109 research outputs found
Wall slip in primitive chain network simulations of shear startup of entangled polymers and its effect on the shear stress undershoot
In some recent experiments on entangled polymers of stress growth in startup
of fast shear flows an undershoot in the shear stress is observed following the
overshoot, i.e., before approaching the steady state. Whereas tumbling of the
entangled chain was proposed to be at its origin, here we investigate another
possible cause for the stress undershoot, i.e., slippage at the interface
between polymer and solid wall. To this end, we extend the primitive chain
network model to include slip at the interface between entangled polymeric
liquids and solid walls with grafted polymers. We determine the slip velocity
at the wall, and the shear rate in the bulk, by imposing that the shear stress
in the bulk polymers is equal to that resulting from the polymers grafted at
the wall. After confirming that the predicted results for the steady state are
reasonable, we examine the transient behavior. The simulations confirm that
slippage weakens the magnitude of the stress overshoot, as reported earlier.
The undershoot is also weakened, or even disappears, because of a reduced
coherence in molecular tumbling. In other words, the disentanglement between
grafted and bulk chains, occurring throughout the stress overshoot region, does
not contribute to the stress undershoot.Comment: 38 pages and 9 figure
Mechanical Properties of End-crosslinked Entangled Polymer Networks using Sliplink Brownian Dynamics Simulations
The mechanical properties of a polymeric network containing both crosslinks
and sliplinks (entanglements) are studied using a multi-chain Brownian dynamics
simulation. We coarse-grain at the level of chain segments connecting
consecutive nodes (cross- or sliplinks), with particular attention to the
Gaussian statistics of the network. Affine displacement of nodes is not
imposed: their displacement as well as sliding of monomers through sliplinks is
governed by force balances. The simulation results of stress in uniaxial
extension and the full stress tensor in simple shear including the (non-zero)
second normal stress difference are presented for monodisperse chains with up
to 18 entanglements between two crosslinks. The cases of two different force
laws of the subchains (Gaussian chains and chains with finite extensibility)
for two different numbers of monomers in a subchain (no = 50 and no = 100) are
examined. It is shown that the additivity assumption of slip- and crosslink
contribution holds for sufficiently long chains with two or more entanglements,
and that it can be used to construct the strain response of a network of
infinitely long chains. An important consequence is that the contribution of
sliplinks to the small-strain shear modulus is about ⅔ of the
contribution of a crosslink
Effetti non lineari nella reologia dei liquidi polimerici concentrati
Dottorato di ricerca in ingegneria chimica. 8. ciclo. Comitato scientifico G. Astarita, N. Grizzuti e G. Marrucci. Relatore G. MarrucciConsiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7, Rome; Biblioteca Nazionale Centrale - P.za Cavalleggeri, 1, Florence / CNR - Consiglio Nazionale delle RichercheSIGLEITItal
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