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Nanocomposites have been made by mixing soft particles (polymer latex) with hard particles (silica) in aqueous dispersions and extracting water to produce a dense film. Segregation between the two kinds of particles can be controlled, and even suppressed. The elongational modulus is strongly increased by such fillers at low deformations, and remains important at large deformations, which the samples can stand without breaking. Since the silica particles are small (200 Å), we can follow their relative displacements under stretching, by Small-Angle Neutron Scattering, through analysis and simulation of the anisotropic patterns. The latter show a crossover from affine displacements to a set of shear displacements that let the particles avoid each other at large deformations. The shear could release the localized stresses (due to polymer confinement) and dissipate more energy. In this way it may contribute to the toughness of the composite against crack propagation.Introduction. -Soft polymeric materials can be reinforced by hard inclusions called "fillers" [1,2]. Common examples are rubber reinforced by carbon black particles, and "silicone" elastomers (PDMS) reinforced by silica. The properties of interest are, at small deformations, the mechanical modulus, and, at large deformations, the resistance to tear and wear. These properties are determined by the mechanical properties of each component, by the interfacial energy [3], and by geometrical factors such as the sizes, shapes and distances of fillers [4]. However, the mechanism of reinforcement is, at present, not understood. The roles proposed for fillers include i) temporary junctions of the polymer chains [5], ii) steric restriction described by modified Einstein laws [1, 2] or more elaborated "concentrated dispersion" models [6], ramified nature of aggregates of particles [3] leading to iii) connection in a "filler network" [7], but also iv) to overlap resistance of the bushy aggregates [8].Remarkable reinforcements can be achieved with fillers that are extremely fine, e.g., nanometric: on the one hand, large increases of the modulus may be achieved, and on the other hand, the composites can still take large deformations before they rupture [1][2][3][4]9]. The origin of this toughness of nanometric composites is still in question. In this letter, we present experimental evidence for relative displacements of nanometric particles in composites that undergo large deformations. These displacements may make it possible for the material to dissipate the energy that is stored at the tip of a fracture, and thereby accommodate large deformations without rupture.

Cellular films have been produced through evaporation of aqueous dispersions containing latex particles. The membranes of the cells are made of hydrophilic species which originate from the surfaces of the particles; they form a periodic structure which separates the cell cores from each other. Thermal treatments have been applied to induce the fragmentation of membranes. The resulting structural changes have been observed through small-angle neutron scattering and electron microscopy. It has been found that the fragmentation of membranes is controlled by three parameters: the mobility of membrane polymers, the anchoring of these polymers on the core, and the mobility of the core. After fragmentation occurs, the hydrophilic membrane material is expelled to large lumps immersed in a continuous latex matrix.

Latex films have been produced through evaporation of aqueous dispersions containing polymeric particles. These films have a cellular structure:  The cell cores are made of hydrophobic polymers, and the cell walls are made of carboxylic monomers that are copolymerized with the core polymers. The mechanical properties of these films are determined by the state of the cell walls. In humid environments, the cell walls take up water and lose their cohesive strength; consequently the films become brittle. The possibility of reinforcing the films through interdiffusion of core polymers across the cell walls was examined. Annealing the films at temperatures where these polymers are mobile produced extensive interdiffusion of low molar mass polymers across the cell walls. This had no effect on the mechanical properties of the films. Annealing at temperatures that caused fragmentation of cell walls produced interdiffusion of high molar mass polymers. At this stage the films became resistant to humidity.