Phase Behavior of Polymer-Grafted Nanoparticles

The fundamental thermodynamic interactions between polymer-grafted nanoparticles and their surroundings—whether in a polymer matrix or a solvent—are crucial to the properties and performance of the resulting material. The phase behavior and conformations of the grafted polymer are governed by both entropic and enthalpic effects that in turn, drive dispersion and aggregation in the system. Although the dispersion-aggregation transition in the athermal case is largely entropic in nature, significant enthalpic interactions exist in many chemically dissimilar graft-matrix and graft-solvent systems that influence their phase transition. In this work, the phase behavior and conformational transitions of polystyrene (PS)-grafted silica nanoparticles in a poly(vinyl methyl ether) (PVME) matrix and cyclohexane are investigated—both systems exhibit conformational changes in the brush as a function of temperature. Through a combination of transmission electron microscopy imaging, small angle x-ray and neutron scattering techniques, a gradual wetting-dewetting transition in the silica-PS/PVME composite is elucidated and found to be distinct from the dispersion-aggregation transition. This is in stark contrast to athermal systems of chemically similar brush and matrix chains where the two transition events are analogous. Moreover, using light and neutron scattering, a coil-to-globule transition of the grafted PS chains in cyclohexane is probed as the solvent quality changes from good to poor. Measurements of the hydrodynamic size reveal a continuous change from a highly-swollen brush to a globule-like configuration before particle aggregation occurs. These observations are consistent with changes in the brush thickness and radius of gyration measured from neutron scattering experiments. This work presents key insights into the thermodynamic behavior of polymer-grafted nanoparticles unique to dissimilar graft-matrix and graft-solvent systems. Furthermore, it highlights fundamentally important concepts crucial to the design and functionality of advanced materials.