Study of Sub-T_g Relaxation in Plasticized Polycarbonate with the Quartz Crystal Microbalance

Quartz crystal resonators are well-known for their sensitivity to the film mass and are commonly referred to as quartz crystal microbalances (QCM). A unique aspect of quartz crystal resonators is that they are also able to accurately probe the viscoelastic properties in the megahertz frequency regime. Here we discuss the use of the rheometric quartz crystal microbalance (RheoQCM) as a fixed frequency rheometer operating at 15 MHz to study the rheological properties of bisphenol-A polycarbonate (PC) from -30 °C up to 180 °C. The effects of dioctyl terephthalate (DOTP) and the solvent, 1,4-dioxane, on the viscoelastic response were investigated. The plasticizer increases the modulus in the glassy regime and decreases the magnitude of a sub- T_g relaxation peak centered near 50 ° C. The data were analyzed with an open-source Python-based interface developed for use with a low-cost vector network analyzer, using methods that can also be applied to data obtained from commercial QCM-D instrumentation.