Polymer equations of state derived from molecular simulation (original) (raw)
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Chemical Physics, 2000
We present an application of Wertheim's Thermodynamic Perturbation Theory (TPT1) to a simple coarse grained model made of flexibly bonded Lennard-Jones monomers. We use both the Reference Hyper-Netted-Chain (RHNC) and Mean Spherical approximation (MSA) integral equation theories to describe the properties of the reference fluid. The equation of state, the density dependence of the excess chemical potential, and the critical points of the liquid--vapor transition are compared with simulation results and good agreement is found. The RHNC version is somewhat more accurate, while the MSA version has the advantage of being almost analytic. We analyze the scaling behavior of the critical point of chain fluids according to TPT1 and find it to reproduce the mean field exponents: The critical monomer density is predicted to vanish as n−1/2n^{-1/2}n−1/2 upon increasing the chain length nnn while the critical temperature is predicted to reach an asymptotic finite temperature that is attained as n−1/2n^{-1/2}n−1/2. The predicted asymptotic finite critical temperature obtained from the RHNC and MSA versions of TPT1 is found to be in good agreement with the Theta\ThetaTheta point of our polymer model as obtained from the temperature dependence of the single chain conformations.
Macromolecular Chemistry and Physics, 2003
A simple expression for the composition dependence of the Flory-Huggins interaction parameter of polymer/solvent systems reported earlier is used to model the demixing of polymer solutions into two liquid phases. To this end the system specific parameters ζ and ν of that approach are calculated as a function of temperature using the thermodynamic expressions resulting for the critical conditions on one side and from experime ntally determined critical data for polymers of different molar mass on the other side. By means of data reported for the system cyclohexane/polystyrene it is demonstrated that binodal and spinodal lines are very accurately modeled at low temperatures (UCSTs) and at high temperatures (LCSTs). The parameters obtained from the demixing behavior match well with that calculated from the composition dependence of the vapor pressure at temperatures where the components are completely miscible. Information on the phase separation of the system transdecalin/polystyrene for different molecular weights and at different elevated pressures is used to show that the approach is also apt to model pressure influences. The thus obtained ζ (T;p) and ν (T;p) enable the prediction of the (endothermal) theta temperature of the system as a function of pressure in quantitative agreement with the data directly obtained from light scattering measurements with dilute solutions.
Study on the molecular models commonly used in the simulation of dilute polymeric solutions in flow
Molecular Simulation, 2017
Three molecular models, commonly used in the simulation of polymeric solutions and melts were employed to describe the rheological behaviour of dilute, elastic and constant viscosity solution formed by bead-andbond chain molecules immersed into a soft-sphere solvent. The intermolecular interactions for the three models were calculated by the Lennard-Jones potential. The differences amongst the models proceeded from the intramolecular restrictions: the simplest one is a Freely-Joined-Chain (FJC) model with harmonic bond potentials, in the second model bonds are restricted by a finite extensible non-linear elastic (FENE) potential plus a repulsive WCA potential, and the third model is conformed by the United Atom (UA) approach which includes bond, flexion and torsion potentials. Both Couette and Poiseuille flows were simulated using Non-Equilibrium Molecular Dynamics. Deformation displayed by the three chain models; defined in terms of their radius of gyration was calculated and according to results it was found that for Couette flow, the three chains exhibit similar response to deformation. In Poiseulle flow, the FJC and FENE models behave similarly but the UA model presents a larger resistance to deformation. For both flow regimes, the forces involved to deform the chains were estimated in terms of the first normal stress differences. From these estimations it was found that the UA model depicted the stiffest chain, followed by the FENE model, and last the FJC model.
Computers & Chemical Engineering, 1998
Thermodynamic and structural properties of polymers are investigated using the molecular dynamics technique to simulate chain-like hydrocarbon molecules. Miscibilities of hydrocarbon blends can be in some instances be predicted from the cohesive energy densities of the constituent pure components. This approach holds great promise; however a number of obstacles must first be overcome. It is not currently known in which instances the mixture miscibility/pure component property relation is valid. In addition, although differences in polymeric cohesive energy densities can be estimated experimentally, measurements of individual polymeric cohesive energy densities are not possible and individual values must be estimated from internal pressure data. These simulations address both of these obstacles. The cohesive energy density, internal pressure, and for the first time their ratio are assessed for chain like hydrocarbons. Intermolecular pair distribution functions are determined, and a correlation between them and those instances where miscibility may be predicted from pure component properties is identified. Correlations between chain architecture, cohesive energy density and intermolecular pair distribution functions are also investigated. 0
Molecular dynamics simulation of a model oligomer for poly(N-isopropylamide) in water
Chemical Physics Letters, 2004
Molecular dynamics (MD) has been used to simulate a dilute aqueous solution of a 50-units oligomer model for the thermoresponsive polymer poly(N-isopropylacrylamide) at 300 and 310 K, i.e., below and above its lower critical solution temperature (LCST) in water. Statistical analyses of the system trajectories show that at 310 K the oligomer exhibits a more compact conformation than at 300 K, in qualitative agreement with experiments, and that it is surrounded by a smaller number of first-hydrationshell water molecules.
Ind Eng Chem Res, 1995
The group contribution-Flory equation of state (GC-Flory EoS) is applied to the prediction of the phase behavior of monodisperse polymer/single solvent systems. The model is capable of predicting with satisfactory accuracy the most common types of phase diagrams typical of liquidliquid equilibria of polymer solutions (i.e., phase diagrams of the UCST, LCST, combined (UCST + LCST), and hourglass types). Combinatorial effects derived from differences in size, shape, and structure of the polymer and the solvent molecules strongly influence the phase behavior of the systems, but the type of a phase diagram of a specific polymerholvent system is primarily governed by the nature of the molecular energy interactions. The GC-Flory EoS predicts the significant effect of the free-volume contributions at high temperatures, which is in very good agreement with the nature of the liquid-phase separation at temperatures near the gas-liquid critical temperature of the solvent, where the highly expanded state of the solvent leads to LCST behavior.