X-ray scattering is a powerful tool to probe the microstructure of polymer nanocomposites; however, the technique has traditionally been used to study composites in which both the filler and the matrix are solid. X-ray scattering investigations of composites based on liquid metal fillers dispersed in a soft elastomer are scarce, despite the growing interest in these liquid metal composites for areas including flexible electronics and recyclable and reconfigurable circuits. In this work, soft polymer composites comprised of a eutectic gallium indium (EGaIn) alloy filler and a PDMS matrix were prepared at various volume loadings of EGaIn filler. Ultra-small angle X-ray scattering experiments were conducted to characterize the bulk morphology of the droplets, aggregates, and interfaces in the composite and to understand how the droplet size varied as a function of filler loading. The scattering data was analyzed using the Unified Fit scattering function and a Monte Carlo method, and the results were in excellent agreement with the composite morphology observed via scanning electron microscopy. All methods revealed that the droplets increase in size as they are incorporated in the composite, and that coalescence or agglomeration occurs during dispersion. This work demonstrates the viability of X-ray scattering to elucidate structural information about liquid metal droplets in polymer composites and thus provides a facile method for characterizing materials in soft robotics and soft electronics.

Quantitative analysis of liquid metal-polymer composite morphology via x-ray scattering

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