Sommaire:

Polymer nanocomposites, systems of polymers containing nanoparticles (NPs), are fascinating systems that have many applications in material science, biology and medicine, but also pose challenges to theoretical physics. One of the fundamental problems in the physics of nanocomposites is to understand how the structure and dynamics of the system depends on key parameters, such as NP size and volume fraction and the typical size of the polymeric mesh. In this thesis we use molecular dynamics simulations to study the structural and dynamic properties of NPs embedded in liquid and solid polymer nanocomposites. We observe that when weakly attractive, well dispersed NPs are added to a dense polymer solution, both the polymers and the NPs experience a dynamical slowing down. We find that, in qualitative agreement with experiments, this dynamical slowing down is captured by a confinement parameter in the form h/λ, where h is the average distance between the surfaces of neighboring NPs (interparticle distance). We are able to show that for the NPs, λ can be interpreted as the hydrodynamic radius of the NP, whereas for the polymers it behaves as a cooperativity length scale. Simulating disordered, polydisperse polymer networks containing purely repulsive NPs, we find that small NPs can freely diffuse through the entanglement mesh, while large NPs are transiently trapped and can only move through a sequence of “jumps” (hopping motion). We find that the parameter controlling NP localization is the ratio between the NP diameter and the localization length of the crosslinks. Finally, we propose a new method to characterize the geometrical mesh size in polymer liquids, a quantity that is important to describe the diffusion of NP in a disordered medium.

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