Free Volume of Interphases in Model Nanocomposites Studied by Positron Annihilation Lifetime Spectroscopy

The free volume studies were performed in well characterized model nanocomposites by positron annihilation lifetime spectroscopy (PALS) to explore the influence of the interphase nanoscale character of the hydrophobically functionalized filler particles and the nanoscale particle size on positron parameters. A weakly repulsive system, which should not form an interphase, was obtained by mixing of low molecular weight poly(ethylene-alt-propylene) (PEP) and hydrophobically modified silica with varying concentration. A low molecular weight sample was chosen. Because of a finite centre of mass diffusion and a low radius of gyration the interstitials between particles are effectively filled and hence a most suitable model system is obtained. The absence of an interphase was confirmed by neutron scattering and neutron spin echo measurements. DSC experiments showed a constant glass transition temperature T g and a decrease in Δc p at T g with increasing filler concentration as expected. In contrast, PALS measurements showed decreasing glass transition temperatures and a strong drop of the thermal expansion coefficient above T g. These seemingly conflicting results are demonstrated to be due to nanoscale character of the hydrophobically functionalized filler particles with sizes in the range of the positronium diffusion length which requires taking into account out-diffusion of positronium from the particles. In particular, it is shown that the changes on PALS parameters such as o-Ps lifetime or its intensity, with increasing filler content cannot be attributed to the formation of an interphase with properties different from the polymer matrix. Like the neutron scattering experiments, PALS does not find any evidence of an interphase between the filler and the polymer within the resolution limit of the present technique, which is in agreement to the neutron scattering experiments.