Buckle Up for Softy

Abstract

MATERIALS SCIENCE Tensile or compressive tests to measure the elastic modulus of a material are often limited by the size and shape of test specimens. Local indentation probing is useful for hard metals or ceramics, but less so for soft materials. The elastic modulus is a stiffness indicator but also reflects such properties as adhesion and swelling. Researchers are seeking alternative methods to measure complex samples such as a contact lens, which is small and soft and may need to be studied under hydrated conditions. Wilder et al. address this problem by inverting a technique used to characterize thin films. They measure the modulus of a compressed polymer by coating the surface with a stiffer material of known modulus. The periodicity of the buckling response depends primarily on the modulus ratio between the stiff film and softer substrate, and thus the unknown modulus can be determined from optical measurements of the buckled film. Modulus values from measurements of a model poly(dimethylsiloxane) system coated with a polystyrene film agreed well with those obtained from compression tests. The technique can also quantify spatial variations in modulus through a single experiment, as demonstrated on a hydrogel sample that was prepared with a spatial concentration gradient of cross-linking agent. — MSL Macromolecules 39 , 10.1021/ma060266b (2006).