Effect of tacticity on the local dynamics of polypropylene melts

Abstract

We present a quasielastic neutron scattering (QENS) study of the effect of tacticity on the local dynamics of polypropylene (PP). QENS measurements were carried out on different spectrometers. On IN10 (ILL, France) we have measured the decrease of the elastic intensity as a function of temperature for atactic (a-PP), isotactic (i-PP), and syndiotactic (s-PP) PP. The results show that the polyproylene sub-Tg dynamics is independent of tacticity. Measurements of the dynamic incoherent structure factor were carried out on the spectrometers IRIS and OSIRIS (ISIS, UK) and, after Fourier transform, the intermediate scattering functions were computed and analyzed. For all samples investigated, the intermediate scattering functions I(Q,t) show good overlap using shift factors that are close to those reported in the literature. Detailed analysis of the incoherent dynamic structure factor in terms of fast and slow decay processes indicates that in the subpicoseconds regime molecular motion is independent of tacticity. The slower segmental process depends on the sample stereoregularity and, consistently with C13 NMR measurements and molecular dynamics simulations, isotactic PP relaxes faster than the other polymers, while s-PP is the slowest. Correlation times display a non-Arrhenius temperature dependence that is described by a Vogel–Fulcher–Tamman relationship, with parameters that depend on tacticity. Thus the ratio between the correlation times is temperature dependent, and while the dynamic behavior of the samples is very similar at 460 K, considerable differences are observed at lower temperature.