Structure of adsorbed layers and of chains in polymer nanocomposites
Résumé
A combined small-angle scattering of neutrons (SANS) and X-rays (SAXS) study of the
structure of polymer layers pre-adsorbed on silica nanoparticles (NP) which are subsequently
incorporated into polymer nanocomposites (PNCs) is presented. Pre-adsorbed chains are found
to promote ideal dispersion, before desorption in the late stages of nanocomposite formation.
The microstructure of the interfacial polymer layer is characterized by detailed modeling of X-ray
and neutron scattering. Only in ideally well-dispersed systems a static interfacial layer of
reduced polymer density over a thickness of ca. 2 nm is evidenced based on the analysis with a
form-free density profile optimized using reverse Monte Carlo simulations. This interfacial
gradient layer is found to be independent of the thickness and mass of the initially adsorbed
polymer, but appears to be generated by out-of-equilibrium packing and folding of the preadsorbed
layer. The impact of annealing is investigated to study the approach of equilibrium,
showing that initially ideally well-dispersed systems adopt a repulsive hard-sphere structure,
while the static interfacial layer disappears.
In parallel, SANS and the use of zero average contrast conditions allow the measurement of
chain conformation in the PNCs with and without pre-adsorption. Surprisingly, we evidenced the
spontaneous formation of thermally-stable adsorbed layers in PNCs containing matrix chains of
different mass, whereas a symmetric matrix of identical chain masses does not show this effect.
This study contributes thus to the fundamental understanding of the interplay between
effects which are decisive for macroscopic material properties: polymer-mediated interparticle
interactions, and particle interfacial effects on surrounding polymer.