Molecular Simulation of Olefin Oligomer Blend Phase Behavior

Chemical Potential of a Flexible Polymer Liquid in a Coarse-Grained Representation

While the excess chemical potential is the key quantity in determining phase diagrams, its direct computation for high-density liquids of long polymer chains has posed a significant challenge. Computationally, the excess chemical potential is calculated using the Widom insertion method, which involves monitoring the change in internal energy as one incrementally introduces individual molecules into the liquid. However, when dealing with dense polymer liquids, inserting long chains requires generating trial configurations with a bias that favors those at low energy on a unit-by-unit basis: a procedure that becomes more challenging as the number of units increases. Thus, calculating the excess chemical potential of dense polymer liquids using this method becomes computationally intractable as the chain length exceeds N ≥ 30. Here, we adopt a coarse-grained model derived from the integral equation theory for which inserting long polymer chains becomes feasible. The integral equation theory of coarse graining (IECG) represents a polymer as a sphere or a collection of blobs interacting through a soft potential. We employ the IECG approach to compute the excess chemical potential using Widom's method for polymer chains of increasing lengths, extending up to N = 720 monomers, and at densities reaching up to ρ = 0.767 g/cm3. From a fundamental perspective, we demonstrate that the excess chemical potentials remain nearly constant across various levels of coarse graining, offering valuable insights into the consistency of this type of procedure. Ultimately, we argue that current Monte Carlo algorithms, originally designed for atomistic simulations, such as configurational bias Monte Carlo (CBMC) methods, can significantly benefit from the integration of the IECG approach, thereby enhancing their performance in the study of phase diagrams of polymer liquids.

Investigation of nanoparticle-polymer interaction in bio-based nanosilica-filled PLA/NR nanocomposites: molecular dynamics simulation

Molecular dynamics (MD) simulation, by employing the COMPASS force field, was utilized to investigate structural and thermal characteristics as well as interfacial interactions between components of nanocomposite consisting of poly(lactic acid) (PLA)/natural rubber (NR)/nanosilica, abbreviated as PSxN, where 1 ≤ x ≤ 7 and it represents the parts of SiO₂ nanoparticles added to the PLA/NR (PN) blend. Analysis of the obtained results including density (ρ), fractional free volume (FFV), glass transition temperature (T_g), interaction energy (E_interaction), and radial distribution function (RDF) of these nanocomposites was performed. Comparing E_interaction of nanocomposites with that of the PN blend showed that the interactions between the chains of the two polymers are highly dependent on the added amounts of silica nanoparticles, so that by adding silica to the PN blend to obtain PS1N and PS3N nanocomposites, the amount of E_interaction was reduced to a smaller positive value, which indicates the tendency of the nanocomposite's components to interact with each other. By further addition of silica nanoparticles to have PS5N and PS7N nanocomposites and then by analysis of the RDF results, it was found that the nanoparticles were not well dispersed in these two nanocomposites and they were accumulated in the NR rubbery phase. Therefore, the percolation threshold for silica loading on the PN blend is at most 3 parts (x = 3). These results as well as the other obtained simulation results were compared with the available experimental data, and the agreement observed between them approved the simulation procedure and validated the obtained results.

A Force Field for Poly(oxymethylene) Dimethyl Ethers (OMEn)

A united atom force field for the homologous series of the poly(oxymethylene) dimethyl ethers (OMEn), H₃C-O-(CH₂O)_n-CH₃, is presented. OMEn are oxygenates and promising new synthetic fuels and solvents. The molecular geometry of the OMEn, the internal degrees of freedom, and their electrostatic properties were obtained from quantum mechanical calculations. To model repulsion and dispersion, Lennard-Jones parameters were fitted to the experimental liquid densities and vapor pressures of pure OMEn (n = 1-4). The critical properties of OMEn (n = 1-4) were determined from the simulation data. Additionally, the shear viscosity of pure liquid OMEn is evaluated and compared with literature data. Finally, the solubility of CO₂ in OME2, OME3, and OME4 is predicted using a literature model for CO₂ and the Lorentz-Berthelot combining rules. The results agree well with experimental data from the literature.

Thermodynamic Analysis of n-Hexane-Ethanol Binary Mixtures Using the Kirkwood-Buff Theory

A complete thermodynamic analysis of mixtures consisting of molecules with complex chemical constitution can be rather demanding. The Kirkwood-Buff theory of solutions allows the estimation of thermodynamic properties, which cannot be directly extracted from atomistic simulations, such as the Gibbs energy of mixing (Δ_mix G). In this work, we perform molecular dynamics simulations of n-hexane-ethanol binary mixtures in the liquid state under two temperature-pressure conditions and at various mole fractions. On the basis of the recently published methodology of Galata [ Fluid Phase Equilib. 2018 , 470 , 25 - 37 ] , we first calculate the Kirkwood-Buff integrals in the isothermal-isobaric ( NpT) ensemble, identifying how system size affects their estimation. We then extract the activity coefficients, excess Gibbs energy, excess enthalpy, and excess entropy for the n-hexane-ethanol binary mixtures we simulate. We employ two approaches for quantifying composition fluctuations: one based on counting molecular centers of mass and a second one based on counting molecular segments. Results from the two approaches are practically indistinguishable. We compare our results against predictions of vapor-liquid equilibria obtained in a previous simulation work using the same force field, as well as with experimental data, and find very good agreement. In addition, we develop a simple methodology to identify the hydrogen bonds between ethanol molecules and analyze their effects on mixing properties.

Computational Design of High-χ Block Oligomers for Accessing 1 nm Domains

Molecular dynamics simulations are used to design a series of high-χ block oligomers (HCBOs) that can self-assemble into a variety of mesophases with domain sizes as small as 1 nm. The exploration of these oligomers with various chain lengths, volume fractions, and chain architectures at multiple temperatures reveals the presence of ordered lamellae, perforated lamellae, and hexagonally packed cylinders. The achieved periods are as small as 3.0 and 2.1 nm for lamellae and cylinders, respectively, which correspond to polar domains of approximately 1 nm. Interestingly, the detailed phase behavior of these oligomers is distinct from that of either solvent-free surfactants or block polymers. The simulations reveal that the behavior of these HCBOs is a product of an interplay between both "surfactant factors" (headgroup interactions, chain flexibility, and interfacial curvature) and "block polymer factors" (χ, chain length N, and volume fraction f). This insight promotes the understanding of molecular features pivotal for mesophase formation at the sub-5 nm length scale, which facilitates the design of HCBOs tailored toward particular desired morphologies.

Predicting Flory-Huggins χ from Simulations

We introduce a method, based on a novel thermodynamic integration scheme, to extract the Flory-Huggins χ parameter as small as 10^{-3}kT for polymer blends from molecular dynamics (MD) simulations. We obtain χ for the archetypical coarse-grained model of nonpolar polymer blends: flexible bead-spring chains with different Lennard-Jones interactions between A and B monomers. Using these χ values and a lattice version of self-consistent field theory (SCFT), we predict the shape of planar interfaces for phase-separated binary blends. Our SCFT results agree with MD simulations, validating both the predicted χ values and our thermodynamic integration method. Combined with atomistic simulations, our method can be applied to predict χ for new polymers from their chemical structures.