3.1. Polymer Characterization
The chemical structure of 6FBBA-TMDAT was confirmed by FTIR and
1H NMR spectroscopy. The FTIR spectrum (
Figure 2a) showed the characteristic absorption bands of the amide group at frequencies in the range 3150–3410 cm
−1 for –NH stretching and 1657 cm
−1 for C=O stretching. The aliphatic and aromatic C–H stretching bands appeared at frequencies of 2925 and 3062 cm
−1, respectively. In the
1H NMR spectrum of 6FBBA-TMDAT (
Figure 2b), the absorption band at 9.81 ppm is assigned to the amide protons (H
h), confirming the formation of amide linkages. The protons of the methyl groups (H
a) appeared as two singlets at 2.08 and 2.36 ppm, whereas the bridgehead protons of the triptycene unit (H
b) appeared at 5.51 and 6.08 ppm. The aromatic protons (H
c–g) resonated in the range of 7.00–8.05 ppm.
Figure 3 displays the thermogravimetric analysis (TGA) of 6FBBA-TMDAT; the absence of weight loss in the range 100–450 °C demonstrated that the methanol treatment successfully extracted all DMAc solvent from the 6FBBA-TMDAT sample. The degradation temperature at 5% weight loss (
Td,5%) was about 480 °C (
Table 1); this
Td,5% is comparable to that of 6FDA-TMDAT [
45] (synthesized by polycondensation of 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and TMDAT) representing the polyimide counterpart of 6FBBA-TMDAT polyamide. TGA was also employed to ensure the absence of any residual solvent in film samples before pure- and mixed-gas permeation tests.
The
amorphous cell module of Material Studio 2017 software was applied to simulate the space-filling ability of 6FBBA-TMDAT using the geometric density of 6FBBA-TMDAT measured in-house (1.15 ± 0.04 g/cm
3). The same simulation procedure was also performed for 6FDA-TMDAT polyimide (see properties in
Table 1). Clearly, the amorphous cell of the polyamide (
Figure 4a) is more densely packed than that of 6FDA-TMDAT polyimide (
Figure 4b); moreover, upon building the amorphous cell, the computer program provided occupied volume (
) vs. free volume (
) estimates that were used to calculate the fractional free volume (FFV) of both polymers. Accordingly, the FFV value for 6FBBA-TMDAT was 0.217 (a high value for polyamides) positioning this polyamide at the high end of the range of FFV defined by correlations of gas permeability vs. fractional free volume reported in the classical literature [
15]; hence, the insertion of the TMDAT building block was very effective in enhancing the FFV. The 6FDA-TMDAT polyimide showed a higher FFV value of 0.254; therefore, we anticipated to observe a higher permeability of this polyimide when compared to the structurally related 6FBBA-TMDAT polyamide, as discussed below.
The difference in FFV between 6FBBA-TMDAT and 6FDA-TMDAT (
Table 1) reflects the chain rigidity and chain interaction properties of these polymers. Generally, FFV (and typically gas permeability) increases as the polymer exhibits increased intrachain rigidity and interchain spacing, and shows less interchain interactions (hydrogen bonding or charge transfer complex formation tend to reduce FVV). In particular, chain motion of the 6FBBA-TMDAT polyamide is described here based on a torsion analysis via computer-assisted molecular simulations (Material Studio 2017 program was used). This description was further extended by including a comparison in terms of torsion energy vs. allowed torsion angles between the 6FBBA-TMDAT polyamide and its equivalent 6FDA-TMDAT polyimide. For both polymers,
Figure 5a shows the rotational freedom around the C-N bond (1) directly linked to the TMDAT unit. The methyl groups of the TMDAT unit practically lock the rotation to ~120° for the polyamide and ~180° for the polyimide; in fact, the hydrogen of the N-H group of the polyamide is located closer to the two methyl groups of the TMDAT than the oxygens of the C=O groups of the polyimide. The polyamide can still rearrange itself, due to the rotational freedom around the C–N (2) and C–C (3) bonds (
Figure 5b).
Because of the higher rotational freedom (i.e., lower intrachain rigidity) around the bonds of the amide group, the 6FBBA-TMDAT polyamide can pack its polymeric chains more efficiently than the 6FDA-TMDAT polyimide. N
2 sorption at 77 K (
Figure 6) confirmed the simulation results; in fact, N
2 uptake in the 6FDA-TMDAT polyimide was higher than in the 6FBBA-TMDAT polyamide; the BET surface area of 6FDA-TMDAT polyimide (620 m
2 g
−1) was ~57% higher than that of 6FBBA-TMDAT polyamide. Nevertheless, because of the presence of the tetra-substituted triptycene block, the 6FBBA-TMDAT polyamide showed a surprisingly high BET surface area of 396 m
2 g
−1.
The WXRD spectrum of the 6FBBA-TMDAT polyamide is shown in
Figure 7; several amorphous peaks with two overlapping amorphous halos with an average chain spacing of 8.0 and 5.95 Å, respectively, can be observed. Also, three additional peaks located around 4.7, 3.5, and 2.1 Å were identified. The two main peaks with large chain spacing derive from the introduction of the bulky triptycene building block in the polymer repeat unit. The weak amorphous peak at about 2.1 Å could be an indication of the occurrence of strong interchain interactions between the amide sites through hydrogen bonding. A similar peak was also found for the triptycene based polyimide with hydroxyl-functionalized diphenyl-hexafluoropropane unit [
59], where –OH group interchain hydrogen bonding was attributed to the formation of this high-angle amorphous peak.
3.2. Pure-Gas Sorption, Permeation, and Physical-Aging
In this section, we describe how chemical structure, chain packing, and free volume distribution correlate with pure-gas transport properties of 6FBBA-TMDAT polyamide.
CH
4 and CO
2 uptakes of 6FBBA-TMDA are displayed in
Figure 8a,b in the range of 0–15 atm at 35 °C. In comparison with a high-free-volume polymers, such as PTMSP [
60] (also plotted in
Figure 8a,b), at pressures lower than 5 atm, 6FBBA-TMDAT polyamide displayed higher CH
4 and CO
2 uptakes possibly reflecting a higher content of microporosity and ultra-microporosity—note: to reduce surface energy, gas molecules tend to sorb in small pores first. On the contrary, at pressures higher than ~7 atm for CH
4 and ~14 atm for CO
2, gas sorption in PTMSP is higher than in 6FBBA-TMDAT, and this behavior agrees with the fact that PTMSP is a polymer with pore size distribution shifted towards larger micropores, i.e., in the range of ~10–20 Å [
41].
Figure 8a,b also reveal that CO
2 and CH
4 uptakes in 6FBBA-TMDAT were significantly higher than in the case of low-free-volume glassy cellulose acetate (CA) [
61] and polysulfone (PSF) [
62] which are the most commonly used commercial gas separation materials. Interestingly, a comparison between the 6FBBA-TMDAT polyamide and CA revealed a greater CO
2/CH
4 solubility selectivity of commercial CA (
Figure 8c). Note: the solubility selectivity at infinite dilution of CA was 2.7 times higher than that of 6FBBA-TMDAT.
The discussion is now extended to the pure-gas permeability properties of 6FBBA-TMDAT polyamide at 35 °C. He, H
2, N
2, O
2, CH
4, and CO
2 pure-gas permeabilities measured at 2 atm are listed in
Table 2. 6FBBA-TMDTA aged for two days (‘
fresh’ sample) exhibited noteworthy gas permeabilities, e.g., 226, 144, and 33 barrer for H
2, CO
2, and O
2, respectively. Pure-gas permeability of 6FBBA-TMDAT increased in the following order: H
2 > He > CO
2 > O
2 > N
2 > CH
4. The fresh 6FBBA-TMDAT polyamide film had moderately high permselectivities, e.g., 39, 25, and 4.7 for H
2/CH
4, CO
2/CH
4 and O
2/N
2, respectively.
Because extensive hydrogen bonding induces tight interchain packing and discourages chains mobility, conventional low-free-volume polyamides typically do not display significant physical aging tendency. However, in view of the high surface area exhibited by 6FBBA-TMDAT, physical aging induced some reduction of 6FBBA-TMDAT permeability (
Table 2). The aging knee—i.e., the starting point of the region of
quasi-steady-state permeability [
44]—of 6FBBA-TMDAT appeared after ~40 days. For example, for the H
2–CH
4 gas pair, this knee can be identified at the right boundary of the highlighted region of
Figure 9a; CH
4 and H
2 permeabilities decreased by 30 and 15% respectively, whereas H
2/CH
4 permselectivity increased by 20% from the fresh sample values. A general analysis of all studied gases revealed that the reduction of permeability (at the aging knee) from fresh sample values approximately followed a linear trend of the kinetic gas diameters (see
Figure 9b).
In
Table 2, pure-gas permeability and permselectivity of 6FBBA-TMDAT polyamide are also compared with the structurally related 6FDA-TMDAT polyimide [
45] and conventional cellulose acetate (CA, with a degree of acetylation of about 2.9 [
64]). The 6FDA-TMDAT polyimide is much more permeable than 6FBBA-TMDAT polyamide because of the higher surface area (
Figure 6), FFV, and chemical bond rotation freedom as discussed earlier in this work (
Figure 4 and
Figure 5). For example, aged films samples of 6FBBA-TMDAT polyamide and 6FDA-TMDAT polyimide exhibited CO
2 permeabilities of 109 and 1150 barrer, respectively. On the other hand, hydrogen bonding ensured higher gas-pair permselectivities for 6FBBA-TMDAT polyamide than 6FDA-TMDAT polyimide (
Table 2); for example, pure-gas O
2/N
2 and CO
2/CH
4 selectivities (4.8 and 26) of aged 6FBBA-TMDA were more than ~30% and ~70% higher than those of its polyimide counterpart 6FDA-TMDAT (3.7 and 15), respectively. Likewise, H
2/CH
4 and H
2/N
2 pure-gas permselectivity of 6FBBA-TMDA was 3.8 and 2.3 fold greater than 6FDA-TMDAT (at the aging knee), respectively, as shown in
Figure 10 and
Table 2. The good combination of both permeability and permselectivity of 6FBBA-TMDAT in both fresh and aged samples placed it close to the 2008 H
2/CH
4 (
Figure 10) and H
2/N
2 (not shown) upper bounds.
Compared to conventional low-free-volume glassy polymers, such as CA, 6FBBA-TMDAT demonstrated (
Table 2) much higher permeability (e.g., ~9 and ~16 times higher for H
2 and CO
2 permeability, respectively) because of both higher solubility and diffusion coefficients (see
Table 3 for the CO
2–CH
4 pure-gas pair at 2 atm and 35 °C). However, enhancement in permeability coupled with a decrease in permselectivity followed the traditional permeability/selectivity trade-off relationship. As a result, CA is generally more permselective than 6FBBA-TMDAT (
Table 2). For the CO
2–CH
4 pure-gas system, the data in
Table 3 indicate that the permselectivity in CA benefitted from the above mentioned solubility selectivity advantage (
Figure 8c); whereas 6FBBA-TMDAT polyamide fills this solubility selectivity gap by a superior diffusion selectivity (
Table 2).
3.3. CO2–CH4 Mixed-Gas Permeation
The 6FBBA-TMDAT film was also tested for mixed-gas permeation of a 50:50 mol% CO
2:CH
4 mixture up to 30 atm total feed pressure (at 35 °C). As shown in
Figure 11a, CO
2 pure-gas permeability decreased with increasing pressure, due to the saturation of Langmuir’s sites. On the other hand, CO
2–CH
4 competitive sorption was responsible for the decrease of CO
2 mixed-gas permeability from the pure-gas values (
Figure 11a). In the range of studied pressures, both pure- and mixed-gas CO
2 permeability isotherms did not reveal a minimum and the subsequent positive slope that generally marks the occurrence of
plasticization. In other words, the occurance of plasticization hides behind the effects on gas permeability of saturation of Langmuir’s sorption sites and competitive sorption. Although in a different range of permeability, pure- and mixed-gas CO
2 permeability of 6FBBA-TMDAT followed the general behavior trends of CA [
64] (
Figure 11a).
For both 6FBBA-TMDAT and CA, pure-gas CH
4 permeability decreased slightly with pressure, due to Langmuir’s sites saturation (
Figure 11b). Contrarily to the just described permeation behavior of CO
2 in the mixture, CH
4 mixed-gas permeability of both 6FBBA-TMDAT and CA
increased with partial pressure. Experimental studies on CO
2–CH
4 pure- and mixed-gas permeability, sorption and diffusion in glassy polyimide and rubbery polydimethylsiloxane membranes [
66,
67] showed that this CH
4 permeability enhancement is directly correlated to CO
2 sorption; i.e., increasing CO
2 content in the polymer matrix activates chain mobility thus boosting CH
4 diffusion coefficient. Hence, the occurrence of this phenomenon explains the decrease in CO
2/CH
4 permselectivity under mixed-gas conditions in both 6FBBA-TMDAT and CA (
Figure 11c).
Finally, it is noteworthy that at 10 atm partial CO
2 pressure (the typical well-head pressure of interest for natural gas applications [
68]), the increase in mixed-gas CH
4 permeability from the corresponding pure-gas values was 2% and 43% for CA and 6FBBA-TMDAT, respectively. Consequently, 6FBBA-TMDAT suffered a considerable loss in CO
2/CH
4 mixed-gas permselectivity compared to CA. This is most likely, due to the fact that the hydrogen-bonding ability of 6FBBA-TMDAT was sterically reduced by the presence of the bulky triptycene building block (
Figure 1).