Physics (Internal)

"Equilibrium Pathway of Spin-Coated Polymer Films"

This event is part of the Biophysics/Condensed Matter Seminar Series.

Abstract: Spin-coating is a prevalent method for making thin polymer films. In this process, a drop of dilute polymer solution is added to a substrate whereupon the substrate is spun at 500 – 4000 rpm to spread the solution. Rapid evaporation of the solvent causes the polymer to vitrify into a residue film in seconds. The abruptness of the process has led many to suggest that polymers in spin-coated films are non-equilibrated or metastable, which may explain some of the more bizarre results, such as negative thermal expansion and a large residual stress comparable to the polymer’s elastic modulus. There have been some speculations about the physical form of the metastable state, but none has been verified. Here, we show that metastability in spin-coated films can arise from their thickness uniformity, causing the surface profile of the films to deviate from the equilibrium, comprising thermally excited surface capillary waves compliant to the equipartition law. As a result, the equilibration process of the films encompasses sequential excitations of the capillary wave modes from the short- (fast) to long-wavelength (slow) modes. We anticipate that more detailed models can be constructed upon the ideas presented here to explain the unusual observations in spin-coated polymer films. A thorough understanding about the metastability of these films and their physical behaviors is necessary to predict and control their stability for practical applications.