Diels–Alder Reactions on Surface-Modified Magnetite/Maghemite Nanoparticles: Application in Self-Healing Nanocomposites

Surface-modified
inorganic nanoparticles with organic groups allowing
thermoreversible cross-linking are versatile building blocks for nanocomposites
with improved lifetimes. We studied the synthesis of various organophosphorus
coupling agents consisting of phosphonic acid anchor groups, different
alkyl spacers (C3, C10), and furan- or maleimide functional groups.
The obtained molecules, which are able to undergo Diels–Alder
(DA) reactions, were attached to the surface of well-defined preformed
magnetite/maghemite nanoparticles by an exchange reaction of the stabilizing
oleic acid ligands under mild conditions. A dilution of the reactive
functional groups with unreactive methyl phosphonic acid resulted
in a cofunctionalization of the surface with nonreactive groups. The
reactivity of the modified particle surface toward DA reactions was
systematically explored by converting these particles with maleimide/methylfuran
or N-dodecylmaleimide/dodecylfuran. Fourier transform
infrared spectroscopy, differential scanning calorimetry, and CHN
elemental analysis measurements revealed a correlation between the
quantity of functional reagent on the surface and the conversion in
DA reactions. DA conversion of the modified particles is higher if
fewer sterical hindered molecules react with the surface, which has
implications on the use of these particles as cross-linking agents
in polymer matrices. A thin nanocomposite film with thermally triggered
healing capacity was successfully prepared by cross-linking the particles
with 1,1′-(methylenedi-4,1-phenylene)­bismaleimide.