Preparation Of PNIPAM Thermosensitive Hydrogel And Its Thermochromic Properties

With the rapid development of human society,energy consumption has increased,CO₂ emissions have risen,and the global climate has deteriorated.In 2017,the world's primary energy consumption for heating,ventilation and refrigeration accounted for43%,and the problem of building energy consumption can not be ignored.The smart window can change the solar transmittance according to changes in the external environment,thereby reducing the energy consumption of summer cooling and winter heating,which is conducive to building energy conservation.Poly?N-isopropylacrylamide??PNIPAM?is a temperature-sensitive hydrogel with a low critical solution temperature?LCST?close to room temperature.The transmittance is abrupt near the LCST and its phase transition does not need to consume extra energy.In addition,it has low preparation cost.Therefore,it is an ideal material for smart windows.However,PNIPAM Smart windows still have some problems in application:slow response rate,low temperature performance and lack of evaluation of cycle characteristics,and undeveloped phase transition mechanism.In order to solve this problems,this paper carried out the following research:A series of different PNIPAM microgels were prepared by soap-free emulsion polymerization under different cross-linking agent dosages,and applied to smart windows.The microgel-type smart window completes visible color to the naked eye within 5 s.The response rate is greatly improved.After the phase change of the PNIPAM hydrogel,the ordinary PNIPAM smart windows have volume shrinkage,but microgel-type smart window solves this problem.The influence of the amount of cross-linking agent on the particle size,LCST and optical properties of the microgel was analyzed.The amount of cross-linking agent increased,the particle size of the micro-gel decreased,the LCST of the microgel increased,and the visible light transmittance of the microgel type smart window decreased before the phase change.A series of interlayers with different thickness and different PNIPAM content were prepared,and transmittance of this interlayers was tested.The effects of PNIPAM content and interlayer thickness on the optical properties of smart windows were investigated.The influence of interlayer thickness and PNIPAM content on the solar window modulation efficiency of smart window is the same:within a certain range,the interlayer thickness or PNIPAM content increases,and the solar modulation efficiency is enhanced.After forty times thermal cycles,the smart window still maintains good solar modulation efficiency.Seven times of low temperature refrigeration cycle,it is found that low temperature freezing has no effect on the cycle characteristics and solar modulation efficiency of PNIPAM microgel smart window,showing good frost resistance performance.In view of the current undeveloped mechanism of PNIPAM hydrogel phase transition,in order to better develop and utilize PNIPAM hydrogel,the temperature sensitivity mechanism of the PNIPAM hydrogel is further studied by computer simulation.First,a monomer model was constructed to analyze the molecular structure and charge distribution.Then,the behavior of the monomer unit before and after the phase change in the micro-solution was simulated by molecular dynamics.Finally,the PNIPAM polymer chain was constructed and the transformation of the molecular chain conformation before and after the phase transition was simulated,which explains the phase transition mechanism of PNIPAM gel.