Alignment of perpendicular lamellae in block copolymer thin films by shearing

Abstract

Lamellar block copolymers, with the lamellae standing perpendicular to the substrate, are attractive candidates as templates for nanostructure array fabrication. However, no process currently exists to impose long-range in-plane alignment of such perpendicular lamellae on simple unpatterned substrates—to align the lamellar normal over macroscopic distances. Here, we have generated such aligned films of perpendicular lamellae in a polystyrene-poly(methylmethacrylate) diblock, PS-PMMA, by neutralizing the substrate with a random terpolymer brush and shearing the film with a moving polydimethylsiloxane (PDMS) pad in contact with the film surface. At sufficiently high shear stresses, the lamellae align over the entire (cm²) area of the pad; the perpendicular orientation of the lamellae is preserved, although for films thicker than the lamellar spacing, a “capping layer” of PS forms in contact with the PDMS pad. However, when compared with typical shear-aligned block copolymers having a morphology of in-plane cylinders, a significantly higher stress is required to align the lamellar PS-PMMA, and the orientational order is poorer and the dislocation density higher; a limiting order parameter ψ₂ ≈ 0.8 is achieved at high stresses.