Synthesis and self assembly processes of aqueous thermoresponsive hybrid formulations

Abstract

New hybrid assemblies with thermothickening properties are designed using poly-(N-isopropylacrylamide) (PNIPA) as responsive trigger. The main components of the formulations are hydrolyzed polyacrylamide grafted with PNIPA side-chains (PgPNIPA) and core–corona silica–PNIPA nanoparticles (SgPNIPA). PNIPA brushes with controlled molar mass are synthesized in a two-step procedure through a “grafting from” approach involving thiol groups chemically anchored on the silica surface. Particles are characterized by FT-IR, ¹H NMR, ¹³C NMR, solid state CP/MAS ¹H–²⁹Si NMR, acid–base and chemical titrations, and dynamic light scattering. Rheology and microcalorimetry studies are used to investigate the self-assembling behaviour of hybrid formulations. For SgPNIPA aqueous dispersions, a collapse of the corona is induced upon heating. It starts from the silica core at low temperature (29 °C) and spreads progressively toward the outer part of the corona, leading to a reversible aggregation of the particles above 34 °C. When the same particles are added into PgPNIPA solutions, original self-assembling behaviours are observed. Contrary to neat silica particles, the presence of a dense PNIPA corona grafted on the silica surface prevents specific interactions between the grafted copolymer and the surface of the silica core. When the formulation is heated, the hybrid mixture exhibits two distinct phase transitions correlated to the PNIPA corona (30–50 °C) and the PNIPA grafts (40–60 °C). A synergy between these two transitions is observed above 30 °C with a noticeable increase of viscosity, followed above 40 °C by the self-assembling behaviour of PNIPA grafts and the formation of a physical network with improved viscoelastic properties.