Thin-Film Morphology of Symmetric Six-Arm Star-Shaped Poly(methyl methacrylate)-block-Polystyrene Copolymers

Abstract

We investigated, via atomic force microscopy and fieldemission scanning electron microscopy, the thin-film morphology of six-arm star-shaped poly(methyl methacrylate)-b-polystyrene [(PMMA-b-PS) 6 ] with the volume fraction of PS block of ∼0.5. At smaller molecular weight (M) but higher than the critical molecular weight (M crit ) above which block copolymers start to microphase-separate, the thin-film morphology greatly depends on the surface tension at the air side when a substrate has preferential interaction with one block (PMMA). For a near-neutral air surface, interesting tube-like nanostructures, instead of vertically oriented lamellae, were formed at the top of the film, while a PMMA layer was formed on the bottom film, contacting the silicon substrate with a native oxide. This is because the combination of vertical and parallel lamellae generates a huge energy penalty at the T-junction connecting these two different lamellar orientations. Tube-like nanostructures were also formed on other substrates that are preferential to one block, for instance, gold or a substrate grafted by a PS brush, when the film thickness does not meet the commensurability. On the other hand, when M is much higher than M crit , vertical lamellae were formed throughout the entire film thickness. The self-consistent field theory supported the experimental findings.

Graphoepitaxy of Symmetric Six‐Arm Star‐Shaped Poly(methyl methacrylate)‐block‐Polystyrene Copolymer Thin Film

“…According to our previous study, [31] (PMMA-b-PS) 6 thin film without confinement showed two different nanostructures (nanotubes and PL) depending on the film thickness and the affinity between the blocks and the substrate when the M of the (PMMAb-PS) 6 is ≈1.23 M crit . In this study, to investigate the effect of the affinity between the walls (and/or the bottom) and both blocks confined in the trench, we fixed M of (PMMA-b-PS) 6 as 1.23 M crit , where nanotubes are well-formed in thin film without confinement.…”

“…[20] Some research groups obtained diverse nanopatterns on a single substrate by controlling the microdomain orientation of the BCPs [21][22][23][24] or employing complex molecular architecture of BCPs. [25][26][27][28][29][30][31] Also, to obtain a long-range ordering of BCP microdomains on a substrate, the BCP self-assembly is combined with geometrical confinement, so-called graphoepitaxy, which is referred to as the directed self-assembly (DSA). [21,22,[32][33][34][35][36][37][38] Confinement of the BCPs can allow the control of the orientation of microdomains by adjusting the affinity between walls and blocks.…”

“…Star-shaped BCPs have been widely investigated because of their unique star-like molecular architecture. [29,31,[43][44][45][46][47][48] Recently, we fabricated nanotubes by controlling the affinity between the air and the substrate toward each block of the star-shaped BCPs. [31] When the molecular weight (M) of a star-shaped BCP was relatively small, but higher than the critical molecular weight (M crit ) forming the microdomain, the thin film morphology was greatly affected by the affinity of both the air and the substrate toward both blocks.…”

“…[29,31,[43][44][45][46][47][48] Recently, we fabricated nanotubes by controlling the affinity between the air and the substrate toward each block of the star-shaped BCPs. [31] When the molecular weight (M) of a star-shaped BCP was relatively small, but higher than the critical molecular weight (M crit ) forming the microdomain, the thin film morphology was greatly affected by the affinity of both the air and the substrate toward both blocks. [48] When a thin film of the star-shaped BCP was placed on a substrate preferential to one block, while the air side became near-neutral for both blocks, vertically oriented lamellae (VL) and parallelly oriented lamellae (PL) were formed at the top and the bottom of the film, respectively.…”

“…[49][50][51] On the other hand, when the M is much larger (M > 1.5 M crit ), welldefined VL are formed because the entropy contribution overcomes the enthalpy contribution. [31] Nanotubes are one of the most appealing structures because they show unique optical [52,53] and electronic [54,55] properties arising from their large surface-to-volume ratio. For device applications, DSA is required to obtain long-range lateral ordering of nanotubes.…”