Assembly of polymer microspheres and the thin-film Micropatterning
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Sometimes, comonomer, such as acrylic acid, is required to reduce the particle size. In this case, the colloid particles may have many surface groups introduced by surfactants and comonomers. According to the electric double layer theory of colloid, the interaction between two particles must be affected by the surface charges. It is therefore worthwhile investigating the effect of the particle surface groups on the film formation. In our experiment, different kinds and concentrations of electrolytes were added into the latex system in order to discuss their effects on the resultant particle arrays. Then metal Al film was deposited on the two-dimensional arrays of microspheres through thermal evaporation. Finally, the particles were removed in an ultrasonic bath and the substrate surface with features located where the interstitial spaces of the spheres had been was left. The polymer microspheres with different sizes can be easily prepared so that the micropatterns can be in nanosize. All the processes are simple and easy to control. EXPERIMENT Copolymer latexes(poly(styrene/acrylic acid)) smaller or larger than I 00nm were prepared by emulsion or soapless polymerization with KPS(potassium persulfate) as initiator and SDS (sodium dodecylsulfate) as emulsifier. The latex arrays were obtained in an experimental cell designed according to reference [15] for controlling of the ordering process. Polymer colloid with a given volume was spread on various substrates over the accessible area encircled by a Teflon ring with an inner diameter of 19mm. The ring was pressed against the substrate (to avoid leakage) by a brass ring and screw, and then the system was put into a vacuum desiccator. Water was allowed to evaporate at 18-19'C. Thermal evaporation of Al was performed at 5x 10- Torr chamber pressure. Then, the microspheres were dissolved using acetone and toluene in an ultrasonic bath. (CSF-3A Ultrasonic Wave Generator, output: IOOv, 2.6A) This process removed the surface material that covered the spheres but did not damage the Al features adhering directly to the substrate. The hydrophilic glass, silicon slides and freshly cleaved mica were used as substrates. Before use, the glass slides were thoroughly cleaned with detergent and chromic acid, then washed with deionized water. Silicon slides were treated to be hydrophilic or hydrophobic. The resulting morphologies of the latex films and the micropatterns were observed with a scanning electron microscope. (Hitachi H-570) Before that the samples were sputter-coated with a 20nm-thick layer of gold. RESULTS Assembly of latex particles When a certain volume of polymer latex is put into a limited area on a substrate, the particles are involved in Brownian motion. With the evaporation of solvent, the center of the area becomes dry and a ring-shaped narrow zone of tightly packed particles is formed over the middle of the substrate. The ordered zone is encircled by a thicker and slightly concave meniscus. The particles located in the meniscus region begin to move toward the ord
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