Permeation through thin polymer films at cryogenic temperatures

• Measured He permeability coefficients of BoPET, PEEK, PEI, and EVOH thin films from 190 – 30 K. • Measured H 2 permeability coefficients of PEEK between 210 – 30 K. • Permeability-temperature curves followed an Arrhenius relation. • Below 90 – 150 K, measured leak rates dropped below the detection limit of a calibrated mass spectrometer. • The above polymer films could serve as barrier membranes for liquid H 2 storage. Cryogenic propellant bladders could substantially improve propellant storage and slosh characteristics, especially in microgravity conditions. However, an experimental effort in the 1970s showed unacceptably high permeation rates of hydrogen and helium through thin polymer films (candidate materials for bladders) at cryogenic temperatures. A qualitative hypothesis for the increase is the relatively large thermal de Broglie wavelength of hydrogen and helium molecules at cryogenic temperatures could increase the hopping rate of the molecules and resulting bulk permeation. This hypothesis was tested by measuring the leak rate of helium through thin films (on the order of 25 µ m thick) of polyether ether ketone (PEEK), polyetherimide (PEI), ethylene vinyl alcohol (EVOH), and biaxially-oriented polyethylene terephthalate (BoPET) from 190 – 30 K. Hydrogen permeation was also tested through PEEK specimens from 210 – 30 K. A calibrated variable mass-spectrometer and custom sample test-cell and apparatus were utilized. The measurements indicate that permeation follows an Arrhenius relation with decreasing temperature until the limit of the sensor resolution was achieved below ∼90 K for He, and below ∼150 K for H 2 . The anomalous increase in permeability with reducing temperature observed in the historical 1970′s study was only reproduced with samples known to have developed leaks in the sealing. Thus, polymer films could be viable permeation barriers for liquid cryogens in aerospace applications, including bladders.