(4cx) • Probing Equilibrium Phase Behavior of Asymmetric Block Copolymer Thin Films

Confining block copolymers to thin films adds geometric frustration to the system and incorporates surfaces effects, which are absent in bulk. However the effect of these added energetic costs are not completely understood or quantified. We have probed how film thickness affects equilibrium behavior for highly asymmetric block copolymer systems with layering parallel to the substrate. In particular we have looked at the arrangement of microdomains in spherical morphology block copolymers and melting behavior of cylindrical morphology block copolymers. In multilayer films of spherical morphology diblock copolymers, competition between hexagonal packing of the microdomains preferred at the surfaces and the BCC (110) packing preferred by the internal ?bulk-like' layers leads to a transition in the packing symmetry from HEX to FCO to BCC as the film thickness is increased. We have found that this transition can be suppressed by blending a small amount of majority block homopolymer, which migrates to the highly stretched interstices of the respective lattices and relieves the packing frustration. Self-consistent-field theory simulations show that the homopolymer reduces the stretching of the PS block and the free energy penalty of HCP relative to BCC inner layers. The other phenomenon that we have probed is the smectic-nematic-isotropic transition in graphoepitaxially confined monolayers films of cylindrical morphology diblock copolymer. We find that in-plane defects can be suppressed and mediated by building multi-layer self assembled structures. We have combined quantitative AFM studies with grazing incidence small angle X-ray diffraction, which is a powerful tool to obtain sample information characteristic of large areas and is not limited to the surface, like most microscopy techniques. Diffraction lineshape analysis allowed us to quantify the decay of translational and orientational order with increasing temperature. The results have been interpreted in the context of the Toner?Nelson theory of melting for layered systems.