Positronium (Ps) annihilation data in molecular substances is simply related to the repulsive barrier (U-0) and the radius (R) of the cavity which the positronium creates in a liquid or finds in a molecular solid. We use an improved version of spherical well model with diffused cavity wall, and an elegant linear relationship is obtained between the size of the cavity and the pick-off lifetime. Through this description, we describe the micro-porous nature of various molecular substances like cross-linked polyamide membrane material and compare the structural transitions of a similar hetero cyclic membrane material through temperature dependent positronium lifetime data.