Control of gyroid forming block copolymer templates: Effects of an electric field and surface topography

The control over 10 nm scale porosity derived from self-assembly of copolymers is an extremely promising method for the synthesis of organic-inorganic hybrid materials applied, for example, in solar cells. Here, we report the thin film behaviour of a poly(4-fluorostyrene)-b-poly(d,l-lactide) PFS-b-PLA block copolymer which adopts the bicontinuous gyroid phase in the bulk and may be used to form a porous template suitable for patterning functional materials by selective degradation of the minority PLA domains. The response of the copolymer morphology to DC electric fields is probed at temperatures where the bulk copolymer adopts either the gyroid or the cylindrical phase. At 150 °C electric field alignment results in vertical arrays of cylinders, lamellae, and perforated lamellae while at 180 °C the gyroid phase coexists with a standing perforated lamellar phase. We show that both polymer-substrate interactions and substrate topography are critical factors determining substrate reconstruction of the gyroid phase. Spontaneous cross-film percolation of the minority network phase on a given substrate, a prerequisite for electrochemical replication, is dependent on surface topology at the scale of the gyroid unit cell. Importantly, under suitable processing conditions all these complex copolymer morphologies can be electrochemically replicated to produce highly ordered freestanding nanostructured arrays over large areas.