Block copolymers (BCPs) and synthetic infiltration synthesis (SIS) are used to double the line density on a substrate. The BCP comprises first and second interconnected BCP components with a functional group at the junction or interface of the components. After deposition of the BCP on the substrate and annealing, a pattern of parallel stripes of first and second BCP components is formed with a pattern of functional group interfaces between the components. Each of the BCP components is non-reactive with atomic layer deposition (ALD) precursors, while the functional group is reactive with the ALD precursors. The ALD results in the infiltration of inorganic material into the interfaces where the reactive functional groups are located but without affecting the BCP components. After removal of the organic material, a pattern of parallel lines of inorganic material remains with a pitch half that of the stripes of BCP components.