In situ visualization and mechanical study of a transparent filled rubber

Filled rubbers are composite materials containing two interpenetrating phases: crosslinked elastomers, and a ‘filler’ consisting of colloidal particle aggregates. Above a critical volume fraction, the colloidal aggregates form a system-spanning subnetwork that reinforces the elastomer network and introduces a new energy loss mechanism at low strains of only 1-5%. This loss mechanism, known as the Payne Effect, is one of the mechanical hallmarks of filled rubbers and is a major contributor to rolling friction in tires.

We create a model filled rubber which exhibits the rheological hallmarks of traditional filled rubbers, but is also transparent. Optical transparency is achieved by matching the refractive index of our filler, fumed silica, and our polymer, PDMS. Visualizing the deformation of the filler subnetwork requires a contrast mechanism with sufficient spatial resolution to resolve the filler microstructure. Quantum dots, whose physical size is comparable to the smallest length scales in the filler subnetwork, provide the desired optical contrast. Using this system, we can directly observe local movements of filler particle aggregates during in situ deformation, and compare those dynamics with bulk rheological tests to gain new insight into the physics of the Payne effect.