Experimental free volume aspects of the polymer rheology as obtained by positron annihilation lifetime spectroscopy

The ortho-positronium (o-Ps) annihilation parameters, i.e, the mean o-Ps li fetime,tau(3),and the o-Ps relative intensity, I-3, in cis-1,4-polybutadien e (cis-1,4-PBD) and polyisobutylene (PIB) over a wide temperature range inc luding the glass-liquid transition have been measured by means of positron annihilation lifetime spectroscopy (PALS). From them the free volume micros tructural characteristics, i.e, the mean free volume hole size,V-h, and the free volume hole fraction, F-h, have been determined via a semiempirical q uantum-mechanical model of o-Ps in a spherical hole or a phenomenological m odel of volumetric and free volume hole properties, respectively. Consequen tly, the literature rheological data for both the above-mentioned polymers have been related to the free volume hole fractions via the WLF-Doolittle t ype equation. It has been found that i) in the case of PIE this equation ho lds over 130K above the glass transition temperature T-g and ii) in the cas e of cis-PBD the WLF-Doolitle equation is valid in the temperature range ov er 60K above 1.3T(g) but below 1.3T(g) down to T-g the modified WLF-Doolitt le-Macedo-Litovitz equation with the additional activation-energy term desc ribes the shift factor data better.