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

Open spaces and relaxation processes in the subsurface region of isotactic polypropylene were investigated by monoenergetic positron beams. From measu rements of the lifetime spectra of positrons, the size of the open spaces i n the subsurface region (less than or equal to 0.2 Gem) was found to be lar ger 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, T-g(upper) and T-g(lower) were determined a s 306 K and 278 K, respectively. These transition temperatures were associa ted with the onset temperatures of the molecular motions under the constrai nt imposed by crystalline regions and those free from the constraint, respe ctively. In the subsurface region, although the onset temperatures of the m olecular motions were close to those in the bulk, the molecular motions abo ve T-g(lower) were found to be suppressed. The annihilation characteristics of positrons with different implantation energies were also discussed. (C) 2000 John Wiley & Sons, Inc.