4. Results and discussion
The films from 0:5 (v/v) acetone/MEK solutions were transparent with a smooth surface. When the volume of acetone increased, the film surface appeared rougher. The membranes spin from 3:2 (v/v) acetone/MEK solutions were opaque and white. Fig. 3 shows the typical SEM photographs of PVDF films obtained from different acetone/MEK mixture ratio solutions. The addition of acetone into a PVDF solution has great effect on the film morphology. The size and density of the pores increased as the content of acetone increased. In Fig. 3a, the film is relatively smooth and flat with pores below 1 m diameter. When 20% v/v acetone was added into the solution, small pores with diameter were generated as showed in Fig. 3b. The average pore diameter in Fig. 3c is 1.6 m. In Fig. 3d, the average diameter of pores became 2.4 m. These SEM data suggests that the membrane mean pore size increased in proportion to the increase in the amount of acetone presence in the polymer solution. When the acetone volume ratio was over 60%, the solution evaporation rate becomes too high for smoothly spin coating. Table 1 shows the addition of acetone results in the increase of PVDF-TrFE film porosity. The porosity of film increased dramatically from 48% to 73% when the volume ratio of acetone changed from 20% to 40%. This trend agrees well with the change of pore configuration observed from SEM images.