Processing-Driven Control of the Properties of Polymer Grafted Nanoparticle Composites

The effect of preparation conditions on the properties of glassy polymers has been a subject of intense research, and these glassy polymers also age with deleterious consequences on properties. Here, we surprisingly find a similar preparation dependence for polymer-grafted nanoparticle melts (PGNP), even when the chains are in the melt. Specifically, we show that processing PGNPs by spin-casting vs slowly casting them from solutions yields temporally stable states with vastly different properties, including surface morphologies, mechanical properties, and gas transport. We propose that these differences arise because the end-grafted polymer brushes, especially for high grafting density and short chain lengths, are in an extremely long-lived colloidal glassy state, even though the chains themselves are mobile. Simulations suggest that there are strong variations in the chain interpenetration states between adjacent nanoparticles driven by solvent evaporation rates. With slower evaporation, there is evidently increased chain interpenetration leading to mechanical property improvements, while collapsed brushes dramatically increase gas permeability properties under fast evaporated conditions. These results strongly argue that processing protocols are an unappreciated control variable in determining the temporally stable properties of this class of materials.