Flexible smart thermo-responsive devices

Abstract

With the rapid civilization and modernization around the world, the global energy usage in buildings increases dramatically. Flexible responsive devices have been utilized to save building energy. These devices achieve responsiveness to external stimuli through the use of stimuli-responsive materials, which can detect changes in temperature, pH, light, or other environmental stimuli, and generate various output signals including shape-morphing, mechanical motions, and optical properties changes to regulate the heat entered into the building. Compared to traditional polymer materials, hydrogels offer a unique set of advantages, including softness, biocompatibility, and multifunctionality The well-researched stimuli-responsive hydrogels include thermo-, moisture-, chemical-responsive hydrogels, as well as other types. Common fabrication methods for these hydrogels involve 3D printing techniques such as direct ink writing (DIW), stereolithography (SLA), and fused deposition modeling (FDM). Particularly, thermo-responsive hydrogels, well-explored for their sensitivity to heat, have been applied in various fields like energy-efficient smart windows, actuators, and drug delivery systems. While current thermo-responsive energy saving devices exhibit promising solar light regulation, they encounter limitations in terms of flexibility, durability, and solar regulation capabilities. Consequently, there is a need for a novel design of flexible smart thermo-responsive devices to enhance both durability and energy-saving performance. In Chapter 4, a composite hydrogel based on physically crosslinked PNIPAm was created with adjustable thermochromic performance achieved by varying PNIPAm concentration (with Tlum ~90% at 15 °C and ΔTsol up to 62.3%). Laminating the hydrogel with PDMS significantly improved its durability, extending its lifespan by nine times compared to a pure PNIPAm hydrogel. Additionally, the physically crosslinked PNIPAm acted as a rheology modifier, enabling the 3D printing of free-standing bilayer structures using the direct ink writing (DIW) technique. Following the composite hydrogel that regulates vis-NIR light, stretchable silver nanowires (AgNWs) coatings were developed to regulate broadband infrared (2.5-25 μm) emissivity in chapter 5. The thin film composed of 15 layers of AgNWs on PDMS exhibited impressive low broadband infrared emissivity (εBroadband) values (0.16 at 40% stretched state, 0.34 at the released state). Moreover, chapter 6 demonstrated an off-plane reconfigurable PDMS kirigami structure with one-sided opening behavior during stretching, which held promise for exposing functional underlying layers in energy-saving devices. Finally in chapter 7, a novel solar/radiative cooling dual-control smart window was fabricated, which comprised a reconfigurable kirigami structure and a PDMS-laminated thermochromic hydrogel coated with AgNWs. The lamination of hydrogel significantly enhanced the durability, extending its lifespan by nine times compared to conventional smart hydrogels. Meanwhile, it still maintained a commendable solar transmittance modulation ability of ~24%. Furthermore, by controlling the opening and closing of the kirigami structure, an effective broadband infrared emissivity regulation of 0.5 was achieved. This thesis presents an innovative design for a flexible thermo-responsive smart window device, which showed prolonged durability and superior energy-saving performance. The outcomes surpassed those of current state-of-the-art designs reported in literature, providing inspirations for the design of future sustainable façade.