Dynamics of Syndiotactic Polypropylene

We present a study of the structure and dynamics of long-chain syndiotactic polypropylene (s-PP) melts by small-angle neutron scattering (SANS), rheology, neutron backscattering, and neutron spin echo spectroscopy (NSE). From SANS, we determine the characteristic ratio of C-infinity = 11.5, which is larger than that obtained from previous experiments but in good agreement with simulation. Concerning dynamics, in the Rouse regime, neutron backscattering and incoherent NSE revealed Rouse rates that very well agree with each other, demonstrating the consistency of measurements in energy and time space. The single-chain dynamic structure factor was also measured by NSE. As none of the conventional data evaluation schemes for local reptation were successfully applicable, we used the novel extended de Gennes dynamic structure factor [Monkenbusch, M. J. Chem. Phys. 2023, 159, 034902]. We found that s-PP displays a strongly asymmetric tube with an aspect ratio of about 0.5. Allowing for a small non-Gaussianity improves the fit. The scaling properties of the Monkenbusch dynamic structure factor are well fulfilled also for s-PP. Contrary to intuition, the tube asymmetry appears not to be related to chain stiffness at least not in the range of C-infinity = 5 (polyisoprene) and 11.5 (s-PP).