidized hydroxylhomopolyimide 6FDA-bisAPAF,14 have also been included. As can be seen, the gas permeability of TR-PBO membrane decreased about 1.5 fold while selectivity remained almost unchanged in comparison with previously reported aPBO.14 From PALS results (Table 4), the different cavity size distribution might be the main reason contributing to this effect: smaller cavity sizes and higher intensities for the larger pores were present for TRPBO sample, whereas size and number of the smaller pores was Table 5. Gas Permeation Properties of TR Membranes gas permeability (Barrer)a

a

ideal selectivityb

sample

He

H2

CO2

O2

N2

CH4

O2/N2

CO2/N2

CO2/CH4

CO2/H2

H2/CH4

N2/CH4

TR-PBO XTR-PBOI-5 XTR-PBOI-10 XTR-PBOI-15 XTR-PBOI-20 aPBO14 tPBO14

269 446 517 404 345 356 2647

294 603 763 515 421 408 4194

261 746 980 668 440 398 4201

52.5 133 193 119 81.9 81 1092

12.6 29.6 50.9 29.8 19.7 19 284

7.5 19.9 33.0 19.4 12.4 12 151

4.2 4.5 3.8 4.0 4.2 4.3 3.8

20.7 25.2 19.3 22.4 22.3 21 15

34.8 37.5 29.7 34.4 35.5 34 28

0.9 1.2 1.3 1.3 1.0 1.0 1.0

39.2 30.3 23.1 26.5 34.0 35 28

1.7 1.5 1.5 1.5 1.6 1.6 1.9

1 Barrer =10−10 cm3 (STP) cm/ (s cm2 cmHg). bIdeal selectivities were obtained by the ratio of two gas permeabilities. I

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occurrence of cross-linking reaction along with TR process, nonequilibrium free volume elements develop and get locked into the polymer structure. PALS analysis confirmed the existence of a bimodal free volume distribution for this series of XTR-PBOI membranes, where large cavity sizes significantly increased due to the cross-linking and small cavity sizes did not change as much. As a result, a synergistic effect of high permeability and high selectivity appeared to be created in one step, in particular for the gas pair CO2/CH4. On the basis of high diffusion rates and sorption capacities from large free volume, XTR-PBOI membranes displayed superior CO2 permeabilities. Besides, the cross-linking reactions tighten and rigidify polymer chains, creating ultrafine micropores with the right characteristics to discriminate CO2 from its mixture with CH4. Therefore, the newly proposed strategy provides a route to facile tuning of free volume distribution in TR polymer membranes to optimize gas separation performance.

tested. This explains the high permeability of XTR-PBOI-10 membrane. Interestingly, the gas selectivity did not significantly decrease with the increase in permeability for the diverse XTR-PBOI compositions in comparison with reference TR-PBO membrane (Table 5 and Figure 8). Only the XTR-PBOI-10 sample with the largest permeability increments presented diminished selectivity. Except for the XTR-PBOI-5 sample, the XTR-PBOI series of membranes presented an inverse P-α relationship: as P increases, α decreases and vice versa. At temperatures like 450 °C, loose diester interchain bridges are prone to degrade while a much more rigid biphenyl crosslinked structure is formed following or alongside the imide-tobenzoxazole rearrangement. As a result, nonequilibrium free volume elements develop and get locked into the polymer structure, as molecular mobility does not allow relaxation to fill the void space created. The larger cavities contribute to the higher rate of gas transport, whereas the presence of small micropores works for the improved gas-pair selectivity. Therefore, a synergistic effect of high permeability and high selectivity appear to be created in one step, principally for the gas pair CO2/CH4.Thus, XTR-PBOI membranes exhibited outstanding gas performance, easily surpassing the most recent polymeric upper bound for CO2/CH4 separation,54 as shown in Figure 9. Although the permeability of these samples still

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ASSOCIATED CONTENT

S Supporting Information *

Characterization of polymers. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y. M. L). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Korea CCS R&D Center, funded by the Ministry of Education, Science and Technology in Korea which we gratefully acknowledge. CMD and AJH thank the Office of the Chief Executive Science Leader Team for support.

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REFERENCES

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Figure 9. Relationship between CO2 permeability and CO2/CH4 selectivity of TR membranes with 2008 upper bounds.53,54

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CONCLUSIONS A novel approach to improve gas permeability in TR polybenzoxazole membranes while retaining selectivity has been demonstrated by transesterification cross-linking reaction of o-hydroxy polyimide precursors with 1,4-butylene glycol in the solid state. During the TR process at 450 °C, loose diester interchain bridges are prone to degrade while much more rigid biphenyl cross-linked structure is formed following or alongside the cyclization reaction to the polybenzoxazole structure. The as-synthesized cross-linked TR poly(benzoxazole-co-imide) (XTR-PBOI) membranes retained much larger free volume elements as compared to previously reported aPBO membrane, from linear 6FDA-bisAPAF hydroxyl-polyimide. With the J

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