Materials synthesis and characterizations
The N atom in the Tröger's base moieties could interact with -CN groups in PIMs, considerably improving the miscibility of PIMs and Tröger's base polymer. The bromomethylated PIMs were prepared through converting methylated PIM-1 to PIMs with bromomethyl groups by using N-bromosuccinimideas bromomethylating agent (represented by PIM-BM-x, x is the degree of bromomethylation and x=70, Fig S1-3)25. Then the blends of PIM-BM-70 with rigid TB polymers (Fig S4-5) with excellent mechanical properties (PIM-BM/TB) (Fig S21 and Table S3) were selected as the framework to develop cross-linked microporous membranes.
Physically blended PIM-BM-70 and TB polymers with excellent miscibility were readily fabricated into transparent membranes by dissolution in chloroform and cast on a glass plate. The resultant PIM-BM/TB membrane could be simply cross-linked when thermally processed in a temperature window of 120 oC-300 oC over varied periods in nitrogen with ppm level of oxygen. The cross-sectional morphology of membranes was characterized by Scanning Electron Microscopy (SEM). As the SEM images show in Fig. 1a-f, all membranes regardless of thermal cross-linking temperatures display a smooth, macrovoid-free surface. With the increase of thermal processing temperature from 120 oC to 300 oC, the membrane color changed from originally yellow, to brown yellow and then black as observed in Fig. 1g. PIM-BM/TB treated at 200 oC over 20 h is partially insoluble in common organic solvents such as chloroform and NMP (Fig. S10) which can easily dissolve the pristine PIM-BM/TB. Upon treated at much higher temperatures between 250 oC - 300 oC, PIM-BM/TB became completely insoluble as seen in Fig. 1g.
We proposed three types of chemical cross-linking mechanisms likely occurring during the heating process, i.e. reactions of tertiary amine with bromomethyl groups with the formation of quaternary ammonium salts, alkylation reactions and oxidative crosslinking reactions within PIM-BM/TB membranes as schematically shown in Fig. 2 and Fig S7-9.
In the first scenario, we found the critical role of reactive sites of CH2Br groups of PIM-BM and tertiary amino groups of TB in PIM-BM/TB blends. One or two -CH2Br groups per repeat units of PIM-BM reacts with tertiary amino groups in TB polymers or phenyl groups in PIM or/and TB. X-ray photoelectron spectrometer (XPS) results (Fig. 3a-e) confirm that the covalent C-Br bonds in fresh PIM-BM/TB membranes are gradually transformed to the Br-containing salt via nucleophilic coupling reactions of tertiary amine with bromomethyl groups during the initial thermal treatment. This reaction occurs at a low temperature from 120 oC to 300 oC. The extent of nucleophilic coupling reactions between C-Br bonds and tertiary amine quantified by XPS results was found to increase from 12% to 40% with reaction temperature increasing from 120 oC to 300 oC (Table S2). Thus, the primary reaction mechanism is proposed that partial tertiary amino groups in TB polymers with bromomethyl groups in PIM-BM (bromomethylated PIMs) were converted to quaternary ammonium salts [N+(R)3]CH2RBr- through coupling reactions (Fig S7). The cross-linking degree regarding to such reaction for XPIM-BM/TB-250 oC-10 h and 300 oC-5 h estimated from XPS results were 25%, and 40%, respectively.
With respect to the second reaction mechanism, the membranes were subjected to inert gas with ppm level of O2 in the temperature range of 250-300 oC. In this case, we proposed a possible thermal cross-linking of C-Br bonds and benzene rings via alkylation reaction route, generating HBr as gaseous product, besides the aforementioned coupling reaction. As evidenced by the ion current corresponding to HBr from TGA/Mass Spectrometry (Fig S15), HBr was released from 230 oC to 300 oC for pristine PIM-BM/TB, corroborating the crosslinking mechanism through alkylation reaction. In contrast, the amount of HBr released during alkylation reaction reduced significantly for the case of crosslinked XPIM-BM/TB-250 oC -10 h, since most of CH2Br groups had participated in the crosslinking reaction with benzene rings, TGA-MS did not detect HBr signal for XPIM-BM/TB-300 oC-5 h (the trace amount of HBr was difficult to detect), suggesting consumption of the majority of C-Br groups as reactions proceeding within polymer blends. The consumption of C-Br groups was also verified by XPS results where Br 3d core signal ratio of C-Br (70.3 ev) to Br- (68.3 ev) in XPS spectra significantly decreased for the membrane thermally treated at 300 oC for 5 h (Fig. 3e). The cross-linking degree regarding to alkylation reaction for XPIM-BM/TB-250 oC-10 h and XPIM-BM/TB-300 oC-5 estimated from XPS results were 22%, and 40%, respectively. Moreover, as depicted by Fourier transform infrared spectra (FTIR) of the membranes (Fig. 3f), the characteristic peak of C-Br near 660 cm-1 for XPIM-BM/TB membranes treated at high temperatures ≥250 oC become less intense comparing with the original PIM-BM/TB.
In the third stage, we proposed an oxidative induced crosslinking mechanism that PIM-BM/TB membranes experience a thermal-oxidative reaction during heating treatment in a temperature of 250-300 oC in the presence of ppm-level oxygen. Such phenomena was also observed in the temperature range of 300-450 oC for pure PIM-1. The oxygen plays a key role to partially decompose polymer chain into polymer fragments. The scissor of spiro linkage of PIM-1 and methylene groups linked to N atoms of TB is likely the dominant decomposition step. Partial backbones were oxidized and led to the formation of COOH groups. Afterwards, fragmented polymer chains are hypothetically thermally rearranged to an energy favorable state. Simultaneously, the chemical reaction took place among reactive groups including the oxidative induced free radicals, CH2Br and N containing groups in the polymer chain, causing extensive covalent crosslinking (Fig 2). The degree of oxidative polymer chain scissor is tunable via changing the O2 concentration in the purge gas as discussed later.
As discussed above, despite the different trends observed in PIM-BM/TB membrane characterizations under various temperatures, the three possible crosslinking mechanisms are inherently coupled, all of which could contribute to the crosslinked network formed within the membranes. Decoupling the cross-linking reaction mechanisms in PIM-BM/TB membranes merits further study to clarify effects of each individual reaction on membrane structures and properties.
The phsychemical properties of membranes were further characterized using a wide range of techniques. As TGA profiles show, the dependence of weight loss on crosslinking temperatures demonstrated that crosslinked membranes had improved thermal stability over uncrosslinked PIM-BM/TB blends (Fig. 4a and Fig S11). In fact, the XPIM-BM/TB-300 oC-5 h membranes start to decompose at a temperature of ~400 oC, significantly higher than the case of untreated PIM-BM/TB membranes or treated at 200 oC with a decomposition temperature of ~300 oC. XRD spectra of PIM-BM/TB before and after thermal crosslinking revealed all polymers were amorphous (Fig. 4b). Referring to XRD profiles of pristine membranes, the peak at the angle of 11.9 o (d-space values of 7.43 Å ) is attributed to loosely packed polymer chains26-27. With increasing crosslinking extent, the broad peaks at 11.9o slightly shifted to a higher angle (i.e. a smaller d-space value), implying crosslinking reactions tighten the chain spacing and potentially enhance molecular sieving properties of membranes. A molecular modelling comparison PIM-BM/TB and XPIM-BM/TB shows reduced free volume fractions with increasing crosslinking degree, which is consistent with PALS results (Table S1).
A typical plot of stress-strain curve of membranes (Fig. 4c) shows a reduction in elongation and tensile strength at break with increasing cross-linking degree, suggesting that the XPIM-BM/TB polymer ﬁlms became less ductile compared with untreated PIM-BM/TB. For example, pristine membranes have an ultimate yield stress of 43 MPa at 8.8% strain whereas the membranes treated at 200 oC for 20 h have a tensile stress of 19 MPa at 1.9% strain at break. When treating membranes above 250 oC, the films became rigid and beyond the testing capability of our experimental apparatus.
Pore structure characterization
As evidenced by the above XRD testing, the formation of covalent crosslinking bonds tends to tighten the polymer chains, leading to a reduced d-spacing in polymer matrix. To further gain insights of microstructures, the pore size distribution of membranes was characterized using combined simulated and experimental approaches based on the MELT program and PALS results. As visualized by molecular simulation in Fig. 5a, the rigid polymer chains in PIM-BM/TB was poorly packed, leading to the formation of irregularly shaped free volume. Molecular modelling comparison of pristine PIM-BM/TB with crosslinked PIM-BM/TB shows the occurrence of efficient packing after cross-linking (Fig. 5a,b). This crosslinking induced tightening effect was consistent with the fractional free volume (FFV) simulation (supplementary Table S1). For instance, with H2 gas for the structure probe, the calculated FFV reduced from 0.228 to 0.187 for the uncrosslinked and crosslinked membranes, respectively.
Fig. 5c reveals the free volume distributions generated from MELT program based on PALS results. The average free volume radii shifted to a lower value upon crosslinking. As a result, crosslinked membranes exhibited a smaller fractional free volume than the pure polymers, as described in Supplemental Table S1. Moverover, a thermal treatment at the temperature of 250 oC and 300 oC results in much narrow pore size distribution over untreated membranes, indicating the more restricted chain motion and smaller cavities in the crosslinked format of membranes28. Indeed, crosslinking not only tightens the inner pores of membranes but also tailors the width of ultramicropores connecting neighboring cavities, potentially allowing selective diffusion of smaller gases such as H2 and CO2 while excluding larger gas molecules like N2, and CH4, which will be discussed the section below.
Gas transport properties
To explore the gas transport properties in the membranes, single gas permeation was first conducted on pristine polymer membranes and thermally cross-linked membranes using gas molecules including H2 (2.89 Å), CO2 (3.3 Å), O2 (3.46 Å) N2 (3.64 Å) and CH4 (3.8 Å) at 35 oC under a feed pressure of 50 psia. Gas permeabilities and ideal selectivities of the membranes are described in Table 1. The pristine PIM-BM/TB membranes exhibit high permeability with moderate gas selectivities, consistent with reported PIMs family membranes. As expected, the crosslinking reaction results in a drastic decrease in gas permeability for XPIM-BM/TB due to pore structure shrinkage as discussed above. Particularly, gas permeability of large gas molecules (CH4, N2) reduced much more significantly over smaller gas molecules like O2, H2, CO2 as shown in Fig. 6a. Nevertheless, the gas selectivity increases remarkably upon thermal crosslinking. Moreover, the sequence of gas permeability follows the order of H2>CO2>O2>N2>CH4 in XPIM-BM/TB regardless of crosslinking temperatures. Such trend is in accordance with the order of gas kinetic diameters, suggesting the molecule sieving property of these membranes. Indeed, the crosslinked membranes with efficiently packed chain geometry display substantially enhanced molecular sieving ability. For instance, the H2/CH4 selectivity of representative XPIM-TM/TB membranes prepared upon heating at 250 oC for 10 h increased from 17.2 to 118.6 and CO2/CH4 selectivity increased from 18.0 to 54.1.
The CO2/CH4 and H2/CH4 separation data are plotted with current upper bounds and compared with literatures. Fig. 6b and c show the overall trends of remarkable enhancement of selectivity with moderate reduction in gas permeabilities over other existing PIM based membranes. In comparison to industrially used polymeric gas separation membranes (e.g. polysulfone, PSF), the XPIM-BM/TB membranes have apparently higher selectivities while maintaining orders-of-magnitude higher permeability, well exceeding present upper bounds29-31. Notably, the H2/CH4 selectivity of XPIM-BM/TB membranes treated at temperature of 300 oC for 5 h is as high as 814.1, increased by 47-fold over un-crosslinked PIM-BM/TB, which is one of the highest values reported for other comparable polymeric membranes. Owing to rigid structures with closely packed chains in these membranes, a significantly high H2/CH4 permselectivity is achieved due to the large difference in kinetic diameters for H2 and CH4 molecules. The substantial reduction of CH4 permeability after thermal treatment indicates the narrow pore structure of XPIM-BM/TB and large reduction of cavities sizes preferentially permeating H2 over CH4.
Other notable gas pairs including O2/N2, and H2/N2 are described in Supplementary Fig. S19-20. Similar to H2/CH4, all these gas pairs achieve highest selectivity for crosslinked PIM-BM/TB treated at 300 oC and substantially exceed the gas separation limits of conventional polymeric membranes. The separation process of O2/N2 is much more difficult as compared to H2/CH4 and CO2/CH4 as O2 and N2 have similar kinetic diameters with only Ångstrom-level differences. Nonetheless, the crosslinked membranes exhibit great promise for O2/N2 separation as the separation performance is located well above the Robeson upper bound. With increasing reaction temperature, O2/N2 selectivities remarkably increased from 3.8 to 11.1 with a reduction of O2 permeability from 422.5 Barrer to 18.2 Barrer. This impressive gas separation performance is similar to that of carbon molecular sieve and other molecular sieving membranes32-33, further confirming the enhanced molecular sieving effects within these cross-linked membranes.
Tunable gas transport properties
One of the important advantages of PIM-BM/TB polymer membranes is the versatility in tailoring the microporous structure and gas separation performance. The gas transport properties intimately correlated with pore structures of membranes could be precisely tuned by controlling cross-linking reaction temperature, reaction time and the atmosphere. Fig. 7 shows the effects of reaction temperature, reaction time and oxygen concentration in the purging gas on gas permeability and selectivities in crosslinked PIM-BM/TB. Evidently, as the cross-linking temperature increased from 80 oC to 300 oC, gas permeability gradually decreased while gas selectivity increased significantly (Fig. 7a-b). In fact, the cross-linked membranes demonstrated the most substantial drop in gas permeability of largest gases of CH4 and the least reduction in H2 permeability depending on reaction temperature (Fig. 7b). Correspondingly, the H2/CH4 gas pair with the largest kinetic diameter difference is most sensitive to reaction temperatures. In addition to the reaction temperature, the gas separation performance can be readily tailored by changing crosslinking time. For the membranes reacted with identical reaction temperature of 250 oC, increasing cross-linking time from 5 h to 20 h leads to the increase of gas selectivites along with a reduction in gas permeability. For example, CO2 permeabilities reduce from 430.7 Barres to 148.5 Barres and CO2/CH4 selectivity increase from 27.5 to 79.7 (Table S6). We further investigated the effects of O2 concentration on the gas separation performance of crosslinked membranes at a temperature of 250 oC with a reaction duration of 10 h as shown in Fig 7e-7f. For example, the CO2 gas permeability reduced when increasing O2 concentration from inert gas to a level of 21000 ppm. The gas selectivity of CO2/CH4 reached the maximum of 56 at 200 ppm O2. The oxygen concentration plays a critical role during thermally oxidative crosslinking reaction. Therefore, adjusting the cross-linking temperature, time and oxygen concentration can simply generate polymeric membranes with commercially attractive gas separation performance. To further improve the membrane performance, blending ratio of PIM-BM/TB and degree of bromomethylation are other potential tunable factors to improve the gas transport properties, which is currently being explored in our lab.
Effects of high feed pressures and ageing behavior
To explore the practical applicability of membranes under aggressive feed conditions, the membranes were subjected to high CO2 pressures. In the case of un-cross-linked PIM-BM/TB membranes, exposure of membranes under high pressure pure CO2 leads to reduced gas permeability owing to the non-ideality under high pressures (Fig. 8a) which is commonly observed in highly porous PIMs34. The CO2/CH4 selectivity reduced with increasing pressure to 500 psia. On the other hand, the cross-linked membranes exhibit much less permeability decline over pristine membranes, indicating considerably stabilized structure in the membranes. The mixed gas permeation tests were performed using CO2/CH4 1:1 gas mixture for PIM-BM/TB-250 oC-10 h (Fig. 8c-d). Mixed gas separation properties were comparable to pure gas permeation results.
With the consideration of long-term stability of membranes, physical aging over a period of 12 months for XPIM-BM/TB treated at 250 oC for 10 h were investigated. Gas permeability of XPIM-BM/TB gradually reduced and gas selectivities increased during aging. As shown in the Robeson plot (Fig. 6b-c), the aged XPIM-BM/TB membranes still demonstrate outstanding gas separation performance well above the upper bound.