Ph'D defence Mme Rachel Le Brouster

Abstract:
For many industrial sectors, the modification of surface properties remains a strategic issue and can be achieved either by directly changing the surface chemistry, in particular via coatings, or by structuring the surface, via the replication of micro- or nanotextures, a route described as physical. The aim of this thesis study was to improve understanding of the influences of surface chemistry and physical structuring on the hydrophobic properties of polystyrene (PS). 

Initially, additives, called POISE-a, consisting of random copolymers of oligomers of -styrene and fluoro-MOM-styrene, were synthesized by controlled radical polymerisation. Formulation with a polystyrene matrix at different weight contents were carried out by solvent casting, and revealed clear transitions in wettability behaviour. Static contact angles with water close to 120° were then achieved for higher POISE-a levels (of the order of a few per cent), a result equivalent to a surface coated with -CFx fluorinated groups, confirming the effectiveness of the additives synthetised using this chemical route. 

The second part of this study was devoted to understanding the origin of the gain in PS hydrophobicity properties. An in-depth analysis of the morphology of the blends revealed the phenomenon of phase separation between the additive and the PS matrix, with the presence of nodules of varying size and quantity in the volume. The improved hydrophobic behaviour of the PS/POISE-a blends was then linked to the migration of the additives to the surface of the films, saturating the surface for additive contents of 10 wt.%. This work has shown that the presence of chemical heterogeneities in solution, some of which migrate to the surface and crystallize during the solvent evaporation process. This segregation mechanism was linked to the architecture of the copolymers and their intra- and/or intermolecular interactions with the polymer matrix. 

The final part of this study was devoted to understanding the contribution of POISE-a and texture replication on the wettability properties of polystyrene injection-moulded parts. It was shown that POISE-a has a positive effect on the rheological behaviour of PS, both by plane/plane rheometry and under real production conditions for injection moulded parts, with an increase in the flow rates of the additivated polymers and better control of the quality of the injected parts. The improvement in the hydrophobic properties of injected PS was achieved either replicating textures or by adding POISE-a to non-textured surfaces.