Design and Characterization of Liquidlike POSS-Based Hybrid

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Design and Characterization of Liquid-like POSS-based Hybrid Nanomaterials Synthesized via Ionic Bonding and Their Interactions with CO2 Camille Petit, Kun-Yi Andrew Lin, and Ah-Hyung Alissa Park Langmuir, Just Accepted Manuscript • DOI: 10.1021/la4007923 • Publication Date (Web): 30 Jul 2013 Downloaded from http://pubs.acs.org on September 4, 2013

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Design and Characterization of Liquid-like POSSbased Hybrid Nanomaterials synthesized via Ionic Bonding and Their Interactions with CO2 Camille Petit, Kun-Yi Andrew Lin† and Ah-Hyung Alissa Park* Department of Earth and Environmental Engineering & Department of Chemical Engineering, Lenfest Center for Sustainable Energy, Columbia University, 500 W. 120th Street, New York, NY 10027, USA !

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ABSTRACT Liquid-like Nanoparticles Organic Hybrid Materials (NOHMs) were designed and synthesized by ionic grafting of polymer chains onto nanoscale silica units called Polyhedral Oligomeric Silsesquioxane (POSS). The properties of these POSS-based NOHMs relevant to CO2 capture, in particular thermal stability, swelling, viscosity as well as their interactions with CO2 were investigated using thermogravimetric analyses, differential scanning calorimetry, NMR and ATR FT-IR spectroscopies. The results indicate that POSS units significantly enhance the thermal stability of the hybrid materials, and their porous nature also contributes to the overall CO2 capture capacity of NOHMs. The viscosity of the synthesized NOHMs was comparable to those reported for ionic liquids, and rapidly decreased as the temperature increased. The sorption of CO2 in POSS-based NOHMs also reduced their viscosities. The swelling behavior of POSSbased NOHMs was similar to that of previously studied nanoparticle-based NOHMs and this generally resulted in less volume increase in NOHMs compared to their corresponding polymers for the same amount of CO2 loading. ! !

KEYWORDS Polyhedral oligomeric silsesquioxane (POSS), CO2 capture, swelling, diffusivity, viscosity, nanomaterial.

INTRODUCTION The continuous increases in greenhouse gases emissions, in particular CO2, cause changes in the delicate balance in the Earth’s system, thereby threatening the fragile balance of our environment. Although considerable research initiatives on the development of renewable energy have been undertaken, their large-scale implementation may take decades. Additionally, fossil 2 !


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fuels, which contribute significantly to anthropogenic CO2 emissions,1 are likely to remain the main source of energy in the near future. Therefore, carbon management represents one of the great challenges faced by the scientific and engineering communities. Due to the scale of CO2 to be sequestered, Carbon Capture and Storage (CCS) is perceived as one of the most promising paths toward CO2 mitigation. However, CO2 capture from point sources still represents an economically challenged step of CCS. The most mature technology for post-combustion CO2 removal in coal-fired power plants is amine scrubbing (e.g., monoethanolamine MEA).2 While MEA exhibits high CO2 capture capacity and favorable kinetics, this approach is currently challenged by a number of drawbacks including the corrosiveness of the solvent and the material loss during the regeneration process, as well as the high parasitic energy penalty due to the high water content (70 – 85 wt%).3 Taking all these into account, researchers are seeking alternative CO2 capture media. The types of materials proposed so far for post-combustion CO2 capture can be classified into: liquid solvents, membranes and solid sorbents.4 As novel solvents, ionic liquids (ILs) have received considerable attention,3 owing to their low vapor pressure, high thermal stability and potential tunability via incorporation of task-specific functional groups.3,5,6 However, at this time the manufacturing cost of ILs remains prohibitively high, and their potential sensitivity to moisture is another issue. Nanoparticle Organic Hybrid Materials (NOHMs) represent another type of liquid CO2 absorbent, whose synthetic steps are rather straightforward and involve readily available chemicals.7-18 These materials consist of an inorganic nanoparticle (core) tethered with polymeric chains (canopy).7-18 The canopy confers NOHMs their liquid-like property without addition of any solvent, and acts as a solvating medium. Like ILs, NOHMs exhibit negligible vapor pressure and enhanced thermal stability, which is achieved by tethering the polymeric

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canopy onto thermally stable inorganic cores. Recent studies indicated that NOHMs with silica nanoparticles cores (7-22 nm) have potential for CO2 capture via functionalization of the canopy with amine groups and proper design of its structural parameters such as the grafting density of the polymer chains.7-10 In particular, it was found that polymer chains in NOHMs exhibit a unique and well-ordered configuration leading to a positive entropic effect that provides enhanced access of CO2 to functional groups along the grafted polymers.7-10 Further enhancement of the efficiency of NOHMs for CO2 capture in terms of lowering the viscosity and required weight of the NOHMs could be reached by minimizing the “inert” portion of NOHMs, i.e. the inorganic cores. Therefore, in this study, in order to reduce the inorganic component, NOHMs with smaller thermal stable inorganic cores were designed. Polyhedral oligomeric silsesquioxane (POSS) units appeared as attractive candidates for this task. They represent a nanometer-size silica-based compound with the formula (RSiO3/2)8, where R can be a hydrogen atom, or any polar or non-polar organic molecules.19, 20 POSS exhibits a rigid cage-like structure which usually consists of a 1-nm cube with silicon atoms at each corner linked to organic arms (or hydrogen atoms).19, 20 It is common to refer to POSS as the smallest silica nanoparticle. Via the proper selection of the organic arms, it was possible to graft polymer chains onto POSS particles, thereby forming new NOHMs structures with minimized inorganic parts. Investigation of POSS/polymer composites have been reported in the literature and the results showed enhanced thermal stability of the materials owing to the presence of POSS.21-24 Tanaka et al. recently reported their study on a synthesized ionically tethered POSS nanomaterial, referred to as a POSS-based ionic liquid, but this study was mostly focused on the materials synthesis and did not include any study on the interaction between CO2 and these POSS-based materials.21,25 There are a number of benefits of using POSS as the inorganic core for this new type of

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NOHMs for CO2 capture in addition to the reduction of the “inactive” part of the hybrid nanomaterial. First, by minimizing the inorganic fraction of NOHMs, the viscosity of the solvent can be reduced. Next, there are eight distinct, equal distance sites available on POSS for polymer grafting, and thus, the grafting density can be well controlled for fundamental studies. The use of POSS as core also allows a higher canopy/core size ratio of NOHMs than the nanoparticle-based NOHMs without the concern of solidifying the final NOHMs product. Furthermore, since the POSS cages are void and their size exceeds that of CO2 molecule (3.3 Å),26 it is expected that they can accommodate molecules of this gas. Recently solid membranes consisting of POSS/polymer composites have been reported for their use in CO2 removal and CO2/N2 and CO2/H2 separations.24,27 The objective of this study was to design and synthesize liquid-like POSS-based NOHMs with different ionically tethered polymeric canopies and to investigate their potential for CO2 capture. Important properties such as thermal stability and viscosity, as well as temperature and CO2induced swellings were investigated for the synthesized POSS-based NOHMs. !

EXPERIMENTAL SECTION Material synthesis. Three POSS-based NOHMs with ionic bonding were synthesized. First, a 11 wt% aqueous solution of octaammonium POSS (Hybrid Plastics) was prepared. An ion exchange resin (DowexTM A OH, Sigma-Aldrich) was then employed to remove the original counter ion (i.e., chloride) from the POSS units and neutralize the amine groups. Next, three different polymers, namely, two glycolic acid ethoxylate lauryl ether (Sigma Aldrich, MW~460 and MW~740) and one poly(ethylene glycol) 4-nonylphenyl 3-sulfopropyl ether potassium salt (Sigma Aldrich, MW~1260), were diluted with deionized water to obtain three 10 wt% 5 !


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polymeric solutions. In the case of the latter polymer, an ion exchange resin (DowexTM HCR W2, Sigma-Aldrich) was employed to remove the potassium ions and protonate the sulfonate groups. Each polymer solution was added dropwise to the POSS solution while monitoring the pH. The addition of polymer solution was stopped when reaching the equivalence point indicating the complete neutralization of the POSS amine groups by the acidic groups (carboxylic or sulfonic) of the polymers. Finally, the samples were dried, first in air at ambient temperature to evaporate most of the water, and then under vacuum at 40 oC for about 48 hours. The NOHMs samples are labeled as POSS-I-PCn or POSS-I-PSn, where n corresponds to the molecular weight of the polymer (n = 460, 740 or 1260). “I” stands for “Ionic” and indicates the type of bonding between the POSS unit and the polymer chains, while “PC” and “PS” refer to polymers (“P”) with carboxylic (“C”) and sulfonic (“S”) endgroups, respectively. In the following, the octaammonium POSS will be referred to as POSS(N), and the polymers as PC460, PC740 and PS1260, following the same nomenclature as described above. The schematic structures of the synthesized NOHMs and their parent materials are shown in Table 1. The findings for POSS-based NOHMs were compared to unbound polymers as well as the previously published data on nanoparticle-based NOHMs.7,10,18 The nanoparticle-based NOHMs consisted of 7 nm silica nanoparticle cores with ionically grafted polyetheramine chains (Jeffamine M2070, MW ~ 2,000). This material is referred to as NP-I-PE2000, where “NP” stands for nanoparticle, “I” for ionic and “PE” for polyetheramine (note that it was listed as NOHM-I-HPE in the previous publication).7,10,18

Thermogravimetric analysis (TGA). The thermal stabilities of the NOHMs and the unbound polymers were evaluated using a thermal analyzer (TA Q50, TA Instruments Inc.). The samples 6 !


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were exposed to an oxygen environment (flow rate: 40 mL·min−1) from 15 oC to 600 oC with a temperature ramping rate of 5 oC·min−1.

Attenuated Total Reflectance (ATR) FT-IR spectroscopy. ATR FT-IR spectra of the synthesized materials were obtained using a Nicolet 6700 spectrometer (Thermo Fisher Scientific Inc.) equipped with an ATR cell consisting of a high-pressure cell (Golden GateTM Supercritical Fluids Analyzer, Specac Ltd). After reaching equilibrium, spectra were collected 32 times with a resolution of 2 cm−1. In addition to confirming ionic bonding and CO2 loading, FTIR spectroscopy was also used to determine the temperature and CO2-induced swelling behaviors of the NOHMs and unbound polymers by obtaining the spectra under various temperatures (from 60 to 120 oC) or under various CO2 pressure conditions (0, 0.3, 0.7, 2.1, 3.5 and 5.5 MPa at 25 o

C), respectively. Details on these experimental methods are provided in a prior study.9

Differential Scanning Calorimetry (DSC). DSC experiments were performed on a Maia 200 F3 calorimeter (Netzsch) equipped with a liquid nitrogen cooling system. The NOHMs and unbound polymers were loaded in aluminum pans, heated from 25 oC to 100 oC, then cooled to 100 oC and finally reheated to 100 oC. The heating and cooling temperature ramping rates were 10 oC·min−1. The DSC data from the second heating cycle are reported in this paper.

Viscosity measurement. The viscosities of the NOHMs samples were measured using a Viscopro 200 viscometer (Cambridge Viscosity) equipped with a water circulation system (Julabo F12) to control the temperature. To study the temperature effect on the viscosities, about

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1 mL of NOHMs and polymer samples were loaded in the sample compartment and the viscosity was measured at various temperatures ranging from 30 to 80 oC. Similarly, to investigate the effect of CO2 on the viscosity, the same amount of sample was used and the viscosity was measured at 40 oC, under various CO2 pressures ranging from 0 to about 5 MPa.

Density and refractive index measurements. In addition to the above analytical techniques, other tools such as a pycnometer (1 mL) and a refractometer (Atago, odel PAL-RI) were employed in order to provide density and refractive index measurements, which are important for the calculations of the CO2 capture capacities.

RESULTS AND DISCUSSION Thermal stability is an important characteristic of CO2 capture media since the regeneration of the absorbent may be achieved via temperature swing, especially when chemisorption is involved for CO2 capture. Thus, the thermal stability of POSS-based NOHMs was investigated and the results are presented in Figure 1. The results were compared to those of nanoparticlebased NOHMs. As previously hypothesized, the presence of the POSS units visibly enhanced the thermal stability of the synthesized hybrid nanomaterials compared to their corresponding polymers. As seen in Figure S1, this enhancement was more pronounced than that observed for a physical mixture of POSS and polymer chains. This indicates that strong interactions were present in the POSS-based NOHMs compared to the physical mixtures. These interactions were likely related to the formation of the ionic bond between the POSS units and the polymer chains, which was later confirmed by the FT-IR measurement. Compared to the nanoparticle-based NOHMs which decomposed around 350 oC, the POSS-based NOHMs exhibited a slightly lower 8 !


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thermal stability. However, the POSS-based NOHMs were still stable at the typical temperature for regeneration of CO2 capture solvents, i.e. 100-140 oC. In order to confirm the successful synthesis of the POSS-based NOHMs, the structural characterization of the synthesized materials was performed using ATR FT-IR spectroscopy. In particular, evidences of reaction between the POSS(N) and the polymers during the synthesis of the POSS-based NOHMs with ionic bonding were investigated (Figure 2). The spectra of all the parent polymers were very similar, and the main vibration bands included: C−O stretching (1090 cm−1), CH2 symmetric and asymmetric stretching (2860 cm−1 and 2930 cm−1), and O−H stretching from water (both commercial polymers and NOHMs may contain traces amount of water as water was used during their synthesis) (3450 cm−1).28 As expected, the spectra of the NOHMs exhibited similar features to those of the parent polymers. However, some differences existed and indicated the presence of POSS(N). For instance, in the case of POSS-I-PS1260 the C-O stretching band at ~1090 cm−1 was broader due to the overlap with the Si-O-Si stretching and S=O vibration bands.29, 30 For PC460 and PC760, the C=O stretching band from the carboxyl groups appeared at 1740 cm−1, indicating that these functional groups were in their acidic form.31, 32

In the spectra of POSS-I-PC460 and POSS-I-PC740, this band was shifted to 1580 cm−1,

indicating that the carboxyl groups were now converted to the basic form (i.e., carboxylate).31,32 This observation again confirmed the acid-base reaction and the formation of the ionic bond between the amine groups of the POSS(N) and the carboxylic groups of the polymers. For POSS-I-PS1260, such band shift was not observed since the vibrational band of the sulfonic group overlapped with those of C-O and Si-O-Si.30 The DSC traces reported in Figure 3 provide interesting information on the physical state of the synthesized hybrid nanomaterials as well as the change in the polymer chains’ mobility 9 !


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induced by the incorporation of POSS particles. As seen from Figure 3, the POSS-based NOHMs have melting temperatures at or below room temperature. No significant difference was noticed for NOHMs synthesized with carboxylic groups compared to their corresponding polymers, while POSS-I-PS1260 melted at a different temperature than PS1260. This phenomenon was also observed by others who studied hybrid polymeric materials made using silica nanoparticles and polymer with carboxylic groups.11 This result suggests that while the difference is not significant, the functional groups that were involved in the ionic bonding and/or the nature of the polymer affected the melting temperatures of the POSS-based NOHMs. In the case of the glass transition temperatures (Tg), both POSS-I-PC740 and POSS-I-PS1260 exhibited glass transition at temperatures similar to those of PC740 and PS1260, respectively. On the contrary, for POSSI-PC460, its glass transition temperature shifted to a higher value compared to that of its parent polymer. This may be due to the higher weight fraction of POSS particles in POSS-I-PC460 compared to POSS-I-PC740 and POSS-I-PS1260. As discussed earlier in the experimental section, each NOHMs sample was prepared by grafting 8 polymer chains per POSS particle, and thus, the overall weight fractions of NOHMs samples were varied depending on their chain lengths. The change in the glass transition temperature at a higher weight fraction of POSS indicates that the presence of large quantity of inorganic phase affects the mobility of the polymer chains. A similar phenomenon has been reported for POSS/polymer composites and in their case the glass transition temperature of the hybrid materials increased as the fraction of POSS increased.33 Another important property of the CO2 capture solvents for large-scale processes is their viscosity. An undesirably high viscosity would increase the energy required to pump the materials throughout the CO2 capture and solvent regeneration units. Moreover, it can also affect

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the CO2 diffusivity during CO2 capture. First of all, the highly viscous character of POSS-IPC460 prevented the measurement of its viscosity under the given temperature range. As shown in Figure 4, the viscosities of POSS-I-PC740 and POSS-I-PS1260 decreased with increasing temperature. At 80 oC, the viscosities of POSS-I-PS1260 and POSS-I-PC740 were decreased to about 600 mPa.s and 100 mPa.s, respectively. The viscosities of these POSS-based NOHMs are significantly lower than those of nanoparticle-based NOHMs. Indeed, as shown in Figure 4, the viscosities of nanoparticle-based NOHMs were about five times higher than that of POSS-IPC740 at 70 oC. The viscosities of POSS-based NOHMs were in the range of those of other novel CO2 capture solvents such as Task-Specific IL [P66614][Ile] (100 mPa·s at 50 oC)33 and Room Temperature IL [bmim][BF4] (80 mPa·s at room 30 oC).5,35 However, they are still significantly higher than that of a 30% MEA solution (2 mPa·s).5,35 To further decrease the viscosity of POSS-based NOHMs, it could be envisioned to add an “inert” low-viscosity NOHMs or a secondary fluid. The latter approach has already been evaluated for various ILs.36 Finally, it is important to note that for the polymers tested for this study, the size of the polymer chains did not seem to be a predominant controlling parameter for the materials’ viscosity. The viscosities of the synthesized POSS-based NOHMs were also evaluated under increasing pressures of CO2 at 40 oC. The results are summarized in Table 2. For POSS-I-PC740, a continuous and progressive decrease in its viscosity was observed as more CO2 was introduced in the measurement chamber and CO2 was absorbed by NOHMs. At the end of the run (PCO2 ~ 5 MPa), the viscosity had decreased by about 80%. The reduction of viscosity under CO2 pressure was also observed in prior studies conducted on polyethylene glycol.37 In the case of POSS-IPS1260, the viscosity decrease was however limited to about 35%. Moreover, the viscosity pattern as a function of CO2 pressure also differed from that of POSS-I-PC740. The viscosity of

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POSS-I-PS1260 remained relatively constant up to PCO2 ~ 2 MPa, and started to decrease noticeably only at pressures greater than 2 MPa. Overall, these findings illustrate that increased CO2 absorption in NOHMs could contribute to a reduction of its viscosity, and this would also further enhance CO2 diffusion in the materials. Besides the viscosity, another interesting property of NOHMs for CO2 capture is their swelling behavior. Prior work has already reported the CO2-induced swelling behaviors of nanoparticlebased NOHMs, which suggested that the frustrated structures of NOHMs’ polymeric canopy contribute to the entropic effect of CO2 capture, and as a result, NOHMs swell significantly less than the parent polymer at the same CO2 loading.8-10 For this study of POSS-based NOHMs, both temperature and CO2-induced swelling behaviors were investigated. By reducing the core size of NOHMs down to 1 nm scale, it was important to confirm whether the entropic effect was still valid for the POSS-based NOHMs. First, the temperature-induced swelling behavior of POSS-based NOHMs was investigated. As explained above, a relatively high temperature may be needed to lower the working viscosity of the NOHMs, and a temperature swing may also be required to regenerate NOHMs after CO2 capture. For these reasons, the temperature-induced swelling behavior of POSS-based NOHMs was investigated by the ATR FT-IR setup and the results are presented in Figure 5. The swelling (S) is calculated as follows: ! % =

∆! !

×100

(1)

where V and ΔV represent the NOHMs’ volume at a given temperature and the volume change due to a temperature increase, respectively. As descried in previous studies,9,38,39 the swelling percent was calculated based on the change in the CH2 stretching band of the polymeric canopy

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(~ 2900 cm-1) found in the FT-IR spectra using Equation (2):8-10,38,39 !(%) =

!! !

!

× !!! − 1 ×100

(2)

!

where A0, de0, A, and de are the absorbance of the CH2 bands and the effective path length at T0 (T0 = 60 oC) and at T (T = 70, 80, 90, 100, 110 and 120 oC), respectively. Equation (2) can be simplified assuming that the effective thickness does not change with the temperature, and thus, the swelling of the materials only depends on the FT-IR absorbance of the CH2 band.8-10,38,39 As shown in Figure 5, increasing the temperature caused a volume increase for all materials tested including NOHMs and their parent polymers. Like previously studied nanoparticle-based NOHMs, POSS-based NOHMs also swelled less than their corresponding polymers. The extent of the difference was however more pronounced in the case of POSS-I-PC460 and POSS-IPC740 than POSS-I-PS1260. Since POSS-I-PS1260 was synthesized with the lowest fraction of inorganic phase, it was expected that it would behave most closely to its parent polymer. Next, the interaction of the POSS-based NOHMs with CO2 was investigated using ATR FT-IR spectroscopy. The same approach as for temperature-induced swelling (see Equation (2)) was used to measure the CO2-induced swelling of the POSS-based NOHMs and their corresponding polymers at different partial pressures of CO2. However, in this case, A0, de0, A, and de represented the absorbance of the CH2 band and the effective path length before and after exposure to CO2 at a given CO2 pressure, respectively. The results are shown in Figure 5. The CO2-induced swelling was overall much more pronounced than that caused by thermal swing, reaching between 25 and 35 % swelling at PCO2= 5.5 MPa, depending on the NOHMs samples. Similar to the temperature-induced swelling behaviors, the CO2-induced swelling was smaller for the POSS-I-PCn NOHMs compared to their parent polymers (Figures 5(a-2) and 5(b-2)). As seen

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in Figure S2 and reported in our previous studies,8,10 this behavior was also observed for nanoparticle-based NOHMs. This trend was however opposite in the case of POSS-I-PS1260. Since the NOHMs’ swelling depends on the amount of CO2 captured in the materials, the swelling data was also plotted as a function of the actual CO2 loading instead of the CO2 partial pressure. The results are presented in Figure 6 and they provide insight into the mechanisms of CO2 absorption in the materials. Note that because of the highly viscous nature of POSS-IPC460, the density of this material could not be measured and thus, the CO2 loading could not be accurately calculated. The CO2 loading was calculated using ATR FT-IR spectroscopy and details on the calculations can be found in Ref. [8] and [10]. As seen in Figure 6(b), POSS-IPS1260 swelled less than its parent polymer for the same amount of CO2 captured. This trend, although less pronounced for the POSS-based NOHMs, was also observed with the nanoparticlebased NOHMs (Figure 6(c)) and the greater ordering of the polymer chains in NOHMs compared to the unbound polymer was proposed as the reason for the reduced CO2-induced swelling of the synthesized hybrid nanomaterials.8-10 In the case of the nanoparticle-based NOHMs, this chain ordering phenomena seemed to create “unoccupied” space where CO2 could be absorbed without significant volume change. Unfortunately, this phenomenon could not be observed here using Raman spectroscopy as previously employed to study the structural effects in nanoparticle-based NOHMs. This may be due to a number of factors such as the greater ratio of the polymer chain size to the POSS size and the cubic shape of the POSS particles compared to spherical particles. The POSS’ cubic shape would lead to a closer packing of the POSS units and this could result in less volume needed to be filled by the grafted polymer chains. However, it is also not conclusive that there was no ordering in the polymeric canopy since the close interaction between polymer chains attached to different POSS cores would also lead to similar

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trend in the Raman spectra. Further investigation may be needed to further scope the interparticle and intraparticle interactions of polymer chains of POSS-based NOHMs. It is also possible that the bulkier benzene rings in PS1260 along with the ordering of the polymer chains resulted in some “free” space around the POSS units, thereby favoring CO2 absorption compared to the unbound polymer. Another potential contributor to the reduced CO2-induced swelling of POSSI-PS1260 compared to the unbound polymer is the absorption of CO2 in the cages of POSS. Since the internal cavities of POSS are rigid, there is no volume change related to this type of CO2 absorption. The absorption of CO2 in the POSS’ porous network has already been described by Li et al. for POSS/polymer composite membranes.27 Consequently, unlike in nanoparticlebased NOHMs for which the inorganic part only served thermal enhancement purposes, in POSS-based NOHMs, the core particles may serve multi-functions enhancing the thermal stability while also participating in the CO2 capture process. As seen in Figure 6(a), unlike POSS-I-PS1260, POSS-I-PC740 swelled more than its parent polymer for the same CO2 loading. This could be related to the absence of benzene rings on PC740, and therefore, the absence of unoccupied space where CO2 could be absorbed without increase in the volume of the material. In addition to this effect, it is possible that the polymer chains in POSS-I-PC740 interacted with the POSS cavity and, as a result, some portion of the polymer chains was unavailable for CO2 capture, as already observed in previous studies.27 This blocking effect could also occur in POSS-I-PS1260 but it may be less pronounced due to the presence of the benzene rings. Overall, if part of the polymer chains in POSS-I-PC740 was unavailable for CO2 capture, it resulted in an overestimation of the CO2 loading. Indeed, the amount of CO2 loading in NOHMs was calculated in mmol of CO2 per gram of solvent where the solvent refers to the polymeric fraction of the POSS-based NOHMs. Therefore, if part of the

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so-called solvent was not available for CO2 capture, the calculation for CO2 loading at each partial pressure of CO2 becomes incorrect (i.e., lower than it really is). Equation (3) provides some details on how the CO2 loading was calculated: !"! !!"#$%&'!(!!"#!"! ∙ !!"#$%&' !! ) =

!!"! (!!"! ∙!"!"#$%&"' !! ) !"!"! (!!"! ∙!"#!"! !! )×!×

!(!!"#$%&"' ∙!"!"#$%&"' !! ) !!!

×1000

(3)

Where CCO2 is the concentration of CO2 in the material, MWCO2 is the molecular weight of CO2, ρ is the density of the material, p is the polymer fraction of the material and S is the welling. One can see that if the fraction of the polymer in POSS-I-PC740 available for CO2 capture decreased, then a corrected and smaller p value should be considered, leading to an increased CO2 loading. At this stage of the study, it is not possible to evaluate the exact amount of material effectively involved in CO2 capture. While further investigation is needed to provide an in-depth understanding of CO2 loading versus swelling behavior of POSS-I-PC740, it is clear that the structure of the grafted polymer chains in POSS-based NOHMs affects the CO2 loading and CO2-induced swelling behaviors.

CONCLUSIONS Liquid-like POSS-based NOHMs were synthesized with ionic bonds via acid-base reactions. These hybrid nanomaterials allowed minimizing the “inert” inorganic core portion compared to nanoparticle-based NOHMs without compromising their thermal stability. In fact, the newly synthesized NOHMs still exhibited enhanced thermal stability compared to their parent polymers with a thermal decomposition temperature at around 300 oC (compared to about 350 oC for the nanoparticle-based NOHMs). In regards to their application for CO2 capture, a number of physical and CO2 capture properties were investigated. The measured viscosity of POSS-based 16 !


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NOHMs was comparable to those reported for ILs currently developed as novel CO2 capture solvents. Both the thermal swing and CO2 absorption enabled a more than fivefold decrease in their viscosities. Finally, both temperature and CO2-induced swelling behaviors of the POSSbased NOHMs were studied and it was found that for the given experimental conditions, the CO2-induced swelling was more pronounced than that induced by temperature swing. Similar to the nanoparticle-based NOHMs, the grafted polymers in POSS-I-PS1260 experienced less volume increase than the unbound polymer for the same CO2 loading. However, the results were not clear in the case of POSS-I-PC740. The results from the POSS-I-PS1260 case suggested the importance of the validation of the assumption that all the polymer chains of NOHMs are involved in CO2 capture. Additional structural investigation may be needed to further scope this complex relationship between interparticle and intraparticle interactions of grafted polymer chains of POSS-based NOHMs.

AUTHOR INFORMATION Corresponding Author * Phone: +1 212 854 8989. Fax: +1 212-854-7081. E-mail: [email protected] Present Address †

Department of Environmental Engineering, National Chung Hsing University, Taiwan R.O.C.,

250 Kuo Kuang Road, South District, Taichung City, Taiwan R.O.C. !

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ACKNOWLEDGMENTS

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This publication was based on work supported by Award No. KUS-C1-018-02, made by King Abdullah University of Science and Technology (KAUST). The authors are grateful to Govind Nadadur for his experimental help in the viscosity measurements. !

SUPPORTING INFORMATION Thermogravimetric curves of the POSS-based NOHMs and the physical mixture of POSS and polymer; swelling of the nanoparticle-based NOHMs as a function of CO2 partial pressure. This information is available free of charge via the Internet at http://pubs.acs.org/. ! !

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Table 1. List of POSS-based NOHMs samples with their chemical composition and schematic representation of their overall structure. Sample*

Polymeric canopy

Inorganic core

Chemical bonding

POSS-I-PC460 (12.8 wt% POSS)

PC460

(6:1 mole ratio of inorganic:organic) POSS-I-PC740 (8.4 wt% POSS)

PC740

(6:1 mole ratio of inorganic:organic) POSS-I-PS1260 (4.1 wt% POSS)

PS1260

POSS(N)

(7:1 mole ratio of inorganic:organic)

* Bonding type (-I-: Ionic), Polymeric canopy (PC: Polymer with Carboxylic endgroup, PS: Polymer with Sulfonic endgroup).

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Table 2. Viscosity of POSS-I-PC740 and POSS-I-PS1260 measured at 40 oC while increasing pressure of CO2. POSS-I-PC740

POSS-I-PS1260

PCO2 [MPa]

Viscosity [mPa·s]

PCO2 [MPa]

Viscosity [mPa·s]

0

595

0

7923

0.43

505

0.39

8163

0.94

423

1.30

7654

1.48

339

1.95

7651

2.28

269

2.32

7054

3.02

210

3.09

6527

4.10

151

4.13

6512

4.86

116

4.92

5309

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Caption to the Figures

Figure 1. Thermogravimetric curves of the POSS-based NOHMs and NP-based NOHMs and their corresponding unbound polymers, obtained in oxygen atmosphere. The POSS contents in POSS-IPC460, POSS-I-PC740 and POSS-I-PS1260 are equal to 12.3 wt%, 8.4wt% and 4.1 wt%, respectively. The silica content in NP-I-PE2000 is equal to 17 wt%.

Figure 2. ATR FT-IR spectra at 298 K of: (a) POSS-I-PC460 and its corresponding unbound polymer, (b) POSS-I-PC740 and its corresponding unbound polymer, and (c) POSS-I-PS1260 and its corresponding unbound polymer. Figure 3. DSC traces of: (a) POSS-I-PC460 and its unbound polymer, (b) POSS-I-PC740 amd its corresponding unbound polymer, and (c) POSS-I-PS1260 and its corresponding unbound polymer. Figure 4. Viscosities of POSS-I-PC740, POSS-I-PS1260 and NP-I-PE2000 as a function of the temperature. The viscosities of NP-I-PE2000 in the temperature range 20 oC - 60 oC are not reported because they were too high to be measured. Figure 5. Temperature (left) and CO2 (right) -induced swelling behaviors of: (a) POSS-I-PC460 and its corresponding unbound polymer, (b) POSS-I-PC740 and its unbound polymer, and (c) POSS-I-PS1260 and its corresponding unbound polymer. Figure 6. CO2-induced swelling behaviors of the synthesized materials as a function of CO2 loading at 298 K: (a) POSS-I-PC740, (b) POSS-I-PS2160, and (c) NP-I-PE2000 and their corresponding unbound polymers.

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Figure 1. Thermogravimetric curves of the POSS-based NOHMs and NP-based NOHMs and their corresponding unbound polymers, obtained in oxygen atmosphere. The POSS contents in POSS-IPC460, POSS-I-PC740 and POSS-I-PS1260 are equal to 12.3 wt%, 8.4wt% and 4.1 wt%, respectively. The silica content in NP-I-PE2000 is equal to 17 wt%.

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Figure 2. ATR FT-IR spectra at 298 K of: (a) POSS-I-PC460 and its corresponding unbound polymer, (b) POSS-I-PC740 and its corresponding unbound polymer, and (c) POSS-I-PS1260 and its corresponding unbound polymer.

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Figure 3. DSC traces of: (a) POSS-I-PC460 and its unbound polymer, (b) POSS-I-PC740 and its corresponding unbound polymer, and (c) POSS-I-PS1260 and its corresponding unbound polymer.

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Figure 4. Viscosities of POSS-I-PC740, POSS-I-PS1260 and NP-I-PE2000 as a function of the temperature. The viscosities of NP-I-PE2000 in the temperature range 20 oC - 60 oC are not reported because they were too high to be measured.

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Figure 5. Temperature (left) and CO2 (right) -induced swelling behaviors of: (a) POSS-I-PC460 and its corresponding unbound polymer, (b) POSS-I-PC740 and its unbound polymer, and (c) POSS-I-PS1260 and its corresponding unbound polymer.

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Figure 6. CO2-induced swelling behaviors of the synthesized materials as a function of CO2 loading at 298 K: (a) POSS-I-PC740, (b) POSS-I-PS2160, and (c) NP-I-PE2000 and their corresponding unbound polymers. 29 !


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TABLE OF CONTENT ONLY

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Design and Characterization of Liquidlike POSS-Based Hybrid

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