Preparation Of Uniform Polymer Microspheres And Their In Situ Self-assembly

The self-assembly of nano-sized and microsized spheres has attracted increasing interest because such behavior can result in 2/3-D ordered structures, which endow materials with special physical and chemical properties. These assembled spheres with highly ordered superstructures have been demonstrated to be useful in many fields. Since the last two decades, various methodologies have been developed for the fabrication of different types of colloidosomes. A common point for the reported studies on colloidosomes is that several steps were required to achieve the expected structures, regardless of the final objectives. The primary particles must be prepared prior to their assembly; these primary particles were practically all surface charged, and the charge density must be carefully tailored in order to make their assembly occur at the interface of the template droplets. In this work, colloidosomes of polymer particles were achieved through a simple one-step route by the in situ self-assembly of primary particles in the precipitation polymerization of methyl methacrylate(MMA) and crosslinker monomer in a H2O-EtOH mixture in the presence of ethyl benzene(EB) as the droplet template.Briefly, polymerizations were carried out at 70℃ with 2 m L of the monomers trihydroxymethyl propane triacrylate(TMPTA) and MMA, with 2 wt% of azobisisobutyronitrile(AIBN) relative to the monomers as the initiator, in 98 m L of a ternary solvent mixture consisting of distilled H2 O, anhydrous ethanol(EtOH) and ethyl benzene(EB). The size and morphology of the colloidosomes were examined using scanning electron microscopy(SEM). The microspheres were separated by centrifugation at 12000 rpm. The supernatant was dried up to evaluate the yield of the soluble polymer by gravimetry. The soluble polymer in EB and in H2O-EtOH was also determined.The prerequisite for the in situ formed primary particles to self-assemble is the presence of the template droplets of a third solvent not miscible with the polymerization medium, i.e. H2O-EtOH. To find an appropriate solvent, 6 solvents of different polarities, including cyclohexane, butylbenzene, xylene, benzene, toluene and EB, were tested. It was found that the self-assembly of the primary particles was detected only when xylene or EB was used as the template solvent, particularly for EB.It is easily perceived that the amount of water in the binary solvent played an important role, because the water amount could greatly change the solubility of the oligomers, and therefore impose an influence on the nucleation of the primary particles; At the same time, the water amount in the solvent could also affect the solubility of EB in the binary solvent, and thus imposes effects on the formation of EB droplets. To this end, polymerization was carried out with 14 mL of EB and varied H2O-EtOH volume ratio from 1/9 to 6/4 under otherwise the same experimental conditions. It was seen that the self-assembly did not occur at low H2O-EtOH ratios(1/9 and 2/8), and the colloidosomes of the primary particles were observed only when this ratio reached 3/7.The experiments were continued to optimize the EB amount for the self-assembly, with EB varied from 6 to 30 mL, while the total volume of the ternary solvent was kept unchanged at 98 m L, and the volume ratio of H2O-EtOH was 3/7. With EB varied from 14 m L to 26 mL, the assembled colloidosomes possess a rough surface with the primary particles obviously protruding from the surface; whereas those with a quite smooth surface were observed when EB amount was beyond the limits from 14 m L to 26 m L.To explore the impact of TMPTA on colloidosomes formation, a set of polymerizations was carried out using varied TMPTA amounts from 0 to 80 wt% in the monomer mixture with the other polymerization conditions unchanged. There were no polymer microspheres being found without the use of TMPTA; and smooth aggregates of the primary particles were detected for the run with high TMPTA. In the rest of the cases, all primary particles were assembled, leaving very few of them remaining unassembled.Polymerizations were also conducted to elucidate the appropriate polymerization time while keeping all the experimental conditions the same as described in the previous section and with TMPTA fixed at 40%. Self-assembly of the primary particles was time dependant. At 0.5 h of polymerization, only tiny individual primary particles were present; initial self-assembly of the primary particles was observed at 1 h of polymerization time, and the surface of the aggregates of the primary particles were pretty smooth. With extended time, the assembly was in progress with a gradual increase in size combined with a rougher surface.