Glasses are materials that have the microscopically disordered structure of a fluid, but the mechanical properties of a solid. These materials are crucial for a number of technological applications. Our research focuses on understanding the dynamics of these materials for the purpose of engineering their structure-function relationships.
In a key recent example of this work, we have shown that photoreaction of a guest molecule can be suppressed by one order of magnitude in a molecular glass mixture by more densely packing the glass, without altering the chemical structures of the guest or host. The strong correlation between glass density and photostability in a two-component system identified here had not been established before, and we anticipate that the results reported in this work will help guide future efforts to increase the lifetime of organic electronic devices.
J. Chem. Phys. 149, 204503 (2018) DOI: 10.1063/1.5052003
When stretching a material along one axis, most natural and synthetic materials will contract in the orthogonal lateral dimension. This contraction behavior can be quantified by the Poisson’s ratio, v, which is defined by the negative ratio of a material’s lateral and axial strains. Auxetic materials have been formed in the past through trial and error techniques, but our group is interested in rationally controlling the auxetic behavior of a material by tuning the topology of disordered networks using simulation.
Recent work in our group has utilized a pruning strategy along with a realistic model of disordered networks that incorporates angle-bending and bond-stretching forces. By selectively pruning bonds that do not contribute to a negative Poisson’s ratio, we show that the Poisson’s ratio can be tuned to arbitrary values. Our experimental colleagues in the Nagel Group at the University of Chicago then laser cut 2D sheets of the exact simulation topologies and corroborated our findings in a macroscopic, experimentally reproducible context. These findings establish that pruned networks are a promising pathway for the generation of unique mechanical metamaterials.
Soft Matter 15, 8084—8091 (2019) DOI: 10.1039/c9sm01241a
Polymers are a critical element of modern society and underlie the vast majority of common technical applications. Studies in our group are broadly concerned with the physics of a variety of polymers, including charged polymers (polyelectrolyte and polyzwitterions), combinations of charged polymers leading to interesting self-assembly behavior, mechanically-interlinked polymers, and the self-assembly of block copolymers at surfaces.
Examples of our collaborative computational and experimental work on polyelectrolyte brushes have been published in Science Advances and Science. Polyelectrolyte brushes provide wear protection and lubrication in many technical, medical, physiological and biological applications. Wear resistance and low friction are attributed to counterion osmotic pressure and the hydration layer surrounding the charged polymer segments. However, the presence of multivalent counterions in solution can strongly affect the interchain interactions and structural properties of brush layers. We evaluated the lubrication properties of polystyrene sulfonate brush layers sliding against each other in aqueous solutions containing increasing concentrations of counterions. The presence of multivalent ions (Y3+, Ca2+, Ba2+), even at minute concentrations, markedly increases the friction forces between brush layers owing to electrostatic bridging and brush collapse. Our results suggest that the lubricating properties of polyelectrolyte brushes in multivalent solution are hindered relative to those in monovalent solution.
Science Advances 3, o1497 (2017) DOI:10.1126/sciadv.aao1497pmid:29226245
Science 460, 1434-1438 (2018) DOI: 10.1126/science.aar5877
In the realm of polymer melt dynamics, motivated by recent achievements in the synthesis of interlocking polymers, the structural features of poly[n]catenanes, polymers composed entirely of interlocking rings (or macrocycles), have been studied by extensive molecular dynamics simulations in the melt state. The polymers themselves possess many unusual structural features at short and intermediate length scales, which can be attributed to density inhomogeneities along the polymer contour. Furthermore, the conformations of the individual macrocycles within poly[n]catenanes are quite different from those of ordinary ring polymers and depend on the topology of the macrocycle, that is, whether it is threaded by one ring (chain end) or two (chain center).
Macromolecules 53, 3390-3408 (2020) DOI: 10.1021/acs.macromol.9b02706
Liquid crystals (LCs) are a phase of matter that flows like a liquid, but the orientations of the molecules are highly ordered over long length scales. This presence of long-range orientational order results in novel macroscopic behaviors of systems that employ LCs. In our group, we model LCs on multiple length and time scales in an effort to engineer new applications for the laboratory and industry. Liquid crystal blue phases (BPs) are three-dimensional soft crystals with unit cell sizes orders of magnitude larger than those of classic, atomic crystals. The directed self-assembly of BPs on chemically patterned surfaces uniquely enables detailed in situ resonant soft x-ray scattering measurements of martensitic phase transformations in these systems. The formation of twin lamellae is explicitly identified during the BPII-to-BPI transformation, further corroborating the martensitic nature of this transformation and broadening the analogy between soft and atomic crystal diffusionless phase transformations to include their strain-release mechanisms.
Science Advances 6 eaay5986 (2020) DOI: 10.1126/sciadv.aay5986
DNA Folding and Hybridization
Our research group excels in the modeling of the biophysics of DNA at multiple length and time scales – ranging from atomistic DNA descriptions, all the way up to mesoscale models of chromatin. Initial coarse-grained studies invoked the 3SPN.2 model, designed within our group, to model the morphological behavior of DNA strands. Recent work has used the more strongly coarse-grained 1CPN model, also developed within our group, to understand chromatin. Recent work to this end can be found in the following publication:
J. Chem. Phys. 150, 215102 (2019) DOI: 10.1063/1.5092976
Development of Advanced Sampling Methods
Molecular simulations are typically limited by the time scale of sampling. With a reasonable amount of computational resources, one can only simulate on the order of hundreds of nanoseconds. On the other hand, in real complex systems most of the phenomena of interest occur at orders-of-magnitude longer time scales. To solve such problems of sampling, we develop new advancing sampling algorithms that can accelerate the sampling of molecular systems using both Monte Carlo and Molecular dynamics techniques. Some recent efforts in this regard are:
J. Chem. Theory Comput. 16, 1448-1455 (2020) DOI: 10.1021/acs.jctc.9b00883
Science Advances 5, eaav1190 (2019) DOI: 10.1126/sciadv.aav1190