Symmetry Transition in Thin Films of Diblock Copolymer/Homopolymer Blends

ABSTRACT: 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 surfaceswith theBCC (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.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 interstice