A new study by a team including scientists from NIST
indicates that thin polymer films can have different properties depending on
the method by which they are made. The results suggest that deeper work is
necessary to explore the best way of creating these films, which are used in
applications ranging from high-tech mirrors to computer memory devices.
Thin films spread atop a surface have many applications in
industry. Inexpensive organic solar cells might be made of such films, to name
one potential use. Typically they’re made by dissolving the polymer, and then
spreading a small amount of the liquid out on a surface, called a substrate.
The solution becomes a film as the solvent dries and the remainder solidifies.
But as this happens, stresses develop within the film that can affect its
Manufacturers would like to know more about how to control
these stresses to ensure the film does what they want. But scientists who study
film formation often use a different method of casting films than a
manufacturer would. One method used in industry is “flow
coating”—similar to spreading frosting across a cake. Another method is
“spin casting”—placing a drop of liquid on a substrate that spins
rapidly and spreads the droplet out evenly by centrifugal force. Both methods
create smooth films generally, but the team decided to examine whether the two
methods create different effects in finished films consisting of a
self-assembling block copolymer.
“It’s an important question because some proposed
applications intend to take advantage of these effects,” Douglas
The team’s comparison led to results that surprised them.
Although the rapid spinning of spin casting is very dynamic, suggesting it
would convey more stress to the resulting film, it actually led to fewer
residual stresses than flow coating did. As previous studies have shown that
leftover solvent can lead to stresses in the film, the team’s new theory is
that because the solvent evaporates from the developing film more slowly in
flow coating, this solvent discourages the film solids from arranging
themselves into the equilibrium structure.
For one example, the practical benefits of this
understanding could help manufacturers who propose making computer memory
devices from thin films in which the solids arrange themselves as tiny
cylinders in the film. Such devices would require the cylinders to stand on
end, not lay down flat.
“We find we can get them to stand up much more easily
with one casting method than another,” Douglas
says. “If we can get better results simply by varying the mode of film
casting, we need to explore more deeply what happens when you make films by