Confined dynamics in poly(ethylene terephthalate): a coherent and incoherent neutron scattering study

Chain Dynamics of Ultrahigh Molecular Weight Polyethylene Composites with Graphene Oxide Nanosheets

We investigate the melt chain dynamics of ultrahigh molecular weight polyethylene (UHMWPE) and its composites with graphene oxide (GO) nanosheets by means of neutron spin echo spectroscopy. For the GO concentrations explored, we observe hindered chain dynamics with respect to the pure UHMWPE. We propose a model where a fraction of the polymer is immobilized on top and at the bottom of GO sheets. This model enables us to provide a microscopic measurement of the adsorbed and free polymer fractions, as well as the thickness of the adsorbed layer. Our experiments provide experimental nanoscopic evidence of GO hindering entanglement formation in a polymer melt, a phenomenon that had been observed at the macroscale before via rheological measurements.

Coarse-grained molecular dynamics simulations of poly(ethylene terephthalate)

We have constructed efficient coarse-grained (CG) models of poly(ethylene terephthalate) (PET), using three mapping schemes, in which a repeat unit is lumped into either three or four beads. The CG potentials are parameterized to reproduce target distributions of an underlying accurate atomistic model [H. Eslami and F. Müller-Plathe, Macromolecules 42, 8241-8250 (2009)]. The CG simulations allow equilibration of long PET chains at all length scales. The CG results on the density of PET in melt and glassy states, chain dimension, local packing, and structure factor are in good agreement with experiment. We have established a link between the glass transition temperature and the local movements including conformational transitions and mean-square displacements of chain segments. Temperature transferabilities of the three proposed models were studied by comparing CG results on the static and thermodynamic properties of a polymer with atomistic and experimental findings. One of the three CG models has a good degree of transferability, following all inter- and intra-structural rearrangements of the atomistic model, over a broad range of temperature. Furthermore, as a distinct point of strength of CG, over atomistic, simulations, we have examined the dynamics of PET long chains, consisting of 100 repeat units, over a regime where entanglements dominate the dynamics. Performing long-time (550 ns) CG simulations, we have noticed the signature of a crossover from Rouse to reptation dynamics. However, a clear separation between the Rouse and the reptation dynamics needs much longer time simulations, confirming the experimental findings that the crossover to full reptation dynamics is very protracted.