Photonic Crystals from Emulsion Templates
Original entry: Lidiya Mishchenko, APPHY 226, Spring 2009
Vinothan N. Manoharan, Arnout Imhof, James D. Thorne, and David J. Pine, Adv. Mater. 2001, 13, No. 6, March 16 []
Emulsion, colloid, templated assembly
"Macroporous titania, which undergoes transition to the rutile phase by calcination without collapse of the pore structure, is obtained by polymerizing a titania sol suspended around 'colloidal crystals' of oil droplets. The deformable template counteracts cracking of the titania phase."
Soft Matter ExampleThe paper's method for developing macroporous titania involved the preparation of a monodisperse emulsion of oil droplets stabilized by surfactant (Figure 1) and suspended in an immiscible liquid, in this case a titania sol (titania precursor). Through sedimentation or crentifugation, oil droplets were close packed and ordered. A catalyst was then added to the emulsion colloidal crystal, allowing the titania sol to gel. The oil droplets were then dissolved away, and the gel was aged, dried, and calcined to transform it to the rutile phase of titania (Figure 2). The reason deformable oil droplets were used for the template was to allow the material to withstand the stresses of shrinkage during gelation without cracking.
The most relevant concepts in this paper involved the creation of a monodisperse emulsion of oil droplets. The two method mentioned for creation of a monodisperse oil emulsion in water were "fractionating" and Mason-Bibette. The "fractionating" method could actually produce emulsions with a polydispersity as low as 10%. This method somehow utilizes the liquid-solid transition with a certain concentration of surfactant in solution. I could not quite understand this paper, but it seems as though the formation of micelles and repeated saturation of a solution could lead to a monodisperse emulsion (see Reference below).
Since the creation of monodisperse emulsions is so difficult, the paper also suggested the use of polymer colloids for the template. If one chooses a polymer template with a low glass transition temperature, then these particles have similar properties to oil droplets. Polymer spheres at these temperatures are deformable and are easily dissolved. (For example, polybutyl methacrylate spheres have a glass transition near room temperature.)
There are two interesting concepets involved with these polymer spheres. One is that polymer spheres are themselves made through emulsion polymerization involving an emulsion stabilized by surfactant. So why are polymer spheres of low polydispersities so much easier to make than emulsions? (Maybe because it is simply easier to separate out the larger spheres from the suspension after they are polymerized?)
Also, I have never really thought about the glass transition temperature of polymers. But it makes sense. Polymers are just glass since you could not really call them a crystalline solid. They just become "infinitely viscous" below a certain temperature.
Reference: J. BIBETTE, "Depletion Interactions and Fractionated Crystallization for Polydisperse Emulsion Purification", Journal of Colloid and Interface Science, Vol. 147, No. 2, December 1991 []