arXiv subjects

David S. Simmons

Publications and source records attributed to David S. Simmons.

18 recordsLinked to original sources

arXiv2026

Anisotropic Nanoparticle Rejamming Triggers Thermodynamic Cavitation in Elastomer Nanocomposites

Nanoparticles can dramatically reinforce elastomers while paradoxically causing cavitation at lower strains. Despite decades of research, the microscopic origin of this behavior has remained unsettled. Here, molecular dynamics simulations reveal that cavitation and failure arise from the nanoparticulate reinforcement mechanism itself. Initial jamming of the nanoparticulate network leads to a buildup of negative pressure in the elastomer matrix, reinforcing it and simultaneously driving it towards a cavitation limit. This crisis is initially averted by yield of the particle network. However, an anisotropic rejamming event of the nanoparticles ultimately drives a runaway negative pressure buildup that leads to cavitation and failure. These results identify nanoparticle-network-induced thermodynamic cavitation as the origin of void formation in elastomeric nanocomposites, and they establish collective filler dynamics as a potential point of control of ultimate failure.

cond-mat.soft↗

arXiv2026

AMDAT: An Open-Source Molecular Dynamics Analysis Toolkit for Supercooled Liquids, Glass-Forming Materials, and Complex Fluids

AMDAT (Amorphous Molecular Dynamics Analysis Toolkit) is an open-source C++ toolkit for post-processing molecular dynamics trajectories, focused on high-performance static and dynamic analyses of amorphous, glassy, and polymer materials, including supercooled liquids and complex fluids. In this paper, we describe AMDAT's design for efficient long-timescale analysis via in-memory trajectory handling and exponential time sampling, and we demonstrate representative workflows for widely used observables such as radial distribution functions, structure factors, intermediate scattering functions, and neighbor correlations.

cond-mat.mtrl-sci↗

arXiv2026

The Interplay Between Liquid-Liquid Phase Equilibria, Sequence, and Tg in Copolymers

Copolymerization is commonly employed to tune polymers' glass formation and improve properties such as ion conductivity and adhesion. Classically, mixing rules such as the Fox equation are employed to explain glass transition temperature (Tg) variations with copolymer composition. However, many copolymers deviate from these mixing rules in a manner that is monomer-sequence sensitive. We perform molecular dynamics simulations to probe the interplay between copolymer sequence, liquid-liquid phase equilibria, and Tg. We find that the direction and sequence-dependence of Tg shift are predicted by the liquid-liquid phase behavior of the comonomers. Systems tending towards Upper Critical Solution Temperature behavior negative Tg deviations, while systems tending towards Lower Critical Solution Temperature behavior exhibit positive Tg deviations. In both cases, this effect is strengthened with increasing alternation - a consequence of bond-induced forced mixing. These results inform strategies for rationally varying copolymer Tg, at fixed composition, via design of polymer chain sequence.

cond-mat.soft↗

arXiv2026

Depth-dependent interplay of dynamical heterogeneity and chain dynamics at the surface of glass-forming polymers

Polymer thin films exhibit pronounced interfacial mobility gradients that modify chain relaxation, yet how these gradients govern chain-scale dynamics across depth remains incompletely understood. Using molecular dynamics simulations of freestanding glass-forming polymer films, we resolve how depth-dependent variations in segmental relaxation shape chain dynamics across a wide range of displacement scales. Near the free surface, accelerated segmental mobility suppresses Rouse-regime scaling exponents to values as low as gamma = 0.4, reflecting transient localization induced by interfacial mobility gradients rather than topological entanglement. In contrast, the film interior exhibits enhanced Rouse scaling exponents consistent with predictions of the Heterogeneous Rouse Model (HRM), indicating that bulk dynamic heterogeneity compresses the Rouse regime. Mapping the minimum scaling exponent gamma_min across depth reveals a linear gradient that separates the bulk-like enhancement regime from the surface-induced suppression regime of chain dynamical scaling. Together, these results demonstrate that bulk and interfacial dynamic heterogeneity modify chain relaxation in opposite ways and establish Rouse scaling as a sensitive, spatially resolved probe of glassy dynamical heterogeneity and interfacial dynamical gradients in polymers.

cond-mat.soft↗

arXiv2025

Reaction/Diffusion Competition Drives Anomalous Relaxation of Vitrimers

Since their discovery in 2011, vitrimers - covalent associative network polymers - have challenged the traditional understanding of soft matter relaxation dynamics: unlike in typical glass-forming liquids, vitrimers' viscous relaxation can be entirely decoupled from their underlying structural (segmental) dynamics. Beyond this fundamental mystery, the origin of vitrimers' Arrhenius viscosity in the presence of super-Arrhenius structural relaxation behavior has been of high interest due to vitrimers' potential to provide readily reprocessable high-performance plastics. Here, we combine simulations, theory, and experiments to establish a foundational understanding of vitrimer relaxation dynamics. We identify two types of transient networks based on the ratio of atomic displacement scales required for bond exchange to those required to relax a segment. In systems where bond exchange only requires sub-segmental motion, we show that network relaxation is governed by a competition between chemical exchange reactions and segmental diffusion. This competition produces vitrimers' signature network/segment decoupling, while also driving a crossover between Arrhenius and super-Arrhenius behavior that is observed for many vitrimers. This work provides an explanation for longstanding puzzling features of vitrimer dynamics and establishes a foundation for rational vitrimer design.

cond-mat.soft↗

arXiv2025

Glassy interphases reinforce elastomeric nanocomposites by enhancing percolation-driven volume expansion under strain

For nearly a century, introduction of nanoparticles to elastomers has yielded extraordinarily tough nanocomposites that are critical to technologies from actuators to tires. The mechanisms by which this reinforcement occurs have nevertheless remained a central open question in material science. One widely debated hypothesis posits that strong interactions between polymer and particles induce "glassy bridges" that cement particles into a cohesive percolating network that resists elongation. Here, molecular dynamics simulations show that glassy particle shells do not primarily provide elongational cohesion. Instead, they amplify an underlying mechanism wherein competition between filler and elastomer networks causes the elastomer's volume to increase on deformation. This induces contributions from the elastomer's bulk modulus, which is of order 1000 times larger than its Young's modulus. These findings establish a unified understanding of low-strain reinforcement in filled elastomers as emanating from volumetric competition between coexisting particulate and elastomeric networks. This reframes and unifies our understanding of low-strain reinforcement, provides a clear-cut diagnostic for the presence of glassy bridging, and offers a new design principle for tough elastomeric nanocomposites.

cond-mat.soft↗

arXiv2025

Reducing Data Requirements for Sequence-Property Prediction in Copolymer Compatibilizers via Deep Neural Network Tuning

Synthetic sequence-controlled polymers promise to transform polymer science by combining the chemical versatility of synthetic polymers with the precise sequence-mediated functionality of biological proteins. However, design of these materials has proven extraordinarily challenging, because they lack the massive datasets of closely related evolved molecules that accelerate design of proteins. Here we report on a new Artifical Intelligence strategy to dramatically reduce the amount of data necessary to accelerate these materials' design. We focus on data connecting the repeat-unit-sequence of a \emph{compatibilizer} molecule to its ability to reduce the interfacial tension between distinct polymer domains. The optimal sequence of these molecules, which are essential for applications such as mixed-waste polymer recycling, depends strongly on variables such as concentration and chemical details of the polymer. With current methods, this would demand an entirely distinct dataset to enable design at each condition. Here we show that a deep neural network trained on low-fidelity data for sequence/interfacial tension relations at one set of conditions can be rapidly tuned to make higher-fidelity predictions at a distinct set of conditions, requiring far less data that would ordinarily be needed. This priming-and-tuning approach should allow a single low-fidelity parent dataset to dramatically accelerate prediction and design in an entire constellation of related systems. In the long run, it may also provide an approach to bootstrapping quantitative atomistic design with AI insights from fast, coarse simulations.

cond-mat.mtrl-sci↗

arXiv2025

On the origin of heating-induced softening and enthalpic reinforcement in elastomeric nanocomposites

Despite a century of use, the mechanism of nanoparticle-driven mechanical reinforcement of elastomers is unresolved. A major hypothesis attributes it to glassy interparticle bridges, supported by an observed inversion of the variation of the modulus E(T) on heating -- from entropic stiffening in elastomers to enthalpic softening in nanocomposites. Here, molecular simulations reveal that elastomer enthalpic softening can instead emerge from a competition over preferred nonequilibrium volumes between elastomer and nanoparticulate networks. A theory for this competition accounting for softening of the bulk modulus on heating predicts the simulated E(T) inversion, suggesting that reinforcement is driven by a volume-competition mechanism unique to co-continuous systems of soft and rigid networks.

cond-mat.soft↗

arXiv2024

Central role of filler-polymer interplay in nonlinear reinforcement of elastomeric nanocomposites

Nanoparticles can greatly enhance the mechanical response of elastomeric polymers essential to a wide range of applications, yet their precise molecular mechanisms of high-strain reinforcement remain largely unresolved. Here we show, based on molecular dynamics simulations, that high-strain reinforcement emerges from an interplay between granular nanoparticulate compressive behavior in the normal direction and polymer incompressibility. This feedback loop, which is initiated by a mismatch in the Poisson ratios of nanofiller and polymer, invokes a contribution from the polymer's bulk modulus to the elongational stress, while the tendency of the polymer to contract in the normal direction maintains a near-jammed filler state. This effect persists even once the direct filler elongational contribution becomes dissipative after the 'Payne effect' yield. These results indicate that direct particle-particle contact effects, even in the absence of potential augmenting mechanisms such as glassy polymer bridges, can drive the mechanical reinforcement effects typical of experimental systems.

cond-mat.soft↗

arXiv2024

Quantitatively Connecting Experimental Time-Temperature-Superposition-Breakdown of Polymers near the Glass Transition to Dynamic Heterogeneity via the Heterogeneous Rouse Model

Polymers near the glass transition temperature Tg often exhibit a breakdown of time-temperature-superposition (TTS), with chain relaxation times and viscosity exhibiting a weaker temperature dependence than segmental relaxation times. The origin of this onset of thermorheological complexity has remained unsettled and a matter of debate. Here we extend the Heterogeneous Rouse Model (HRM), which generalizes the Rouse model to account for dynamic heterogeneity, to make predictions for the relaxation modulus G(t) and complex modulus G*($ω$) of unentangled polymers near Tg. The HRM predicts that G(t) and G*($ω$) exhibit enhanced effective scaling exponents in the Rouse regime in the presence of dynamic heterogeneity, with a more rapid decay from the glassy plateau emerging as the system becomes more dynamically heterogeneous on cooling. This behavior is predicted to emerge from a strand-length dependence of the moment of the segmental mobility distribution probed by chain dynamics. We show that the HRM predictions are in good accord with experimental complex modulus data for polystyrene, poly(methyl methacrylate), and poly(2-vinyl pyridine). The HRM also predicts the onset of distinct temperature dependences among chain scale quantities such as terminal relaxation time and viscosity in our experimental systems, apparently resolving one of the most significant standing objections to a heterogeneity-based origin of TTS-breakdown. The HRM thus provides a generalized theory of the chain-scale linear rheological response of unentangled polymers near Tg, accounting for the origin of TTS-breakdown at a molecular mechanistic level. It also points towards a new strategy of inferring the dynamic heterogeneity of glass-forming polymeric systems based on the temperature-evolution of modified scaling in the Rouse regime.

cond-mat.soft↗

arXiv2024

Mixed equilibrium/nonequilibrium effects govern surface mobility in polymer glasses

The temperature at which supercooled liquids turn into solid-like glasses ($T_g$) can change at the free surface, affecting the properties of nanostructured glasses and their applications. However, inadequate experimental resolution to determine the $T_g$ gradient and a longstanding debate over the role of nonequilibrium effects have hindered fundamental understanding of this phenomenon. Using spatially resolved $T_g$ measurements and molecular dynamics simulations, we reveal a crossover from equilibrium behavior to a new regime of near-surface nonequilibrium glass physics on cooling. This crossover causes the form of the nonequilibrium $T_g$ gradient to change, highlighting the need to include these physics for rational understanding of the properties of realistic nanostructured glass-forming materials. They also potentially recast the interpretation of decades of experimental data on nanoconfined glasses.

cond-mat.soft↗

arXiv2024

Is the Molecular Weight Dependence of the Glass Transition Temperature Caused by a Chain End Effect?

The immense dependence of the glass transition temperature $T_g$ on molecular weight $M$ is one of the most fundamentally and practically important features of polymer glass formation. Here, we report on molecular dynamics simulation of three model linear polymers of substantially different complexity demonstrating that the 70-year-old canonical explanation of this dependence (a simple chain end dilution effect) is likely incorrect at leading order. Our data shows that end effects are present only in relatively stiff polymers and, furthermore, that the magnitude of this end effect diminishes on cooling. Instead, we find that $T_g(M)$ trends are instead dominated by shifts in $T_g$ throughout the entire polymer chain rather than through a chain end effect. We show that these data are consistent with a generic two-barrier model of $T_g$ and its $M$-dependence, motivated by the Elastically Collective Nonlinear Langevin Equation (ECNLE) theory. More broadly, this work indicates both a need to reassess the canonical understanding of $T_g(M)$ in linear polymers (and macromolecules at large) and an opportunity to reveal new glass formation physics with renewed study of $M$ effects on $T_g$.

cond-mat.soft↗

arXiv2020

The microscopic origins of stretched exponential relaxation in two model glass-forming liquids as probed by simulations in the isoconfigurational ensemble

The origin of stretched exponential relaxation in supercooled glass-forming liquids is one of the central questions regarding the anomalous dynamics of these fluids. The dominant explanation for this phenomenon has long been the proposition that spatial averaging over a heterogeneous distribution of locally exponential relaxation processes leads to stretching. Here we perform simulations of model polymeric and small-molecule glass-formers in the isoconfigurational ensemble to show that stretching instead emerges from a combination of spatial averaging and locally nonexponential relaxation. Results indicate that localities in the fluid exhibiting faster-than-average relaxation tend to exhibit locally stretched relaxation, whereas slower-than-average relaxing domains exhibit compressed exponential relaxation. We show that local stretching is predicted by loose local caging, as measured by the Debye-Waller factor, and vice versa. This phenomenology in the local relaxation of in-equilibrium glasses parallels the dynamics of out of equilibrium under-dense and over-dense glasses, which likewise exhibit an asymmetry in their degree of stretching vs compression. On the basis of these results, we hypothesize that local stretching and compression in equilibrium glass-forming liquids results from evolution of particle mobilities over a single local relaxation time, with slower particles tending towards acceleration and vice versa. In addition to providing new insight into the origins of stretched relaxation, these results have implications for the interpretation of stretching exponents as measured via metrologies such as dielectric spectroscopy: measured stretching exponents cannot universally be interpreted as a direct measure of the breadth of an underlying distribution of relaxation times.

cond-mat.soft↗

arXiv2016

Rational approximation of affine coordinate subspaces of Euclidean space

We show that affine coordinate subspaces of dimension at least two in Euclidean space are of Khintchine type for divergence. For affine coordinate subspaces of dimension one, we prove a result which depends on the dual Diophantine type of the basepoint of the subspace. These results provide evidence for the conjecture that all affine subspaces of Euclidean space are of Khintchine type for divergence.

math.NT↗

arXiv2016

Diophantine approximation in Banach spaces

In this paper, we extend the theory of simultaneous Diophantine approximation to infinite dimensions. Moreover, we discuss Dirichlet-type theorems in a very general framework and define what it means for such a theorem to be optimal. We show that optimality is implied by but does not imply the existence of badly approximable points.

math.NT↗

arXiv2015

Diophantine approximation and the geometry of limit sets in Gromov hyperbolic metric spaces

In this paper, we provide a complete theory of Diophantine approximation in the limit set of a group acting on a Gromov hyperbolic metric space. This summarizes and completes a long line of results by many authors, from Patterson's classic '76 paper to more recent results of Hersonsky and Paulin ('02, '04, '07). Concrete examples of situations we consider which have not been considered before include geometrically infinite Kleinian groups, geometrically finite Kleinian groups where the approximating point is not a fixed point of the group, and groups acting on infinite-dimensional hyperbolic space. Moreover, in addition to providing much greater generality than any prior work of which we are aware, our results also give new insight into the nature of the connection between Diophantine approximation and the geometry of the limit set within which it takes place. Two results are also contained here which are purely geometric: a generalization of a theorem of Bishop and Jones ('97) to Gromov hyperbolic metric spaces, and a proof that the uniformly radial limit set of a group acting on a proper geodesic Gromov hyperbolic metric space has zero Patterson--Sullivan measure unless the group is quasiconvex-cocompact. The latter is an application of a Diophantine theorem.

math.DS↗

arXiv2014

$\mathbf{Bad}(s,t)$ is hyperplane absolute winning

J. An (2013) proved that for any $s,t \geq 0$ such that $s + t = 1$, $\mathbf{Bad}(s,t)$ is $(34\sqrt 2)^{-1}$-winning for Schmidt's game. We show that using the main lemma from An's paper one can derive a stronger result, namely that $\mathbf{Bad}(s,t)$ is hyperplane absolute winning in the sense of Broderick, Fishman, Kleinbock, Reich, and Weiss (2012). As a consequence one can deduce the full dimension of $\mathbf{Bad}(s,t)$ intersected with certain fractals.

math.NT↗

arXiv2013

Determinacy and indeterminacy of games played on complete metric spaces

Schmidt's game is a powerful tool for studying properties of certain sets which arise in Diophantine approximation theory, number theory, and dynamics. Recently, many new results have been proven using this game. In this paper we address determinacy and indeterminacy questions regarding Schmidt's game and its variations, as well as more general games played on complete metric spaces (e.g. fractals). We show that except for certain exceptional cases, these games are undetermined on Bernstein sets.

math.LO↗