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.

We use nonequilibrium molecular dynamics simulations to investigate the structural properties of an oriented melt of n-eicosane under steady-state planar elongational flow. The flow-induced structure was evaluated using the structure factor s k taken as the Fourier transform of the total pair correlation function g r . We found that the equilibrium liquid structure factor is in excellent agreement with the one determined via x-ray diffraction. Moreover, a new x-ray diffraction experiment has been performed on a crystalline n-eicosane sample. The resulting intramolecular contribution to the structure factor was found to be in very good agreement with the simulated one at a high elongation rate, indicating the existence of a possible crystalline precursor structure. DOI: 10.1103/PhysRevLett.96.037802 PACS numbers: 61.20.Ja, 36.20.Ey, 61.10.ÿi, 61.20.Gy The crystallization of polymer melts under flow has generated a tremendous amount of interest over the years. Understanding crystallization mechanisms, kinetics, and crystallite morphologies are just a few of the problems that present an ongoing interest among research communities [1][2][3]. With the rapid advancements in computational capabilities and the development of new algorithms, molecular simulation techniques are playing an increasingly important role in elucidating these problems. Short and long chain n-alkanes have been extensively used to model the behavior of polyethylene, in particular, and polymers in general. In practice, polymers are known to form ordered domains when subjected to deformation either in the melt or solid states. In industrial applications such as fiber spinning or film blowing, this phenomenon is desired, and a precise control over the nucleation rates, crystallite growth, and morphology is critical. From an experimental perspective, it is very challenging to investigate the individual phenomena taking place during polymer crystallization, given the different length and time scales involved. This is why molecular simulation is potentially the ideal tool for investigating these processes.Crystallization of long chain molecules from quiescent melts is particularly difficult to attain with the molecular simulation techniques available today, due to the long simulation times and atomistic-level detail needed to observe such phenomena. Extensive studies have been dedicated to characterizing melting and crystallization of n-alkanes under equilibrium conditions using molecular dynamics [4][5][6][7][8][9][10][11] or Monte Carlo [3,12,13] techniques. Even for the relatively short alkane chains, the simulation times needed to observe ordered phase formation are on the order of tens of nanoseconds, which is prohibitive on most supercomputers today. To this end, alternative methods have been proposed in order to enhance the crystallization rates, which include crystallization in the presence of a surface [2] or increasing the melting point by driving the system away from equilibrium via uniaxial stretching [7,14,15] or shear flow [6,16,17].I...

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