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Insights into polymer film instability could aid high tech industries
January 14, 2009While exploring the properties of polymer formation, a team of scientists at the National Institute for Standards and Technology (NIST) has made a fundamental discovery* about these materials that could improve methods of creating the stable crystalline films that are widely used in electronics applications-and also offer insight into a range of other phenomena.
The team has determined that temperature can play a decisive role in determining which of two competing processes-called crystallization and dewetting-will "take the lead" when a semicrystalline polymer film hardens, thereby granting qualitatively different properties to the finished film. The findings could lead to better control of these two processes, which can cause imperfections in polymer films during their formation.
Such imperfections can hinder the performance of potential new technologies, such as solar cells or thin film transistors, that employ organic polymer films on their surfaces, according to research chemist Christopher Soles. At this point, he said, the organic semiconductor industry is being hindered by a lack of understanding of crystal formation in thin polymer films.
"If organic photovoltaics-to take just one example-are ever to be realized and marketed, we need to understand how the film formation process works," said Soles. "You have to know the properties of these materials first in order to control their stability."
As a polymer film cools, two different things can happen locally within it: Either its molecules can crystallize, starting from some nucleation center (such as a scratch) and then expanding into the surrounding unstable film. Or, because of chemical differences between the polymer and its underlying substrate, such as a silicon wafer, the film's molecules can "dewet"-similar to the beading up of water droplets on a windshield. If crystallization and dewetting occur simultaneously, they can couple to create imperfections that can be a nuisance for applications that rely on film uniformity, such as organic solar cells. The challenge is to bring these instabilities under control for constructive purposes.
Using model polymers with well understood crystallization behavior, the team discovered that a few degrees' variation in temperature controls whether crystallization or dewetting will dominate the hardening process. They also found that when two growing crystals expand and collide, the stress created where they contact each other can cause dewetting-but if the angle between the two expansion fronts is small enough, then this localized dewetting might be averted.
"What's cool about this discovery is not just that we have better understanding of polymer films, which are widely used in a range of coating and interface technologies," said coauthor Jack Douglas, also of NIST's polymers division. "There's a whole class of mathematical problems in which you have multiple effects that are all fighting for domination of some field of action-the spreading of languages, for example, or the growth of different tissues within organisms. It's a very common phenomenon, and this research could provide theoretical insight into those problems as well."
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Funding for this study was provided by the National Research Council.
* B.C. Okerberg, B.C. Berry, T.R. Garvey, J.F. Douglas, A. Karim and C.L. Soles. Competition between crystallization and dewetting fronts in this polymer films. Soft Matter, November 2008, 1-7. DOI: 10.1039/b806074f
National Institute of Standards and Technology (NIST)
"If organic photovoltaics-to take just one example-are ever to be realized and marketed, we need to understand how the film formation process works," said Soles. "You have to know the properties of these materials first in order to control their stability."
As a polymer film cools, two different things can happen locally within it: Either its molecules can crystallize, starting from some nucleation center (such as a scratch) and then expanding into the surrounding unstable film. Or, because of chemical differences between the polymer and its underlying substrate, such as a silicon wafer, the film's molecules can "dewet"-similar to the beading up of water droplets on a windshield. If crystallization and dewetting occur simultaneously, they can couple to create imperfections that can be a nuisance for applications that rely on film uniformity, such as organic solar cells. The challenge is to bring these instabilities under control for constructive purposes.
Using model polymers with well understood crystallization behavior, the team discovered that a few degrees' variation in temperature controls whether crystallization or dewetting will dominate the hardening process. They also found that when two growing crystals expand and collide, the stress created where they contact each other can cause dewetting-but if the angle between the two expansion fronts is small enough, then this localized dewetting might be averted.
"What's cool about this discovery is not just that we have better understanding of polymer films, which are widely used in a range of coating and interface technologies," said coauthor Jack Douglas, also of NIST's polymers division. "There's a whole class of mathematical problems in which you have multiple effects that are all fighting for domination of some field of action-the spreading of languages, for example, or the growth of different tissues within organisms. It's a very common phenomenon, and this research could provide theoretical insight into those problems as well."
###
Funding for this study was provided by the National Research Council.
* B.C. Okerberg, B.C. Berry, T.R. Garvey, J.F. Douglas, A. Karim and C.L. Soles. Competition between crystallization and dewetting fronts in this polymer films. Soft Matter, November 2008, 1-7. DOI: 10.1039/b806074f
National Institute of Standards and Technology (NIST)
Polymer Films Current Events and Polymer Films News RSSCaltech scientists solve decade-long mystery of nanopillar formations
Scientists at the California Institute of Technology (Caltech) have uncovered the physical mechanism by which arrays of nanoscale (billionths-of-a-meter) pillars can be grown on polymer films with very high precision, in potentially limitless patterns.
Bottoms up: Better organic semiconductors for printable electronics
Researchers from the National Institute of Standards and Technology (NIST) and Seoul National University (SNU) have learned how to tweak a new class of polymer-based semiconductors to better control the location and alignment of the components of the blend.
MIT crafts bacteria-resistant films
Having found that whether bacteria stick to surfaces depends partly on how stiff those surfaces are, MIT engineers have created ultrathin films made of polymers that could be applied to medical devices and other surfaces to control microbe accumulation.
UW paper in Science shows how some solids mimic liquids on nanoscale
A University of Waterloo physics and astronomy research team, in a paper to be published Friday in Science Magazine, shows how some solids behave like liquids on the nanoscale.
Scientists use nanomaterials to localize and control drug delivery
Using nanotechnology, scientists from UCLA and Northwestern University have developed a localized and controlled drug delivery method that is invisible to the immune system, a discovery that could provide newer and more effective treatments for cancer and other diseases.
New paper reveals nanoscale details of photolithography process
Scientists at the National Institute of Standards and Technology (NIST) have made the first direct measurements of the infinitesimal expansion and collapse of thin polymer films used in the manufacture of advanced semiconductor devices.
Emory physicist opens new window on glass puzzle
When most people look at a window, they see solid panes of glass, but for decades, physicists have pondered the mysteries of window glass: Is glass a solid, or merely an extremely slow moving liquid" An Emory University research team led by physicist Eric Weeks has yielded another clue in the glass puzzle, demonstrating that, unlike liquids, glasses aren't comfortable in confined spaces.
A new wrinkle in thin film science
A remarkably simple experiment devised by scientists yields important information about the mechanical properties of thin films--nanoscopically thin layers of material that are deposited onto a metal, ceramic or semiconductor base.
UD scientists discover new class of polymers
For years, polymer chemistry textbooks have stated that a whole class of little molecules called 1,2-disubstituted ethylenes could not be transformed into polymers-the stuff of which plastics and other materials are made.
Virginia Tech chemists create new polymers by adding DNA base pairs
Chemists at Virginia Tech are creating new polymers by adding DNA base pairs. Attributes include improved stretchable behavior and self-healing polymer films and coatings.
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