Colloidal polymer composites: Are nano-fillers always better for improving mechanical properties?

We find that the inclusion of nano-fillers leads to brittle fracture at a lower ϕ than when μm-size fillers are used. The data interpretation is supported by Weibull analysis. However, for a given ϕ, the storage modulus is higher in the rubbery regime, and the creep resistance is higher when nanoparticles are used. Using scanning electron microscopy to support our arguments, we show that the properties of colloidal composites are correlated with their microstructure, which can be altered through control of the filler:polymer particle size ratio. Hard nanoparticles pack efficiently around larger particles to provide reinforcement (manifested as a higher storage modulus and greater creep resistance), but they also introduce weak points that lead to brittleness.