Effect of functional group ratio in PEBAX copolymer on propylene/propane separation for facilitated olefin transport membranes

PEBAX-5513/AgBF4/Al(NO3)3 membranes were fabricated for mixed olefin/paraffin separation. In order to improve the selectivity of the membranes utilizing PEBAX-1657, PEBAX-5513, which increased the ratio of amide groups from 40% to 60% in the copolymer, was used. The selectivity and permeance of the membranes were 7.7 and 11.1 GPU, respectively. Furthermore, the PEBAX–5513/AgBF4/Al(NO3)3 membranes had long-term stability because of Al(NO3) to have the stabilizing effect on Ag+ ions acting as an olefin carrier. Unexpectedly, the performance of the membrane selectivity was not improved, and the permeance became rather lower. Generally, when Ag+ ions was added to the polymer containing amide groups, the selectivity increased with the content of the amide groups. However, Al(NO3)3 was added for the stability of Ag+ ions and there was no increase in selectivity. Since the ratio of amide was high, Ag+ ions were favorably in coordination with the oxygen of the carbonyl group, but the NO3− ions in Al(NO3)3 had the enhanced interaction with Ag+ ions as obstacles for olefin complexation. Therefore, the composition ratio of amide/ether in the polymer matrix was negligible for olefin separation.

Poly(ether-block-amide) resin, a typical thermoplastic elastomer, was widely known as PEBAX. PEBAX was consisted of a linear chain combination of polyamide, a hard segment responsible for mechanical strength, and polyehter, a soft segment responsible for high permeability due to high chain mobility of the ether linkage 17 .
In case of PEBAX-1657, 60% of the permeable ether group and 40% of the hard amide group were able to obtain a high permeance, but the selectivity tended to be relatively low 6 .
In this study, to increase the selectivity, PEBAX-5513, which increased the ratio of the amide groups capable of showing high selectivity, was selected as the polymer matrix.

Results and Discussion
SEM images analysis. SEM images were obtained to confirm that the selective layer was uniformly distributed. Figure 1 shows the cross-section of the selective layer of PEBAX-5513/AgBF 4 /Al(NO 3 ) 3 coated on the porous polysulfone. The image shows that the selective layer of PEBAX-5513/AgBF 4 /Al(NO 3 ) 3 was uniformly coated and the thickness was 3.2 µm.
Separation performance. Figure 2 shows the permeance and selectivity of the PEBAX-5513/AgBF 4 / Al(NO 3 ) 3 membrane for a propylene/propane mixture. The permeance and selectivity of the membrane were about 11.1 GPU and 7.7, respectively. Furthermore, Ag + ions could act as olefin carriers, owing to Al(NO 3 ) 3 , a reduction inhibitor. Thus, the long-term stability of the membrane was confirmed over 100 h. In another earlier study, a high selectivity was observed due to the coordination of the carbonyl oxygens of the amide group with the Ag + ions [18][19][20][21][22] . However, similar results were not observed in this study. The reason for these phenomena is discussed in the next section.
FT-IR analysis. Figure 3 shows the FT-IR spectrum of the neat PEBAX-5513, PEBAX-5513/AgBF 4 , and PEBAX-5513/AgBF 4 /Al(NO 3 ) 3 . For the neat PEBAX-5513 spectrum, the peak of C=O bond was observed at 1635 cm −1 , but the peak moved to 1612 cm −1 when AgBF 4 was added. This was because the intensity of the C=O bond peak was weakened as the Ag + ions of AgBF 4 react with the oxygen atom of the C=O bond in the amide  www.nature.com/scientificreports www.nature.com/scientificreports/ group of the polymer. Thereafter, when Al(NO 3 ) 3 was added to PEBAX-5513/AgBF 4 , the peak of C=O bond shifted to 1623 cm −1 . This was due to the NO 3 − ions interacting with the Ag + ions bound to the oxygen atom of the C=O. Thus, the intensity of the C=O bond peak was restored, and the C=O bond peak shifted to a higher. For PEBAX-1657, when AgBF 4 was added, the wavenumber of the C=O bond peak slightly decreased from 1637 to 1620 cm −1 6 ; however, for PEBAX-5513, it decreased significantly from 1635 to 1612 cm −1 . This was because PEBAX-5513 had a higher proportion of amide groups than that of PEBAX-1657, since Ag + more favorably coordinates to the amide group than the ether group. However, when Al(NO 3 ) 3 was added, the recovered wavenumber of the C=O bond peak was similar for both PEBAX-5513 and PEBAX-1657. Hence, it was thought that the degree of coordination of the NO 3 − ions of Al(NO 3 ) 3 with Ag + ions was similar.

TGA analysis.
To investigate the thermal stability of neat PEBAX-5513, PEBAX-5513/AgBF 4 , and PEBAX-5513/AgBF 4 /Al(NO 3 ) 3 , TGA analysis was performed from 0 to 600 °C as shown in Fig. 4. In PEBAX-5513/AgBF 4 , although the Ag + ions formed a bridge between the polymer chains, the Ag + ions coordinated to the amide group weakened the existing intermolecular attractions in the polymer. The enhanced free volume of the polymer caused plasticization. As a result, the thermal stability seemed to decrease. However, when Al(NO 3 ) 3 was added, the results were different from those for PEBAX-1657. The thermal stability of PEBAX-1657 slightly increased 6 , since the NO 3 − ions of Al(NO 3 ) 3 formed a new bond with the Ag + ions coordinated to the C=O bond, weakening the bonds between oxygen and Ag + ions. On the contrary, when Al(NO 3 ) 3 was added to PEBAX-5513/AgBF 4 , even the crosslinking formed between the Ag + ions and polymer chain collapsed, and the thermal stability was expected to be lower. Figures 5 and 6 showed the Raman graph used to confirm the state of the BF 4 − and NO 3 − ions. Figure 5 indicated the state of BF 4 − ions, which meant free ions, ion pairs, and ion aggregates at 765, 770, and 774 cm −1 , respectively. In the case of PEBAX-1657/AgBF 4 , BF 4 − ions exist mainly in the form of free ions, but also in the form of ion pairs and ion aggregates 6 . On the other hand, in PEBAX-5513/AgBF 4, it was observed that  www.nature.com/scientificreports www.nature.com/scientificreports/ BF 4 − ions exists only as a free ion. This was due to the abundant amide groups in PEBAX-5513; therefore, the Ag + ion could be strongly coordinated, being free of BF 4 − . In the case of PEBAX-1657/AgBF 4 /Al(NO 3 ) 3 , the interaction between the Al 3+ ions and BF 4 − ions were generated. Then, BF 4 − ions became the ion pairs state with Al 3+ ions 6 . However, in PEBAX-5513/AgBF 4 /Al(NO 3 ) 3 , the Al 3+ ions was bound to the abundant amide; hence, BF 4 − ions was expected to be able to maintain a free state. Figure 6 showed the state of the NO 3 − ions, which indicated free ions, ion pairs, and ion aggregates, corresponding to 1034, 1040, and 1045 cm −1 , respectively 23 . In PEBAX-5513/AgBF 4 /Al(NO 3 ) 3 , the NO 3 − ions react with Ag + ions to form ion aggregates, corresponding to 1046 cm −1 .

Conclusions
The polymer matrix of PEBAX-1657/AgBF 4 /Al(NO 3 ) 3 used in an earlier study was changed to PEBAX-5513, with a higher amide group ratio in PEBAX, to improve the selectivity as shown in Fig. 7. The membrane permeance decreased because the proportion of the ether group, the permeable segment in PEBAX, decreased. However, there was no increase in selectivity. Since the ratio of amide was high, Ag + ions were favorably in coordination with the oxygen of the carbonyl group, but the NO 3 − ions had the enhanced interaction with Ag + ions as obstacles for olefin complexation. As a result, the effect of the ratio of amide/ether in the polymer matrix could be negligible for Ag + ions to act as olefin carriers for polymer/Ag salt/Al(NO 3 ) 3 complex membranes. Therefore, these results should be considered for the future design of block-copolymer syntheses to obtain high-selectivity/ high-permeability membranes.   Preparation of membranes. PEBAX-5513 was dissolved in a mixed solvent of distilled water and ethanol at 90 °C to prepare a solution having a concentration of 3 wt%. The solution was allowed to cool at room temperature, and then AgBF 4 was added as an olefin carrier so as to have a concentration of 17 wt% of the total solution. Al(NO 3 ) 3 was added as a reduction inhibitor of Ag + ions so as to have a 0.1 mol ratio with respect to AgBF 4 . The solution was then stirred for 1 hour at room temperature and coated on the polysulfone support (Toray chemical, Inc., Korea) using an RK Control Coater (Model 202, Control Coater RK Print-Coat Instruments Ltd., UK). The solution-coated complex was dried in vacuum oven for 24 hours at room temperature.
Gas separation experiments. The performance of PEBAX-5513/AgBF 4 /Al(NO 3 ) 3 membranes was tested using a propane/propylene mixed gas(50:50 vol%). The permeance of the membranes was obtained using a bubble flow meter and the selectivity was obtained using gas chromatography(GC). The flow of the mixed gas was controlled by a mass flow controller (MFC), and the gas permeance was expressed as GPU (1 GPU is 1 × 10 −6 cm 3 (STP)/(cm 2 s cmHg)).

Characterization
The prepared membranes were characterized by scanning electron microscopy (SEM; JSM-5600LV, JEOL). A VERTEX 70 FT-IR spectrometer (Bruker Optics Inc.) was used to investigate the chemical change in the membranes. The thermal stability of the membranes was measured by thermogravimetric analysis (TGA; Universal V4.5 A, TA Instruments). Raman spectra of the PEBAX-5513/AgBF 4 /Al(NO 3 ) 3 complex membrane were collected at room temperature using a Bruker Optics Ram II Raman module with a resolution of 4 cm −1 .