Polymers in Confined Geometries
Confining polymer molecules to geometries that are smaller than a few times the molecules' size influences properties such as viscosity and morphology. Films of polymer blends often exhibit more desirable characteristics than individual homopolymers; however, most blend components are also highly incompatible and will undergo demixing and phase separation. The degree of phase separation in polymer blends can have either adverse or beneficial effects, depending on the resulting morphology. In order to achieve direct and quantitative chemical characterization of polymer blends in confined geometries, researchers from several institutions collaborated at the ALS to use a photoemission electron microscope (PEEM) for near-edge x-ray absorption fine structure (NEXAFS) microscopy of surfaces, and scanning transmission x-ray microscopes (STXMs) for NEXAFS microcopy of the interior. In one study, the researchers investigated the thickness dependence of the dewetting characteristics of annealed bilayers comprising a layer of brominated polystyrene (PBrS) 30 nm thick on top of a polystyrene (PS) layer 34 nm thick, all on a silicon substrate.
Left: NEXAFS spectra from boxes in PEEM image; only PS is on the surface. Middle: PEEM image showing advanced stages of dewetting via topography contrast. Right: Schematic of the deduced morphology across "spines" in image (PBrS detected with STXM).
The researchers observed that the PBrS dewets the PS layer upon annealing, and the PS eventually encapsulates the PBrS with the formation of PBrS "spines" covered by PS prior to the breakup into PBrS droplets. However, as the PS thickness decreases the encapsulation slows down. This type of research provides the information needed to investigate the pathways leading to reproducible fabrication of polymer nanostructures and to study the morphological characteristics and dynamics of such nanostructures.
Research conducted by H. Ade, D. A. Winesett, and A. P. Smith (North
Carolina State University); S. Anders, T. Stammler, and C. Heske (Berkeley Lab); D. Slep (Hilord Chemical Corp.); J. Asselta, M. H. Rafailovich, and J.
Sokolov (State University of New York at Stony Brook), and J. Stöhr (IBM), using Beamlines 7.0.1 and 8.0.1. Some work was also conducted on Beamline X-1 at the National Synchrotron Light Source.
Funding: National Science Foundation (Young Investigator Award) and U.
S. Department of Energy, Office of Basic Energy Sciences.
Publications about this experiment: H. Ade et al., Appl. Phys. Lett. 73, 3775 (1998).
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