Polymers make perfect building blocks for manufacturing nanostructures because of their variable chemical functionality and the size of the polymer molecules. Moreover, the morphology and other important properties of polymers, such as wetting, adhesion, or bio-compatibility, can be externally modified, e.g., by solvents. Such adaptive behavior is very promising for the engineering of smart surfaces for biomedical applications and nanodevices. For example, a mixed brush of hydrophilic and hydrophobic homopolymers that is exposed to a hydrophilic solvent should chemically segregate, and the hydrophilic component should accumulate at the surface, a process called perpendicular segregation. Although the reversible switching from hydrophilic to hydrophobic character has been observed, the local chemical structure has remained unknown.

Sketch of a mixed polymer brush comprising hydrophilic and hydrophobic homopolymers.

To Bead or Not to Bead

How water behaves on a surface is a big deal to some of us, so we buy dishwasher detergents and rinses that promise to leave dishes without water spots and the like. But how about a way to make a "smart" surface that adapts to its environment--it is either hydrophobic, so the water beads, or hydrophilic, so it spreads out, depending on the surrounding chemical environment. Or it might either stick to or slide off another object, again depending on what's around it. Such adaptive behavior would be just the ticket in medicine and biological science for designing smart surfaces that mimic the permeability of living cell membranes or that selectively recognize and stick to cells or their constituents, as well as in applications such as self-cleaning clothes. To make this dream come true, Minko et al. started by binding different polymers (long chain molecules) to a solid surface (substrate) to form a polymer "brush." Then they exposed the brush to chemical solvents that caused the film structure to adopt the particular desired property, such as being hydrophilic or hydrophobic. They used x-ray microscopy at the ALS to image the chemical changes in the brush films induced by the solvents.

Photoemission electron microscopy based on near-edge x-ray absorption fine structure (NEXAFS) provides a way to identify differences in local chemical structure. By combining NEXAFS, which has been very successfully used for years in spectroscopic studies of polymer materials, with the high surface sensitivity and the high nanometer spatial resolution (< 50 nm) of the PEEM-2 on ALS Beamline 7.3.1.1, the researchers were able to image the chemical structure at the surface of a mixed polymer brush, including the lateral and perpendicular segregation predicted by self-consistent-field (SCF) calculations. They were also able to correlate the chemical morphology of the sample with its topography, as detected by atomic force microscopy.

The samples, a mixed brush whose two components were a random copolymer of styrene and pentafluorostyrene (PSF) and of polymethyl methacrylate (PMMA), were fabricated at the Institute for Polymer Research in Dresden, Germany. The German collaborators used a sophisticated technique of grafting the two incompatible polymers randomly on the surface of a silicon wafer, which prevents macroscopic phase separation of the hydrophobic PSF and hydrophilic PMMA.

An AFM image of a PSF/PMMA brush after treatment with toluene shows that exposure to toluene creates a "ripple phase" (a). The PEEM images show inverted contrast (arrows) at x-ray energies specific for PSF (b) and PMMA (c).

Utilizing the chemical sensitivity of PEEM, the researchers observed that after exposure to the nonselective solvent toluene, the components of the mixed polymer brush created a laterally segregated "ripple" phase comprising worm-like domains, 150 to 160 nm in width. This lateral segregation showed up in PEEM images, which were acquired at two specific x-ray energies corresponding to characteristic absorption peaks for the two polymers at the carbon absorption edge, as a reversal in contrast. This observation was corroborated by atomic force microscope images, which showed the same ripple phase in the topography of the sample. The observed lateral segregation was predicted by the SCF calculations, which considered, among other parameters, the length of the polymer chains, the repulsion between the chains, and the selectivity of the solvent.

An AFM image of a PSF/PMMA brush after treatment with acetone shows that exposure to acetone creates a "dimple phase" (d). The surface is dominantly PMMA, and PEEM images (e,f) show no indication of contrast reversal.

When the polymer was exposed to the selective solvent acetone, a hydrophilic solvent that preferentially dissolves the PMMA component, a new polymer phase was observed that could be described as a "dimple phase," with both lateral and perpendicular segregation. The segregation of the two polymer components perpendicular to the surface resulted in an enhancement of the PMMA at the top of the brush. This segregation appeared in the microscopic PEEM images as a strong reduction in contrast, owing to the prevalence of one polymer component at the surface. Calculations were again able to explain the formation of this new polymer phase.

Research conducted by S. Minko, D. Usov, C. Froeck, and M. Stamm (Institut für Polymerforschung Dresden); M. Müller (Johannes Gutenberg Universität); and A. Scholl (Berkeley Lab).

Research funding: Federal Ministry for Education and Research (BMBF) and German Research Foundation (DFG). Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

Publication about this research: S. Minko, M. Müller, D. Usov, A. Scholl, C. Froeck, and M. Stamm, "Lateral versus Perpendicular Segregation in Mixed Polymer Brushes," Phys. Rev. Lett. 88, 035502 (2002).