Highly selective polymer electrolyte membranes consisting of poly(2-ethyl-2-oxazoline) and Cu(NO3)2 for SF6 separation

Polymer electrolyte membranes consisting of Cu(NO3)2 and poly(2-ethyl-2-oxazoline) (POZ) were prepared for SF6/N2 separation. It was anticipated that repulsive forces would be operative between the negative charge of water and the F atoms of SF6 when Cu(NO3)2 in the composite was solvated by water, and that the barrier effect of Cu2+ ions would be activated. In fact, Cu(NO3)2 solvated by water in the POZ membrane was observed to have more higher-order ionic aggregates than free ions or ion pairs, as confirmed by FT-Raman spectroscopy. Thus, when Cu(NO3)2 solvated by water was incorporated into the POZ matrix, the N2/SF6 selectivity increased to 28.0 with a N2 permeance of 11.2 GPU at a POZ/Cu(NO3)2 mole ratio of 1:0.7. The coordinative interaction of Cu(NO3)2 with the carbonyl group in POZ was confirmed by FT-IR spectroscopy and TGA, and the film thickness of the membrane was determined from SEM analysis.


Results and Discussion
FT-IR. FT-IR measurements were performed on a Varian FTS3100 spectrometer; 64-200 scans were averaged at a resolution of 4 cm −1 . Thermogravimetric analysis (TGA) was performed with Mettler Toledo TGA devices at a heating rate of 10 °C/min. FT-Raman measurements were performed for Cu(NO 3 ) 2 and POZ/Cu(NO 3 ) 2 solutions at room temperature using a Horiba Jobin Yvon LabRam Aramis with a diode laser beam at an excitation wavelength of 785 nm.
The aggregates of copper nitrate were dispersed in the polymer chain. The ion aggregates were solvated by the partial positive charge of water. Therefore, the partial negative charge of water was localized on the exterior of the ion aggregates. Non-polar N 2 gas was transported through the polymer membrane by Fickian transport, but SF 6 gas having a high quadruple moment experienced higher repulsion with the partial negative charge of water. Furthermore, the SF 6 molecules could be blocked by activated Cu cations in the polymer chains.
To confirm the formation of the polymer electrolytes consisting of POZ and Cu(NO 3 ) 2 , the interaction between copper ions and the carbonyl groups in the amide of the polymer was investigated using FT-IR spectroscopy. Figure 2 shows that the spectrum of neat POZ has an intense free C= O peak at 1607 cm −1 . When Cu(NO 3 ) 2 was incorporated into POZ, the shift of the free C= O peak at 1607 cm −1 was negligible. Thus, the copper ion appeared not to interact with the carbonyl group. Furthermore, it was known that the stretching frequency of free NO 3 − occurs at 1336 cm −1 . When Cu(NO 3 ) 2 was incorporated into POZ, the intensity of the peak at 1392 cm −1 , which is the aggregated NO 3 − , diminished and the peak of free NO 3 − at 1336 cm −1 became dominant. This observation indicated that the ionic bonding of nitrate ions with the copper ions became weakened by interaction with the hydrogen bonds formed with water, resulting in the observed decrease of the stretching frequency. Thermal analysis. Thermogravimetric analysis (TGA) was carried out to confirm the thermal stability of the polymer electrolyte using a Universal V4.5A (TA Instruments) apparatus. Figure 3 shows the TGA data for  neat POZ and POZ/Cu(NO 3 ) 2 (1:0.7 mol ratio), demonstrating 78% weight loss for neat POZ between 0 °C and 100 °C, attributed to evaporation of water. On the other hand, when copper nitrate was incorporated into the POZ polymer, water evaporation progressed at a higher temperature of above 100 °C, possibly due to solvation of copper nitrate by water. Therefore, the increase in the decomposition temperature was attributed to the strong interaction of copper with water. The boiling point of copper nitrate is known to be 170 °C. Thus, the loss of about 20% weight at about 170 °C was thought to be due to copper nitrate. Also, weight loss was apparent for all samples at around 400 °C due to degradation of the POZ polymer. It is deduced from this result that copper nitrate did not interact with the POZ polymer. Copper nitrate was solvated by water and thus the decomposition temperature of copper nitrate and water increased.
FT-Raman spectroscopy. To investigate the ionic constituents (i.e. free ions, ion pairs, and higher-order aggregates) of NO 3 − in neat Cu(NO 3 ) 2 and POZ with Cu(NO 3 ) 2 , FT-Raman spectra were acquired as shown in Fig. 4. The ionic constituents of NO 3 − are known to present stretching bands at 1034, 1040, and 1045 cm −1 , assigned to free ions, ion pairs, and ion aggregates, respectively 20 . When copper nitrate was incorporated into the POZ polymer, the wavenumber of the NO 3 − species increased negligibly from 1045.5 to 1047.5 cm −1 as shown in Fig. 4(a), indicating that Cu(NO 3 ) 2 incorporated into POZ existed mostly as ion aggregates. In the case of Cu(NO 3 ) 2 incorporated into POZ, the relative percentage of free ions, ion pairs, and aggregates is shown in Fig. 4(b). New peaks were observed at 1029 and 1034 cm −1 . These peaks were attributed to free ions and ion pairs. The peak areas for each configuration were measured to be 14.829, 15.664, and 69.501% as shown Fig. 4(b). Based on these results, Cu(NO 3 ) 2 incorporated into the POZ polymer formed more higher-order ion aggregates than free ions or ion pairs and the ion aggregates were dispersed in the membrane chain.  Separation performance. After the electrolyte membrane was coated with the control coater, the membrane was applied as the SF 6 separation membrane. Figure 6 showed the single gas permeance and selectivity of N 2 /SF 6 through the copper nitrate ion aggregate membrane. The N 2 gas permeance at each mol ratio (1:0.5 to 1:0.7) was about 4.2 and 11.2 GPU while the SF 6 gas permeance was about 0.7 and 0.4 GPU. 1:0.7 mol ratio of POZ/Cu(NO 3 ) 2 gave rise to the highest N 2 gas permeance with the lowest SF 6 gas permeance. In addition, after 2 d, the SF 6 gas permeance decreased to about 0.2 GPU. Therefore, the selectivity of the POZ/Cu(NO 3 ) 2 (1:0.7 mol ratio) membranes was the best at 28.0. Thus, because Cu(NO 3 ) 2 was solvated by water, the repulsive force between the negative charge of water and the F atoms of SF 6 increased, and the barrier effect of activated Cu 2+ ions was also enhanced as shown in Fig. 7.
This study presented a simple method for the preparation of an electrolyte membrane containing Cu(NO 3 ) 2 aggregates as a barrier material. The membrane was simply fabricated by adding Cu(NO 3 ) 2 to the POZ solution followed by preparation of the composite by using a control coater. The N 2 permeance achieved with a 1:0.5 mol ratio of POZ/Cu(NO 3 ) 2 was 4.2 GPU, which increased to 11.204 GPU in the case of a 1:0.7 mol ratio of POZ/ Cu(NO 3 ) 2 . On the other hand, the SF 6 permeance decreased at a 1:0.7 mol ratio of POZ/Cu(NO 3 ) 2 . Therefore, the highest selectivity of 28.0 was achieved with a 1:0.7 mol ratio of POZ/Cu(NO 3 ) 2 . Fabrication process. A polymer solution was prepared by dissolving POZ in water (20 wt%). To prepare the polymer electrolyte solution, the amount of Cu(NO 3 ) 2 ·xH 2 O added to the polymer matrix was determined according to the mole ratio of the copper ion to the monomeric unit of POZ. This solution was stirred for one day at room temperature. This polymer electrolyte solution was coated onto polysulfone macroporous membrane supports (Toray Inc., Japan) using an RK Control Coater (Model 101, Control Coater RK Print-Coat Instruments Ltd., UK). To contain the water moistly, the polymer electrolyte membranes were dried for 10 minutes at room temperature. The gas flow rates were measured using a bubble flow meter. The unit of gas permeance is GPU, where 1 GPU = 1 × 10 −6 cm 3 (STP)/(cm 2 ·s·cmHg).  Characterization FT-IR measurements were performed on a Varian FTS3100 spectrometer; 64-200 scans were averaged at a resolution of 4 cm −1 . Thermogravimetric analysis (TGA) was performed with Mettler Toledo TGA devices at a heating rate of 10 °C/min. FT-Raman measurements were performed for Cu(NO 3 ) 2 and POZ/Cu(NO 3 ) 2 solutions at room temperature using a Horiba Jobin Yvon LabRam Aramis with a diode laser beam at an excitation wavelength of 785 nm.