The hierarchically structured polymers are porous at both micron- and nano-length scales. This provides both high flux, which means materials can flow through it efficiently, as well as high surface area – both important, for example, in rapid catalytic conversions. The materials were self-assembled from a series of block copolymers, which are large molecules comprising "blocks" of repeating units.
Researchers have made such porous materials before, according to Ulrich Wiesner, the Spencer T Olin Professor of Engineering, but the methods usually involve post-processing; for example, once the polymer is made, the pores need to be etched into the material.
With their new method, the researchers achieved dual porosity by evaporating a solvent from a mixture of a block copolymer with a small molecule additive that is chemically similar to one of the blocks. The two components form two coexisting phases, like water and oil, with a continuous interface between them separated by tens of microns. This method of phase separation is known as spinodal decomposition.
One of the two phases rich in the block copolymer then phase-separates on the tens of nanometers scale into two domains formed by the two blocks of the copolymer. One of the blocks, a polyethylene oxide (PEO) block, is swollen with the small molecule additive, which is immiscible – doesn't mix – with the other block of the polymer. When the additive is washed away, what remains is a continuous pattern of porosities on two lengths scales – tens of microns, and tens of nanometers.
The researchers tried the method with multiple diblock and even triblock polymers, and contend that the method can be generalised for making many versions of this material. They were also able to easily tune the nanostructure of the resulting polymer by adjusting the temperature at which the original organic solvent was evaporated.
Electron microscopy confirmed that the calcite infiltrated the entire structure, small and large pores, thus demonstrating transport of calcite precursors through the porous structure.