| Monodisperse polymer microspheres and supraparticles fabricated from microsphere assembly had significant application values in many fields. The traditional methods to prepare monodisperse microspheres all needed surfactants or stabilizers, which made the components of the polymerization system more complicated and led to microspheres with lower purity. Precipitation polymerization developed in the 1990's made a breakthrough in the preparation of monodisperse microspheres because it didn't need surfactants or stabilizers. However, the development of precipitation polymerization was limited with the commonly existing problems, such as high toxicity of solvents and low yield of microspheres. Facing this challenge, we used low-toxic solvents, ethanol, water and their mixture to explore the new way to obtain monodisperse microspheres with high yield. Based on this work, a facile route to achieve self-assembly of primary particles on an organic droplet was employed to prepare golf-ball-like supraparticles.First, monodisperse microspheres were prepared through precipitation polymerization of trihydroxymethyl propane triacrylate (TMPTA)-styrene (St) in ethanol and ethanol-water mixture with azobisisobutyronitrile (AIBN) as initiator when TMPTA amounts was kept at 10%～60% relative to the total mass of monomers. Impact of TMPTA amounts, initiator concentrations and water content in the solvent mixture were studied. Monodisperse microspheres were obtained within 4 hours along with higher monomer conversion. Results demonstrated clearly that increase in initiator amount and use of water as a cosolvent were indeed very effective to promote the polymerization to high conversions and to get uniform microspheres. With no water under otherwise the same experimental conditions, only about 79% of monomer conversion was detected; while monomer conversion was remarkably increased to 95% when 28 vol% of water was added into ethanol. To reach to this high conversion with no water in the medium,6 wt% of AIBN was needed.Secondly, with pentaerythritol triacrylate (PETA),pentaerythritol tetraacrylate (PETRA) or dipentaerythritol pentaacrylate (DPHA) as crosslinkers, St as co-monomer and AIBN as initiator, the precipitation polymerizations were carried out in ethanol and ethanol-water mixture. It was found that uniform microspheres were obtained when the amounts of PETA, PETRA and DPHA were kept at 5 wt%～35 wt%,5 wt%～35 wt% and 5 wt%～15 wt%, respectively. Results indicated that increasing functionality of crossslinkers was an effective method to promote the polymerization to high conversions. Using 35 wt% of PETA or PETRA and 6 wt% of AIBN, uniform microspheres and high conversions of 93% and 99% respectively were obtained by precipitation polymerization in the mixed solvents with 25 vol% of water. Under otherwise the same experimental conditions but 15 wt% of DPHA, precipitation polymerization in the mixed solvents containing 20 vol% of water led to a conversion of 77%.Thirdly, N-isopropylacrylamide (NIPAM) and an aqueous crosslinker, methylenebisacrylamide (MBAAM) were employed to prepare thermal-sensitive hydrogel microspheres via precipitation polymerization in water with ammonium persulfate (APS) as initator. Poly(NIPAM-MBAAM) microspheres with narrow size distribution were synthesized in a defined range of MBAAM levels, namely between 5% and 10% relative to the total mass of monomers. With increase in MBAAM, the polymerization conversion was significantly promoted and the swelling ratio of mcirospheres gradually decreased, which indicated that thermal-sensitivity of hydrogel microspheres was gradually weakened. Photometer was carried out to measure the thermal-sensitivity of these microspheres. It was found the hydrogel microspheres had lower critical temperatures (LCST) at 30～36℃, but the LCST occurred in a wider temperature range with increase in crosslinker. Amounts of APS imposed an import influence on morphology of poly(NIPAM-MBAAM). Uniform hydrogel microspheres were obtained when 0.5 wt%～2 wt% of APS was used with other conditions kept unvaried. In this range, adding more APS into the polymerization system led to increase of monomer conversion and decrease of microsphere size. Thermogravimetric analysis (TGA) results indicated that the structure of the microspheres was composed of two parts of polymers:one is of high-crosslinking in the core and the other is of low-or non-crosslinking on the surface layer. The experiment results demonstrated that the adsorption and desorption of protein on the microsperes could be controlled by adjusting the crosslinking of polymer, pH and temperature of the complex system of microsphere and water.Finally, an easy way to achieve self-assembly of primary particles on an organic droplet was presented. With the mixed solvents of ethanol and water as continuous phase, toluene was added into them to fabricate the droplet template. Self assembly was then optimized with regard to shaking frequency, polymerization time, toluene volume in the ternary solvent system as well as the amount of TMPTA in the monomers. Golf-ball-like supraparticles were achieved by assembly of poly(TMPTA-St) primary particle obtained from the precipitation polymerization when the experimental condition was fixed at:TMPTA amount of 20 wt%, reaction media composed of 18 mL of toluene, 56 mL of ethanol and 24 mL of water, shaking frenquency at 120/min and reaction time of 4 h. The investigation of formation mechanism of golf-ball-like supraparticles indicated that the primary particles were formed in the mixture of ethanol and water. The compatibility of the primary particles and toluene acted as a driving force making the primary particles directionally migrate into toluene droplet. The golf-ball-like supraparticles were demonstrated with a core-shell structure through the observation with scanning electronic microscope. After coating the supraparticles on the surface of the film of pressure sensitive adhesive, its hydrophobicity was significantly enhanced. |
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