Extending the fused-sphere SAFT-γ Mie force field parameterization approach to poly(vinyl butyral) copolymers.

SAFT-γ Mie, a molecular group-contribution equation of state with foundations in the statistical associating fluid theory framework, is a promising means for developing accurate and transferable coarse-grained force fields for complex polymer systems. We recently presented a new approach for incorporating bonded potentials derived from all-atom molecular dynamics simulations into fused-sphere SAFT-γ Mie homopolymer chains by means of a shape factor parameter, which allows for bond distances less than the tangent-sphere value required in conventional SAFT-γ Mie force fields. In this study, we explore the application of the fused-sphere SAFT-γ Mie approach to copolymers. In particular, we demonstrate its capabilities at modeling poly(vinyl alcohol-co-vinyl butyral) (PVB), an important commercial copolymer widely used as an interlayer in laminated safety glass applications. We found that shape factors determined from poly(vinyl alcohol) and poly(vinyl butyral) homopolymers do not in general correctly reproduce random copolymer densities when standard SAFT-γ Mie mixing rules are applied. However, shape factors optimized to reproduce the density of a random copolymer of intermediate composition resulted in a model that accurately represents density across a wide range of chemical compositions. Our PVB model reproduced copolymer glass transition temperature in agreement with experimental data, but heat capacity was underpredicted. Finally, we demonstrate that atomistic details may be inserted into equilibrated fused-sphere SAFT-γ Mie copolymer melts through a geometric reverse-mapping algorithm.

Development of a fused-sphere SAFT-γ Mie force field for poly(vinyl alcohol) and poly(ethylene).

SAFT-γ Mie, a group-contribution equation of state rooted in Statistical Associating Fluid Theory, provides an efficient framework for developing accurate, transferable coarse-grained force fields for molecular simulation. Building on the success of SAFT-γ Mie force fields for small molecules, we address two key issues in extending the SAFT-γ Mie coarse-graining methodology to polymers: (1) the treatment of polymer chain rigidity and (2) the disparity between the structure of linear chains of tangent spheres and the structure of the real polymers. We use Boltzmann inversion to derive effective bond-stretching and angle-bending potentials mapped from all-atom oligomer molecular dynamics (MD) simulations to the coarse-grained sites and a fused-sphere version of SAFT-γ Mie as the basis for non-bonded interactions. The introduction of an overlap parameter between Mie spheres leads to a degeneracy when fitting to monomer vapor-liquid equilibria (VLE) data, which we resolve by matching polymer density from coarse-grained MD simulation with that from all-atom simulation. The result is a chain of monomers rigorously parameterized to experimental VLE data and with structural detail consistent with all-atom simulations. We test our approach on atactic poly(vinyl alcohol) and polyethylene and compare the results for SAFT-γ Mie models with structural detail mapped from the Optimized Potentials for Liquid Simulations (OPLS) and Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies (COMPASS) all-atom force fields.

High-throughput quantification of quasistatic, dynamic and spall strength of materials across 10 orders of strain rates.

The response of metals and their microstructures under extreme dynamic conditions can be markedly different from that under quasistatic conditions. Traditionally, high strain rates and shock stresses are achieved using cumbersome and expensive methods such as the Kolsky bar or large spall experiments. These methods are low throughput and do not facilitate high-fidelity microstructure-property linkages. In this work, we combine two powerful small-scale testing methods, custom nanoindentation, and laser-driven microflyer (LDMF) shock, to measure the dynamic and spall strength of metals. The nanoindentation system is configured to test samples from quasistatic to dynamic strain-rate regimes. The LDMF shock system can test samples through impact loading, triggering spall failure. The model material used for testing is magnesium alloys, which are lightweight, possess high-specific strengths, and have historically been challenging to design and strengthen due to their mechanical anisotropy. We adopt two distinct microstructures, solutionized (no precipitates) and peak-aged (with precipitates) to demonstrate interesting upticks in strain-rate sensitivity and evolution of dynamic strength. At high shock-loading rates, we unravel an interesting paradigm where the spall strength vs. strain rate of these materials converges, but the failure mechanisms are markedly different. Peak aging, considered to be a standard method to strengthen metallic alloys, causes catastrophic failure, faring much worse than solutionized alloys. Our high-throughput testing framework not only quantifies strength but also teases out unexplored failure mechanisms at extreme strain rates, providing valuable insights for the rapid design and improvement of materials for extreme environments.

Folding Coarse-Grained Oligomer Models with PyRosetta.

Non-biological foldamers are a promising class of macromolecules that share similarities to classical biopolymers such as proteins and nucleic acids. Currently, designing novel foldamers is a non-trivial process, often involving many iterations of trial synthesis and characterization until folded structures are observed. In this work, we aim to tackle these foldamer design challenges using computational modeling techniques. We developed CG PyRosetta, an extension to the popular protein folding python package, PyRosetta, which introduces coarse-grained (CG) residues into PyRosetta, enabling the folding of toy CG foldamer models. Although these models are simplified, they can help explore overarching physical hypotheses about how oligomers can form. Through systematic variation of CG parameters in these models, we can investigate various folding hypotheses at the CG scale to inform the design process of new foldamer chemistries. In this study, we demonstrate CG PyRosetta's ability to identify minimum energy structures with a diverse structural search over a range of simple models, as well as two hypothesis-driven parameter scans investigating the effects of side-chain size and internal backbone angle on secondary structures. We are able to identify several types of secondary structures from single- and double-helices to sheet-like and knot-like structures. We show how side-chain size and backbone bond angle both play an important role in the structure and energetics of these toy models. Optimal side-chain sizes promote favorable packing of side chains, while specific backbone bond angles influence the specific helix type found in folded structures.

Using a Coarse-Grained Modeling Framework to Identify Oligomeric Motifs with Tunable Secondary Structure.

Coarse-grained modeling can be used to explore general theories that are independent of specific chemical detail. In this paper, we present cg_openmm, a Python-based simulation framework for modeling coarse-grained hetero-oligomers and screening them for structural and thermodynamic characteristics of cooperative secondary structures. cg_openmm facilitates the building of coarse-grained topology and random starting configurations, setup of GPU-accelerated replica exchange molecular dynamics simulations with the OpenMM software package, and features a suite of postprocessing thermodynamic and structural analysis tools. In particular, native contact analysis, heat capacity calculations, and free energy of folding calculations are used to identify and characterize cooperative folding transitions and stable secondary structures. In this work, we demonstrate the capabilities of cg_openmm on a simple 1-1 Lennard-Jones coarse-grained model, in which each residue contains 1 backbone and 1 side-chain bead. By scanning both nonbonded and bonded force-field parameter spaces at the coarse-grained level, we identify and characterize sets of parameters which result in the formation of stable helices through cooperative folding transitions. Moreover, we show that the geometries and stabilities of these helices can be tuned by manipulating the force-field parameters.

Effect of Poly(vinyl butyral) Comonomer Sequence on Adhesion to Amorphous Silica: A Coarse-Grained Molecular Dynamics Study.

Modulating a comonomer sequence, in addition to the overall chemical composition, is the key to unlocking the true potential of many existing commercial copolymers. We employ coarse-grained molecular dynamics (MD) simulations to study the behavior of random-blocky poly(vinyl butyral-co-vinyl alcohol) (PVB) melts in contact with an amorphous silica surface, representing the interface found in laminated safety glass. Our two-pronged coarse-graining approach utilizes both macroscopic thermophysical data and all-atom MD simulation data. Polymer-polymer nonbonded interactions are described by the fused-sphere SAFT-γ Mie equation of state, while bonded interactions are derived using Boltzmann inversion to match the bond and angle distributions from all-atom PVB chains. Spatially dependent polymer-surface interactions are mapped from a hydroxylated all-atom amorphous silica slab model and all-atom monomers to an external potential acting on the coarse-grained sites. We discovered an unexpected complex relationship between the blockiness parameter and the adhesion energy. The adhesion strength between PVB copolymers with intermediate VA content and silica was found to be maximal for random-blocky copolymers with a moderately high degree of blockiness rather than for diblock copolymers. We attribute this to two main factors: (1) changes in morphology, which dramatically alter the number of VA beads interacting with the surface and (2) a non-negligible contribution of vinyl butyral (VB) monomers to adhesion energy because of their preference to adsorb to zones with low hydroxyl density on the silica surface.