The Nealey group is a pioneer of the emerging field of block copolymer (BCP) directed self-assembly (DSA). A block copolymer is a molecule consisting of two or more blocks of dissimilar chemical composition. By careful selection of these chemistries, BCPs can be fabricated such that the blocks have some degree of thermodynamically unfavorable interaction; this interaction is encapsulated by the product of χ (chi), the Flory-Huggins interaction parameter, and N, the molecular weight of the polymer. Above a critical χN where the dissimilar blocks have sufficient strength to repel one another, a BCP system will spontaneously phase-separate into chemically pure ordered domains of regular geometry. Specifically, a diblock copolymer system self-assembles into an ordered array of spherical, cylindrical, or lamellar domains, depending on the relative size of the two blocks. While self-assembled BCPs can take on a variety of nanoscale structures, the Nealey group works actively to engineer, control, and direct the size, spacing, orientation, and shape of these structures for many applications.