Targeted sequence design within the coarse-grained polymer genome.

The chemical design of polymers with target structural and/or functional properties represents a grand challenge in materials science. While data-driven design approaches are promising, success with polymers has been limited, largely due to limitations in data availability. Here, we demonstrate the targeted sequence design of single-chain structure in polymers by combining coarse-grained modeling, machine learning, and model optimization. Nearly 2000 unique coarse-grained polymers are simulated to construct and analyze machine learning models. We find that deep neural networks inexpensively and reliably predict structural properties with limited sequence information as input. By coupling trained ML models with sequential model-based optimization, polymer sequences are proposed to exhibit globular, swollen, or rod-like behaviors, which are verified by explicit simulations. This work highlights the promising integration of coarse-grained modeling with data-driven design and represents a necessary and crucial step toward more complex polymer design efforts.

Humidity transforms immobile surface charges into mobile charges during triboelectric charging.

Triboelectric charging - which children see when they rub balloons on their hair - has important consequences in many industries and natural phenomena. Despite its importance, the identity of the charge carriers that lead to triboelectric charging is uncertain. For polymers, previous X-ray photoelectron spectroscopy studies definitively show that bonds break during triboelectric charging. Others have argued that a pair of co-located bond breaks release a charged fragment that acts as the charge carrier for triboelectric charging. We describe an alternative process based on density functional theory results showing that charged fragments, in the presence of water, will react to form neutral fragments and H+ or OH- ions. These results show that a single bond break, which is more likely than a pair of co-located bond breaks, can then create tethered polymer fragments that in humidity will generate mobile H+ or OH- charge carriers for triboelectric charging.

Single-stranded DNA oligomer brush structure is dominated by intramolecular interactions mediated by the ion environment.

Single-stranded DNA (ssDNA) brushes, in which ssDNA oligomers are tethered to surfaces in dense monolayers, are being investigated for potential biosensing applications. The structure of the brush can affect the selectivity and the hybridization efficiency of the device. The structure is commonly thought to result from the balance of intramolecular interactions, intermolecular interactions within the monolayer, and molecule-surface interactions. Here, we test the hypothesis that ssDNA oligomer brush structure is dominated by intramolecular interactions. We use AFM to measure the height of an ssDNA brush and molecular dynamics to simulate the end-to-end distance, both as a function of ionic strength of the surrounding solution. The brush height and the molecule end-to-end distance match quantitatively, providing evidence that the brush structure is dominated by intramolecular interactions (mediated by ions). The physical basis of the intramolecular interactions is elucidated by the simulations.

Effect of surfactant shape on solvophobicity and surface activity in alcohol-water systems.

Here we study the relationship between a surfactant's molecular shape and its tendency to partition to the interface in ethanol-water mixtures. In general, finding surfactants that are effective in alcohol-water mixtures is more challenging than finding ones that are effective in pure water. This is because the solvophobic effect that partitions surfactants from bulk solution to the interface becomes weaker as ethanol concentration increases. We use experiments and molecular dynamics to observe the effects of increasing surfactant tail length or width. The results show that increasing surfactant tail length causes the surfactant to partition to the surface better in low ethanol concentrations, but not at high ethanol concentrations. In comparison, increasing surfactant tail width causes the surfactant to partition to the surface better at higher concentrations of ethanol. We examine the liquid structure to elucidate the mechanisms that weaken the partitioning effect as ethanol concentration increases. Ethanol-water mixtures are nanoscopically heterogeneous with protic and aprotic regions in the bulk solution. We see that the surfactant tail is most likely to be solvated in the aprotic regions where it perturbs fewer hydrogen bonds.

Surface activity of octanoic acid in ethanol-water solutions from molecular simulation and experiment.

The surface activity of a typical surfactant, octanoic acid (OA), in ethanol-water solutions is investigated with a combined experimental and molecular simulation approach. The experiments show that OA reduces the surface tension of ethanol-water solutions at low ethanol concentration, but the effectiveness decreases with increasing ethanol concentration and vanishes for ethanol concentrations above 60%. Molecular dynamics simulations are used to obtain free energy landscapes for OA as a function of the distance from the surface. The free energy driving force pushing OA to the surface decreases with increasing ethanol concentration, and becomes insignificant (i.e., less than kT) for ethanol concentrations above 70%. Thus, the decrease in the effectiveness of OA in reducing the surface tension at higher ethanol concentrations can be attributed to the decrease in the free energy driving force keeping OA at the surface. We expect these results to apply generally to hydrocarbon-based surfactants.