N2 Separation Pe

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Surfaces, Interfaces, and Applications

Interfacial Property Modulation of PIM-1 Through Polydopamine-derived Sub-microspheres (PDASS) for Enhanced CO2/N2 Separation Performance Guanying Dong, Jingjing Zhang, Zheng Wang, Jing Wang, Peixia Zhao, Xingzhong Cao, and Yatao Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02281 • Publication Date (Web): 02 May 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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ACS Applied Materials & Interfaces

Interfacial Property Modulation of PIM-1 Through Polydopamine-derived Sub-microspheres (PDASS) for Enhanced CO2/N2 Separation Performance Guanying Dong,

†#

Jingjing Zhang,

†#

Zheng Wang,

†

Jing Wang,

†

Peixia Zhao,

†

Xingzhong Cao, ‡ Yatao Zhang, † * † School ‡

of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, China

Key Laboratory of Nuclear Analysis Techniques, Institute of High Energy Physics, Chinese

Academy of Science, Beijing 100049, China

*Corresponding

author:

E-mail: [email protected]

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Polydopamine modified additives have been thus far widely used in the mixed matrix membranes (MMMs) for gas separation. However, very few reports focus on the polydopamine alone and investigate its contribution to the gas separation performance. Herein, the polydopamine-derived sub-microspheres (PDASS) were paired with polymers of intrinsic microporosity (PIM-1) to fabricate high-performance gas separation membranes, through which the effects of PDASS on gas permeability and CO2/N2 separation performance were systematically investigated. The addition of PDASS provides a 1.6-fold enhancement in CO2/N2 selectivity together with acceptable gas permeability as compared to the original polymeric membrane. Such enhanced separation behavior is supposed to stem from the densified membrane microstructure induced by the strong intermolecular interactions between PIM-1 and PDASS (i.e. charge transfer, π-π stacking and hydrogen bonding). Importantly, the physical aging behavior, as judged by gas permeability, is retarded for PIM/PDASS membranes after 4 months of testing.

Key words: polymers of intrinsic microporosity; polydopamine sub-microspheres; gas separation; polymer chain packing; physical aging

2


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Introduction Membrane-based gas separation has been regarded as a powerful solution today in solving global environmental problems due to its inherent advantages such as low energy intensity, simple operation and environmental friendliness.1-2 Advanced membrane materials with improved gas separation properties are urgently needed in the membrane-based gas separation field. Though much fundamental research has focused on the systematical optimization in polymer backbone structure over the last several decades, polymeric membrane materials still suffer from a near-universal trade-off between gas permeability and selectivity, that is, the Robeson upper bound.3-4 Among those materials, polymers of intrinsic microporosity (PIMs) have been discovered to be promising materials for gas separation5. Unlike conventional microporous materials (e.g. zeolite), PIMs are soluble and easily processed. Most importantly, PIMs allow for the preferential passage of CO2, enabling them to be good candidates for CO2 capture.6 Subsequently, Du et al reported a series of PIM derivatives, including DSPIM,7 carboxylated PIMs,8 CoPIMs9 and TZ-PIM,10 through the post-modification method to further optimize gas separation properties. McKeown’s group also synthesized new PIM-EA-TB films with Tröger’s base (TB) monomer for gas separation in 2013,11 and then Jin et al designed new TB-based copolymer and found that the introduction of TB enhanced CO2 solubility.12 In addition, the thermal oxidative crosslinking of PIMs has been recently proved to be another effective strategy for CO2 capture by rationally 3



tuning the micropore structure of membranes.13-14 However, in the light of controllability and complexity in practice, a more straightforward approach than either the post-treatment of polymeric membranes or developing new polymeric membranes, is through the mixed matrix membranes (MMMs), which combine the merits of both polymer phase and inorganic filler phase. The investigation of MMMs allows to exploit the transport properties of phases with different nature,15 while the expected membrane performance strongly depends on the appropriate match between polymeric matrix and filler, as well as filler content.16-17 Ideal MMMs should take full advantage of the inorganic fillers and allow target gases to preferentially penetrate. The unexpected non-selective voids in the polymer-filler interface, however, are usually generated due to their poor compatibility, which therefore may increase gas permeability to some extent but lose the selectivity. Moreover, high filler content usually result in agglomeration. Similarly, these problems can be also encountered in the PIMs derived MMMs. For instance, by directly embedding non-porous fumed silica into PIM-1, a permeability-selectivity trade-off relationship was observed. This is because the newly created void volume is greater than the permeable space of the unfilled polymer chain packing.18 According to Freeman’s theoretical prediction,19 improving the rigidification of polymer chains while maintaining large interchain spacing is an alternative strategy to achieve simultaneous improvement in both the permeability and selectivity for polymeric membranes. This prediction has been demonstrated by embedding functionalized fillers into PIMs, such 4


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as amine-functionalized metal-organic framework (MOF)20-21 and functionalized polyhedral oligomeric silsesquioxane (POSS).22-23 In contrast to pure PIMs, the resulting MMMs showed outstanding gas separation performance benefiting from the improved compatibility and rigidification of polymer chains. Accordingly, interfacial issues remain to be further addressed when the PIMs and/or PIM derivatives are used as membrane matrix. To date, Mussel-inspired polydopamine (PDA) coating has opened a universal route for the modification of various substrates due to its strong adhesion nature, film-forming feasibility and durable stability.24-25 Besides, abundant functional groups (e.g. catechol, amine and imine) in the PDA matrices could serve as the effective sites for further modifications. Recently, two approaches, namely “rapid deposition of polydopamine” and “co-deposition of polydopamine and polyethyleneimine”,26-27 have largely accelerated the development of PDA for interfacial modification.28-29 In the membrane-based gas separation field, PDA coated PIM-1 membrane has been reported for hydrogen separation.30 In addition, PDA modified fillers have been also utilized to avoid the issue of compatibility in MMMs.31-32 Though PDA is now widely employed as an interface modifier or medium for membrane design, very few reports have focused on PDA itself and the effects of PDA on the gas separation performance thereof. In this work, a new hybrid membrane with the combination of PIM-1 and PDA-derived sub-microspheres (PDASS) is proposed, through which knowledge of how PDA content can affect the

membrane microstructure and gas transport mechanisms would be clear. 5



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Since the PIM-1 is already a highly gas permeable material, we also anticipate the improved CO2 selectivity against N2 through PDASS.

Experimental methods Materials Two

monomers,

2,3,5,6-tetra-fluoroterephthalonitrile

(TFTPN,

99%)

and

5,5′,6,6′-tetrahydroxy-3,3,3′,3′-tetramethyl-1,1′-spirobisindane (TTSBI, 97%) were purchased from TCI and Macklin, respectively. TFTPN was purified by sublimation at 150oC

under

vacuum,

and

TTSBI

was

recrystallized

with

methanol

and

dichloromethane before usage. Methanol (>99.5%), ethanol (≥99.7), chloroform (≥99), and ammonium hydroxide (25%-28%) were purchased from Kewei Chemistry Co., Ltd. Anhydrous potassium carbonate (K2CO3, 99.99%), N, N-dimethylformamide (DMF, 99.5%) and dopamine hydrochloride (DOPA, 98%) were obtained from Shanghai Aladdin Bio-Chem Technology Co., LTD (Shanghai, China). Deionized water was used throughout the experiment. Synthesis of PDA-based sub-microspheres (PDASS) PDASS was fabricated through the oxidation and self-polymerization of dopamine monomer in NH3OH solution. To be specific, a mixed solution consisting of NH3OH (25%, 1.8 mL), deionized water (180 mL), and ethanol (80 mL) was firstly prepared by stirring at room temperature for 30 min. Afterward, 1 g of DOPA was dispersed into 20 mL of deionized water, which was subsequently injected into the abovementioned mixed solution for the formation of PDASS. In this work, different reaction time (5 h, 6


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10 h and 15 h) were adopted to control the PDASS size. Finally, PDASS was washed with deionized water and ethanol sequentially by centrifugation (8000 rpm, 15 min) and stored in the vacuum oven at 50oC prior to use. A density range of ca. 1.0-1.32 g/cm3 for PDASS was determined by a simple buoyancy method using water and dichloromethane at room temperature. Preparation of PIM/PDASS membranes PIM-1 polymer was synthesized according to the reported method by Budd et al.33 This reaction is based on the polycondensation between 2,3,5,6-tetra-fluoroterephthalonitrile (TFTPN,

2

g,

10

mmol)

and

5,5′,6,6′-tetrahydroxy-3,3,3′,3′-tetramethyl-1,1′-spirobisindane (TTSBI, 3.4 g, 10 mmol) in DMF (70 mL) with the presence of anhydrous K2CO3 (4.15 g, 30 mmol) (Fig. S1). The reaction mixture was stirred at 65oC in a 150 mL three-necked flask equipped with a Dean-Stark trap under nitrogen atmosphere for 72 h. Afterward, the reaction mixture was added to 650 mL of water and the precipitate was collected by filtration. Lastly, the polymer was purified and filtrated three circles by dissolving in chloroform, re-precipitation in methanol, and then dried in vacuum oven at 80oC for 48 h (83% yield). 1H NMR (600MHz, CDCl3-d) δH: 1.23-1.61 (12H, br. m), 2.17-2.34 (4H, br. m), 6.44 (2H, br. s), 6.83 (2H, br. s) (Mn= 65000 g/mol, Mw= 112000 g/mol, polydispersity, Mw/Mn= 1.72). PIM/PDASS membranes were prepared via a solution-casting method. Initially, a certain amount of PDASS was dispersed into 58 g of chloroform and sonicated for 2 h. 7



Subsequently, 1.184 g of PIM-1 powder was added into the PDASS dispersion, followed by continuously stirring at room temperature for 48 h. The homogeneous dispersion was again sonicated for 2 h before casting to eliminate the air bubbles. Then 13 mL of the obtained dispersion was transferred to a PTFE Petri dish (19.62 cm2) with pipette. The resulting membranes were then immersed into methanol to remove the residual solvent, and thoroughly dried in an enclosed cabinet (30oC, 24 h) and vacuum oven (80oC, 24 h) sequentially. The as-prepared membranes are referred to PIM/PDASS-X, where X represents the weight fraction of PDASS in membranes. In addition, pure PIM-1 was also prepared according to aforementioned procedures, which has a density of 1.12 g/cm3 determined by a buoyancy method in a calcium nitrate solution at room temperature.

Characterization methods. Characterizations of PDASS The size of the prepared PDASS with different reaction time was evaluated through dynamic light scattering (DLS) analysis using NanoPlus Zeta/nano particle analyzer (Particulate System). N2 adsorption/desorption isotherms were measured volumetrically at 77 K in the range 1.0010-5 ≤ P/P0 ≤ 1.00 by an ASAP 2420 Surface area & Pore size analyzer. A FEI model TECNAI G2 transmission electron microscope (FEI, USA) was applied to observe the morphological structure. Characterizations of membranes The structure of PIM-1 was characterized using nuclear magnetic resonance (NMR) on 8


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a Bruker AVⅢ HD 600 (Switzerland) operating a field of 14.09 T (600 MHz). Molecular weights and the dispersity index for PIM-1 were determined by gel-permeation chromatography (PL-GPC 50) using THF as eluent at 40oC. The functional groups of as-prepared membranes were monitored by Fourier Transform infrared spectrometer (FTIR) (Thermo Nicolet Corporation, USA) over a range of 400-4000 cm-1. The cross-sectional morphology of membranes was examined on cryogenically (liquid nitrogen) fractured samples by scanning electron microscopy (SEM) (JSM-6700F, JEOL, Japan) at 3 kV, and the surface morphology was investigated by atomic force microscopy (AFM) operated under Fast-scan mode using a FASTSCAN-C probe. Samples were prepared by sticking onto a single crystalline Si substrate. Fluorescence spectra were performed using fluorescence spectrophotometer (F-4500, Hitachi, Japan), and the emission intensities were normalized with the membrane thickness. Gas sorption was performed using an ASAP 2420 with high-purity N2 and CO2 at 77 K in liquid nitrogen and at 273 K in an ice-water bath, respectively. X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance diffractometer using Cu Kα radiation in the range of 5-70 ° at a step size of 0.02 ° . Membrane surface hydrophilicity was also confirmed by an optical instrument (OCA 25, Dataphysics, Germany) equipped with video capture at room temperature. The weight loss of membrane samples was recorded by Thermogravimetric-differential scanning calorimetry analysis (TG-DSC) (Shimadzu, Japan) from 30°C to 800°C with a heating rate of 10oC/min under nitrogen atmosphere. The mechanical properties of membranes 9



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were investigated with a tensile tester AG-2000A (Shimadzu, AUTO graph) at room temperature. Each sample was prepared with a width of 10 mm and a length of

40 mm,

and each test was repeated three times. Positron annihilation lifetime spectroscopy (PALS) measurements The free volume size and distribution of the hybrid membranes were measured by PALS. Experiments were conducted by using a conventional fast-low coincidence system with a time resolution of 250 ps at room temperature. A positron source (22Na isotope) was sandwiched between membrane samples which were cut into pieces with a dimension of 1 cm1 cm. PALS data was recorded with total counts of 2 million and analyzed using a routine LT-9 software. In this work, four components were assumed for the positron lifetimes to obtain the better fitting results. The positron electron bound state could be divided into the spin-antiparallel para-positronium (p-Ps) and spin-parallel ortho-positronium (o-Ps). Para-positronium (p-Ps) lifetime (τ1) was determined as 0.125 ns, and the lifetime longer than 1ns due to the ortho-positronium was determined as τ3 and τ4 with their respective intensity I3 and I4. Pore radius (rPs, Å) was calculated from τPs based on a semiempirical correlation equation derived from a spherical-cavity model as follows34

[

𝑟𝑃𝑠

1

(

𝜏𝑃𝑠 = 0.5 1 ― 𝑟𝑃𝑠 + 1.66 + 2𝜋𝑠𝑖𝑛

2𝜋𝑟𝑃𝑠 𝑟𝑃𝑠 + 1.66

)]

―1

(1)

Then the relative fractional free volume (FFV) was obtained according to the assumption of spherical cavities

(

4

𝐹𝐹𝑉 = ∑𝑛0.0018𝐼𝑛 3𝜋𝑟3𝑃𝑠

)

10


(2)

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where In represents the lifetime intensity (I3 or I4). Gas permeation measurements Gas permeability was measured by a “time lag” apparatus (Suzhou Faith & Hope Membrane Technology Co., Ltd, China) via the constant volume approach (Fig. S2). Pure gases (CO2 and N2) were measured at a feed pressure of 1 bar and 30oC. Initially, the upstream pressure was much larger than the downstream pressure, and an equilibrium was reached between upstream and downstream at the end. The measurement time for each sample was set as 8 h. Gas transport properties of the as-prepared membranes were analyzed based on the classical solution-diffusion mechanism. Gas permeability (P) with the unit of barrer [1 barrer= 10-10 cm3 (STP) cm/ (cm2 s cm Hg)] can be expressed by 𝑃=𝐷×𝑆=

273.15 × 1010 𝑉𝑙 𝑑𝑃 𝐴𝑇∆𝑃 𝑑𝑡 760 𝑙2

𝐷 = 6𝜃

(3) (4)

where D is the diffusion coefficient with a unit of cm2/s, and S is the solubility coefficient with a unit of cm3 (STP)/(cm3 cm Hg). V is volume of the downstream chamber (cm3). l is membrane thickness (cm). A is effective membrane area (cm2). T is operation temperature (K). △P is the effective pressure difference between upstream and downstream. θ is the diffusion time lag obtained from the plot of pressure with time. Ideal selectivity (αx/y) was determined as the ratio of pure gas permeabilities under steady state as follows 11



𝑃𝑥

𝛼𝑥/𝑦 = 𝑃𝑦 =

[

𝐷𝑥 𝐷𝑦

𝑆𝑥

]

× 𝑆𝑦

Page 12 of 42

(5)

It should be noted that each reported data here is the average value of three samples, and the variation in gas permeability during each measurement was found to be less than 9.8%.

Results and discussion To confirm the successful preparation of PDASS, we analyzed the geometrical morphology of PDASS with different reaction time in ethanol by TEM, as shown in Fig. 1. It is observed that PDASS are dispersed individually without obvious aggregation. With an increase in reaction time, PDASS become irregular. Also, an increase in hydrodynamic diameter with the reaction time is confirmed by DLS measurement ranging from 500 to 640 nm, consistent with the observed size by TEM. Fig. 2 shows N2 sorption isotherms for the prepared PDASS samples, in which the Type Ⅲ isotherms are observed for all PDASS samples, indicating the weak interaction between N2 molecule and PDASS. Results, including BET surface area, pore volume and pore size of each sample, are summarized in Table 1. The BET surface area decreases from 51.80 m2/g to 16.41 m2/g with increasing the reaction time. Besides, a very low N2 adsorbed volume for all samples indicates their near-nonporous structure. In general, PDASS with larger BET surface area could provide more adsorption sites, that is, the amine, imine, catechol and quinone groups. In other words, these sites could function largely as a resource to increase the CO2 solubility. Additionally, it has been reported that severe agglomeration would occur for PDASS with a short reaction time.35 In this regard, the as-prepared PDASS with a reaction time of 5 h was decided as the filler and used for the 12


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following membrane preparation.

Fig. 1 TEM images of the prepared PDASS with different reaction time of (a) 5 h, (b) 10 h, and (c) 15 h; (d) Hydrodynamic diameter distribution of PDASS as determined by DLS measurements.

Fig. 2 N2 sorption isotherms of the prepared PDASS with different reaction time (Open symbols: adsorption; Closed symbols: desorption). 13



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Table 1 BET surface area, pore volume and average pore size of the as-prepared PDASS with different reaction time. Reaction time

BET surface area

Pore volume

Average pore size

[h]

[m2/g]

[cm3/g]

[nm]

5

51.80

0.0093

7.20

10

22.34

0.0052

3.06

15

16.41

0.0043

1.06

Fig. 3 Cross-sectional SEM images with different magnifications of (a, b) pure PIM-1 membrane, (c, d) PIM/PDASS-5, (e, f) PIM/PDASS-15 and (g, h) PIM/PDASS-25. 14


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The cross-sectional morphology and integrity of membranes were assessed by SEM as displayed in Fig. 3. It is observed that membrane thickness ranges from 65 to 81 μm, and PDASS could maintain their structural integrity. The incorporation of PDASS even at high content does not induce the formation of observable interfacial voids or defects, indicative of the good compatibility with the PIM-1 matrix. In addition, AFM was performed to investigate the membrane surface morphology, as shown in Fig. 4. Pure PIM-1 membrane has a relatively smooth surface, while the addition of PDASS changes the membrane surface morphology significantly. Small protruding spherical bumps are generated with a height of 10-30 nm. On the other hand, the smaller surface roughness, evaluated by Ra and Rq, occurred at 5 and 10 wt% of PDASS as compared to pure PIM-1 membrane (Table 2). This seems counterintuitive, but similar phenomena have been reported in other nanoparticles-embedded polymeric membranes.36-37 The PDASS may be regularly collocated in membrane at a low content, and the membrane surface becomes smooth.38 While PDASS agglomerates would occur at a high content, resulting in the increased membrane surface roughness.

15



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Fig. 4 AFM images of membrane surface of (a) pure PIM-1 membrane, (b) PIM/PDASS-5, (c) PIM/PDASS-15 and (d) PIM/PDASS-25; Scan area: 20ⅹ20 μm (3D) and 5ⅹ5 μm (2D). Table 2 Surface roughness parameters of pure PIM-1 and PIM/PDASS membranes. Membrane

Ra (nm)

Rq (nm)

Pure PIM-1

20.4

26.2

PIM/PDASS-5

5.0

13.8

PIM/PDASS-15

15.4

19.7

PIM/PDASS-25

24.7

31.4

Water contact angle measurement indicates that the addition of PDASS does not obviously alter the hydrophilicity of membranes (Fig.S3), which is possibly because the PDASS are covered by the polymeric phase. Additionally, all membranes show almost similar spectra pattern based on ATR-FTIR measurement (Fig.S4), where no visible peaks contributed by PDASS are observed. This is likely due to the fact that the depth of PDASS in the membranes is beyond the ATR-FTIR detection depth which is approximately several microns.

Fig. 5 Emission spectra of PIM-1 and PIM/PDASS membranes excited at 370 nm. 16


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The ability to fluoresce exhibited by PIM-1 membrane leads us to investigate the photostability of the material, as shown in Fig. 5. It is observed that membranes excited at 370 nm show decreased fluorescence intensity with the PDASS content. In the meanwhile, the membrane color changes from yellow (pure PIM-1) to dark-yellow (PIM/PDASS) (Fig. S5). It is well known that fluorescence yield decrease suggests a charge transfer in the solid-state polymers, which is especially common for the polymer blends.39-40 Therefore, it is reasonably believed that there are strong intermolecular interactions between PIM-1 and PDASS via a charge transfer.

Fig. 6 XRD patterns of pure PDASS, PIM-1 and PIM/PDASS membranes. XRD patterns were used to further investigate the effects of PDASS content on the membrane microstructure. The evolution of the polymer microstructure generally could be reflected by d-space value, where a smaller d-space value represents a tighter chain packing. As presented in Fig. 6, pure PDASS shows a broad diffraction peak at 22.8o, while two characteristic peaks from PIM-1 are observed for all membranes, revealing the amorphous nature of PIM-1 membrane matrix. Generally, the typical Bragg peaks at 17



13.4o and 17.9o for PIM-1 correspond to the d-space values of 6.6 Å and 4.9 Å, respectively. The broad band at 4.9 Å represents the chain-to-chain distance of the space-efficiently packed chains, and the other broad band at 6.6 Å reflects more loosely packed polymer chains originated from contorted polymer backbone.41 Initially, similar XRD patterns could be observed for PIM/PDASS membranes at low PDASS contents (

N2 Separation Pe

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