Correlating the Macrostructural Variations of an Ion Gel with Its Carbon Dioxide Sorption Capacity

We report on a direct correlation between the macroscale structural variations and the gas sorption capacities of an ion gel. Here, we chose 1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide ([Emim][TF2N]) and poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP) as the ionic liquid and host polymer, respectively. The CO2 sorption in the thin films of the IL-polymer was measured using the gravimetric method. The results of our experiment showed that the trend in CO2 uptake of these mixtures was nonlinearly correlated with the content of IL. Here, we highlight that the variations in the molecular structure of the polymers were the main reason behind the observed trend. The presented data suggested the possibility of using the composition of mixtures containing IL and polymers to realize a synergistic gain for gas sorption in these mixtures.


Introduction
Macromolecules and their mixtures are essential for the development of media used in membrane-based separation. Among various polymer mixtures, those containing ionic liquids (IL) present an interesting class of materials with many applications [1]. Over the past two decades, many studies have been devoted to investigating the properties of these mixtures [2][3][4]. Because IL are widely explored as reversible CO 2 absorbents and their mixtures have been considered selective solvents for CO 2 sorption [5,6], polymeric membranes containing IL gained substantial attention [7]. In the simplest form, mixing IL with polymers and casting films from these mixtures led to the development of membranes that are referred to as supported ionic liquid membranes (SILMs) [8,9].
The use of IL within polymeric domains was first explored for developing ionogels [10] and later in SILM systems [11]. Using PVDF-based SILM for CO 2 separation, it was observed that the solubility of CO 2 in SILM could be improved twofold when compared with that of IL [12]. When confining IL in a lyotropic liquid crystal, a similar observation was made [13]. The above observations pointed to the possibility of tuning gas sorption capacities in IL phases by mixing IL with complex fluids. Classical theory suggests that preparing such mixtures can lead to a reduction in the cohesive forces of the IL phase and in the energy required to form a cavity for the guest gas molecules [14]. Additionally, ordering the IL close to a solid wall is frequently reported and is thought to be the explanation for the enhanced solubility of gas in IL in confinement [15][16][17][18][19][20].
Herein, we report on the significance of the composition of a well-known polymer gel composed of poly (vinylidene fluoride-co-hexafluoropropylene) and 1-ethyl-3methylimidazolium bis(trifluoromethyl sulfonyl)imide on its CO 2 sorption capacities. We note that the swelling of polymers and configuration changes that led to the structural variation in polymers were responsible for the observation of a 1.5-fold enhancement in the CO 2 sorption in these mixtures. By controlling the composition of the mixture, we demonstrated a clear macrostructure-to-function relationship for CO 2 solubility in these mixtures. Utilizing carbon-based additives as nucleating agents, we further explored the following steps were taken to cast the films: the cast polymer was transferred to a heating vacuum oven preset at 60 • C; then, the oven temperature was ramped to 90 • C at a ramp rate of 1.3 • C/min and the wells were left in the oven for two hours and 30 min. Then, the oven temperature was increased to 150 • C at a ramp rate of 1.4 • C/min and the samples were heated for 24 h to remove the residual TEP. At this point, the oven temperature was decreased to 110 • C. It was evacuated using a rotary vane pump and then the drying process continued for another 72 h. The samples were kept in the vacuum oven until they were used for measurement. The thick films were prepared for the X-ray diffraction (XRD) measurement.

Characterizations
The macroscale structural variations in the films were evaluated by observing the samples under an Olympus BX51 polarizing microscope equipped with a Mettler Toledo FP900 thermal system with a temperature range of room temperature to 375 • C. Additionally, the films' topography was evaluated using atomic force microscopy (AFM). The AFM was performed using a Bruker Dimension Icon AFM (Billerica, MA, USA) in tapping mode with a range of 15 µm. The Bruker Dimension Icon AFM was operated under ambient conditions using a commercial silicon microcantilever tip on a nitride lever in ScanAsyst-Air mode. Both height and in-phase images were obtained using a scan rate of 0.988 Hz and 512 samples/line. The degree of crystallinity of the films was evaluated by using the X-ray diffraction (XRD) method. The XRD measurements were performed on Rigaku SmartLab (Rigaku Co., Tokyo, Japan).
The measurement of CO 2 absorption was conducted by using both dynamic and static methods. The dynamic method was adopted from a previous report [21]. A high-pressure chamber equipped with a quartz crystal microbalance (QCM) was used to measure the gas sorption within the film using the gravimetric. The static method was a variation of the pressure drop approach [22,23]. Because obtaining reliable data for IL via the dynamic approach was challenging, we only relied on the results from the static method and compared those results against the reported data in the literature [24].
Before each experiment, the QCM's placeholder was cleaned with acetone and purged with nitrogen gas to remove solvent residues. The test samples that were spin-coated on the QCM were weighed on an analytical balance (Sartorius, Bohemia, NY, USA, MSA225P100DI Cubis Analytical Balance), and its weight was recorded with two digits past decimal point. Then the samples were loaded into the QCM placeholder. Initially, the system was evacuated using a rotary vane pump and the chiller's temperature was set. When the frequency of the coated film on the QCM was stabilized (~3.5 h in continuous vacuum), the frequency and temperature of the module were logged via Eon-LT software. The pressure of chamber was also recorded in LabVIEW. Then, the pressure was set on the pressure regulator and CO 2 was introduced into the system. The ranges for pressure and temperature set points were between 50 and 250 psi and 10 and 40 • C, respectively. For a desorption, the QCM sensor was heated at 60 • C for 20 min while the system was continuously evacuated.

Results and Discussion
The CO 2 absorption capacities of the polymer-IL films were measured; the data are presented in Figure 1A-C. The average values for the CO 2 uptake slightly decreased as we increased the IL content of the mixture; however, this trend was reversed when the IL content was above 30%. In this range, the increase in CO 2 uptake was nonlinearly correlated with the IL content. For example, for a mixture of 50 wt % IL in the polymer at 200 psi and 10 • C, the specific molar sorption was 1.18 ± 0.06 mol/kg while the molar sorption for the polymers and IL were 0.6 ± 0.08 and 1.28 ± 0.08 mol/kg, respectively. When further examining the data, we noted that the measured values for CO 2 sorption in the polymer samples had a large uncertainty when compared with the CO 2 sorption values for the samples containing IL. Given that the PVDF-HFP was a semicrystalline polymer, we attributed the standard deviation in the CO 2 sorption in the polymer to the polymorphism of the samples [25]. In the samples that contained [Emim][TF 2 N], which is a known plasticizer of PVDF-HFP, this variability was not pronounced. This minor variability was attributed to the role of IL in suppressing film crystallinity. It was also reported that the addition of salts to PVDF-HFP favored the formation of one polymorph over the others, which loosely translated into having a more homogenous film compared to the neat polymer.
To gain further insight into the effects of the composition of a mixture on the film structure, we conducted optical microscopy. PVDF-HFP is well known for demonstrating birefringence under polarized light [26]; therefore, we used a polarized microscope for this purpose. Figure 1D presents the morphology of polymeric films as a function of their compositions. As shown, a birefringence of PVDF-HFP appeared as the weakest compared to other samples. As the content of IL in the film was increased, a stronger birefringence was apparent and the spherulitic domains became more discernible. This change matched the data reported in the literature and was attributed to the preferential assembly of the polymer lamellae into the edge-on configuration [27,28]. Additionally, in Figure 1D shows that with an increase in the concentration of the IL in the film, the spherulites became larger in size. Here, IL molecules swelled polymers and altered the polymer structures. Thus, larger spherulites were observed.  To gain further insight into the effects of the composition of a mixture on the film structure, we conducted optical microscopy. PVDF-HFP is well known for demonstrating birefringence under polarized light [26]; therefore, we used a polarized microscope for this purpose. Figure 1D presents the morphology of polymeric films as a function of their compositions. As shown, a birefringence of PVDF-HFP appeared as the weakest compared to other samples. As the content of IL in the film was increased, a stronger birefringence was apparent and the spherulitic domains became more discernible. This change matched the data reported in the literature and was attributed to the preferential assembly of the polymer lamellae into the edge-on configuration [27,28]. Additionally, in Figure 1D shows that with an increase in the concentration of the IL in the film, the spherulites became larger in size. Here, IL molecules swelled polymers and altered the polymer structures. Thus, larger spherulites were observed.
To further probe the film structure at the submicron level, we conducted atomic force microscopy (AFM) experiments; these data are presented in Figure 2 and the details of the sample preparation are described in Section S3.1.2 of the Supplementary Materials. As shown in Figure 2A, the morphology of the PVDF-HFP consisted of dendrites composed of small short multibranched structures, which are typically associated with flat-on lamellae [29]. The lamellae originated from spherulite centers and aggregated into small fibrils in dense clusters. When the IL content was increased to 10 wt % ( Figure 2B), the dendrite-like patterns were replaced by refined and tiny fibrils. The fibrils grew parallel to each other and formed a stack of fibrils near the nuclei center. To better visualize this variation, the in-phase images are presented in Figure S7 of the Supplementary Materials. By increasing the IL content above 30%, the tiny fibrils from the spherulites' center became omnipresent (see Figure S7). These results suggested that the addition of IL favored the formation of edge-on oriented lamellae [28,30], which led to a transition in the polymer's structure that explained the changes in the birefringence and swelling, as shown in the optical images presented in Figure 1D.
Membranes 2022, 12, x FOR PEER REVIEW To further probe the film structure at the submicron level, we conducte microscopy (AFM) experiments; these data are presented in Figure 2 and th sample preparation are described in Section S3.1.2 of the Supplementary shown in Figure 2A, the morphology of the PVDF-HFP consisted of dendr of small short multibranched structures, which are typically associate lamellae [29]. The lamellae originated from spherulite centers and aggrega fibrils in dense clusters. When the IL content was increased to 10 wt % ( dendrite-like patterns were replaced by refined and tiny fibrils. The fibril to each other and formed a stack of fibrils near the nuclei center. To bette variation, the in-phase images are presented in Figure S7 of the Supplemen By increasing the IL content above 30%, the tiny fibrils from the spherulites' omnipresent (see Figure S7). These results suggested that the addition of formation of edge-on oriented lamellae [28,30], which led to a transition in structure that explained the changes in the birefringence and swelling, a optical images presented in Figure 1D. Motivated by the previous works on the effect of the inclusion of car within IL and polymer phases [31,32], we used graphene nanoplatelets additive to our mixtures and probed the variations in CO2 uptake and the f For the control experiment, we chose 50 wt % IL in the polymer and to this a small amount of GNPs ranging between 0.1 and 0.4 wt %. Figure 3A s absorption capacity for the different mixtures. For pressures of 100 an observed a monotonic decline in the absorption capacity of the mixtures a their GNP content, suggesting that CO2 interaction with the film was we GNP content was increased. As with the polymer-IL mixtures, we morphology of the films under a polarized microscope to elucidate the Motivated by the previous works on the effect of the inclusion of carbon allotropes within IL and polymer phases [31,32], we used graphene nanoplatelets (GNPs) as an additive to our mixtures and probed the variations in CO 2 uptake and the film structures. For the control experiment, we chose 50 wt % IL in the polymer and to this mixture added a small amount of GNPs ranging between 0.1 and 0.4 wt %. Figure 3A shows the CO 2 absorption capacity for the different mixtures. For pressures of 100 and 200 psi, we observed a monotonic decline in the absorption capacity of the mixtures as a function of their GNP content, suggesting that CO 2 interaction with the film was weakened as the GNP content was increased. As with the polymer-IL mixtures, we evaluated the morphology of the films under a polarized microscope to elucidate the effect of GNP content. Figure 3B(i-iv) present polarized images of samples prepared with different amounts of GNPs ranging from 0 to 0.4 wt %. The figure shows that when GNPs were added to the polymeric mixtures, the density and size of the spherulitic domains is changed. This observation matched the one reported in the literature [33]. To establish a relationship between the structures and CO2 absorption ca the polymeric films, we studied the thin films' structures and properties usin characterization methods. We first examined the growth rate and average spherulites grown from different mixtures. The spherulite growth rate was obt the slope of the plots of radii of spherulites as a function of time, which is ill Figure 4A. The size of the spherulitic domains, as shown in Figure 4B, was d using the polarized optical images shown in Figures 1D and 3B-E; more de experimental preparations and calculations are available in Section S3 Supplementary Materials. As shown in Figure 4A, we observed that the conten GNP in the mixtures governed the growth rate and size of the spherulites. concentration of IL was increased to 30 wt %, the nucleation of the spher delayed; increasing the IL content further to 50 wt % shifted the onset time of backward. We attributed this change to the plasticization effect of IL; the add beyond the swelling capacity of polymer [34,35] enhanced the mobility of the chain while the latter led to an enhanced nucleation rate for the spheruli contrast, the growth rates were depreciated.
Concurrent with these changes, as shown in Figure 4A, we noted that growth kinetics of the polymer in mixtures containing only the polymer and a the formation of larger spherulites. Figures 4B and 1D clearly demonstrate the A reduction in the growth rate is often associated with the thermal mobility resulting in a longer time for the chains to fold into lamellae. This phenomenon To establish a relationship between the structures and CO2 absorption capacities of the polymeric films, we studied the thin films' structures and properties using different characterization methods. We first examined the growth rate and average size of the spherulites grown from different mixtures. The spherulite growth rate was obtained from the slope of the plots of radii of spherulites as a function of time, which is illustrated in Figure 4A. The size of the spherulitic domains, as shown in Figure 4B, was determined using the polarized optical images shown in Figures 1D and 3B-E; more details on the experimental preparations and calculations are available in Section S3.2 of the Supplementary Materials. As shown in Figure 4A, we observed that the content of IL and GNP in the mixtures governed the growth rate and size of the spherulites. When the concentration of IL was increased to 30 wt %, the nucleation of the spherulites was delayed; increasing the IL content further to 50 wt % shifted the onset time of the growth backward. We attributed this change to the plasticization effect of IL; the addition of IL beyond the swelling capacity of polymer [34,35] enhanced the mobility of the PVDF-HFP chain while the latter led to an enhanced nucleation rate for the spherulites [36]. In contrast, the growth rates were depreciated.
Concurrent with these changes, as shown in Figure 4A, we noted that the slower growth kinetics of the polymer in mixtures containing only the polymer and an IL led to the formation of larger spherulites. Figures 4B and 1D clearly demonstrate these changes. A reduction in the growth rate is often associated with the thermal mobility of chains, resulting in a longer time for the chains to fold into lamellae. This phenomenon facilitates branching and reduces the macroscopic growth rates [37]. Our calorimetry data, which are presented in Section S3.5 of Supplementary Materials, supported this point. In contrast to the polymer-IL mixtures, for the GNP-containing mixtures, a reduction in the spherulite growth rate as a function of the GNP content was attributed to the existence of GNPs and their agglomerates within the polymers, which restrained the mobility of the polymer chains. This trend has been reported for other nanocomposites [38]. When GNPs were introduced to the mixtures, the required activation energy for the chains to pack To establish a relationship between the structures and CO2 absorption capacities of the polymeric films, we studied the thin films' structures and properties using different characterization methods. We first examined the growth rate and average size of the spherulites grown from different mixtures. The spherulite growth rate was obtained from the slope of the plots of radii of spherulites as a function of time, which is illustrated in Figure 4A. The size of the spherulitic domains, as shown in Figure 4B, was determined using the polarized optical images shown in Figures 1D and 3B-E; more details on the experimental preparations and calculations are available in Section S3.2 of the Supplementary Materials. As shown in Figure 4A, we observed that the content of IL and GNP in the mixtures governed the growth rate and size of the spherulites. When the concentration of IL was increased to 30 wt %, the nucleation of the spherulites was delayed; increasing the IL content further to 50 wt % shifted the onset time of the growth backward. We attributed this change to the plasticization effect of IL; the addition of IL beyond the swelling capacity of polymer [34,35] enhanced the mobility of the PVDF-HFP chain while the latter led to an enhanced nucleation rate for the spherulites [36]. In contrast, the growth rates were depreciated.
Concurrent with these changes, as shown in Figure 4A, we noted that the slower growth kinetics of the polymer in mixtures containing only the polymer and an IL led to the formation of larger spherulites. Figures 4B and 1D clearly demonstrate these changes. A reduction in the growth rate is often associated with the thermal mobility of chains, resulting in a longer time for the chains to fold into lamellae. This phenomenon facilitates branching and reduces the macroscopic growth rates [37]. Our calorimetry data, which are presented in Section S3.5 of Supplementary Materials, supported this point. In contrast to the polymer-IL mixtures, for the GNP-containing mixtures, a reduction in the spherulite growth rate as a function of the GNP content was attributed to the existence of GNPs and their agglomerates within the polymers, which restrained the mobility of the polymer chains. This trend has been reported for other nanocomposites [38]. When GNPs were introduced to the mixtures, the required activation energy for the chains to pack To establish a relationship between the structures and CO 2 absorption capacities of the polymeric films, we studied the thin films' structures and properties using different characterization methods. We first examined the growth rate and average size of the spherulites grown from different mixtures. The spherulite growth rate was obtained from the slope of the plots of radii of spherulites as a function of time, which is illustrated in Figure 4A. The size of the spherulitic domains, as shown in Figure 4B, was determined using the polarized optical images shown in Figures 1D and 3B-E; more details on the experimental preparations and calculations are available in Section S3.2 of the Supplementary Materials. As shown in Figure 4A, we observed that the content of IL and GNP in the mixtures governed the growth rate and size of the spherulites. When the concentration of IL was increased to 30 wt %, the nucleation of the spherulites was delayed; increasing the IL content further to 50 wt % shifted the onset time of the growth backward. We attributed this change to the plasticization effect of IL; the addition of IL beyond the swelling capacity of polymer [34,35] enhanced the mobility of the PVDF-HFP chain while the latter led to an enhanced nucleation rate for the spherulites [36]. In contrast, the growth rates were depreciated. we observed a larger number of spherulites with smaller sizes. This behavior was in line with a previous observation [41]. To gain more information on the films' compositions, we analyzed the films using Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). Figure 5A presents a comparison of the IR spectra of the polymeric films. The assignment of the vibrational bands is presented in Section S3.3 of the Supplementary Materials. The characteristic band at the wavenumbers of 612 and 875 cm −1 were assigned to the α-phase and β-phase of the PVDF-HFP, respectively. Additionally, three distinct bands were assigned to the SO2 vibration of the IL. The peak appeared at 1348 cm −1 , which corresponded to an antisymmetric SO2 vibration mode of the IL; this was used to estimate the IL content of the films. The procedure to estimate the composition of polymer-IL film using the FTIR signal is reported in Section S3.3 of the Supplementary Materials. Figure  5B presents the XRD pattern of these films; the predominant peaks at 2θ were equal to ~18°, 20°, and 27°, which corresponded to the α-phase (020), β-phase (200), and α-phase (200) of the PVDF-HFP, respectively [41,42]. The small broad peak at 38° corresponded to the α-phase (021) diffraction [43]. The peak at 27.5° represented π-π spacing of the GNPs [44]; this diffraction peak became stronger as the concentration of GNP was increased. A clear change in the diffractograms was observed as the compositions were varied. We attributed this change in the structure of the polymer to the strong van der Waals (vdW) interaction between the imidazolium cations of the IL and the negative dipoles of the CF2 groups of the PVDF-HFP, which controlled the crystallization kinetics and stabilized the formation of the α-phase of the PVDF-HFP [45,46].  we observed a larger number of spherulites with smaller sizes. This behavior was in line with a previous observation [41]. To gain more information on the films' compositions, we analyzed the films using Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). Figure 5A presents a comparison of the IR spectra of the polymeric films. The assignment of the vibrational bands is presented in Section S3.3 of the Supplementary Materials. The characteristic band at the wavenumbers of 612 and 875 cm −1 were assigned to the α-phase and β-phase of the PVDF-HFP, respectively. Additionally, three distinct bands were assigned to the SO2 vibration of the IL. The peak appeared at 1348 cm −1 , which corresponded to an antisymmetric SO2 vibration mode of the IL; this was used to estimate the IL content of the films. The procedure to estimate the composition of polymer-IL film using the FTIR signal is reported in Section S3.3 of the Supplementary Materials. Figure  5B presents the XRD pattern of these films; the predominant peaks at 2θ were equal to ~18°, 20°, and 27°, which corresponded to the α-phase (020), β-phase (200), and α-phase (200) of the PVDF-HFP, respectively [41,42]. The small broad peak at 38° corresponded to the α-phase (021) diffraction [43]. The peak at 27.5° represented π-π spacing of the GNPs [44]; this diffraction peak became stronger as the concentration of GNP was increased. A clear change in the diffractograms was observed as the compositions were varied. We attributed this change in the structure of the polymer to the strong van der Waals (vdW) interaction between the imidazolium cations of the IL and the negative dipoles of the CF2 groups of the PVDF-HFP, which controlled the crystallization kinetics and stabilized the formation of the α-phase of the PVDF-HFP [45,46]. Concurrent with these changes, as shown in Figure 4A, we noted that the slower growth kinetics of the polymer in mixtures containing only the polymer and an IL led to the formation of larger spherulites. Figures 4B and 1D clearly demonstrate these changes. A reduction in the growth rate is often associated with the thermal mobility of chains, resulting in a longer time for the chains to fold into lamellae. This phenomenon facilitates branching and reduces the macroscopic growth rates [37]. Our calorimetry data, which are presented in Section S3.5 of Supplementary Materials, supported this point. In contrast to the polymer-IL mixtures, for the GNP-containing mixtures, a reduction in the spherulite growth rate as a function of the GNP content was attributed to the existence of GNPs and their agglomerates within the polymers, which restrained the mobility of the polymer chains. This trend has been reported for other nanocomposites [38]. When GNPs were introduced to the mixtures, the required activation energy for the chains to pack from the surface significantly increased [39]. The interaction between the partial positive charge on the C-H bonds of the PVDF-HFP and the negative charge on the surface of the GNPs created a higher free-energy barrier for nucleation, which slowed the crystallization kinetics of the polymer chains [40]. However, when the GNP content was increased, the increase in the nucleation density led to the indiscriminate growth of lamellae. As a result, we observed a larger number of spherulites with smaller sizes. This behavior was in line with a previous observation [41].
To gain more information on the films' compositions, we analyzed the films using Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). Figure 5A presents a comparison of the IR spectra of the polymeric films. The assignment of the vibrational bands is presented in Section S3.3 of the Supplementary Materials. The characteristic band at the wavenumbers of 612 and 875 cm −1 were assigned to the α-phase and β-phase of the PVDF-HFP, respectively. Additionally, three distinct bands were assigned to the SO 2 vibration of the IL. The peak appeared at 1348 cm −1 , which corresponded to an antisymmetric SO 2 vibration mode of the IL; this was used to estimate the IL content of the films. The procedure to estimate the composition of polymer-IL film using the FTIR signal is reported in Section S3.3 of the Supplementary Materials. Figure 5B presents the XRD pattern of these films; the predominant peaks at 2θ were equal to~18 • , 20 • , and 27 • , which corresponded to the α-phase (020), β-phase (200), and α-phase (200) of the PVDF-HFP, respectively [41,42]. The small broad peak at 38 • corresponded to the α-phase (021) diffraction [43]. The peak at 27.5 • represented π-π spacing of the GNPs [44]; this diffraction peak became stronger as the concentration of GNP was increased. A clear change in the diffractograms was observed as the compositions were varied. We attributed this change in the structure of the polymer to the strong van der Waals (vdW) interaction between the imidazolium cations of the IL and the negative dipoles of the CF 2 groups of the PVDF-HFP, which controlled the crystallization kinetics and stabilized the formation of the α-phase of the PVDF-HFP [45,46]. ~18°, 20°, and 27°, which corresponded to the α-phase (020), β-phase (200), and α-phase (200) of the PVDF-HFP, respectively [41,42]. The small broad peak at 38° corresponded to the α-phase (021) diffraction [43]. The peak at 27.5° represented π-π spacing of the GNPs [44]; this diffraction peak became stronger as the concentration of GNP was increased. A clear change in the diffractograms was observed as the compositions were varied. We attributed this change in the structure of the polymer to the strong van der Waals (vdW) interaction between the imidazolium cations of the IL and the negative dipoles of the CF2 groups of the PVDF-HFP, which controlled the crystallization kinetics and stabilized the formation of the α-phase of the PVDF-HFP [45,46].  Additionally, we estimated the degree of crystallinity and the β-phase of the films; the detailed procedures are presented in Section S3.4 of the Supplementary Materials. As shown in Figure 5C, increasing the IL content led to a reduction in the β-phase, which was in line with a previous report [47]. Furthermore, increasing the IL content enhanced the flexibility of the polymer chains and resulted in a reduction in the degree of crystallinity of the PVDF-HFP [42]. The latter was confirmed by the broadening of the XRD peaks, which was an indication of an increase in the volume of the disordered domains within the films [48,49]. Notably, the addition of GNPs led to a reduction in the degree of crystallinity but favored the formation of the β-phase. Here, we expected that the interaction between partial positive charges on the C-H bonds of the PVDF-HFP and the negatively charged surfaces of the GNPs led to a higher probability of the formation of "all-trans" segments of the PVDF-HFP [50,51]. As shown in Figure 5C, a reduction in the crystallinity of the polymer-IL films initially led to an increase in the CO 2 absorption capacity of the films. Upon the addition of GNPs, although the degree of crystallinity was further reduced, the trend of the CO 2 absorption was reversed. In this case, it was expected that the GNPs would induce a variation in the dispersion of the IL within the polymer matrix, weakening the solvation interactions. A similar observation when dealing with GNPs in complex fluid mixtures suggested that the addition of GNPs weakened the interactions between the components of the mixtures [39]. Here, we believe that the observed configurational changes of the PVDF-HFP due to the addition of GNPs resulted in an increase in the effective cohesion of the IL phase through reducing the polymer-IL interaction. As a result, the CO 2 absorption in these films was decreased.
To gain more quantitative information about the CO 2 sorption in the films, we estimated the enthalpies of absorption of the CO 2 in the polymeric films. Figure 6 presents the variations in the natural logarithm of pressure at steady states as a function of the inverse temperature for a 1.75 and 2.75 equivalent excess molar concentration of CO 2 (mole CO 2 per kg of IL). The isosteric enthalpies of absorption were estimated from the linear fits to the Clausius-Clapeyron equation [52] and are presented in the graph; additional data at different mole uptake are presented in Section S2.3.3 of the Supplementary Materials. Here, at the constant excess molar concentration of CO 2 , a systematic increase in the enthalpy of absorption as a function of the GNP concentration was noted. From a design perspective, sorbents that require a higher heat of regeneration (enthalpy of absorption) and provide a lower capacity for gas absorption are not attractive [53]; however, the observed (mole CO2 per kg of IL). The isosteric enthalpies of absorption were estimated from t linear fits to the Clausius-Clapeyron equation [52] and are presented in the grap additional data at different mole uptake are presented in Section S2.3.3 of t Supplementary Materials. Here, at the constant excess molar concentration of CO2 systematic increase in the enthalpy of absorption as a function of the GNP concentrati was noted. From a design perspective, sorbents that require a higher heat of regenerati (enthalpy of absorption) and provide a lower capacity for gas absorption are not attracti [53]; however, the observed trends pointed to the sensitivity of the configuration properties of these mixtures to their compositions.

Conclusions
In summary, we demonstrated the effects of the macrostructures of an ion gel on its CO 2 sorption capacity as a function of its composition. We observed a significant enhancement in the CO 2 sorption capacity for the IL in the polymer phase. The CO 2 sorption increased nonlinearly with increasing IL content. This nonlinearity of the CO 2 sorption was not only based on the hole-filling process, but it was also strongly influenced by both interactions of CO 2 -Polymer-IL and the swelling behavior of the polymer. In addition, we observed a strong impact of an addition of a carbon allotrope; even at a small mass fraction, this addition led to a structural change at the macroscale that reduced the CO 2 sorption capacity. Finally, the latter result of the heat of sorption highlighted the nonideality of these mixtures and the opportunity to choose the mixture composition as a design parameter with macroscopic fingerprints and tune the gas sorption properties of these mixtures.

Supplementary Materials:
The following are available online at https://www.mdpi.com/article/ 10.3390/membranes12111087/s1, Figure S1: (A) FTIR fingerprint of polymeric films; the red box indicates the range used for developing a calibration curve for estimating films' composition; (B) the established calibration curve for estimating the film composition cast from known mixtures; Figure S2: A schematic of an experimental apparatus for the dynamic method. The QCM flange shown in this figure only displays for the representative. Please refer to an experimental apparatus for an accurate dimension. Figure S3 Table S1: Conditions for the spin-coating method; Table S2: The static gravimetric isotherm sorption results of [EMIM][TF 2 N] (Mole CO 2 per kg IL) at 4 different pressures (psi) and temperatures (K). At 100 and 150 psi, an average and standard deviation were reported based on 3 different prepared tests. At 200 psi, an average and standard deviation were reported based on 2 different prepared tests. Data is compared with interpolated reported values; Table S3: Sorption isotherms dataset of 50:50 Polymer:IL at different temperatures and 11 different pressures. The data are the average of five different measurements presented with one standard deviation; Table S4: The enthalpy of adsorption of 50:50 Polymer:IL at different temperatures and 1.75 mole CO 2 / kg IL. The data are the average of five different measurements presented with one standard deviation; Table S5: Spherulite growth rate of polymeric mixture with various IL and GNP contents. The average and standard deviation of all samples were reported from two spherulites that appeared first on the surface; Table S6: Characteristic FTIR peaks of polymeric film; Table S7: Defined peak locations for the peak convolution; Table S8: Relative fraction of β-phase of polymeric films. An asterisk (*) indicates that a value is estimated from an interpolation; Table S9: Parameters applied to AMORPH simulation to calculate the crystallinity and amorphousity of the polymeric film; Table S10: Pattern XRD angles of the polymeric film; Table  S11: The degree of crystallinity of polymeric films. For the films with 0, 10, and 50 wt % IL content. The "Half-width (modelling)" indicates the standard deviation of how close the model to an actual dataset is; Table S12: Melting temperature of various films. For the films with 50 wt % IL content the data are representative of average values of three measurements from different samples, presented with one standard deviation; Table S13: Melting temperatures, crystallization temperatures, and solidification of various films. For the films with 50 wt % IL content, the data are representative of average values of three; Table S14: Degree of crystallinity of Polymer:IL (GNP). References  are cited in Supplementary Materials.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.