Pentiptycene-Based Polyurethane with Enhanced Mechanical

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 17366−17374

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 Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Isfahan, Iran

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‡

S Supporting Information *

ABSTRACT: The 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 the 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−2 s−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 result 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 twofold enhanced plasticization resistance compared to non-pentiptycene-containing PU membranes. KEYWORDS: gas separation membranes, thin film composite, CO2 induced plasticization, CO2 capture, polyurethane, pentiptycene

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. The 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 the introduction of bulky side groups or a contorted structure into the polymer backbone. The restriction of the chain mobility through cross-linking, annealing, and mixed matrix membrane fabrication aims to stop the plasticization and improve the thermomechanical properties. However, a great loss in the permeability and the limitations for scaling up these methods should be considered.1−4 © 2018 American Chemical Society

High free-volume polymers for membrane applications are highly desired. The creation of free volume is generally obtained by the 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 paddlelike triptycene moiety exhibit improved gas permeability compared to the traditional polyimides (e.g., Matrimid).9 The enhancement is greater for pentiptycene-based polyimides with nearly Received: December 5, 2017 Accepted: April 30, 2018 Published: April 30, 2018 17366

DOI: 10.1021/acsami.7b18475 ACS Appl. Mater. Interfaces 2018, 10, 17366−17374

Research Article

ACS Applied Materials & Interfaces

Scheme 1. (a) Synthesis of Pentiptycene Diol (PEN-OH) and Pentiptycene Diamine (PEN-NH2); (b) Demonstration of the Synthesized PU Structures; and (c) Chain Threading of the PU Backbone through the Pentiptycene Clefts

N2 selectivity up to 50 which is comparable to the separation performance of the commercial membrane materials, such as Polyactive24,25 and Pebax 1657.26,27 However, the practical application of the PU membranes is limited by weak thermomechanical stability and low plasticization resistance under realistic gas separation conditions. In our previous work,28 the post-cross-linking process was introduced to overcome the low thermal and mechanical stability of the PU membranes. The chemical cross-linking rigidified the PU chains and showed a threefold enhanced plasticization resistance compared to the non-cross-linked PU membrane. However, the difficulties associated with the removal of byproducts, complicated reaction, and moisture-sensitive components make the post-cross-linking method undesirable, especially for the development of thin film gas separation membranes. A simpler approach than the post-cross-linking is through the direct synthesis of the 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 the PU membranes.

50% higher CO2 permeability than that of the triptycene-based polyimide membrane.10 The H-shaped molecular structure of pentiptycene provides larger fractional free volume and thereby higher gas permeability.11,12 In addition, 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 the 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 the semicrystalline 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 a high CO2 permeability of 150 barrer and the CO2/ 17367


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ACS Applied Materials & Interfaces 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 plasticizationresistant PU membranes using the pentiptycene diamine chain extender. The rigid structure of pentiptycene with five arene rings and H-shaped 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 the PU structures with high mechanical properties and enhanced CO2-induced plasticization resistance that can be easily applied for the thin film membrane. In contrast, current methods to improve mechanical properties of polymer membranes (e.g., mixed matrix membranes, cross-linking) are not suitable for thin film fabrication.

α=

(1) where n and M indicate the mole and molecular weight of the components, respectively. The PU membranes with an average thickness of 60 μm 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 24 h. Thin film composite (TFC) membranes were prepared from 1 wt % PU solution using a roller blade coater (RK Print Coat Instrument, UK) on a polyvinylidene fluoride (PVDF) support (molecular weight cutoff 50 kDa, AMI Co. US). Poly[1-(trimethylsilyl)-1-propyne] (PTMSP, 2 wt % in cyclohexane) was applied on the PVDF sheets to serve as a gutter layer for smoothing the surface and preventing the pore penetration of the 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 100 nm by using cross-sectional scanning electron microscopy (SEM) images. 2.4. Characterization Methods. The molecular weight of the synthesized PUs was monitored by gel permeation chromatography (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). Attenuated total reflection Fourier transform infrared (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 Kα 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 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 50−800 °C under an N2 atmosphere. The mechanical properties of the synthesized PUs were probed by using both the universal testing machine (84-76 Series Tension Tester, TMI Co.) and the 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. The membrane surface morphology was recorded by atomic force microscopy (AFM) (NanoWizard III, JPK Instruments, Japan) operating in a tapping mode at room temperature with a 125 μm length silicone AFM cantilever. The morphology of TFC membranes was studied by a scanning electron microscope (FESEM, Hitachi S4800, Japan). The pure gas permeability of CO2, H2, CH4, and N2 was studied using the constant volume method at 25 °C and 4 bar. The separation performance of the membranes was also evaluated under CO2/N2 and CO2/CH4 mixed gas feeds at 25 °C (Supporting Information, 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.

2. MATERIALS AND METHODS 2.1. 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, pbenzoquinone, p-fluoro nitrobenzene, and isophorone diisocyanate (IPDI) were purchased from Wako Pure Chemical Industries (Japan) and used as received. Polyethylene glycol-block-polypropylene glycolblock-polyethylene glycol (Pluronic L-61, Mw = 2000 g/mol, SigmaAldrich) was dried at 80 °C under vacuum prior to use. o-Tolidine (Sigma-Aldrich) and dibutyltin dilaurate (Sigma-Aldrich) were used as the chain extender and catalyst, respectively. Glacial acetic acid (Alfa Aesar), anhydrous dimethylacetamide, and dimethylformamide (DMF) (Wako Pure Chemical Industries) were used as received. 2.2. Synthesis of Pentiptycene-Containing Monomer. Pentiptycene-based diol (PEN-OH) and pentiptycene-based diamine (PEN-NH) chain extenders were synthesized according to the previous reports (Supporting Information, Experimental Method, Scheme 1a).11,30 In general, the pentiptycene skeleton containing two phenolic groups at 1,4-positions was constructed via Diels−Alder cycloaddition of anthracene to p-benzoquinone, followed by a reduction to PEN-OH (1). The hydroxyl group was then substituted with p-fluoro nitrobenzene by aromatic nucleophilic reaction to synthesize the dinitro compound (2), which was further reduced by hydrazine to obtain PEN-NH monomer (3). 1H NMR spectra confirmed the structures of the PEN-OH and PEN-NH monomers (Figure S1). 2.3. Synthesis of Polymers. A series of pentiptycene-containing PUs with different hard segment contents were synthesized by a bulk polymerization method between Pluronic L-61, IPDI, and the synthesized PEN-OH and PEN-NH (1 and 3) (Supporting Information, Experimental method). To compare the effect of threedimensional 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 (PDI) 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-poly(urethane urea) (PUU)α (synthesized with PEN-NH), and TOL-PUU, respectively, where α, the weight percent of the hard segment, is calculated by:

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, as shown in Figure 1a. In all the spectra, a broad halo peak at 2θ = 20° suggests an amorphous structure of the synthesized PUs. This result is consistent with the DSC profiles, in which no melting peaks are observed (Figure 1c). For PEN-PUU samples, a new diffraction peak appears at 2θ = 11°, which corresponds to the d-spacing

Table 1. Molecular Weight and Tg of the Synthesized PUs sample

chain extender

Mw (g/mol)

PDI

Tg (°C)

TOL-PUU PEN-PUU50 PEN-PUU59 PEN-PUU65 PEN-PU50

tolidine PEN-NH PEN-NH PEN-NH PEN-OH

49 600 28 900 20 500 19 300 31 200

1.7 1.5 1.4 1.2 1.5

−77.5 −47.0 −27.0 −6.5 −46.3

nIPDIMIPDI + nchain extenderMchain extender nIPDIMIPDI + nchain extenderMchain extender + nPluronicMPluronic

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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.

indicating a higher phase separation between the soft and hard segments. This is in agreement with the previously reported structures since stronger hydrogen bonding in the hard domains of PUUs was observed.17,36 Moreover, the intensity of the bonded carbonyl peak for TOL-PUU is increased and clearly distinguished from the free-carbonyl band, suggesting a higher phase separation than the synthesized PEN-PUU samples. The bulky pentiptycene units may hinder the formation of strong interchain interactions in the hard segments, leading to a 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 the 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 the DSC profiles (Figure 1c) suggests an amorphous structure in all the synthesized PUs. It was shown that the presence of polypropylene glycol 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 the hard and

value of 8.4 Å. It explains different chain packings for PENPUUs compared with the TOL-PUU structure. The disruption of chain packing is induced by pentiptycene as the d-spacing of polymer chains, 8.4 Å, correlates well with the spatial dimension of the pentiptycene unit (Figure S4). The intensity of the peak decreases at the lower content of the soft segments. FTIR spectra are utilized to investigate the interactions and phase separation of the synthesized PUs (Figure 1b). 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 the lower content of polyol. In contrast, the intensity of aromatic C−H bending (∼750 cm−1) increases with increasing pentiptycene content. The type and strength of hydrogen bonding in PUs can be specified by the 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 a 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 a 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 a lower frequency for PEN-PUU50, 17369


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ACS Applied Materials & Interfaces Table 2. Single Gas Separation Properties, Diffusivity, and Solubility Coefficients of the PU Membranes gases sample TOL-PUU

PEN-PUU50

PEN-PUU59

PEN-PUU65

PEN-PU50

a

CO2 a

P Db Sc P D S P D S P D S P D S

95.2 8.3 11.5 36.6 4.4 8.3 28.7 4.0 7.2 18.0 3.9 4.6 12.4 2.8 4.4

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

H2 7.6 0.5 1.1 2.6 0.3 0.8 2.3 0.3 0.8 1.4 0.3 0.5 0.9 0.2 0.5

23.5 52.4 0.4 11.4 37.9 0.3 9.6 36.5 0.3 6.7 34.0 0.2 4.8 26.8 0.2

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

ideal selectivity CH4

1.6 0.5 0.0 0.9 3.0 0.0 0.8 2.9 0.0 0.5 2.4 0.0 0.4 1.9 0.0

10.0 4.5 2.2 2.7 1.7 1.6 2.0 1.4 1.4 1.1 1.0 1.3 1.0 0.7 1.4

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

N2 0.7 0.3 0.2 0.2 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.0 0.1 0.0 0.0

3.0 6.0 0.5 1.3 3.3 0.4 1.1 2.8 0.4 0.9 2.2 0.4 0.5 1.6 0.3

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

CO2/CH4 0.2 0.4 0.0 0.1 0.2 0.0 0.1 0.2 0.0 0.1 0.2 0.0 0.0 0.1 0.0

9.5 1.8 5.2 13.6 2.6 5.2 14.4 2.9 5.1 16.3 3.9 3.5 12.2 4.0 3.1

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.0 0.2 0.7 1.4 0.2 0.8 1.5 0.3 0.7 1.5 0.5 0.4 1.3 0.4 0.4

CO2/N2 31.7 1.4 23.0 28.2 1.3 20.8 26.1 1.4 18.0 20.0 1.8 11.5 24.8 1.8 14.7

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

3.3 0.1 2.2 2.9 0.1 2.0 3.2 0.1 2.0 2.7 0.2 1.3 2.3 0.2 1.7

CO2/H2 4.1 0.2 28.8 3.2 0.1 27.7 3.0 0.1 24.0 2.7 0.1 23.0 2.6 0.1 22.0

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.4 0.0 2.8 0.3 0.0 2.7 0.3 0.0 2.7 0.3 0.0 2.5 0.3 0.0 2.5

Permeability (10−10 cm3(STP) cm/cm2 s cmHg). bDiffusivity coefficient (10−7 cm2/s). cSolubility coefficient (10−3 cm3(STP)/cm3 cmHg).

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 the AFM images (Figure S3) where randomly distributed soft and hard domains throughout the film inferred the incomplete phaseseparated morphology. The motion of chains is less hindered by the hard domains of PUs in the well-phase-separated morphology structures.39 The TOL-PUU shows a lower Tg at −77 °C with regard to the value of −47 °C for the PEN-PU50 and PEN-PUU50 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 PENPU50 displays noticeable changes in the phase separation behavior. In addition, a higher Tg of pentiptycene-based PUs is associated with the chain threading and interlocking with pentiptycene units which decreases the chain mobility (Scheme 1c). The effect of hard segment content on the Tg of PENPUUs is complicated. It was reported that the phase separation of copolymers increases with the increase in the hard segment content.40 However, a different trend is observed here. For example, the Tg of PEN-PUU50 is increased to −27 and −6 °C for PEN-PUU59 and PEN-PUU65, respectively, indicating a microphase mixing. This behavior could be rationalized by the chain-threading effects of the 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 1d shows the elastic modulus and hardness of the pentiptycenebased PUs measured by nanoindentation testing. The results are compared with those of the non-pentiptycene counterparts. An isotropic response within the indentation load displacement was assumed for the calculation of modulus and hardness.41 The PEN-PUU50 exhibits more than 500% increase in elastic modulus and 300% enhancement in hardness compared to the TOL-PUU. The tensile properties also show a similar trend (Supporting Information, 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 Pebax1657.42 More interestingly, the PEN-PUUs show better mechanical properties than the chemically cross-linked 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 phase separation of PEN-PU50 is responsible for lower mechanical properties compared to PEN-PUUs. The susceptibility of the polymers to thermal degradation is a big concern toward the membrane applicability. The thermal stability of the PUs was evaluated by TGA, and the results are shown in Figure 1e. The differential TGA (DTGA) in Figure 1f 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 the 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 synthesizing PUs, it can be concluded that the soft segment of the PEN-PUU shows higher thermal stability than other structures. In addition, the char yield of PEN-PUU59 and PEN-PUU65 is greater than 70%, whereas the char yield of TOL-PUU is relatively low. It is supposed that the 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 the PU membranes along with the diffusivity and solubility coefficients calculated from the solution-diffusion model (see Experimental Method, Supporting Information). Gas permeabilities of all the PU structures follow the order of P(CO2) > P(H2) > P(CH4) > P(N2). A 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 a 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 17370


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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 of the CO2/N2 and CO2/H2 selectivities of the bulk and TFC membranes of TOL-PUU and PEN-PUU50, (c) comparing the performance of the PEN-PUU50 and TOL-PUU thin membranes (red filled circles) with some developed commercial membranes (the CO2/N2 upper-bound limit is reproduced from ref 60): (upper-half filled squares) poly(amidoamine) dendrimer composite membrane,61,62 (black filled triangle) supported ionic liquid membrane of aqueous diethanolamine,63 (open circle) polyvinylamine thin film membrane,64 (open triangle) polyallylamine blend thin membrane,65 (two-half circle) GKSS membrane,66 (half-filled diamonds) data of Hendriks et al.,67 (black filled circle) cross-linked PU,28 (black filled square) Polaris,60 (d) cross-sectional SEM image of the TFC membrane of PEN-PUU on an α-alumina porous substrate, and (e) CO2 plasticization of the bulk and TFC membranes of TOL-PUU and PEN-PUU50

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 have great influence on the gas separation properties of the PU membranes.17,21 Compared with the pentiptycene-based PU membranes, TOL-PUU exhibits the highest permeability for all gases. For example, the CO2 permeability of TOL-PUU is about 3 times higher than that of PEN-PUU50 with the same molar ratio of hard to soft segments (Figure 2a). The higher phase separation between the hard and soft segments is responsible for the 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 the block copolymers.50 For example, the permeability reduction in PEN-PU50 with the lowest phase separation is larger compared to the 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).

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 phaseseparated PU structures.23 It is thought that the intermixing of the two domains, 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 the PU membranes increases with decreasing 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 2a). The mixed gas separation performance of the PU membranes was measured using CO2/N2 and CO2/CH4 gas mixtures (50/ 50 vol %) at 25 °C and 8 bar, as shown in Table S3. The mixed gas permeability and selectivity are slightly lower with regard to the pure gas data. For example, in the 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, 17371


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Table 3. Single Gas Separation Properties of the TOL-PUU and PEN-PUU50 Thin Film Composite Membranes gas permeance (GPU)

ideal selectivity

sample

CO2

H2

CH4

N2

CO2/CH4

CO2/N2

CO2/H2

TOL-PUU PEN-PUU50

1433.8 ± 100.4 458.3 ± 36.7

700.9 ± 54 169.7 ± 12.9

183.7 ± 12.5 38.2 ± 3.1

65.6 ± 4.9 17.9 ± 1.3

7.8 ± 1.0 12.0 ± 1.4

21.8 ± 2.2 25.5 ± 2.8

2.1 ± 0.2 2.7 ± 0.3

undergo plasticization faster than the bulk membranes. The bulk PEN-PUU50 membrane (thickness ≈ 60 μm) was plasticized at a CO2 feed pressure of 28 bar, whereas in the TFC configuration (thickness ≈ 100 nm), the CO 2 plasticization started at 16 bar. It has been reported that the membranes with lower thickness exhibits a higher cohesive energy density and consequently higher gas solubility coefficients.58,59 Therefore, the TFC membranes of PENPUU50 and TOL-PUU have higher CO2 sorption capacity that results in an accelerated plasticization behavior.

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 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 TFC configuration at 25 °C and 4 bar. A higher degree of nonequilibrium-free volume during the fast film formation makes the gas selectivity of the TFC membranes lower than the bulk separation properties.17 However, it is found that the differences in the gas selectivity of the TFC and bulk membranes for the PEN-PUU50 are smaller than the TOL-PUU. Figure 2b exhibits that the PEN-PUU50 membrane with a 100 nm thickness (see Figure 2d) retains about 90% of its bulk CO2/N2 and CO2/H2 selectivities, whereas 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 PEN-PUU membranes increased the thin film stability. Figure 2c shows the trade-off plot for CO2/N2 selectivity versus CO2 permeance for some recently reported membranes in the literature for CO2 capture. In the case of PU membranes, the filled black circle indicates the cross-linked PU membrane,28 and the red circles are the data of developed membranes 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 PENPUU50 is more economical to resolve the issue of pressure ratio limit of carbon capture and storage (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 the membranes.55 The CO2-induced plasticization behavior of the bulk and thin membranes of TOL-PUU and PEN-PUU50 was tracked at the range of 1−40 bar. The CO2 fugacity was used to calculate the gas permeability due to nonideal behavior of CO2 at high pressure.56 The CO2 permeability of the membranes in Figure 2e 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 demonstrated that the plasticization and aging behavior of the polymer membranes are significantly influenced by the thickness.52,57 The thin film membranes (∼100 nm thickness)

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 structures with pentiptycene groups were synthesized in order to improve the plasticization resistance of the thin film membranes while its separation performance is not changed compared to the bulk membrane values. The PENPUU50 thin film membrane (thickness ≈ 100 nm) showed a high CO2 permeance of 450 GPU with a little decrease in the CO2/N2 selectivity (∼25.5) compared to the selectivity of the thick membrane (CO2/N2 ≈ 28.2). Moreover, the thin film and bulk membrane of PEN-PUU50 resist against the CO2-induced plasticization up to 16 and 28 bar CO2 pressure, respectively. The PEN-PUU65 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 TOL-PUU 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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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b18475. Experimental details for the synthesis of monomers and polymers, gas permeability measurement, AFM images and tensile properties of the membranes, and mixed gas permeability data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.S.). *E-mail: [email protected] (B.G.). *E-mail: [email protected] (E.S.). ORCID

Behnam Ghalei: 0000-0003-2848-9138 17372


Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The 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.

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