Study On Controllable Nucleation And Particle Size Growth During Styrene Emulsion Polymerizatio

This paper originated from the national natural science fund project "Secondary nucleation mechanism of one step preparation of high solids polymer emulsions”.One-step method of producing high solids emulsions uses secondary nucleation in the interspaces of large particles,to form part of small particles,and finally results in bivariate distribution.The purpose of this paper is to synthesize monodisperse emulsions with certain number and size of latex particles.For this purpose,the process of polymerization is divided into two stages,a)the stage of nucleation:the preparation of seed latex with certain solids content;b)the stage of particle size growth:adding monomer,surfactant and initiator into the seed latex to keep the number of particles unchanged,and make the particle size grow,then the solids content of latex increases.The effects of concentrations of insoluble monomer(styre ne)and anionic emulsifier(lauryl sodium sulfate)on the nucleation modes,number and size of latex particles in the two stages are discussed in detail.The results are as follows:1.Effects of emulsifier concentration on the nucleation modes,particle number and size1)Effects of emulsifier concentration on the nucleation modes,number of particlesThe stage of nucleation:a)When the emulsifier concentration is below its critical micelle concentration(CMC),the primary particles formed earlier will agglomerate together.This can deduce that those latex particles are produced via homogenous nucleation.The intensity of the agglomeration would "self-adjust" according to different emulsifier concentrations to keep the surface coverage between 20%-40%.The number of latex particles is proportional to the surfactant concentration under this condition.b)When the emulsifier concentration exceeds its CMC,the agglomeration does not exist,so those particles are produced via micelle nucleation.And the number of particles is proportional to 0.6 exponential of the emulsifier concentration([S]0.6).The stage of particle size growth:As the surfactant added can supply the needs for covering new surface during particle growth,the number ofparticles is invariable.That means no new particles form.2)Effects of emulsifier concentration on particle sizeThe stage of nucleation:The polymerization rate in a single particle increases with the decrease of the emulsifier concentration.So the particle size also increases.However the total rate of reaction will decrease.Hence the low emulsifier concentration will be disadvantageous for the total polymerization.The stage of particle size growth:When the monomer is sufficient and the surfactant amount is well controlled to limit new particles formation,the particle size is proportional to the reaction time.2.Effects of monomer concentration on the nucleation modes,particle number and size1)Effects of monomer concentration on the nucleation modes,number of particlesThe stage of nucleation:When the emulsifier concentration is below its CMC,the primary particles will agglomerate together,so those particles are produced via homogenous nucleation.However,the increase of the monomer concentration will weaken the agglomeration.It can lead to a slight increase of the particle number.But the influence of monomer concentration is not so distinct compared to that of the emulsifier concentration.The stage of particle size growth:If the rate of dropping monomer is too fast,new nucleation will occur.It can result in the increase of the particle number.2)Effects of monomer concentration on the particle sizeThe stage of nucleation:When the emulsifier concentration is below its CMC,the increase of monomer concentration can raise both the rate in a single particle and the rate of the total reaction.It can increase the particle size,so it is beneficial to obtain the large size latex particles.The stage of particle size growth:Enhancing the rate of adding monomer produces little effect on the particle size,but may cause new particles form.According to above results,it is concluded that controllable nucleation is possible using different reaction conditions.So the seed latex with expected particle number and size may be obtained.