Elastic modulus of amorphous polymer thin films: Relationship to the glass transition temperature

Understanding the mechanical properties of polymers at the nanoscale is critical in numerous emerging applications. While it has been widely shown that the glass transition temperature (T_g) in thin polymer films generally decreases due to confinement effects in the absence of strong favorable interactions between the polymer and substrate, there is little known about the modulus of sub-100 nm polymer films and features. Thus, one might use this depressed T_g as a surrogate to estimate how the modulus of nanoconfined polymeric materials deviates from the bulk, based on constructs such as Williams-Landel-Ferry (WLF) time-temperature superposition principles. However, such relationships have not been thoroughly examined at the nanoscale where surface and interface effects can dramatically impact the physical properties of a material. Here, we measure the elastic modulus of a series of poly(methacrylate) films with widely varying bulk T_g's as a function of thickness at ambient temperature, exploiting a wrinkling instability of a thin, stiff film on an thick, elastic substrate. A decrease in the modulus is found for all polymers in ultrathin films (<30 nm) with the onset of confinement effects shifting to larger film thicknesses as the quench depth (T_gbulk - T) decreases. We show that the decrease in modulus of ultrathin films is not correlated with the observed T_g decrease in films of the same thickness.