PHASE SEPARATION MECHANISMS OF SOLUTIONS OF
THERMOSENSITIVE WATER-SOLUBLE POLYMERS: A ROUTE TO GREEN
MEMBRANES
Phase separation of polymer solution, induced by an abrupt quench
in temperature, leads to a macroscopic segregation of phases. The
growth of the phases and the associated structures when time
elapses since the beginning of the quench are still badly known
for hydrosoluble polymer solutions. However phase separation
processes are widely used in manufacturing polymeric material like
gels1 or membranes
Mastering the temperature-induced morphology and porosity
needs to fully understand how phase separation proceeds and to
establish the relationship between phase separation mechanism and
processing parameters during material preparation.
For some years, we studied a neutral hydrosoluble polymer the
poly(N-isopropylacrylamide) or PNIPAM. When aqueous solutions of
this polymer are heated above a temperature of the order of 42°C,
a phase separation occurs. However the PNIPAM does not follow the
macroscopic phase separation. The phase separation stops at a
stage where it leads to porous stable PNIPAM colloids (spheres
having size of the order of 200 nm). A probable mechanism,
consistent with the colloidal size variation with temperature, is
that the phase separation is blocked at an early stage (Cahn
regime of spinodal decomposition) by adsorption of residual ions
at the grain’s surface2.
Our ultimate goal is now to make membrane manufacturing safer and
more economic on atoms by using modified natural polymers like
cellulose ethers and water as the only solvent, thus lowering
wastes and recycling. Consolidation of the film structure will be
carried out by radiation induced cross-linking. For the thesis
work, on the basis of their relevance for membrane preparation and
accessible phase boundaries, the biocompatible and neutral polymer
selected is the hydroxypropyl cellulose (HPC). Aqueous
hydroxypropyl cellulose is a fascinating system which exhibits an
isotropic phase in dilute solutions, but forms an ordered liquid
crystalline phase with cholesteric structure in concentrated
solutions at room temperature. As PNIPAM the HPC has a lower
critical solution temperature (LCST) close to room temperature and
it undergoes reversible phase separation upon heating3.
Phase diagrams of HPC solutions will be established and bulk phase
separations will be studied to determine what kind of bicontinuous
phases are obtained as a function of the temperature,
concentration and time. The characterization will be done in a
large concentration range in order to change the topologies of the
predominant phase (in volume) rich in polymer or in solvent. Phase
diagrams and bulk phase separations of HPC solutions will be
studied by varying the physico-chemical parameters: degree of
polymerization and polydispersity.
The boundaries of the phase diagrams (cloud points) will be
determined by turbidity measurements. The morphologies and the
growth laws with time of phases will be established using
scattering techniques (Neutron, Light, XRay), Diffusive Wave
Spectroscopy, Optical and Confocal Microscopies.
Quantitative models will be proposed to rationalize the phase
separation processes in order to provide roadmaps for membrane
manufacturing.