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Coarse-grained (CG) computer simulation models of polyamide-6,6 (PA-6,6) and graphene have been developed to simulate long chains of polymer confined between surfaces. Here, groups of atoms are mapped onto a smaller number of beads, allowing simulation of nanoconfined polymers over the length scales and time scales much longer than what is achievable in atomistic simulations. The CG force field has been obtained using the iterative Boltzmann inversion method, in which the distribution functions for different degrees of freedoms are iteratively matched to the corresponding distributions obtained from atomistic simulations. Taking into account the detailed chemical structures of both polymer and confining surfaces, the resulting CG force field is shown to be transferable and applicable to simulate the confined polymer systems over a wide range of temperatures and intersurface distances. Employing this force field, CG simulations have been performed on long chains of PA-6,6 confined between graphene surfaces, at constant temperature, constant parallel component of pressure, and constant surface area of the confining surfaces. It is shown that the present CG model describes well the layering of polymers confined between the surfaces. The conformations of confined polymers have been analyzed by calculating the radius of gyration and the orientation of end-to-end vectors relative to the surface normal. It is shown that the CG model allows efficient and fast equilibration of even very long chains of PA-6,6 in very narrow pores. [ABSTRACT FROM AUTHOR]

Our new simulation scheme in isosurface−isothermal−isobaric ensemble [Eslami, H.; Mozaffari, F.; Moghadasi, J.; Müller-Plathe, F. J. Chem. Phys.2008, 129, 194702], developed to simulate confined fluids in equilibrium with bulk, is applied to simulate polyamide-6,6 oligomers confined between graphite surfaces. The reverse nonequilibrium molecular dynamics simulation technique is employed to shear the graphite surfaces. In this work, six confined systems, with different surface separations, as well as the bulk fluid are simulated. Our results show a viscosity increase with respect to the bulk fluid, with decreasing distance between surfaces. Also, the calculated viscosities of the confined systems show an oscillatory behavior with maxima corresponding to well-formed layers between the surfaces. We observe a substantial slip at the surfaces, with the slip length depending on the shear rate and on the slit width. The slip length and the slip velocity show oscillatory behavior with out-of-phase oscillations with respect to the solvation force oscillations. Moreover, the temperature difference between coldest and hottest parts of the simulation box depends on the shear rate and on the layering effect (solvation force oscillations). An analysis of oligomer deformation under flow shows preferential alignment of oligomers parallel to the surfaces with increasing shear rate. [ABSTRACT FROM AUTHOR]

An accurate force-field has been parametrized for PET and is employed to simulate a long chain, consisting of 120 repeat units, over a wide temperature and pressure range, for long time scales. The force field has been validated against the experimental results for the pressure−volume−temperature properties, the characteristic ratio, the dipolar ratio, and the population of ethylene glycol bond conformer. In all cases the agreement with experiment is quite good. The glass transition temperature is calculated very accurately as a function of pressure. The local dynamics of the chain has been investigated by calculating phenylene reorientation, collective dipole moment, and individual dipole moment correlation functions. The calculated relaxation times for collective dipole moment and phenylene reorientation correlation functions show activation energies close to the corresponding experimental β-relaxation value. Local translational mobilities of phenylene groups in the backbone and end phenylene groups have also been investigated. It is concluded that the end groups have much higher mobility than the polymer backbone, and increasing the distance from the chain ends decreases the mobility dramatically. The long-time asymptotic slope of chain displacement at temperatures higher than the glass transition temperature is around 0.65 in close agreement with Rouse model. On the other hand at the glassy state, the polymer segments perform very restricted movements in the cages formed by their surrounding segments. [ABSTRACT FROM AUTHOR]

A molecular dynamics simulation method with coupling to an external bath is used to simulate polyamide-6,6 trimers confined between graphite surfaces. In this simulation method, the temperature and the parallel component of pressure are kept fixed, and the distance between the confining graphite surfaces is changed to achieve equilibrium. The simulation results on the oscillatory behavior of solvation force, the number density of confined oligomers, and stepwise variation of the oligomer numbers as a function of distance between the confining graphite surfaces are reported and discussed. The hydrogen bonding in confined oligomers has also been studied in detail, and it is shown that hydrogen-bond formation depends on the layering effect and on the geometrical restrictions and reveals an oscillatory behavior like the solvation force oscillations. The autocorrelation functions for NH, CO, and end-to-end vectors are also studied, and it is concluded that the confined fluid has a considerably lower relaxation time than that of the bulk fluid, and the relaxation times for confined fluid show an oscillatory behavior with maxima corresponding to well-formed structures parallel to the surfaces. The study of local dynamics via calculating the autocorrelation functions for bins parallel to the surfaces reveals that the fluid layers close to the surfaces have higher relaxation times than the fluid in the central region. Our calculated center-of-mass diffusion coefficients also show oscillatory behavior with out-of-phase oscillations with respect to the solvation force oscillations. [ABSTRACT FROM AUTHOR]

A computational method for the calculation of the solubility of gases, including argon, hydrogen, nitrogen, carbon dioxide, methane, and propane, in polystyrene over a wide range of temperatures and pressures is described. The excess chemical potentials and the partial molar volumes of gases in polystyrene were calculated using Widom's test-particle insertion method. Using the calculated chemical potentials and partial molar volumes of the sorbed gases at a fixed temperature, the chemical potentials in the polymer phase were expanded in terms of the pressure. Performing a grand-canonical ensemble molecular dynamics simulation of the gas phase, we calculated the phase coexistence point using a recent method by Vrabec and Hasse Mol. Phys.2002, 100, 3375−3383. The results on the calculated solubility coefficients and solubilities over a wide range of temperatures and pressures are compared with experimental data. [ABSTRACT FROM AUTHOR]