February 2, 1999
Ultrathin diblock copolymer films have garnered much attention from researchers in recent years as a system for studying the growth of order. These materials exhibit a rich and complex set of kinetic processes during formation that also hold promise for diverse technological applications. Copolymer films will become more useful as their mechanical properties are determined and their morphology becomes more predictable and hence controllable. Particularly elusive to researchers has been the discovery of the types of defects that arise and evolve over time in these films. Four MRSEC researchers from the Departments of Chemistry and Physics at the University of Chicago, with efforts spearheaded by graduate student Jongin Hahm, have observed the temporal evolution of individual defects in ultrathin diblock copolymer films for the very first time.
Diblock copolymers are composed of two chemically distinct polymer blocks. When films of diblock copolymers are annealed in an oven, nanometer-scale patterns called "microdomains" can be observed. The stripe patterns result from repulsion between the two halves of each polymer molecule. The width of these stripes is determined by the length of the polymer chains in the film. Initially the patterns amount to no more than a random dappling of the film, but with time, the dark and light regions organize themselves into the swirling fingerprint patterns shown on the right. As time passes, the swirls become smoother and the defects become fewer. In this way the film progresses towards a regular parallel stripe pattern. The drive to form theis regular pattern comes from the system's desire to reach its most stable, that is lowest-energy form.
But how do the polymers know where to go? If each polymer just went downhill in energy, the pattern would get stuck. Downhill motion cannot remove the defects, yet somehow the defects become fewer. With this experiment we can watch the defects in the act of reorganizing and see exactly how the polymers go about reducing their disorder.
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.
Reference:
- "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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