Deformable liquid metal polymer composites with tunable electronic and mechanical properties
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INVITED PAPER Deformable liquid metal polymer composites with tunable electronic and mechanical properties Amanda Koh Autonomous Systems Division, Vehicle Technology Directorate, U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, USA
Jennifer Sietins Manufacturing Science and Technology Branch, Weapons and Materials Research Directorate, U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, USA
Geoffrey Sliphera) Autonomous Systems Division, Vehicle Technology Directorate, U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, USA
Randy Mrozekb) Polymers Branch, Weapons and Materials Research Directorate, U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, USA (Received 9 May 2018; accepted 7 June 2018)
Room-temperature liquid metals, such as eutectic gallium–indium–tin (galinstan), dispersed in a polymer matrix present unique potential as conductors that may have minimal influence on the host polymer mechanical performance while providing enhanced electrical performance. Work described herein systematically evaluates the influence of uncured polydimethylsiloxane (PDMS) viscosity and galinstan loading on final dispersion viscosity and cured modulus. Dispersions of up to 80 vol% galinstan were obtained with relative permittivity values up to 170 that otherwise exhibited similar uncured rheological changes to a solid filler. Cured galinstan-in-PDMS dispersions, however, exhibited a reduced stiffness increase with respect to the host polymer relative to a solid filler. At a critical PDMS viscosity and metal, loading phase inversion to a conductive PDMS-in-metal dispersion was observed. It is anticipated that this work will enable the development of liquid metal polymer composites with independently controlled mechanical and electrical properties for a wide variety of stretchable electronic applications.
I. INTRODUCTION
Composites of soft compliant polymers and conductive fillers have long been investigated to enable devices with a range of functionalities including sensors,1,2 inductors,3 conductive components,1,4,5 actuators,6 and EMI shielding.7 Polymer composites are commonly based on hard spherical particles dispersed homogenously in a continuous polymer phase.8,9 These systems, while conceptually simple, are limited in their functionality by an upper filler concentration limit (maximum random packing fraction).10 At high filler concentrations, the deformation characteristics of the composite material often suffer from increased stiffness, reduced elongation at break, and decreased mechanical damping that, for elastomers, can render the device no longer compliant.11,12 Nonspherical solid fillers, such as metal flakes,13 graphene,14,15 and carbon nanotubes,16 Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2018.209 J. Mater. Res., Vol. 33, No. 17, Sep 14, 2018
generally have similar material/electronic challenges even though lower concentrations are
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