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1. Structural search for stable Mg–Ca alloys accelerated with a neural network interatomic model

   https://doi.org/10.1039/c8cp05314f  

   Ibarra-Hernández, Wilfredo; Hajinazar, Samad; Avendaño-Franco, Guillermo; Bautista-Hernández, Alejandro; Kolmogorov, Aleksey N.; Romero, Aldo H. (November 2018, Physical Chemistry Chemical Physics)

   We have combined a neural network formalism with metaheuristic structural global search algorithms to systematically screen the Mg–Ca binary system for new (meta)stable alloys. The combination of these methods allows for an efficient exploration of the potential energy surface beyond the possibility of the traditional searches based on ab initio energy evaluations. The identified pool of low-enthalpy structures was complemented with special quasirandom structures (SQS) at different stoichiometries. In addition to the only Mg–Ca phase known to form under standard synthesis conditions, C14-Mg 2 Ca, the search has uncovered several candidate materials that could be synthesized under elevated temperatures or pressures. We show that the vibrational entropy lowers the relative free energy of several phases with magnesium kagome layers: C15 and C36 Laves structures at the 2 : 1 composition and an orthorhombic oS36 structure at the 7 : 2 composition. The estimated phase transition temperatures close to the melting point leave open the possibility of synthesizing the predicted materials at high temperatures. At high pressures up to 10 GPa, two new phases at the 1 : 1 and 3 : 1 Mg : Ca stoichiometries become thermodynamically stable and should form in multi-anvil experiments. 

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2. Tuning conformational asymmetry in particle-forming diblock copolymer alloys

   https://doi.org/10.1039/d2sm01332k  

   Case, Logan J.; Bates, Frank S.; Dorfman, Kevin D. (December 2022, Soft Matter)

   Self-consistent field theory is employed to compute the phase behavior of binary blends of conformationally asymmetric, micelle-forming diblock copolymers with miscible corona blocks and immiscible core blocks (a diblock copolymer “alloy”). The calculations focus on establishing conditions that promote the formation of Laves phases by tuning the relative softness of the cores of the two different Laves phase particles via independent control of their conformational asymmetries. Increasing the conformational asymmetry of the more spherical particles of the Laves structure has a stabilizing effect, consistent with the expectations of increased imprinting of the Wigner–Seitz cells on the core/corona interface as conformational asymmetry increases. The resulting phase diagram in the temperature-blend composition space features a more stable Laves phase field than that predicted for conformationally symmetric systems. The phase field closes at low temperatures in favor of macrophase separation between a hexagonally-packed cylinder (hex) phase and a body-centered cubic phase. Companion calculations, using an alloy whose components do not produce a hex phase in the neat melt state, suggest that the Laves phase field in such a blend will persist at strong segregation. 

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3. Symmetry breaking in particle-forming diblock polymer/homopolymer blends

   https://doi.org/10.1073/pnas.2006079117  

   Cheong, Guo Kang; Bates, Frank S.; Dorfman, Kevin D. (July 2020, Proceedings of the National Academy of Sciences)

   Compositionally asymmetric diblock copolymers provide an attractive platform for understanding the emergence of tetragonally close-packed, Frank–Kasper phases in soft matter. Block-polymer phase behavior is governed by a straightforward competition between chain stretching and interfacial tension under the constraint of filling space at uniform density. Experiments have revealed that diblock copolymers with insufficient conformational asymmetry to form Frank–Kasper phases in the neat-melt state undergo an interconversion from body-centered cubic (bcc) close-packed micelles to a succession of Frank–Kasper phases (σ to C14 to C15) upon the addition of minority-block homopolymer in the dry-brush regime, accompanied by the expected transition from bcc to hexagonally packed cylinders in the wet-brush regime. Self-consistent field theory data presented here qualitatively reproduce the salient features of the experimental phase behavior. A particle-by-particle analysis of homopolymer partitioning furnishes a basis for understanding the symmetry breaking from the high-symmetry bcc phase to the lower-symmetry Frank–Kasper phases, wherein the reconfiguration of the system into polyhedra of increasing volume asymmetry delays the onset of macroscopic phase separation. 

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4. Aggregation and Adsorption Behavior of Organic Corrosion Inhibitors studied using Molecular Simulations

   Himanshu Singh, Yathish Kurapati (March 2019, NACE Corrosion conference)

   We have performed all-atom classical molecular dynamics simulations of aggregation and adsorption of different corrosion inhibitor molecules on metal surfaces. We report free energies of aggregation and adsorption of imidazolinium-type (henceforth referred to as imid) and quaternary ammonium-type (referred to as quat) corrosion inhibitors of different alkyl tail lengths. Corrosion inhibitor molecules show a strong tendency to adsorb onto metal surfaces in the unaggregated state. Inhibitor micelles, on the other hand, experience a free energy barrier to adsorption. The quat micelles are found to be thermodynamically stable in the adsorbed state whereas the imid micelles are only metastable in the adsorbed state. Quat micelles deform and partially disintegrate upon adsorption, which renders stability, while the imid micelles do not deform. The inhibitor molecules demonstrate a strong tendency to aggregate into micelles in the aqueous phase. The micellization free energy is found to be ~68 kBT for a micelle comprising of 18 molecules of imid molecules. 

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5. Equilibrium phase behavior of gyroid-forming diblock polymer thin films

   https://doi.org/10.1063/5.0224767  

   Magruder, Benjamin R; Ellison, Christopher J; Dorfman, Kevin D (August 2024, The Journal of Chemical Physics)

   Thin-film confinement of self-assembling block polymers results in materials with myriad potential applications—including membranes and optical devices—and provides design parameters for altering phase behavior that are not available in the bulk, namely, film thickness and preferential wetting. However, most research has been limited to lamella- and cylinder-forming polymers; three-dimensional phases, such as double gyroid (DG), have been observed in thin films, but their phase behavior under confinement is not yet well understood. We use self-consistent field theory to predict the equilibrium morphology of bulk-gyroid-forming AB diblock polymers under thin-film confinement. Phase diagrams reveal that the (211) orientation of DG, often observed in experiments, is stable between nonpreferential boundaries at thicknesses as small as 1.2 times the bulk DG lattice parameter. The (001) orientation is stable between modestly B-preferential boundaries, where B is the majority block, while a different (211)-oriented termination plane is stabilized by strongly B-preferential boundaries, neither of which has been observed experimentally. We then describe two particularly important phenomena for explaining the phase behavior of DG thin films at low film thicknesses. The first is “constructive interference,” which arises when distortions due to the top and bottom boundaries overlap and is significant for certain DG orientations. The second is a symmetry-dependent, in-plane unit-cell distortion that arises because the distorted morphology near the boundary has a different preferred unit-cell size and shape than the bulk. These results provide a thermodynamic portrait of the phase behavior of DG thin films. 

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