NEXAFS Depth Profiling of Surface Segregation in Block Copolymer Thin Films

Journal Article · Fri Jan 01 04:00:00 EST 2010 · Macromolecules

DOI:https://doi.org/10.1021/ma902866x· OSTI ID:1019919

Krishnan, S; Paik, M; Ober, C; Martinelli, E; Galli, G; Sohn, K; Kramer, E; Fischer, D

NEXAFS spectroscopy was used to probe the surface composition and under-water surface reconstruction of thin films of comb-like diblock copolymers with cylindrical and spherical microphases. The polymers consisted of a polystyrene block, and a second block prepared from a styrenic monomer grafted with fluoroalkyl-tagged poly(ethylene glycol) side chains. Compositional depth profiling of the microphase separated block copolymer films, in the top 1-3 nm of the film, was performed to understand the role of block copolymer microstructure and self-assembly on surface composition. Using experimentally determined concentration profiles, the surface concentration of phenyl ring carbon atoms was quantified and compared with those of homopolymer and random copolymer controls. The carbon atoms from the relatively high surface energy phenyl groups were depleted or excluded from the surface, in favor of the low surface-energy fluoroalkyl groups. While it is expected that block copolymer surfaces will be completely covered by a wetting lamellar layer of the lower surface energy block, a significant amount of the higher surface energy polystyrene block was found to be present in the surface region of the cylinder-forming block copolymer. Evidently, the spontaneous formation of the cylindrical polystyrene microdomains in the near-surface region compensated for the lowering of the free energy that could have been achieved by completely covering the surfaces with a lamellar layer of the lower surface energy fluorinated block. All surfaces underwent molecular reconstruction after immersion in water. The experimental concentration depth profiles indicated an increased surface depletion of phenyl ring carbon atoms in the water-immersed thin films, due to the tendency of hydrophilic PEG side groups to be present at the polymer-water interface. Such a detailed characterization of the outermost layers of the block copolymer surfaces was possible because of the exceptional depth resolution of the NEXAFS depth profiling technique.

• Research Organization: Brookhaven National Laboratory (BNL) National Synchrotron Light Source
• Sponsoring Organization: DOE - OFFICE OF SCIENCE
• DOE Contract Number: AC02-98CH10886
• OSTI ID: 1019919
• Report Number(s): BNL--95765-2011-JA
• Journal Information: Macromolecules, Journal Name: Macromolecules Journal Issue: 10 Vol. 43; ISSN 0024-9297
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ATOMS
CARBON
CHAINS
COPOLYMERS
FREE ENERGY
MICROSTRUCTURE
MONOMERS
POLYMERS
POLYSTYRENE
PROBES
RESOLUTION
SEGREGATION
SPECTROSCOPY
SURFACE ENERGY
THIN FILMS
WATER
national synchrotron light source