3.1 SEM images of PI membranes
Whether HCPs can be uniformly dispersed in MMMs is an important indicator for evaluating the quality of MMMs. Gas separation performance will greatly reduce once the HCPs get together and form agglomeration. Fig. 1a and 1b show the overlooking SEM image and the cross-sectional SEM image of pure PI film, respectively. A pure PI film with a homogenous structure is relatively smooth and dense. Fig. 1c, 1e, 1g, and 1i show the surface SEM images of HCPs-0.02/PI MMMs, HCPs-0.04/PI MMMs, HCPs-0.06/PI MMMs, and HCPs-0.08/PI MMMs, respectively. The results show that the amount of HCPs in MMMs increases with continued addition of HCPs. When the amount of HCPs is 0.08 wt.%, the size of HCPs becomes relatively larger because slight agglomeration occurs due to many more HCPs. Fortunately, HCPs have no large-scale agglomeration and are still well dispersed in PI films indicating good compatibility between HCPs and PI films.
Fig. 1d, 1f, 1h, and 1j show cross-sectional SEM images of HCPs-0.02/PI MMMs, HCPs-0.04/PI MMMs, HCPs-0.06/PI MMMs, and HCPs-0.08/PI MMMs, respectively. The distribution and sizes of HCPs agree with the SEM images. All SEM images show that the as-prepared HCPs/PI MMMs have no defects after adding HCPs, which can avoid side leakage during gas separation and provide a better guarantee for the next step of gas separation.
3.2 FT-IR spectra of PI membranes
FT-IR spectra can assess the degree of amidization of the polymer. The FT-IR spectra of pure PI film and different HCPs/PI MMMs are shown in Fig. 2. The wavenumber at 1785 cm-1 and 1725 cm-1 is the symmetric and asymmetric stretching vibration peaks of C=O in the imides groups, while the wavenumber at 1370 cm-1 and 718 cm-1 is the stretching and bending vibration peaks of C-N[43, 44]. The characteristic peak at about 3300–3500 cm-1 is the stretching vibration of N-H in poly(amic acid), which is not detected. The result shows that the polymers achieved amidation. In addition, the assigned peaks at 1250 cm-1 and 1140 cm-1 are the stretching vibration of C-F in PI, demonstrating that the fluorinated polyimides have been successfully obtained. The characteristic peaks of different HCPs/PI MMMs are the same as those of pure PI film, indicating that no new chemical bond was formed between HCPs and PI.
3.3 Mechanical strength of PI membranes
The MMMs need to withstand a certain pressure during gas separation. Therefore, the MMMs should have sufficient mechanical strength. The mechanical strength of HCPs/PI MMMs is necessarily assessed. Elongation at breaking and the tensile strength of pure PI film and different PI/HCPs MMMs are displayed in Fig. 3. As shown in Fig. 3, after adding to HCPs, the elongation at breaking of HCPs/PI MMMs gradually decreases with increasing amounts of HCPs. However, the elongation at breaking of HCPs/PI MMMs is higher than that of pure PI film. However, the value of the elongation at breaking of pure PI film and HCPs/PI MMMs is not large, and the value of the elongation at breaking of the HCPs/PI-0.02 is the highest (only 3.9%). This may be because PI has high rigidity, and molecular chain motion is difficult. The tensile strength of HCPs/PI MMMs increases in the beginning and then decreases. The tensile strength of HCPs/PI MMMs reaches up to the maximum about 44.2 MPa when the amount of HCPs is 0.04 wt.%. HCPs are a kind of pure organic fillers. They have good compatibility with PI, and therefore HCPs/PI MMMs have no defects, and their tensile strength does not decrease. In addition, due to the presence of 2-phenylimidazole in HCPs and hydrogen bonds between the N-H bond on the imidazole ring and C=O bond in PI, they can increase the tensile strength of HCPs/PI MMMs due to incorporation of HCPs fillers; the result is similar to previous reports in which filler can enhance the mechanical strength of MMMs[46, 47]. The tensile strength of HCPs/PI MMMs will reduce because of excessive HCPs that may be related to the interfacial effect forming agglomeration of HCPs.
3.4 Gas separation performance
Gas separation performance is an important and ultimate index to evaluate the quality of MMMs. Fig. 4a shows gas separation performance of pure PI film and HCPs/PI MMMs for CO2/CH4 at 35°C and 0.1 MPa of feed gas pressure. The permeation flux of pure PI film toward CO2 is 62.41 barrer, and the ideal selectivity to CO2/CH4 is 20.76. Simultaneously, as the amount of HCPs varies from 0.02 wt.% to 0.08 wt.%, the permeability of HCPs/PI MMMs to CO2 is 63.36 barrer, 105.85 barrer, 86.22 barrer, and 68.58 barrer. It increases at first and then decreases. Moreover, the permeability of all HCPs/PI MMMs is higher than that of pure PI film. When the amount of HCPs is 0.04 wt.%, the permeability of HCPs/PI MMMs to CH4 and CO2 enhances 125% and 69.6%, respectively. The results demonstrate that HCPs/PI MMMs facilitate CH4 and CO2 transport. Besides, HCPs/PI MMMs have better transport than pure PI film. Fortunately, the ideal selectivity is 14.57, 15.67, 14.66, and 13.59. The ideal selectivity of all HCPs/PI MMMs only decreases slightly than a pure PI film.
To better assess gas transport properties of MMMs, the CO2 and CH4 permeability and ideal selectivity data are displayed in Fig. 4b in a Robeson’s diagram for the corresponding CO2 and CH4 pair. The ideal selectivity obviously decreases as Robeson defines the linear relationship as “upper bound” accompanied by an improvement of gas permeability in MMMs. Here, the CO2 and CH4 are below the upper-bound line and represent the perfect combination of permeability and ideal selectivity for the fixed gas pair. However, they show a favorable tendency to increase the gas permeability, while the ideal selectivity basically remains unchanged after adding HCPs, which is different from the typical trade-off phenomenon of polymer membranes. In this respect, the HCPs are used to prepare MMMs in an effective approach to successfully improve gas transport and break the trade-off effect.
Next, O2 and N2 permeability experiments were carried out to prove that HCPs can promote different gas transport. The O2/N2 separation performance of PI film and different HCPs/PI MMMs and Robenson’s upper bound correlation (2008) for O2/N2 separation are shown in Fig. 5. Fig. 5a shows the gas separation performance of pure PI film and HCPs/PI MMMs for O2/N2. According to Fig. 5a, the pristine PI film exhibits that the permeability of O2 is 13.85 barrer, and the ideal selectivity of O2/N2 pair is 3.29. Meanwhile, in terms of HCPS/PI MMMs, the permeability of HCPs/PI MMMs to O2 increases at first and then decreases as the amount of HCPs changes from 0.02 wt.% to 0.08 wt.%. Moreover, the permeability of the HCPs/PI MMMs to O2 is higher than that of pristine PI film. When the HCPs were 0.04 wt.%, the permeability of the HCPs/PI MMMs to O2 is 24.03 barrer and plateaus. Compared with the pristine PI film, the permeability of HCPs/PI MMMs toward O2 and N2 increases by 73.5% and 91.6%, respectively, while the ideal selectivity of HCPs/PI MMMs toward O2/N2 pair only reduces 9.7%. The results demonstrate that HCPs/PI MMMs also facilitate O2 and N2 transport, improve gas permeability, and maintain stable selectivity.
Obviously, as shown in Fig. 4a and Fig. 5a, HCPs/PI MMMs have relatively weaker transport abilities to CH4, O2, and N2 than CO2. HCPs with imidazole-type poly-ionic liquid structures have certain catalytic properties for the cyclization of CO2 to carbonate. These can be used as a CO2 capture and conversion material[49-52]. This makes CO2 easy to be adsorbed in HCPs. When HCPs are used as the filler of PI MMMs, the solubility coefficient of PI MMMs to CO2 can be enhanced, and it may increase the gas permeability and accelerate the CO2 gas transport.
The results on permeability and ideal selectivity in the Robeson’s diagram for the corresponding O2 and N2 pair (Fig. 5b) are analogous to the corresponding CO2 and CH4 pair; however, the permeability and ideal selectivity of HCPs/PI MMMs to O2/N2 are obviously lower than those of CO2/CH4. This also demonstrates that the interaction between HCPs and CO2 leads to better permeability and selectivity than that of HCPs and O2.
3.5 Analysis of gas separation process
The gas separation performance of MMMs strongly depends on MMM structures. To gain an insight into the relation between the permeability and selectivity of MMMs and microstructure, the average interspacing distance (d-spacing) stands for the distance between polymer chain segments that reflects the permeability coefficient of MMMs. The d-spacing value of MMMs can be calculated by Bragg’s equation. XRD patterns of the PI film and HCPs/PI MMMs are shown in Fig. 6. The results demonstrate that the PI film and the HCPs/PI MMMs have broad amorphous peaks that are attributed to the presence of bulky -C(CF3)2- groups in PI; thus, the PI film and HCPs/PI MMMs have loose chain packing.
Generally, the amorphous nature of PI is favorable to gas permeation. The gas permeability of polymer membranes will strengthen with increasing d-spacing. The d-spacing values of PI film and HCPs/PI MMMs are 5.58 Å, 5.60 Å, 5.70 Å, 5.70 Å, and 5.46 Å(Table S1). The results show that the d-spacing trend for the PI films and the HCPs/PI MMMs gradually increases and then reduces, which agrees with different gas permeability observations except for HCPs-0.06/PI MMMs. Although the d-spacing value of HCPs-0.04/PI MMMs is equal to that of HCPs-0.06/PI MMMs, the gas permeability of HCPs-0.04/PI MMMs is higher than that of HCPs-0.06/PI MMMs. It is very possible that the free spaces in the microstructure besides the d-spacing of polymer chain segments have a fundamental effect on the gas permeability of polymer membranes.
Fig. 7 shows the molecular simulation diagrams of the PI and HCPs/PI systems after MD simulation. In this simulation, Fig. 7a and 7b are schematic representations of simulated molecular cells of the PI and HCPs/PI systems, in which twenty-three molecular chains composed of 6FDA-DAPI with 30 repeating units for the PI are placed in a cube box. The blue and gray parts indicate free volume and occupied volume, respectively. Fig. 7 shows that the free volume of HCPs/PI MMMs is significantly larger than that of pure PI films. According to the literature, the large free volume provides a fast channel for gas transport, thereby enhancing gas permeability[27, 45]. The other simulation results of MMMs containing different HCP amounts are shown in supporting information Fig. S2; the specific values obtained by simulation are shown in supporting information Table S1.
The density of HCPs/PI mixed polymers in stable state varied from 1.5358 to 1.3553 g/cm3, which is higher than 6FDA-based polyimides, and the FFV of MMMs (0.1892–0.2498) is also higher than most of 6FDA-based polyimide. Combining supporting information Fig. S2 with Table S1, we see that the FFV of MMMs containing different HCP amounts is larger than that of pure PI films. As the amount of HCPs increases, the FFV of PI MMMs first increases and then slightly decreases. Therefore, although the permeation flux of HCPs/PI MMMs to different gases is higher than that of pure PI films, the gas transport of HCPs/PI MMMs does not always increase with more HCPs. When the amount of HCPs added is 0.04%, the FFV of HCPs/PI MMMs reaches a maximum (0.2498). Therefore, the permeability of different gases is the largest. The permeability gradually reduces as even more is added. This also proves that the gas permeability not only is related to the d-space of the membranes but also has a greater relationship with the FFV of the membrane. In contrast, the ideal selectivity of different gas pairs reduces according to the large FFV; this result agrees with Fig. 4 and Fig. 5.