Consequences of Surface Neutralization in Diblock Copolymer Thin Films

Two high-chi block copolymers, lamella-forming poly(styrene-block-[isoprene-random-epoxyisoprene]) (PS-PEI78, with 78 mol % epoxidation) and lamella-forming poly(4-trimethylsilylstyrene-block-D,L-lactide) (PTMSS-PLA), were used to study three combinations of interfacial neutrality involving at least one neutral interface. PS-PEI78 annealed on a nonpreferential polymer mat (SMG) produced perpendicular lamellae independent of film thickness, indicating a neutral substrate and neutral free surface. In contrast, the presence of only one neutral interface results in the formation of surface topography (islands and holes) with 0.5L(0) step heights. PS-PEI78 (neutral free surface) annealed on PS brush (PS block preferential) forms half' islands and holes. The inverse experiment, PTMSS-PLA (with a PTMSS preferential free surface) annealed on a neutral (or near neutral) substrate surface, also generates 0.5L(0) topography. These half' island and hole structures are stable to extended thermal annealing. PS-PEI78 exposes both blocks at the free surface in contrast to PTMSS-PLA, which exposes just one. All three combinations of interfacial neutrality are explained by the precise balancing of the wetting tendencies of the two blocks. Evolution of the 0.5L(0) motifs appears to be facilitated by a preference to form half-period thick nuclei in the initial stages of morphological development.