Clément Robin

Poly(methacrylic acid) (PMAA) is a polyelectrolyte which has been rather scarcely investigated in the literature, despite its various and very interesting physical properties. The behavior of PMAA solutions is drastically different from the one displayed by the extensively studied poly(acrylic acid), due to the methyl group located in the alpha position of the carboxylic acid function. At low ionization degrees in aqueous solvent, synergetic effects resulting from both hydrophobic interactions (methyl groups) and intramolecular hydrogen bonds between acidic groups are responsible for the hypercoiled conformation of the PMAA chains. Nevertheless, above a critical ionization degree, the PMAA macromolecule behaves as an extended chain similar to a usual highly charged polyelectrolyte. The main purpose of this work is to correlate the rheological properties to the structure of the chains for semi-dilute solutions of PMAA.

In the semi-dilute regime, above a critical concentration, PMAA solutions display a sharp increase in their viscosity over time under shear. This phenomenon known as antithixotropy occurs only when the macromolecules adopt a hypercoiled conformation. The influence of both concentration and ionization degree on the rheological behavior of PMAA solutions was studied. Shear induces the formation of intermolecular bonds which are responsible for the formation of a physical gel. We have demonstrated that the critical shear stress at which the gel is formed does not depend on the concentration. Interestingly, the results obtained remind some features observed in the case of the shear thickening of charged colloids.

Neutral PMAA chains in water also display a Lower Critical Solution Temperature (LCST) around 67 ◦C. The reorientational dynamics of water molecules above the LCST was studied by means of NMR experiments, which evidenced the dehydration of the polymer coils during phase separation. The formation of interfaces between polymer-rich and polymer-poor domains was investigated by small-angle neutron scattering experiments. A possible mechanism describing the transition at the molecular level was derived from the results obtained throughout the use of complementary experimental approaches. This mechanism is very similar to the one proposed for the LCST transition of poly(N-isopropylacrylamide) (PNIPAM). The cloud point temperature of PMAA solutions is the same as the gel temperature at which a viscoelastic gel is formed.

The understanding of the behavior of PMAA solutions under shear as well as under temperature is an essential prerequisite to the design of PMAA/silica nanocomposites. These nanocomposites were are prepared in the solution state. The interactions between PMAA chains and silica were clearly increased by the functionalization of the silica particles. Such interactions between PMAA and filler particles were found to be related to the pH of the solutions, due to the nature of the chemical groups at the surface of the functionalized silica. A possible perspective to this work would be the correlation between the structural features of the nanocomposites and their rheological properties during the drying process.

Keywords: polymer, polyelectrolyte, poly(methacrylic acid), rheology, SAXS, SANS, NMR, LCST, colloids, adsorption