Paul Sotta

Filler–elastomer interaction in model filled rubbers, a 1H NMR study

Abstract 1 H NMR experiments on filled rubbers allow the observation of two features: the introduction of supplementary topological constraints by the fillers, and a layer of immobilised segments at the particle surface. In order to assess the role of the filler–elastomer interaction on the local chain dynamics, silica filled elastomer model systems were prepared. The synthesis methods used allowed us to test vastly different interaction strengths. It is shown that in the case of a weak interaction, the influence of the fillers is small, whereas a strong anchoring at the interface leads to an increase in the density of the topological constraints. The latter is correlated to a layer of highly immobilised segments whose thickness decreases with temperature. This layer is seen as a glassy shell.

Mechanical properties of thin confined polymer films close to the glass transition in the linear regime of deformation: theory and simulations.

Over the past twenty years experiments performed on thin polymer films deposited on substrates have shown that the glass transition temperature Tg can either decrease or increase depending on the strength of the interactions. Over the same period, experiments have also demonstrated that the dynamics in liquids close to the glass transition temperature is strongly heterogeneous, on the scale of a few nanometers. A model for the dynamics of non-polar polymers, based on percolation of slow subunits, has been proposed and developed over the past ten years. It proposes a unified mechanism regarding these two features. By extending this model, we have developed a 3D model, solved by numerical simulations, in order to describe and calculate the mechanical properties of polymers close to the glass transition in the linear regime of deformation, with a spatial resolution corresponding to the subunit size. We focus on the case of polymers confined between two substrates with non-negligible interactions between the polymer and the substrates, a situation which may be compared to filled elastomers. We calculate the evolution of the elastic modulus as a function of temperature, for different film thicknesses and polymer-substrate interactions. In particular, this allows to calculate the corresponding increase of glass transition temperature, up to 20 K in the considered situations. Moreover, between the bulk Tg and Tg + 50 K the modulus of the confined layers is found to decrease very slowly in some cases, with moduli more than ten times larger than that of the pure matrix at temperatures up to Tg + 50 K. This is consistent with what is observed in reinforced elastomers. This slow decrease of the modulus is accompanied by huge fluctuations of the stress at the scale of a few tens of nanometers that may even be negative as compared to the solicitation, in a way that may be analogous to mechanical heterogeneities observed recently in molecular dynamics simulations. As a consequence, confinement may result not only in an increase of the glass transition temperature, but in a huge broadening of the glass transition.

Connectivity of the Hexagonal, Cubic, and Isotropic Phases of the C12EO6/H2O Lyotropic Mixture Investigated by Tracer Diffusion and X-ray Scattering

The connectivity of the hydrophobic medium in the nonionic binary system C12EO6/H2O is studied by monitoring the diffusion constants of tracer molecules at the transition between the hexagonal mesophase and the fluid isotropic phase. The increase in the transverse diffusion coefficient on approaching the isotropic phase reveals the proliferation of bridgelike defects connecting the surfactant cylinders. This suggests that the isotropic phase has a highly connected structure. Indeed, we find similar diffusion coefficients in the isotropic and cubic bicontinuous phases. The temperature dependence of the lattice parameter in the hexagonal phase confirms the change in connectivity close to the hexagonal−isotropic transition. Finally, an X-ray investigation of the isotropic phase shows that its structure is locally similar to that of the hexagonal phase.

Fatigue crack growth dynamics in filled natural rubber

Abstract We present fatigue experiments performed on filled natural rubber and study the correlations between crack growth dynamics and fracture morphologies imprinted by an irregular crack path. Slow crack growth dynamics is obtained by cyclic fatigue in a pure shear test. We will show that an unstable crack growth regime exists for high loads. We will also discuss the appearance of sawtooth striations which follow a scenario that significantly differs from previous results reported in the literature.

Fatigue Behavior in Filled Natural Rubber: Study of the Mechanical Damage Dynamics

Rupture dynamics in reinforced elastomers is a much more complex process than in pure elastomers due to the intrinsic heterogeneous mixture of a rubber matrix with filler particles at submicronic scale. In the case of natural rubber, an additional source of heterogeneity is the strain-crystallization effect. How rupture dynamics and crack path are affected by filler particles and strain-crystallization is still a matter of debate. Actually, understanding how rupture dynamics and crack path are correlated to each other is probably an important key in order to improve long time resistance of reinforced rubbers.

Effect of intermolecular interactions on the viscoelastic behavior of polyamide melts

Polyamides are engineering thermoplastics which exhibit good mechanical and barrier properties. However, the viscoelastic behavior over the complete range of polyamide relaxation is a topic rarely mentioned in the literature due to the presence of a crystalline phase and lack of thermal stability.The rheological behavior of amorphous PA 6I with increasing molecular weight was studied. As molecular weight increases, a clear rubbery plateau appears and the longest relaxation time is shifted to lower frequencies, as expected by the Rouse and reptation models.Interactions were added to the PA by copolymerizing PA 6I’s monomers with different substituted isophthalic acids. Ionic copolyamides were synthesized in molar fractions from 5 to 20 mol%, inciting an increase of about 10 to 40°C in the glass transition temperature. Master curves of unentangled PA 6I and substituted polyamides, with similar molecular weight, overlap in the complete frequency range using an appropriate reference temperature, which is close to, but not identical to Tg. Ionic groups have an effect on the Angell’s dynamical fragility, i.e. on the temperature variation of the rheological response close to Tg. Dynamic moduli of unentangled polyamides were fitted with Rouse model, showing no effect of hydrogen bonds or ionic groups on the shape of the rheological master curves. The molecular weight between entanglements increases for ionic copolyamides due to an increase of chain rigidity. Small-angle X-ray scattering shows that no segregation of ionic domains occurs in unentangled ionic copolyamides, while entangled copolyamides show only weak segregation.Stronger hydrogen bonding resulted in a decrease of the molecular weight between entanglements. Nevertheless no significant difference was observed in the shape of master curves, which were fitted using the Rouse model.The effect of interacting groups on the local dynamics in the solid state (below Tg) was studied by dielectric spectroscopy. Ionic groups have no effect on secondary relaxations, while alpha-relaxation is shifted accordingly to Tg. No difference was observed between ionic copolyamides and PA 6I during aging experiments, as ionic groups do not act as dynamic heterogeneities, i.e., zones where the local dynamics are heterogeneous due to different local ionic fractions

2H NMR Line Shape in Polymer Networks

Some properties of polydimethylsiloxane (PDMS) polymer networks, which have been put into evidence by use of Deuterium Nuclear Magnetic Resonance (2H NMR), will be described in this paper. The anisotropy of molecular motions (in the time range 10−6 to 10−5s) may be investigated with this technique. It has been already shown that deuteriated probes(solvent or free PDMS chains)dissolved in uniaxially deformed networks, acquire a uniaxial orientational order, which means that their motions, in the range 10−5s, become uniaxial around the applied force direction 1–3. This property and specifically the free chain orientation has been interpreted as the effect of orientational interactions between chain segments.