Network Connectivity and Long-Range Continuity of Lamellar Morphologies in Block Copolymer Thin Films

The connectivity, and thus long-range continuity of the domains, in a lamellar polystyrene-block-poly(methyl methacrylate) copolymer in thin films depends on the volume fraction of each block and can be shifted by homopolymer addition to substrate-spanning continuity of either the polystyrene (PS) or poly(methyl methacrylate) (PMMA) domains. Essential features of the lamellar morphology were captured by a simple network analysis that quantified the number of branch points and end points in the lamellar domains. The transition in network continuity from the PS to PMMA domain as a function of copolymer. volumetric composition (from f(PMMA) = 0.45 to 0.55) was correlated with a 5-fold increase in the PMMA branch point density and a concomitant 3-fold reduction in the PMMA end point density. These results indicate that the copolymer's composition drastically impacts the self-assembled lamellar morphology in thin films and is an important design consideration when using such materials for lithographic applications, including for directed assembly to generate long-range, defect-free order.