Homogeneous Dispersion of Magnetic Nanoparticles Aggregates in a PS Nanocomposite: Highly Reproducible Hierarchical Structure Tuned by the Nanoparticles’ Size

Abstract

We present here the synthesis and structural characterization of new nanocomposites made of spherical magnetic nanoparticles of maghemite (γ-Fe 2 O 3) dispersed in a polystyrene (PS) matrix. The γ-Fe 2 O 3 nanoparticles, synthetized in aqueous media, were first gently transferred by dialysis in dimethylacetamide (DMAc), a polar solvent which is a good solvent for PS. Electrostatic repulsions enable to keep colloidal stability in DMAc. The nanocomposites were then processed by a controlled evaporation of DMAc of binary mixtures of γ-Fe 2 O 3 nanoparticles and PS chains. The size of the nanoparticles ranges from 3.5 to 6.5 nm and can be changed without any modification of the nanoparticles' surface. The structural organization of the nanoparticles inside the polymer was determined as a function of the nanoparticles' size. It was performed by combining very high resolution SAXS measurements which permit to decrease the nanoparticles content down to very low values (Φ mag ~ 10-5) and TEM microscopy. Whatever the size, the nanoparticles are organized with a hierarchical structure that shows that their aggregation has been driven by a two-step process. At low spatial scale, dense primary aggregates composed of some tens of nanoparticles are formed whatever Φ mag , resulting from the first aggregation step. For Φ mag > 10-4 , these primary aggregates underwent a second aggregation step and are organized at larger scale in fractal aggregates of finite size of ~ 200nm of radius, with a dimension of 1.7. The size of the dense primary aggregates is almost constant when changing the nanoparticles radius, i.e the mean aggregation number of primary aggregates decreases with an increase of the radius.

Effect of hybrid nanoparticles on glass transition temperature of polymer nanocomposites

“…The scattering intensity of the solutions decreases monotonously as the modulus of the scattering vector increases, which indicates that the particles are well‐dispersed in the solutions and that no aggregates form. For the composites, the scattering curves are characterized by a maximum that corresponds to a certain mutual ordering of the scattering particles .…”

Perspective: Outstanding theoretical questions in polymer-nanoparticle hybrids

“…[251][252][253][254][255][256] However, to our knowledge, only a few works have considered the corresponding phenomena when magnetic NPs are placed in a polymer matrix in an external field. 25,[257][258][259] Jestin et al, for example, incorporated iron oxide NPs in a polymer matrix in the presence of solvent (Fig. 9).…”

Destructive changes of polymer matrices during preparation, storage, and mechanical testing of neat and C60‐filled polystyrene films

“…Since the filler aggregation is a temporally continuous process (proceeding up to complete removal of solvent), it might be supposed that the structures visualized at the microlevel began to form at the nanoscale level. Indeed, resembling structures were observed at the nanoscale level for different fillers dispersed in PS matrix, such as spherical magnetic nanoparticles of maghemite (γ‐Fe 2 O 3 ) and silica nanoparticles grafted with PS chains as well as for ultrathin PS + C 60 composite films during dewetting . Formation dynamics of aggregates shows that both filler concentration and rate of solvent evaporation exert influence on their sizes.…”

“…Nanofiller aggregation cannot be prevented if films are prepared by solvent evaporation from mixed solutions of polymer and nanofiller (even when filler was initially dispersed at the molecular level, as in the case of the mixed PS + C 60 solution) . The striking example which exhibits the inevitable filler aggregation is the composite of PS with spherical magnetic nanoparticles (maghemite) . Stable state of nanoparticle colloid in PS solution was initially sustained by electrostatic repulsion.…”