Pentiptycene-Based Polyurethane with Enhanced Mechanical

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Applications of Polymer, Composite, and Coating Materials

Pentiptycene-based polyurethane with enhanced mechanical properties and CO2-plasticization resistance for thin film gas separation membranes Ali Pournaghshband Isfahani, Morteza Sadeghi, Kazuki Wakimoto, Binod Babu Shrestha, Rouhollah Bagheri, Easan Sivaniah, and Behnam Ghalei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18475 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on April 30, 2018

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ACS Applied Materials & Interfaces

Pentiptycene-Based Polyurethane with Enhanced Mechanical Properties and CO2-Plasticization Resistance for Thin Film Gas Separation Membranes Ali Pournaghshband Isfahani,† Morteza Sadeghi,*,‡ Kazuki Wakimoto,† Binod Babu Shrestha,† Rouhollah Bagheri,‡ Easan Sivaniah,*,† and Behnam Ghalei*,† †

Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, 606-8501, Kyoto, Japan; Email: [email protected]; [email protected] ‡

Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Isfahan, Iran; Email: [email protected]

Abstract Development of thin film composite (TFC) membranes offers an opportunity to achieve the permeability/selectivity requirements for optimum CO2 separation performance. However, the durability and performance of thin film gas separation membranes are mostly challenged by weak mechanical properties and high CO2 plasticization. Here, we designed new polyurethane (PU) structures with bulky aromatic chain extenders that afford preferred mechanical properties for ultra-thin film formation. An improvement of about 1500% in Young’s modulus and 600% in hardness was observed for pentiptycene-based PUs compared to typical PU membranes. Single (CO2, H2, CH4, and N2) and mixed (CO2/N2 and CO2/CH4) gas permeability tests were performed on the PU membranes. The resulting TFC membranes showed a high CO2 permeance up to 1400 GPU (10-6 cm3(STP) cm-2s-1 cmHg-1) and the CO2/N2 and CO2/H2 selectivities of about 22 and 2.1, respectively. The enhanced mechanical properties of pentiptycene-based PUs results in high performance thin membranes with the similar selectivity of the bulk polymer. The thin film membranes prepared from pentiptycene-based PUs also showed a two-fold enhanced plasticization resistance compared to non-pentiptycene containing PU membranes.

Keywords: gas separation membranes, thin film composite, plasticization, CO2 capture, polyurethane, pentiptycene

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1. Introduction Gas separation via polymer membranes is considered to be more energy efficient over conventional phase change separation technologies. However, CO2-induced plasticization under realistic operating conditions is the main problem. Current research is focused on the development of new materials with high gas separation performance that retain the overall properties in the wide range of process conditions. The key to reach this target lies in significantly high free volume polymers by introduction of bulky side groups or contorted structure into the polymer backbone. Restriction of the chain mobility through crosslinking, annealing, and mixed matrix membrane fabrication are aimed to stop the plasticization and improve the thermo-mechanical properties. However, a great loss in the permeability and the limitations to scaling up these methods should be considered. 1-4 High free volume polymers for membrane applications are highly desired. The creation of free volume is generally obtained by disruption of chain packing in the polymer structure. Molecular modeling analysis and experimental studies suggest that adding bulky side groups or highly contorted structures effectively disrupt the chain packing and result in higher gas permeability.5-6 Recently, new microporous polyimides with iptycene units have been designed to achieve fast and selective gas transport. 7-8 The polyimides with paddle-like triptycene moiety exhibit improved gas permeability compared to traditional polyimides (e.g. Matrimid).9 The enhancement is greater for pentiptycene-based polyimides with nearly 50% higher CO2 permeability than that of triptycene-based polyimide membrane.10 The H-shape molecular structure of pentiptycene provides larger fractional free volume and thereby higher gas permeability.11-12 Also, iptycene-based polyimides show high CO2 plasticization resistance and excellent mechanical properties.13-14 The rigidity of the polymer backbone and highly restricted chain mobility are the main reasons for these improvements.15-16 Polyurethane (PU) is a well-known material for CO2 capture which consists of hard and soft segments.17-20 In previous studies, the structure-property relationship of PU membranes was established through different variations of chemical structure. The soft domains are responsible for the gas transport through the PU membranes, so higher phase separation between the hard and soft segments are preferred.17, 21 A promising CO2 selectivity but a relatively low gas permeability was observed for semi-crystalline polyethylene glycol (PEG)based PU membranes.22 A new approach was recently reported by our group to suppress the crystallization of PEG-based PUs.23 The resulting membranes exhibited high CO2 permeability of 150 barrer and the CO2/N2 selectivity up to 50 which is comparable to the separation performance of commercial membrane materials, such as Polyactive® 24-25 and Pebax 1657.26-27 However, the practical application of PU membranes is limited by weak thermo-mechanical stability and low plasticization resistance under realistic gas separation conditions. In our previous work,28 the post-crosslinking process was introduced to overcome the low thermal and mechanical stability of PU membranes. Chemical crosslinking 2 ACS Paragon Plus Environment

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rigidified the PU chains and showed a three-fold enhanced plasticization resistance compared to the non-crosslinked PU membrane. However, the difficulties associated with the removal of by-products, complicated reaction and moisture-sensitive components make the post-crosslinking system undesirable, especially for development of thin-film gas separation membranes. A simpler approach than the post-crosslinking is through the direct synthesis of new PU structures with rigid functional moieties. The rigid backbone could provide high mechanical properties and good CO2 plasticization resistance.29 Additionally, the presence of impermeable hard domains adversely affects the overall transport properties of PU membranes. Therefore, it is imperative to design new PUs with permeable hard segments. Herein, we explore the idea of finely tuning the gas transport properties, and developing high plasticization resistant PU membranes using the pentiptycene diamine chain extender. The rigid structure of pentiptycene with five arene rings and H-shape structure allowed us to effectively hinder the chain packing in PU structures. To the best of our knowledge, this is the first report of pentiptycene-based PU structures. An important aspect of this research is developing PU structures with high mechanical properties and enhanced CO2-induced plasticization resistance that can be easily applied for thin film membrane. In contrast, current methods to improve mechanical properties of polymer membranes (e.g. mixed matrix membranes, crosslinking) are not suitable for thin film fabrication. 2. Materials and methods Materials. Anthracene, sodium bicarbonate, sodium dithionite, palladium on carbon

(Pd/C, 10 wt.%) and hydrazine monohydrate were obtained from Sigma-Aldrich. Tetrachloro-1,4-benzoquinone, p-benzoquinone, p-fluoro nitrobenzene, isophorone diisocyanate (IPDI) were purchased from Wako Pure Chemical Industries (Japan) and used as received. Polyethylene glycol-block-polypropylene glycol-block-polyethylene glycol (Pluronic® L-61, Mw=2000 g/mol, Sigma-Aldrich) were dried at 80°C under vacuum prior to use. o-Tolidine (Sigma-Aldrich) and dibutyltin dilaurate (DBTDL, Sigma-Aldrich) were used as the chain extender and catalyst, respectively. Glacial acetic acid (Alfa-acer), anhydrous dimethylacetamide and dimethylformamide (Wako Pure Chemical Industries) were used as received. Synthesis of petiptycene-containing monomer. Pentiptycene-based diol (PEN-OH) and

pentiptycene-based diamine (PEN-NH) chain extenders were synthesized according to the previous reports (ESI, Experimental method, Scheme 1(a)).11, 30 In general, pentiptycene skeleton containing two phenolic groups at 1,4-positions was constructed via Diels-Alder cycloaddition of anthracene to p-benzoquinone, followed by reduction to pentiptycenebased diol (1). The hydroxyl group was then substituted with p-fluoro nitrobenzene by aromatic nucleophilic reaction to synthesize dinitro compound (2), which was further reduced by hydrazine to obtain pentiptycene-based diamine monomer (3). 1H-NMR spectra confirmed the structures of the PEN-OH and PEN-NH monomers (Figure S1). 3 ACS Paragon Plus Environment


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Synthesis of polymers. A series of pentiptycene-containing PUs with different hard

segment contents was synthesized by a bulk polymerization method between pluronic® L61, IPDI and the synthesized PEN-OH and PEN-NH (1 and 3) (ESI, Experimental method). To compare the effect of three-dimensional pentiptycene monomers, o-tolidine with a planar rigid structure was used as the chain extender to synthesize PU (Scheme 1). The molecular weight and polydispersity index of the synthesized polymers are reported in Table 1. Pentiptycene-based PUs and tolidine-based PU were designated as PEN-PUα (synthesized with PEN-OH), PEN-PUUα (synthesized with PEN-NH) and TOL-PUU, respectively, where α, the weight percent of the hard segment, is calculated by: α=

(1)

where n and M indicate the mole and molecular weight of the components. The PU membranes with an average thickness of 60µm were fabricated by casting the polymer solution (10 wt% in DMF) in a Teflon petri dish, followed by slow evaporation at 60°C and vacuum drying at 80°C for 24h. Thin film composite membranes (TFC) were prepared from 1wt% PU solution using roller blade coater (RK print coat instrument, UK) on PVDF support (MWCo. 50kDa, AMI Co. US). PTMSP (2 wt% in cyclohexane) was applied on PVDF sheets to serve as a gutter layer for smoothing the surface and preventing the pore penetration of PU solution. PTMSP with high permeability and low CO2 selectivity is a wise choice for the gutter layer as it almost has no effect on the separation performance.31-32 The thickness of the selective layer was found to be around 100nm by using cross-sectional SEM images.

Table 1. Molecular weight and Tg of the synthesized PUs Sample

Chain extender

Mw (g/mol)

PDI

Tg (°C)

TOL-PUU

Tolidine

49600

1.7

-77.5

PEN-PUU50

PEN-NH

28900

1.5

-47.0

PEN-PUU59

PEN-NH

20500

1.4

-27.0

PEN-PUU65

PEN-NH

19300

1.2

-6.5

PEN-PU50

PEN-OH

31200

1.5

-46.3


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Scheme 1: (a) Synthesis of pentiptycene diol (PEN-OH) and pentiptycene diamine (PEN-NH2); (b) demonstration of the synthesized PU structures; (c) the chain threading of PU backbone through the pentiptycene clefts.

Characterization methods. The molecular weight of the synthesized PUs was monitored by gel

permeation chromatography (GPC, Shimadzu, 800 series), calibrated with polystyrene standards. 1H NMR spectra were recorded on a Bruker 500 MHz spectrometer in deuterated dimethyl sulfoxide (DMSO-d6) and chloroform (CDCl3). ATR-FTIR spectra of the synthesized PUs were obtained in the range of 4000-600 cm-1 with a resolution of 4 cm-1 and 128 scans on an IRTracer-100 Shimadzu spectrometer. Wide angle X-ray diffraction (WAXD, Rigaku RINT XRD) analysis of the PUs was performed by monitoring the diffraction patterns at an angle 2θ from 5° to 40° and a scanning rate of 10°/min using Cu K-alpha radiation under a voltage of 40 kV and a current of 200 mA. Differential scanning calorimetry (DSC) of the synthesized PUs was carried out on a Bruker DSC 3100SA instrument at a heating rate of 10°C/min over a temperature range from -100°C to 200°C. Thermal transitions were recorded at a second heating program to remove the thermal history. Thermal stability of the PUs was analyzed by thermogravimetric analysis (TGA, Rigaku TG8120, Japan) in the temperature range of 50°C to 800°C under N2 atmosphere. The mechanical properties of the synthesized PUs were probed by using both universal testing machine (84-76 Series Tension 5 ACS Paragon Plus Environment


Tester, TMI Co.) and nanoindentation instrument (ENT 2100, Elinoix) with a 100 nm diamond tip. Each indent was made in the coated polymer samples on the surface of silicon wafer up to a maximum depth of around 1 µm. For each sample, 25 testing points were selected and the average value was finally reported. Membrane surface morphology was recorded by atomic force microscopy (AFM) (Nanowizard III, JPK instruments, Japan) operating in tapping mode at room temperature with a 125µm length silicone AFM cantilever. The morphology of TFC membranes was studied by scanning electron microscope (FESEM, Hitachi S-4800, Japan). The pure gas permeability of CO2, H2, CH4 and N2 was studied using the constant volume method at 25°C and 4bar. The separation performance of the membranes was also evaluated under CO2/N2 and CO2/CH4 mixed gas feeds at 25°C (ESI, Experimental method). The permeate composition was analyzed using the gas chromatography machine (GC-2014, Shimadzu, Japan). The pure and mixed gas permeability measurements were replicated three times for each sample to ascertain repeatability.

3. Results and discussion 3.1 Chemical and physical characterizations.

The influence of pentiptycene units on the chain packing of PUs was examined by WAXD patterns in Figure 1 (a). In all spectra, a broad halo peak at 2θ=20° suggests an amorphous structure of the synthesized PUs. This result is consistent with DSC profiles, in which no melting peaks are observed (Figure 1 (c)). For PEN-PUU samples, a new diffraction peak appears at 2θ=11°, which corresponds to the d-spacing value of 8.4 Å. It explains different chain packings for PEN-PUUs compared with the TOL-PUU structure. The disruption of chain packing is induced by pentiptycene since the d-spacing of polymer chains, 8.4 Å, is correlated well with the spatial dimension of pentiptycene unit (Figure S4). The intensity of the peak decreases at lower content of soft segments. FTIR spectra are utilized to investigate the interactions and phase separation of the synthesized PUs, (Figure 1 (b)). The intensity of the C-O-C stretching peak (at 1100 cm-1) and C-H stretching vibrations in the range of 2800-3000 cm-1 decreases at lower content of polyol. In contrast, the intensity of aromatic C-H bending (~750 cm-1) increases with increasing the pentiptycene content. The type and strength of hydrogen bonding in PUs can be specified by analysis of the intensity and the shift of the double carbonyl bands in the region of 1600-1800 cm-1.33-34 The peak at lower frequency (around 1620 cm-1) corresponds to the bonded carbonyl groups with NH functionalities. In contrast, the peak related to the free carbonyl groups appears at higher frequency (around 1730 cm-1) which suggests the hydrogen bonding interactions between the NH groups and C-O-C moieties in the soft segments.35 The free carbonyl band of PEN-PU50 is diminished and shifted to lower frequency for PEN-PUU50, indicating higher phase separation between the soft and hard segments. This is in agreement with previously reported structures since stronger hydrogen bonding in the hard domains of poly(urethane urea)s (PUUs) was observed.17, 36 Moreover, 6 ACS Paragon Plus Environment

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the intensity of bonded carbonyl peak for TOL-PUU is increased and clearly distinguished from the free carbonyl band, suggesting higher phase separation than the synthesized PENPUU samples. The bulky pentiptycene units may hinder the formation of strong interchain interactions in the hard segments, leading to reduced phase separation.37 For example, it was reported that triptycene-based PUs have weaker hydrogen bonding than PUs with planar structures.38 By comparing the intensity of carbonyl bands, it is evident that the hydrogen bonds are disrupted more by increasing the concentration of pentiptycene moieties. DSC measurements were conducted to study the thermal transition and phase separation behavior of copolymers. The absence of melting peaks in DSC profiles (Figure 1(c)), suggests an amorphous structure in all synthesized PUs. It was shown that the presence of polypropylene glycol (PPG) in the soft segments disrupts the regular chain packing and PEG crystallization.23 The soft segment glass transition temperature (Tg) of PUs is related to the phase separation of hard and soft domains. However, in most PU structures, the Tg of the hard segment cannot be detected because of the relatively weak signal of the transition from the incomplete phase separated hard domains.20 This observation is consistent with AFM images (Figure S3) where randomly distributed soft and hard domains throughout the film inferred the incomplete phase separated morphology. The motion of chains is less hindered by the hard domains of PUs in well phase separated morphology structures.39 The TOL-PUU shows a lower Tg at -77°C in regards to the value of –47°C for PEN-PU50 and PENPUU50 samples, which is attributed to its higher phase separation. This is in agreement with the FTIR spectra. Moreover, exploring the AFM images of TOL-PUU and PEN-PU50 displays noticeable changes in the phase separation behavior. In addition, a higher Tg of pentiptycene-based PUs is associated to the chain threading and interlocking with pentiptycene units which decreases the chain mobility (Scheme 1(c)). The effect of hard segment content on the Tg of PEN-PUUs is complicated. It was reported that the phase separation of copolymers increases with increasing the hard segment content.40 However, a different trend is observed here. For example, the Tg of PEN-PUU50 is increased to -27°C and -6°C for PEN-PUU59 and PEN-PUU65 respectively, indicating microphase mixing. This behavior could be rationalized by chain threading effects of pentiptycene units. It seems that the chain flexibility of polyol is more confined in samples with higher pentiptycene content, increasing the Tg value. 3.2 Thermal stability and mechanical properties

Mechanical properties are important toward the applicability of membranes under realistic industrial condition. Figure 1(d) shows the elastic modulus and hardness of the pentiptycene-based PUs measured by nanoindentation testing. The results are compared with those of non-pentiptycene counterparts. An isotropic response within the indentation load displacement was assumed for the calculation of modulus and hardness.41 The PENPUU50 exhibits more than 500% increase in elastic modulus and 300% enhancement in 7 ACS Paragon Plus Environment


Figure 1 Membrane characterization. (a) X-ray diffraction patterns, (b) FTIR spectra, (c) DSC results, (d) Young’s modulus and hardness, (e) TGA, and (f) DTGA thermograms of the synthesized PUs.

hardness compared to the TOL-PUU. The tensile properties also show the similar trend (ESI, Tensile property, Table S2). As a general observation, the PEN-PUUs are much more mechanically robust than other commercial rubbery polymer membranes. For example, the elastic modulus of PEN-PUU59 is more than 17 times higher than that of Pebax1657.42 More interestingly, the PEN-PUUs show better mechanical properties than chemically crosslinked PU membranes.28 The improved mechanical properties of the PEN-PUUs are explained by molecular reinforcement as a result of the chain threading and interlocking mechanism with pentiptycene units.43-44 However, weak molecular interactions and low


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phase separation of PEN-PU50 is responsible for lower mechanical properties compared to PEN-PUUs. The susceptibility of polymers to thermal degradation is a big concern toward the membrane applicability. Thermal stability of PUs was evaluated by TGA and the results are shown in Figure 1(e). The differential thermogravimetric analysis (DTGA) in Figure 1(f) exhibits two major steps of weight loss for all PUs. The first stage that starts around 300°C is attributed to the cleavage of urethane bonds.45 This stage is followed by decomposition of the soft segments at around 400°C. The weight loss value is consistent with the polyol content in the PUs. Since the same polyol was used for synthesis of PUs, the soft segment of PEN-PUU shows higher thermal stability than other structures. In addition, the char yield of PEN-PUU59 and PEN-PUU65 is greater than 70%, whilst the char yield of TOL-PUU is relatively low. It is supposed that chain interlocking between the pentiptycene clefts results in the chain reinforcement and consequently increases the energy required for decomposition. Obviously, PEN-PUU65 has the highest thermal stability because of the higher pentiptycene content. 3.3 Gas transport properties

Single gas permeability of the synthesized PUs was measured with various gases (CO2, CH4, N2 and H2) at 4 bar and 25°C. Table 2 presents the gas transport data of PU membranes along with the diffusivity and solubility coefficients calculated from the solution-diffusion model (see experimental section, ESI). Gas permeabilities of all PU structures follow the order of: P (CO2)> P (H2)> P (CH4)> P (N2). Higher gas permeability of CO2 than H2 is typical for rubbery polymers like PUs, where transport properties greatly rely on the gas solubility. A similar trend is also observed for CH4 with higher permeability than smaller N2 molecules. The sequence of gas permeability is consistent with the critical temperature of the gas molecules rather than the kinetic diameters: CO2 (304.2 K) > CH4 (190.9 K) > N2 (126.3 K) > H2 (33.2 K).46 The similar trend was reported for Pebax and other PU structures.23, 47-49 The variations in the chain extender structure greatly influence on the gas separation properties of PU membranes.17, 21 Compared with pentiptycene-based PU membranes, TOLPUU exhibits the highest permeability for all gases. For example, CO2 permeability of TOLPUU is about three times higher than the PEN-PUU50 with the same molar ratio of hard to soft segments (Figure 2(a)). Higher phase separation between the hard and soft segments is responsible for higher gas permeability, where the gas molecules are less hindered by the impermeable hard domains. In general, a decrease in the phase separation efficiently decreases the gas permeability of block copolymers.50 For examples, the permeability reduction in PEN-PU50 with the lowest phase separation is larger compared to other samples. Moreover, the low gas permeability of PEN-PUU65 and PEN-PUU59 with lower soft segment contents is likely attributed to the chain threading of polyols through the pentiptycene clefts (i.e. higher phase mixing).



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TOL-PUU has the highest CO2/N2 and CO2/H2 selectivities of 31.7 and 4.1, respectively which indicates a large contribution of solubility selectivity (Table 2). It was shown that high CO2/light gas selectivity is obtained for phase-separated PU structures.23 It is thought that the intermixing of the two domains, i.e. soft and hard, would adversely affect the accessibility of CO2 gas molecules to the adsorption sites of polyol, leading to a reduced solubility selectivity. Moreover, the diffusivity selectivity of PU membranes increases with decreasing the phase separation which is not favorable for CO2/H2 separation. For example, CO2/N2 and CO2/H2 selectivities of PEN-PUUs are lower than the values of TOL-PUU (Figure 2(a)). Mixed gas separation performance of PU membranes was measured using CO2/N2 and CO2/CH4 gas mixtures (50/50 vol.%) at 25°C and 8 bar, shown in Table S3. The mixed gas permeability and selectivity are slightly lower as regards to the pure gas data. For example, in TOL-PUU sample, the CO2 permeability of 95.2 and CO2/N2 selectivity of 31.7 for single gas measurement decreased to 80.0 and 28.6, respectively, under the CO2/N2 (50/50) mixed gas feed. The lower gas permeability and selectivity in the feed gas mixtures is due to the interactions and competition between gases to pass through the membrane.51-52 This reduction from the single gas measurement is more significant for CO2/CH4 gas mixture which is due to the competitive sorption of CH4 molecules.53 Table 2. Single gas separation properties, diffusivity and solubility coefficients of the PU membranes Gases

Ideal selectivity

Sample

a

CO2

H2

CH4

N2

CO2/CH4

CO2/N2

CO2/H2

a

TOL-PUU

P b D c S

95.2±7.6 8.3±0.5 11.5±1.1

23.5±1.6 52.4±0.5 0.4±0.0

10.0±0.7 4.5±0.3 2.2±0.2

3.0±0.2 6.0±0.4 0.5±0.0

9.5±1.0 1.8±0.2 5.2±0.7

31.7±3.3 1.4±0.1 23.0±2.2

4.1±0.4 0.2±0.0 28.8±2.8

PEN-PUU50

P D S

36.6±2.6 4.4±0.3 8.3±0.8

11.4±0.9 37.9±3.0 0.3±0.0

2.7±0.2 1.7±0.1 1.6±0.2

1.3±0.1 3.3±0.2 0.4±0.0

13.6±1.4 2.6±0.2 5.2±0.8

28.2±2.9 1.3±0.1 20.8±2.0

3.2±0.3 0.1±0.0 27.7±2.7

PEN-PUU59

P D S

28.7±2.3 4.0±0.3 7.2±0.8

9.6±0.8 36.5±2.9 0.3±.0

2.0±0.1 1.4±0.1 1.4±0.1

1.1±0.1 2.8±0.2 0.4±0.0

14.4±1.5 2.9±0.3 5.1±0.7

26.1±3.2 1.4±0.1 18.0±2.0

3.0±0.3 0.1±0.0 24.0±2.7

PEN-PUU65

P D S

18.0±1.4 3.9±0.3 4.6±0.5

6.7±0.5 34.0±2.4 0.2±0.0

1.1±0.1 1.0±0.1 1.3±0.0

0.9±0.1 2.2±0.2 0.4±0.0

16.3±1.5 3.9±0.5 3.5±0.4

20.0±2.7 1.8±0.2 11.5±1.3

2.7±0.3 0.1±0.0 23.0±2.5

PEN-PU50

P D S

12.4±0.9 2.8±0.2 4.4±0.5

4.8±0.4 26.8±1.9 0.2±0.0

1.0±0.1 0.7±0.0 1.4±0.0

0.5±0.0 1.6±0.1 0.3±0.0

12.2±1.3 4.0±0.4 3.1±0.4

24.8±2.3 1.8±0.2 14.7±1.7

2.6±0.3 0.1±0.0 22.0±2.5

−10

3

2

Permeability (10 cm (STP) cm/(cm s cmHg); 3 3 cm (STP)/cm cmHg)

b

-7

2

Diffusivity coefficient (10 cm /s);


c

Solubility coefficient (10

-3

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3.4 Thin film composite membrane and plasticization study

Practical membranes for commercial gas modules are generated using a thin selective layer coated onto a porous support. The thickness of the selective layer is normally a few hundred nanometers to provide high gas flux and productivity, where the porous support is required for mechanical stability. Table 3 presents the gas transport properties of TOL-PUU and PEN-PUU50 in the thin film composite (TFC) configuration at 25°C and 4bar. Higher degree of non-equilibrium free volume during the fast film formation makes the gas selectivity of TFC membranes lower than the bulk separation properties.17 However, it is found that the differences in the gas selectivity of TFC and bulk membranes for the PENPUU50 are smaller than the TOL-PUU. Figure 2 (b) exhibits that the PEN-PUU50 membrane with 100 nm thickness (see Figure 2(d)) retains about 90% of its bulk CO2/N2 and CO2/H2 selectivities, whilst the decrease in TOL-PUU thin film is nearly 30% of the bulk membrane values. This might be due to the fact that the enhanced mechanical properties in the PENPUU membranes increased the thin film stability. Figure 2 (c) shows the trade-off plot for CO2/N2 selectivity versus CO2 permeance for some new developed membranes reported in literatures for CO2 capture. In the case of PU membranes, the filled black circle indicates the crosslinked PU membrane,28 and the red circles are data for new developed materials in this work. The PEN-PUU50 and TOL-PUU have the modest CO2/N2 selectivity but much higher CO2 permeance than other polymer membranes considered for flue gas treatment. Practically, using high permeance membranes such as PEN-PUU50 is more economical to resolve the issue of pressure-ratio limit of CCS from flue gas rather than high selective but low permeance membranes.54 Implementation of TFC in aggressive industrial condition requires high stability against CO2-induced plasticization. An increase in the CO2 permeability with pressure indicates the plasticization which reduces the mechanical properties and the durability of membranes.55 CO2-induced plasticization behavior of the bulk and thin membranes of TOL-PUU and PENPUU50 was tracked at the range of 1 to 40 bar. The CO2 fugacity was used to calculate the gas permeability due to non-ideal behavior of CO2 at high pressure.56 The CO2 permeability of the membranes in Figure 2(e) indicates that PEN-PUU50 resists more against CO2 plasticization. This result suggests that the chain stiffening of the polymer backbone mitigates the plasticization effect. Generally, the restriction of the chain mobility is known as a successful strategy to suppress the CO2-induced plasticization.2-3 It has been Table 3. Single gas separation properties of the TOL-PUU and PEN-PUU50 thin film composite membranes Gas permeance (GPU) Sample TOL-PUU PEN-PUU50

CO2 1433.8±100.4 458.3±36.7

H2

Ideal selectivity

CH4

700.9±54 169.7±12.9

N2

CO2/CH4

CO2/N2

CO2/H2

183.7±12.5

65.6±4.9

7.8±1.0

21.8±2.2

2.1±0.2

38.2±3.1

17.9±1.3

12.0±1.4

25.5±2.8

2.7±0.3



demonstrated that the plasticization and aging behavior of polymer membranes are significantly influenced by the thickness.52, 57 The thin film membranes (~100nm thickness) undergo plasticization faster than the bulk membranes. The bulk PEN-PUU50 membrane (thickness~60µm) was plasticized at CO2 feed pressure of 28 bar, whilst in the TFC configuration (thickness~100nm), the CO2 plasticization started at 16 bar. It has been reported that membranes with lower thickness exhibits a higher cohesive energy density (CED) and consequently higher gas solubility coefficients.58-59 Therefore, the TFC membranes of PEN-PUU50 and TOL-PUU have higher CO2 sorption capacity that results in an accelerated plasticization behavior.

4. Conclusion The objective of this research was to address the fundamental issue of ultra-thin film membranes for gas separation, namely the lack of physical durability and low plasticization resistance. New types of PU chemistry with pentiptycene groups were synthesized in order to improve the plasticization resistance of the thin film membranes whilst its separation performance is not changed compared to the bulk membrane values. The PEN-PUU50 thin film membrane (thickness~100 nm) showed a high CO2 permeance of 450 GPU with a little decrease in CO2/N2 selectivity (~25.5) compared to the selectivity of bulk membrane (CO2/N2~28.2). Moreover, the thin film and bulk membrane of PEN-PUU50 resist against CO2-induced plasticization up to 16 bar and 28 bar CO2 pressure, respectively. The PENPUU65 with the highest amount of pentiptycene moieties showed a 1600% increase in the modulus and more than 50°C improvement in the thermal stability compared to the TOLPUU sample. The chain threading and interlocking mechanism with the pentiptycene units rigidified the polymer chains and provided higher mechanical and thermal properties. The results support the potential of the synthesized PU structures to apply in the thin film configuration to overcome the challenges that limit the use of PUs for practical applications. The PEN-PUU and TOL-PUU structures with high CO2 permeance (up to 1400 GPU) and improved mechanical properties are promising for a wide range of CO2 capture process.


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Figure 2 Thin film study. (a) Normalized CO2 separation for the bulk membranes at 4 bar and 25°C, (b) Comparison the CO2/N2 and CO2/H2 selectivities of the bulk and TFC membranes of TOL-PUU and PEN-PUU50, (c) Comparing the performance of PEN-PUU50 and TOL-PUU thin membranes (red filled circles) with some 60 developed commercial membranes (The CO2/N2 upper bound limit is reproduced from ref. ): (() 61-62 poly(amidoamine) dendrimer composite membrane , (7) supported ionic liquid membrane of aqueous 63 64 65 diethanolamine , (−) polyvinylamine thin film membrane , (8) polyallylamine blend thin membrane , (1) 66 67 28 TM 60 GKSS membrane , (Υ)data of Hendricks at al. , (,) crosslinked PU , (!) Polaris (d) Cross-sectional SEM image of the thin film composite membrane of PEN-PUU on α-alumina porous substrate, (e) CO2 plasticization of the bulk and TFC membranes of TOL-PUU and PEN-PUU50.



Associated content Supporting information is available free of charge on the ACS publication website. Experimental details for the synthesis of monomers and polymers, gas permeability measurement, AFM images and tensile properties of the membranes, mixed gas permeability data.

Acknowledgment Authors gratefully acknowledge JST-Mirai project funding and JST-A-Step funding. iCeMS is supported by the World Premier International Research Initiative (WPI), MEXT, Japan. References (1) Swaidan, R.; Ghanem, B.; Al-Saeedi, M.; Litwiller, E.; Pinnau, I. Role of Intrachain Rigidity in the Plasticization of Intrinsically Microporous Triptycene-Based Polyimide Membranes in Mixed-Gas CO2/CH4 Separations, Macromolecules 2014, 47, 7453-7462. (2) Japip, S.; Wang, H.; Xiao, Y.; Chung, T. S. Highly Permeable Zeolitic Imidazolate Framework (ZIF)71 Nano-Particles Enhanced Polyimide Membranes for Gas Separation, J. Membr. Sci. 2014, 467, 162-174. (3) Wind, J. D.; Sirard, S. M.; Paul, D. R.; Green, P. F.; Johnston, K. P.; Koros, W. J. Carbon DioxideInduced Plasticization of Polyimide Membranes: Pseudo-Equilibrium Relationships of Diffusion, Sorption, and Swelling, Macromolecules 2003, 36, 6433-6441. (4) Bachman, J. E.; Smith, Z. P.; Li, T.; Xu, T.; Long, J. R. Enhanced Ethylene Separation and Plasticization Resistance in Polymer Membranes Incorporating Metal-Organic Framework Nanocrystals, Nat. Mater. 2016, 15, 845-849. (5) Arabi Shamsabadi, A.; Seidi, F.; Nozari, M.; Soroush, M. A New Pentiptycene-Based Dianhydride and Its High-Free-Volume Polymer for CO2 Removal, ChemSusChem 2018, 11, 472-482. (6) Shrestha, B. B.; Wakimoto, K.; Wang, Z.; Isfahani, A. P.; Suma, T.; Sivaniah, E.; Ghalei, B. A Facile Synthesis of Contorted Spirobisindane-Diamine and Its Microporous Polyimides for Gas Separation, RSC Adv. 2018, 8, 6326-6330. (7) Ma, X.; Ghanem, B.; Salines, O.; Litwiller, E.; Pinnau, I. Synthesis and Effect of Physical Aging on Gas Transport Properties of a Microporous Polyimide Derived from a Novel Spirobifluorene-Based Dianhydride, ACS Macro Lett. 2015, 4, 231-235. (8) Rose, I.; Carta, M.; Malpass-Evans, R.; Ferrari, M.-C.; Bernardo, P.; Clarizia, G.; Jansen, J. C.; McKeown, N. B. Highly Permeable Benzotriptycene-Based Polymer of Intrinsic Microporosity, ACS Macro Lett. 2015, 4, 912-915. (9) Sydlik, S. A.; Chen, Z.; Swager, T. M. Triptycene Polyimides: Soluble Polymers with High Thermal Stability and Low Refractive Indices, Macromolecules 2011, 44, 976-980. (10) Mao, H.; Zhang, S. Synthesis, Characterization, and Gas Transport Properties of Novel IptyceneBased Poly [Bis (Benzimidazobenzisoquinolinones)], Polymer, 2014, 55, 102-109. (11) Luo, S.; Liu, Q.; Zhang, B.; Wiegand, J. R.; Freeman, B. D.; Guo, R. Pentiptycene-Based Polyimides with Hierarchically Controlled Molecular Cavity Architecture for Efficient Membrane Gas Separation, J. Membr. Sci. 2015, 480, 20-30. (12) Luo, S.; Wiegand, J. R.; Gao, P.; Doherty, C. M.; Hill, A. J.; Guo, R. Molecular Origins of Fast and Selective Gas Transport in Pentiptycene-Containing Polyimide Membranes and their Physical Aging Behavior, J. Membr. Sci. 2016, 518, 100-109. (13) Swaidan, R.; Al-Saeedi, M.; Ghanem, B.; Litwiller, E.; Pinnau, I. Rational Design of Intrinsically Ultramicroporous Polyimides Containing Bridgehead-Substituted Triptycene for Highly Selective and Permeable Gas Separation Membranes, Macromolecules 2014, 47, 5104-5114. (14) Wiegand, J. R.; Smith, Z. P.; Liu, Q.; Patterson, C. T.; Freeman, B. D.; Guo, R. Synthesis and Characterization of Triptycene-Based Polyimides with Tunable High Fractional Free Volume for Gas Separation Membranes, J. Mater. Chem. A 2014, 2, 13309-13320.


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N2

CO2

Polyurethane selective laye

CO2 separation

r

mbrane Ceramic suppme ort

Gutt

y er La

er

Pentiptycene-based Polyurethane


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Pentiptycene-Based Polyurethane with Enhanced Mechanical

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