Local free volumes in boron-bombarded Kapton polyimide: A positron lifetime study

The bombardment of thin films of Kapton polyimide with energetic boron ions can lead to an increase in the maximum mass uptake from 3.9 to 14% at 100% relative humidity. It can be estimated that the number of N_o of free-volume sites increases from 1.8 to 6.5 nm on boron bombardment, assuming that each sorption site accommodates exactly one water molecule. Boron irradiation also leads to a notably increased curvature of the convex absorption isotherm indicating that the mean hole size υ decreases significantly below a value of 0.1 nm³. Position annihilation lifetime (PAL) spectroscopy is used to study the microstructure of the modified films after boron bombardment.