Structure and dynamics of polymer nanocomposites with different interactions

Polymer nanocomposites (PNCs) are made by the dispersion of hard fillers in a polymer matrix, where the fillers are usually added to enhance mechanical properties. Such materials attract industrial interest and their investigation is essential for the improvement of their performances. The latter are strongly related to the structural properties of PNCs: the dispersion state of nanoparticles (NPs) in the polymer melt is governed by the mixing protocols (solid phase mixing or solvent casting) and thermodynamics of the system (i.e., particle miscibility) which influence both filler-filler and filler-polymer interactions.
The present study concerns PNCs obtained using hydrophilic silica NPs dispersed in two different polymer matrices: 1) poly(2-vinylpyridine) forming attractive NP/polymer interfaces or 2) a weakly interacting styrene-butadiene (SB) rubber. In the first system, the dynamical properties of the interfacial layer surrounding NPs are obtained using broadband dielectric spectroscopy (BDS). Moreover, we propose an original approach combining small-angle scattering experiments of X-rays and neutrons and simulations to investigate the static properties. Only in ideally well-dispersed systems using pre-adsorption of the polymer chains, a static interfacial layer with a gradient of density extending over 2 nm is evidenced based on the analysis with a form-free density profile. This gradient is found to be generated by out-of-equilibrium packing of the preadsorbed layer.
In the second polydisperse and disordered SB system, we propose a reverse Monte-Carlo analysis of the scattering data giving access to NP aggregate mass distributions. In opposition to the previous system, BDS experiments are not conclusive regarding the impact of the industrial filler on the segmental relaxation. We used neutron spin-echo spectroscopy (NSE) and demonstrate – here for the first time – that incoherent NSE can be used to resolve small modifications of the segmental dynamics of PNCs.