Open spaces and relaxation processes in the subsurface region of polypropylene probed by monoenergetic positron beams

Abstract

Open spaces and relaxation processes in the subsurface region of isotactic polypropylene were investigated by monoenergetic positron beams. From measurements of the lifetime spectra of positrons, the size of the open spaces in the subsurface region (≤0.2 μm) was found to be larger than that in the bulk; their differences were estimated as 20% at 295 K and 10% at 395 K. From conventional positron annihilation experiments, the glass-transition temperatures, Tg (upper) and Tg (lower) were determined as 306 K and 278 K, respectively. These transition temperatures were associated with the onset temperatures of the molecular motions under the constraint imposed by crystalline regions and those free from the constraint, respectively. In the subsurface region, although the onset temperatures of the molecular motions were close to those in the bulk, the molecular motions above Tg (lower) were found to be suppressed. The annihilation characteristics of positrons with different implantation energies were also discussed.