Preparation And Performance Of High Phase Change Enthalpy And High Thermal Conductivity Polyethylene Glycol-based Shape-setting Phase Change Material

Phase change materials(PCMs)have important applications in solar energy storage,energy saving buildings,and smart textiles due to their high energy storage density and near isothennal heat storage.Commonly used solid-liquid Phase change materials have problems such as melt leakage and low thermal conductivity.It is an effective solution to this problem by converting them into form stable phase change materials(FSPCMs)by physical or chemical methods.FSPCMs have received increasing attention in recent years.However,FSPCMs caused decreasing phase change enthalpy compared to pristine PCMs in different extents.On the other hand,there is still a lack of cheap and convenient method to significantly improve the thermal conductivity of FSPCMs.Currently,how to obtain FSPCMs with both high phase change enthalpy and high thermal conductivity is an important challenge in the field.In this work,Polyethylene glycol(PEG)-based FSPCMs with high phase change enthalpy and high thermal conductivity have been successfully prepared by combining chemical cross-linking,synergistic crystallization and metal foam as thermal conductive network.Specifically,this thesis includes the following three parts:(1)A series of acryloxy terminated polyethylene glycol(PEG)with different molecular weights and functionalities are prepared by esterification of PEG with acryloyl chloride.The products can undergo free radical polymerization for the preparation of FSPCMs with chemical cross-linking structure.The effect of end group modification on the phase transition properties of PEG was investigated.It was found that it can significantly reduce the degree of subcooling of the PEG having double hydroxyl groups at its end,but has no effect on polyethylene glycol monomethyl ether(mPEG)having only one hydroxyl group at its ends.Further studies indicate that transforming the hydroxyl groups of PEG and mPEG to acetoxy and-Cl groups using acetyl chloride and thionyl chloride has similar effects on subcooling.The mechanism may be that the change of the PEG end group affects the hydrogen bond network between the molecules.(2)PEG-based FSPCMs with different chemical crosslinking network structures were prepared based on acryloxy terminated PEG monomers of different molecular weights and functionalities.Chemically crosslinked PEG-based FSPCMs with PEG moieties locating at the backbone were prepared from PEG diacrylate(PEGA).PEG-based FSPCMs with PEG moieties in the side chain were prepared by copolymerization of PEG monomethyl ether acrylate(mPEGA)and divinyl benzene(DVB).FSPCMs of different crosslinking densities were obtained by varying the molecular weights of PEGA and mPEGA.The effects of different chemical cross-linking structures on the phase transition properties,crystal structure and mechanical properties of PEG-based FSPCMs were investigated.The phase transition characteristics and crystallization characteristics of FSPCMs are changed due to the restriction of PEG molecular chain by cross-linking structure.The influence is more obvious for FSPCMs with PEG moieties on the main chain rather than the side chain.As the crosslinking density decreases,the reduction ratio of Phase change enthalpy compared with that of pristine PEG decreases.When the crosslinking degree of the main chain type PEG-based FSPCMs is 33 mol/m3,the melting enthalpy {?Hm)is 115.5 J/g,decreasing by 28.26%The main chain type PEG-based FSPCMs have excellent tensile properties,while the side chain type PEG-based FSPCMs are very weak in mechanical properties.As the crosslinking density decreases,the tensile strength of the main chain type PEG-based FSPCMs increases first and then decreases,while the elongation at break increases monotonously.When the degree of crosslinking is 95 mol/m3,the mechanical properties of FSPCMs are optimal,having a tensile strength of 18.10 MPa,and the elongation at break is 672%.(3)PEG-based FSPCMs with high phase change enthalpy and high thermal conductivity are prepared by combining chemical crosslinking,synergistic crystallization and metal foam as thermal conductive network.P(PEG4000A)was selected as a crosslinking network,and additional pristine PEG was introduced to enhance the phase change enthalpy of PEG-based FSPCMs by synergistic crystallization.The maximum amount of extra PEG that P(PEG4000A)crosslinked network can accommodate,and the effect of different content of pristine PEG on the phase transition and crystallization characteristics of FSPCMs are investigated.In addition,composite FSPCMs with high phase change enthalpy and high thermal conductivity were prepared by impregnation of the FSPCMs in copper foam which has high thermal conductivity.The effect of the porosity of copper foam on the thermal conductivity of the FSPCMs is studied.It is found that the thermal conductivity of the FSPCMs increases with decreasing porosity of copper foam.When the porosity of copper foam is 93.58%,the thermal conductivity of FSPCMs reaches 3.96 W/m·K,which is 13.7 times higher than that of the original PEG(0.29 W/m·K).Meanwhile,a high phase change enthalpy(?m=154.5 J/g)is also obtained.