Simultaneous Wrinkle-Patterning and In Situ Foaming in Polymer Films Using Supercritical Carbon Dioxide

Unique and random wrinkle features are desired for applications, especially in anticounterfeiting. Multifunctional wrinkled materials as well as simple wrinkling methods are desired for more applications. In this study, free-standing wrinkle-patterned porous polymer films were prepared using a modified supercritical CO₂ foaming strategy. Spontaneous wrinkling was generated by applying moderate normal stress to the polymer films while foaming in situ. The wrinkle characteristics (wavelength and amplitude) were shown to be governed by the film thickness and the magnitude of the applied stress. Excessive (>5000 N) or no stress produced no obvious wrinkling or foaming whatsoever. Moreover, different morphologies of the wrinkles such as brain coral and herringbone patterns were achieved depending upon the shape of the mold used to apply the stress. Thus, the patterns show unique features for each batch process. Notwithstanding, geometric shape of the foamed wrinkle area is customizable. The reported wrinkling strategy offers the complementary advantage of patterning a foam in a polymer film and templating a PDMS stamp for different applications such as "mask-less" soft lithography, anticounterfeiting, pattern transfer, flexographic printing, and displays.