The Preparation And Properties Of Colloidal Crystals With Well-defined Hairv Microparticles

The preparation of colloidal crystals with hairy microspheres as building blocks was mainly investigated in this paper. The hairy microspheres grafted with environmental sensitive brushes were synthesized by Reversible Addition-Fragmentation Transfer (RAFT) polymerization in three steps. Firstly, the monodisperse polystyrene (PS) microspheres were obtained by soap-free emulsion polymerization. Secondly, the hydroxyethyl methacrylate (HEMA) containing hydroxyl groups was dropwised into reaction system and loaded on the surface of PS microspheres. Then, acyl chloride group of RAFT agent reacted with hydroxyl groups and the RAFT agent was anchored on the surface of PS microspheres. Finally, the monomer acrylic acid (AA) and photoinitiator were added into the reaction system. Then the pH-sensitive PAA brush which is collapsed under acidic condition and stretched in the base was obtained by UV light-initiated RAFT polymerization. With the same method, the thermo-sensitive PNIPA brush was prepared successfully by adding functional monomer N-isopropyl acrylamide (NIPA) and photoinitiator. The PNIPA brush is collapsed when the temperature is high and stretched when the temperature was low (below32℃). The above polymerization process demonstrated the characteristics of living free radical polymerization. After that, the above two kinds of hairy microspheres were used as building blocks to prepare the colloidal crystals by vertical deposition method, respectively. The optical properties of the colloidal crystals and the influence of microspheres particle size, emulsion concentration, pH value or temperature, the thickness of brushes and the substrate on the self-assembly were investigated. The results indicated that four kinds of PS microspheres (170nm,251nm,330nm and697nm) can self-assemble into ordered arrangement, but it’s much easier to form ordered arrays for the larger particles. PS microspheres can self-assemble into a face-centered cubic close packed (FCC) of [111] arrays not only on the surface of colloidal crystals, but also in their sections. What’s more, with the increase of particle size, the position of the diffraction peak occurred red shift, as a result, colloidal crystals presented different colors. When hairy microspheres grafted with PAA brushes were taken as building blocks, the distance between colloidal crystals’ particles increased and the original regular arrangement was disturbed because of the strong interaction among these brushes. However, the particles were re-arranged into ordered [100] plans by adjusting the emulsion concentration to a suitable value. Besides, the arrangement could also be controlled by adjusting pH value, when the pH value is equal to10,[100] plans were obtained and when the pH is equal to4,[111] plans were obtained. When hairy microspheres grafted with thermo-sensitive PNIPA brushes were taken as building blocks, the ordered [111] plans were also obtained with the temperature of40℃during the self-assembly process. Reducing the temperature to20℃, the distance between particles increased and the arrangement was disturbed due to the effect of the interaction among PNIPA brushes. When trying with different substrates, colloidal particles can self-assemble into [111] plans on the mica.On the other hand, the preparation of crosslinked colloidal crystals with hairy microspheres as building blocks was also investigated. The hairy microsphere containing a PS microsphere grafted with copolymer brushes was synthetized by conventional free radical polymerization. Firstly, the monodispersed PS particles loaded with photoinitiator were obtained by the same method as mentioned above. Secondly, functional monomers AA and GMA were added, then the copolymer PAA-co-PGMA brush was synthetized by UV light-initiation. After that, this hairy particles were used as building blocks and colloidal crystals were prepared by vertical deposition method. Finally, when the solvent evaporated to a certain state, the carboxylic acid groups reacted with epoxy groups by increasing the temperature, then, a crosslinked colloidal crystal was obtained. The results showed that the hardness of the film, increased from0.055GPa to0.067GPa and a Young’s modulus from2.03GPa to3.30GPa after crosslinking. In addition, the mechanical strength also increased and could be peeled off from the substrate. While, colloidal crystals hardly have any mechanical strength on the surface of substrate before cross-linking.