Preparation And Characterization Of Highly Crosslinked Monodisperse Polymer Microspheres

Monodisperse and crosslinked polymeric microspheres are receiving significant attention because of their wide applications. Among available methods, precipitation polymerization is of specific interest because this process enables one to get micron-sized microspheres with clean surfaces via a single step without use of any stabilizers or surfactants. However, the development of precipitation polymerization was limited with the commonly existing problems, such as high toxicity of solvents and low yields of microspheres. In this work, low-toxic solvents ethanol is used to explore the possibility to get monodisperse microspheres with high yield. And the following are the main results.Monodisperse P(TMPTA-St) microspheres were synthesized by precipitation polymerization with styrene (St) as the main monomer, trihydroxymethyl propyl triacrylate (TMPTA) as cross-linker and azobisisobutyronitrile (AIBN) as initiator in ethanol. Results demonstrated clearly that, the monomer conversion can be improved by extending reaction time, increasing initiator concentration and crosslinker amount. Keeping 80 wt% TMPTA, 2 wt% AIBN, the mixture was purged with nitrogen for 5 min, and the polymerization was carried out under shaking rate at 120 osc./min at 70 oC for 4 h, the monomer conversion reached up to 90.17 %. Under the same precipitation polymerization condition, the monomer conversion increased to 94.44 % when 26 vol% of water was added into ethanol.By adding second solvent into ethanol gradually, the Hansen's tri-dimension solubility parameters of the mixed solvent changed continuously. Through examination on the formation of the yield microspheres and tti-dimension solubility parameters of solvent, the following was concluded: To obtain monodispersed microspheres, when the differences between the solubility parameters of the solvent mixture and those of the monomers must be controlled in the range of 4.4¹2.6 forδ(the general solubility parameter),-3.4^-2.1 forδd (the dispersion solubility parameter),4.6¹0.9 forδp (the polar solubility parameter), and7.3²0.1 MPa^1/2 forδ^H (the hydrogen bonding solubility parameter), respectively. Monodisperse P(TMPTA-St) microspheres were prepared via precipitation polymerization. Replacing TMPTA by pentaerythritol triacrylate or dipentaerythritol pentaacrylate, monodisperse microspheres were obtained, which provided solid support to the above conclusion.To understand the mechanisms of the polymerization, the concentration of initiator at different time of polymerization was determined by liquid chromatograph, and compared with the concentration of ABIN during self-decomposition unedr the same conditions. Both showed similar evolution with the former slightly larger. The residual concentration of AIBN was 73.46 % out after 4 h of polymerization with 80 wt % of TMPTA. This confirmed that the low conversion of the monomers was not due to the exhaust of the initiator AIBN.