Modes of deformation in a soft/hard nanocomposite: A SANS study

Abstract

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.

Study on two-dimensional small-angle X-ray scattering of in situ silica filled nanocomposite elastomer during deformation

“…The morphology of in situ silica particles became the buckling structure from the random state by elongating to the stretching direction and compressing from the lateral directions during uniaxial stretching. These observed morphological changes were in good agreement with those of the simulation on the basis of the displacement model by Rharbi et al (1999). The buckling structure of silica particles was clearly observed during the retracting process.…”

“…It was not a butterfly pattern, which was detected by stretching of carbon black filled styrene-butadiene vulcanizate by Morfin et al (2006), for example. It is worth noting that a similar two-dimensional SAXS pattern with our result was obtained by a computer simulation for a soft nanocomposite by Rharbi et al (1999). The simulation was conducted as follows: hard sphere particles were placed at random in a box that represented the polymer matrix, then the box was stretched in the parallel direction and compressed in the perpendicular direction and the centers of mass of the particles were displaced affinely in accord with the macroscopic deformation.…”

“…For the moment, we can not clarify the mechanism of buckling for our nanocomposite. However, the four-spot pattern obtained was very similar to the computer simulated one on the basis of the shear displacement model by Rharbi et al (1999). Thus, the latter phenomenon can be ascribed to the deformation mechanism for our in situ silica filled nanocomposite.…”

“…This morphological change of in situ silica particles is ascribable to the elongation in the stretching direction and the compression from the lateral directions occurring simultaneously during uniaxial stretching. On the appearance of a four-spot pattern, Rharbi et al (1999) proposed two processes; either some long aggregates buckle under the lateral compression, or the space in the perpendicular direction to the stretching between two aggregates is filled up by another aggregate that forced between them. For the moment, we can not clarify the mechanism of buckling for our nanocomposite.…”

“…For examples, one of the pioneering works is a study on the deformation behavior of nanocomposites composed of Nanolatex and silica of ca 20 nm diameter by smallangle neutron scattering (SANS) under a slow elongation, where the obtained scattering patterns were compared with those by a computer simulation (Rharbi et al, 1999). Together with several experimental studies (Rharbi et al, 1996;Clement et al, 2001;Kishimoto et al, 2006;Morfin et al, 2006;Harrak et al, 2006), active simulations (Oberdisse et al, 2000) were also conducted by changing the geometry, phase hardness and degree of aggregation of nanoparticles in the composites, where the experimental results by SANS, small-angle X-ray scattering (SAXS), ultra-small-angle neutron scattering (USANS), ultra-small-angle X-ray scattering (USAXS) and atomic force microscopy (AFM) were utilized.…”