Page 72 - ILL Annual Report 2019

THEORY
Nicolás A. García. Argentinian
The ILL
‘I was awarded a PhD degree in Physics from the Universidad Nacional del Sur in 2015. My research interests include polymer physics and colloidal systems. Nowadays, I am a postdoctoral researcher at the ILL, my investigation
focus being the fundamental properties of heterogeneous polymers such as block copolymers, nanocomposites and polymer rings. Always working in close collaboration with experimentalists, I try to reveal the secrets of these fascinating systems.’
Confinement disentangles polymer chains in thin films
Polymer thin films are ubiquitous in our daily life, encompassing applications ranging from packaging and food wrapping
to adhesives, lubricants and protective coatings for furniture and glasses. They are also widely used in microelectronics and nanotechnology. On a day-to-day basis, the thickness of the films decreases to dimensions comparable with the size of a single polymer chain. Hence, an important question arises as to whether such small dimensions under confinement changes the fundamental properties of polymers. In this paper, we shed light on this intriguing issue.
AUTHORS
N.A. García (ILL)
J.-L. Barrat (UGA, Grenoble University, France)
ARTICLE FROM
Macromolecules (2018)—doi: 10.1021/acs.macromol.8b01884
REFERENCES
[1] M. Doi and S.F. Edwards, Oxford University Press: Oxford (1986) [2] A. Korolkovas, P. Gutfreund and J.-L. Barrat, J. Chem. Phys. 145
(2016) 124113
[3] M. Kröger, Comput. Phys. Commun. 168:3 (2005) 209 [4] Y.H. Lin, Macromolecules 20 (1987) 3080
The mechanical and viscoelastic response of polymers in melts and concentrated solutions depends fundamentally on the molecular weight of the chains. Indeed, when their molecular weight increases the chains’ mobility with respect to each other is constrained by the simple fact that they cannot cross each other. In this way, so-called entanglements occur naturally [1]. Nowadays, we know that these topological constraints dictate the fundamental properties of polymers. Thus, a logical approach to answering our question is to investigate the network of entanglements and how it is affected by confinement.
As entanglements are not directly observable via experiments, numerical simulations are essential for exploring their nature.
We performed numerical simulation using a recently proposed coarse-graining method [2], which models the chains
as pseudo-continuous bodies interacting through ultrasoft potentials to speed up the simulation. The motion is then resolved using Brownian dynamics with large time-steps.
The topological analysis was performed using the Z1 algorithm [3], a method that detects entanglement through geometrical minimisation. Furthermore, the algorithm provides the position of entanglements within the film.
Figure 1
a) Comparing monomer and entanglement density profiles for three different films constructed with chains of length N = 1 024. Two different scales (left, blue and right, red axes) are used to plot these quantities in the same figure to aid comparison. The labels on the curves indicate the film thickness.
b) Normalised reduction of entanglements per chain as a function of confinement for all free-standing films’ thicknesses studied here for chain lengths N = 512, 1 024, and 2 048.
ANNUAL REPORT 2019