A transparent silica colloidal crystal/PDMS composite and its application for crack suppression of metallic coatings.

A silica colloidal crystal (SCC)-polydimethylsiloxane (PDMS) composite with a heterogeneous surface of silica and PDMS was prepared by spreading a premixed PDMS solution on the 3D structured SCCs and curing the solution in-situ. Although the SCCs had a light blue color, the obtained composite of SCC and PDMS, due to the close effective refractive indexes of the materials, was colorless and transparent; the UV-vis spectra indicated a negligible effect of the added SCC on the transmittance of the PDMS sheet (1% reduction). Interestingly, the transparent composite sheet became translucent under stress and became clear again when relaxed. It was found that the wrinkles formed on the surface under stress were responsible for the optical change; and, the formation of the wrinkles was ascribed to the rigid nature of the SCC layer embedded in PDMS. We had applied this SCC/PDMS composite as a substrate to support a thin gold film of nanoscale thickness and found that the embedded SCC layer worked well as a transitional interface for bonding materials of mismatched mechanical properties. The incorporation of SCC layer significantly suppressed the crack generation and propagation of the gold film. The results demonstrated a potential approach for fabricating compliant and crackfree metallic films on polymeric substrates.