(72d) Thermomechanical Behavior of Polymer Films at Cryogenic Temperatures

The purpose of this study was to develop a theoretical approach for estimation of mechanical properties of polymer films at extreme cryogenic temperatures (e.g. 4 K). Time-temperature superposition (TTS) was used as a mathematical tool for data extrapolation. TTS is commonly used to extrapolate polymer data taken at high temperatures towards long times which represent data experimentally difficult to obtain. Here, however, the method was used for predictions in the opposite direction, extrapolating data taken at short times towards very cold temperatures. Polymer core films of the electronic laminates, ULTRALAM® 3850HT (LCP) and Pyralux AP8525R (polyimide), were tested on a TA Instruments Q800 dynamic mechanical analyzer to obtain creep compliance data, which was then extrapolated using TTS. The Arrhenius equation basis was used for the TTS model. Master curves were generated for the polyimide and for the machine and cross-machine direction for LCP by first calculating time-based shift factors for each set of isothermal creep data. Activation energy of flows, E_a, were then calculated using the Arrhenius model. All data were accumulated within the range of testing temperatures varying from about 150K to 293 K. Both polymers were found to have relaxation transitions within this range and predictions therefore initiated from the colder side of the transition temperature. Predictive equations are proposed for creep compliance. These compliances will be useful for overall failure models at 4K associated with cryogenic devices ranging from electronics to magnetic cavities.