Control of the orientation of symmetric poly(styrene)-block-poly(d, l-lactide) block copolymers using statistical copolymers of dissimilar composition

Abstract

The interactions of block copolymers with surfaces can be controlled by coating those surfaces with appropriate statistical copolymers. Usually, a statistical copolymer comprised of monomer units identical to those of the block copolymer is used; that is, typically a poly(styrene)-stat-poly(methyl methacrylate) (PS-stat-PMMA) is used to direct the alignment of poly(styrene)-block-poly(methyl methacrylate) (PS-block-PMMA), and poly(styrene)-stat-poly(2-vinylpyridine) (PS-stat-P2VP) has been used for poly(styrene)-block-poly(2-vinylpyridine) (PS-block-P2VP). Reports of controlling the orientation of block copolymers with statistical copolymers with a dissimilar composition are limited. Here, we demonstrate that this method can be further extended to show that PS-stat-PMMA can be used to control the wetting properties of poly(styrene)-block-poly(d,l-lactide) (PS-block-PDLA). Surfaces were modified with a series of cross-linked PS-stat-PMMA-stat-glycidyl methacrylate terpolymers, and the surface chemistries and energies were assessed using angle-dependent X-ray photoelectron spectroscopy and the two-liquid harmonic method, respectively. From these experiments, an expected neutral compositional window was identified for symmetrical PS-block-PDLA. Moreover, high-resolution SEM, AD-XPS, and grazing-incidence SAXS measurements were used to evaluate the morphology of PS-block-PDLA as a function of the surface composition of the underlying cross-linked copolymer films, and the neutral composition was found to range from 32 to 38 mol % of PS, in the bulk polymer. Ultimately, we demonstrated the determination of nonpreferential surface compositions that allow the self-assembly of lamellae with sizes in the sub-10 nm regime that are oriented perpendicular to the substrate. These findings have important implications for the use of PS-block-PDLA block copolymers in directed self-assembly, most specifically in advanced lithographic processes. © 2012 American Chemical Society.