Many processes in industry, for example catalytic conversions, require high surface area materials. These materials generally have lots of small pores, which in turn means that only a limited amount of gases or liquids can pass through them. One way around this problem is "hierarchically" porous materials that have connected pores on multiple (at least two) length scales, something that allows for higher fluxes.

Corning Inc., which produces catalytic converters for cars, uses a multistep process to generate these hierarchically porous materials, explains team leader Uli Wiesner. In a first step, a macroscopic ceramic monolith with porous channels measuring hundreds of microns across is produced. These channels are then coated with a high surface area material with nanoscale-sized holes.

"What we have shown in our work is how to generate such hierarchical porous materials out of polymers in a single step, plus a subsequent water rinsing/washing step to pull out low mass additives to generate porosity," he told "In this approach, the multiple tedious steps used in earlier techniques to generate porosity by removing, for instance, one block of the copolymer at a time in post-processing procedures are thus replaced by the simple rising/washing step with water. What is more, since our technique is based on well established thermodynamic principles, it should be quite general and could lend itself to other block copolymer systems, which would be very exciting too."

"Spinodal" decomposition and mesophase separation

The Cornell University’s method involves a combination of "spinodal" decomposition between a small molecule additive and a block polymer, and so-called mesophase separation of the block copolymer swollen in one block with the small molar mass additive. The two ingredients - in this case, polystyrene-block-polyethylene oxide (PS-b-PEO), and a PEO oligomer (o-PEO) - are mixed together in a solvent, which is then evaporated off. "Since the additive we chose is water soluble, simply rinsing the resulting film with water allows to make a film with micron-scale pores (from the spinodal decomposition part) and nanoscale pores (by removing the additive from one of the domains of the block copolymer)."

Spinodal decomposition is a method used to separate out phases between two components – typically two polymers. During the process, a continuous 3D interface between these components is generated. The phase separation usually occurs over distances of microns. Block copolymers, on the other hand, are macromolecules in which the blocks typically phase separate on the nanometre length scale.

The new technique could help develop better catalytic converters, says Wiesner, and be used in bioengineering where the hierarchical scaffolds could be used to grow 3D cell structures, for example.

Spurred on by its preliminary results, the team says that it is now busy trying to better understand the physics underlying the process they have developed and how it works in detail. "We would also like to extend this method to other materials and broaden out the scope of the approach," said Wiesner.

The researchers published their work in Science DOI: 10.1126/science.1238159.

Further reading

Researchers crack the nanocrystal challenge (Oct 2010)
Sol-gel makes nanostructured metallics (Mar 2012)
Universal assembly directs ultrahigh-density storage devices (Nov 2010)
Li-ion batteries benefit from hierarchical LiFePO4/C (Nov 2012)