HARNESSING SURFACE WRINKLING TO MEASURE THE VISCOELASTIC PROPERTIES OF POLYMER FILMS
Edwin P. Chan,1 Kirt M. Page,1 Qinghuang Lin,2 and Christopher M. Stafford1
1Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD 20899
2IBM T. J. Watson Research Center, Yorktown Heights, NY 10598
Polymer films are ubiquitous in many technological applications such as optics, microelectronics, and biomedical devices. Since the viscoelastic response of the polymer film can impact device performance, it is important to understand and control the viscoelastic properties of the confined polymer layer. However, there are limited approaches that can measure the viscoelastic properties of confined polymer films. In this work, we present a new measurement technique based on surface wrinkling that can quantify the viscoelastic properties of confined polymer films. By heating a polymer film that is confined by both a rigid substrate and a thin superstrate to a temperature that facilitates polymer mobility, a thermal mismatch stress develops that leads to the formation of wrinkles on the superstrate surface. The wavelength and amplitude of these surface wrinkles reflect the mechanical properties of this composite film. Based on this approach, we demonstrate that thermal wrinkling can be used to measure the elastic modulus and shear viscosity of glassy, styrene-based polymer films at annealing temperatures slightly above their glass transition temperature. By monitoring the time- and temperature-dependent wavelength and amplitude evolution of the surface wrinkles, we can extract the viscoelastic properties of the polymer film with the aid of a theoretical model.