CASE STUDY

Molecular dynamics
simulations accelerate
the development andoptimization of recyclable
tire materials
Scientists from Evonik and Schrödinger gain a deeper understanding of the impact ofadditives and macrocyclic structures on trans-polyoctenamer rubber (TOR)

Executive Summarya)• Accurately predicted key thermophysical andstructural properties such as glass transition
temperature (Tg), density, free volume, anddiffusion coefficient for rubber mixture systems• Explored the miscibility of transpolyoctenamer rubber (TOR) with different
crosslinkers and base polymers• Developed an understanding of the impactof macrocyclic polymer chains on the structuraland thermophysical properties• Accelerated rational design of highperformance rubber formulations and provided
insights into the key factors for processability ofrecycled rubbersb)

Snapshots of final configuration after the production MD
run for a) linear chains, and b) macrocyclic chains of 2000
molecular weight.1

Challenges

ApproachTrans-polyoctenamer rubber (TOR), commerciallyknown as VESTENAMER®, has been widely used asa tire additive to improve the processability andrecyclability of tire rubbers. In order to efficiently
co-vulcanize different rubbers to obtain improved
properties, it is vital to understand the interactionof TOR with different components of rubber
formulations, including crosslinkers and basepolymers. However, experimental characterizationof these formulations is time-consuming andresource-intensive.

Molecular modeling provides an economicalalternative to long experiments and lengthy
development cycles for new tire formulations.
Scientists from Evonik and Schrödinger worked
together to digitally evaluate the miscibility of TOR
with various crosslinkers and polymers, and to look
into the impact of macrocyclic polymer chains on
TOR properties. With GPU-accelerated moleculardynamics (MD) engine, scientists were able to run
the simulations at high-speed, e.g. 50ns of diffusion
calculations, in only a few hours.
All models and simulations were performed using theSchrödinger Materials Science Suite and Desmondfor MD according to the following procedure:Constructed models for polymers andcrosslinkers included in Figure 1, andequilibrated the systems
Computed cohesive energy, solubility
parameter distance, energy of mixing, andinteraction energy for different mixtures topredict miscibility
Predicted thermophysical (Tg) and structural(density, diffusion, free volume) properties
to understand the impact of macrocyclic
chains (greater than 8000 g/mol molecularweight) on TOR

Figure 1. Structures of crosslinkers and polymers used in simulations.1

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Results
The computational approach revealed new
insights into the compatibility of various
polymers/crosslinkers and provided an accurate
evaluation of the structural and thermophysicalproperties of TOR systems as summarized to
the right. The work serves as an example ofhow digital molecular simulations can provide
unique insights that are not readily available oreconomical to study in an experimental setting.

Summary

The simulation methods and workflows presented
in this work can be easily extended to study otherpolymer/crosslinker systems.

• SBR, NR, and TOR are likely not miscible with eachother. A mixture of these three polymers would
most likely result in a three-phase system, whichis in line with experimental studies• The miscibility order for the crosslinkers inSBR, NR, and TOR is TBzTD > SA > CBS > S8,suggesting S8 likely has a limited miscibility in all
three polymers, which is also demonstrated bycomputing the energy of mixing• Crosslinkers diffuse more in TOR when comparedto SBR and NR allowing for better crosslinking• Interaction energy and density analysis indicatedideal mixing at low crosslinker concentration for
all the mixtures• TOR macrocycles can potentially affect theviscosity as well as other mechanical properties,
and may contribute to lower glass transition
temperature of TOR

Figure 2. Predicted diffusivity of different crosslinkers in TOR, NR, and SBR

3References
1. Using molecular simulation to assess the impact of additives andmacrocyclic structures on trans-polyoctenamer rubber (TOR)
Mohammad Atif Faiz Afzal, Benjamin J Coscia, Andrea R Browning,Thomas S. Asche, Alexander Paasche, Mathew D. Halls
International Elastomers Conference, Oct 4-7, 2021, Pittsburgh

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