Researchers at the University of Waterloo in Ontario, Canada, have found that even when the bulk of a polymer material is solid, its surface behaves like a liquid. This finding has profound implications for applications such as nanolithography in which thin polymer films are used.
“The properties of polymer surfaces are very important in any application where you have physical contact with the surface region,” says physicist James Forrest, who carried out the study with his former research student Zahra Fakhraai, now at the University of Toronto. “This might be in a machining process, or even having two glassy polymers in contact below the glass transition temperature.”
With sliding between a glassy polymer and a solid surface, the level of energy dissipation depends on the surface properties in the first few nanometres. In the case of nanomachines, the stability and strength of the devices depends not on the bulk mechanical properties of the materials from which they are constructed, but rather their nanoscale properties.
The researchers are now measuring similar effects for other polymers, and are finding some surprising results. “For poly(methylacrylate), a common polymer in lithography, the surface properties depend on the substrate material even for films as thick as 200 nm,” says Forrest. “That is a huge distance for the type of forces expected to have an influence on these systems.”
Sheffield University’s Richard Jones saw this coming. “It’s been suspected for some time that polymer chains near a surface are free to move even when the bulk of the material is glassy. But the evidence for this has been circumstantial and the issue controversial,” he says. “Fakhraai and Forrest’s paper provides definitive proof, challenging theorists to explain this striking effect, and reminding us that we may not be able to predict the behaviour of polymer nanostructures on the basis of their bulk properties.”
Further reading: “Measuring the Surface Dynamics of Glassy Polymers”, Z. Fakhraai and J. A. Forrest, Science 319, 5863 (2008).
Figure: Nanoholes made by gold nanospheres in a polystyrene surface. By probing the surface dynamics of the material, researchers have shown that polymer surfaces at the nanoscale behave like liquids, even when the bulk of the material is solid (source: James Forrest/University of Waterloo).
Article first published in Nanomaterials News.