Thermally Rearranged Polybenzoxazoles Containing Bulky

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Thermally Rearranged Polybenzoxazoles Containing Bulky Adamantyl Groups from Ortho-Substituted Precursor Copolyimides Carla Aguilar-Lugo,† Cristina Á lvarez,† Young Moo Lee,*,‡ José G. de la Campa,*,† and Á ngel E. Lozano*,†,§,∥ †

Instituto de Ciencia y Tecnología de Polímeros, ICTP-CSIC, Juan de la Cierva 3, E-28006 Madrid, Spain Department of Energy Engineering, Hanyang University, 04763 Seoul, Republic of Korea § SMAP, UA-UVA_CSIC, Associated Research Unit to CSIC, Fac. de Ciencias, Univ. de Valladolid, Paseo Belén 7, E-47011 Valladolid, Spain ∥ IU CINQUIMA, Univ. de Valladolid, Paseo Belen 5, E-47011 Valladolid, Spain ‡

S Supporting Information *

ABSTRACT: A new nucleophilic monomer (2,2-bis(3amino-4-hydroxyphenyl)adamantane, ADHAB) having bulky adamantane groups has been synthesized following an efficient synthetic methodology. The main target of this work was to employ a high thermal stable bulky cycloaliphatic moiety as adamantane to obtain aromatic ortho-hydroxypolyimides (poly(o-hydroxyimide)s) able to thermally rearrange to give polybenzoxazole (TR-PBO) materials that could be tested as gas separation membranes. Thus, an array of orthoacetylcopolyimides, o-acetyl PIs) were prepared by reaction of ADHAB and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (APAF) with 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) via chemical imidization. Copolyimides and homopolyimides showed inherent viscosities ranging from 0.49 to 0.70 dL/g and provided good-quality dense membranes. Glass transition temperatures of these o-acetyl copolyimides were higher as the amount of ADHAB increased. The thermal stability of the adamantane moiety during the TR process was evaluated by directly synthesizing PBOs, which were made from the reaction, and ulterior thermal cyclization, of 2,2-bis(4-chlorocarbonylphenyl)-hexafluoropropane with ADHAB/APAF. TR-PBO membranes made through a thermal treatment at 450 °C for 30 min showed excellent gas separation properties for the CO2/CH4 gas pair with values close to the 2008 Robeson limit. structures, have not surpassed the 2008 Robeson upper limit,17 and they are not able to accomplish industrial requirements for many gas separation applications, particularly due to their tendency to plasticization, relatively high physical aging, and its difficulty to use at high temperatures.18 In 2007, a new approach paved the way for obtaining materials with outstanding gas separation properties.19−22 The methodology consisted of thermal rearrangement (TR) of ortho-hydroxy polyimides (HPIs) in the solid state above 400 °C (named the TR process) to generate polybenzoxazole (PBO) groups together with release of CO2.23−25 This thermal process led to the formation of an increased number of fine-tuned free volume elements, which yielded materials with very high permeability19,20 and good selectivity. The observed improvement was so high that the gas separation properties clearly surpassed the 2008 Robeson limit for some gas pairs.17,26 Because of the elevated temperature required for this conversion, cross-linked materials were obtained, which

1. INTRODUCTION Aromatic polymers are widely employed for advanced applications. Within the vast array of aromatic polymer structures, aromatic polyimides (PIs) are considered to be materials of great technological and academic interest due to their good mechanical properties, excellent thermal stability, elevated chemical resistance, and high glass transition temperatures.1−5 Furthermore, it is possible to design and easily obtain tailor-made aromatic polyimides by synthesis of new monomers or by simple modification of known polymers.5−7 Regarding their use in gas separation applications, it is feasible to design aromatic polyimides with elevated rigidity and high fractional free volume (FFV), which leads to materials with high permeability and good permselectivity.7−15 An excellent approach to improve the gas separation properties of polymers consists of introducing bulky substituents into the main chain or as pendent groups.7,13,16 Bulky substituents decrease the ability of the macromolecular chains to pack, and therefore the FFV of the material is augmented. Moreover, when the bulky group is well chosen, the rigidity of the chain can be enhanced, causing an increase in selectivity. Nevertheless, aromatic polyimides, even those with well-designed © XXXX American Chemical Society

Received: November 21, 2017 Revised: February 2, 2018

A

DOI: 10.1021/acs.macromol.7b02460 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Structures of Monomers, Precursor Polyimides, and TR Polymers Employed in This Work

properties that permit the formation of materials with high FFV and a narrow and controlled free volume distribution.46 Based on these antecedents, this work describes the synthesis y characterization of new TR-PBO materials that incorporate bulky adamantane groups into the main chain. Adamantane (tricyclo[3.3.1.1]decane) is a bulky, highly symmetric molecule composed of three connected rigid cyclohexane rings in a chair-type conformation, quite free of conformational stress.47 This moiety, due to its molecular volume (144.3 Å3, 1.7 times the molecular volume of a phenyl group, 85.1 Å3), tends to inhibit the packing of polymer chains and to decrease polymer crystallinity, which is conducive to enhanced processability.48,49 Furthermore, the special atomic disposition of this cycloaliphatic structure increases thermal resistance well above that of most aliphatic structures and achieves a thermal stability comparable to aromatic moieties. For instance, Plaza-Lozano et al. obtained aromatic polyimides from spiro-(adamantane-2,9′(2′,7′-diamino)fluorene, and they observed the thermal degradation of these polyimides to be close to 500 °C.47 Additionally, the introduction of an adamantane group conveniently placed in the macromolecular chain has increased both rigidity and FFV,48,49 providing good gas separation properties. The chosen TR monomer for this work was 2,2-bis(3-amino4-hydroxyphenyl)adamantane (ADHAB). The monomer, easily synthesized from relatively affordable 2-adamantanone, was

improved its plasticization resistance and decreased its physical aging.27,28 These results encouraged research in this area, and new structures were developed.29−32 In this way, the search for relationships between structure and properties was envisioned, and the feasibility of carrying out the TR process on oacyl-21,28,29 or o-alkoxypolyimides35 was realized. Additionally, important efforts on the design and synthesis of new TR polymers have been accomplished by incorporating contorted monomers with high rigidity to produce o-hydroxy polyimides with high FFV, which subsequently produced thermally rearranged polybenzoxazoles (TR-PBOs) with excellent gas separation properties.29,36,37 In this way, TR-PBOs derived from polyimides with bulky 9,9-diphenylfluorene moieties have been obtained.38,39 In order to improve the properties of the final TR-PBO materials, classical polymer science approaches, such as copolymerization of well-designed monomers, have been employed, and the results were promising.40−42 Finally, classical technologies employed in the field of gas separation membranes such as the cross-linking of precursor polyimides43,44 or the formation of high-quality hollow fibers have been used to obtain high-productivity TR-PBO materials.41 However, it is necessary to improve the balance of properties of TR-PBOs polymers in order to reduce the cost of precursor monomers, decrease their rearrangement temperature,37 understand degradation processes that accompany the TR process, and obtain insight into relationships between structure and B


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Macromolecules

75 °C. After the addition was complete, the solution was heated to reflux for 24 h. The final hot solution was filtered through Celite to remove the Pd/C catalyst and partially evaporated with a rotary evaporator before pouring into cold distilled water. The product precipitated as a white solid, which was filtered, washed with water, dried, and recrystallized in ethanol. 87% yield. 1H NMR (400 MHz, DMSO-d6) δ 8.57 (s, 2H), 6.63 (s, 2H), 6.50−6.40 (m, 4H), 4.28 (s, 4H), 2.99 (s, 2H), 2.01−1.61 (m, 12H). 13C NMR (100 MHz, DMSO-d6) δ 141.31, 140.65, 136.25, 114.64, 114.18, 112.63, 49.07, 38.20, 33.67, 31.77, 27.46. 13C NMR spectrum of ADHAB is included in the Supporting Information (Figure 2-SI). Synthesis of 2,2-Bis(4-chlorocarbonylphenyl)hexafluoropropane (6FCl). The synthesis of 6FCl was carried out by chlorination of the acid groups of 2,2-bis(4-carboxyphenyl)hexafluoropropane (6F) with thionyl chloride in accordance with the published procedure.51,52 Synthesis of o-Acetylpolyimides and o-Acetylcopolyimides Derived from 6FDA and ADHAB/APAF. A three-necked flask equipped with a mechanical stirrer and gas inlet and outlet was charged with 5.0 mmol of diamine (i.e., ADHAB, APAF, or a mixture of both) and 5 mL of NMP. The mixture was stirred at room temperature under a nitrogen atmosphere until the diamine was completely dissolved. Then, the solution was cooled to 0 °C, and the required amounts of CTMS (1 mol/mol reactive group), Py (1 mol/ mol reactive group), and DMAP (0.1 mol/mol Py) were added. After warming to room temperature, the solution was stirred for 15 min to ensure formation of the silylated diamine. The solution was then recooled to 0 °C, and 6FDA (5.0 mmol) was added, followed by 5 mL of NMP. The reaction mixture was stirred for 15 min at this temperature, allowed to reach room temperature, and left overnight to form the o-hydroxypoly(amic acid) solution. The poly(amic acid) was chemically imidized by adding a solution of acetic anhydride (40 mmol, 4 mol/mol reactive group) and Py (40 mmol, 4 mol/mol reactive group). The solution was stirred for 6 h at room temperature and 1 h at 60 °C to promote complete imidization. The polyimide solution was cooled to room temperature and poured into water, and the resulting precipitate was washed with water and a mixture of water/ethanol (1/1) and finally dried in a vacuum oven at 150 °C overnight. For ADHAB/APAF-6FDA copolyimides, the naming convention is AD followed by the mole fraction of ADHAB used in the copolymer synthesis. For example, AD(0.75) refers to a copolyimide prepared with 75 mol % ADHAB and 25 mol % APAF (Scheme 1). Synthesis of o-Hydroxy Precursor Polyamides of PBO (HPAs) Derived from 6FCl and ADHAB/APAF. A three-necked flask equipped with a mechanical stirrer and gas inlet and outlet was charged with 5.0 mmol of diamine (ADHAB or APAF) and 5 mL of NMP. The mixture was stirred at room temperature under a nitrogen atmosphere until the diamine was completely dissolved. The solution was then cooled to 0 °C, and the required amounts of CTMS (1 mol/ mol reactive group) and Py (1 mol/mol reactive group) were added. After warming to room temperature, the solution was stirred for 15 min to ensure formation of the silylated diamine. The solution was recooled to 0 °C, and 6FCl (5.0 mmol) was added, followed by 5 mL of NMP. The reaction mixture was stirred for 15 min at this temperature, allowed to reach room temperature, and left overnight to form the polyamide solution. The very viscous polyamide solution was poured into water, and the resulting precipitate was washed with water and a mixture of water/ethanol (1/1) and finally dried in a vacuum oven at 120 °C (schematics of synthesis of these materials is included in the Supporting Information, Figure 1-SI). Film Casting and Thermal Treatment. Films were prepared from 10 wt % polyimide solutions in chloroform, filtered through a 3.1 μm glass-fiber syringe filter, and cast onto a leveled glass plate. The solvent was evaporated at room temperature, and the resulting film was removed from the glass plate, rinsed with deionized water, and dried in a vacuum oven 5 h at 120 °C, 1 h at 150 °C, 1 h at 180 °C, 1 h at 200 °C, and 10 min at 250 °C. Solvent removal was confirmed by thermogravimetric analysis. o-Acetylhomopolyimides and o-acetylcopolyimides were converted to PBOs by thermal treatment in a

obtained in excellent purity on a multigram scale. oAcetylcopolyimides from ADHAB and 2,2-bis(3-amino-4hydroxyphenyl)hexafluoropropane (APAF) were obtained, by reaction with 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), with high molecular weight using the in situ silylation method (Scheme 1). All the polyimides were synthesized by chemical imidization. For comparative purposes, the o-acetylhomopolyimide from APAF and 6FDA was prepared. The copolyimides were characterized by spectroscopic methods, and their thermal properties were determined by DSC and dynamic and isothermal TGA. Finally, the aim of this work was to figure out the fundamental aspect of degradation at the thermal rearrangement temperature on the calculation of thermal conversion and thus fractional free volume to find out the effect of the incorporation of adamantly group on the transport properties of small gases and thus to compare the gas separation properties of precursor polymers and TR-PBO polymers containing adamantyl groups and 6F groups in TR-PBOs.

2. EXPERIMENTAL SECTION Materials. Concentrated nitric acid (65%), glacial acetic acid, toluene, methanol, dichloromethane, and chloroform were reagent grade and were used without purification. Phenol (99%), 2adamantanone (99%), methanesulfonic acid (99%), 3-mercaptopropionic acid (99%), trifluoromethanesulfonic acid (98%), hydrazine monohydrate (98%), palladium 10 wt % on activated carbon, chlorotrimethylsilane (CTMS), pyridine (Py), N,N-dimethylaminopyridine (DMAP), N,N-dimethylacetamide (DMAc), and N-methyl-2pyrrolidinone (NMP) were purchased from Aldrich and were used as received. 2,2-Bis(3-amino-4-hydroxyphenyl)hexafluoropropane (APAF) and 2,2-bis(4-carboxyphenyl)hexafluoropropane (6F) were purchased from TCI Europe. 2,2-Bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) was provided by Cymit ́ Quimica. APAF and 6FDA were sublimed at 220 °C at high vacuum (0.05 mmHg) prior to use. Synthesis of 2,2-Bis(4-hydroxyphenyl)adamantane. For this synthesis, 6.0 g (40 mmol) of 2-adamantanone was added to 25 mL of toluene and 9.4 g (100 mmol) of molten phenol at 50 °C under a nitrogen atmosphere, and the mixture was stirred until it became homogeneous. Then, 0.3 mL (3.4 mmol) of 3-mercaptopropionic acid, 3.0 mL of methanesulfonic acid, and 0.3 mL of trifluoromethanesulfonic acid was added dropwise. The reaction mixture was kept at 50 °C for 12 h, during which a white solid precipitated. The precipitate was filtered, washed with hot water, and recrystallized from ethanol to afford colorless needles. 78% yield; mp (316 °C). 1H NMR (400 MHz, DMSO-d6) δ 9.01 (s, 2H), 7.18 (d, J = 8.7 Hz, 4H), 6.60 (d, J = 8.7 Hz, 4H), 3.16 (s, 4H), 1.91 (d, 4H), 1.73 (s, 2H), 1.64 (d, 6H). 13C NMR (100 MHz, DMSO-d6) δ 154.81, 139.85, 126.81, 114.74, 49.52, 38.25, 33.53, 31.90, 27.60. Synthesis of 2,2-Bis(3-nitro-4-hydroxyphenyl)adamantane. Concentrated nitric acid (9.9 mL) was added dropwise over 1 h to a stirred solution of 9.0 g (28 mmol) of 2,2-bis(4-hydroxyphenyl)adamantane in toluene (30 mL) and glacial acetic acid (30 mL) at 0 °C. After stirring an additional hour, the mixture was allowed to warm to room temperature and maintained for 2 h. The resulting dinitro compound was filtered, washed with cold water and methanol, and purified on a silica gel column with dichloromethane as eluent to give a yellow product. 70% yield. 1H NMR (400 MHz, DMSO-d6) δ 10.72 (s, 2H), 7.86 (d, 2H), 7.65 (dd, 2H), 7.04 (d, 2H), 3.23 (s, 2H), 1.82− 1.60 (m, 12H). 13C NMR (100 MHz, DMSO-d6) δ 149.88, 139.65, 137.42, 133.30, 122.37, 119.87, 49.43, 37.69, 33.00, 31.23, 27.20. Synthesis of 2,2-Bis(3-amino-4-hydroxyphenyl)adamantane (ADHAB). A three-necked flask was charged with 8.0 g (20 mmol) of 2,2-bis(3-nitro-4-hydroxyphenyl)adamantane, 100 mL of ethanol, and 0.8 g of 10% palladium on carbon (Pd/C). Hydrazine monohydrate (18 mL) was added dropwise to the mixture over a period of 30 min at C


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Macromolecules

Density was determined using a XS105 Dual Range Mettler Toledo balance coupled with a density kit based on Archimedes’ principle. Samples were weighed in air and into a known-density liquid (high purity isooctane). Measurements were performed at room temperature, and the densities were calculated from the following expression:

Carbolite tube furnace under high-purity nitrogen. To ensure complete imide ring closure, all samples were heated at a rate of 5 °C/min to 300 °C and maintained at this temperature for 1 h. Afterward, the films were heated at 5 °C/min to the final rearrangement temperature of either 350 °C for 1 h or 450 °C for 30 min and cooled to room temperature at a rate no greater than 10 °C/min. Thermally treated membranes were designated as TRX, where X indicates the final temperature applied to the samples. Films of HPAs were prepared from 10% w/w solutions in DMAc, filtered through a 3.1 μm glass-fiber syringe filter, and cast onto a leveled glass plate. The solvent was removed by evaporation at 60 °C overnight, and the resulting film was removed from the glass plate, rinsed with deionized water, and dried in a vacuum oven 5 h at 120 °C, 1 h at 150 °C, 1 h at 180 °C, and 1 h at 200 °C. HPA samples were converted to PBOs by thermal treatment in a Carbolite tube furnace under high-purity nitrogen. To ensure complete conversion of ohydroxy(polyamide) to polybenzoxazole, all samples were heated at a rate of 5 °C/min up to 375 °C and maintained at this temperature for 15 min. Afterward, they were slowly cooled to room temperature. The thermally formed PBO membranes made from HPAs were designated X-PBO, where X indicates the employed diamine (ADHAB or APAF). Characterization. 1H and 13C NMR spectra were recorded on a Varian Inova 400 at 400 and 100 MHz, respectively. A Spectrum-One PerkinElmer Fourier transform infrared spectrometer (FT-IR) with Universal ATR sampling accessory was used to characterize the polyimides and TR-PBO films. The scan range was from 4000 to 650 cm−1. The inherent viscosity of homopolyimides and copolyimides was measured at 30 °C using an automated Canon-Ubbelohde suspended level viscometer with NMP as the solvent. The polymer concentration was 0.5 g/dL. Computer simulations were carried out by first drawing the molecules in Hyperchem55 and then optimizing the molecular and intermediate structures at the AM1 level.56 Subsequently, electronic energies of the optimized geometries were calculated by density functional theory (DFT) (without any geometrical constraint (use of Opt keyword) for starting molecules and final molecules) using the Becke’s three-parameter hybrid function with the 6-31G(d,p) basis set (B3LYP/6-31G(d,p))57 by means of the Gaussian 09 program.58 Molecular depictions were created using the Arguslab 4.01 freeware program59 and the Gaussview 5 program.60 Differential scanning calorimetric (DSC) data were obtained on a TA Instruments DSC Q-2000 Analyzer. Experiments were conducted at a heating rate of 20 °C/min under nitrogen, using approximately 6 mg of sample in hermetic aluminum pans. Thermogravimetric analyses (TGA) and differential thermogravimetric analyses (DTG) were performed on a TA Instruments Q-500 Thermobalance. Dynamic ramp scans were run at 10 °C/min under nitrogen (60 mL/min), using approximately 5 mg of sample. Isothermal thermogravimetric analyses were performed to establish thermal conditions for the preparation of TR samples. Thereby, polyimide films were thermally treated at 300 °C for 1 h and then heated to the desired temperature (350 and 450 °C) and held isothermally. Thermogravimetric analyses/mass spectrometry (TGA-MS) was conducted on a TA Q-500 thermobalance (TA Instruments) combined with a mass spectrometer (MS) ThermoStar GSD 301T (Pfeiffer Vacuum GmbH, Germany). Dynamic ramp scans were run at 10 °C/min. Interchain distances of the copolyimides and TR-PBO membranes were determined by wide-angle X-ray scattering (WAXS) at room temperature with a Bruker D8 Advance system fitted with a Goebel mirror and a PSD Vantec detector. A Cu Kα (λ = 1.542 Å) radiation source was used. A step scanning mode was employed for the detector from 2° to 55°, with a 2θ step of 0.024° and a scan rate of 0.5 s per step. The average d-spacing was obtained from Bragg’s equation:

nλ = 2d sin θ

ρsample = ρliquid

Wair − Wliquid Wair

(2)

The density data were used to evaluate chain packing using the fractional free volume (FFV), which was calculated using the following relation: FFV =

Ve − 1.3Vw Ve

(3)

where Ve is the polymer specific volume and Vw is the van der Waals volume, which was obtained by molecular modeling applying the semiempirical method Austin Model 1 (AM1) in the Hyperchem computer program, version 8.0. Gas permeation properties were determined for single gas feeds using a constant volume/variable pressure apparatus at 30 °C. The initial downstream pressure was maintained below 10−2 mbar, while the upstream pressure was kept at 3.0 bar for all gases. He, O2, N2, CH4, and CO2 were used. The purities of CH4 and O2 were greater than 99.95%, and the others gases were greater than 99.99% pure. Helium permeation tests at three upstream pressures (1, 3, and 5 bar) were carried out to verify the absence of pinholes. Permeability values (P) were determined from the slopes of downstream pressure vs time (dp(t)/dt), plotted when steady state had been achieved, according to the expression

P=

273 Vl dp(t ) 76 ATp0 dt

(4)

where A (cm2), V (cm3), and l (cm) are respectively the effective area, the downstream volume, and the thickness of the film; T is the temperature in K; and p0 (cmHg) is the pressure of the feed gas in the upstream chamber. P is usually expressed in barrer (1 barrer = 10−10 (cm3 (STP) cm)/(cm2 s cmHg). The ideal selectivity for a gas pair was calculated from the ratio of the pure gas permeabilities PA and PB. αA/B =

PA PB

(5)

3. RESULTS AND DISCUSSION Chemical reactivity of ADHAB was calculated by quantummechanical calculations using the semiempirical method AM1 and the DFT method using the B3LYP/6-31G(d,p) basis set (Table 1). These calculations also showed that the conformaTable 1. Electronic Parameters of Aromatic oHydroxydiamines polymer

HOMO energy (AM1) (eV)

HOMO energy (B3LYP/6-31G(d,p)) (eV)

ADHAB APAF HAB mHAB

−8.59 −8.30 −8.02 −8.27

−5.03 −5.16 −4.93 −4.96

tional freedom of aromatic rings in ADHAB is severely restricted; thus, ADHAB is a very rigid moiety compared with analogous structures. APAF showed the lowest value of the highest occupied molecular orbital (HOMO) energy and, hence, was expected to be a less nucleophilic monomer, from both AM1 and B3LYP/631G(d,p) calculations. This finding was expected due to the

(1)

where d is the d-spacing, θ is the scattering angle, and n is an integer number related to the Bragg order. D


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Macromolecules presence of the electron-withdrawing hexafluoropropane or 6F groups. ADHAB gave a HOMO energy (from both AM1 and B3LYP/6-31G(d,p)) very close to that of 3,3′-diamino-4,4′dihydroxybiphenyl (mHAB) and slightly lower than that of 3,3′-dihydroxybenzidine (HAB) (HAB and mHAB are highly reactive o-dihydroxydiamine monomers employed for making TR-PBOs). Thus, on the basis of electronic features, ADHAB should produce higher molecular weight polymers than APAF. Monomer Synthesis. The diamine monomer 2,2-bis(3amino-4-hydroxyphenyl)adamantane (ADHAB) was synthesized using an efficient three-step method (Figure 1). 2,2-Bis(4-

Figure 2. 1H NMR spectrum of ADHAB (400 MHz, DMSO-d6).

increase the reactivity of the amino groups. In the second step of the process, poly(amic acid)s were chemically imidized by the addition of a mixture of acetic anhydride and Py. As a result of the imidization method, the resulting polyimides had acetylated hydroxyl groups. Precursor o-acetylpolyimides showed values of inherent viscosity between 0.5 and 0.7 dL/g (Table 2), which were satisfactory for the preparation of dense membranes. Table 2. Inherent Viscosity and Thermal Properties of Precursor o-Acetylpolyimide Membranes

Figure 1. Synthesis of 2,2-bis(3-amino-4-hydroxyphenyl)adamantane (ADHAB).

sample

hydroxyphenyl)adamantane was prepared by condensation of 2-adamantanone with excess phenol in the presence of very low pKa acids and a thiol derivative. This reaction took place easily with aromatics when a superacid is used. Because the aromatic ring is activated in phenol, a mixture of a strong acid (methanesulfonic acid) and a small amount of superacid (trifluoromethanesulfonic acid) gave the best condensation result. Moreover, it has been shown that the addition of thiols as cocatalysts improves both the rate of reaction and the selectivity to the desired isomer,61,62 and thus, a small catalytic amount of 3-mercaptopropionic acid was employed. Subsequently, direct nitration with nitric acid in glacial acetic acid afforded 2,2-bis(3-nitro-4-hydroxyphenyl)adamantane in excellent yield and purity. Finally, catalytic reduction of the nitro derivative with hydrazine monohydrate and 10% Pd/C as catalyst produced, after recrystallization, a high purity monomer in good yield. Structures of the diamine and synthetic intermediates were confirmed by 1H and 13C NMR spectroscopy (Figure 2 and Figure 2-SI). Synthesis and Characterization of Precursor Polyimides and Copolyimides. Precursor polyimides were prepared using a two-step low-temperature polycondensation method, where an initial condensation reaction of diamine (ADHAB and/or APAF) and dianhydride (6FDA) was carried out in a chemical imidization process (Scheme 1). Polymer APAF-6FDA is a well-known polyimide, used in the preparation of TR polymers, and has been prepared in this case as reference polymer. In order to obtain high-MW poly(amic acid) intermediates, in situ silylation of the diamines was used.63,64 The in situ silylation method required the use of a silylating agent (CTMS) and Py and DMAP as activating reagents to

ADHAB6FDA AD(0.75) AD(0.5) AD(0.25) APAF6FDA

ηinh (dL/g)

Tg (°C)

theor wt lossa (%)

measd wt lossb (%)

measd wt lossc (%)

0.53

285

20.4

17.6

20.1

0.49 0.56 0.61 0.70

270 267 252 240

20.3 20.2 20.1 20.0

14.8 13.5 12.3 11.6

16.1 15.3 13.4 12.5

a

Theoretical weight loss of polybenzoxazole formation from oacetylpolyimides. bDetermined by dynamic TGA at a heating rate of 10 °C/min under a nitrogen atmosphere from 250 to 450 °C. c Determined by isothermal TGA at 450 °C for 30 min.

A similar in situ silylation methodology was employed to prepare HPAs, which were used to test the thermal stability of polybenzoxazoles. In this case, DMAP was not added in order to decrease the reactivity of the silylated o-hydroxydiamines and hence avoid formation of cross-linked structures by reaction of hydroxyl moieties with the electrophilic monomer. This synthetic methodology has been used in our group for a long time, and polyamides ranging from high to very high MW are obtained.65 Chemical structures of the precursor polyimides and copolyimides were confirmed by 1H NMR and FT-IR. All polyimides showed characteristic absorption bands: asymmetric and symmetric stretching vibration bands of CO (1780 and 1720 cm−1), stretching of C−N (1370 cm−1), transverse stretching of C−N−C groups at 1100 cm−1, and out-of-plane bending of C−N−C groups at 720 cm−1. ADHAB polyimide and copolyimides presented a C−H stretching band of the adamantane moiety around 2900 cm−1, and the other vibrational bands were overlapped by bands of the polymers. Additionally, C−F stretching of the hexafluoroisopropylidene moiety was represented by absorption peaks at 1250−1100 E


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Macromolecules cm−1. 1H NMR confirmed the chemical structures of precursor polyimides, as can be seen in Figure 3 for ADHAB-6FDA

differential TGA (DTG) curve, was 0.08%/min, while it was higher for ADHAB-PBO, 0.20%/min. Thus, after 30 min at 450 °C, APAF-PBO underwent a weight loss of 3.4%, while ADHAB-PBO lost 5.7%. Therefore, it can be concluded that (a) both PBOs suffer a certain amount of degradation at the TR temperature, (b) the thermal stability of the ADHAB-PBO is lower than that of the APAF-PBO, which can probably be attributed to partial degradation of adamantane moieties, and (c) as the theoretical weight loss corresponding to complete elimination of adamantane would be 17.9%, the degradation that takes place during this thermal treatment is only partial. Moreover, the weight losses were low at 425 °C and negligible at 400 °C.50 It is well accepted that the thermal conversion of ohydroxypolyimides or o-acylpolyimides to TR-polybenzoxazoles is not a clean process due to the high temperatures employed for the rearrangement. Therefore, plausible thermal processes for o-acetyl-PIs substituted with adamantane groups could include the following: conversion of the o-acetylimides to benzoxazole groups;46,69 conversion of the o-acetylimide groups to o-hydroxyimides and thermal conversion of these groups to benzoxazole units;69 conversion of the o-hydroxyimide moieties to lactams;35,70 conversion of the o-acetylimide moieties to lactams;35 cross-linking; degradation of nonrearranged imide moieties; degradation of the adamantane moieties; degradation of the 6F groups and other moieties; and other types of degradation processes. Thus, the TR process, although it is commonly simplified in published studies, is very complex and dependent on the moieties included in the precursor structures, the treatment temperature, and the residence time at that temperature. For instance, alkyl groups should have a much lower degradation temperature than aromatic groups; hence, more weight loss would be expected. In contrast, cross-linking associated with the TR process should bring about an increase in thermal stability. Furthermore, when very rigid and stable structures (multiaromatic condensed moieties) are used, the thermal conversion starts at higher temperature and overlaps with the degradation, and it is cumbersome to separate these processes.38 In our case, the set of TR materials has adamantane groups that possess a higher thermal resistance than common aliphatic moieties.48,50,52,71 However, as commented above, isothermal treatment of our reference ADHAB-PBO at 450 °C highlighted a partial degradation, with a weight loss around 6%. On the other hand, thermal treatment of APAF-PBO showed a weight loss greater than 3%. Therefore, a reasonable way of treating these materials should take into account degradation determined from the most stable PBOs made from the corresponding HPA. Figure 4 shows TGA and DTG curves for the precursor polymers at a heating rate of 10 °C/min under a nitrogen atmosphere. TGA scans of TR polymers typically display a twostage mass loss. The first stage, between 300 and 450 °C, is associated primarily with formation of the polybenzoxazole structure by thermal rearrangement, and the second stage, starting around 450−490 °C, is attributed to thermal degradation of the formed polybenzoxazole backbone and degradation of nonrearranged precursors. As seen in the figure, the initial weight loss related to the thermal rearrangement process occurred at a slightly higher temperature for ADHAB6FDA; the temperature at the maximum conversion rate for this polymer was around 385 °C, while that for APAF-6FDA was 370 °C, according to the DTG curves. Previous studies

Figure 3. 1H NMR spectra (400 MHz, CDCl3) and peak assignments for ADHAB-6FDA, AD(0.5), and APAF-6FDA.

(AD(100)), APAF-6FDA (AD(0)), and AD(0.5) polyimides. Protons from the acetate functionality appear at 2.07−2.14 ppm. Aromatic proton resonances produce peaks between 7.2 and 8.0 ppm, with signals corresponding to the hydrogens of phenyl moieties of the diamines shifted upfield relative to signals related to phenyl groups of the dianhydride. Aliphatic signals ascribed to the adamantane could also be observed. No peak above 10 ppm was observed, confirming complete acetylation of hydroxyl groups. Thermal Properties of Precursor Polyimides. Glass transition temperatures (Tg) of the polyimides considered in this study were determined by DSC (Table 1). A single Tg was observed for each copolyimide, consistent with that observed for typical random copolymers. Tg increased with ADHAB content, plausibly due to the increase of polymer chain rigidity imparted by the bulky adamantyl groups. In order to quantify the thermal stability of TR-PBOs and to estimate the amount of degradation that takes place during the TR process, PBO samples, synthesized by reaction of 6FCl with APAF and ADHAB followed by thermal treatment at 375 °C for 15 min for conversion to polybenzoxazoles, were thermally evaluated by performing isothermal TGAs under nitrogen at 450 °C for 2 h. The idea was to evaluate these materials at that high temperature to quantify the thermal degradation of authentic polybenzoxazoles. Polybenzoxazoles can be formed by chemical cyclization of o-hydroxypolyamides in a one-step reaction in several media such as polyphosphoric acid or Eaton’s reagent (a mixture of methanesulfonic acid and phosphorus pentoxide)66,67 or by thermal treatment at temperatures below 400 °C. In this work, the thermal methodology was used to attain PBOs68 (TGA (Figures 3-SI and 4-SI) and differential TGA curves (DTG) (Figures 5-SI and 6-SI) of HPAs and PBOs made from HPAs are included in the Supporting Information). It was observed that thermal degradation of these PBOs, under dynamic conditions, began at very similar temperatures: 490 °C for ADHAB-PBO and 500 °C for APAF-PBO. Nevertheless, when the materials were isothermally treated at 450 °C, the degradation of the PBOs was dependent on the polymer structure. The maximum degradation rate for APAF-PBO, derived from the isothermal F


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Macromolecules

have shown that the degree of conversion at a given temperature is strongly dependent on precursor polyimide rigidity and chemical structure. Therefore, the presence of bulky substituents could restrict the mobility of the polymer chains, hindering the rearrangement process. Moreover, a new region of weight loss was observed between the two previously commented in the polymers bearing ADHAB, which depended on the proportion of this monomer. This loss was observed at a temperature close to 440 °C and could be associated with degradation of the adamantane moiety. Different proportions of adamantane caused significant differences in the weight loss rates (rWL) determined from DTG curves. For ADHAB-6FDA, rWL was 0.33%/°C at 441 °C, whereas a very low value of rWL = 0.06%/°C was found for APAF-6FDA at the same temperature. The values for the copolymers were between the homopolymer values; AD(0.75) rWL = 0.20%/°C, AD(0.5) rWL = 0.14%/°C, and AD(0.25) rWL = 0.09%/°C at the same temperature. ADHAB-6FDA exhibited the highest weight loss at 450 °C (18.5%) compared to APAF-6FDA (11.5%), while the

Figure 4. Thermogravimetric analysis of precursor polymers considered in this study.

Figure 5. Isothermal thermogravimetric analysis of ADHAB-6FDA, APAF-6FDA, precursor o-acetyl-PI membranes, and their copolymers under a nitrogen atmosphere. G


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Macromolecules

Table 3. TR Conversion of Precursor o-Acetyl-PIs and TRPBO Derived from Them

copolyimides showed intermediate weight loss values (Table 2). However, in spite of the partial degradation of adamantane, in all of the cases, these values were below the theoretical value of full conversion to the final PBO structure (20.4% for ADHAB-6FDA and 20.0% for APAF-6FDA), which indicates that a dynamic run at 10 °C/min is not sufficient to obtain full thermal rearrangement at 450 °C. Consequently, we must consider whether the degradation process observed for APAF-PBO and ADHAB-PBO should be included within the weight losses of HPIs. It is clear that the TGA data of these PBOs could provide better knowledge of the thermal processes involved, and consequently, this information could permit to design new materials based on thermal treatment of o-hydroxy- or o-acetyl-PIs. To optimize the thermal treatment variables for TR preparation in the tubular furnace, an isothermal study was performed by heating film samples to the desired temperature (350, 400, 425, and 450 °C) at a heating rate of 5 °C/min and holding them at that temperature for 3 h. Figure 5 shows isothermal thermograms of the precursor polyimides, showing weight loss as a function of time at a given temperature. The theoretical mass loss for complete PBO conversion is shown by dashed lines: 20.4% for ADHAB-6FDA and 20.0% for APAF-6FDA. As expected, weight loss increased with rearrangement temperature. At 350 and 400 °C, the weight loss was low and remained very far from the theoretical value. In contrast, at 450 °C, all of the samples exceeded the theoretical value at treatment times that depended on the copolymer structure, which could denote degradation processes. Thus, it is obvious that at high temperatures and prolonged heating times thermal degradation is likely to occur concurrently with the thermal rearrangement process, as several studies have suggested. Based on this data, thermal treatment protocols were selected to ensure that samples with thermal histories similar to those reported in previous studies experienced minimal thermal degradation and also that the mechanical properties were sufficient to permit their characterization as gas separation membranes. Two rearrangement temperatures and times were chosen: 350 °C for 1 h and 450 °C for 30 min. The percent conversion of precursor polyimides to TR-PBO membranes was estimated based on TGA data using eq 6: % TR conversion =

actual weight loss × 100 theoretical weight loss

polymer code

TR conversiona (%)

TR conversionb (%)

ADHAB-6FDA PI250 ADHAB-6FDA TR350 ADHAB-6FDA TR450 AD(0.75) PI250 AD(0.75) TR350 AD(0.75) TR450 AD(0.5) PI250 AD(0.5) TR350 AD(0.5) TR450 AD(0.25) PI250 AD(0.25) TR350 AD(0.25) TR450 APAF-6FDA PI250 APAF-6FDA TR350 APAF-6FDA TR450

0 22 99 0 26 79 0 20 76 0 30 67 0 34 63

0 22 71 0 26 54 0 20 53 0 30 47 0 34 46

TR conversion considering no degradation of TR-PBOs at 450 °C/ 30 min. bTR conversion considering degradation of PBOs made from HPAs at 450 °C/30 min (5.7% for ADHAB-PBO and 3.4% for APAFPBO).

a

% TR conversion actual weight loss − PBO observed weight loss = × 100 theoretical weight loss (7)

where PBO observed weight loss is the observed degradation weight loss of PBOs made by thermal treatment of HPAs (5.7% for ADHAB-PBO and 3.4% for APAF-PBO, taking into consideration the molar fraction of each repeat unit in the copolymers). For additional information, TGA-MS spectra of ADHAB6FDA and APAF-6FDA have been included in the Supporting Information (Figures 7-SI and 8-SI). Characterization of Thermally Rearranged Polybenzoxazole (TR-PBO) Membranes. Structural changes in precursor polyimides during thermal treatment were monitored using FTIR. Figure 6 presents ATR-FTIR spectra of precursor polyimides and thermally treated samples. Samples treated at 350 °C gave new peaks at wavenumbers around 1620 cm−1 (CN stretching, oxazole I) and 1475 and 1060 cm−1 (−O−C stretching), which are characteristic of the PBO structure. The presence of broad hydroxyl peaks at 3400 cm−1 in these samples seems to indicate that some of the acetate groups were converted to hydroxyl groups during the thermal rearrangement at this temperature. When the rearrangement temperature was increased to 450 °C, PBO bands were more intense for all of the samples; in some cases, the appearance of a new band at 1555 cm−1 associated with benzoxazole (oxazole II) was observed. In addition, the imide peaks substantially decreased, and the absorption band from hydroxyl groups around 3400 cm−1 disappeared. There is not general agreement on the mechanism for the thermal rearrangement of precursor polyimides with acyl groups. Recently, a parallel mechanism that suggested the formation of rigid aromatic lactams as well as polybenzoxazoles during the TR process was proposed based on experimental and theoretical data.35,70,72 Kostina et al. reported the appearance of bands at 1670 cm−1 (amide I) and 1550 cm−1 (amide II), attributable to lactams, in

(6)

This calculation is based, as all authors have considered in previous TR studies, on the assumption that any polyimide segment that undergoes thermal rearrangement is converted to the corresponding polybenzoxazole segment.8,19 Conversion values for o-acetyl polyimides and o-acetyl copolyimides treated at 450 °C/30 min are collected in Table 3. At 350 °C, very low degrees of conversion were obtained for all of the precursor films, in accordance with dynamic TGA curves. At 450 °C, the conversion of ADHAB-6FDA was almost complete, whereas for APAF-6FDA, conversion was 63%. This result could be due to the differences in rWL. Copolymers exhibited the same behavior: the rWL and the PBO conversion at 450 °C decreased as APAF content increased. However, if weight loss associated with the degradation of ADHAB-PBO and APAF-PBO was considered, the actual conversion rate would be lower in all cases. Thus, in Table 3, both thermal conversion values are depicted. eq 7 was employed to calculate the corrected TR conversions: H


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Macromolecules

Figure 6. ATR-FTIR spectra of membranes of precursor o-acetylpolyimides and copolyimides and TR-PBO analogues obtained at different temperatures.

samples of o-hydroxypolyimides treated at temperatures above 400 °C.72 Some of the copolymers treated at 450 °C in this study showed a small band at 1675 cm−1 that could be related to amide I of lactam, but in all of the cases, the intensity of this peak was low compared to the imide and benzoxazole bands. It is important to note that a band around 2900 cm−1, associated with the adamantane moiety, was present in all of the copolymers after thermal treatment, indicating that if degradation of this moiety took place, it was minimal. Wide-angle X-ray scattering patterns of thermally treated membranes and precursor polyimide films are compared in Figure 7. All samples showed a broad amorphous halo, which indicates the amorphous nature of the membranes. The preferential intersegmental distance (d-spacing) was calculated according to Bragg’s equation (eq 1), and data are presented in Table 4. All precursor polyimides exhibited similar preferential intersegmental distances, with values around 0.55−0.57 nm. Thermal rearrangement from o-acetyl-PIs to TR-PBOs at both rearrangement temperatures resulted in slightly larger values of the maxima, but a significant increase in the contribution of low angle values was observed, which implies an increase in larger intersegmental distances. The broadening of signals was strongly dependent on the treatment temperature.

Fractional free volumes were initially estimated with eq 8, which Liu et al. proposed for the calculation of van der Waals volumes (VW) for partially rearranged TR copolymers: VW = wADHAB[cADHABVW,ADHAB‐PBO + (1 − cADHAB) × VW,ADHAB‐PI] + (1 − wADHAB)[cAPAFVW,APAF‐PBO + (1 − cAPAF)VW,APAF‐PI]

(8)

where wADHAB is the mass fraction of ADHAB-6FDA in the copolymer, and cADHAB and cAPAF are the fractional TR conversions of ADHAB-6FDA and APAF-6FDA, respectively, from eq 6 (Table 2). VW,ADHAB‑PI and VW,ADHAB‑PBO are the van der Waals volumes of ADHAB-6FDA polyimide and its corresponding PBO (cm3/g), respectively. Likewise, VW,APAF‑PI and VW,APAF‑PBO are the van der Waals volumes of APAF-6FDA polyimide and its corresponding PBO (cm3/g), respectively.46 Use of this equation for TR materials is rather cumbersome because no degradation is taken into account and because, during the conversion to polybenzoxazole, a portion of the oacetylimide groups are converted to o-hydroxylimide groups, which should evolve to benzoxazole moieties. FFV calculations were performed following the methodology of Liu et al.,46 which assumes that both ADHAB-6FDA and APAF-6FDA moieties in a copolyimide will undergo the same I


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Figure 7. Wide-angle X-ray diffraction (WAXD) patterns of membranes of precursor o-acetylpolyimides and TR-PBO derived from them.

Table 4. Physical Properties of Precursor o-Acetyl-PIs and TR-PBOs polymer code

d-spacing (nm)

density (g cm−3)

Vwa (cm3 g−1)

FFVb

Vwc (cm3 g−1)

FFVd

ADHAB-6FDA PI250 ADHAB-6FDA TR350 ADHAB-6FDA TR450 AD(0.75) PI250 AD(0.75) TR350 AD(0.75) TR450 AD(0.5) PI250 AD(0.5) TR350 AD(0.5) TR450 AD(0.25) PI250 AD(0.25) TR350 AD(0.25) TR450 APAF-6FDA PI250 APAF-6FDA TR350 APAF-6FDA TR450

0.57 0.57 0.59 0.57 0.58 0.62 0.56 0.57 0.61 0.56 0.57 0.61 0.55 0.57 0.64

1.318 1.314 1.254 1.355 1.335 1.302 1.391 1.375 1.335 1.423 1.404 1.370 1.458 1.449 1.420

0.470 0.473 0.487 0.456 0.460 0.466 0.442 0.444 0.449 0.429 0.431 0.432 0.415 0.416 0.416

0.195 0.192 0.207 0.197 0.203 0.211 0.200 0.206 0.220 0.207 0.214 0.230 0.212 0.216 0.232

0.470 0.473 0.482 0.456 0.460 0.463 0.442 0.444 0.449 0.429 0.431 0.432 0.415 0.416 0.416

0.195 0.192 0.215 0.197 0.203 0.216 0.200 0.206 0.220 0.207 0.214 0.231 0.212 0.216 0.231

a

van der Waals volumes calculated from TR conversion (ratio of polyimide and polybenzoxazole) considering no degradation of TR-PBOs at 450 °C/30 min. bFFV calculated from TR conversion (ratio of polyimide and polybenzoxazole) considering no degradation of TR-PBOs at 450 °C/30 min. cvan der Waals volumes calculated from TR conversion (ratio of polybenzoxazole and polyimide) considering degradation of TR-PBOs at 450 °C/30 min (5.7% for ADHAB-6FDA and 3.4% for APAF-6FDA). dFFV calculated from TR conversion (ratio of polybenzoxazole and polyimide) considering degradation of TR-PBOs at 450 °C/30 min (5.7% for ADHAB-6FDA and 3.4% for APAF-6FDA).

extent of TR conversion at a given thermal treatment (cADHAB = cAPAF). However, TGA curves indicated different rates of weight loss for ADHAB-6FDA and APAF-6FDA. ADHAB-6FDA seems to have a higher rWL, which suggests a higher TR

conversion compared to APAF-6FDA segments if no degradation is considered. Despite this, the difference between the VW of polyimides and their corresponding PBOs was less than 3.6% for ADHAB-6FDA PI and its corresponding PBO, J


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Macromolecules Table 5. Gas Permeation Properties of Precursor o-Acetyl-PIs and Thermally Treated Membranes permeabilities (barrer) polymer code ADHAB-6FDA

AD(0.75)

AD(0.5)

AD(0.25)

APAF-6FDA

PIHPI TR350 TR450 HPI TR350 TR450 HPI TR350 TR450 HPI TR350 TR450 HPI TR350 TR450

ideal selectivities

He

O2

N2

CH4

CO2

αCO2/CH4

αO2/N2

αCO2/N2

53 100 178 60 123 240 66 146 308 109 169 485 126 187 645

4.5 8.7 35 5.1 9.4 51 6.7 10 68 8.7 12.1 152 12 19 195

0.74 1.4 7.3 0.89 1.6 11 1.1 1.7 16 1.5 2.0 38 2.3 3.5 48.6

0.48 0.40 3.6 0.59 0.60 5.9 0.67 0.73 11 0.87 0.80 26 1.2 1.3 38.4

20 27 151 23 35 235 27 43 406 35 51 702 51 65 974

42 68 42 39 58 40 40 59 37 40 64 27 43 50 25

6.1 6.2 4.8 5.7 5.9 4.6 6.1 5.9 4.3 5.8 6.1 4.0 5.2 5.4 4.0

27 19 21 26 22 21 24 25 25 23 26 18 22 19 20

Figure 8. CO2 permeability and CO2/CH4 selectivity as a function of treatment temperature for all precursor o-acetyl-PIs and TR polymers.

between 7 and 11% at 450 °C (3% and between 7 and 12% when the corrected TR conversion is included). Gas Permeation Properties. Table 5 presents permeability coefficients and ideal selectivities of the polymers at 35 °C and an upstream pressure of 3 bar. For all of the precursor polyimides and thermally rearranged polymers, good correlation of gas permeability with APAF content was observed. Permeability coefficients of all gases increased with APAF content, which could be associated with the increase in FFV, probably due to disruption of polymer chain packing imparted by the bulky 6F units in APAF. Pure-gas selectivities of CO2/ CH4 and O2/N2 were quite similar for precursor polyimides, irrespective of APAF content, but decreased in the thermally treated polymers at 450 °C as the APAF content was increased. Gas selectivity values for CO2/CH4 and O2/N2 gas pairs initially increased at low TR conversions before decreasing at higher TR conversions (450 °C). This decrease was very important compared to the initial values for APAF-6FDA and AD(0.25), while it was lower for the other TR converted polymers. These effects can also be seen in Figure 8, which presents pure CO2 permeability and CO2/CH4 selectivity as a function

whereas for APAF-6FDA PI and APAF-6FDA PBO, this difference was less than 0.45%. Therefore, the assumption that both segments achieve the same TR conversion has a minor influence on the global FFV values. The estimated FFV values of precursor polyimides and TR-PBO polymers are displayed in Table 4. In addition, we calculated a modified FFV following two assumptions: (1) total conversion of acetate to hydroxyl groups and final conversion to TR-PBO and (2) 5.7% degradation for ADHAB-6FDA and 3.4% degradation for APAF-6FDA. The corrected FFVs are depicted in the last column of Table 4. FFV increases with APAF content in the precursor copolyimides, and this trend can be attributed to the extra 6F groups in APAF that cause considerable hindrance to polymer chain packing. Consequently, it appears that the effect of the 6F moiety is greater than the effect of the adamantane groups. Similar to previous reports in other TR polymers, FFV increased after TR conversion, suggesting that the thermal rearrangement process leads to a more open morphology (FFV, along with d-spacing, is considered an index of the degree of openness of the polymer matrix and correlates with gas transport properties). This increase was around 3% for samples treated at 350 °C and K


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Macromolecules of temperature treatment. CO2 permeability widely increased in parallel with TR conversion. It should be commented at this point that the gas separation properties of TR materials are very dependent on several factors such as molecular weight of the PI, solvent employed in the casting, the amount of solvent present in the PI precursors, purity of the inert gas, thermal treatment and residence time of the sample at the final temperature, and so on. Thus, the final gas separation properties of these TR materials could be quite different. Regarding APAF-6FDA, Kim et al. tested a acetyl APAF-6FDA having CO2 permeability above 5000 barrer after its thermal treatment at 450 °C for 1 h.22 For the same o-acetylHPI precursor, Liu et al. gave a CO2 permeability value for the APAF-6FDA TR-PBO of 1400 barrer with a CO 2/CH4 selectivity of around 15.47 Therefore, the values observed in the bibliography show a strong dependence on the experimental conditions. Because of that, we need to prepare the reference TR-PBO in the same conditions than the other members of the series in order to obtain comparable results for all the set. The CO2/CH4 separation performance of the materials considered in this study can also be seen in Figure 9. Thermal

Figure 10. Relationship between O2 permeability and O2/N2 selectivity for precursor o-acetyl-PIs and TR-PBOs derived membranes. Open symbols represent precursor polyimides, half-filled symbols correspond to samples treated at 350 °C, and filled symbols belong to samples treated at 450 °C.

4. CONCLUSIONS A set of high molecular weight o-acetylcopolyimides have been prepared from a new TR monomer, 2,2-bis(3-amino-4hydroxyphenyl)adamantane (ADHAB), and 2,2-bis(3-amino4-hydroxyphenyl)hexafluoropropane (APAF) by reaction with 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA). The new monomer was easily synthesized in good yield and excellent purity from 2-adamantanone. Moreover, to accurately determine the inherent thermal degradation of these TR materials, two related PBOs were prepared by thermal treatment of o-hydroxypolyamides (HPAs), which were obtained by reaction of ADHAB and APAF with 2,2-bis(4chlorocarbonylphenyl)hexafluoropropane. These PBOs obtained from HPAs were studied by TGA, which showed thermal degradation processes occurring at the thermal rearrangement temperature (450 °C for 30 min). From these results, a new method for studying TR materials was proposed in this work that included the amount of observed degradation for authentic PBOs in the calculation of TR conversion and FFV determination. This method is very easy to be implemented, and it permits to determine in an accurate way the thermal degradation of the studied TR-PBO at the thermal treatment temperature, which can be translated to a better design of macromolecular structures able to be employed in many applications including gas separation. Finally, the precursor polyimides and the TR-PBOs prepared by thermal treatment were evaluated as gas separation membranes. A dependence between the amount of ADHAB present in the polymer and its final properties was observed. These TR-PBOs showed excellent separation properties for the CO2/CH4 gas pair, with values very close to the 2008 Robeson limit, even though the permeability values were lower for ADHAB materials than for the membrane made from APAF. The gas separation properties of these materials were sufficiently effective for them to be considered promising candidates for industrial applications.74 In this work, it was clearly stated that the bulky and cycloaliphatic adamantane group is a moiety able to be

Figure 9. Relationship between CO2 permeability and CO2/CH4 selectivity for precursor o-acetyl-PIs and TR-PBOs derived membranes. Open symbols represent precursor polyimides, half-filled symbols correspond to samples treated at 350 °C, and filled symbols belong to samples treated at 450 °C.

rearrangement at 350 °C led to highly selective polymers that exceed the 1991 upper bound.73 Increasing the temperature of the PBO rearrangement to 450 °C produced materials that approach the 2008 upper bound17 and, in the case of AD(0.5), surpass it. The trade-off plot for the O2/N2 separation is presented in Figure 10. The TR-PBO followed a well-defined tendency: permeability values increased with increased TR conversion, approaching the 1991 upper bound. The effect of APAF content is more noticeable at high rearrangement temperature (450 °C), an improvement in O2/N2 separation performance being observed as APAF content increased. This effect caused a shift to high permeabilities of samples treated at this temperature, such that polymers with higher APAF content, like AD(0.25) and APAF-6FDA, crossed the 1991 upper bound. L


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Macromolecules

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incorporated in TR materials attending to their excellent thermal stability.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02460. Schematics of HPAs and TR-PBOs, NMR characterization of ADHAB (1H NMR and 13C NMR), dynamic TGA of HPAs, isothermal TGA of HPAs treated at 375 °C (PBOs), and dynamic TGA-MS of o-acetyl-PIs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.M.L). *E-mail: [email protected] (J.G.C). *E-mail: [email protected] (A.E.L). ORCID

Young Moo Lee: 0000-0002-5047-3143 Á ngel E. Lozano: 0000-0003-4209-3842 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support provided by Spain’s MINECO (CTQ2013-48406-P, MAT2016-76413-C2-R1, and MAT2016-76413-C2-R2). This research was also supported by the Korea Carbon Capture & Sequestration R&D Center (KCRC) administered through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF2014M1A8A1049305). We are also indebted to the Spanish Junta de Castilla y León for funding help (Projects VA248U13 and VA256U13). C. Aguilar-Lugo gratefully acknowledges a CONACYT postdoctoral fellowship (264013).

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REFERENCES

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