Investigating the mechanical and optical properties of thin PDMS film by flat-punched indentation

We investigated mechanical and optical properties of a thin polydimethylsiloxane (PDMS) film through a flat punched indentation experiment and a finite element simulation. A microscopic imaging method was used to measure the compressive strain of thin PDMS films, and its corresponding bulk refractive index (RI) was estimated using the relation between density and refractive index. A total internal reflection (TIR) experiment was conducted to estimate the local RI values near the bottom surface of PDMS film. We obtained the correlations between force and indentation displacement in thin PDMS films (10-70 micron). Stress-strain curves present the non-linear deformation of PDMS film with the instantaneous modulus depending on the load and the thickness. Poisson's ratio of PDMS film is estimated by fitting with Yang's asymptotic solution and is inversely proportional to the aspect ratio between the radius of the flat punch probe and the film thickness. RI of PDMS film increases with the decreasing film thickness as well as the increasing load. The bulk RI was increased up to 1.476 by the compressing load while the bottom-surface RI was increased only up to 1.435. This difference is explained qualitatively through the localized stress-strain distribution from finite element analysis. An inhomogeneous stress-strain distribution is observed in the simulation with lower strain at the bottom of the film, corresponding to experiment. A lower strain leads to lower local density resulting in a lower refractive index. Our research provides fundamental insights into the correlation between mechanical and optical properties of thin polymer films.