Porous Materials with Stress and Temperature Activated Porosity

The fabrication of micro-structured films with dynamic porosity is useful in various applications. This article presents fabrication of microcellular films exhibiting dynamic porosity upon applying an external pressure stimulus. Interestingly, we show that the dynamic porosity is in conjunction with an opaque to transparent transition (OTT) that imparts the material with a unique optomechanical behavior. The OTT experienced foams will have a transparency almost equal of the as-cast films. The foaming process used for this study is solid-state foming technique with CO2 gas as physical blowing agent. A styrene-ethylene-butylene-styrene (SEBS) triblock copolymer was used as the foaming material. Polystyrene (PS) blocks in SEBS play a key role in this process since CO2 reduces the T-g of PS and enable ethylene-butylene (EB) parts to swell during saturating stage while upon depressurization their T-g increases again that prevents the forming pores from collapse. A custom-made in-situ optomechanical setup was used to characterize the optical and mechanical behavior of the produced foams. Optomechanical tests at different strain rates and loads revealed that the films undergo a gradual OTT behavior indicating their ability to be used as optical pressure-sensitive films. Quenching temperature is of great importance in this process since the foams show various OTT behavior once the quenching temperature changes. Moreover, here we show after pressing the foams, the transparent films can be re-foamed by the same process for many cycles.