Thermosensitive Polyurethane Applied To Thermal Phase Transition Smart Windows

With the increasing demand for building energy conservation,smart windows which can adjust solar transmittance and indoor temperature have become a research hotspot in the field of energy conservation.Due to the differences of phase transition mechanism and material properties,there are many kinds of smart window materials.However,it is still a great challenge to prepare smart window materials with all desired properties,including high compliance,wide tunability and autonomous regulation.Based on the temperature sensitive polyether molecules,we have constructed the long-chain structure and cross-linked network structure of polyurethane with thermal response,and finally successfully prepared the thermotropic phase change materials with transparency at low temperature and opaque at high temperature.Furthermore,we set up multiple sets of controls and optimized molecular design to improve material properties.Thus,we selected one group of polyurethane with the best optical performance and thermal response as the phase change material for thermochromic smart windows.The specific research content of this paper includes the following two systems:The first system is waterborne polyurethane emulsion.We designed and synthesized waterborne polyurethane emulsion with thermotropic phase change based on polyether(PPG-PEG-PPG)containing hydrophilic and hydrophobic segments.Since the hydrophilic segments and amide bonds of polyurethane can form intermolecular hydrogen bonds with water molecules at low temperature,the hydrophobic interaction of the polymer will increase after the temperature rises,which causes the aggregation of colloidal particles to achieve thermally induced phase transition and results in scatter sunlight.After comparing the optical properties of different components and concentrations of emulsion,WPU-IPDI(I-0.5)emulsion is selected as a perfusion thermochromic liquid material applied to smart window.The emulsion has higher light transmittance at lower room temperature(T_lum=60.4%),which can ensure normal lighting in the room.With the increase of temperature,it changes from clear transparent to white and cloudy,and has a greater degree of solar modulation(?T_sol=54.2%).Its ultra-high water content in emulsion can realize heat storage,so as to effectively block the solar energy into the room.In addition,the emulsion can achieve thermochromism in a relatively short time,and shows good stability and insignificant thermal hysteresis in the heating and cooling cycle,which is conducive to the long-term use of thermochromic smart windows.The second system is a cross-linked polyurethane hydrogel network.We still chose polyethylene oxide/polypropylene oxide(PEO/PPO)based nonionic triblock copolymers as the soft segment,and introduced polyfunctional N-phenylaminomethyl hybrid silica(NPAM-silica)with main structure which is POSS to build a cross-linking network.The PU-POSS films were successfully synthesized and completely swollen in water to obtain highly elastic and transparent hydrogels.Due to the thermal responsiveness of the hydrophilic and hydrophobic segments and the structural stability of the chemical cross-linking network,this hydrogel has excellent thermochromic properties and elasticity,and can even exhibit excellent solar modulation in the stretched state.At the same time,the hydrogel exhibits good stability in hundreds of cold and heat cycles.In addition,the hydrogel is used as a sandwich material for small smart windows,and is made into a housing model.The result shows that this thermochromic smart window has a greater ability to adjust the indoor temperature,which proves that it has good energy-saving properties.Finally,based on the current research on waterborne polyurethane emulsion and polyurethane hydrogel,we put forward more ideas and future research directions for thermochromic smart window materials.In summary,this paper reports that two kinds of thermochromic sandwich materials for smart windows were designed and synthesized based on temperature-sensitive polyurethane.Both of which exhibit excellent solar modulation and heat shielding capabilities.This kind of polyurethane with thermally induced phase change and high water content has opened up a new way for the research of new materials in the field of smart windows.