Synthesis And Properties Of Monodisperse Crosslinked Polystyrene Microspheres

The monodisperse crosslinked polymer microspheres have received much attention because of their superior thermal and solvent resistance, mechanical strength, surface activity and adsorption properties. They are of wide prospects for using in many fields such as biomedicine, electronic science, information technology, analytical chemistry, standard measurement and environment protection etc. In this paper, mono-size crosslinked polystyrene (CPS) microspheres were firstly prepared by dispersion polymerization using crosslinker-post-addition method. During the experiments, some pretty polymerization systems were chosen, and then, the influences of polymerization parameters on system stability, particle size and size distribution were investigated to find out the reasonable polymerization way of monodisperse CPS microspheres.A series of experiments were conducted to get the effects of polymerization parameters on particle size and size distribution. In the ethanol/water system, the average size of CPS microspheres increases with increasing initial monomer concentration, initiator concentration, crosslinker concentration, the ratio of ethanol/water, the agitation rate and it also increases with the decrease of stabilizer concentration. Using crosslinker-post-addition method could lead to the preparation of monodisperse CPS microspheres, whose size increases with the delay of the point to carry out post-addition but decreases with extending the addition time. However, the size almost keeps constant with the increasing amount of ethanol in the post-addition recipe. Through the properties characterization, it is found that the crosslinking degree of microsheres obtained reaches the maximum of 92%. The glass transition temperature and thermal decomposition temperature reach up to 105.7℃and 428.7℃, respectively.The kinetic characteristics and mechaninsm of dispersion polymerization with crosslinker-post-addition method were primarily discussed. The results showed that the primary polymerization rate and final conversion increase with the increase of crosslinker and initiator concentration. The existence of PVP-g-PS graft copolymer and physics adsorption of stabilizer were found on the surface of polymer microspheres by Fourier transform infrared spectroscopy (FTIR) and Energy-dispersive spectroscopy (EDS); the occurrence of copolymerization of Styrene (St) and divinylbenzene (DVB) leads to the remaining of some double bond on the surface of or inside the microspheres. Based on the experimental results, the mechanism of dispersion polymerization using crosslinker-post-addition method was proposed. In the beginning, monomer, stabilizer and initiator all dissolve in the dispersed medium. As soon as the polymerization begins, primary radicals, generated by decomposition of the initiator, grow in the continuous phase by the addition of monomer units until they reach their critical chain length where they precipitate to form nuclei. In this process, the chain-transfer reaction of oligomeric radicals with PVP molecules leads to the generation of some PVP-g-PS molecules. The nuclei formed are unstable and aggregate with each other and finally become stable particles which are stabilized by the spatial steric hindrance of PVP-g-PS molecules and the stabilizer adsorbed from the medium. Subsequently, the crosslinker DVB was brought into the reaction system. One part of DVB with monomer, initiator and radicals were absorbed by the particles and polymerized inside them, the other DVB polymerized or copolymerized with monomer in the medium generating oligomer radicals which were soon captured by the existing particles. The microspheres grow by the above ways until the exhaustion of the monomer and crosslinker.The preparation of highly crosslinked monodisperse polystyrene microspheres by distillation-precipitation polymerization was also studied and the influences of various parameters on particle size and size distribution were investigated. The results show that the size and yield of microspheres increase with the increasing total monomer concentration, crosslinker concentration and initiator concentration. The particle size distribution decreases and yield of particles increases with the increasing amount of ethanol in the solvent mixture. The crosslinker reaches the maximum concentration of 63.8% based on the total volume of monomer, which demonstrates the feasibility of distillation-precipitation polymerization to prepare highly crosslinked monodisperse polystyrene microspheres. At last the similarities and differences of distillation-precipitation polymerization and dispersion polymerization were briefly summarized.