Control of the Orientation of Symmetric Poly(styrene)-block-poly(D,L-lactide) Block Copolymers Using Statistical Copolymers of Dissimilar Composition

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 SEMI 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.