The effect of surface constraints on the ordering of block copolymer domains

Abstract

The effect of surfaces on the ordering of block copolymer microdomains has been studied by TEM observation of thin-film droplets. Poly(styrene butadiene) block copolymers exhibiting spherical, cylindrical and lamellar microdomain morphologies were formed by solvent evaporation on a carbon substrate, annealed aboveT g, and stained with OsO4 prior to TEM observation. Samples with lamellar morphologies exhibited orientations of the lamellae in which the domains were either perpendicular to the substrate surface, or parallel to the substrate surface in which a layered structure resulted. Specimens in which cylinders were formed revealed sheets in which the cylinder axes lie parallel to the substrate, as well as parallel to each other. Stacking of such layers was observed, with exceptional registration between layers such that the cylinders in one layer were located over the inter-domain regions of the cylinders in adjacent layers. Specimens with spherical microdomain morphologies revealed a layer structure that consisted of hexagonally packed arrays of the spherical micelles. The stacking of layers of discrete thickness of all three types of microdomain morphology to produce the observed geometries indicates that the exterior of bulk samples may also exhibit such surface steps that reflect the ordering of the microdomains within the sample. Regardless of the microdomain type, all samples revealed a peripheral region in the thinnest area of the droplets in which the formation of the normal microdomain morphology (i.e. that of spheres, cylinders or lamellae) was inhibited. The uniform thickness of this region provides evidence that preferential segregation of the block component with the lowest surface energy takes place regardless of the sample composition.