Molecular hybridization of polydimethylsiloxane with zirconia for highly

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Molecular hybridization of polydimethylsiloxane with zirconia for highly gas permeable membranes Roman Selyanchyn, and Shigenori Fujikawa ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00178 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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ACS Applied Polymer Materials

Molecular hybridization of polydimethylsiloxane with zirconia for highly gas permeable membranes Roman Selyanchyn,†, * and Shigenori Fujikawa†, ‡,§ †WPI

International Institute for Carbon-Neutral Energy Research (WPI-I2CNER) Kyushu

University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ‡Center

for Molecular Systems (CMS), Kyushu University, Kyushu University, 744 Motooka,

Nishi-ku, Fukuoka 819-0395, Japan §NanoMembrane

Technologies Inc., 4-1, Kyudai-Shimachi, Nishi-ku, Fukuoka, 819-0388, Japan

ABSTRACT Inorganic-organic nanocomposite hybrids containing zirconium dioxide (ZrO2) as inorganic crosslinker/filler and polydimethylsiloxane (PDMS) as a polymeric matrix have been synthesized using the in-situ sol-gel reaction between silanol-terminated PDMS and zirconium normal butoxide (Zr(OC4H9)4). Hybrid materials were used to fabricate gas separation membranes which were characterized by scanning electron microscopy, dynamic scanning calorimetry, nanoindentation, ATR-FTIR, and XPS spectroscopies. Amorphous structure of incorporated ZrO2 fillers was verified by x-ray diffraction. Small gases (He, H2, O2, N2, and CO2) permeability experiments were carried out to study the effect of the inorganic component amount on the

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properties of the ZrO2@PDMS hybrids. The permeability of the developed hybrids considerably exceeded the permeability of conventional PDMS which is known as “gold standard” highly gaspermeable rubbery polymer. Depending on the ZrO2 content fabricated hybrids demonstrated increased permeability for all gases with improvement inversely proportional to the kinetic diameter of gas molecules, i.e., highest permeability increase (relatively to PDMS) was observed for H2 and lowest for N2. Such behavior suggests the formation of the size-sieving amorphous zirconia domains within PDMS which do not impede gas transport due to the nanosize of the fillers. As a result, gas separation membranes prepared using the developed materials demonstrated better separation performance for CO2/N2, H2/N2, and O2/N2 pairs compared to the conventional PDMS. KEYWORDS: metal oxide, PDMS, nanocomposite, gas separation, membrane

1. INTRODUCTION Carbon dioxide emissions into the atmosphere are considered as a major contributor to planetary scale greenhouse effect resulting in global warming [1]. Large scale and economically feasible CO2 capture at mass-emission points (e.g., fossil fuel 　 power stations, cement plants, etc.) and subsequent storage of captured gas (CCS) is recognized as a necessary prerequisite for sustainable energy use in a future. Well-established technology of CO2 scrubbing from flue gases by liquid amines does not meet the cost requirements since it uses the heating process to release captured CO2, resulting in high energy consumption. Therefore, the development of different CO2 capture solutions is critically needed. Gas separation by membranes is one of the alternatives, which potentially 2 ACS Paragon Plus Environment

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can reduce the capture costs and energy down to affordable level for the industry, incentivized by governments tax credits [2]. Gas separation membranes made of pure organic polymers demonstrate the trade-off behavior between the permeability and selectivity of the target gas in a pair being separated [3-5]. For these reasons, recent research works are seeking new materials that are able to break the ‘upper bound’ limit of performance. Metal organic frameworks (MOFs), polymers of intrinsic microporosity (PIMs), microporous carbons are some of the promising materials for the gas separation. However, there are still challenges of defects formation in MOF membranes or aging effects in PIMs both having negative influence on gases separation. Metal oxides (MO) are the class of materials with excellent chemical and thermal stability, widely investigated for the energy-related and catalysis applications [6]. There is also a significant interest to utilize MO in membranes, however, rigid structure leads to macro-fragility which is a significant material disadvantage. Therefore, bulk MO materials or thick MO-based membranes become fragile and more difficult to process, compared to that made of organic polymers. Combination of useful properties of metal oxides with the flexibility of organic polymers offers great potential for many useful applications. In particular, transition metal oxides such as TiO2 and ZrO2 get increased attention due to their abundance and low cost. In part, it is inspired by the application of silicon oxide fillers for the various purposes such as polymers reinforcement [7], novel biomaterials for bone tissue engineering [8] molecular imprinting [9], gas transport improvement [10], etc. Polydimethylsiloxane (PDMS) represents a valuable inorganic polymer (according to IUPAC classification) [11]. High gases permeability, rubbery nature and low-cost are



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attractive features of the polymer causing its broad application in membrane science and technology field [12]. PDMS membranes deposited on the porous supports are particularly reliable for volatile organic compounds capture and have been marketed for more than several decades [13]. Also, PDMS is widely used as a gutter layer in the thin film composites (TFC) membranes. The main purpose of the gutter material is to seal the defects of the porous support layer and smooth it for the subsequent selective layer deposition [1416]. In our previous work, for example, ca. 1 micron thick PDMS layer was used to support the thin 100 nm TiO2 membrane incorporated with CO2-philic molecules [17]. Despite its ca. ten times higher thickness compared to the selective layer, the PDMS gutter layer does not make a significant resistance to gas flow in most of the cases. However, when the target permeances of the separation membranes are high (e.g., for large-scale CO2 capture) the resistance of the gutter layer as well as porous support becomes meaningful [18, 19]. Therefore, more permeable materials suitable to substitute PDMS in the thin-film composite membranes are needed. Hybrids of PDMS and metal oxides are a relatively novel class of materials aimed to combine useful properties of both components to obtain improved material functions. Combination of a very flexible PDMS with hard MO is possible via well-known sol-gel process between a silanol terminated PDMS (PDMS-OH) and metal alkoxides (MAO) as material precursors. This type of elastic hybrids with substantially high content of metal oxide component is often referred to as flexible ceramics due to increased flexibility in contrast to rigid pure ceramic materials [20] even though the materials are not pure ceramics. Due to the high reactivity of MAO with water, chelating agents (e.g., ethyl acetoacetate, EtAcAc) are commonly used to control the hydrolysis rate of MAO during


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the hybrids formation making it possible to add liquid water to initiate hydrolysis [21, 22, 23]. It is, however, difficult to remove hydrolysis suppressors from the final materials. The synthesis, however, can follow a simplified route by first mixing of PDMS-OH and metal alkoxide in a suitable solvent, achieving complexation at an increased temperature following the solvent evaporation and slow alkoxide hydrolysis in a reaction with atmospheric water [24]. In this work, we report the preparation of zirconium oxide-PDMS hybrids membranes with high gas permeabilities and improved CO2/N2 selectivities, via the simple sol-gel process. The chemistry behind the material formation follows the conventional sol-gel reaction pathway as given in Fig.1. The process is started from complexation reactions between zirconium n-butoxide (ZrBO) and PDMS-OH that may be promoted by mild heating. At this time, hydroxyl groups of PDMS-OH react with the alkoxide moieties of ZrBO and butanol is eliminated upon the complexes formation. Following the complexation, the mixture is dried from solvent under ambient conditions with the slow exposure to atmospheric humidity (i.e. partially closed vessel). During this process, most of the unreacted alkoxide groups are hydrolyzed and alcohol eliminated. The process of composite formation is finalized by the prolonged heating at a higher temperature (150 C) to eliminate the water from the zirconium hydroxide (condensation reaction), therefore the final state of the material is considered to be a well-dispersed mixture of ZrO2 and PDMS. As a result of the given reactions in these types of materials ZrO2 itself plays the role of PDMS cross-linker as shown schematically in Fig. 1.



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Fig. 1. Schematic illustration of the ZrO2@PDMS hybrid formation in the sol-gel reaction between zirconium alkoxide with hydroxyl terminated PDMS-OH, and the final structure assuming complete hydrolysis of alkoxide into ZrO2.

The synthesized hybrids were used to prepare the membranes, and their structure and properties were investigated. The nanocomposites with the amount of crosslinker up to 20 wt% despite the material uniformity, can be viewed as mixed matrix membranes for the gases separation. The surprisingly high gases permeability was observed in some compositions concerted with the optimal mechanical properties that make these nanocomposites promising materials for the utilization in gas separation membranes. In the most immediate applications, we suggest that synthesized hybrids can be used for gutter layer formations in TFC as alternatives to PDMS.


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2. EXPERIMENTAL METHODS 2.1 Materials Silanol-terminated polydimethylsiloxane (PDMS-OH) with different molecular weights (550 – 37500 g/mol) and respective viscosities (16-2000 cst) were purchased from Gelest. Two-component kit for the fabrication of silica filled PDMS (Sylgard 184) was purchased from Dow Corning. Zirconium n-tetrabutoxide (ZrBO, Kanto Chemical) was used as a metal oxide precursor. Hexane, chloroform, isopropanol (IPA, Wako) were used as solvents for the precursor mixtures. Poly-sodium styrene sulfonate (PSS) was purchased from Sigma-Aldrich and used for the fabrication of water-soluble sacrificial layer on glass. Pure deionized water (18.3 M cm−1, Millipore, Direct-QTM) was used for PSS sacrificial layer dissolution.

2.2 Hybrid membranes fabrication For the preparation of precursor mixture, ZrBO was first mixed with a certain amount of hexane while stirring. To prevent possible abrupt gel formation, silanol terminated PDMS was then slowly added by small portions while the solution was intensively stirred. Hybrids with 5, 10, 15 and 20 wt.% of ZrO2 in PDMS were obtained using molar ratios between ZrBO:PDMS-OH approximately equal to 2.5:1, 5.4:1, 8.6:1, 12.1:1 respectively, considering molecular weight of PDMS-OH precursor is 6000 gmol-1. After proper mixing, the solution was stirred for additional 2 hours for the complexation of alkoxide and PDMS-OH. The solution concentrations were adjusted to obtain 5%, 10%, 15% or 20% of the mass content of ZrO2 in the final composition (assuming the complete hydrolysis of alkoxide). To prepare the membrane (detailed schematics in Fig. S1), a certain amount of



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hybrid precursor solution was cast in Teflon dish or glass with PSS sacrificial layer and slowly dried at 60 C for 24 hours. Subsequently, membranes were heated at 150 C in the oven for two days. After that, membranes were peeled off the Teflon dish (or detached from glass upon PSS dissolution), characterized and assembled for the gas permeability measurements. In a preliminary study, PDMS-OH with different molecular weights (from 550 Da up to 37500 Da) were used to obtain the membranes with optimal mechanical properties. Membranes used in this work were prepared by using PDMS-OH with the molecular weight of ca. 6000. Two component PDMS kit (Sylgard 184) was used for the reference membrane preparation (further referred as conventional PDMS). Material was prepared according to the kit instructions by mixing A and B components with 10:1 ratio. Before casting, mixture was degassed by sonication. Casted membrane was cured at the temperature 125 C for 20 minutes as recommended by provider. Although exact composition of Sylgard 184 precursors is unknown, it does contain silica fillers. Thermal gravimetric analysis conducted by Wang et al. suggested the content of silica fillers in Sylgard 184 ~30 wt% [25] which makes it a good reference for our ZrO2-filled membranes.

2.3. Characterization The chemical structure of fabricated composite membranes was analyzed by scanning FTIR Microscope (Nicolet iN10 MX) in the attenuated total reflectance (ATR) mode with the scan range of 4000–650 cm−1. The chemical composition of the composite membranes was also determined using XPS (PHI 5000 Versa Probe/Scanning ESCA microprobe with Mg Kα radiation, 12 kV, 10 mA).


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Morphology of the membranes cross-section was observed using field emission scanning electron microscope (JSM-7900F, JEOL, Japan). For cross-section observation, thick membranes were cut using a sharp knife and attached to the holder using double-sided conductive carbon tape. In order to prevent charge-up, the samples were coated with a thin platinum layer using an ion sputter (Hitachi E-1030) before the observation. X-ray diffraction (XRD) of all samples was measured using a Rigaku Smartlab X-ray diffractometer (Japan) with Cu Kα radiation. Small-angle X-ray scattering (SAXS) data on a 10%-ZrO2@PDMS were collected using Rigaku NANO-Viewer (Japan) X-ray system. The acquisitions were performed in transmission mode, with the distances of 14 cm from sample-to-detector. The mechanical properties (effective Young’s modulus and hardness) of the nanocomposite films were measured by the nanoindentation with Berkovich geometry indenter (Ultra-nanoindentation hardness tester, OPX-UNHT, Anton-Paar TriTec SA, Switzerland). The calculations of elastic modulus and hardness were done using the UNHT software that is based on the Oliver and Pharr theory [26, 27]. Differential scanning calorimetry (DSC) was carried out using a Netzsch instrument (DSC 204 F1 Phoenix®) under a nitrogen atmosphere (50 mL·min−1). The samples (8–10 mg) were cooled in an aluminum pan from room temperature to −150 °C and re-heated to 150 °C. This cycle was repeated 3 times. The heating and cooling rate was set to 10 °C min−1 with an empty aluminum pan used as a reference. The Tg value was determined as the midpoint value between the onset and the end of a step transition using the Netzsch instrument analysis software (Proteus Analysis).



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2.4. Gas permeability tests For the gas permeance measurements, membrane was masked with aluminum tape to provide the circle of desired diameter and active area (d = 1 cm, S = 0.785 cm2). Additionally, to prevent bending upon vacuuming membrane was placed on the porous polycarbonate support filter (with 1.2 µm pores diameter), see details in Supporting Information, Fig. S2. The dry gases (He, H2, O2, N2, CO2) permeation through the membranes was measured at 35 °C was using the GTR-11A/31A gas barrier testing system (GTR Tec Corp., Japan, see Fig. S3 for details). Permeability (P) of gases in barrer units (1 barrer = 10-10 cm3(STP)·cm/cm2·s·cmHg) was estimated for membranes of different composition and compared to the literature data for PDMS membranes reported by Merkel et al. [28] as well as to conventional Sylgard 184 PDMS membranes fabricated in the same lab and measured on the same equipment. The ideal selectivity between two different gases in a hybrid membrane was calculated using the ratio of the permeability of a single gas, A/B= PA/PB. where PA and PB are the permeabilities of gases A and B, respectively. The permeability of CO2 and N2 in mixed gas setting was also measured at 35 C using a calibrated gas mixtures used as a feed gas (supplied by Fukuoka Sanso Ltd.). In addition to pure gases, three different CO2/N2 mixtures were used containing respectively 50%– 50%, 20%–80% and 5%–95% of CO2 and N2.

3. RESULTS AND DISCUSSION Variety of hybrid materials representing PDMS crosslinked by metal oxide were reported earlier. In most cases, metal alkoxides are modified with a hydrolysis suppression agent ethyl acetoacetate, prior to the crosslinking reaction [21, 22]. We have found that in 10 ACS Paragon Plus Environment

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principle the crosslinking can also be achieved without any extra agents. To prevent the abrupt gelation and hydrolysis, the mixture of PDMS-OH and ZrBO should be sufficiently diluted using the suitable solvent. For the low molecular weight PDMS-OH precursors, anhydrous alcohols such as EtOH, IPA of BuOH can be used. For higher molecular weights of PDMS-OH, the relative amount of hydroxylation is significantly smaller, therefore CHCl3 or n-hexane are more appropriate to dissolve the hydrophobic PDMS as well. Moreover, utilization of non-polar solvent prevents hydrolysis of alkoxide. All mixtures prepared in hexane are stable for a long time without hydrolysis or gelation. After being mixed in solution, the complexation reaction between PDMS-OH and ZrBO can be accelerated by the heating of the solution, generally below the boiling point of the used organic solvent (less than 60 C). In our experiments, we have used the excess amount of the metal-oxide, larger than stoichiometrically needed for the crosslinking (molar ratio ZrBO:PDMS-OH = 1:2). It is important, in order to confirm the effect of the incorporated ZrO2 in PDMS matrix on the gas permeation. Hybrids with 5, 10, 15 and 20 wt.% of ZrO2 in PDMS were obtained using molar ratios between ZrBO:PDMS-OH approximately equal to 2.5:1, 5.4:1, 8.6:1, 12.1:1 respectively, considering molecular weight of PDMS-OH precursor is 6000 g/mol.

3.1. Morphology of the nanocomposites Following the scheme described in the experimental section, we were able to prepare the membranes by different methods such as casting or spin-coating. Figure 2 (a-d) shows the digital photo of the thick casted membranes. It is seen that even with higher content of the metal oxide in the material matrix membranes are still transparent, suggesting that they



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don’t contain large aggregates or particles of zirconia in the PDMS matrix. Based on the physical appearance, the size of the inorganic particles of ZrO2 or its hydroxides in these membranes are supposed to be smaller than 10 nm [21]. SEM observation, given in Fig.3 (a-d) supports this suggestion. All materials show smooth morphology without obvious particulates. The only visible texture change was observed in case of 20%-ZrO2@PDMS (Fig. 3d), namely there are some inhomogeneity signs at high magnifications (higher magnifications for all samples provided in the insets of Fig.3). Transparency of the hybrids and inability to distinguish ZrO2 and PDMS using SEM supports fine dispersion between organic and inorganic phases. Katayama et.al. reported the morphology of similar materials observed by high resolution TEM [22] and confirmed that dispersion is indeed in the microscale, and extremely small size of particles (2-3 nm). Due to modified synthetic procedure morphology may be altered in our membranes and more detailed characterization will be reported elsewhere.


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Fig. 2. Photography of the casted membranes with size ca. 3x3 cm and thickness ca. 1 mm: (a) ZrO2@PDMS nanocomposites prepared using different amount of ZrO2 (5, 10, 15 and 20 wt%) and PDMS-OH with Mw = 6000 Da; (b) Flexible response of the thick ZrO2@PDMS nanocomposite membranes depending on the ZrO2 content; (c) ZrO2@PDMS nanocomposites prepared using different molecular weight (Da) of PDMS-OH and 10% of ZrO2; (d) Flexible response of the thick ZrO2@PDMS nanocomposite membranes made of different Mw of PDMS-OH.



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Fig. 3 SEM images of the cross-section of the ZrO2@PDMS nanocomposites prepared using a different amount of ZrO2, (a) 5wt%, (b) 10wt%, (c) 15 wt%, (d) 20 wt% and PDMSOH with Mw = 6000 Da. Insets show higher magnifications at which only 20 wt% ZrO2 containing membrane stands out demonstrating specific texture while other hybrids are smooth without visible particulates.

3.2. ATR-FTIR and XPS The FT-IR spectra of the hybrids synthesized from different mixtures of ZrBO and PDMS-OH were acquired to confirm the formation of bonds between the inorganic component (ZrO2) and the PDMS, as shown in Fig. 4a. The absorption corresponding to Zr-O-Si bonds was observed as a characteristic shoulder at around 940 cm-1 [21, 22]. The existence of the shoulder and its increase with the content of ZrO2 confirm that inorganic component derived from ZrBO is chemically bonded to PDMS chains via Zr-O-Si bonds. Interesting observation compared to the reference studies is the absence of absorption peaks 14 ACS Paragon Plus Environment

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around 1600 and 3500 cm-1 that are usually present in metal oxide containing composites and attributed to the stretching vibration of hydroxyl groups OH. This fact is signifying that zirconium alkoxide mixed with PDMS-OH was completely transformed into the oxide form in the hybrid without remnants of the hydroxides typical for ZrO2 containing materials. For further confirmation of complete hydrolysis, we have performed the XPS analysis of the casted membranes. The wide-range XPS spectra of the ZrO2@PDMS composite membranes and reference PDMS are given in Fig. 4b. There we can see clearly the increase of the Zr signals proportionally to the content of alkoxide on a precursor solution. Atomic composition of the hybrids calculated from survey spectra also confirms complete zirconium butoxide hydrolysis. The result shows a small difference between experimental and expected results confirming that nanocomposites indeed are composed of ZrO2 and PDMS without hydroxyl groups present in final materials (Supporting information, Fig. S5). Additionally, correspondence of the experimentally measured atomic concentration with material assuming completely hydrolysed ZrBO also verifies the purity, proving also that there are no remnants of organic solvent or butyl functional moieties left in the material.



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Fig. 4 (a) ATR-FTIR of ZrO2@PDMS nanocomposites with different amount of ZrO2. (b) Wide range XPS spectra of the ZrO2@PDMS membranes with different content of ZrO2 compared to Sylgard 184 PDMS.

3.3.DSC and mechanical properties The thermal properties of the composites were analysed to assess and confirm their rubbery nature, and the result is shown in Fig. 5. The step-like change of the heat flow around -120C was observed for all compositions and indicates the glass transition temperature characteristic for the conventional crosslinked PDMS (Sylgard 184), which was also measured in the same regimen (Fig. 5e). The DSC curves of the nanocomposites with 15 and 20 wt% ZrO2 are similar to PDMS, signifying complete confinement of the PDMS chains between metal oxide regions and restricted polymer chains movement (full crosslinking). In contrast – significant differences are observed for the nanocomposites with the lower content of the ZrO2. Namely, crystallization behaviour was detected during the cooling cycle (exothermic peaks around -73C and -87C respectively) for 5%- and 10%-ZrO2@PDMS composites. Respective endothermic peaks were detected during heating cycles (around -43C and -50C) in case of both 5% and 10% ZrO2 containing materials. As these peaks correspond to crystallization and melting of PDMS chains, the obtain result suggests that hybrids with a lower amount zirconium oxide contain some amount of free PDMS chains with sufficient degree of freedom that may be only partially bound or even not bound completely to the ZrO2 within the material [22, 29, 30, 31] Comparison of the melting and crystallization peak areas shows that 5%-ZrO2@PDMS composite possesses about two times higher amount of PDMS available for crystallization


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compared to 10%-ZrO2@PDMS. Also, samples with 5% and 10% zirconia demonstrated slightly higher Tg, however the correlation with ZrO2 content is not clear. Major conclusion form the DSC analysis is that glass transition temperatures are very close to the value of conventionally prepared PDMS which supports rubbery nature on the materials at room temperature, similar extent of polymer chain movement and not suppressed gas transport. Mechanical properties of the membranes were measured by nanoindentation [32], have also demonstrated dependence on the content of the ZrO2 in the hybrid (Supporting information, Fig. S6). Namely the low content ZrO2 hybrids showed both lower hardness and modulus compared to PDMS. In general, mechanical properties changed exponentially with the increase of the metal oxide content. Compared to the reference PDMS, 10%- and 15%- ZrO2@PDMS hybrids are more close to it in terms of hardness and modulus. The extreme materials with 5% ZrO2 and 20% are respectively much more soft and rigid. Such results indicate that ZrO2 does not play a role of simple physical filler in PDMS, because the incorporation of inorganic fillers usually promotes more linear hardness increase in polymer matrices with the amount of added filler [33].



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Fig. 5. DCS curves of the nanocomposites with different amount of ZrO2, namely (a) 5 wt%, (b) 10 wt%, (c) 15 wt%, (d) 20 wt% and (e) reference material – Sylgard 184 PDMS (cooling rate 5 K/min, heating rate 10 K/min were used in every case).

3.4. XRD and SAXS The microstructure of the developed nanocomposites was also analyzed by x-ray diffraction (XRD). As seen in the resulting patterns given in Fig.6, the XRD patterns of all membranes are demonstrating broad diffuse diffraction halos around 11.9 and 30 that are characteristic for PDMS [34,35]. The very broad X-ray signals are typical for materials 18 ACS Paragon Plus Environment

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with disordered structures. Another broad peak evolving around 6 may signify the formation of larger ZrO2 domains in the PDMS matrix when the amount of ZrBO in the precursor is increasing. Important to note also that despite the relatively high temperature of crosslinking, none of the samples, demonstrated sharp peaks, characteristic for crystalline ZrO2. Small-angle X-ray scattering (SAXS) was performed on one sample in order to verify nanodomains formation and estimate their size. SAXS profile of the 10wt%-ZrO2@PDMS nanocomposite membrane (given in Supporting Information Fig. S7a) displayed two broad maxima that can be assigned to the ZrO2 (qmax = 0.056 Å-1) and PDMS (qhalo = 0.835 Å-1) correlation length respectively. Correlation lengths calculated for PDMS and ZrO2 are equal to ca. 7.53 Å and 110 Å that is in accordance with the literature data for similarly fabricated TiO2@PDMS nanocomposites [24]. Furthermore, assuming the ZrO2 domains have a quasi-spherical shape, the average radius of nanodomains can be estimated from SAXS profiles. The calculation of domains average radius, however, requires the information of the ZrO2 volume ratio. Despite known mass ratio, calculation of volume ratio is not possible directly because the density of ZrO2 in the nanocomposites is unknown. Dalod et al. who developed nanocomposites consisting of TiO2 and PDMS assumed that TiO2 embedded in the material could be approximated as crystalline anatase form [24]. However, there is no evidence that ZrO2 in our composites is in crystalline form. Therefore, we believe much lower ZrO2 density estimate should be used for domain size calculation. For instance, the value of 1.7 g/cm3, was reported as a density of dry and amorphous TiO2 gel [36]. With this value, used for 10 wt% sample, the approximated volume ratio is 6.3%, and the diameter of the nanodomain is close to 100 Å. However, this estimation should be



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taken with care as real density has a big influence on the result (see Supporting Information Fig. S6b).

Fig. 6. XRD patterns of nanocomposite membranes with different content of ZrO2 crosslinker compared to PDMS (Sylgard 184).

3.5. Pure and mixed gas transport through the hybrid materials Apart from the achieving satisfactory material mechanical properties, the primary purpose of this study was to measure gas permeability of the zirconia crosslinked PDMS nanocomposites. Results of pure gases permeability in the developed nanocomposites are given in Fig.7 except for the 5%-ZrO2@PDMS sample that was too soft for the measurements in the standard conditions (200 kPa transmembrane pressure). Due to this softness, membrane material was deforming under pressure and penetrating into the supporting membrane pores. Among the immediate observations for other samples, one can notice that nanocomposite materials have demonstrated significantly higher gas permeability compared to PDMS. In particular, all nanocomposites showed higher 20 ACS Paragon Plus Environment

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permeability to small gases (He and H2). For larger gases only 20%-ZrO2@PDMS nanocomposite membrane demonstrates smaller permeability compared to PDMS. It is obvious that improvement depends both on the type of gas and ZrO2 content. Sample containing 10% ZrO2 the highest permeability for all tested gases. As the main purpose of the work is to develop the material able to substitute the commonly used PDMS in the gutter layer of composite membranes for post-combustion CO2 capture, we have also studied the permeability towards CO2 and N2 in the mixed gas setting. The results of the measurement are given in the Fig.8, in form of mixed gas permeability relative to pure gas permeability. In mixed gas condition the CO2 permeability decreased compared to the pure gas and increased for N2. Such behavior is expected as PDMS is known to have a very high CO2 solubility resulting in a increase of the permeability with the CO2 pressure [28]. The decrease of CO2 permeability in case of the mixed gas experiment concluded ca. 5% for all ZrO2@PDMS nanocomposites while the increase of N2 permeability is below 4%. Same behaviour was observed for the Sylgard 184 PDMS membrane. Finally, the separation performance of the nanocomposite membranes was compared to the state-of-art PDMS in the form of selectivity vs. permeability of the fast gas and is given in Fig.9. Here we can see that for all useful gas separations (H2/N2, CO2/N2, O2/N2), membranes behave differently from expectations for the common mixed matrix, i.e., both permeability and selectivity have increased.



900

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Fig. 7. Pure gases (He, H2, CO2, O2, N2) permeability measured at 35C in the ZrO2@PDMS nanocomposite membranes. Dashed reference lines represent the data reported in the literature (marked with label “Lit.”) for PDMS [28] and data of PDMS prepared in our lab using commercial Sylgard 184 kit (marked with label “PDMS”). Error bars represent standard deviation of the replicate measurements on the same membrane. 1.04

Relative permeability (Pmixed/Ppure)

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

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10% ZrO2@PDMS

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Fig. 8. Relative permeability change (Pmixed/Ppure) for the mixed CO2/N2 gas setting measured in the hybrid and Sylgard 184 PDMS membranes demonstrate similar tendency, slightly increased N2 permeability (squares) and decreased CO2 permeability (circles) for the mixed gas tests. 22 ACS Paragon Plus Environment

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(a)

(b)

6

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wt% ZrO20 @PDMS 2

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wt% ZrO10 @PDMS 2 wt% ZrO15 @PDMS 2

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H2

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3.30 3.46 3.64 Kinetic diameter (angstrom)

Fig. 9. Robeson diagrams (selectivity vs. permeability) comparison of the separation performance of the fabricated nanocomposite membranes and PDMS for (a) H2/N2; (b) CO2/N2 and (c) O2/N2 gas pairs. (d) Improvement of the gas permeability (PcompositePPDMS)/PPDMS(%) in the developed nanocomposites compared to PDMS (nonhighlighted area – positive improvement, light grey highlight – decreased permeability).

3.6. Mechanism of improved gas permeability Permeability of gases in the thick organic polymers is usually described by a solubilitydiffusion mechanism. While for glassy polymers separation ability is almost always related to a free volume of material, kinetic diameter of gas species, gas permeability and 23 ACS Paragon Plus Environment


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selectivities of rubbery polymers (e.g. PDMS) are generally attributed to the condensability (critical temperature) of the gas species and related high solubility in the polymer matrix [13]. Result obtained for the ZrO2@PDMS nanocomposites, suggests the significant increase of the gases solubility scalable with the size of gas molecules. However, this is unlikely as it should proportional to condensation temperatures. From the other hand, DSC results for developed nanocomposites suggests the larger degree of freedom of PDMS polymer chains in the nanocomposites with smaller crosslinking density [22, 30]. However, the solubility of gases usually does not depend strongly on the crosslinking condition, moreover, not crosslinked PDMS liquids showed smaller CO2 solubility compared to the crosslinked membranes [37]. Therefore, we have to attribute the observed improved separation performance of the ZrO2@PDMS nanocomposites to the metal oxide component. The trend observed for the gas separation parameters change is the most desirable for material development as both selectivity and permeability increase. Although, we do not clearly observe the phase separated ZrO2 domains in the developed materials, one can relate the developed nanocomposites to the class of mixed matrix membranes, where PDMS is a matrix and ZrO2 is the inorganic filler. In such case, the filler should be considered as the main contributor to better gas separation. Metal oxides have been used as physical fillers in different polymers, PDMS in particular, and have been widely investigated for mixed matrix membrane applications. The particles itself (especially in crystalline form) are not contributing to the selectivity improvement but can lead to the increased permeability [38]. For example, Wang et al. used surface modified mesoporous silica (KIT-6) as fillers in PDMS [39] and showed that 2% loading could significantly improve the permeability towards C4H10 and C4H10/N2


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selectivity and mild improvement of CO2/N2 separation. The mechanism of improved performance was relying on the plasticization of rigid PDMS/mesoporous silica interfaces that are closed for less soluble gases (e.g., N2). In our case, the improvement of the permeability does not depend on the gas solubility but on the gas size. This leads to the conclusion that the internal structure of the ZrO2 domains in the PDMS (which are likely well mixed with polymer) plays a mild size-sieving role [40]. As a result, we observe the increased selectivity, especially in the case of the H2/N2 gas pair, where the difference of kinetic diameters is much higher (0.75 Å). Important to note that ZrO2@PDMS nanocomposites, despite higher permselectivity compared to PDMS, are still far from the Robeson upper bounds (therefore lines are not depicted in Fig. 9). It is not surprising, as the majority of polymers reaching the Robeson upper bound in the high gas permeability region, are glassy polymers namely, thermally reversed (TR) polymers and polymers of intrinsic microporosity (PIMs) [41]. However, for gutter layer in TFCM – the most important property is high gas permeability and most widely used polymers are PDMS (rubbery) and poly(1-(trimethylsilyl)-1-propyne) (PTMSP, glassy polymer) [19]. And while PTMSP has much higher permeability than PDMS, its glassy nature prevents reliable fabrication of few-hundred nanometer thin layers. As an example, PTMSP was used as ca 2 micron think gutter layer on (PVDF) to support graphene oxide-ionic liquid [42] or metal-organic framework based MMM [43] selective layers. From the other hand, much thinner layers of PDMS can be fabricated on large scale and therefore predominantly used as gutter layers to support even thinner polymeric or mixed matrix selective layers [44, 45]. In a similar fashion highly permeable protective (sealing) coating if often required on the selective layer. Its main function is to seal defects



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in selective layer and protect it from damage during the handling [41]. The most critical disadvantage of PTMSP (despite its high gas permeability) is accelerated aging [46]. Therefore, silicone rubber retains exclusive use as a gutter layer in the majority of reported TFCM for CO2 separation [47]. Taking to account that ZrO2@PDMS composites developed in this work are uniform, homogenous, have ca. 50% higher permeability for CO2 and ca 100% higher for H2, combined with slightly higher selectivities, we believe they can improve the performance of TFCM considerably if utilized as alternative gutter layers. For the construction of high permeance TFCM, resistance of gutter/protective layers becomes an important factor [44-45] and therefore even relatively small increase of separation performance of gutter layer can make a big improvement in total TFCM permeance/selectivity and ultimately the cost of gas separation. Based on the microscopy observation and DSC of the nanocomposites we can schematically describe the internal structure of the hybrid membranes as given in Fig. S8. As the amount of the ZrO2 in the material is increased, the size of the domains also increases which results in an increased probability of gas interaction with the ZrO2. This leads to the higher selectivity of the 20% ZrO2@PDMS nanocomposite. Elucidation of the higher permeability in the developed materials would require the detailed study of gases solubility and diffusivity in the nanocomposites. In contrast to the conventional MMMs materials, developed nanocomposites do not contain the isolated ZrO2 particles that can be viewed as independent contributors to permeability and selectivity. Therefore, we are unable to characterize them directly, and as a result we cannot apply the common MMM theoretical models [48] to explain the behavior. The detailed elucidation will require a separate study, and it is planned in the future. However, even based on the reported findings, the


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indisputable advantage of the nanocomposite chemical nature would be the possibility of alkoxide chemical modification before the crosslinking with PDMS-OH. This approach would provide a potential tool to modify the separation selectivity towards the target gas.

CONCLUSIONS In this work, we have investigated nanocomposite, mixed matrix membranes, where ZrO2 filler plays an additional crosslinking role. Morphologically, materials prepared in a described way overcome common drawbacks of MMM such as the presence of leaky/blocking interfaces. Moreover, ZrO2@PDMS composite materials exhibited significantly higher gases permeability and slightly higher selectivities compared to conventional PDMS (Sylgard 184) which is the state-of-art material for gutter/caulking layers in composite membranes. Improvement of gas separation performance, consistent with the difference in kinetic diameter of the gas pair under separation, suggests the formation of nanosized gas size-sieving ZrO2 domains in PDMS matrix. Chemical modification of these simplest inorganic crosslinkers could be considered as a way towards separations with higher selectivities/permeabilities that are critically needed in order to achieve the membranes attractive for industrial application, especially for high scale CO2 capture.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge at http://pubs.acs.org. Scheme of membrane fabrication and assembly for gas permeation measurements, gas27 ACS Paragon Plus Environment


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permeation apparatus, additional SEM images, XPS results, mechanical properties of the membrane assessed by nanoindentation and SAXS.

AUTHOR INFORMATION Corresponding Author * Tel/Fax: +81 92 8026877. E-mail: [email protected] † Tel/Fax: +81 92 8026872. E-mail: [email protected] Author Contributions The manuscript was written through contributions of both authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by World Premier International Research Center Initiative (WPI), sponsored by the Japanese Ministry of Education, Culture, Sports, Science, and Technology. R. Selyanchyn acknowledges the Japan Society for Promotion of Science (JSPS) for a Grant-in-Aid for Research Start-up (no. 26889045). Authors also want to acknowledge Prof. Benny Freeman, Dr. Heewook Yoon and Dr. Jaesung Park from the University of Texas for useful discussions about the PDMS based materials. We would like to thank Dr. Ikuo Taniguchi from I2CNER, Kyushu University for his help with DSC


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measurements and useful discussions throughout the research period. Ms. Nao Hirakawa is acknowledged for help with SAXS measurement.

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TOC Image: Molecularly dispersed ZrO2 in PDMS with enhanced gas separation ability

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Molecular hybridization of polydimethylsiloxane with zirconia for highly

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