(103a) Coarse-Graining of Nanocoating Using MARTINI Force Field for Mechanical Property Evaluation

Nanocomposite coatings, either thermoplastics or thermosets, have been proven to possess significantly improved performance or novel functionalities, such as self cleaning, self healing, etc.  Dramatically increased surface area of nanosize fillers inside the coating matrix is the main reason for achieving superior mechanical properties.  Molecular simulation can be performed to identify the root cause behind enhancement in properties of nanocomposite coatings, which is increase in polymer number density around nanoparticles.^[1]  But these results of coarse-grained simulations can be difficult to validate with real experimental results due to loss of physical meaning and degrees of freedom of the system.  Hence, effective coarse graining methodology needs to be developed to study molecular behavior of polymer systems and model with experimental validation.

Recently, a MARTINI based approach has been introduced to develop successfully coarse grained models for biomolecular systems.^[2]  A coarse grained model is generated from atomistic models, which should be more reliable in model accuracy and model-based prediction, and thus it can be compared with experimental data.  In this paper, the MARTINI technique is adopted for coarse grained simulations of polymer nanocomposite material.  Due to increasing environmental concerns, water based resin systems, such as acrylics, are used tremendously for nanocoating syntheses.  Thus polymethyl methacrylate (PMMA) resin with spherical silica nanoparticles dispersed in it is modeled for simulating a nanocoating.  The volume fraction of nanofillers is kept at 5%.  All atomistic simulations are performed using the CHARMM force field.  The verification to this model is obtained by obtaining density and glass transition temperature consistent with experimental data.  The equilibrated structures of a polymer from all-atom model are used to generate the coarse grained model by mapping onto coarse-grained beads.  The parameters from the MARTINI model are modified, and a new set of parameters are generated to obtain a coarse grained model with comparable densities and radius of gyration with that of atomistic structures.  All molecular dynamics simulations are performed with NAMD, version 2.7.

The MARTINI models for pure polymer samples and nanocoating samples of different nanoparticle sizes are deformed with external force at a fixed strain rate in longitudinal and transverse directions to evaluate its mechanical properties.  The Young’s modulus of nanocomposite is observed to increase at lower temperatures.  The relative change in stiffness of nanocomposite is lower below the glass transition (T_g) compared to above T_g due to increased mobility of polymer chains.  It is also seen that the Young’s modulus increases with decreases in the size of the nanoparticle.  This justifies the increase in the surface area leading to improved mechanical properties.  During the deformation testing, the externally applied stress reaches a peak and goes down, which dictates the onset of plastic flow of the nanocomposite.  The behavior of polymer models under applied stress is in agreement with experimental results.  This demonstrates the applicability of the use of coarse-grained potentials in the design of nanocoating formulations with superior mechanical properties.  

References

1. Xiao J., Y. L. Huang, and C. Manke, "Computational Design of Thermoset Nanocomposite Coatings: Methodological Study on Coating Development and Testing," Chemical Engineering Science, 65, 753-771, 2010.
2. Marrink S. J., Risselada H. J., Yefimov S., Tieleman P. D. and Vries A. H., “The MARTINI Force Field: Coarse Grained Model for Biomolecular Simulations”, Journal of Physical Chemistry B, 111, 7812-7824, 2007.