Hélène Montes

Solid particles in an elastomer matrix: impact of colloid dispersion and polymer mobility modification on the mechanical properties

The reinforcement of elastomers by inorganic fillers, a concept of very high technological importance, is commonly understood to result from the presence of a mechanical network of partially aggregated filler particles. The non-linear mechanical properties, in particular the decrease of the modulus at high strain (Payne effect), are further interpreted to be a consequence of the breakdown of this filler network. There are, however, many open questions concerning the actual nature of the interparticle connections, where a modified polymer layer forming “glassy bridges” constitutes one possibility. In this work, we address this issue with a suitable silica-filled model elastomer, where we characterize the silica dispersion by SANS in combination with reverse Monte-Carlo modeling, and the mobility modification of the polymer by low-field proton NMR spectroscopy. In our samples, we identify a glassy layer as well as a region of intermediate mobility (possibly modified Rouse modes). Based on the structural information from SANS, we are able to quantify the amount of interparticle connections, and correlate it with the magnitude of the Payne effect taken from shear rheology. This works only if we assume that these connections comprise both, the glassy layer as well as the region of intermediate mobility. The amount of glassy immobilized polymer only does not suffice to explain the mechanical properties.

Particles in model filled rubber: Dispersion and mechanical properties

We have been able to design model filled rubbers with exactly the same chemical structure but different filler arrangements. From these model systems, we show that the particle arrangement in the elastomeric matrix controls the strain softening at small strain amplitude known as the Payne effect, as well as the elastic modulus dependence on the temperature. More precisely, we observed that the Payne effect disappears and the elastic modulus only weakly depends on the temperature when the particles are well separated. On the contrary, samples with the same interfacial physical chemistry but with aggregated particles show large amplitudes of the Payne effect and their elastic modulus decreases significantly with the temperature. We discuss these effects in terms of glassy bridge formation between filler particles. The observed effects provide evidence that glassy bridges play a key role on the mechanical properties of filled rubbers.

Revealing the respective effect of aging and cyclic deformation through the memory effect in glassy polymers

It has been known for 40 years that the state of a glass cannot be characterized by a single parameter such as its density and depends on its whole thermal history in a complex way. This phenomenon, known as the memory effect, reveals that the spatial distribution of the dynamics in a glass is deeply heterogeneous. Among the various memory effects, we will focus on the following one. After an annealing at a temperature T, a glassy system will exhibit a specific signature of susceptibility—around the temperature T of annealing. This effect shows that relaxations during annealing occur only in some domains of specific dynamics. This has been observed in a variety of glasses, with different techniques.The memory effect, here, is observed for the first time through the dynamical elastic modulus. We show that the dynamical elastic modulus can be described by the simple phenomenological so-called Tool–Narayanaswamy–Moynhian (TNM) model. We evidence the competition between plastic deformation and annealing by applying cyclic strain during the annealing. As a result, we establish that deformation exhibits an effect that is opposite to thermal annealing and less selective in temperature.