• Kharrat, Delphine El
• Cabuil, Valérie
• Sandre, Olivier

Abstract

We are interested to in a new type of composite material based on the association of a thermosensitive polymer, poly(N-isopropylacrylamide) noted PNIPAM and a colloidal dispersion of magnetic iron oxide nanoparticles called an “ionic ferrofluid”. On the one hand, PNIPAM exhibits a volume phase transition at the threshold temperature 32°C called LCST (Lower Critical Solution Temperature); on the other hand, the nanoparticles made of maghemite (Γ-Fe2O3) bring different properties among which superparamagnetism and the possibility to induce heating at distance under radiofrenquency waves. Thus combing the two components in the same formulation could lead to capsules that could deliver a drug on-demand. This design is challenging because of unfavourable interactions between the poorly polar network of PNIPAM (even hydrophobic above the LCST) and the highly polar magnetic oxide nanoparticles, which are stabilized against aggregation at pH 7 by the use of surface electrostaticaly charged ligands sodium citrate. The macroscopic study of gels chemically cross-linked in the presence of a ferrofluid reveals that with pure PNIPAM almost all the nanoparticles are released in the surrounding. On the contrary, the magnetic nanoparticles can be retained in gels based on a copolymerization of NIPAM with the mush more polar polymer poly(acrylamide) PAM. To reach this goal, the copolymer should not be simply statistical as the swelling transition would loses both its sharpness and its amplitude. By studying the interactions between the magnetic nanoparticles and linear PAM chains, we show that the concentration of sodium citrate needs to be sufficiently low so that the citrate ligands desorb and can be replaced by PAM chains adsorption at the iron oxide surface. Dealing with gels, we used this phenomenon to stop the release of nanoparticles outside the gel by creating a semi-interpenetrated network of NIPAM and PAM. More precisely, linear hydrophilic chains of PAM are adsorbed on the nanoparticles creating small clusters (about 60-100 nm in size) which are sterically trapped within the meshsize of NIPAM. This type of system permits us to retain more than half of the introduced nanoparticles inside the gels. The swelling volume of theses nanocomposites gels are followed both at a macroscopic and nanoscopic scales, by Small Angle Neutron Scattering (SANS) experiments using the nanoparticles as internal structure probes. Passing through the volume transition of the gel matrix, the internal structure of the magnetic clusters undergoes a collapse completely reversible when the temperature is lowered back.

Topics

• nanoparticle
• nanocomposite
• impedance spectroscopy
• dispersion
• surface
• cluster
• phase
• experiment
• Sodium
• phase transition
• iron
• copolymer
• small-angle neutron scattering