R. Bärenwald, S. Goerlitz, R. Godehardt, A. Osichow, Q. Tong, M. Krumova, S. Mecking, K. Saalwächter.
Local Flips and Chain Motion in Polyethylene Crystallites: A
Comparison of Melt-Crystallized Samples, Reactor Powders, and
Macromolecules 47, 5163-5173 (2014).
The crystallization and the mechanical properties of polyethylene,
which is one of the most important commodity polymers, are influenced by the
crystalline α-relaxation. This process originates from the diffusive chain transport
through the crystallites as mediated by local 180° flips. Recent studies have stressed
the relevance of the chain folding morphology on the chain diffusion, but its relation
to the rate of jumps of the individual repeat units has not yet been addressed. In this
study, we compare samples with very different morphology, including nanocrystals
as a unique new model system, and use proton low-field and carbon-13 high-field
solid-state NMR spectroscopy to determine the rate of local jumps and the largescale
crystalline-amorphous diffusion coefficient, respectively. We find that samples
with tight folds (reactor powders and nanocrystals) display on average lower activation energies of the local jumps. Nanocrystals
stand out in that they feature a significantly broader distribution of local jump rates, which we attribute to the location of stems in
the finite nanocrystal. Our results for the crystalline-amorphous long-range diffusion are at partial variance with previous
findings in that samples with tight folds do not generally exhibit the fastest diffusion, and we discuss the related ambiguities. Our
data suggest that the higher chain mobility in the amorphous domain of melt-crystallized samples has an accelerating effect on
intracrystalline chain dynamics at high temperatures but is accompanied by a more progressive slowdown at low temperatures
due to cooperativity effects.