University of Southern Denmark
Effects of Graphite and Plasticizers on the Structure of Highly Entangled Polyisoprene Melts
Giunta, G.; Svaneborg, Carsten; Ali Karimi-Varzaneh, Hossein; Carbone, Pietro
Published in:
ACS Applied Polymer Materials
DOI:
10.1021/acsapm.9b00815
Publication date:
2020
Document version:
Accepted manuscript
Citation for pulished version (APA):
Giunta, G., Svaneborg, C., Ali Karimi-Varzaneh, H., & Carbone, P. (2020). Effects of Graphite and Plasticizers
on the Structure of Highly Entangled Polyisoprene Melts.
ACS Applied Polymer Materials
,
2
(2), 317-325.
https://doi.org/10.1021/acsapm.9b00815
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Article
Effects of Graphite and Plasticizers on the
Structure of Highly Entangled Polyisoprene Melts
Giuliana Giunta, Carsten Svaneborg, Hossein Ali Karimi-Varzaneh, and Paola Carbone
ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00815 • Publication Date (Web): 23 Dec 2019
Downloaded from pubs.acs.org on January 6, 2020
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1
Effects of Graphite and Plasticizers on the
Structure of Highly Entangled Polyisoprene Melts
G. Giunta
a*
, C. Svaneborg
b
, HA. Karimi-Varzaneh
c
, P. Carbone
a*
a
School of Chemical Engineering and Analytical Science, The University of Manchester, Oxford Road, M13
9PL, Manchester, United Kingdom
b
Department of Physics, Chemistry and Pharmacy, University of Southern Denmark (SDU), Campusvej 55,
Odense M 5230, Denmark
c
Continental Reifen Deutschland GmbH,Jädekamp 30, D-30419 Hannover, Germany
KEYWORDS Polyisoprene, graphite, plasticizers, coarse-grained, entanglements, loops,
trains, tails
Abstract
Using a simple and efficient way to optimise a chemically-specific bead-and-spring model for
polymer/surface systems, we analyse the structural properties of high molecular weight
polyisoprene (PI) in contact with graphite. We find that, in the vicinity of the graphite, the
adsorbed PI chains assume a pancake structure, are highly packed and highly entangled. The
addition of plasticizers even with moderate surface affinity guarantees an almost complete
surface coverage and forces the polymer chains to detach from the surface and to become less
entangled. The softening effect of the plasticizers is observed also in bulk when they are added
to the system but are not adsorbed on the surface. Finally, we show that the definition of the
thickness of the interface is not unambiguous but depends on the observable used to
characterized the melt: it is function of the polymer molecular weight if defined looking at the
chain conformation but it becomes independent on the polymer chain length if defined looking
at the entanglement density.
*
giuliana.giunta@manchester.ac.uk
*
paola.carbone@manchester.ac.uk
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Introduction
In order to improve mechanical and rheological properties of polymeric materials, inorganic
particles with linear dimension up to 1m can be mixed in the polymeric matrix
1,2
. The addition
of these fillers enhances the mechanical response of the compound by improving for example
hardness and tear resistance but may be also used to modify other properties such thermal,
electrical conductivity and optical properties
1,3
. The polymer/filler interfacial area and the
chemical nature of the particles-polymer matrix interactions
4
are the two parameters directly
responsible for the modification of the properties. However due to the complexity of the
experimental characterization and of the material itself, which often contains polymer chains
with large distribution of molecular weights, particles of different sizes and geometry and
different type of additives, it is currently difficult to identify design rules for such materials
1
and thus their development is still very much empirical
5
.
To cut short the laboratory testing experimental time, there is therefore an increasing need of
predicting computational tools able to provide reliable models linking filler geometrical and
chemical characteristics with composite properties. Computer modelling has been widely
applied to achieve a rapid and accurate prediction of the properties of the nano-composites
before their preparation, processing and characterization
4,5
. Nevertheless, molecular
simulations of many body systems using chemically detailed all-atom models are still limited
to polymers with low molecular weight due to the prohibitively long relaxation times associated
to the macromolecules
6
. The use of efficient coarse-grained (CG) models, where atomic details
are lost by grouping atoms into single interaction sites (beads, superatoms), is therefore
invaluable in order to overcome the spatiotemporal limitations of large systems whose
behaviour is in the mesoscale
7–9
. Several examples of systematic CG models are already in the
literature. Bisphenol-A-polycarbonate on nickel
10
and PI on graphite
11
are examples of CG
models of specific polymer-surface systems derived by means of the Iterative Boltzmann
Inversion (IBI) technique
12
. Although these CG models accurately reproduced the structural
properties of atomistic simulations, the IBI technique is generally a high time consuming CG
method as it is based on a bottom-up approach that requires reference atomistic simulations to
be performed
7,13,14
. On the other hand, top-down CG methods such as the SAFT-γ
15
and
MARTINI
16
in principle, do not require atomistic trajectories as a reference to develop the CG
intermolecular potential derivation. These approaches are instead based on the reproduction of
macroscopic thermodynamic properties of small molecules that can be considered as building
blocks to construct bigger molecular systems. In both cases, the intramolecular potentials
usually require a structure-based parameterisation
6
. These models are used when maintaining
some chemical details of the system is essential to investigate the properties of interest
14
. For
example in the MARTINI model developed by Gobbo et al.
17
the authors describe the
adsorption of organic molecules on graphite. In this model the interaction parameters are
obtained by fitting the adsorption enthalpies from the gas phase and the wetting enthalpies of
the pure compounds.
For the specific case of CG of polymer melts, one of the most used CG model is the bead and
spring (or Kremer-Grest, KG) model
18
where bonded interactions are described by the finite-
extensible non-linear elastic (FENE) potential and the non-bonded beads interact through the
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3
Weeks-Chandler-Andersen (WCA) potential
19
. This model not only reduces the number of
degrees of freedom, but it is also computationally advantageous since does not include the
calculations of the expensive long-range interactions between polymer beads and the square
root operations involved in the harmonic potential
6
. In the majority of cases, this simple CG
model is used to study generic (i.e. non-chemically specific) polymeric and polymer/surface
systems
20–23
. The lack of chemical details in the model is not a problem when looking for
scaling laws or universal behaviour and the KG model has helped to clarify many aspects of
the structure and dynamics of polymer chains in the immediate vicinity of surfaces
11,20–22,24,25
.
Svaneborg and co-authors
26,27
have recently showed that is it possible, by tuning the chain
stiffness (i.e. the Kuhn length), to adapt a generic bead-and-spring polymer model to simulate
melts of chemically-specific (at and above the Kuhn length) polymers. The authors developed
a set of KG model parameters for several commodity polymers and showed that following their
parameterization procedure it is possible to use the KG model to reproduce entanglement
moduli comparable with the experimental data.
Here we are interested in investigating the structural features of a typical elastomeric composite
comprised of (cis-)polyisoprene and graphite. This composite material is widely used in the
automotive industry, as the incorporation of carbon black (nano) fillers in natural rubber
enhances the mechanical properties of tyres (rolling and wear resistance, wet grip, etc.)
28
. The
graphite is in this instance, considered as a model for the surface of carbon black fillers
11
.
The aim of this work is double folds: on one side we propose a new coarse-graining procedure
for polymer/surface systems that allows the inclusion of chemical specificity into the
mesoscopic description of the KG model; we then use the model to investigate the structural
properties of highly entangled PI chains in contact with an homogeneous graphitic surface
paying particular attention to the effect that low molecular weight diluents (plasticizers) have
on the conformation and entanglement density of the adsorbed polymer chains
29
.
Models and Methodology
PI (100% cis-1,4) is represented using a KG bead-and-spring model adapted to match the
universal properties of a wide range of specific polymers species developed by Svaneborg et
all
26
. The PI KG model is parameterized to match the conformation properties of cis-PI at and
above the Kuhn scale. In particular the entanglement density will emerge naturally since the
simulation model reproduce the correct Kuhn length and the correct density of Kuhn segments
of cis-PI. The details of the model can be found in reference 26 and a summary of the model
strategy is reported in the Supporting Information. Here we just summarize the parameters
relevant for the current work.
In the PI KG model, atoms are lumped together into spherical beads in such a way that a CG
bead contains 67% of the mass of polymer repeating unit (i.e. the mass of the bead is m
b
=45.62
g mol
−1
). The choice of the bead mass, along with other parameters of the force field is the
result of the coarse-graining scheme that allows the reproduction of the Kuhn length and the
Kuhn segment density of the real polymer
26
.
The CG sites interact through the WCA pair potential:
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