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.

Neutron scattering investigations on methyl group dynamics in polymers

“…FEW scans reported in Ref. [153] for different tacticities of PP did not show apparent differences, within the experimental noise, in the temperature range dominated by methyl group dynamics. This result is supported by the rather close values of the measured librational peaks for the isotactic [144] and atactic [145] conformations.…”

Tacticity as a Factor Contributing to the Thermal Stability of Polystyrene

“…The tacticity‐induced conformational changes are accompanied by a change in the chain dynamics. MD simulations and NMR experiments demonstrate an increase in the rate of the segmental relaxation dynamics of PP in the row: syndiotactic, atactic, isotactic 10, 15. Rheological measurements show that syndiotactic PP exhibits a 10 times higher zero shear rate viscosity16 and 10–15 kJ · mol −1 larger activation energy of viscous flow16, 17 than isotactic PP.…”

Generation of well relaxed all atom models of stereoregular polymers: a validation of hybrid particle-field molecular dynamics for polypropylene melts of different tacticities

“…Even in the case of fully amorphous materials or polymer melts, the tacticity plays an important role in influencing the final material behavior. In particular, in this case, tacticity is effective in the behavior of several properties such as: glass transition temperature (T g ), [6][7][8] self-diffusion coefficient (D), [8] entanglement molar mass (M e ), [9] and other rheological properties. [10,11] Since tacticity mainly influences the torsional angle distributions of the polymer backbone, the main molecular effects on single chain properties are connected to changes in chain stiffness.…”