University Park, Pa. -- Industry's push for thinner and thinner polymer films may one day hit the boundary where these films no longer behave the same as bulk polymers, but according to Penn State polymer scientists, the location of that boundary is still unknown.
"We are trying to see if the molecules of polymers in ultra thin films of only 14 nanometers had the same shape and size as the molecules in the bulk material," says Ronald L. Jones, graduate student in polymer science.
A thin film of 14 nanometers is slightly thinner than the smallest virus. Polymer thin films are used as coatings, adhesives, parts lubricants, in inks and in the semiconductor industry. Biological applications of polymers in thin films include biomembranes and lipid biolayers. Characteristics that may differ between bulk and ultra thin films include strength, permeability, diffusion of solvents and optical properties which could influence applications.
Polymers are chains of organic molecules that tend to form balls and entangle with other molecules forming an amorphous solid with very little order.
"In bulk form, the molecules are sort of spherical, and previous computer simulations suggested that the shape in these thin films would not change much, however, indirect measures by some researchers implied that this picture is not correct," says Dr. Sanat K. Kumar, professor of material science and engineering, in the College of Earth and Mineral Sciences. "We found that the simulations were correct and the shape of the molecule does not change very much."
The researchers, who also include Derek L. Ho and Robert M. Briber of the University of Maryland and Thomas P. Russell of University of Massachusetts, reported their findings in today's (July 8) issue of the journal Nature.
Working with Kumar, Jones used polystyrene, the polymer used to make Styrofoam cups, plates and packing material, to create ultra thin films on small silicon disks. The researchers chose silicon because its surface can be polished to an almost molecularly smooth finish and it is transparent to a neutron beam. Using a small-angle neutron scattering device at the National Institute of Standards and Technology, the researchers aimed a beam of neutrons at the thin film surface and measured the angle of deflection off the material to determine the size and shape of the molecules.
While the 14 nanometer thin film is much thicker than a single atom, the diameter of the polymer ball in the bulk material is larger than the film's thickness. The researchers believe that the polymer spheres have a lot of space inside so that while the basic shape remains the same, the packing of the molecules inside the ball is more efficient.
"The molecules in the thin film may also be less tangled than in the bulk polymer," says Jones. "However, we have not yet tested that possibility."
Tighter packing of molecules and less entanglement may alter the characteristics of the thin film materials, but not as much as if the thin film structure were very different from the bulk structure. With even thinner films, which finally cross the boundary into thin film structure, the molecules might not have a ball shape, but might be more linear and more ordered.
"We know that the bulk material is organized in a very random way," says Kumar. "We can measure the randomness of the molecular organization in the 14 nanometer thin film and we know that they are still random. These thin films are not ordered."
EDITORS: Dr. Kumar is at sxk11@psu.edu by email; Mr. Jones is at (814) 863-4381 or rljones@planck.plmsc.psu.edu by email.
Journal
Nature