Preparation Of Polyurea Microspheres By Precipitation Polymerization And Formation Mechanism Of The Microspheres

Monodisperse polyurea microspheres,prepared by precipitation polymerization without surfactant or stabilizer,are surface-clean and have attracted great interests.Most of the polymer microspheres reported so far have been prepared through the free radical polymerization,of which the production has been extremely low because of the low monomer loading,leading to very low yield of the microspheres and long polymerization time.The development of the polymer microspheres has been therefore severely restricted.In this work,monodisperse polyurea?PU?microspheres were prepared by stepwise precipitation polymerization with isophorone diisocyanate?IPDI?as the monomer,and the polymerization was conducted in water and N,N-dimethylformamide?DMF?as the solvent under quiescent conditions,i.e.without stirring or shaking with the reactor standing still in a water bath ser at desired temperature.The experimental conditions were optimized,and the mechanism of microsphere formation and growth was also studied.Microspheres were characterized by optical microscopy and scanning electron microscopy?SEM?.The results showed that the size and the yield of the microspheres increased with an increase in monomer loading and in the amount of DMF in the mixed solvent of DMF-H₂ O.With the temperature of polymerization fixed at 30 oC and H₂O/DMF mass ratio at 25/75?i.e.75 wt% of DMF in the binary solvent?,the maximal IPDI loading allowed to have monodisperse microspheres was 3.0 wt%;This allowed IPDI loading was increased to 6.0 wt% when DMF in the mixed solvent was further increased to 85 wt%.When the reaction temperature was increased from 30 °C to 50 °C while the rest of the experimental condition was unchanged,the maximal monomer loading allowed to have monodisperse microspheres was practically the same.DMF has an obvious catalytic effect on the polymerization.The microsphere size and yield reached their top value within 15 min of polymerization time and remained unchanged with extended polymerization.This polymerization time of 15 min to reach the final size and the yield of the microspheres was obviously shortened comparing with the polymerization conducted in H₂O.The precipitation polymerization in this work is based on the stepwise polymerization by its mechanism,which is different from that of free radical polymerization.In order to understand better the polymerization,the formation and growth mechanisms of the microspheres were studied.For this purpose,the microspheres at different stage of the polymerization must be separated so that to their characterization.To this end,three methods of microspheres separation were designed.It was found that the microsphere separation by quick filtration by a membrane with pore size of 0.22 ?m was the most effective method.The separation was fast?about 5 s?and the results were reproducible.Through this separation,the size of the microspheres obtained at 63 min of polymerization was 3.226 ?m and the yield of the microsphere was 2.5%.The microsphere size and the yield increased rapidly within the following 25 min.The size reached 8.814 ?m with the yield increased to 65% at 90 min of the polymerization.These increases in the size and the yield of the microspheres indicated that the polymerization was progressing rapidly in the time period of 63 min to 90 min.The number of the microspheres remained substantially constant?7.6 × 10⁷ / m L?during this period,and the size of the microspheres was kept uniform without aggregation observed by SEM and optical microscopy.The growth of the microspheres was achieved by absorbing oligomers or by reaction of the oligomers with the functional and residual groups on the surface of the microspheres.Once the polymerization extended beyond 90 min,the microspheres started to precipitate out from the system and deposited to the bottom of the reactor,and new polymer particles were formed,,the number of the microspheres was increased?to 8.9 × 10⁷ / mL?with also a deterioration in the monodispersity of the microspheres.