Oligomer/Polymer Blend Phase Diagram and Surface Concentration Profiles for Squalane/Polybutadiene: Experimental Measurements and Predictions from SAFT-γ Mie and Molecular Dynamics Simulations.

The compatibility and surface behavior of squalane-polybutadiene mixtures are studied by experimental cloud point and neutron reflectivity measurements, statistical associating fluid theory (SAFT), and molecular dynamics (MD) simulations. A SAFT-γ Mie model is shown to be successful in capturing the cloud point curves of squalane-polybutadiene and squalane-cis-polybutadiene binary mixtures, and the same SAFT-γ Mie model is used to develop a thermodynamically consistent top-down coarse-grained force field to describe squalane-polybutadiene. Coarse-grained molecular dynamics simulations are performed to study surface behavior for different concentrations of squalane, with the system exhibiting surface enrichment and a wetting transition. Simulated surface profiles are compared with those obtained by fitting to neutron reflectivity data obtained from thin films composed of deuterated squalane (d-sq)-polybutadiene. The presented top-down parametrization methodology is a fast and thermodynamically reliable approach for predicting properties of oligomer-polymer mixtures, which can be challenging for either theory or MD simulations alone.

Assessing the transferability of common top-down and bottom-up coarse-grained molecular models for molecular mixtures.

The performance of three methods for developing new coarse-grained models for molecular simulation is critically assessed. Two bottom-up approaches are employed: iterative Boltzmann inversion (IBI) and the multiscale coarse-graining method (MS-CG), using an atomistic n-octane-benzene reference system. Results are compared to a top-down coarse-graining approach employing the SAFT-γ Mie equation of state. The performance of each methodology is assessed against the twin criteria of local structure prediction and accurate free energy representation. In addition, the transferability of the generated potentials is compared across state points. We examine the extent to which the IBI methodology can be improved by using a multi-reference approach (MS-IBI), and demonstrate how a pressure correction can be employed to improve the results for the MS-CG approach. Additionally, we look at the effect of including angle-terms in the SAFT-γ Mie model. Finally, we discuss in detail the strengths and weaknesses of each method and suggest possible ways forward for coarse-graining, which may eventually address the problems of structure prediction, thermodynamic consistency and improved transferability within a single model.

Correction: Predicting oligomer/polymer compatibility and the impact on nanoscale segregation in thin films.

Correction for 'Predicting oligomer/polymer compatibility and the impact on nanoscale segregation in thin films' by Elise F. D. Sabattié et al., Soft Matter, 2017, 13, 3580-3591.

Predicting oligomer/polymer compatibility and the impact on nanoscale segregation in thin films.

Compatibility between oligomers and polymers was systematically assessed using differential scanning calorimetry (DSC) and was correlated with similarity in saturation and solubility parameter. These measurements enabled validation of detailed volume of mixing calculations using Statistical Association Fluid Theory (SAFT-γ Mie) and molecular dynamics (MD) simulations, which can be used to predict behaviour beyond the experimentally accessible conditions. These simulations confirmed that squalane is somewhat more compatible with poly(isoprene), "PI" than poly(butadiene), "PB", and further enabled prediction of the temperature dependence of compatibility. Surface and interfacial segregation of a series of deuterated oligomers was quantified in rubbery polymer films: PI, PB and hydrogenated poly(isoprene) "hPI". A striking correlation was established between surface wetting transition and mixtures of low compatibility, such as oligo-dIB in PB or PI. Segregation was quantified normal to the surface by ion beam analysis and neutron reflectometry and in some cases lateral segregation was observable by AFM. While surface segregation is driven by disparity in molecular weight in highly compatible systems this trend reverses as critical point is approached, and surface segregation increases with increasing oligomer molecular weight.

A Green's-Function Approach to Exchange Spin Coupling As a New Tool for Quantum Chemistry.

Exchange spin coupling is usually evaluated in quantum chemistry from the energy difference between a high-spin determinant and a Broken-Symmetry (BS) determinant in combination with Kohn-Sham density functional theory (KS-DFT), based on the work of Noodleman. As an alternative, an efficient approximate approach relying on Green's functions has been developed by one of the authors. This approach stems from solid-state physics and has never been systematically tested for molecular systems. We rederive a version of the Green's-function approach originally suggested by Han, Ozaki, and Yu. This new derivation employs local projection operators as common in quantum chemistry for defining local properties such as partial charges, rather than using a dual basis as in the Han-Ozaki-Yu approach. The result is a simple postprocessing procedure for KS-DFT calculations, which in contrast to the BS energy-difference approach requires the electronic structure of only one spin state. We show for several representative small molecules, diradicals, and dinuclear transition metal complexes that this method gives qualitatively consistent results with the BS energy-difference approach as long as it is applied to high-spin determinants and as long as structural relaxation effects in different spin states do not play an important role.