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Long-range ordering of block copolymer cylinders driven by combining thermal annealing and substrate functionalization.

This work presents a new method for forming well-defined nanostructured thin films from self-assembled polystyrene-block-poly(l-lactide) (PS-PLLA) on Si wafers with a functionalized SiO2 surface. Large, well-ordered, perpendicular PLLA cylinders in PS-PLLA thin films can be formed using the functionalized substrate. In contrast to random copolymers, a neutral substrate for the PS and PLLA blocks is formed by functionalizing a substrate with hydroxyl-terminated PS (PS-OH) followed by hydroxyl-terminated PLLA (PLLA-OH). The heterogeneous grafting of PS-OH and PLLA-OH can be substantially alleviated using this two-step functionalization. Accordingly, the surface properties can be fine-tuned by controlling the ratio of grafted PS-OH to PLLA-OH to control the orientation of the PLLA cylinders on the functionalized SiO2. Nevertheless, the orientation that is driven by the neutral substrate is surprisingly limited in that the effective length of orienting cylinders is less than twice the interdomain spacing. Thermal annealing at high temperature can yield a neutral air surface, rendering perpendicular PLLA cylinders that stand sub-micrometers from the air surface. Consequently, the neutral substrate can be used to enable truly film-spanning perpendicular cylinders in films to be fabricated using the high-temperature thermal treatment. In addition, the perpendicular cylinders can be laterally ordered by further increasing the annealing temperature. The ability to create these film-spanning perpendicular cylinders in films with a well-ordered texture and sub-micrometer thickness opens up possible applications in nanotechnology.