Thickness Dependent Transitions in Asymmetric Block Copolymer Films

The effect of blending small weight fractions of low molecular weight majority block homopolymer on the structure of multilayer films of spherical morphology poly(styrene-b-2vinylpyridine) [PS-P2VP] has been studied. The structure of the films was characterized with grazing-incidence small-angle X-ray scattering (GISAXS) and transmission electron microscopy (TEM). In multilayer films of PS-P2VP, competition between hexagonal packing of the spherical domains preferred at the surfaces with the BCC (110) packing preferred by the internal layers leads to a transition in the packing symmetry as the number of sphere layers (n) is increased.(1) Neat PS-P2VP exhibits hexagonal close-packed (HCP) symmetry up through n = 4, but at four layers coexistence of hexagonal and face-centered orthorhombic phases is observed for short annealing times. At n = n* = 5 the face-centered orthorhombic structure (FCO) is the stable phase. On increasing n further, the FCO phase continuously distorts to approach the arrangement of the BCC (110) plane. We observe that blending a small weight fraction of low molecular weight PS homopolymer with PS-P2VP suppresses this transition and stabilizes the hexagonal close-packed arrangement beyond four layers. Moreover, n* increases with increasing weight fraction of incorporated homopolymer for the small weight fractions of homopolymer used in this study. Self-consistent-field theory simulations designed to mimic the experimental system corroborate that n* is expected to increase and show that the PS homopolymer segregates to the interstices of the HCP unit cell. This suggests that the homopolymer reduces the stretching of the PS block and the free energy penalty of HCP relative to BCC inner layers. This result is consistent with the hypothesis that the excessive stretching requirement in an HCP arrangement is the cause of its higher free energy as compared to the BCC lattice. (1) Stein, G. E.; Kramer, E. J.; Li, X. F.; Wang, J. Macromolecules 2007, 40, 2453– 2460