Free Volume Profiles at Polymer–Solid Interfaces Probed by Focused Slow Positron Beam

Abstract

The properties of materials involving polymer–solid interfaces are determined not only by the properties of the constituents but also by the interfacial region of the polymer near the solid, the so-called interphase. Here we report on positron annihilation lifetime spectroscopy (PALS) measurements of the free volume near the interface with a focused positron beam. The measurements were performed at an interface between a Teflon AF 1600 film and a 30 nm thick SiN membrane from the backside of a Si substrate which was locally etched away to expose the ultrathin SiN membrane. We were able to focus the positron beam onto the exposed region and to probe the SiN–Teflon AF 1600 interfacial region with low implantation energies, resulting in very narrow implantation profiles and excellent depth resolution. We found a pronounced decrease in orthopositronium lifetime close the interface. On the basis of modeling, we could estimate the interphase width and the average density increase in the interfacial region as ∼12 nm and 5–7%, respectively.