Projecting Frequency Dependent Mechanical Properties of an Aromatic Polyimide at 4 Kelvin

The purpose of this study was to investigate a parameterized mathematical approach for estimating the complex modulus of a polyimide film at extreme cryogenic temperatures (e.g. 4 K). Such films are used in extreme cryogenic applications such as quantum computing, superconducting magnets and electronic devices. A polymer core of the electronic laminate, Pyralux AP8525R (polyimide), was tested on a TA Instruments Q800 dynamic mechanical analyzer from 140 K to 293K to obtain complex tensile moduli, and these data were then extrapolated using multiple types of fits. For one predictive method, activation energy of flows, E_a, were calculated using an Arrhenius model and frequency-temperature superposition. All fits were projected to 4 K. A second type of fit was a T³ model. A validation study using Vespel, a material where dynamic modulus had been previously reported at 4K, was also performed. The validation study indicated that for similar materials (e.g. amorphous aromatic polyimides) a cubic parametrized fit might be the most useful predictive tool. Other polymers reported as tested to 4K in the literature exhibited a plateau in modulus at low temperatures (sigmoidal fit). From the literature, an amorphous aromatic polyimide (Kapton) and polystyrene showed an increase in modulus at low temperature which is inconsistent with a plateau or sigmoidal fit. The T³ or cubic model was chosen as the best predictive method for the modulus of the Pyralux core polymer. The theoretical rationale for a cubic fit will be discussed.