Effect of membrane preparation method on the outer surface roughness of cellulose acetate butyrate hollow fiber membrane

Cellulose acetate butyrate (CAB) hollow fiber membrane was prepared via both thermally induced phase separation (TIPS) and nonsolvent induced phase separation (NIPS) methods. Atomic force microscope (AFM) and scanning electron microscope (SEM) were used to observe the outer surface morphology of the membranes obtained by different preparation methods (TIPS and NIPS) and various preparation conditions. A tapping mode was operated for studying the polymeric membranes when AFM was applied to image the outer surface of a hollow fiber membrane in the air. Both nodular structure and bumpy structure were observed. AFM images have revealed that both the nodular structure size and the roughness of the membrane outer surface were effected by the air gap distance, coagulation bath temperature and composition. Furthermore, the membrane preparation method (TIPS or NIPS) has profound effect on membrane surface roughness.

CAB hollow fiber membranes were prepared via both NIPS and TIPS methods. The outer surface roughness was observed by AFM. Nodular structure was obtained due to polymer precipitation and the bumpy structure was obtained by phase separation. For NIPS membrane, the roughness on the outer surface became higher with adding more solvent into the coagulation bath and the higher bath temperature. The effect of air gap distance on membrane morphology was not so pronounced. For TIPS membrane, the air gap distance had great effect on membrane morphology. The membrane obtained when air gap was 0 mm had Fig. 9. Effect of bath temperature. Air gap: 5 mm. Bath temperature: (a) 50 C; (b) 0 C.