Time-Dependent Orientation Coupling in Equilibrium Polymer Melts

Abstract

The motion in concentrated polymer systems is described by either the Rouse or the reptation model, which both assume that the relaxation of each polymer chain is independent of the surrounding chains. This, however, is in contradiction with several experiments. In this Letter, we propose a universal description of orientation coupling in polymer melts in terms of the time-dependent coupling parameter κ(t). We use molecular dynamics simulations to show that the coupling parameter increases with time, reaching about 50% at long times, independently of the chain length or blend composition. This leads to predictions of component dynamics in mixtures of different molecular weights from the knowledge of monodisperse dynamics for unentangled melts. Finally, we demonstrate that entanglements do not play a significant role in the observed coupling.

Inter-Chain Cross-Correlation in Multi-Chain Slip-Link Simulations without Force Balance at Entanglements

“…In the previous papers 4,6,11,12) , the origin of the cross-correlation was suggested to be the force balance and the osmotic force. These two forc- On the other hand, the cross-correlation between different chains is observed for the present model.…”

“…However, the molecular simulations by Cao and Likhtman 4) mentioned above have revealed that the cross-correlation does exist even in melts of bead-spring chains for which nematic interaction is rather weak. Recent studies by the multi-chain slip-link simulations 6,11,12) suggest that the cross-correlation is due to the force balance around entanglement and/or the osmotic force that suppresses density fluctuations.…”

“…S (t) can be decomposed into the self-correlation for the same chain A (t) and the cross-correlation between the different chains C (t) as, Following the study by Cao and Likthman 4) , the contribution of the cross-correlation is expressed by the coupling parameter κ (t) defined as, Because similar trends have been reported for the bead-spring simulations 4) and for the original model 6,11) , the upturn in κ (t) might be originated from the creation of entanglement rather than the inter-chain forces. The mechanism for such a slow relaxation nature is unknown, though.…”

“…The effect of cross-correlation between different chains in entangled polymers has been reported [1][2][3] and its contribution to rheology is not negligible [4][5][6] . In the tube model 7) and its extensions 8,9) , the test chain is assumed to move independently and the cross-correlation is not considered, at least explicitly.…”

“…On the other hand, the cross-correlation has been reported via the infrared dichroism and birefringence for polystyrenes 1,2) , and via the H double-quantum NMR for polybutadienes 3) . Cao and Likhtman 4) have reported for the bead-spring simulations that the contribution of cross-correlation in the relaxation modulus is around 40 %.…”