CH4 separation

Journal Pre-proof Preparation of thermally rearranged poly(benzoxazole-co-imide) membranes containing heteroaromatic moieties for CO2/CH4 separation Feng Gan, Jie Dong, Xiaochen Xu, Mengmeng Li, Xin Zhao, Qinghua Zhang PII:

S0032-3861(19)30951-6

DOI:

https://doi.org/10.1016/j.polymer.2019.121945

Reference:

JPOL 121945

To appear in:

Polymer

Received Date: 16 September 2019 Revised Date:

23 October 2019

Accepted Date: 25 October 2019

Please cite this article as: Gan F, Dong J, Xu X, Li M, Zhao X, Zhang Q, Preparation of thermally rearranged poly(benzoxazole-co-imide) membranes containing heteroaromatic moieties for CO2/CH4 separation, Polymer (2019), doi: https://doi.org/10.1016/j.polymer.2019.121945. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Preparation of thermally rearranged poly(benzoxazole-co-imide) membranes containing heteroaromatic moieties for CO2/CH4 separation Feng Gan, Jie Dong*, Xiaochen Xu, Mengmeng Li, Xin Zhao, Qinghua Zhang* State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, P. R. China *Corresponding author: [email protected]; [email protected]

Abstract: Simultaneously achieving excellent mechanical properties and superior gas separation performance remains a great challenge for thermally rearranged polymers when applied as the membrane materials in large scale gas separation applications. Herein, different heteroaromatic non-TR-able codiamines containing benzimidazole/benzoxazole structures were incorporated into ortho-hydroxyl functionalized

polyimide

backbones,

which

were

subsequently

transformed

into

the

poly(benzoxazole-co-polyimide) copolymers by thermal rearrangement (TR). In all cases, the solid-state TR reaction induces the increased preferential intersegmental distances (d-spacing) and fractional free volumes of polymer chains, favorable for the gas permeation. While, the π-π stacking distance exhibits opposite variation tendencies for benzimidazole-based and benzoxazole-based TR-PBOI membranes, which is demonstrated to make a significant effect on the gas selectivity. These TR-PBOI membranes thermally treated at 420 °C for 1 h exhibit higher tensile properties in relative to most of previously reported TR-PBOI membranes with the tensile strength of 97 to 118 MPa and initial modulus of 2.0-2.4 GPa. Moreover, incorporating heteroaromatic non-TR-able codiamines endows the resultant TR-PBOI membranes with excellent gas separation properties for the CO2/CH4 gas pairs with the CO2 permeability and CO2/CH4 ideal selectivity values exceeding the 1991 upper bound and close to 2008 upper bound. We anticipate this facile method will facilitate the large-scale preparation and application of TR membranes for gas separation.

Keywords: poly(benzoxazole-co-imide); heterocyclic codiamine; gas separation

1

Introduction Natural gas consisting primarily of methane (CH4) is an indispensable fuel and chemical feedstock. Prior to the utilization of natural gas, an essential step is the removal of some impurities, e.g., the acid gas CO2, to improve the combustion efficiency and to prevent corrosion of the pipelines [1]. Due to the important implication for natural gas separating in energy conversion and applications, several approaches including amine adsorption [2], cryogenic distillation [3], and membrane separation [4-8] were explored and elaborated in recent years. Among of them, polymeric membrane-based gas separation technology has attracted great interests owing to its overall advantages in terms of high energy efficiency, operational simplicity, agile architecture designability, flexibility, smaller footprint and so on [9, 10]. In general, the efficacious CO2 selectivity, high permeability, good mechanical property as well as other chemophysical properties are all crucial parameters for an ideal gas separation membrane. Over past years, numerous polymer membranes such as cellulose acetate (CA) [11], polycarbonates (PC) [12], polysulfones (PSF) [13], polyamides (PA) [14], poly(1-trimethylsilyl-1-propyne) (PTMSP) [15], polyimides (PIs) [16, 17] and polybenzoxazole (PBO) [18, 19] are routinely used for natural gas purification (CO2/CH4), hydrogen recovery and nitrogen production from air (O2/N2). However, almost all of the reported polymer membranes face a limit of trade-off between gas permeability and selectivity, as expressed by the empirical upper bond proposed by Robeson in 1991 and subsequently revised in 2008 [20, 21]. To advance beyond these limits, recent efforts in developing gas separation membranes are mainly focused on (i) designing new polymer backbones [22], (ii) post-treatment of polymers [23, 24] and (iii) blending of fillers to produce mixed matrix membranes [25], and many of which have exhibited separation performance well beyond the Robeson's upper bonds. Among these polymer matrices, polyimides (PIs) have drawn considerable attention for membrane-based gas separation owing to the combination of properties, such as high mechanical property, superior thermal stability and excellent chemical resistance, making them suitable for use in some harsh conditions. Nevertheless, among commonly available polyimides, including Kapton, Upilex and Matrimid, always exhibit an unsatisfied gas separation performance, especially with a low permeability of <10 Barrer. Thus, much of the research work is being addressed to develop novel polyimide membrane materials with a high permeability and selectivity. Workable approaches to 2

improve the gas separation properties of polyimide membranes consist of fabricating highly microporous materials with high fractional free volume (FFV), such as Tröger's base (TB) polyimide backbones containing benzene rings fused by bicyclic units [26, 27], polyimides with intrinsic microporosity (PIM-PIs) featuring ladder-like structures [28], and thermally rearranged polybenzoxazole (TR-PBO) membranes derived from ortho-hydroxy polyimide precursors [4, 18, 19, 29, 30]. During the structural rearrangement of materials in solid state, the distortion of the imide moiety into a rigid-rod heterocyclic benzoxazole ring always leads to the significant configurational and conformational changes and formation of microcavity with diameters ranging from 3 Å to 9 Å and an increased molecular free volume, which are advantageous from the high gas permeability point of view, and many of which have shown separation properties well surpassing the upper bound limits [24]. In general, TR-PBO membranes exhibit more obvious advantages in large-scale gas separation applications because of their facile preparation, low cost, easily tailored microporous structures and superior gas separation properties compared to the Tröger's base polyimides and PIM-PIs, both of which are always prepared via rather complicated monomer synthesis and polymerization. Additionally, due to the elevated temperature in the thermal conversion of TR-PBOs, cross-linking reaction easily takes place in the materials, which improves the plasticization resistance and decreases the physical aging of the resultant materials. While, a drawback that can not be ignored for the TR polymers is their greatly deterioration of mechanical properties compared to their ortho-hydroxy polyimide precursors attributed to the possible chain decomposition at high temperatures, which seriously limits the stability and durability of the membrane for industrial gas separation. In this sense, it is urgent to improve the balance of properties of TR-PBO membranes for improving the gas separation performance, maintaining excellent mechanical behaviors and reducing the cost of precursor monomers as well as obtaining insight into relations between structure and properties for their widespread industrial implementation. Recently, several strategies have been adopted to fabricate mechanically strong TR polymer membranes, and the results were always promising. One attempted way is reducing the conversion temperature. A representative work was reported by Meis and the co-workers [31], and they demonstrated that using the allyl derivatives (allylation of the hydroxyl group in ortho-position to the imide group) rearrangement could reduce the conversion temperature by 200 °C, and the final highly mechanically stable TR membrane showed a permeability almost four times higher than that of the 3

original TR-PBO membrane. Another possible solution to fabricate mechanically strong TR membranes

is

via

the

copolymerization

of

well-designed

monomers

to

create

poly(benzoxazole-co-imide) (TR-PBOI) [32, 33], which means copolymerizing a commercial dianhydride and an ortho-hydroxy diamine with a non-ortho-hydroxy diamine (non-TR-able). In this way, the stiff polybenzoxazole segments and polyimide portions are simultaneously constructed in the backbone when treated under a high temperature. The gas separation performance and mechanical behavior of the TR-PBOIs can be synergistically regulated, in which the permeability of resulting membranes can be imparted by the TR-PBO and the selectivity along with the mechanical property can be optimized by the PI portion. Numerous TR-PBOI membranes have been successfully fabricated, and the effects of non-TR-able diamine chemical structures, copolymerization modes (random or block copolymerization) and thermal treatment condition as well as the additional cross-linking reaction on the gas separation and other chemophysical properties of the resulting TR-PBOI membranes have been investigated in detail [34-36]. For instance, Zhuang et al. [36] incorporated two different non-TR-able diamines containing benzoxazole and benzimidazole units into the ortho-hydroxy polyimides, the TR-PBOI membranes exhibited a high CO2 permeability of 114 Barrer, a high ideal selectivity for CO2/CH4 of 42, and a high tensile strengths of 113.9 MPa as well as an outstanding heat resistance (Tg reached 362 °C) by optimizing the polymerization condition, copolymerization mode and thermal holding time of the co-polyimide precursors. Jo et al. [33] prepared a series of TR-PBOIs with different backbone rigidities, and their results revealed that higher flexibility of the ortho-hydroxy polyimide precursor allowed the rearrangement to occur at a faster rate and at a lower temperature, and meanwhile, increasing the backbone rigidity easily led to an increase in ideal selectivity. Despite the above mentioned efforts, the effects of rigidity and functionality of non-TR-able diamines on the thermal rearrangement of TR-able components, chains packing behavior, the formation of microcavities and their impact on gas separation properties of the resultant TR-PBOI membranes are needed to be further explored comprehensively. Previously reported works have shown that increasing the polymer backbone rigidity is beneficial for improving membranes' selectivity [37, 38]. Benzimidazole and benzoxazole moieties are two typical

rigid-rod-like

structures

composed

of

connected

benzene

and

five-membered

imidazole/oxazole rings. Lee and the co-workers illustrated that polyimide membranes containing benzoxazole units exhibited an excellent combination of properties, such as good processibility, high 4

mechanical properties and superior gas separation performance [39]. Additionally, the incorporation of benzimidazole and benzoxazole moieties in polyimide backbones is expected to improve the oxidative stability of the co-polyimide precursors, which makes a great sense in decreasing the possibility of chains decomposition in the high temperature rearrangement [40]. In this study, co-polyimide precursors were synthesized by four different heterocyclic non-TR-able diamines containing

either

benzimidazole

or

benzoxazole

2,2'-bis(3-amino-4-hydroxyphenyl)hexafluoropropane

moieties,

(APAF)

with

and

the a

TR-able dianhydride

4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), and the TR-PBOI membranes were successfully prepared via the solid-state TR process. In particular, TR-PBOIs with the preformed bis-benzimidazole and bis-benzobisoxazole moieties are incorporated into the polymer backbones, and their effects on thermal rearrangement reaction, inter-chain packing behavior, microcavity size, mechanical behaviors, gas separation performances for some specific gas pairs (CO2/CH4 and O2/N2) of the resultant TR-PBOI membranes were systematically investigated and compared with those of the samples containing the similar benzimidazole and benzoxazole units. The higher content of benzimidazole or benzoxazole moieties in the polymer backbones is anticipated to endow the membranes with some exceptional properties. As a result, we extend our investigation to show that controlling the chemical structures of non-TR-able diamines is a promising way to produce TR membranes with both good enough gas separation property and superior mechanical strength.

Experimental Section Materials.

4,4'-(hexafluoroisopropylidene)

5-amino-2-(4-aminobenzene)

diphthalic

benzimidazole

5-amino-2-(4-aminobenzene)benzoxazole

(BOA,

anhydride (BIA,

99%)

were

(6FDA, 99%)

purchased

from

99%), and Sunlight

Pharmaceutical Co., Ltd. (Changzhou, China). The 2,2'-p-phenylenebis (5-aminobenzimidazole) (PBABI), 2,6-(4,4'-diaminodiphenyl) benzo[1,2-d:5,4-d'] bisoxazole (PBOA) were synthesized in our laboratory according to our previous works [41, 42]. 2,2'-Bis(3-Amino-4-hydroxy-phenyl) hexafluoropropane (APAF, 98%) was purchased from Energy Chemical Co., Ltd. (Shanghai, China). N-Methyl pyrrolidone (NMP) was obtained from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). All monomers were dried in vacuum at 60 °C prior to use. Synthesis of ortho-hydroxy co-polyimide precursors. The objective ortho-hydroxy 5

co-polyimide (co-PI) precursors were synthesized by using dianhydride 6FDA and the diamine APAF with four different non-TR-able diamines, BIA, PBABI, BOA and PBOA via a two-step azeotropic imidization process as shown in Scheme 1. The molar ratio of the TR-able diamine (APAF) to non-TR-able diamine (either BIA, PBABI, BOA, or PBOA) was set to 5:5, and the molar amount of diamines was tantamount to the dianhydride so as to obtain high molecular weight polymers. Using the copolymerization of 6FDA-APAF/BOA as a typical example, in detail, 4.163 g (0.0114 mol) APAF and 2.56 g (0.0114 mol) BOA were dissolved in NMP (120 mL) in a three necked flask, and then 10.099 g (0.0227 mol) 6FDA was added. After reaction at 5 °C for 12 h, the mixture was then heated to 190 °C for 12 h under a dry N2 atmosphere, and the byproduct water was removed using a Dean-Stark trap, finally, the co-polyimides were obtained. The co-polyimide solutions were precipitated in a mixture of methanol and deionized water at room temperature, and finally were filtrated and dried in a vacuum at 120 °C. For comparison, 6FDA-APAF homo-PI was synthesized in a similar method. Preparation of poly(benzoxazole-co-imide) membranes. The co-PI precursor powders were dissolved in NMP to prepare PI solutions with a 8 wt% weight concentration. The PI solution was cast onto a clean glass plate, and co-PI membranes was obtained after dried in the vacuum oven at 60 °C for 12 h, and then at 120 °C for 6 h and finally at 200 °C for 12 h to ensure that the NMP was removed completely. All precursor membranes were obtained using the similar procedure with the thickness ranging from 50 to 75 µm, and the obtained co-PI membranes containing BIA, PBABI, BOA and PBOA were abbreviated as PI-a, PI-b, PI-c and PI-d, respectively. Afterwards, the ortho-hydroxy co-PI precursors were converted to corresponding TR-PBOI membranes by thermal treatment in a tube furnace under high-purity nitrogen. All membranes were heated at 5 °C/min to the final rearrangement temperature of either 380 or 420 °C for 1 h, respectively, and cooled to room temperature at a rate of 10 °C/min. Thermally treated membranes were singed as TR-PBOI-(a-d)-380 and TR-PBOI-(a-d)-420, respectively.

6

Scheme 1. (A) Synthetic route of co-polyimide precursors and the resultant thermally rearranged poly(benzoxazole-co-imide)s; (B) typical procedure for fabricating TR-PBOI membranes. Measurements. 1H-NMR spectra were performed on a Bruker Avance 600 nuclear magnetic resonance spectrometer using d6-DMSO as the solvent, and the solid-state

13

C-NMR spectra were

recorded on a Bruker Avance 400 nuclear magnetic resonance spectrometers. FTIR spectra were obtained on a Nicolet 8700 spectrometer. Molecular weights of co-polyimide precursors were evaluated by gel permeation chromatography in a Malvern Viscotek GPCmax analyzer with DMAc as the eluant. TGA-FTIR scans were obtained under the nitrogen atmosphere from 40 to 900 °C at a heating rate of 10 °C/min. TGA data were obtained on a Netzsch TG 209 F3 thermogravimetric 7

analyzer instrument. The two-dimensional correlation analysis of FTIR spectra were generalized using 2DCS-Professional software composed by Zhou Tao (Polymer Research Institute, Sichuan University, China) [43]. Dynamic mechanical analysis (DMA) was performed on a DMA Q800 (TA Instruments) at a scan rate of 5 °C/min under a nitrogen atmosphere. X-ray photoelectron spectroscopy (XPS) data were acquired on an Escalab 250Xi spectrometer (ThermoFisher Scientific, USA), equipped with an analyzer (pass energy of 50 eV) and an Al Kα X-ray source. Wide-angle X-ray diffraction (WAXD) data were recorded at 16B1 Beamline in Shanghai Synchrotron Radiation Facility (SSRF), in detail, the wavelength was 1.24 Å, and the distances between the CCD X-ray detector (Specifications PILATUS3) and samples were 202.2 mm. Mechanical properties of membranes were analyzed by an Instron 5969 instrument with a drawing rate of 5 mm/min. Positron Annihilation Lifetime Spectroscopy (PALS) was performed to explore average radius and size of the free volume cavity of the co-PI precursors and the resultant TR-PBOI membranes, and the LT-9 computer program was adopted to resolve the PALS data. According to numerous previous reports, average free volume radius (R) could be correlated to the third lifetime τ3 (>0.5 ns) of the pick-off ortho-Positronium (o-Ps) annihilation and could be calculated by the following semi-empirical Tao-Eldrup model (3) [44]: 1 2

τ 3-1= [1-

R 1 2π R + sin( )] R + ∆R 2π R + ∆R

(3)

where τ3 represents the lifetime of o-Ps, and ∆R is empirical constant of 1.656 Å fitted by empirical electron layer thickness. As a consequence, the average size of free volume cavity (Vf(PLAS)) could be calculated by equation (4): V f (PALS) =4π R 3 /3

(4)

Gas separation properties were performed by a Labthink VAC-V1 instrument, in detail, the gas permeability coefficients (P) for four gases, CO2, O2, N2 and CH4, were recorded by a time-lag method under 1 atm upstream pressure (standard constant-volume) at 35 °C using a variable-pressure permeation method, as the follow equation (5): P=

T ∆p V D × × 0 × ∆t S T × p0 ( p1 − p2 )

(5)

where ∆p/∆t is the rate of the pressure change at steady state, V (cm3) is the downstream chamber volume, S (cm2) is the effective area of test samples, T (K) is the measurement temperature, p0 and 8

T0 is the standard pressure and temperature, respectively, D (cm) is the thickness of membrane, p1 and p2 (cmHg) represent the pressure between the two sides of membranes, respectively. The ideal selectivity (αa/b) for component a and b were defined as the ratio of pure gas permeability of the two gases as shown in the following:

α a /b =

Pa Pb

(6)

Results and Discussion Synthesis of ortho-hydroxy co-polyimide precursors Precursors ortho-hydroxy co-polyimides were easily synthesized in a straightforward in-situ solution thermal imidization approach by reacting 6FDA with APAF, and non-TR-able diamines containing benzimidazole or benzoxazole moieties, as shown in Scheme 1. Specifically, the new diamines PBABI and PBOA exhibiting symmetrical structures and containing higher benzimidazole or benzoxazole contents would lead to much more rigid backbones for the TR-PBOIs, and more permeable and mechanically stronger films are expected. The 6FDA-APAF homo-PI is a typical polymer used in the preparation of TR membranes, which has been synthesized and utilized as the reference polymer. The molecular-weights of co-polyimides containing benzimidazole or benzoxazole moieties are summarized in Table 1, and the of Mw and Mn values are in the range of (11.9-23.5)×104 and (8.4-16.6)×104 g·mol-1, respectively, with the PDIs of 1.4-1.6, indicating high molecular weights of these precursors. Clearly, the co-polyimides containing benzoxazole exhibit much

lower

molecular-weights

compared

to

the

samples

synthesized

from

the

benzimidazole-containing monomers, which is probably attributed to the higher nucleophilicity of BIA and PBABI than that of the BOA and PBOA. For evaluating their reactivities, 1H-NMR spectra of these diamines were carried out and shown in Figure S1. Clearly, the PBOA diamine exhibits an downfield and single proton signal at around 6.02 ppm in relative to other diamines. Two proton signals for -NH2 in asymmetric BOA are located at 5.27 and 5.80 ppm, respectively, which exhibit downfield shifts in comparison with the amine proton signals of the PBABI (a single peak at 5.03 ppm) and BIA (at 4.83 and 5.47 ppm), revealing that the nucleophilicity of benzimidazole-containing diamines is higher than that of the benzoxazole-containing diamines. Similar results have been reported in Zhuang's previous results [36]. 9

The chemical structures of ortho-hydroxy co-polyimides were conformed by 1H-NMR. As shown in Figure 1, the presence of the ortho-hydroxy proton in 6FDA-APAF homo-PI and the co-polyimides was confirmed by a single at 10.4 ppm. The co-polyimide precursors containing BIA and PBABI exhibit an additional proton peak at 13. 26 and 13.33 ppm, respectively, representing the -N-H in benzimidazole moieties from BIA and PBABI, which can not be observed in precursors containing benzoxazole units. Several overlapped signals are located at 7.1-8.4 ppm due to the benzene rings as well as other aromatic moieties incorporated in polymer backbones. Besides, the amide proton peaks at around 6.5 ppm for poly(amic acid)s (PAAs) are not found in all cases, indicating that the PAAs are completely converted to co-polyimide in this case.

Table 1. Molecular weights and PDIs of ortho-hydroxy co-polyimide precursors Code

Non-TR-diamine

Mw (10 g·mol-1)

Mn (10 g·mol-1)

PDI

PI-a

BIA

23.5

16.6

1.4

PI-b

PBABI

22.4

13.9

1.6

PI-c

BOA

16.2

11.3

1.4

PI-d

PBOA

11.9

8.4

1.4

4

4

Figure 1. 1H-NMR spectra of ortho-hydroxy co-polyimide precursors with different non-TR-able 10

diamines

Thermal rearrangement from co-polyimide precursors to TR-PBOI membranes Figure 2 shows TGA and DTG curves for different precursor polymers at a heating rate of 5 °C/min under the N2 atmosphere. Typically, a two-step weight loss can be distinguished for all of the cases, in detail, an initial mass loss occurring at 300 to 500 °C, which is due to the elimination of hydroxyl groups and formation of benzoxazole structures by the thermal rearrangement, and the second weight loss observed in the range of 500-750 °C is mainly attributed to the thermal decomposition of poly(benzoxazole-co-imide) backbones. As seen in the DTG curves, a clear difference in the rearrangement temperature was observed for these ortho-hydroxy co-polyimides containing different non-TR-able diamines, namely, the temperatures at the maximum conversion rate for PI-a and PI-b precursors containing benzimidazole moieties are 400 and 415 °C, respectively, while those for PI-c and PI-d are 368 and 386 °C, which indicates that benzoxazole-based precursors can be readily thermally rearranged at a much lower temperature compared to the benzimidazole-based ones. A lower TR reaction temperature is not only beneficial for resulting in a large gap between the thermal rearrangement and the chains decomposition temperature, but also reducing the possible chains degradation that is instrumental in maintaining excellent mechanical properties of the resultant TR-PBOI membranes. Reported works [32, 33] have illustrated that the TR reaction temperature is closely related to the glass transition temperatures of precursor polymers, and a more rigid-rod-like backbone with the higher Tg always requires a higher rearrangement temperature. The DSC curves of the precursor membranes are shown in Figure 3 (A). Obviously, the PI-a and PI-b precursors containing benzimidazole have higher Tg values of 294 and 300 °C than those of the PI-c (260 °C) and PI-d (268 °C), which are likely attributed to the strong hydrogen-bonding interaction between N-H and C=N of benzimidazole or N-H in benzimidazole and C=O in imide rings. Therefore, because of the lower thermal rearrangement temperature, the degrees of conversion for PI-c and PI-d are 96.2% and 95.1%, respectively, when isothermally treated at 420 °C for 1 h, which are higher than those of the PI-a (86.1%) and PI-b (78.4%), as shown in Figure 3(B). Of particular interest is that TR-PBOI membranes derived from the benzoxazole-containing non-TR-able diamines exhibit a better thermal stability represented by the higher decomposition temperatures in relative to benzimidazole-based membranes, and this can be ascribed to the more 11

rigid-rod benzoxazole structures.

Figure 2. Thermogravimetric analysis of different ortho-hydroxy co-polyimide precursors

Figure 3. (A) DSC thermograms of ortho-hydroxy co-polyimide and (B) the degree of thermal rearrangement of precursor polymers containing different non-TR-able diamines. Chemical evolution from precursor co-polyimides to TR-PBOIs after the thermal treatment were monitored by ATR-FTIR spectroscopy. As shown in Figure 4(A), all precursor membranes exhibit the typical hydroxyl peak at 3450 cm−1, imide C=O asymmetric stretching at 1785 cm-1 and C-N-C stretching of imide ring at 1370 cm-1. When thermally treated at 420 °C for 1 h, 12

the

corresponding TR-PBOI-a and TR-PBOI-b samples give two new bands at wavenumbers around 1554 and 1477 cm-1 attributed to the characteristics of benzoxazole structure, and meanwhile, the TR-PBOI-c and TR-PBOI-d specimens show increased intensities of benzoxazole band at wavenumbers of around 1504 cm-1 and 1556 cm-1 accompanied by the declined characteristic imide bands and disappeared hydroxyl peaks, indicating the formation of new benzoxazole moieties from the thermal rearrangement of ortho-hydroxy co-polyimides. For further clarifying the changes of different groups in the polymer backbone during the thermal rearrangement, a powerful 2D-COSY FTIR spectra of PI-a was carried out. Figure 4(B) shows the synchronous map of PI-a treated in the temperature region of 300-450 °C during the heating process. Clearly, two strong auto peaks along the diagonal at 1720 and 1370 cm-1 (marked as red color) representing the imide units of precursor polymers make a synchronous change with the temperature rising, and meanwhile, two additional cross-peak located at (1720, 1472) and (1720, 1550) cm-1 (marked as the blue color) can be in off-diagonal region, indicating that the 1720 and 1472 or 1550 cm-1 bands have an opposite variation response when the sample suffering the heating-induced perturbation. Thus, it can be speculated that the typical benzoxazole vibration modes at 1550 and 1472 cm-1 increase while the imide band at 1720 and 1370 cm-1 reduces in the post thermal rearrangement, which is in accordance with the ATR-FTIR spectra in Figure 4(A).

Figure 4. (A) FTIR spectra of ortho-hydroxy co-polyimide precursors and the corresponding TR-PBOIs with different non-TR-able diamines; (B) the synchronous 2D-COSY FTIR spectra of PI-a in the temperature region of 300-450 °C. Actually, the ATR-FTIR resonances are relatively weak as revealed in Figure 4(A), it is difficult 13

to distinguish the conversion process in more details. In addition, the TR-PBOI membranes become insoluble in common solvents after thermal treatment. Therefore, other structure characterization tools including the solid state

13

C-NMR and XPS analyses were performed to further confirm the

occurrence of benzoxazole cyclization (Figure 5(A)). As shown in Figure 5(B), typical carbon peaks for imide carbonyl at 165 ppm (Ca) and the carbon atom (Cb) linked to hydroxyl group at 153 ppm can be observed for the 6FDA-APAF homo-polyimide precursor. When thermally treated at 420 °C, the intensity of carbonyl peak decrease and subsequent formation of characteristic benzoxazole resonances at 163 (a*), 150 (b*) and 142 ppm (c*), respectively. For PBOIs derived from the co-polyimide precursors, typical resonances for benzoxazole moieties are easily found, and besides, the imide ring signal (d*) can also be observed, confirming the resultant TR-PBOIs simultaneously containing the polybenzoxazole and polyimide portions in the backbone. As expected, the 13C-NMR result is strong support of the proposed poly(benzoxazole-co-imide) molecular structure and in accordance with the ATR-FTIR analysis.

Figure 5. (A) Structural transformation from co-polyimide precursor to TR-PBOI and (B) solid-state 13

C-NMR spectra of the TR-PBOI membranes. Significant differences among unprocessed precursors and the thermally rearranged specimens

were observed in the XPS chemical state scans. Figure 6 compares the fitted N 1s XPS spectra of precursor co-polyimides and TR-PBOIs-420 by analyzing the binding energy around nitrogen related bonds. Clearly, the N 1s core peaks relative to all samples show a main contribution located at 400.5 eV, which is originated from the sp3-hybridized C-N-C structure in imide ring. In PI-a and PI-b precursors, two additional peaks with the binding energy of 400 and 398.4 eV can be assigned to the 14

C=N and C-N groups in the imidazole structure, however, these two contributions can not be observed in PI-c and PI-d, both of which only show the presence of two main components at 400.5 eV (C-N-C in imide ring) and 398.9 eV (C=N in benzoxazole groups), respectively. After thermal rearrangement, both the TR-PBOI-a and TR-PBOI-b exhibit an extra peak at 398.9 eV belonging to the C=N group in benzoxazole moieties [45], which confirms the presence of the benzoxazole moieties in these two thermally cyclized specimens. Meanwhile, the benzoxazole component increases significantly in TR-PBOI-c and TR-PBOI-d as revealed by the enhanced C=N intensity at 398.9 eV compared to their corresponding precursors, further illustrating the successful benzoxazole cyclization. Experimental intensity Computed intensity ••••• • • • C-N-C (imide) • • C=N (imidazole) • 400 eV • • • C-N (imidazole) •

Intensity (a.u.)

• •

•• • •• • ••• •••••••••••••••••••••••••••••••

Experimental intensity TR-PBOI-a-420 Computed intensity C-N-C (imide) • •••• • 400 eV •• • C-N (imidazole) • ••• • • •• • • • C=N (imidazole) 398.9 eV • • •• C=N (benzoxazole)•• •

•• • 398.4 eV • •• • • •••• •• • • •••••••• ••••• • • • •

400.5 eV••

• • •• • ••••••••• ••••••••• •••• •••••• • •• •• •

404

402

400

398

Experimental intensity Computed intensity •• • •• C-N-C (imide) • • • • C=N(imidazole) • • • C-N(imidazole) • 400.5 eV••

• • • • •• •••••••••• •••• •••••••••••••••••• • • •

404

396

Experimental intensity Computed intensity C-N-C (imide) C=N(benzoxazole ) •••••••• •

400.5 eV ••

• • • • • • • •• •• • • ••••••••••••••••••••••••••••

404

402

•

PI-c

400

398

Experimental intensity Computed intensity 400.4 eV ••••• C-N-C (imide) •• • C=N (benzoxazole) • •

TR-PBOI-c-420

398

402

396

• • • • 398.9 eV • •• •• • •• • • • • ••• •• ••• ••• •••••••••• • •• • •• • ••• •

• • • ••• •• ••• •• •• ••••••••••• • • •• • ••••••• • •

Experimental intensity TR-PBOI-d-420 •• Computed intensity • •• • ••••• • C-N-C (imide) • • •• •• • •• • 398.9 eV C=N(benzoxazole ) • • •

• •

400.4 eV •

•

• • • • •• •• • ••••••••••••••• ••••••••••••••••• •

404

396

PI-d

• •

• •

•• •••••• 398.9 eV •• • • • • • • • • • • •• ••• ••••••••••••••••••••••••

400

eV

•• •• • •••••• • ••• ••••••••••••••

Binding Energy (eV)

• •• • • • • • • • • • • •• • 398.9 eV • •• ••• • •••••• ••• •• •• •• • ••• •• • ••••••••••••••••••••••••••• •••••••••••••• ••••••••••••• •

• • 398.4 • •

• • • •• •• ••••••• •••••• • •••• ••••••••• • •• • •

400.5 eV

Intensity (a.u.)

Intensity (a.u.)

Experimental intensity Computed intensity C-N-C (imide) C=N (benzoxazole)•• 400.5 eV

400 eV

Experimental intensity TR-PBOI-b-420 • Computed intensity ••• •••• • • 400 eV • • C-N-C (imide) •••• •• C=N (imidazole) • • 398.9 eV •• C-N (imidazole) • •• • • C=N (benzoxazole) • •

Binding Energy (eV) •

PI-b

• • • • 398.4 eV •••• ••••••• ••• • ••• •• • •• • • ••••••••• ••••• ••

•

• • • • •• 398.4 eV • •••• •• •• ••• ••• ••• ••• • ••• ••••••••••••

400.5 eV •

•

•

PI-a

Intensity (a.u.)

•

402

• • • • ••• • •• •• •••••••••• ••• ••• • •

400

398

396

Binding Energy (eV)

Binding Energy (eV)

Figure 6. High-resolution spectra of N 1s of co-polyimide precursors and the corresponding TR-PBOIs. 15

Chain packing and FFV of TR-PBOI membranes Wide-angle X-ray diffraction (WAXD) measurements were conducted to explore the polymer chain packing behavior during the thermal rearrangement. As shown in Figure 7(A), the TR-PBOI-a treated at 420 °C shows a broad amorphous halo in the typical 2D WAXD pattern and other samples exhibit analogous X-ray results, proving the primarily amorphous nature of these samples. In addition, the ordered π-π stacking domains also can be observed [46]. In detail, the 1D WAXD distribution files integrated from 2D WAXD patterns and the preferential intersegmental distance (d-spacing) calculated according to Bragg's equation are shown in Figure 7(B)-(F). All precursor and thermally rearranged membranes exhibit two diffraction peaks located at around 2θ=13.5° and 20.6°, representing the average interchain packing in the ordered domains and the π-π stacking order, respectively. In all cases, the thermal rearrangement to TR-PBOI results in larger d-spacing values calculated by the diffraction peak at around 2θ=13.5°, which indicates the enlargement in fractional free volume (FFV) for PBOIs in relative to their corresponding precursors as the d-spacing can be considered as an index of openness of the polymer matrix. Additionally, higher treatment temperature inclines to result in larger d-spacing value and lower packing density. For example, when thermally treated at 420 °C for 1 h, the TR-PBOI-a-420 exhibits a preferential intersegmental distance of 5.61 Å, which increases by 0.36 Å with regard to the corresponding PI-a precursor. Previous studies have reported that during the solid-state structural rearrangement of precursor polymers, the distortion of the polyimide chain into a rigid-rod polybenzoxazole easily resulted in the disrupted chain packing and microcavity, leading to the increased FFV. Comparatively, the benzoxazole-based TR-PBOIs exhibit larger d-spacing values than those of the benzimidazole-based ones. In detail, the d-spacing of TR-PBOI-c-420 and

TR-PBOI-d-420 containing BOA and PBOA

are 5.79 and 5.68 Å, as compared to 5.61 and 5.60 Å for thermally treated samples containing BIA and PBABI, respectively. Thus, the TR-PBOI-c-420 shows the largest distance mainly attributed to its asymmetric chemical structure. However, from the point of view of π-π stacking, which represents some parallel arranged aromatic heterocyclic rings in the backbone, the benzimidazole-containing PI-a and PI-b show opposite variation tendencies in relative to the benzoxazole-containing PI-c and PI-d. For example, the π-π stacking interplanar distance for PI-a decreases from 3.53 Å for the precursor to 3.32 Å for TR-PI-a-420, whereas both PI-c and PI-d exhibit a clear increase in face-to-face π-π stacking 16

distance in the thermal rearrangement, indicative of their looser molecular packing nature. According to the previous work [47], the strong hydrogen-bonding interaction between the proton donor (N-H) and the proton acceptor (C=N) in BIA or PBABI may acted as physical cross-linking points in inter-chains, which would limit the conformation adjustment of benzene or imide rings. However, it can be proposed that high mobility of polymer chains and the destroyed hydrogen-bonding interaction under high temperature in the thermal rearrangement may favor the closer packing of heterocyclic benzimidazole rings. Of note is that the π-π stacking distance is between the gas kinetic diameter of CO2 (3.30 Å) and CH4 (3.80 Å), which may endow these membranes with a typical molecular sieving effect. Therefore, the increased π-π stacking distance for benzoxazole-containing TR-PBOIs inclines to simultaneously promote their CO2 permeability and CO2/CH4 selectivity when applied for gas separation.

17

Figure 7. (A) Typical 2D WAXD pattern for TR-PBOI-a, and (B-F) 1D WAXD curves of the obtained membranes with different non-TR-able diamines. While the evolution of molecular packing behavior in the thermal rearrangement has been well-understood by the WAXD measurements, only limited information is about the free volume of macromolecular chains. As is well known, the positron annihilation lifetime spectroscopy (PALS) has developed into a very powerful technique for probing atomic- or subnanometer-sized microporosity in materials. Generally, the positron intensity and lifetime differ systematically with variations in polymer backbone structure and chains free-volume, and the larger lifetime values always mean larger spherical free volume elements. In this work, the positron lifetime spectra 18

measured for co-polyimide precursors and TR-PBOI-420 membranes are shown in Figure 8. Madzarevic et al. [44] reported that the o-Ps (o-Ps, positronium annihilation from the ortho state) long-lifetime data (τ3) was highly sensitive correspondence to the void size in materials and could be utilized to estimate the dimensions of local free volumes. As shown in Table 2, the TR-PBOI-420 membranes exhibit higher τ3 and intensity (I3) values compared to the corresponding precursors, indicative of distinctly increased free volume sizes. Based on the Tao-Eldrup model, the detailed parameters has been calculated. Obviously, both the mean hole radius (R) and the average size (Vf PLAS)

of free volume for the TR-PBOI-420 membranes follow the sequence of TR-PBOI-c-420 >

TR-PBOI-d-420 > TR-PBOI-a-420 > TR-PBOI-b-420, which can be interpreted as reflecting an sequence in free volume. The results correlate well with the d-spacing values from the WAXD measurements, conforming the significance of rigid-rod-like benzoxazole moieties in constructing microporous structures. The enhanced cavity size and adequate free volume make the TR-PBOI membranes favorable for permeation of small gaseous molecules such as CO2 and O2. Non-TR monomer: BIA

(B) 100k

PI-a TR-PBOI-a-420

Normalized Counts

Normalized Counts

(A) 100k 10k

1k

100

10

1 2500

3000

3500

4000

Non-TR diamine: PBABI PI-b TR-PBOI-b-420

10k

1k

100

10

1 2500

4500

3000

Non-TR diamine: BOA

(D) 100k

PI-c TR-PBOI-c-420

Normalized Counts

Normalized Counts

(C) 100k 10k

1k

100

10

1 2500

3000

3500

4000

3500

4000

4500

Channel Number (13ps/chn)

Channel Number (13ps/chn)

Non-TR diamine: PBOA PI-d TR-PBOI-d-420

10k

1k

100

10

1 2500

4500

3000

3500

4000

4500

Channel Number (13ps/chn)

Channel Number (13ps/chn)

Figure 8. PALS curves of the co-polyimide precursors and the corresponding TR-PBOI-420 membranes.

Table 2. Analyzed data for the positron lifetime in the co-polyimide precursors and the corresponding TR-PBOI-420 membranes 19

Code

Non-TR diamine

τ3 (ns)

I3 (%)

R (Å)

Vf (Å3)

PI-a PI-b

BIA PBABI

1.917 1.613

1.488 1.412

2.771 2.464

89.12 62.66

PI-c PI-d

BOA PBOA

1.986 1.089

1.483 1.423

2.835 2.664

95.44 79.19

TR-PBOI-a-420 TR-PBOI-b-420

BIA PBABI

2.116 2.018

3.713 3.776

2.953 2.865

107.9 98.51

TR-PBOI-c-420 TR-PBOI-d-420

BOA PBOA

2.359 2.136

3.578 3.712

3.157 2.971

131.8 109.8

Thermal and mechanical properties of TR-PBOI membranes Dynamic mechanical analysis (DMA) was adopted in order to determine the glass transition temperature (Tg) of the thermally converted TR-PBOI membranes. As shown in Figure S2, when thermally treated at 420 °C for 1 h, the Tg taken from the peak temperature of Tan δ curves ranges from 420 to 451 °C depending on the backbone chemical structures, indicating the superior thermal resistance of the obtained separation membranes, which are even much higher than some commercial polyimides, such as Kapton® (377-399 °C) [48], Upilex-R® (330 °C) [49] and Matrimid® (313 °C) [50]. Meanwhile, the TR-PBOI-a-420 and TR-PBOI-b-420 containing benzimidazole structure have better thermal stabilities than the benzoxazole-based ones mainly attributed to that the more closer molecular packing and strong hydrogen-bonding interaction arisen from the benzimidazole moiety can accelerate the diffusion of heat. For the gas separation membranes, a sufficient mechanical strength is always required to withstand the high operating pressure and enable membrane module preparation in their practical application. However, most TR membranes inevitably experience the deterioration of mechanical properties in the high temperature thermal rearrangement. In this work, the designed polymer backbones containing rigid-rod-like benzimidazole and benzoxazole structures are expected to improve their tolerance to decomposition at high temperatures and contribute to the mechanical properties. Mechanical performances in terms of tensile strength, modulus and elongation at break for co-polyimide precursors and TR-PBOI membranes at different TR conditions were tested and the detailed results are shown in Figure 9. It is clear to identify that the mechanical properties are not only influenced by chemical structures of membranes but also by the thermal rearrangement 20

conditions in all cases. The co-polyimide precursors display high tensile properties with the tensile strength of 123-136 MPa, elongation at break of 13-16% and initial modulus of 1.6-1.9 GPa. When thermally treated at 380 or 420 °C for 1 h, the tensile strength decreases accompanied by a sharp decrease in the elongation at break for the resultant membranes. It is gratifying to note that the mechanical properties of the TR-PBOI-420 membranes still remain at a high level with the tensile strength ranging from 97 to 118 MPa, and meanwhile, the initial modulus of the thermally rearranged membranes (2.0-2.4 GPa) are much higher as compared with their corresponding co-polyimide precursors due to the chain structural transformation from the ortho-hydroxy polyimide to a more rigid polybenzoxazole structure. Additionally, slightly higher tensile strength and elongation at break are observed for the benzimidazole-containing TR-PBOI-a-420 and TR-PBOI-b-420 membranes relative to the benzoxazole-based TR-PBOI-c-420 and TR-PBOI-d-420. Moreover, it should be mentioned that the prepared membranes in this work either containing benzimidazole or benzoxazole units exhibit higher tensile strength values than most previously reported TR-PBO or TR-PBOI membranes, including TPI-PBO-0.5 (16 MPa) [30], PBO-DAM-400-2h (64.32 MPa) [51], HAB-6FDA400 (85 MPa) [52], spiroTR-PBO-PM (79.2 MPa) [53] and 6FDA-HAB-Allyl (59.3 MPa) [24]. The excellent mechanical strengths and thermal stabilities of these TR-PBOI membranes are of great interests for commercial gas separation applications.

21

Figure 9. (A-D) Typical strain-strength curves of the co-polyimide precursor and TR-PBOI membranes with different non-TR-able diamines, and their detail tensile strength (E) and (F) modulus.

Gas separation properties of TR-PBOI membranes The gas permeability (P) and ideal separation factors (selectivity) for some common gas pairs of thermally rearranged and their corresponding precursor polymer membranes are listed in Table 3. For comparative purpose, the gas separation properties of some commercial membranes have also been included. Obviously, the sequence of permeability coefficients for all of the precursors and thermally rearranged membranes is P(CH4) < P(N2) < P(O2) < P(CO2), following the reverse gas kinetic 22

diameter order of CH4 (3.80 Å) > N2 (3.64 Å) > O2 (3.46 Å) > CO2 (3.30 Å). Both the chemical structure and thermal rearrangement condition have influences on the membranes' permeability coefficients. As expected, permeability coefficients of all gases increase after the thermal rearrangement of ortho-hydroxy copolyimide precursors, which could be associated with the increased FFV and looser chains packing as illustrated in WAXD and PALS. For instance, the CO2 permeability of the TR-PBOI-d-420 containing PBOA reaches around 58.9 Barrer, nearly 16 times higher than that of the primary PI-d precursor. In addition, the permeability coefficient increases as the thermal treatment temperature raised mainly attributed to the higher benzoxazole conversion degree, hence, the CO2 permeability for TR-PI-c increases by 79% from 34.6 to 61.9 Barrer when raising the temperature from 380 to 420 °C. Of course, the non-TR-able diamines also show significant effects on gas permeabilities, and BOA and PBOA based membranes are more permeable to gases than those with BIA and PBABI codiamines. For example, the CO2 permeabilities of TR-PBOI-c-420 reaches around 61.9 Barrer, roughly 1.5 and 1.8 times as high as that of the TR-PBOI-a-420 and TR-PBOI-b-420, respectively, illustrating that the incorporation of highly rigid preformed benzoxazole rings contribute to the improved gas permeation. A similar trend can be found for the rest of gases tested, namely, CH4, O2 and N2. To gain a better understanding on the fundamentals governing gas separation performance of the resultant TR-PBOI membranes, permeability of the four tested gases were deconvoluted into diffusivities and sorption coefficients based on sorption-diffusion theory and the detailed results are shown in Figure 10. By comparison, these materials have nearly the same CH4 diffusivity of (5.4-7) × 109 cm2/s, while the TR-PBOI-c-420 and TR-PBOI-d-420 show higher CO2 diffusion coefficients in relative to the benzimidazole-containing TR-PBOI membranes, which correlates well with FFV values and their larger free-volumes providing abundant sorption sites for CO2. As shown in Figure 10(B), the CO2 solubility coefficients is the highest among tested gases due to its strong interaction with the polar groups in polymers. The CO2 solubility coefficients change in the these membranes in the order of TR-PBOI-d-420 > TR-PBOI-c-420 > TR-PBOI-b-420 > TR-PBOI-a-420, thus, high concentration of benzoxazole structure in the backbone promotes high CO2 solubility. Consequently, thermally rearranged membranes containing BOA and PBOA facilitate the CO2 permeation.

23

Figure 10. (A) Pure gas diffusivity and (B) solubility of various TR-PBOI-420 membranes The gas selectivity for two different gas pairs also strongly depends on the chemical structure and thermal reaction protocol. Obviously, all prepared membranes have a higher CO2/CH4 ideal selectivity than the O2/N2 gas pairs, which is mainly arisen from the difference of the gas kinetic diameters. For the ortho-hydroxy precursor membranes, the CO2/CH4 gas pair selectivity ranges from 20.8 to 36.3, in which the PI-c exhibits the highest selectivity value. It can be noticed that after treatment at 380 °C all of TR-PBOI membranes show higher CO2/CH4 gas pair selectivities than their corresponding precursors ranging from 32.6 to 66.5. Of interest is that thermal conversion at 420 °C implies an further increase in the ideal CO2/CH4 selectivity values except for the TR-PBOI-c-420, meaning that the TR reaction has a synergistic effect on permeability and selectivity of the CO2/CH4 gas pair, which seems different from the well-known trade-off relation between the permeability and selectivity. Specifically, the CO2/CH4 selectivities of TR-PBOI-a-420 and TR-PBOI-b-420 are 58 and 62.3, respectively, slightly higher than those of the TR-PBOI-c-420 (41.8) and TR-PBOI-d-420 (54.5). As mentioned before, the thermal rearrangement at a higher temperature not only brings about an increase in the average interchain packing distance, and thus, in the permeability coefficients, but also leads to the decreased π-π stacking distance, which may produce a marked molecular sieving effect for the CO2/CH4 gas pair. Thus, larger-sized molecule such as CH4 is difficult to permeate, while, this barrier effect barely influences the small gas molecules.

24

Table 3. Single gas permeability and ideal selectivity for the membranes in this work and several previously reported membranes

Code

Permeability (Barrera) N2 O2 0.50 1.99 0.35 1.18 0.52 1.87 0.22 1.21

PI-a PI-b PI-c PI-d

CH4 0.22 0.14 0.24 0.17

TR-PBOI-a-380 TR-PBOI-b-380 TR-PBOI-c-380 TR-PBOI-d-380

0.50 0.41 0.52 0.47

0.93 1.20 2.03 1.01

4.60 3.80 8.52 4.87

17.8 14.5 34.6 15.3

35.6 35.4 66.5 32.6

4.95 3.17 4.20 4.82

TR-PBOI-a-420 TR-PBOI-b-420 TR-PBOI-c-420 TR-PBOI-d-420

0.71 0.56 1.48 1.08

2.09 1.75 3.20 2.68

9.80 8.27 14.3 10.8

41.2 34.9 61.9 58.9

58.0 62.3 41.8 54.5

4.69 4.73 4.47 4.03

Kapton [54] Matrimid 5218 [55] Polysulfone [56] t-TR-TFMB-400-2 [32] 6FDA/p-BFO [39] 6FDA/p-BOA [39]

0.06 0.28 0.25 2.61 1.06 0.54

0.11 0.32 0.25 5.44 1.98 0.98

0.64 2.10 1.4 20.4 10 5.61

3.51 10 5.6 85.1 47 25

58.5 36 22 32.6 44 46

5.8 6.6 5.6 2.75 5.11 5.71

a

CO2 7.38 2.99 8.70 3.53

Ideal selectively CO2/CH4 O2/N2 33.5 3.98 21.4 3.37 36.3 3.60 20.8 5.50

Barrer: 1 Barrer=10-10 cm3 (STP)/(cm s cmHg); Ideal selectively=P1/P2

Figure 11 compares the CO2/CH4 gas pair separation properties of the fabricated membranes with the Robeson upper bound and other state-of-the-art membranes including commercial polyimide Kapton® [54], Matrimid 5281® [55] and polysulfone [56] membranes as well as some reported poly(benzoxazole-co-imide) membranes [32, 39]. Clearly, the gas separation performance of the TR membranes has been enhanced by rising the temperature from 380 °C to 420 °C, which exceed the 1991 upper bound and is close to 2008 upper bound. The effect of nob-TR-able codiamine on the gas separation performance is more noticeable at high rearrangement temperature, namely, the benzimidazole-based BIA and PBABI codiamines endow the fabricated membranes better selectivities, while benzoxazole-based BOA and PBOA cause the membranes higher permeabilities. In addition, the present TR-PBOI-420 membranes exhibit better overall gas transport properties as compared to those commercial polymer membranes including Kapton®, Matrimid 5281® and polysulfone, as well as previously reported poly(benzoxazole-co-imide) samples, such as 25

6FDA/p-BFO, 6FDA/p-BOA and t-TR-TFMB-400-2. The superior gas separation performance demonstrates that the TR-PBOI membranes have great potential for natural CO2/CH4 gas purification.

Figure 11. CO2 permeability-selectivity upper bond plots of prepared TR-PBOI membranes for CO2/CH4. Other commercial Kapton®, Matrimid 5281® and polysulfone membranes as well as some poly(benzoxazole-co-imide) membranes are included for comparison.

Conclusions In this work, four different designed poly(benzoxazole-co-imide) membranes bearing either benzimidazole- or benzoxazole-based non-TR-able codiamines in the polymer backbones were prepared by the thermal rearrangement. Evolution of the TR reaction for ortho-hydroxy copolyimide precursors with different chemical structures has been systemically investigated by TGA, 2D-COSY FTIR and XPS. It has been demonstrated that rearrangement temperatures for the precursors containing preformed benzoxazole unit (BOA or PBOA) were much lower than those of the precursors containing benzimidazole structure (BIA or PBABI), depending on their glass transition temperatures.

The

rigid-rod-like

structures

and

excellent

oxidative

stabilities

of

benzimidazole/benzoxazole units are designed to induce a strong interchain interaction and improve their tolerance to decomposition at high temperatures. Thus, a series of mechanically strong TR-PBOI membranes have been successfully obtained with the tensile strength ranging from 97 to 118 MPa and initial modulus of 2.0-2.4 GPa. Meanwhile, these TR-PBOI membranes exhibited 26

excellent separation properties for the CO2/CH4 gas pair, with the values close to 2008 Robeson upper bound. The resultant membranes derived from benzoxazole-containing non-TR-able diamines have higher d-spacing and FFV values compared to the benzimidazole-based membranes, and resulting in larger gas permeability, while, the later ones exhibited relatively higher CO2/CH4 selectivity mainly attributed to their improved molecular sieving effect arisen from the decreased π-π stacking distance. It is hoped that this work can provide a in-depth understanding and a meaningful insight into the structure-property relationship for TR polymer membranes and make a contribution to them for their actual gas separation applications.

Acknowledgements This work was supported by National Natural Science Foundation of China (No. 21774019), China Postdoctoral Science Foundation (2019M651326), Scientific Research Innovation Plan of Shanghai Education Commission (2019-01-07-00-03-E00001) and the Program of Shanghai Academic Research Leader (18XD1400100).

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Highlights: Heterocyclic non-TR-able diamines were utilized to prepare TR-PBOI membranes. The π-π stacking has a marked effect for the selectivity of CO2/CH4 gas pair. Rigid-rod codiamines endow the TR membranes with superior mechanical properties. These strong TR-PBOI membranes exhibit excellent CO2/CH4 separation performance.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: