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Directed Assembly of Ternary Blends on Chemically Patterned Surfaces

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— 2 min read

Recent research from the University of Wisconsin has made significant discoveries in the field of directed assembly. The study, published in ACS Nano, examined ternary blends of cylinder-forming polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) and low molecular weight PS and PMMA. The researchers found that the blends assembled on chemically patterned surfaces with hexagonal symmetry, and that the addition of homopolymer increased the range of L-s values over which assemblies with high orientational order parameter (OPo) values could be achieved.

Key Takeaways:

  • The researchers created ternary blends of PS-b-PMMA and low molecular weight PS and PMMA, and directed them to assemble on chemically patterned surfaces with hexagonal symmetry.
  • The chemical patterns consisted of strongly PMMA preferential spots, patterned by electron-beam lithography, in a matrix of PS.
  • The spot-to-spot spacing of the chemical patterns (L-s) was varied between 0.9L(0) and 1.1L(0), where L-0 is the cylinder-to-cylinder spacing of the pure block copolymer in bulk.
  • The homopolymer volume fraction of the blends (4) was varied between 0 and 0.3.
  • The assemblies were analyzed with top-down SEM, from which OPo values were determined. The SEM analysis was complemented by Monte Carlo simulations, which offered insights into the shapes of the assembled cylindrical domains.
  • In comparison to pure block copolymer, adding homopolymer increased the range of L-s values over which assemblies with high OPo values could be achieved for 1:1 assemblies.
  • However, the corresponding simulations showed that in the 1:1 assemblies the shape of the cylinders was more uniform for pure block copolymer than for blends.
  • In the case of the 4:1 assemblies, the range of L-s values over which assemblies with high OPo values could be achieved was the same for all values of phi(H) tested, but the domains of the pure block copolymer had a more uniform shape.
  • U. Nagpal and colleagues concluded that the results provided insights into the blend composition to be used to meet technological requirements for directed assembly with density multiplication.

Statistics:

  • The L-s values varied between 0.9L(0) and 1.1L(0) for the chemical patterns.
  • The homopolymer volume fraction of the blends (4) was varied between 0 and 0.3.
  • The range of L-s values over which assemblies with high OPo values could be achieved increased with the addition of homopolymer for 1:1 assemblies.
  • The shape of the cylinders was more uniform for pure block copolymer than for blends in the 1:1 assemblies.
  • The domains of the pure block copolymer had a more uniform shape in the 4:1 assemblies.

Sources:

  • U. Nagpal et al., "Pattern Dimensions and Feature Shapes of Ternary Blends of Block Copolymer and Low Molecular Weight Homopolymers Directed To Assemble on Chemically Nanopatterned Surfaces." ACS Nano 2011;5(7):5673-5682.
  • American Chemical Society, 1155 16th St., NW, Washington, DC 20036, USA.

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