Membrane and Desalination Technologies - TCE Moodle Website

Preparation of Polymeric Membranes 87
Fig. 2.32 Velocity distribution of nascent hollow fiber membranes verse distance at the air gap.
I. relative low extension viscosity
II. relative high extension viscosity
III. solidification (high extension viscosity)
At region I, the nascent hollow fiber membranes are just spun from the exit of the
spinneret, they have the same rheological characteristics as the dope solution inside the
spinneret. When the nascent hollow fiber membranes goes into region II, the elongation
viscosity increases greatly with the time because of the solvent exchange, temperature change
and the phase inversion induced by the exchange of solvent and nonsolvent between the
nascent membrane and the bore fluid. Due to the high elongation viscosity, the elongation rate
will decrease, which causes a lower slope of spinline in region II. When the vitrifiction or
solidification occurs in region III, the polymer structure is solidified and the velocity can be
kept at the drawing rate.
For different polymer solutions, bore fluids and spinning conditions, the different spinline
velocities can be achieved at the same draw ratio. When the instantaneous demixing occurs
inside the air gap, three distinct regions exist in the air gap and the drawing speed is mainly
accelerated in the beginning part of the air gap. For the delay demixing in the air gap, the
region I and II may describe its spinline velocity in the air gap, whereas the region II may be
extended into the coagulation bath. When the nascent polymer cannot be solidified because of
the poor coagulant or short air gap, the region III will not exist and the region II is extended
into the coagulation below the water surface and the final velocity is attained under the water
surface (96).