Beyond The Spherical Approximation: Elongated Free Volume Holes In Rubbers: A Positron Annihilation Study

The free volume fraction, a key parameter for the understanding of mechanical and transport properties of polymers," has been evaluated in a fluoroelastomer and a cis-polyisoprene rubber by means of positron annihilation lifetime" spectroscopy and dilatometry. The evaluation showed that the assumption of elongated holes allows one to get a very good" agreement between the free volume fraction experimentally determined and the theoretical expectation based on the latticehole" model. On the other hand, systematic discrepancies are found using the spherical approximation. Moreover, the average" hole size is found to be correlated to the effective bond length leff, a parameter connected to reptation motions and largely" independent of the polymer structure. The result sheds some light on conformational statistics for the most flexible linear" polymers that approach Gaussian chain behavior.