Isothermal compression of poly (dimethylsiloxane), 1,4-poly(butadiene), and a model Estane (in both pure form and a nitroplasticized composition similar to PBX-9501 binder) at pressures up to 100 kbars has been studied using atomistic molecular dynamics (MD) simulations. Comparison of predicted compression, bulk modulus, and U(s)-u(p) behavior with experimental static and dynamic compression data available in the literature reveals good agreement between experiment and simulation, indicating that MD simulations utilizing simple quantum-chemistry-based potentials can be used to accurately predict the behavior of polymers at relatively high pressure. Despite their very different zero-pressure bulk moduli, the compression, modulus, and U(s)-u(p) behavior (including low-pressure curvature) for the three polymers could be reasonably described by the Tait equation of state (EOS) utilizing the universal C parameter. The Tait EOS was found to provide an excellent description of simulation PVT data when the C parameter was optimized for each polymer. The Tait EOS parameters, namely, the zero-pressure bulk modulus and the C parameter, were found to correlate well with free volume for these polymers as measured in simulations by a simple probe insertion algorithm. Of the polymers studied, PDMS was found to have the most free volume at low pressure, consistent with its lower ambient pressure bulk modulus and greater increase in modulus with increasing pressure (i.e., crush-up behavior).
The effects of nanoparticles on the rates of gas diffusion through glassy polymers were studied by a combination of molecular dynamics and kinetic Monte Carlo techniques designed to overcome the computational limitations in obtaining long-time trajectories in the diffusive regime of gas molecules in glassy polymer systems. Using such a methodology, we studied the effect of fullerene nanoparticles upon the diffusivities of N(2) and CO(2) in a polystyrene matrix. The addition of nanoparticles was found to cause a lowering of the diffusion coefficients of both N(2) and CO(2). However, the magnitudes of this lowering and their volume fraction dependencies are seen to depend explicitly on the nature of the penetrant and temperature. We discuss the possible physical mechanisms underlying such behavior.
Copolyesters are a subset of polymers that have the desirable properties of strength and clarity while retaining chemical resistance, and are thus potential candidates for enhancing the impact resistance of soda-lime glass. Adhesion between the polymer and the glass relates to the impact performance of the system, as well as the longevity of the bond between the polymer and the glass under various conditions. Modifying the types of diols and diacids present in the copolyester provides a method for fine-tuning the physical properties of the polymer. In this study, we used molecular dynamics (MD) simulations to examine the influence of the chemical composition of the polymers on adhesion of polymer film laminates to two soda-lime glass surfaces, one tin-rich and one oxygen-rich. By calculating properties such as adhesion energies and contact angles, these results provide insights into how the polymer-glass interaction is impacted by the polymer composition, temperature, and other factors such as the presence of free volume or pi stacking. These results can be used to optimize the adhesion of copolyester films to glass surfaces.
Zheng and Thompson have recently reported a comparison of three atomistic force fields for prediction of physical properties of dimethylnitramine (DMNA) from molecular dynamics (MD) simulations. 1 Specifically, they compared the rigid molecule force field by Sorescu, Rice, and Thompson (SRT); 2 the generalized AMBER-(with partial charges assigned by Zheng and Thompson); and a fully atomistic quantum chemistry (QC)-based force field developed by Smith, Bharadwaj, Bedrov, and Ayyagari (SBBA) 3 for prediction of unit cell parameters, bulk modulus, and melting temperature for crystal DMNA, as well as volumetric properties of liquid DMNA. The authors found that simulations using the SBBA force field predict DMNA properties that significantly deviate from experimental data. This was particularly apparent for the melting temperature of DMNA, which was found to be ∼70 K lower than the experimental value. The two other force fields investigated overestimated the melting temperature of DMNA, but to a lesser extent. In an attempt to improve prediction of the DMNA melting temperature, Zheng and Thompson have adjusted the AMBER dihedral potential, arguing that the original potential predicts dihedral barriers that are off a factor of ∼3 from predictions of QC calculations and experiments.In this comment, we show that (1) simulations using the SBBA force field predict physical properties of crystal DMNA in very good agreement with experiment, including melting temperature; (2) the interpretation of Zheng and Thompson of conformational properties using SBBA and AMBER force fields are incorrect; and (3) the AMBER and AMBER-modified force fields do not provide an adequate description of DMNA conformational properties.SBBA Force Field. To make sure that parameters published in the original SBBA force field manuscript 3 are consistent with those that were used in the simulations described below, we have set up input files for DNMA simulations from scratch using information provided in the original force field reference. It is necessary to point out that in the original force field paper, it was implicit that intramolecular nonbonded (van-der-Waals and Coulomb) interactions are excluded between atoms separated by one or two bonds and are included without any scaling for those that are separated by three (the so-called 1-4 interactions) and more bonds. The former is a standard practice in atomistic MD simulations, whereas the latter (the scaling factor for the 1-4 nonbonded interactions) can depend on a particular force field. Incorrect implementation of the 1-4 intramolecular interactions 4 in Zheng and Thompson's work for the SBBA force field resulted in poor predictions of DMNA properties for this force field.MD Simulations of Liquid DMNA. MD simulations in the NPT ensemble have been run for the system of 160 molecules at 400 K and atmospheric pressure. Zheng and Thompson have reported that for liquid DMNA, they obtained a density of 1.16 g/cm 3 from simulations using the SBBA force field. They acknowledged that this value is abou...
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