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.