Free Volume Distribution

Size distribution of free volume in glassy membrane materials is accessible to estimation by computer simulations (molecular dynamics, MD) and to experimental studies by the probe methods. In low permeable and highly selective conventional glassy polymers like polyimides, size distribution of free volume elements (microcavities), according to MD, is located within the range of microcavity radius 1–5 Å and can be represented by the Gauss function (Heuchel et al. 2004) (see Fig. 1). For polymers with greater gas permeability such as poly(trimethylsilyl propyne) (PTMSP) or amorphous Teflon AF, according to MD, it is much wider and extends up to microcavity radius 10–12 Å (Hofmann et al. 2003) (see Fig. 2). Positron annihilation lifetime spectroscopy (PALS) also indicates widely distributed sizes of free volume in highly permeable polymers. However, there are two interpretations of wide size distribution. According to Shantarovich et al. (1993) and Consolati et al. (1996), lifetime...