Many of the most characteristic properties of polymer materials like plastics come from the fact that their long chain molecules get tangled up. Entanglements between different polymer chains behave like knots, which make a polymer liquid behave like a solid over quite perceptible time scales, just like silly putty. The results of a new experiment show that when you make the polymer film very thin – thinner than an individual polymer molecule – the chains become less entangled with each other, with significant effects on their mechanical properties.
The experiments are published in this weeks Physical Review Letters; I’m a co-author but the main credit lies with my colleagues Lun Si, Mike Massa and Kari Dalnoki-Veress at McMaster University, Canada. The abstract is here, and you can download the full paper as a PDF (this paper is copyright the American Physical Society and is available here under the author rights policy of the APS).
This is the latest in a whole series of discoveries of ways in which the properties of polymer films dramatically change when their thicknesses fall towards 10 nm and below. Another example is the discovery that the glass transition temperature of polymer films – the temperature at which a polymer like polystyrene changes from a glassy solid to a gooey liquid – dramatically decreases in thin films. So a material that would in the bulk be a rigid solid may, in a thin enough film, turn into a much less rigid, liquid-like layer (see this technical presentation for more details). Why does this matter? Well, one reason is that, as feature sizes in the microelectronics industry fall below 100 nm, the sharpness with which one can define a line in a thin film of a polymer resist could limit the perfection of the features one is making. So the fact that the mechanical properties of the polymer themselves change, purely as a function of size, could lead to problems.