Free-volume holes in hydrophilic polymer thin films under humidified conditions as seen by low-energy positron annihilation

Low-energy positron annihilation is a powerful method for characterizing the nanoscopic hole properties of thin films. Some of the positrons introduced into insulating materials like oxides and organic polymers may form ortho-positronium (o-Ps), the spin parallel positron-electron bound state, which tends to localize at open spaces in substances. In free space, o-Ps annihilates into 3{gamma} rays with an intrinsic lifetime of 142 ns. If o-Ps is trapped in a closed hole, it annihilates with a lifetime shorter than 142 ns by a 2{gamma} pick-off process, so that the lifetime is well correlated with the hole dimension. The size of holes, especially smaller than 1 nm in radius R [nm], can be evaluated with the o-Ps lifetime {tau} [ns] based on the following equation, {tau}=0.5[1 - R/(R + 0.1656) + 1/(2{pi})sin(2{pi}R/(R + 0.1656))]{sup -1}. This equation has been invoked frequently for evaluating the free-volume holes of various polymers. Recently our group has developed a controlled-environment positron probe micro-analyzer (atmospheric PPMA) which can be used with the positron annihilation lifetime technique to explore the free-volume holes in functional thin films at atmospheric pressure. In this system, positrons are formed into a brightness-enhanced, focused, short-pulsed beam while their energies are controlled. The obtained positron beam is extracted via an extremely thin vacuum window into the atmosphere, and then irradiated onto the sample surface. In this work, the atmospheric PPMA was applied to measuring the o-Ps lifetimes of polyvinyl alcohol and polycaprolactam films with a thickness of a few hundred nm as a function of relative humidity in the ambient atmosphere for examining the free-volume holes of functional thin materials in consideration of the effect of water molecules on the polymer chain mobility. This work was supported by NEDO. (author)