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.

Can ionic liquid additives be used to extend the scope of poly(styrene)-block-poly(methyl methacrylate) for directed self-assembly?

“…Statistical copolymers of polystyrene, poly(methyl methacrylate), and poly(glycidyl methacrylate) (PS-stat-PMMAstat-PGMA) were prepared, as reported by Keen et al 28 The statistical copolymers (0.5 wt/v%) were dissolved in propylene glycol monomethyl ether acetate (PGMEA) along with triphenylsulfonium triflate (5 wt% with respect to the polymer). The solutions were spin coated at 4000 rpm onto silicon wafers.…”

“…Our initial efforts involved identifying the interfacial interactions of PS-b-PLA with substrates that were modified with a series of crosslinked mattes of statistical copolymers of PS and PMMA. 28 Lamellae oriented perpendicular to the substrate with domain sizes as small as 8 nm could be achieved. Graphoepitaxybased DSA was also carried out with this system, where no resist freezing step was required when using a polyhydroxystyrene-based photoresist template.…”

Advances in chemical modifications of polylactide biodegradable materials

“…Hence the copolymerization of lactic acid with other monomers has aroused tremendous research interest [9,10]. By adjusting the molecular weight of PLA copolymers, and the type and quantity of comonomers, various functional groups can be introduced to PLA molecular chains, and the properties such as crystallinity, hydrophility, and degradation rate can be greatly enhanced accordingly.…”

Directed Self-assembly of Topcoat-free, Integration-friendly High-^|^chi; Block Copolymers

“…Perpendicularly oriented domains of high- BCPs have been reported from PS-b-PLA [10], partially epoxidized PS-b-PI [11], poly(ethyleneoxideb-fluorinated methacrylate) [12] and PS-b-PHEMA [13] on neutral underlayer substrates. However, relatively long bake times ranging from a few hours to one day were employed to achieve the desired structures.…”