 point), the major driving force of membrane formation mechanism was NIPS process. As for MSPSF-0-25, dense skin layer and nger-like structure were formed. When the coagulation water bath temperature was 60 C (higher than the cloud point), the membrane formation process was dominated by RTIPS process. As shown in Fig. 8, a bi-continuous cross-section and homogeneous porous top surface in MSPSF-2-60 were obtained, which was the sign of high pure water ux and good mechanical property of the membrane. These observations indicated that instantaneous phase separation happened in NIPS process as well as dense skin surface and nger-like structure occurred in the membrane, while homogeneous porous top surface and sponge-like structure appeared in the membrane by RTIPS method. Fig. 9 showed the SEM morphology of at-sheet membrane with different SPSF content when coagulation water bath temperature was fastened at 60 C. As shown in Fig. 5, the cloud point of the cast solution with different SPSF content was between 47 C and 49 C, so the membrane formation mechanism was dominated by RTIPS method when the membrane phase separation temperature was 60 C. The main driving force of NIPS method and RTIPS method were mass transfer and heat transfer, respectively. Due to the main driving force of the phase separation was heat transfer instead of mass transfer, as shown in Fig. 9, the morphology of MSPSF-0-60, MSPSF-1-60, MSPSF-2-60, MSPSF-3-60 and MSPSF-4-60 were all bi-continuous crosssection and homogeneous porous top surface structure. SEM image of MSPSF-0-60-b, as presented in Fig. 9, exhibited a dense sponge-like cross-section, this phenomenon indicated a rapid exchange speed during the membrane formation procedure because of the lower viscosity. To investigate the effect of SPSF on the at-sheet membrane morphology, SEM images, as shown in Fig. 9, turned dense sponge-like cross-section into sponge-like structure with the increased SPSF content. This nding was due to the crosslinking structure between SPSF and PES chains, more heat was needed to wreck the interaction during the phase separation, which retarded the phase separation speed and then sponge-like structure emerged in the membrane. Finger-like cross section structure appeared in the bottom of MSPSF-2-60-b, this was attributed to the increased hydrophilic sulfonic group with the addition of SPSF, which accelerated the phase separation process, then leaded to the formation of nger-like structure. The number of pores in PES/ SPSF membrane top surface were more than that in PES membrane, while the size of pores were smaller than that in PES membrane. As for MSPSF-1-60, MSPSF-2-60, MSPSF-3-60 and MSPSF-4-60, the holes on the top surface rst became bigger then became smaller with the increased SPSF content. These phenomena were consistent with the pure water ux and BSA rejection rate in Fig. 12.
The membrane surface topography of PES/SPSF at-sheet membrane with different SPSF content was illustrated in Fig. 10. In the scan area range 20 mm Â 20 mm, the R a of MSPSF-0-60, MSPSF-1-60, MSPSF-2-60, MSPSF-3-60 and MSPSF-4-60 were 46.2 nm, 68.3 nm, 76.1 nm, 83.7 nm and 104.6 nm, respectively. It could be seen that the membrane surface roughness, especially the surface roughness value of the pristine PES membrane (MSPSF-0-60) was lower than the PES/SPSF  membranes. This could be explained by the presence of hydrophilic SPSF which migrated spontaneously to the interface of the at-sheet membrane and then generated rougher membrane top surface. It could also be observed that the surface roughness values of the PES/SPSF at-sheet membrane increased with the increased SPSF content. An increase in the at-sheet membrane roughness indicated an increase in effective membrane surface area causing by the ridges and valleys of nodular shapes, which is benecial in the improvement of ux. 28

Pore size and porosity
The porosity and pore size of at-sheet membrane were listed in Table 3, in which the content of SPSF and coagulation water bath temperature were varied. For membranes prepared by NIPS method (MSPSF-0-25, MSPSF-2-25, MSPSF-4-25), the porosity increased with the content of SPSF. The reason was that the pore gap and hydrophilicity increased with the addition of SPSF. With regard to the membranes by RTIPS method, the porosity rst increased and then changed a little with the increase of SPSF content in the following order: MSPSF-0-60, MSPSF-1-60, MSPSF-2-60, MSPSF-3-60, MSPSF-4-60. This phenomenon indicated that the porosity of the membrane by RTIPS was higher than NIPS method, the porosity reached the maximum and then the growth rate slowed down with the increased SPSF. As for MSPSF-2-25 (by NIPS method) and MSPSF-2-60 (by RTIPS method), the porosity of the MSPSF-2-60 was higher than MSPSF-2-25, this phenomenon was consist with the dense skin surface by NIPS method and homogeneous porous surface by RTIPS method.
The maximum pore size (r max ) and mean pore size (r m ) of at-sheet membrane prepared by RTIPS method (MSPSF-2-60) were both higher than that by NIPS method (MSPSF-2-25), this phenomenon consistent with the morphology obtained by NIPS and RTIPS method (Fig. 8). For the membranes prepared by RTIPS method, the r m and r max reached the maximum at MSPSF-2-60, this appearance was consistent with the variation of pure water ux in Fig. 12. When the mass ratio of SPSF was higher than 2 wt%, the r m and r max, especially the r max, showed a decreasing trend, this was attributed to the increased viscosity with the addition of SPSF.

Hydrophilicity
The effect of SPSF content on static pure water contact angle was displayed in Fig. 11, the static pure water contact angle was measured by testing the top surface of the PES/SPSF at-sheet membrane. The static pure water contact angle of MSPSF-0-60, MSPSF-1-60, MSPSF-2-60, MSPSF-3-60 and MSPSF-4-60 were 91.2, 77.6, 69.3, 61.9 and 62.2, respectively. The result was mainly because SPSF had sulfonic acid hydroxyl group and SPSF transferred to the top surface of the at-sheet membrane during membrane formation process. It is wellknown that the smaller the contact angle, the better the hydrophilicity of the at-sheet membrane, 29 therefore, the SPSF improved the hydrophilicity of the PES/SPSF at-sheet membrane.

Permeation performance
The inuence of SPSF and membrane formation mechanism on the permeation properties and BSA rejection rate were illustrated in Fig. 12. The pure water ux (J w ) of the at-sheet membrane, as illustrated in Fig. 12a, showed a rst increasing  and then decreasing trend with the increase of SPSF, this result matched with the mean pore size (r m ) listed in Table 4. The pure water ux of PES/SPSF membrane was much higher than that of pure PES membrane, however, the BSA retention decreased slightly, this was attributed to homogeneous porous surface and spongy like cross-section obtained by RTIPS method. The maximum pure water ux (966 L m À2 h À1 ) and BSA rejection rate (79%) were obtained from the PES/SPSF at-sheet membrane with 2 wt% SPSF.
When the coagulation water bath temperature was 25 C, the membrane formation mechanism underwent NIPS process, while the membrane formation mechanism followed RTIPS process when the coagulation water bath temperature was 60 C. The pure water ux and BSA rejection rate of the atsheet membrane, as shown in Fig. 12b and c, were both higher than that of the membrane by NIPS method.
Based on these results, it could be seen clearly that PES/SPSF at-sheet membrane by RTIPS process could obtain high pure water ux as well as BSA interception kept at a high value. RTIPS method could be applied as a new method for high pure water ux membrane preparation. Fig. 13 showed time-dependent ux during