Impact of homopolymer pore expander on the morphology of mesoporous carbon films using organic-organic self-assembly

Soft templated mesoporous carbon films (ca. 60 nm thick) are fabricated by the cooperative assembly of poly(styrene-block-ethylene oxide) (PS-b-PEO) with phenolic resin, where the pore sizes are tuned by the addition of polystyrene oligomers (hPS) to this self-assembling system. Significant differences in the morphology and pore expansion are found for the thin films in comparison to previously reported bulk powders using a similar system. The film porosity is nearly independent of swelling agent content for addition of up to 10 wt % hPS; this limited effect is attributed to surface segregation of the hPS on the basis of wetting considerations, which decreases the effective concentration of hPS in the hydrophobic domains. The templated synthesis yields a relatively narrow pore size distribution for <10 wt % hPS, but then the pore size distribution broadens and begins to shift to larger pore sizes for >10 wt % hPS. Elliptical pores are formed due to the uniaxial contraction that occurs during carbonization; interestingly, the apparent contraction increases as hPS loading increases for the films, which also is attributed to surface wetting of the hPS oligomer. Additionally, the pores are not as well-ordered in the films as for the powders. We hypothesize that the poor order is a result of rapid vitrification during spin-coating that leads to initially a disordered mesostructure and subsequent heating leads to cross-linking of the phenolic resin that occurs more rapidly than the chains in the thin films can reorganize into a highly ordered structure, thus yielding a kinetically trapped mesostructure.