Characterizing the Thickness and Temperature Dependence of Elastic Modulus in Polymer Films Using Picosecond Ultrasonics

Abstract Polymer nanofilms have found extensive applications in fields such as medical science and microelectronics, making the exploration of their mechanical properties highly significant. In this study, we examine the dependency of the out-of-plane elastic modulus of polymer films on both thickness and temperature, utilizing picosecond ultrasonics. Multiple acoustic modes of a polymer film are employed to measure its elastic modulus with heightened precision. Measurements are conducted on samples with different thicknesses and at different temperatures. The results indicate that the modulus of polymer nanofilms strongly depends on both thickness and temperature. For polymethyl methacrylate (PMMA) films with a thickness of less than 81 nm, the modulus determined exhibits a decrement correlating with thickness and is quantifiably inferior to the analogous bulk value. Additionally, as the temperature rises, the elastic modulus declines at clearly varying rates, which are described by two distinct modulus-temperature coefficients.