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.