Anisotropic Reinforcement of Nanocomposites Tuned by Magnetic Orientation of the Filler Network

We present a new material which displays anisotropic and mechanical properties tuneable during synthesis under magnetic field. It is formulated by mixing aqueous suspensions of polymer nanolatex and magnetic nanoparticles, coated by a thin silica layer to improve their compatibility with the polymeric matrix, followed by casting. The magnetic properties of these nanoparticles enable their pre-orientation in the resulting nanocomposite when cast under magnetic field. Detailed insight on dispersion by Small Angle Neutron Scattering (SANS) shows chainlike nanoparticle aggregates aligned by the field on the nanometer scale. Applying strain to the nanocomposite parallel to the particle chains shows higher mechanical reinforcement, than when strain is transverse to field. . SANS from strained samples shows that strain parallel to the field induce an organization of the chains while strain perpendicular to the field destroys the chain field-induced ordering. Thus improved mechanical reinforcement is obtained from anisotropic interconnection of nanoparticle aggregates. ha l-0 03 71 97 1, v er si on 1 31 M ar 2 00 9 Introduction Progress in both design of innovative materials and understanding of their properties has been achieved in the past two decades by extending polymer reinforcement by fillers to nanofillers. Among the still open challenges, the improvement and possibly tuning of a specific material property through a simple external trigger is of particular importance. A promising pathway is to bring in a new degree of freedom, e.g., by using magnetic, instead of classical inorganic particles as filler. The design of a functional nanocomposite material with the possibility to orient the filler inside the matrix should allow to tune macroscopic material properties, and in particular the mechanical ones, using a simple external magnetic field. This concept has been explored in the past twenty years by many research teams, mostly studying the effect of micron-size particles or fibres (needles). Two routes are possible: applying the field on the material after synthesis or applying it during the synthesis. We use here this second approach for nanoparticles; in such case, the question of the mechanical consequences of local anisotropy due to specific orientation of a nanofiller, i.e. particles with a size comparable to polymer network mesh sizes, has still to be solved. Here we present a new material based on a dispersion of magnetic nano-particles in a polymer matrix. This is achieved in a controlled way by adapting the knowledge on these systems in solution (“Ferrofluids”) to our own method of dispersion of silica nano-particles in a polymer matrix made of latex beads: via solvent (water) casting of a suspension of magnetic nanoparticles and polymer latex, a “Ferrolatex nano-composite” is formed. In this article, we will show that achieving these steps under the action of a weak magnetic field induces orientation responsible of a strong anisotropy of the reinforcement. Moreover, this mechanical anisotropy can be understood by measuring the spatial arrangement and orientation of the ha l-0 03 71 97 1, v er si on 1 31 M ar 2 00 9 particles on a scale between one nanometer and two hundred nanometers, using Small Angle Neutron Scattering (SANS). Sample preparation The synthesis of a composite made of a latex film filled with magnetic particles requires two main steps (fig. 1). The first main step is the synthesis of the magnetic particles according to a well known process described in the Methods Section. In a second time, in the same batch, the particles are covered with a thin layer of silica. This silica shell makes the particles pH compatible with the latex beads while keeping the initial size and the magnetic properties of the core. Hence we can use the technique developed for introducing silica particles in a latex film. The colloidal suspension of particles is mixed with a polymer bead (R200 Å) suspension at a fixed, high pH, followed by evaporation of the (aqueous) solvent. The latex nanospheres come in contact, and the capillary forces compress them into adjacent polyhedrons. We know that under deformation above Tg, such matrix deforms affinely (homothetically) to the macroscopic deformation, due to the elasticity of the membrane. The colloidal mixture, with a proportion of magnetic nano-particles to the latex spheres of 5% by volume is poured into rectangular 160.1 cm moulds and dried (at T  60°C), i.e. above the Film Formation Temperature which is itself close to the glass temperature transition of the