Mechanical properties of thin confined polymer films and elastomer reinforcement
By studying model systems, we have shown that the temperature dependence of the modulus of filled elastomers can be explained by a long-range gradient of the elastomer matrix glass transition temperature Tg in the vicinity of the particles. We have shown thereby that the mechanisms of reinforcement are the same ones as those which lead to an increase of the glass transition temperature of strongly
adsorbed thin polymer films.
We have developed a 3D model, solved by numerical simulations, to describe the mechanical properties of polymers close to Tg, with a spatial resolution corresponding to the size of dynamical heterogeneities, that is a few nm. We focus on the case of polymers confined between two substrates with 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 Tg, up to 20 K in the considered situations. Moreover, between the bulk Tg and Tg +50K the modulus of the confined layers is found to decrease very slowly as T increases, 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. 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.*
- A Microscopic Model for the Reinforcement and the Nonlinear Behavior of Filled Elastomers and Thermoplastic Elastomers (Payne and Mullins Effects), S. Merabia, P. Sotta, D. R. Long, Macromolecules 2008, 41, 8252-8266