Formation of Nanoscale Protrusions on Polymer Films after Atomic Oxygen Exposure: Observations with Positron Annihilation Lifetime Spectroscopy

Atomic oxygen (AO)is one of the dominant components of the residualatmosphere in low Earth orbit. AO collides with spacecraft with atranslational energy of 5 eV, forming nanoscale protrusions on polymericmaterials. To clarify the influences of a polymer's chemicalstructure on the formation of AO-induced microstructures, this studyinvestigated the size of free-volume holes and the layer thicknessthat interacted with AO for polyethylene (PE), polypropylene (PP),and polystyrene (PS) by positron annihilation lifetime spectroscopy.The injection energies of positrons varied from 1.3 to 10 keV to adjustthe injection depth (range) into the polymers (40 nm-1.6 & mu;m).For the pristine films, the lifetime of ortho-positronium(o-Ps, & tau;(3)) was longer in the orderof PS, PP, and PE regardless of the injection energy of positrons,showing the different sizes of free-volume holes with radii of 0.29,0.31, and 0.32 nm, respectively. The fraction of the decay componentcorresponding to o-Ps in all decay components (relativeintensity of o-Ps, I (3)) was used to investigate the chemical change induced by AO exposure.The I (3) values for the three polymers weredecreased by AO exposure of (2-5) x 10(18) atoms/cm(2) or more at a depth of 40-48 nm, obtained by 1.3 keVpositrons. This indicates that AO formed polar groups (i.e., an oxidizedlayer) on the polymer surfaces. The maximum depths of such chemicalchange for PE and PP were deeper than that for PS. The different sizesof free-volume holes would affect the diffusion or ballistic penetrationof AO, resulting in the difference in the oxidized layers'thicknesses and surface morphologies.