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Solution processable liquid metal nanodroplets by surface-initiated atom transfer radical polymerization

Abstract

Eutectic gallium indium (EGaIn) is a liquid metal alloy at room temperature. EGaIn microdroplets can be incorporated into elastomers to fabricate highly stretchable, mechanically robust, soft multifunctional composites with high thermal stability1 and electrical conductivity2,3,4 that are suitable for applications in soft robotics and self-healing electronics5,6,7. However, the current methods of preparation rely on mechanical mixing, which may lead to irregularly shaped micrometre-sized droplets and an anisotropic distribution of properties8. Therefore, procedures for the stabilization of sub-micrometre-sized droplets of EGaIn and compatibilization in polymer matrices and solvents have attracted significant attention9,10,11,12. Here we report the synthesis of EGaIn nanodroplets stabilized by polymeric ligand encapsulation. We use a surface-initiated atom transfer radical polymerization initiator to covalently functionalize the oxide layer on the surface of the EGaIn nanodroplets13 with poly(methyl methacrylate) (PMMA), poly(n-butyl acrylate) (PBMA), poly(2-dimethylamino)ethyl methacrylate) (PDMAEMA) and poly(n-butyl acrylate-block-methyl methacrylate) (PBA-b-PMMA). These nanodroplets are stable in organic solvents, in water or in polymer matrices up to 50 wt% concentration, enabling direct solution-casting into flexible hybrid materials. The liquid metal can be recovered from dispersion by acid treatment. The nanodroplets show good mechanical, thermal and optical properties, with a remarkable suppression of crystallization and melting temperatures (down to −80 °C from 15 °C).

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Acknowledgements

The authors acknowledge financial support from the National Science Foundation (DMR 1501324, DMR-1709344 and CMMI-1663305) and the Air Force Office of Scientific Research (AFOSR) Multidisciplinary University Research Initiative (FA9550-18-1-0566; programme manager, K.Goretta). The authors also acknowledge the use of facilities in the Colloids, Surfaces and Polymer Laboratory at Carnegie Mellon, supported by grant no. CMU 678083-769798.

Author information

J.Y. and M.H.M. conceived and designed the experiments. J.Y. performed the synthesis and kinetic studies. J.Y., M.H.M. and Z.L. fabricated and characterized the materials. Z.W. performed the microscopic characterization. N.K. and C.P. were involved in discussions at various stages of the work. M.R.B., C.M. and K.M. supervised the work. All authors discussed the results and commented on the manuscript.

Competing interests

The authors declare no competing interests.

Correspondence to Michael R. Bockstaller or Carmel Majidi or Krzysztof Matyjaszewski.

Supplementary information

Supplementary information

Supplementary Figs. 1–5

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Further reading

Fig. 1: SI-ATRP from EGaIn and EGaIn-PMMA hybrid droplets.
Fig. 2: EGaIn–PBMA hybrid droplets and their tensile performances.
Fig. 3: EGaIn–PBA-b-PMMA thermoplastic elastomer.
Fig. 4: EGaIn–PDMAEMA water-soluble hybrid droplets and cationic gel-wrapped EGaIn.