Determination Of Monomer Concentration In Particles During Soap-free Emulsion Polymerization

Large-diameter granules and monodisperse high-molecular-weight polystyrene microspheres are with properties of strong adsorption,transportation,aerospace,medical,pharmaceutical,building materials,printing,plastic additives due to their strong adsorption,large specific surface area and strong cohesive properties.It is widely used in many fields such as seawater desalination,agriculture,forestry and horticulture.The method of preparing polymer microspheres can be divided into physical methods and chemical methods.The physical method is to prepare a polymer microsphere by spray drying a polymer solution or precipitating a precipitant,or to prepare a microsphere by dropping and cooling the polymer melt through a capillary.The chemical method is automatically converted into microspheres during the formation of the polymer,and various polymerization methods such as emulsion polymerization,suspension polymerization,soap-free emulsion polymerization,dispersion polymerization,and precipitation polymerization are mainly used.The microspheres prepared by physical methods are often polydisperse,and large-scale industrial production is difficult.Therefore,the chemical method is the main preparation method of polymer microspheres.However,due to the complexity of the chemical system,the polymerization mechanism has been controversial.Now the mechanism that can fully explain the experimental results of chemical methods is the nano-submicron monomer droplet nucleation and monomer transport theory proposed by Ni et al.This theory unifies the mechanism of all chemical methods,but its kinetic model has not yet been established.Therefore,in order to explore the dynamics law,it is necessary to study the monomer transport during the polymerization process and the particle size and distribution of the microspheres.In this paper,a simple styrene soap-free polymerization and its seed polymerization system were selected.The effect of polymerization conditions on the particle size and distribution of microspheres was studied with potassium persulfate as initiator.The research contents are as follows:(1)The effects of initial styrene monomer mass fraction,initiator polymerization temperature,stirring speed,initiator concentration,mass fraction and other reaction conditions on the dispersibility and diameter of polystyrene microspheres were investigated.The appearance and monodispersity of the microspheres were characterized by scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The conclusion is as follows: within a certain range,the mass fraction of monomer increases,the diameter of PS microspheres increases;the mass fraction of initiator increases,the particle size of PS microspheres decreases gradually;with the increase of stirring speed,the PS diameter of the microspheres decreased;as the temperature increased,the PS microspheres showed an increasing trend.Through a large number of experiments,the ideal conditions and formula of soap-free emulsion polymerization were screened out,and monodisperse PS with diameters of 100 nm and 460 nm was prepared as the seed of the second step.(2)The first monomer single-swelling soap-free seed emulsion polymerization method is used to investigate the ability of seed microspheres to control monomer during polymerization.That is,traditional seed polymerization is to disperse seeds in water,and this paper directly disperses seeds in monomer.In the middle,the seeds are allowed to swell and then seed polymerization is carried out.The effects of monomer to seed mass ratio,temperature,stirring speed and reaction time on the dispersibility and diameter of polystyrene microspheres were discussed.As a result,it was revealed that the monomer concentration and the polymerization temperature were too large,and the particle diameter of the polymerized polystyrene microspheres was increased,but the monodispersity was deteriorated.Therefore,the monomer to seed mass ratio ranges between 1 to 5,and the polymerization temperature ranges between 65 to 75 ?.The increase of reaction time results in increases of the diameter of the polystyrene microspheres,and the monodispersity is relatively uniform.If the stirring speed is too large or too small,the monodispersity of the polystyrene microspheres is deteriorated,so the stirring speed is preferably between 100 to 550 rpm.