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 lifetime,tau(3),and the o-Ps relative intensity, I-3, in cis-1,4-polybutadiene (cis-1,4-PBD) and polyisobutylene (PIB) over a wide temperature range including the glass-liquid transition have been measured by means of positron annihilation lifetime spectroscopy (PALS). From them the free volume microstructural 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 quantum-mechanical model of o-Ps in a spherical hole or a phenomenological model of volumetric and free volume hole properties, respectively. Consequently, the literature rheological data for both the above-mentioned polymers have been related to the free volume hole fractions via the WLF-Doolittle type equation. It has been found that i) in the case of PIE this equation holds over 130K above the glass transition temperature T-g and ii) in the case of cis-PBD the WLF-Doolitle equation is valid in the temperature range over 60K above 1.3T(g) but below 1.3T(g) down to T-g the modified WLF-Doolittle-Macedo-Litovitz equation with the additional activation-energy term describes the shift factor data better.