NAMS 2002 Workshop - ICOM 2008

Asymmetric Polymeric Membrane Formation – 6
Tuesday July 15, 11:30 AM-12:00 PM, Wai’anae
Phase Separation Microfabrication
M. Bikel (Speaker), Membrane Technology Group - University of Twente, The Netherlands
R. Lammertink, Membrane Technology Group - University of Twente, The Netherlands,
r.g.h.lammertink@utwente.nl
M. Wessling, Membrane Technology Group - University of Twente, The Netherlands
Phase Separation Micro Fabrication (PSuF) entails the phase separation of
polymer solutions that are cast onto structured supports which serve as molds. In
this way, microstructured membranes can be obtained without the use of
cleanroom technology. The replication of the pattern on the mold makes these
membranes asymmetric from a structural point of view, as one side is flat and the
other one is structured. The structural asymmetry can be obtained independently
from the morphological one. Here, we focus on the effects of this pattern and
other variables on the final structural and morphological asymmetry of
membranes obtained from a PES/PVP/NMP/water system.
Many kinds of features can be replicated down to the micron range, even with
high aspect ratios. These features can be indentations into the membrane
surface as well as protrusions of polymeric material emerging from the patterned
surface. With small adaptations, this process is extendable to the structuring of
hollow fibers, whether be it for structuring the outside of the fiber, its lumen or
both.
By means of introducing different features on the molds, the phase separation
mechanism could be studied and changed. The presence of a small amount of
features allowed us to study the process by observing the effect of the final
morphology on the replication of the features. Several types of distortion were
observed when the polymeric matrix shrunk away from the mold walls. Generally,
indentations on the membranes were larger than the features and polymeric
protrusions, smaller.
An increase in the number or total area of the features decreased the
deformation of the replicas to a minimum. In this case, the pattern has an effect
on the morphology. This has to do with the lack of access to the space between
the mold and the polymer solution, implying that less non-solvent comes directly
in contact with the features. Instead, the whole phase separation takes place
through the diffusion of the non-solvent through the polymer solution, yielding
membranes with a different morphology. To further analyze this, we used a third
mold with deeper features. In this case, local thinning of the film above the
features was observed. This can be related to pulling forces during shrinkage.