Fig 1 illustrated the schematic for the preparation of the PVA/Cu2+/PAMAM membranes (PPCm) and the process of gases separation. PAMAM has high concentrations of amine groups which can enable the membrane to increase the selectivity of CO2. Therefore, PAMAM was introduced into the PVA/Cu2+ membranes. Cu2+ and PAMAM complexed with PVA polymer chains to form the PPCm with significantly enhanced mechanical properties and separation efficiency11, 20-22. Generally, small molecular gases such as CO2, N2 and H2 permeated through the membrane by the physical solution-diffusion mechanism. However, when PAMAM was introduced into the membrane, the amine groups in PAMAM could react reversibly with CO2, so as to promote the permeation and separation of CO2.
Structure characterization
Fig 2 showed the FTIR spectra of PPCm with different PAMAM contents. For the PVA/Cu2+ base membranes without PAMAM, there was 3416cm-1 stretching vibration attributed to -OH, 2919 cm-1 stretching vibration attributed to -C-H, the stretching vibration of 1434cm-1 was H-C-H, 912cm-1 and 842 cm-1 belonged to the stretching vibration of C-O-H and C-C respectively. But for the PPCm two new absorption peaks position at 1645 cm-1 and 1565 cm-1 belonged to the stretching vibration of -C-O- and bending vibration of -N-H in PAMAM. With the increase of PAMAM, they shifted to 1655 cm-1 and 1555 cm-1. At this time, the peak position of -OH gradually moved to a lower wave number. The shift of these functional group peaks was mainly due to the hydrogen bond interaction between the -NH-CO- group in the PAMAM and the -OH group in the PVA. The strong hydrogen bonding interaction could act as physical crosslinking agents, which would enhance the mechanical performances of the matrix.
Fig 2 showed the SEM images of PPCm with different PAMAM contents. The crosssection were smooth and glossy, and no PAMAM agglomeration was observed. It can be found that the surface roughness of PPCm increased with increasing of PAMAM contents, indicating the tough fracture. Therefore, the excellent compatibility between PAMAM and PVA/Cu2+ matrix can be concluded.
DSC analysis
Fig 3 showed the DSC curves and crystallinity of PPCm with different PAMAM contents. From the endothermic curves of the PPCm, PVA crystallization peak at ca. 180℃ could be observed. With the introduction of PAMAM, the crystallization onset temperature and the crystallization peak temperature decreased with the increase of PAMAM. The crystallinity of PPCm could be calculated from the follows:
ΔH0 is the melting enthalpy of the PVA membrane at 100% crystallinity; the ΔHc is the melting enthalpy of the transfer membrane; and the f is the mass fraction of the polymer matrix.
It could be concluded that the crystallinity of PPCm was obviously higher than that of PVA/Cu2+ based membrane without PAMAM, as shown in Fig 3 (c). With the increase of PAMAM, the crystallinity of PPCm increased obviously. The results showed that the PAMAM with high degree of branching exhibited heterogeneous nucleation effect in the PVA matrix, which could increase the crystallinity.
Mechanical properties
Fig 4 illustrated tensile strength, Young's modulus and elongation at break of the PPCm with different PAMAM contents. Fig 4 (a) showed that the tensile strength of 5% PAMAM increased to the maximum. And then the tensile strength of PPCm decreased continuously with the increase of PAMAM content. PAMAM exhibited reinforcing effect in the matrix, and increased the rigidity of the membrane. As shown in Fig 4 (b), with the increase of PAMAM, the Young's modulus of PPCm increased obviously. The Young’s modulus of the transfer promoting membrane with the 30% PAMAM content increased by 132% compared with the PVA/Cu2+ base membrane without PAMAM. The fracture elongation of PPCm decreased significantly with the increase of PAMAM content, as shown in Fig 4(c). When the existence of PAMAM was too much, it destroyed the structure of the membrane, resulting in the decrease of tensile strength and elongation at break. Besides, the results of Young's modulus and tensile strength were mainly due to the increase of crystallinity of the PPCm, as well as the strong hydrogen bonding interactions. The raise of intermolecular force leaded to the promotion of Young's modulus and the decline of tensile strength.
Gas Separation Properties
Separation properties for a single gas
The gas permeation unit (GPU) of three single gases, CO2, H2 and N2, were measured, as shown in the Fig 5. The CO2 GPU of the PPCm increased with the increase of PAMAM. The gas permeation unit of CO2 was as high as 120 GPU when the PAMAM concentration was 30%. It was much greater than that of N2 and H2. Even with the increase of PAMAM concentration, the gas permeation unit slightly decreased. The gas permeation of N2 and H2 was independent of the PAMAM content. This was mainly due to the reduction of physical solution-diffusion in the process of gas permeation. However, in the course of CO2 permeation through PPCm with amine carriers, there was not only physical solution-diffusion mechanism, but also facilitated transport mechanism. This fully illustrated that the introduction of PAMAM would have an efficient effect on the separation of mixed gases.
Separation properties of the mixed gas
As shown in Fig 6, the gas permeation unit of CO2/N2 with a volume ratio of 15/85 was measured under different inlet gas pressures. The GPU of CO2 and N2 increased significantly with the increase of PAMAM contents. And the CO2 and N2 permeation unit increased at higher inlet gas pressure. When the PAMAM was 10% and the inlet gas pressure was 0.5 MPa, the CO2 permeation unit was 7.3 GPU and the N2 permeation unit was 1.6 GPU, as illustrated in Fig 6 (a, b). Separation factor was an important parameter to measure the separation ability of membranes. The separation factors of the CO2/N2 of PPCm were calculated, as shown in Fig 6(c). As the PAMAM content increasing, the separation factor of CO2/N2 increased obviously. When the PAMAM was 10% and the inlet gas pressure was 0.5 MPa, the separation factor of the PPCm was 14.
The permeation of small molecular gases such as CO2 and N2 through the membrane was the physical-diffusion mechanism. Beyond that, there was another facilitated transport mechanism for CO2 permeation through the PPCm. Therefore, the gas permeation unit of CO2 was obviously larger than that of the N2. And with the increase of PAMAM contents, the separation factor increased. Because it increased the density of effective amine groups which were capable of interacting with CO2. The CO2 and N2 permeation unit increased at higher inlet gas pressure, because of the coupling of two gases in the mixture. In general, the separation factors of PPCm increased with the increase of PAMAM.
Under different inlet gas pressures, the gas permeation unit of CO2/H2 with a volume ratio of 50/50 was illustrated in Fig 7 (a, b). The CO2 permeation unit generally decreased and then stabilized with the increase of PAMAM content, while the H2 permeation unit generally decreased with the increase of PAMAM content. The CO2/H2 separation factors of the PVA/PAMAM/Cu2+ transfer membranes were calculated, as shown in Fig 7 (c). And when the PAMAM was 20% and the inlet gas pressure was 0.5 MPa, the separation factor of the PPCm was 1.3, reaching the maximum. The separation factor of CO2/H2 increased steadily, with the increase of PAMAM contents. When the PAMAM content exceeded 20%, the separation factor of PPCm for CO2/H2 decreased.
The small size of H2 gas molecules were easier to permeate through the membrane than CO2 and N2 molecules by the dissolution-diffusion mechanism. When the PAMAM content increased, the tightness of the membrane increased, resulting in the decline of H2 permeation unit. However, CO2 mainly permeated through the PPCm by the facilitated transport mechanism. Therefore, the separation factor of the PPCm of the CO2/H2 was much smaller than that of CO2/N2. When the PAMAM content was too much, the effective amine density decreased. And it had resulted in the decline of the separation factors.