Evaluation of Force Fields for Molecular Simulation of Polyhedral Oligomeric Silsesquioxanes

Abstract

Polyhedral oligomeric silsesquioxanes (POSS) are nanometer-size molecules suitable for the production of organic-inorganic nanocomposite materials. These organic-inorganic nano-building blocks show promise for enabling the production of polymeric materials of exceptional mechanical properties as well as novel composite materials. While the experimental studies of these materials have rapidly evolved in the past decade, their theoretical investigation is still in its infancy. Toward the validation of force fields for the molecular simulation of POSS-containing systems, we present the charge-transfer reactive (CTR) force field for the molecular simulation of polyhedral oligomeric silsesquioxane (POSS) molecules and compare the ability of this, and several force fields taken from the literature, to predict the thermophysical properties of POSS-containing systems. The literature force fields compared include the universal force field (UFF) and the COMPASS and Hybrid-COMPASS force fields. Predictions from molecular dynamics simulations of the structural parameters (unit cell vectors), melting temperature, and FT-IR spectra of crystals of POSS monomers are presented. The POSS monomers investigated are octahydride, octamethyl, and octapropyl POSS. Predicted quantities are compared to experimental results where available and provide molecular-level physical insight into several aspects of the behavior of POSS molecules. While all the force fields tested perform reasonably well, our results indicate that the Hybrid-COMPASS and CTR force fields predict structural properties that are in good agreement with experimental data.

Thermal Behavior of Caged Silsesquioxane (POSS) Studied by Molecular Dynamics Simulations

“…As presented in Table 2, our results were in consistent to those reported by Ionescu et al [36], in which the differences were below 0.2%. Our data were also in a good agreement to the data reported based on the single crystal X-ray analyses [41].…”

“…In the present work, UFF models were used without a Coulombic term, as in the work by Ionescu et al [36] For the H-POSS system, they have discussed that the effect of the term is negligible for the structure of the system using a different force field. However, further refinements of the force fields and studies of the role of Coulombic term for the long-range structure of the systems will be necessary.…”

“…Ionescu et al [36] have demonstrated the MD calculations of H-POSS using this UFF model to estimate the volume change of crystals as a function of temperature. This method would also provide thermal behavior of crystals of POSSs having various substituents and it might be possible to estimate the effect of POSS substituents.…”

“…Cummings and coworkers [34] simulated thermodynamic and transport properties of POSS in poly(dimethylsiloxane) and obtained radial distribution functions, potentials of mean force, and self-diffusion coefficient for POSS monomers dissolved in n-hexadecane by the MD simulations [35]. They also calculated H-POSS using different force fields to simulate the melting behaviors [36].…”

Parameter optimization in molecular dynamics simulations using a genetic algorithm

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a b s t r a c tIn this work, we introduce a genetic algorithm for the parameterization of the reactive force field developed by Kieffer [12][13][14][15][16]. This potential includes directional covalent bonds and dispersion terms.

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