The SEM images in the upper panel of Figure S-1a exhibit the dense side
of the three PAN substrates. It can be observed that there exist
nano-sized pores on the surface of both the PAN-O and PAN-H substrates,
though the sizes of the pores on the surface of the PAN-H substrate are
substantially smaller than those on the surface of the PAN-O substrate.
On the other hand, there are no discernable pores in the SEM image for
the surface of the PAN-A substrate. All these observations indicate that
the PAN polymer network was shrunk owing to the stress relaxation of PAN
polymer in the water bath at an elevated temperature of
45ºC31; the swelling of PAN polymer network resulting
from the alkaline treatment further reduced the pore
size39. The cross-sectional SEM images shown in the
lower panel of Figure S-1a confirms that the heat and alkaline
treatments could have marginal effects on the figure-like macrovoids
beneath the dense skin.
The representative AFM images in Figure S-1b indicate that all the PAN
substrates have similar values of the roughness in terms of the root
mean square (RMS); the RMS roughness values for the PAN-O and PAN-H
substrates are nearly the same (approximately 2.4±0.2 nm), while that of
the PAN-A substrate is increased to 3.2±0.2 nm. This supports the
assumption that the geometrical changes resulting from the
post-treatments could be ignored when analyzing the effects of the
substrate on the IP process.
The water permeabilities of all the PAN substrates were evaluated in
terms of the flux-transmembrane pressure (TMP) relationship and the
evaluation results are compared in Figure 2. It is evident that both the
heat and alkaline treatments substantially increased the hydraulic
resistance of the substrate. In particularly, the hydrolysis in the NaOH
solution resulted in an even lower value of the water permeability
(82.2±3.9 LMH/bar) when compared with that for the PAN-H substrate
(113.0±2.6 LMH/bar). These observations are consistent with the changes
in the surface pore sizes on the dense side as revealed by the SEM
images in Figure S-1a.