Reinforcement and Polymer Mobility in Silica-Latex Nanocomposites with Controlled Aggregation
Résumé
The tunable structure of silica-latex nanocomposites made of silica nanoparticles (radius ≈ 80 Å) and a copolymer of methyl methacrylate and butyl acrylate-latex beads (radius≈210 Å) has been studied by small-angle neutron scattering and transmission electron microscopy. An aggregation diagram as a function of the control parameters--silica volume fraction and precursor solution pH--has been established. In this aggregation diagram, isoaggregation lines have been identified. It was used to express the small-strain reinforcement factor measured with stress-strain isotherms as a function of volume fraction at fixed aggregation number in the range from 50 to 100. The large-strain properties have been rationalized using the energy needed to rupture samples, and this quantity has been found to present an optimum at intermediate volume fractions (15%). In order to understand the striking rheology of the system, a neutron contrast-matching study has been undertaken by adding deuterated polymer beads. The bead demixing kinetics during annealing has been used to characterize the dynamics in various environments defined by the hard silica structure. In particular, in nanocomposite samples containing 15 vol % of silica the dynamics is found to be blocked.