Poly (N-isopropylacrylamide) Based Hybrid Microgel Stabilized Water-in-Oil Pickering Emulsion

Microgel-stabilized Pickering emulsions have received extensive attention due to their good stimulus responsiveness and stability.However,microgels are too hydrophilic to stabilize w/o Pickering emulsion.The w/o emulsion can be prepared with specific oil phases,binary particles or surface-modified microgels,but there are issues such as oil limitation,too many influence factors,and complicated methods.Therefore,it is crucial to propose a simple method to hydrophobize microgels to stabilize w/o emulsions.In this work,hydrophobic poly（N-isopropylacrylamide）based hybrid microgels were prepared by inverse Pickering emulsion template,and w/o Pickering emulsions were successfully stabilized with the hybrid microgels.Mechanism of the formation and phase inversion of the w/o Pickering emulsion is revealed.Finally,the application of the Pickering emulsion stabilized with the hybrid microgel is developed.The main results are as follows:（1）Controllable preparation and characterization of PNIPAM-Based hybrid microgelsThe hydrophobic PNIPAM-based hybrid microgels were prepared in one step by inverse Pickering emulsion template.The size and structure of hybrid microgels were studied in relation to parameters such as oil type,emulsification method,particle concentration,oil-water ratio,and crosslinker concentration.It’s worth noting that PNIPAM-based hybrid microgels can disperse well not only in water,but also in a variety of organic solvents.The contact angle revealed that the surface wettability of the water-dispersed and oil-dispersed hybrid microgels differs.The PNIPAM-based hybrid microgels can be loaded with dyes or nanoparticles and dispersed in oil,and the Fe₃O₄ nanoparticles can provide magnetic responsiveness to the hybrid microgels.Furthermore,the PNIPAM-based hybrid microgels have good redispersibility and can be stored in solid form.The good oil dispersibility of PNIPAM-based hybrid microgels makes them suitable for water removal from biodiesel.（2）Preparation of w/o Pickering emulsions stabilized with PNIPAM-based hybrid microgelsThe w/o Pickering emulsion was successfully stabilized with hydrophobic PNIPAM-based hybrid microgels.The microstructure and dynamic interfacial tension of Pickering emulsions suggest that the hybrid microgels are firmly adsorbed at the oil-water interface and reduce the interfacial tension.The effect of water-oil volume ratio on Pickering emulsions was studied.The type of Pickering emulsions stabilized by the hybrid microgels could be switched faciley by adjusting the oil/water volume ratios.PNIPAM-based hybrid microgels are temperature-sensitive.The ambient temperature rises,the volume shrinkage of the hybrid microgel leads to the denser surface coverage of the hydrophobic sites and thus the hybrid microgel would become more hydrophobic.Therefore,the Pickering emulsion stabilized with the PNIPAM-based hybrid microgel is temperature-responsive and can be transformed between w/o,o/w,and demulsification.The temperature phase inversion points of emulsions with different water-oil volume ratios were studied,and phase inversion diagrams were obtained.Furthermore,the hybrid microgels dispersed in different organic solvents were all able to stabilize different w/o Pickering emulsions.（3）Application of w/o Pickering emulsions stabilized with PNIPAM-based hybrid microgelsThe Pickering interfacial catalytic system was developed by w/o Pickering emulsions stabilized with PNIPAM-based hybrid microgels.As the concentration of the hybrid microgels increases,the catalytic efficiency of the Pickering emulsion increases significantly.The magnetic hybrid microgels endow the Pickering catalytic system with magnetic responsiveness,which facilitates the separation and circulation of the system.After ten cycles,the catalytic efficiency hardly changed.The phase inversion release of the Pickering emulsions stabilized with hybrid microgels was developed.Emulsions stabilized with pure microgels and hydrophobic silica nanoparticles could not achieve this application due to the phase inversion could not occur.The magnetic hybrid microgels can be adsorbed on the droplet interface of the surfactant-stabilized w/o emulsion,and the sorting of emulsion droplets can be realized under a static magnetic field.