Fig. 3 Characterization of MMCM-A (20 wt% MOF loading). Morphologies of MMCM-A at spinning speed of 1000 rpm: (a, b) cross-sectional images, (c) surface image. Morphologies of MMCM-A at spinning speed of 2000 rpm: (d, e) cross-sectional images, (f) surface image. (Scale bar=500 nm)
With regard to MMCM-B, the MOF-protruding structure could also be observed. The cross-sectional images (Fig. 4a-b) show that the selective layer is only ~140 nm thick, and NH2-ZIF-8 crystals could nearly penetrate the polymer matrix. Moreover, we notice that facets of NH2-ZIF-8 crystals appearing on the thin film and no defects or pinholes were generated (Fig.4c). The cross-sectional images of the MMCM-B with a thinner selective layer is shown in Fig 4d-e. The selective layer is ~80 nm thick, and NH2-ZIF-8 crystals could nearly penetrate the selective layer, while defects and pinholes appear on the membrane surface (Fig. 4f), which are detrimental to the separation performance.
The FT-IR spectra clearly demonstrate the interfacial interactions in MMCMs. As shown in Fig. S4, aromatic imide structure exists in polyimide at 1780 and 1720 cm-1, indicating the asymmetric and symmetric stretching vibration of C=O, respectively. The peak corresponding to the stretching vibration of C-N in imide rings appears at 1357 cm-1, and the peak corresponding to the bending vibration of C=O appears at 720 cm-1. Compared with pure 6FDA-DAM, the imide characteristic peak of the MMCM-A remains after the incorporation of NH2-ZIF-8 crystals, confirming the existence of the chemical structure of polyimide. Moreover, the C=O characteristic peak of the MMCM-A exhibits a slight blue shift, confirming the hydrogen-bonding interaction between the amine groups of MOF particles and polyimide chains.
Fig. S5 shows the infrared spectra of PI-Br and MMCM-B with different MOF loading. The characteristic peak of imide ring retains in the membranes, indicating the existence of polyimide structure. Meanwhile, the peak corresponding to the stretching vibration of C-Br appears at 650 cm-1, confirming the successful functionalization of bromine on 6FDA-DAM chains. In contrast with the infrared absorption peak of MOF crystals, the peaks of the amine weaken after crosslinking, while the band strength of the aryl-N increases, confirming the covalent-bonding exist between PI-Br and NH2-ZIF-8.
Overall, through optimizing the fabrication parameters, we prepared pinhole-free MMCM-A and MMCM-B wherein the membrane thickness is ~280 and ~140 nm, respectively. Because the interfacial covalent bonds in MMCM-B are much stronger than the hydrogen bonds in MMCM-A, wherein the stronger interfacial bonds benefit the formation of compatible interface structure, MMCM-B membrane materials could be manufactured into much thinner film.