About (Dis)Entanglement and Knots in Confined Polymer Films.

The reptation mechanism introduced by de Gennes and Edwards, where a polymer diffuses along a fluffy tube defined by the constraints imposed by its surroundings, convincingly describes the relaxation of long polymers in concentrated solution and melts. We propose that the scale for the tube diameter is set by local chain segregation which we study analytically. We show that the concept of local segregation is especially operational for confined geometries, where segregation extends over mesoscopic domains drastically reducing binary contacts, and provide an estimate of the entanglement length, which increases with decreasing film thickness. Our predictions are quantitatively supported by extensive Molecular Dynamics simulations on systems consisting of long, entangled, chains [1]. A strong disentanglement is found for strong confinement. We now combine this finding with a recently proposed knot analysis [2] which shows an increase of self-entanglements in thin films. Their impact on dynamics, however, seems to be limited.

[1] N.K. Lee, D. Diddens, H. Meyer, A. Johner, Phys. Rev. Lett. 118 (2017) 067802.
[2] H. Meyer, E. Horwath, P. Virnau arXiv:1710.11077 [cond-mat.soft]