Free volume-size dependence on temperature and average molecular-weight inpoly(ethylene oxide) determined by positron annihilation lifetime spectroscopy

Positron lifetime measurements were carried out in a series of poly(ethylen e oxide)-PEO-of different average molecular weights ((M) over bar(w)): 1000 , 1500, 6000, 10,000, 300,000, and 4 M. The mean radius ((R) over bar) and the mean free volume size (Vf) values were determined using a semiempirical equation that correlates the orthopositronium (o-Ps) lifetime (tau(3)) and size of holes existing in the amorphous phases. The hole mean radius value s determined at room temperature from lifetime spectra were found to be bet ween 2.68 and 2.97 Angstrom, and the hole volumes between 80 and 110 Angstr om(3). Free volume size evolution was studied with temperature variation un til the melting temperature of the PEO samples. The degree of crystallinity and the melting temperatures were determined by Differential Scanning Calo rimetry. (C) 1999 John Wiley & Sons, Inc.