Atomic force microscope image of outer side of hollow fiber membrane. Pressure applied on polymer solution: 6.9 kPa gauge; air gap: 80 cm; bore fluid flow rate: 0.4 ml/min; temperature: 20–22 ◦ C; coagulation bath: distilled water. Arrow shows the direction of bore 

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... 5. Atomic force microscope image of inner side of hollow fiber membrane (hollow fiber spinning conditions, same as Fig. 3; arrow shows the direction of bore liquid flow).  ...

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... fiber spinning conditions and the corre- sponding hollow fiber dimensions are summarized in Table 1. It is clear from the table that both outer and inner diameters of the hollow fibers decreased with an increase in the bore fluid flow rate. The hollow fiber wall thickness decreased also with an increase in bore liquid flow rate. This is understandable since the polymer solution velocity increased with an increase in the bore fluid flow rate. The thickness of the polymer solution should decrease to maintain a constant flow rate. These results agree with earlier experimental data [1]. However, the above results are opposite to the observation of Chung et al. [11]. This could be because the polymer solution and the spinning conditions used in present experiments are different from those used by Chung et al. [11]. It should be noted that Chung et al.’s work is different from the present work at least in the following aspects. 1. The polymer solution and coagulation media system are different. 2. Their work is wet-spinning without air gap. 3. The polymer solution flow rate was changed to- gether with the bore liquid flow rate. Thus, it seems that the spinning conditions are very important factors for hollow fiber spinning. Even a small change in spinning condition, may have a strong effect on hollow fiber membrane. Fig. 2 shows the AFM image of the outer surface of the hollow fiber when the bore fluid flow rate was 0.1 ml/min. Fig. 3 is a similar image when the bore fluid flow rate was 0.4 ml/min. In these figures, nodules and nodule aggregates are observed. These two figures further indicate the following. 1. The nodule and nodule aggregates are elongated to the direction of bore fluid flow. The elongation is more pronounced when the bore fluid flow is low. 2. The outer surfaces of the hollow fibers are very porous and pores are elongated to the direction of the bore fluid flow. 3. The pore size becomes smaller as the bore fluid flow rate increases from 0.1 to 0.4 ml/min. The average pore size calculated from the AFM image was 218.4 and 93.4 nm, respectively, correspond- ing to the bore fluid flow rate of 0.1 and 0.4 ml/min. It should be noted that this observation agrees with that of Chung et al. [11]. However, their fabrication method was different than the present one. Similar AFM images of the outer surfaces were observed for the hollow fibers that were spun at the bore fluid rates of 0.2 and 0.3 ml/min. The mean roughness parameters, R a , of the outer surfaces scanned at 1 ␮ m range are given in Table 2. It seems that the roughness parameter decreased with an increase in the bore fluid flow rate. The degree of elongation of the nodules and nodule aggregates depends on the time required for hollow fibers to traverse the distance from the spinneret to the gelation bath, which is called air-gap. The longer the time, the more is the elongation. When the bore fluid flow rate is increased the time becomes shorter and the degree of elongation decreases. Figs. 4 and 5 show the AFM images of the inner surfaces of the hollow fibers when the bore fluid rate was 0.1 and 0.4 ml/min, respectively. Both figures show clearly nodules and nodule aggregates. These figures also indicate the following. 1. Nodule aggregates are aligned to the direction of bore fluid flow, forming small rows of nodule aggregates. The average size of nodule aggregates is 63 nm and the average length of row is 224.1 nm for 0.1 ml/min bore fluid rate. The average ...