3.1.1. Polymerization and Characterization
Polynorbornene (pNB) was easily synthesized using the catalyst Ni(C
6F
5)
2(SbPh
3)
2 [
10] (
Table S1, entry 1). However, synthesizing polymers made with 5-vinyl-2-norbornene proved to be more difficult. Neither Ni(C
6F
5)
2(SbPh
3)
2 or Pd
2dba
3/AgSbF
6/PPh
3 [
25] were capable of obtaining large molecular weights with a monomer mixture of NB and VNB (ratio 50:50) or with only VNB (
Table S1, respectively entries 2-3 and 5-6). VNB is known to be challenging to polymerize as the exocyclic vinyl group can act as a reactive site for chain transfer [
26] or it can slow the catalyst down by sterical hindrance or coordination [
26,
27]. Since large molecular masses are required to obtain mechanically stable membranes, another catalyst system was employed using Pd
2dba
3/TTPB/PCy
3 [
26] (
Table S1, entries 4 and 7), resulting in number average molecular weights above 167 kg/mol. This proved to be sufficient for producing mechanically stable free-standing membranes. Therefore, with this catalyst system, a series of polynorbornenes with varying VNB content were synthesized. During workup, aliquots were taken for GPC analysis in THF (
Table 2,
Figure S1). The aliquots did not precipitate in THF, but had difficulty passing through a 0.2 µm filter. Possibly, the higher molar mass chains were retained, resulting in an underestimation of the real molar masses. Still, high molar masses were obtained for all polymers.
The solubility of the polymers after drying proved to be challenging. pNB, pNB-VNB-25 and pNB-VNB-50 were soluble in chloroform. pNB-VNB-75 and pVNB were stored in solution as they would become insoluble after drying. As a consequence, not all solvent could be removed, resulting in large residual solvent signals in the NMR spectra (
Figure 2). The
1H NMR spectra showed that the polymers were monomer free as no sharp correlating monomer signals are present in the polymer spectra. Also, by equalizing the vinyl proton signal intensity (4.5–6.5 ppm), a gradual decrease in aliphatic signals (0.5–2.5) is visible. This indicates that an increasing amount of VNB to NB is built in, which is in accordance with the monomer composition. The integration of the signals indicate that for pNB-VNB-25 and for pNB-VNB-50 respectively about 31% and 55% VNB was built in (
Figures S10 and S11). These values may be slightly overestimated due to the presence of solvent signals in the aliphatic region (e.g., water).
The free-standing membranes were analyzed by Fourier-transform infrared spectroscopy (FT-IR) (
Figure 3). Analyzing the spectra with increase in VNB content, it is clear that the purple regions, related to the unsaturated C=C and C–H bonds, increase in intensity while the orange regions, related to the saturated C–H bonds, decrease in intensity. This confirms the conclusion from the NMR spectra: NB-VNB copolymers can be synthesized by controlling the VNB content in the polymerization feed.
Raman, which is highly sensitive to the C=C stretching vibration, allow for a deeper analysis of the VNB content in the polymer membranes, by analyzing the relative changes in the C=C signal at 1637 cm
−1. For this, the Raman spectra were recorded across different VNB feed concentrations using their common and invariant CH
x signatures just below 3000 cm
−1 and were subsequently normalized. As such, the relative intensity of the C=C signal in the material can be quantified and compared directly. This analysis is presented in
Figure 4, which exhibits a near perfect correlation between VNB content in the monomer feed and the C=C signal strength at 1637 cm
−1. This correlation is in accordance with the VNB content analysis by NMR signal integration for pNB-VNB-25 and pNB-VNB-50. Such an approach may facilitate a direct means of analyzing the VNB content when such details are not known.
Scanning Electron Microscopy (SEM) was used to view the cross-sections of the membranes (
Figure 5). All membrane cross-sections show homogeneous dense films.
Thermogravimetric analysis (TGA) showed no discernable pattern with increasing VNB content (
Figure S3). The degradation temperature of pVNB at 5% mass loss was 350 °C, indicating high thermal stability. No
Tg was detected under 340 °C, confirming the rigidity of the polynorbornenes (
Figure S5).
The wide-angle X-ray scattering (WAXS) pattern of the polynorbornenes included two broad peaks, indicating that the polymers are completely amorphous (
Figure 6). It is generally accepted that the peak at higher 2θ correlates with intrasegmental interactions while the peak at lower 2θ correlates with intersegmental interactions [
15,
28,
29]. Three interesting trends are visible with an increase in exocyclic vinyl group presence in the polymers. First, the intrasegmental peak broadens at higher 2θ. This can be explained by the increasing presence of the exocyclic vinyl groups which may be capable of increasing the amorphous character of the intrasegmental packing. Second, the intersegmental peak shifts from 10° to 8.4°. This corresponds, using Bragg’s law, with a
d-spacing shift from 8.8 to 10.5 Å, indicating that the increase in exocyclic vinyl group presence considerably increases intersegmental
d-spacing. Third, relatively increasing scattering intensities of the low 2θ peak with increase in VNB content indicates that more numerous intersegmental scattering events took place compared to the intrasegmental scattering. Therefore, based on these observations, an increase in exocyclic vinyl group content seems to benefit larger intersegmental spacing. Comparing this data with other addition type polynorbornenes with different side groups, it is clear that the X-ray scattering data for pNB is similar to previously published pNB X-ray data (
Table 3) [
17]. Also, the intersegmental
d-spacing of pVNB is situated between the intersegmental
d-spacings of polynorbornene with a methyl group and with a butyl group, further confirming the trend that intersegmental
d-spacing increases with increasing side-group bulkiness [
28,
30].
3.1.2. Gas Separations
First, the membranes were tested for CO
2/CH
4 and CO
2/N
2 separations using a mixed-gas feed (
Figure 7) to investigate the influence of the exocyclic vinyl group. The permeability quadrupled for both gas pairs with increasing VNB content, while α
CO2/CH4 declined slightly and α
CO2/N2 remained largely constant, even though trends hardly exceed the experimental error. Single-gas permeabilities were collected as well to easily compare them with other published data (
Table 4). Unfortunately, the membrane pNB-VNB-25 had become defective and was left out of the analysis. For the permeabilities, a trend is visible confirming the previous mixed-gas permeabilities: By increasing the VNB content, the permeability increases. pVNB even achieved a CO
2 permeability of 104 Barrer. This is also in line with the WAXS trend which shows that a larger VNB content resulted in larger
d-spacings. The permeabilities obtained in this work for pNB are close to those previously published [
15]. The permeabilities of pVNB are close to the one of the polynorbornenes with a methyl side group [
28]. The ratio of two single-gas permeabilities gives the ideal selectivity of that gas pair. The ideal selectivity differs from mixed-gas selectivity in the sense that it allows for a more direct comparison with other single gas measurements and the resulting ideal selectivities, but it does not take into account any interactions between the mixture components as is the case with mixed-gas selectivity. These ideal selectivities show a slight decrease with increase in VNB content (
Table 5). In the case of H
2/CO
2, this even results in a reverse selectivity for pVNB.