Constructing Lignin-Modified Nanocellulose Encapsulated Phase Change Microcapsules By Pickering Template Method And Its Application

It is necessary to utilize energy effectively although human existence depends on energy.As society gradually industrializes,energy resources are becoming increasingly scarce.Phase change materials(PCMs)have demonstrated promising potential for energy storage due to their latent heat that can be utilized to store and release energy.However,PCMs are vulnerable to leakage during the phase change process.To combat this problem,researchers prefer to encapsulate PCMs in microcapsules,which also improve PCMs utilization.Cellulose nanofiber(CNF)has been employed to stabilize Pickering emulsions because of its high aspect ratio,amphiphilicity,and flexibility.However,due to the excessive hydrophilicity of CNF,it is difficult to produce high internal phase emulsions because of their high viscosity.Lignin is also a biomassbased surfactant,characterized by a hydrophobic backbone consisting of phenyl propyl alkyl groups,whereas its hydrophilic side chains are composed of hydroxyl and methoxy groups,which allows it to effectively reduce the surface tension at the interface between oil-water interface.Therefore,the possibility for CNF to modify the hydrophobicity by complexing with lignin,resulting in emulsions with high internal phase and high gravity resistance,shear thinning ability,and energy storage.In this paper,lignin was combined with CNF as a compound emulsifier to create a phase change Pickering emulsion,which is characterized by high energy storage,stable phase change,and excellent shear thinning properties.The main findings are as follows:(1)Firstly,to enhance the emulsification performance of CNF and prepare emulsions with higher oil-water ratios.This paper modified CNF with aminated lignin(AL)and successfully prepared CNF/AL phase change Pickering emulsions with oil to water ratio of 50%.As the shear rate increased in the range of 0.1～1000 1/s,the viscosity of CNF/AL phase change Pickering emulsions decreased,exhibiting typical shear thinning behavior.The phase change capabilities of paraffin were retained by the phase change Pickering emulsions,which displayed a reversible phase change process during the heating/cooling cycle in the range of 25 to 70 °C.Further,phase change Pickering emulsions of CNF/AL also exhibited excellent thermal storage properties with58.22 J/g latent heat,as compared to 241.9 J/g latent heat for microcapsules.The sensible heat of phase change emulsion was 3.20 J/(g·K)in the range of 40～54 ℃.Besides,the heat effect produced by latent heat and sensible heat combined could reach 90.98 J/g,which was higher than that of 58.8 J/g produced by the sensible heat of the water.However,the huge number of amine groups on the AL surface functions as a link between the CNF and the interaction,resulting in enhanced agglomeration of the CNF/AL phase change Pickering emulsions.(2)Secondly,to improve the emulsification performance of CNF and resolve the problem of CNF/AL emulsions agglomerating and gelling,phase change Pickering emulsions were developed by depolymerizing lignin(DL)to decrease its molecular weight and enhance surface reactivity in conjunction with CNF.With typical shear thinning behavior,the emulsion viscosity decreased as the shear rate increased in the range of 0.1～1000 1/s,and there also occurred a reversible phase change in the heating/cooling cycle in the range of 25 to 70 °C.With the phasechange characteristics of paraffin in CNF/DL phase change Pickering emulsions remaining unchanged,a latent heat storage capacity of 50.23 J/g was achieved,with a matching microencapsulation latent heat of 201.3 J/g.The sensible heat of phase change emulsion was 2.52J/(g·K)in the range of 40～55 ℃.Besides,the heat effect produced by latent heat and sensible heat combined could reach 79.11 J/g,which was higher than that of 63 J/g of water.(3)It has been demonstrated in the preceding two chapters that lignin improves the emulsification performance of CNF significantly.By combining lignin as a retained component of CNF(LCNF)and compounding with CNF,a phase change Pickering emulsion was developed.The synergistic emulsification of LCNF with CNF,as well as the stabilization of paraffin Pickering emulsions were investigated.As a result of a highly entangled network structure formed by CNF and LCNF,the stability performance of CNF/LCNF phase change Pickering emulsions improved substantially.The emulsion demonstrated typical shear thinning behavior,as the viscosity decreased as the shear rate increased in the range of 0.1～1000 1/s and the emulsion underwent a reversible phase change during heating/cooling cycles in the range of 25 to 70 °C.Additionally,the CNF/LCNF phase change Pickering emulsion had the same phase-change characteristics as paraffin and has up to 55.85 J/g latent heat storage,while the microcapsules can store up to 196.9 J/g.The sensible heat of phase change emulsion was 2.36 J/(g·K)in the range of 40～58 ℃.Besides,the heat effect produced by latent heat and sensible heat combined could reach 84.6 J/g,which was higher than that of 75.6 J/g of water.