Direct measurement of free‐volume hole distributions in polymers by using a positronium probe

We have directly measured the free‐volume hole distributions in semicrystalline polypropylene by positron lifetime annihilation spectroscopy. A Laplace inversion technique was engaged to analyze the positron lifetime spectra measured under quasi‐isotropic external pressures of 0, 4.2, and 14.7 kbar into continuous lifetime distributions. The hole radii distributions as determined from the ortho‐positronium lifetime distributions are found to be between 4.0 and 0.5 Å and to have maxima at 3.0, 1.9, and 1.1 Å under the external pressures of 0, 4.2, and 14.7 kbar, respectively.