Methods for investigation of the free volume in polymers

Experimental probe methods for investigation of the free volume in polymers (positron annihilation lifetime spectroscopy, ¹²⁹Xe NMR spectroscopy and inverse gas chromatography) and relevant theoretical approaches (computer simulation) are considered. Various definitions of the free volume and related terms are given. The physical principles that underlie particular probe methods as well as the scope and limitations of each method are discussed. A systematic comparison of the size of free-volume elements in polymers determined by different methods is carried out.