February 2, 1999
How does one "see" individual defects that may be only 0.5 µm across or less? Producing images of microdomain patterns in materials that are only 50 nanometers thick has been a source of frustration for polymer scientists. Two types of microscopy, transmission electron and scanning electron (TEM and SEM) have been used to image individual defects, but only at a single point in time. The microdomains in diblock copolymer films are destroyed by staining needed for contrast enhancement and the irradiation damage by beams of electrons used by TEM and SEM. The researchers in this study overcame these difficulties using atomic force microscopy (AFM). In earlier work the group demonstrated that AFM techniques successfully capture the details previously seen only in electron microscope images. An image of a certain film area can be sampled, annealed so it evolves further, and quenched back to room temperature. The exact same film area can then be imaged again, allowing for the first time the tracking of the evolution of an individual defect over time. For details on how an atomic force microscope works, click on the image of the AFM just to the right.
Atomic force microscopy has allowed the researchers to describe individual defects and their evolution in diblock copolymer films in considerable detail. Direct evidence of two defect types has been found, although it is highly probable that several other forms exist. The first step in defect evolution involves the formation of a temporary "Y-joint" in the sample. This joint forms when the open ends of an initial defect connect with one or both ends of a neighboring microdomain. This type of initial defect may be quite common, as many open ends evolved into Y-joints during the second anneal. After a y-joint has formed, it can grow through the microdomains until it encounters and destroys another defect.
If two single open-ended defects are one spacing apart, they can simply "join". If the defects are further apart, the geometrical arrangement of a microdomain next to the defect can be rearranged via a "relinking" process. Both of these processes were found in the same 2 µm by 1.5 µm diblock copolymer sample. The researchers observed that the y-joint configuration was the most stable defect form, followed by single-ended and then double-ended domains.
With this new information on defect patterns, the researchers can begin to propose likely scenarios for defect evolution across multiple annealing sessions. The findings of this work by MRSEC researchers offers the beginning of understanding of morphological changes in polymer films. Such information, coupled with the kinetics of molecular movement, will lead to the prediction of heat-activated structural changes that enhances our theoretical knowledge and increases the technological value of these materials. Future research by this group will look at the effects of annealing conditions, stress fields, and electric fields on the kinetics of microdomain evolution. The possibility of tuning the substrate upon which the polymer film is cast in order to create facets, a technique known as "nanopatterning", will also be explored.
- "Defect evolution in ultrathin films of polystyrene-block-polymethylmethacrylate diblock copolymers observed by atomic force microscopy" Jongin Hahm, Ward A. Lopes, Heinrich M. Jaeger, Steven J. Sibener Journal of Chemical Physics. 109:23, 10111-10114 (15 December 1998).
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