Metamaterials are man-made materials that possess properties not found in nature, for example a negative refractive index or characteristics allowing for the realization of a lens with a sub-wavelength diffraction limit. Soft mechanical metamaterials have been prepared from commercially available polymers including poly(lactic acid), poly(amide), and polysiloxanes. Specifically, for porous polysiloxane acoustic metamaterials, a strong dependence of the longitudinal sound speed on the porosity and material properties has been established. A simple and robust method to prepare both stiff and soft porous polymer networks is using the emulsion templating technique known as polymerized high internal phase emulsions (polyHIPEs). While soft acoustic polyHIPE-based metadevices have been reported, these examples have been limited to commercially available curable PDMSs, where only the volume fraction of the pores in the final polyHIPEs can be controlled to prepare the targeted wave shaping capabilities. In the work in this presentation, we have manipulated the network chemistry of PDMS-based polyHIPE elastomers to control the storage modulus of the materials. We used thiol-ene “click reactions” as a simple and robust polymerization technique to control the crosslinking in the network which in turn controlled the moduli of materials, with the goal of ultimately accessing a wider scope of desirable sound velocities. Both porous materials and non-porous control materials showed identical trends in mechanical properties where an equal thiol to ene molar ratio produced materials with the highest storage modulus of ~225 and ~1150 kPa respectively. In acoustic testing results, non-porous PDMS elastomers were found to all obtain similar longitudinal sound speeds of ~1000 m/s during ultrasonic characterization. Acoustic measurements of polyHIPEs show that all materials obtain exceptionally low values of sound velocity between ~40 and ~60 m/s, regardless of the thiol to ene ratio. These results show that these polyHIPE formulations have promise as acoustic metamaterials.

Mechanically tunable PDMS-based polyHIPE acoustic materials

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ACS Fall 2022

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