Investigation Of Thermo-responsive Microgels With Linear Transition Behavior And Embedded Mechanism

Recently,thermo-responsive microgels have attractted more and more attentions due to their potential applications in the field of smart textiles,drug controlled release,environmental protection and so on.In this paper,the thermo-responsive microgels with linear transition behavior were prepared by precisely designing the core-shell structure of microgels.Because of the unique transition behavior,these microgels can be used as the nano-actuators and nano-sensors.In this paper,various types of thermo-responsive microgels and temperature and pH dual responsive microgels were prepared by selecting different monomers.The structure and transition behaviors of these microgels were thoroughly investigated,including partical size,size dispersion and thermal sensitivity.Moreover,the linear transition behavior mechanism of core-shell microgels was preliminary explored.The main results obtained are presented as following:The core of microgels was prepared by precipitation polymerization of poly（N-isopropylacrylamide）（PNIPAM）.Afterwards,the shell was further synthesized on the PNIPAM microgels via precipitation polymerization of poly（ethylene glycol）methyl ether methacrylate(OEGMA₃₀₀).The properties of PNIPAM microgels are first investigated,showing that the obtained PNIPAM microgels are thermo-responsive.It should be noted that the crosslinker N,N-methylene bis-acrylamide（MBA）significantly influence the properties of microgels.Increasing the amount of MBA results in a diminishing transmittance,pariticle size and shrinking ratio of PNIPAM microgels.The structure investigation of PNIPAM/POEGMA₃₀₀ microgels confirms that the microgels possess core-shell structure.These microgels show a linear transition behavior between 35 oC and 65 oC,which is realized by the dragging effect of the collapsed PNIPAM core to the POEGMA₃₀₀ shell when the temperature is above the volume phrase transition temperature（VPTT）of PNIPAM.The properties of the core-shell microgels are affected by the amount of MBA as well.When the amount of MBA is decreased,the transmittance,particle size,and shrinking ratio of microgels all present a increasing tendency.Moreover,the absolute value of the linear fitting slope of the particle size is bigger,indicating the linear response of the core-shell microgels is more prominent.Besides PNIPAM/POEGMA₃₀₀ microgels,poly（N-isopropylmethacrylamide）（PNIPMAM）and 2-（2-methoxyethoxy）ethyl methacrylate（PMEO₂MA）are also used to prepare core and shell of the PNIPMAM/PMEO2 MA microgels,respectively.The preparation process is similar as the former PNIPAM/POEGMA₃₀₀ microgels.Unlike the PNIPAM/POEGMA₃₀₀ microgels previously investigated,PNIPMAM core possesses a higher VPTT than that of PMEO2 MA shell.For this reason,PMEO2 MA shell will collapse and press the PNIPMAM core when the temperature is above the VPTT of PMEO₂MA（25 oC）.During the increase of temperature from 25 oC to 45 oC,the pressure of the PMEO2 MA shell to the PNIPMAM core will continue,inducing the transition behavior to be linear as well.It should be noted that the cross-linker MBA also influence the particle size and shrinking behavior of the microgels as well.Furthermore,in order to realize pH-responsive property in the PNIPAM/POEGMA₃₀₀ microgels,monomer acrylic acid（AAc）was added during the preparation of the POEGMA₃₀₀ shell.The obtained PNIPAM/P(OEGMA₃₀₀-co-AAc)microgels present both thermo-and pH-responsive behaviors.The microgels show a linear transition behavior in the temperature range between 35 oC and 65 oC,which is similar as that of PNIPAM/POEGMA₃₀₀ microgels.Additionally,when the pH is below the pKa of PAAc,the microgels is thermo-resposive.On the contrary,when the pH is above the pKa of PAAc,transition behavior of the microgels is not observed,indicating the microgels do not possess thermo-responsive property in this scenario.