Figure 1. PXRD, N2 adsorption and desorption, and SEM image of Ni-ZIF-8 (a), (b), and (c), while (d), (e), and (f) showing the WXRD, FTIR, and TGA analysis of Ni-ZIF-8 MMMs.
The DSC test will observe the effect of ZIFs on polymer melting and glass transition temperatures, as well as help us to estimate the polymer-MOF interfacial interaction46. Table S3 representing the T g (glass transition temperature), T m (melting temperature), andT c (crystallization temperature) of pure PDMS and MMMs. With the increase in Ni-ZIF-8 loading, the DSC results showed the increasing trends in the crystallization and melting temperatures of MMMs as compared with the pure PDMS. The changes in melting and crystallization temperatures were attributed to the reduction of chain mobility of PDMS due to the addition of Ni-ZIF-8 nanoparticles47. The PDMS chain mobility reduction may cause the rigid packings of polymer chains, which has shown excellent interfacial interaction between Ni-ZIF-8 and PDMS phases.
SEM images were used at low and higher magnification to check the surface morphologies of the synthesized membranes, as shown in Figure 2. The PDMS control membrane surface was defect-free, dense, and homogeneous (Figure 2 a and b). In comparison, Ni-ZIF-8 nanoparticles observed on the membrane surface, which were well dispersed within the PDMS matrix, implying their good compatibility. The incorporation of Ni-ZIF-8 nanoparticles has resulted in some surface roughness of the MMMs and has increased with respect to the loadings of Ni-ZIF-8. The membrane showed good homogeneity even at high loading up to 15 wt% of Ni-ZIF-8 (Figure 2 (c) and (d)). It was observed that the nanoparticles of Ni-ZIF-8 started to agglomerate marginally at a loading of 20 wt% (Figure S6 and S7), which may be due to the stronger surface tension of ZIF particles than to the polymer affinity. The overall well scattered ZIFs in the polymer is 15 wt% loadings. At lower magnification, the nanoparticles of Ni-ZIF-8 tend to be aggregated, but at a higher magnification, they are very well dispersed from each other, and no defects were observed in the MMMs. Cross-sectional morphologies of the 15% Ni-ZIF-8 MMM have been shown in Figure 2 (e and f) with low and high magnifications, respectively. There was no sign of voids observed between the membrane active layer and the PVDF support. The average thickness of the 15% Ni-ZIF-8 MMM active layer was 13 µm. At higher magnification, Ni-ZIF-8 nanoparticles can be seen in the active layer of the membrane within the PDMS.