Topological Effect on Self-Assembly of High Interaction Parameter Block Copolymers*

Block copolymers (BCPs) with high χ value, polystyrene-block-polydimethylsiloxane (PS-b-PDMS), have drawn intensive attention due to their rich phase behaviors. Lamellar phase with an interdigitating chain packing can be found in the PS-b-PDMS where a superlattice structure can be formed in a diblock but the star-block (PS-b-PDMS)n (n = 3 and 4) might constrain the superlattice formation due to configurational topology. Owing to the high χ value of the PS-b-PDMS, metastable cubic network phases including double primitive phase (DP), double diamond phase (DD) and double gyroid phase (DG) can be obtained from a single-composition, lamellar PS-b-PDMS. The order-order transitions from DP (hexapod network) to DD (tetrapod network), and finally to DG (trigonal planar network) are attributed to the reduction on the degree of packing frustration (entropic penalty) within the junction (node), different to liquid crystals. By taking advantage of the topological effect with increasing the arm number (star-block (PS-b-PDMS)n (n = 6)), it is possible to overcome the packing frustration, giving higher accessibility for the formation of network phases. It is highly appealing to exploit the forming network phases as templates for fabrication of metamaterials. Controlling the orientation of nanostructured BCP thin films is essential for BCP lithography. According to conventional wisdom, the orientation of BCP thin films is mainly determined by molecular interactions (enthalpy-driven orientation). By taking advantage of the entropic effect due to the topology of star-block copolymers, it is feasible to regulate the entropic contribution with the number of arms, giving self-assembled PS-b-PDMS thin films with perpendicularly oriented PDMS cylinders. With combination of configurational topology effect (entropy effect) and surface air plasma treatment or vacuum effect (enthalpy effect) to create a neutral air surface, it is possible to give the formation of the film-spanning perpendicular cylinders upon thermal annealing through self-alignment mechanism.