**abstract:**
It has long been known that the variation of density and stiffness between
different species signifies changes in the degree of entanglement of long-chain
flexible polymer melts. An overlooked point is that, as chain length N increases,
the contour length density (or mass density), and the stiffness of the system (or
characteristic ratio), increase, until they reach chain length-independent values
(large N asymptotic regime). Here we examine how the variation of these
quantities, as chain length increases, is related with the onset of entanglements,
and the associated changes in the underlying system topology.
Our analysis is based on the reduction of Dissipative Particle Dynamics
trajectories of a coarse-grained polymer melt to Primitive Paths (PPs). The latter
are generated by using the CReTA algorithm, which constructs PPs by reducing
chain conformations to the corresponding shortest paths.
We will show that, as N increases, the density and stiffness exhibit an Ndependence
which leads to larger chain overlap in the short, than in the long chain
regime. For large N, chain overlap falls gradually to the scaling law expected of
long-chain systems.
At the level of PPs, the increasing overlap leads to a crossover in the system
topology which can be described as a gradual transformation of PP conformations
from thin rods (short chains), to random walks (long chains), A simple scaling
model predicts that this transformation leads to a Rouse to reptation transition in
dynamics and rheology. The entanglement molecular weight (MW), Me is
interpreted as the crossover length of this transition. The predicted critical to
entanglement MW ratio, Mc * Me is one, which, though small, is compatible with
packing length independence and the suppression of contour length fluctuations
within the model. The comparison between a dynamical and a static topological
analysis reveals a slowing down of Rouse modes, which is maximum at the length
scale where the underlying system of PPs appears as a network of topological
constraints.
*

* References : *Macromolecules

Wed 8 Jun, 18:15 - 18:45, Aula Dini

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