Vapor induced phase separation for super-hydrophobic polymeric membrane preparation
Résumé
Introduction
Membrane distillation (MD) is a promising process for water desalination, which is not limited by
osmotic pressure - contrary to reverse osmosis - and hence by salt concentrations. In MD, pure vapor
water permeates through the membrane thanks to a transmembrane water partial pressure difference
while liquid salty water remains on the membrane feed side. MD process requires thus membranes
with high porosity, micrometer pore size, low thickness to maximize permeate fluxeswhereas it
requires membranes with high hydrophobicity (or even better super-hydrophobicity) and important
liquid entry pressure in order to prevent wetting of the membrane during MD operation. However, there
is still today a lack of specific polymeric membranes designed for MD.
The aim of this work was to develop macro-porous highly hydrophobic membranes specifically
prepared for MD by Non-solvent Induced Phase Separation (NIPS).
Material and Methods
20%wt. PVDF was initially dissolved in solvent N,N-dimethylacetamid (DMAc). This dope solution was
then casted on a glass support and pure water was used as non-solvent in two successive NIPS
steps. First, the casted dope solution was exposed to water vapor at fixed temperature and humidity
(Vapor Induced Phase Separation step or VIPS step) during a fixed duration. Casted doped solution
was then immersed into liquid water bath during 4 hours at ambient temperature (Liquid Induced
Phase Separation step or LIPS step). Influence of the VIPS step parameters (duration, temperature
and humidity) was investigated and the obtained membranes were thoroughly characterized by
scanning electron spectroscopy observations, liquid drop contact angle, liquidentry pressure (LEP),
porosity and roughness measurements.
Results
Succession of VIPS and LIPS steps allowed successfully obtaining macro-porous membranes with
high porosity (70 – 80%). Figure 1a presents the influence of the VIPS duration (at 20°C and 99% RH)
on (i) the liquid drop contact angle and (ii) the Liquid Entry Pressure. Duration of the VIPS step is a
crucial parameter in the control of the morphology and properties of final membranes. For high VIPS
durations (more than 2 minutes), VIPS step is predominant and controls the membrane morphology:
an interconnected nodule structure is obtained. In such case, membrane surface
had a near super-hydrophobicity (with liquid drop contact angle around 125°C) while the LEP is very
low. These two properties are directly related to the nodule structure of the membrane surface. On the
contrary, for shorter VIPS durations (less than 1 minute), influence of the VIPS step is weak since the
water intake is too low during this first VIPS step. In this case, the membrane morphologies were very
similar to those of membranes obtained by LIPS step only :a three-layer
morphology with a very thin dense layer, a finger-like macro-void layer and a macro-porous nodule
sublayer. These membranes exhibited higher LEP thanks to the thin dense upper layer while
membrane surface was more hydrophilic. LIPS step seems dominant and influence of the VIPS step
may be neglected.
For VIPS duration between 1 and 2 minutes, both VIPS and LIPS affect the membrane morphology
simultaneously. When the VIPS duration was increased from 1 to 2 minutes, typical LIPS process
finger-like macro-void layer was reducing while the typical VIPSprocess nodule structure was
increasing. In relation to the change of structure, the liquid drop contact angle increased and the LEP
decreased when the VIPS duration was increasing. The time limit between the dominance range of the VIPS and LIPS steps was shown to be strongly related to the operating conditions of the VIPS step
(RH, temperature…)
Conclusion
In order to obtain both high surface hydrophobicity and important liquid entry pressure, an optimum
must be researched for intermediate VIPS duration when both VIPS and LIPS affect the final
membrane morphology. More studies of the influence of the operating parameters during the VIPS
step at intermediate VIPS durations are currently underway.