Honeycomb Porous Films Prepared from Porphyrin-Cored Star Polymers: Submicrometer Pores Induced by Transition of Monolayer into Multilayer Structures

Honeycomb porous films prepared by the breath figure method show great potential in fields such as templating, separation, microanalysis, and superhydrophobic materials. This method is of significant simplicity but generally generates films with pores larger than 1 mu m. Here we report an approach to formation of films with pore diameter as small as about 240 nm, which is based on the transition of monolayer into multilayer structures. Porphyrin-cored star polymers poly(styrene-block-2-hydroxyethyl methacrylate) having on average only 1-2 hydrophilic monomer units were synthesized by atom transfer radical polymerization (ATRP) and used for preparing honeycomb films. We investigated the effects of polymer structure and concentration on the pore diameter and film morphology. A tentative mechanism for the formation of multilayer structures was proposed and experimentally verified by in situ observing the breath figure process and measuring the glass transition temperature of the stars using differential scanning calorimetry (DSC). This work is helpful in understanding the mechanism of the breath figure method and preparing honeycomb films with submicrometer-sized pores which show perspectives as advanced microfiltration membranes.