Non-Gaussian and Cooperative Dynamics of Entanglement Strands in Polymer Melts

We present a study on tracer diffusion of short poly(ethylene oxide (PEO) tracers in a strongly entangled PEO melt. We find that within the entanglement volume, the dynamics of entanglement strands is significantly non-Gaussian. Following theoretical predictions of Guenza [Guenza, M. G. Phys. Rev. E 2014, 895 052603], we quantify the non-Gaussian correction alpha(2)(t) in terms of a logarithmic Gaussian function with a maximum at a time in the order of the Rouse times of different tracers and a width that is independent of the tracer's length. The strength of the non-Gaussian correction is found to be equal for all tracers providing another proof that tracers mirror the host dynamics, which needs to be independent of the tracer length. As for polyethylene [Zamponi, M. et al. Phys. Rev. Lett. 2021, 12618 187801], independent of their molecular weight, the tracer's center-of-mass mean square displacements are subdiffusive at short times until they have reached the size of the reptation tube d; then, a crossover to Fickian diffusion takes place indicating cooperative chain motion within the entanglement volume d(3). Thus, the host dynamics within the tube is not only cooperative but also significantly non-Gaussian.

Automated summary

Short PEO tracers in a PEO melt exhibit significantly non-Gaussian dynamics within the entanglement volume, with a non-Gaussian correction quantified by a logarithmic Gaussian function that is independent of tracer length. The strength of the non-Gaussian correction is equal for all tracers, indicating that tracers mirror the host dynamics. At short times, the center-of-mass mean square displacements of tracers are subdiffusive until reaching the size of the reptation tube, then transitioning to Fickian diffusion, implying cooperative chain motion within the entanglement volume.