Shape memory polymers (SMP) have been extensively implemented for applications ranging from biomedical devices and soft robotics to deployable structures. However, these materials mostly rely on heat for shape recovery and programming. Here, we introduce a new class of smart SMPs with pressure-responsive characteristics based on dynamic porosity–an ability to configure a pore size within a solid– that allows for optical monitoring of the stimuli. Introducing pores in a structure reduces the relative density of the material and introduces interfaces resulting in a reduction in optical transparency of the bulk through light scattering. We show that utilizing the dynamic porosity, a macroporous film transitions instantly from an optically opaque (25% transmittance) to a transparent (96% transmittance) state by applying an out-of-plane contact pressure of 3.8 MPa at room temperature. The new SMP also presents an anomalous ‘cold’ shape recovery of the pressure-activated films through repeated in-plane stretching that renucleates the pores and transitions the material back to the initially opaque state. The presented concept ushers a new class of responsive materials that interlace optical sensitivity features with micro- and nanoscale material deformations, establishing new research opportunities in the field of SMPs.
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 Tg of PS and enable ethylenebutylene (EB) parts to swell during saturating stage while upon depressurization their Tg 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.
Emerging interests in hardware security as well as environmental concerns have given rise to the field of transient or temporary electronics, which can be decommissioned by an external stimulus with minimal impact to the surrounding environment. In this study, an all graphene based film is produced by a one-step deposition process. The conversion of graphene oxide (GO) to reduced graphene oxide (rGO) depends on an interfacial reduction reaction. Control of processing conditions such as the underlying substrate, pH of GO and the film drying environment results in an ability to tailor the internal architecture of the films and their electronic properties. Furthermore, the ability to create masks for selective reduction of GO during deposition was also demonstrated, which was used to create intricate yet well-defined patterns and connections required in electronic circuits and devices. All graphene based freestanding films with selectively reduced GO were used in transient electronics application as circuitry and RFID tag patterns. Furthermore, the reversible and controllable hygromorphic actuation ability of GO films is demonstrated along with application in humidity sensing.
Emerging interests in hardware security as well as environmental concerns have given rise to the field of transient or temporary electronics, which can be decommissioned by an external stimulus with minimal impact to the surrounding environment. In this study, an all graphene based film is produced by a one-step deposition process. The conversion of graphene oxide (GO) to reduced graphene oxide (rGO) depends on an interfacial reduction reaction. Control of processing conditions such as the underlying substrate, pH of GO and the film drying environment results in an ability to tailor the internal architecture of the films and their electronic properties. Furthermore, the ability to create masks for selective reduction of GO during deposition was also demonstrated, which was used to create intricate yet well-defined patterns and connections required in electronic circuits and devices. All graphene based freestanding films with selectively reduced GO were used in transient electronics application as circuitry and RFID tag patterns.
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