NAMS 2002 Workshop - ICOM 2008

Absorption of water was determined both by gravimetric and by NIRS
measurements. Experiments were carried out in a static mode, i.e. without any
air circulation. The open cell was placed in a closed vessel at 40°C with an
atmosphere with fixed RH controlled by a saturated salt solution (NaCl for 75%
RH).
Fourier transform NIRS in a transmission mode was used to measure the
transfer of water within an elementary volume of polymer solution. NIR spectra
were recorded each 30 min using a Perkin Elmer Spectrum One NTS equipped
with a tungsten- halogen lamp with a quartz envelope and a deuterated triglycine
sulfate (DTGS) detector. The instrument was controlled via the software
Spectrum v3.02 from Perkin Elmer, which permits acquisition and processing of
spectra. Four scans were averaged at a 4 cm -1 resolution in the range of 2600-
10000 cm -1 . The size of the laser spot was set equal to 2.5 mm diameter, and the
analyses were performed at 7 mm under the air/solution interface. Gravimetric
measurements were used as a reference method to evaluate the NIR method.
The NIR measurements performed on different systems during VIPS exhibit the
following points:
The composition of the solution, in terms of polymer, solvent and non-solvent
local concentration has been successfully followed in- line and in-situ. Before the
phase separation takes place, whatever the system the evolution curves
representing the local concentration of water gradually increase whereas the
solvent concentration decreases and the concentration of the polymer remains
almost constant.
The concentration of the three components can be followed not only before but
also after the phase separation has occurred at the top surface of the polymer
solution, since the points of measurement are placed under the top surface. This
aspect is of special interest for studying the mass transport phenomena at
different stages of the membrane formation. Nevertheless, as soon as the
solution phase separates at the point of measurement, the NIR analysis is no
more possible.
The time needed for reaching the phase separation at the top surface of the
polymer solution can be easily determined from the experimental curves. Indeed,
demixing leads to a drastic change in the mass transport phenomena due to the
formation of a polymer precipitated surface layer. The penetration rate of nonsolvent
is also reduced resulting in a change of slope in the absorption curves,
the critical point indicating complete surface phase separation.
Using three points of measurements within the polymer solution, concentration
profiles can be plotted during time. These results could help validating numerical
mass transfer model using the VIPS process. In addition, the method allows the
apparent diffusion coefficients to be determined.