Fabrication of Hierarchical Structures by Wetting Porous Templates with Polymer Microspheres

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Original entry: William Bonificio, AP 225, Fall 2009

Information

Fabrication of Hierarchical Structures by Wetting Porous Templates with Polymer Microspheres Jiun-Tai Chen, Dian Chen, Thomas P. Russell Langmuir 2009 25 (8), 4331-4335


Soft matter keywords

Polystyrene, Microsphere, Heirarchical structure, Glass transition temperature

Summary

The researchers used a very simple method using capillarity to create a hierarchical structure. Polystyrene spheres were first allowed to self assemble over a silicon substrate, this being the first length scale of the structure. After this a nanoporous sheet of aluminum was placed over the top of the spheres. The spheres were then heated above their glass transition temperature. At this point capillary action occurred within the pores on the polystyrene, causing the polystyrene to rise up into the spheres. This created many 'bumps' on the spheres - giving rise to the second length scale of the hierarchical structure.

Soft matter discussion

Schematic of creating hierarchical structure. Top left, the spheres lie on substrate. Top right a nanoporous aluminum template is placed on top of the spheres. Bottom right the spheres are heated and capillary action drives the spheres up into the nanopores. Bottom left the template is removed leaving the structure intact.
SEM image of the final hierarchical structures of the polystyrene spheres at two different length scales.

Two soft matter phenomenons of self assembly and capillarity were utilized by the researchers in this experiment to create their hierarchical structure.

First, a suspension of polystyrene spheres in ethanol was placed on a clean silicon wafer, and allowed to dry over night. As the ethanol evaporated the spheres self assembled into an hexagonal monolayer across the wafer. This occurred because of the capillary forces driving the polystyrene together as the ethanol evaporated. Different experiments were performed using different radii from .3 to 2 microns.

After this, an aluminum template that had 50nm pores spaced 100nm apart was placed over the top. The system was then heated to a temperature above the polystyrene glass transition temperature (Tg for polystyrene = 378K) and the researchers allowed capillary forces to do their work, lifting the polystyrene into the pores of the aluminum to form nanopores. The flow rate of the polymer melt in the pores can be calculated using the following equation:

<math>\frac{dz}{dt} = \frac{R\gamma cos \theta }{4 \eta z}</math>

where z is the height, t is the time, R is the radius of the nanopores, <math>\gamma</math> is the surface tension, and <math>\eta</math> is the viscosity.

The aluminum template was subsequently dissolved, leaving the polystyrene spheres behind.

The sizes of both length scales of the hierarchical structure can easily be controlled by altering the size of the polystyrene sphere, or the size of the nanopores.