Ion Complexation: A Route to Enhanced Block Copolymer Alignment with Electric Fields

We investigate the enhanced alignment of lamellar microdomains under an electric field by addition of lithium chloride (LiCl) into polystyrene-block-poly(methyl methacrylate) ($\mathrm{PS}\mathrm{\text{−}}b\mathrm{\text{−}}\mathrm{PMMA}$) copolymers. A significant increase of dielectric contrast resulting from the formation of lithium-PMMA complexes markedly reduces the critical electric field strength required to overcome the preferential interactions of one block with the substrate, providing a route to achieve the complete alignment of microdomains in block copolymer thin films.

Figure 1

Left: ir spectra of $\mathrm{PS}\mathrm{\text{−}}b\mathrm{\text{−}}\mathrm{PMMA}$ thin films cast from solution (A), (B), and (C), respectively. Right: Cross-sectional TEM images of $\mathrm{PS}\mathrm{\text{−}}b\mathrm{\text{−}}\mathrm{PMMA}$ thin films spin coated from solution (B) and solution (C) after applying a $\sim 40\mathrm{V}/\mu \mathrm{m}$ dc electric field at $175\pm 5°\mathrm{C}$ under ${\mathrm{N}}_2$ for 20 h. Scale bar: 200 nm.

Figure 2

Left: ir spectra of $\mathrm{PS}\mathrm{\text{−}}b\mathrm{\text{−}}\mathrm{PMMA}$ films with different molar ratio of added LiCl to the carbonyl group in PMMA: (a) 0, (b) $1:10$, (c) $1:6$, and (d) $1:1$. Right: Corresponding cross-sectional TEM images of $\mathrm{PS}\mathrm{\text{−}}b\mathrm{\text{−}}\mathrm{PMMA}$ thin films after applying a $\sim 40\mathrm{V}/\mu \mathrm{m}$ dc electric field at $175\pm 5°\mathrm{C}$ under ${\mathrm{N}}_2$ for 20 h. Scale bar: 200 nm.

Figure 3

Dielectric constants of PMMA with different molar ratios of added LiCl to the carbonyl group (a) 0, (b) $1:10$, and (c) $1:6$ at 170 °C.

Figure 4

Cross-sectional TEM images of (a) pure $\mathrm{PS}\mathrm{\text{−}}b\mathrm{\text{−}}\mathrm{PMMA}$ thin film and (b) $\mathrm{PS}\mathrm{\text{−}}b\mathrm{\text{−}}\mathrm{PMMA}$ copolymer thin film containing lithium complexes with a thickness of $\sim 260\mathrm{nm}$ after applying a $\sim 40\mathrm{V}/\mu \mathrm{m}$ dc electric field at $175\pm 5°\mathrm{C}$ under ${\mathrm{N}}_2$ for 20 h. Scale bar: 200 nm.