Publications

Current Preprints

Md Mushfiqul Islam, Nishat N. Labiba, Lawrence O. Hall, David S. Simmons, Reducing Data Requirements for Sequence-Property Prediction in Copolymer Compatibilizers via Deep Neural Network Tuning, ArXiv, doi: 10.48550/arXiv.2507.21902.

ABSTRACT: 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.

Pierre Kawak, Harshad Bhapkar, David S. Simmons, Glassy interphases reinforce elastomeric nanocomposites by enhancing percolation-driven volume expansion under strain, ArXiv, doi: 10.48550/arXiv.2509.04755.

ABSTRACT: 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.

Pierre Kawak, Harshad Bhapkar, David S. Simmons, On the origin of heating-induced softening and enthalpic reinforcement in elastomeric nanocomposites, ArXiv, doi: 10.48550/arXiv.2501.06971.

ABSTRACT: 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.

Makayla R. Branham-Ferrari, Shinian Cheng, Alexei P. Sokolov, David S. Simmons, Reaction/Diffusion Competition Drives Anomalous Relaxation of Vitrimers, doi:10.48550/arXiv.2509.19496.

ABSTRACT: Vitrimers are an emerging class of crosslinked polymer networks in which dynamic covalent bonds can exchange and rearrange, without any significant change in population of bonded states.1 Because these bonds can rearrange at high temperatures without disintegration of the network, vitrimers could combine the mechanical stability of traditional permanently crosslinked networks with the recyclability of glasses and physical networks. However, unlike classical transient/physical networks, vitrimers can relax at a rate that is decoupled from the mobility of their polymer segments. This challenges our fundamental understanding of how dynamic covalent networks relax and deform. Here we combine simulations, theory, and experiments to show that vitrimer dynamics exhibit two very different regimes: (i) a high temperature regime controlled by the local bond-exchange reaction rate and (ii) a low temperature regime controlled by segmental diffusion. This fundamental difference from traditional physical networks explains why vitrimers can exhibit Arrhenius dynamics, providing a foundation for their rational design.

2025

Yue, P., Mangalara J. H., Branham-Ferrari, M., and Simmons, D. S., Oligomeric Antiplasticization of Poly(methyl methacrylate) with Long-Chain Chloroparaffins, ACS Applied Polymer Materials 7 (2025) 9503-9516. doi: 10.1021/acsapm.4c03751

ABSTRACT: Small-molecule diluents are an essential and long-standing tool in the improvement of polymer properties in applications ranging from coatings to structural plastics. In recent years, however, concerns regarding the potential environmental and health risks associated with leaching of small-molecule additives have driven an increasing need to scale back or eliminate diluents’ use in a range of polymer applications. This poses a significant threat to multiple polymer industries, as in many cases, equally effective (and cost-effective) means of achieving necessary polymer properties are lacking. Here, we demonstrate a pathway to resolution of this challenge via use of oligomeric diluents that innately exhibit reduced leaching risk due to their larger molecular size and thus reduced matrix mobility, vapor pressure, and environmental solubility as compared to similar small molecules. Via combination of course-grained molecular simulations, atomistic molecular simulations, and experiments, we identify long-chain chloroparaffins as an excellent class of antiplasticizer for poly(methyl methacrylate) (PMMA) and likely for other carbonyl-containing polymers. We report enhancements in the glassy modulus approaching 50%, combined with reductions in glass transition temperature Tg. Our results indicate that introduction of oligomeric additives exhibiting strong interactions with the host polymer, but possessing high flexibility and small segmental size relative to the host polymer, may offer a general pathway to leaching-resistant antiplasticization.

Hung, Jui-Hsiang and Simmons, D. S., Does the Naı̈ve Mode-Coupling Power Law Divergence Provide an Objective Determination of the Crossover Temperature in Glass Formation Behavior?, The Journal of Physical Chemistry B, 129 (2025). 3018-3027. doi: 10.1021/acs.jpcb.4c06623.

ABSTRACT: The glass formation temperature range is commonly divided into a weakly supercooled regime at higher temperatures and a deeply supercooled regime at lower temperatures, with a change in the physical mechanisms that govern dynamics often postulated to occur at the crossover between these regimes. This crossover temperature Tc is widely determined based on a fit of relaxation time vs temperature data to a power law divergence form predicted by the naı̈ve mode coupling theory (MCT). Here, we show, based on simulation data spanning polymeric, small molecule organic, metallic, and inorganic glass formers, that this approach does not yield an objective measure of a crossover temperature. Instead, the value of Tc is determined by the lowest temperature Tmin employed in the fit, and no regime of stationary or convergent Tc value is generally observed as Tmin is varied. Nor does the coefficient of determination R2 provide any robust means of selecting a fit range and thus a value of Tc. These results may require a re-evaluation of published results that have employed the fit MCT Tc value as a metric of temperature-dependent dynamics or a benchmark for depth of supercooling, and they highlight a need for the field to converge on a more objective determination of any posited crossover temperature between high and low temperature regimes of glass formation.

2024

Peijing Yue, and David S. Simmons, Quantitatively Connecting Experimental Time–Temperature–Superposition–Breakdown of Polymers near the Glass Transition to Dynamic Heterogeneity Via the Heterogeneous Rouse Model, Macromolecules 58 (2024) 109-122. doi: 10.1021/acs.macromol.4c00751

ABSTRACT: 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-vinylpyridine). 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 toward a new strategy of inferring the dynamic heterogeneity of glass-forming polymeric systems from the temperature–evolution of modified scaling in the Rouse regime.

Jianquan Xu, Asieh Ghanekarade, Li Li, Huifeng Zhu, Hailin Yuan, David S. Simmons*, Ophelia K. C. Tsui*, and Xinping Wang*, Mixed equilibrium/nonequilibrium effects govern surface mobility in polymer glasses, Proceedings of the National Academy of Sciences, 121 (2024), e2406262121. doi: 10.1073/pnas.2406262121.

ABSTRACT: Using angle-resolved X-ray photoelectron spectroscopy, sum-frequency generation vibrational spectroscopy, contact angle measurements, and molecular dynamics simulations, we verify that the glass transition temperature (Tg) of polymer glass is lower near the free surface. However, the experimental Tg-gradients showed a linear variation with depth (z) from the free surface, while the simulated equilibrium Tg-gradients exhibited a double exponential z-dependence. In typical simulations, Tg is determined based on the relaxation time of the system reaching a prescribed threshold value at equilibrium. Conversely, the experiments determined Tg by observing the unfreezing of molecular mobility during heating from a kinetically arrested, nonequilibrium glassy state. To investigate the impact of nonequilibrium effects on the Tg-gradient, we reduced the thermal annealing time in simulations, allowing the system to fall out of equilibrium. We observe a decrease in the relaxation time and the emergence of a modified z-dependence consistent with a linear Tg-gradient near the free surface. We further validate the impact of nonequilibrium effects by studying the dependence of the Tg on the heating/cooling rate for polymer films of varying thickness (h). Our experimental results reveal significant variations in the Tg-heating/cooling rate dependence with h below the bulk Tg, which are also observed in simulation when the simulated system is not equilibrated. We explain our findings by the reduction in mass density within the inner region of the system under nonequilibrium conditions, as observed in simulation, and recent research indicating a decrease in the local Tg of a polymer when placed next to a softer material.

Pierre Kawak, Harshad Bhapkar, and David S. Simmons*, Central Role of Filler–Polymer Interplay in Nonlinear Reinforcement of Elastomeric Nanocomposites, Macromolecules, 57 (2024) 9466-9475. 10.1021/acs.macromol.4c0048.

ABSTRACT: 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. In particular, long-standing questions endure over the extent to which glassy bridges or tie chains between particles are needed to facilitate reinforcement. 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 the nanofiller and the 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, as a consequence of enduring filler normal stresses mediated by direct particle–particle contacts. 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.

WF Drayer, DS Simmons*, Is the Molecular Weight Dependence of the Glass Transition Temperature Driven by a Chain End Effect?, Macromolecules, 57 (2024) 5589-5597. 10.1021/acs.macromol.4c00419.

ABSTRACT: The immense dependence of the glass transition temperature Tg on molecular weight M is one of the most fundamentally and practically important features of polymer glass formation. Here, we report on molecular dynamics simulations 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 show that end effects are present only in relatively stiff polymers and, furthermore, that the magnitude of these end effects diminish on cooling. We find that Tg(M) trends are instead dominated by shifts in Tg throughout the entire polymer chain rather than through a chain end effect. We show that these data can be rationalized via a generic two-barrier model of Tg and its M-dependence, motivated by the Elastically Collective Nonlinear Langevin Equation theory. More broadly, this work indicates need to reopen the question of the origin of the Tg(M) dependence in linear polymers (and macromolecules at large), as well as an opportunity to reveal new glass formation physics with renewed study of M effects on Tg.

2023

AD Hartley, WF Drayer, A Ghanekarade, DS Simmons*, Interplay between dynamic heterogeneity and interfacial gradients in a model polymer film, The Journal of Chemical Physics, 159 (2023) 204905 . doi: /10.1063/5.0165650.

ABSTRACT: Glass-forming liquids exhibit long-lived, spatially correlated dynamical heterogeneity, in which some nm-scale regions in the fluid relax more slowly than others. In the nanoscale vicinity of an interface, glass-formers also exhibit the emergence of massive interfacial gradients in glass transition temperature Tg and relaxation time τ. Both of these forms of heterogeneity have a major impact on material properties. Nevertheless, their interplay has remained poorly understood. Here, we employ molecular dynamics simulations of polymer thin films in the isoconfigurational ensemble in order to probe how bulk dynamic heterogeneity alters and is altered by the large gradient in dynamics at the surface of a glass-forming liquid. Results indicate that the τ spectrum at the surface is broader than in the bulk despite being shifted to shorter times, and yet it is less spatially correlated. This is distinct from the bulk, where the τ distribution becomes broader and more spatially organized as the mean τ increases. We also find that surface gradients in slow dynamics extend further into the film than those in fast dynamics—a result with implications for how distinct properties are perturbed near an interface. None of these features track locally with changes in the heterogeneity of caging scale, emphasizing the local disconnect between these quantities near interfaces. These results are at odds with conceptions of the surface as reflecting simply a higher “rheological temperature” than the bulk, instead pointing to a complex interplay between bulk dynamic heterogeneity and spatially organized dynamical gradients at interfaces in glass-forming liquids.

Asieh Ghanekarade and David S. Simmons*, Glass formation and dynamics of model polymer films with one versus two active interfaces, Soft Matter, 19 (2023) 8413-8422. doi: 10.1039/D3SM00719G.

ABSTRACT: Polymers and other glass-forming liquids can exhibit profound alterations in dynamics in the nanoscale vicinity of interfaces, over a range appreciably exceeding that of typical interfacial thermodynamic gradients. The understanding of these dynamical gradients is particularly complicated in systems with internal or external nanoscale dimensions, where a gradient nucleated at one interface can impinge on a second, potentially distinct, interface. To better understand the interactions that govern system dynamics and glass formation in these cases, here we simulate the baseline case of a glass-forming polymer film, over a wide range of thickness, supported on a dynamically neutral substrate that has little effect on nearby dynamics. We compare these results to our prior simulations of freestanding films. Results indicate that dynamical gradients in our simulated systems, as measured based upon translational relaxation, are simply truncated when they impinge on a secondary surface that is locally dynamically neutral. Altered film behavior can be described almost entirely by gradient effects down to the thinnest films probed, with no evidence for finite-size effects sometimes posited to play a role in these systems. Finally, our simulations predict that linear gradient overlap effects in the presence of symmetric dynamically active interfaces yield a non-monotonic variation of the whole free standing film stretching exponent (relaxation time distribution breadth). The maximum relaxation time distribution breadth in simulation is found at a film thickness of 4–5 times the interfacial gradient range. Observation of this maximum in experiment would provide an important validation that the gradient behavior observed in simulation persists to experimental timescales. If validated, observation of this maximum would potentially also enable determination of the dynamic gradient range from experimental mean-film measurements of film dynamics.

Asieh Ghanekarade, Anh D. Phan*, Kenneth S. Schweizer*, and David S. Simmons*, Signature of collective elastic glass phyics in surface-induced long-range tails in dynamical gradients, Nature Physics, 19 (2023) 800-806. doi: 10.1038/s41567-023-01995-8

ABSTRACT: Understanding the underlying nature of dynamical correlations believed to drive the bulk glass transition is a long-standing problem. Here we show that the form of spatial gradients of the glass transition temperature and structural relaxation time near an interface indeed provide signatures of the nature of relaxation in bulk glass-forming liquids. We report the results of long-time, large-system molecular dynamics simulations of thick glass-forming polymer films with one vapour interface, supported on a dynamically neutral substrate. We find that gradients in the glass transition temperature and logarithm of the structural relaxation time nucleated at a vapour interface exhibit two distinct regimes: a medium-ranged, large-amplitude exponential gradient, followed by a long-range slowly decaying tail that can be described by an inverse power law. This behaviour disagrees with multiple proposed theories of glassy dynamics but is predicted by the ‘elastically collective nonlinear Langevin equation’ theory as a consequence of two coupled mechanisms: a medium-ranged interface-nucleated gradient of surface-modified local caging constraints, and an interfacial truncation of a long-ranged collective elastic field. These findings support a coupled spatially local–nonlocal mechanism of activated glassy relaxation and kinetic vitrification in both the isotropic bulk and in broken-symmetry films.

2022

Asieh Ghanekarade and David S. Simmons, Combined Mixing and Dynamical Origins of Tg Alterations Near Polymer–Polymer Interfaces, Macromolecules, 56 (2022) 379-392. doi: 10.1021/acs.macromol.2c01621

ABSTRACT: The extent to which altered glass formation behavior in block copolymers and layered polymers results from local compositional mixing vs from longer-ranged dynamical correlations has long been unresolved. Here, we perform molecular dynamics simulations at model polymer–polymer interfaces to understand the relative roles of these mechanisms. Results indicate a crossover from the high-χ regime, where near-interface Tg alterations are driven purely by dynamical gradient effects, to the low-χ regime, where both local mixing and dynamical correlations play a role. Observed Tg gradient ranges are asymmetric, exceed the range of composition gradients, and modestly grow with decreasing χ. These results provide new insight into the design of nanostructured polymers with targeted local dynamics. They also emphasize an apparent dichotomy between many studies, including ours, pointing toward Tg gradient ranges on the order of 10 nm, and a second set of studies reporting on longer ranges up to hundreds of nanometers.

William F. Drayer and David S. Simmons, Sequence Effects on the Glass Transition of a Model Copolymer System, Macromolecules,55 (2022) 5926-5937. doi: 10.1021/acs.macromol.2c00664

ABSTRACT: Achieving rational control of the glass transition temperature Tg and associated dynamics is a major fundamental and practical challenge in polymer science, with applications ranging from ion conductivity of polymer electrolytes to thermal stability and processability of engineering plastics. Here we employ molecular dynamics simulations to elucidate a new strategy for rational control of glass transition temperatures by varying copolymer sequences─an approach viable due to advances enabling synthesis of copolymers with increasingly controlled sequences. Our results point to two regimes of sequence control on Tg. For sequences sufficiently blocky to allow microphase separation, Tg alterations emerge from interfacial effects on dynamics, an extensively studied phenomenon, with sequence effects on domain size playing a central role in determining Tg. For sequences that approach an alternating copolymer, we identify a second regime in which Tg is directly tuned by segmental packing...

Tamara D. Jaeger and David S. Simmons, Temperature dependence of aging dynamics in highly non-equilibrium model polymer glasses, Journal of Chemical Physics, 156 (2022) 114504. doi: 10.1063/5.0080717.

ABSTRACT: A central feature of the non-equilibrium glassy “state” is its tendency to age toward equilibrium, obeying signatures identified by Kovacs over 50 years ago. The origin of these signatures, their fate far from equilibrium and at high temperatures, and the underlying nature of the glassy “state” far from equilibrium remain unsettled. Here, we simulate physical aging of polymeric glasses, driven much farther from equilibrium and at much higher temperatures than possible in experimental melt-quenched glasses. While these glasses exhibit Kovacs’ signatures of glassy aging at sufficiently low temperatures, these signatures disappear above the onset TA of non-Arrhenius equilibrium dynamics, suggesting that TA demarcates an upper bound to genuinely glassy states. Aging times in glasses after temperature up-jumps are found to obey an Arrhenius law interpolating between equilibrium dynamics at TA and at the start of the temperature up-jump, providing a zero-parameter rule predicting their aging behavior and identifying another unrecognized centrality of TA to aging behavior. This differs qualitatively from behavior of our glasses produced by temperature down-jumps, which exhibit a fractional power law decoupling relation with equilibrium dynamics. While the Tool–Narayanaswamy–Moynihan model can predict the qualitative single-temperature behavior of these systems, we find that it fails to predict the disappearance of Kovacs signatures above TA and the temperature dependence of aging after large temperature up-jumps. These findings highlight a need for new theoretical insights into the aging behavior of glasses at ultra-high fictive temperatures and far from equilibrium.

Davindra K. Tulsi and David S. Simmons, Hierarchical Shape-Specified Model Polymer Nanoparticles via Copolymer Sequence Control, Macromolecules, 2022. doi: /10.1021/acs.macromol.1c02215.

ABSTRACT: Nature employs copolymer sequence to realize exquisite control of molecular shape in nanostructures such as protein assemblies, membranes, and DNA complexes. Synthesis of artificial sequence-controlled polymers has recently become viable, paving the way for synthetic materials mimicking biological nanostructures. Further advances are hindered by the poorly understood relationship between sequence and molecular shape. Here, we employ Brownian dynamics simulations to probe the relationship between the sequence of long copolymer chains and the resultant globular shapes realized in a selective solvent. Similar to prior work in multimolecular assembly of lower molecular weight chains, we find that sequence can mediate assembly into a range of single-molecule nanoparticulate shapes ranging from vesicles to sponges to necklaces...

David S. Simmons*, Macromolecular Modeling, in Macromolecular Engineering: From Precise Synthesis to Macroscopic Materials and Applications , 2022, Wiley. doi 10.1002/9783527815562.mme0049.

ABSTRACT: Polymer molecular modeling is particularly challenging due to the role of multibody effects and the large range of time and length scales involved. Mean-field descriptions of many polymer phenomena are well established; however, they typically do not provide for quantitative prediction and often miss or do not address important qualitative features of polymer behavior. Given the massive chemical space available, improvements in modeling and prediction are crucial to polymer design. To overcome these challenges, modelers increasingly employ hybrid approaches in which multiple complementary theoretical frameworks are applied to the same molecular model. This article provides a tutorial introduction to this area. We discuss a conceptual taxonomy for classifying and understanding modeling approaches, survey key theoretical frameworks and molecular models, and discuss factors based on which one might choose among them...

2021

Zhiwei Hao†, Asieh Ghanekarade†, Ningtao Zhu1, Katelyn Randazzo, Daisuke Kawaguchi, Keiji Tanaka, Xinping Wang, David S. Simmons*, Rodney D. Priestley*, and Biao Zuo*, Mobility gradients yield rubbery surfaces on top of polymer glasses, Nature, 596, 2021, 372-376. doi:/10.1038/s41586-021-03733-7. († co-first authors).

ABSTRACT: Many emerging materials, such as ultrastable glasses1,2 of interest for phone displays and OLED television screens, owe their properties to a gradient of enhanced mobility at the surface of glass-forming liquids. The discovery of this surface mobility enhancement3,4,5 has reshaped our understanding of the behaviour of glass formers and of how to fashion them into improved materials. In polymeric glasses, these interfacial modifications are complicated by the existence of a second length scale—the size of the polymer chain—as well as the length scale of the interfacial mobility gradient6,7,8,9. Here we present simulations, theory and time-resolved surface nano-creep experiments to reveal that this two-scale nature of glassy polymer surfaces drives the emergence of a transient rubbery, entangled-like surface behaviour even in polymers comprised of short, subentangled chains. We find that this effect emerges from superposed gradients in segmental dynamics and chain conformational statistics. The lifetime of this rubbery behaviour, which will have broad implications in constraining surface relaxations central to applications including tribology, adhesion, and surface healing of polymeric glasses, extends as the material is cooled. The surface layers suffer a general breakdown in time−temperature superposition (TTS), a fundamental tenet of polymer physics and rheology. This finding may require a reevaluation of strategies for the prediction of long-time properties in polymeric glasses with high interfacial areas. We expect that this interfacial transient elastomer effect and TTS breakdown should normally occur in macromolecular systems ranging from nanocomposites to thin films, where interfaces dominate material properties5,10.

Asieh Ghanekarade, Anh D. Phan*, Kenneth S. Schweizer*, and David S. Simmons*, Nature of dynamic gradients, glass formation, and collective effects in ultrathin freestanding films, Proceedings of the National Academy of Sciences, 118 (31), 2021, e2104398118. doi:10.1073/pnas.2104398118.

SIGNIFICANCE STATEMENT: The origin of spatially heterogeneous alterations in the dynamics of nanoconfined liquids has been a major question in physics, chemistry, and materials science for over 30 y. We combine simulation and theory to establish a unified predictive microscopic understanding in freestanding films at low to medium molecular weight, spanning from thick to ultrathin films. The simulation-based phenomenology of altered dynamics is theoretically predicted and understood based on a combination of coupled near-interface gradients of local caging constraints and interfacial and finite size-induced changes in a collective elastic barrier to motion. No invocation of any realized or incipient critical-like phenomenon is necessary, which may have broad implications for other confined systems and the mechanism of the bulk glass transition.

Tamanna Rahman, David S. Simmons, Near-Substrate Gradients in Chain Relaxation and Viscosity in a Model Low-Molecular Weight Polymer, Macromolecules, 54 (13), 2021, 5935-5949. doi:10.1021/acs.macromol.0c02888

ABSTRACT: Polymers in the nanoscale vicinity of interfaces exhibit a broad range of alterations in their dynamics and glass-formation behavior. A major goal in the study of these effects is to understand their strong apparent dependence on chemistry, measurement time scale, and metrology. Here we employ molecular dynamics simulations of thin freestanding polymer films over a range of thicknesses and polymer backbone stiffnesses to probe these dependences. Results suggest that a chemistry- and metrology-dependent onset of strong nanoconfinement may play an important role in chemical and metrological variations in the apparent strength of nanoconfinement effects..

2020

Daniel Diaz Vela, David S. Simmons, The microscopic origins of stretched exponential relaxation in two model glass-forming liquids as probed by simulations in the isoconfigurational ensemble, The Journal of Chemical Physics, 153 (2020) 234503. doi: 10.1063/5.0035609

ABSTRACT: 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. The 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 more compressed relaxation. We show that local stretching is predicted by loose local caging...

Daniel Diaz Vela, Asieh Ghanekarade, David S. Simmons, Probing the Metrology and Chemistry Dependences of the Onset Condition of Strong "Nanoconfinement" Effects on Dynamics, Macromolecules, 153 (2020) 4158-4171. doi:10.1021/acs.macromol.9b02693

Jui-Hsiang Hung, Tarak K. Patra, David S. Simmons, Forecasting the experimental glass transition from short time relaxation data, Journal of Non-Crystalline Solids, 544 (2020) 120205. doi:10.1016/j.jnoncrysol.2020.120205 .

ABSTRACT: While molecular simulations have contributed to the modern understanding of the glass transition, they are constrained in prediction of experimental glass temperatures Tg because they are limited to times far shorter than those associated with experimental glass formation. Here, we bridge this gap via a model-based forecasting approach...

Jui Hsiang Hung, David S Simmons, Do String-like Cooperative Motions Predict Relaxation Times in Glass-Forming Liquids?, Journal of Physical Chemistry B, 124, 1 (2020) 266-276. doi:10.1021/acs.jpcb.9b09468.

ABSTRACT: The Adam–Gibbs theory of glass formation posits that the growth in the activation barrier of fragile liquids on cooling emerges from a loss of configurational entropy and concomitant growth in “cooperatively rearranging regions” (CRRs). A body of literature over 2 decades has suggested that “string-like” cooperatively rearranging clusters observed in molecular simulations may be these CRRs—a scenario that would have profound implications for the understanding of the glass transition. The central element of this postulate is the report of an apparent zero-parameter relationship between the mass of string-like CRRs and the relaxation time. Here, we show, based on molecular dynamics simulations of multiple glass-forming liquids, that this finding is the result of an implicit adjustable parameter...

2019

Kenneth S. Schweizer, David S Simmons, Progress towards a phenomenological picture and theoretical understanding of glassy dynamics and vitrification near interfaces and under nanoconfinement, Journal of Chemical Physics, 151 (2019) 240901. doi:10.1063/1.5129405.

ABSTRACT: The nature of alterations to dynamics and vitrification in the nanoscale vicinity of interfaces—commonly referred to as “nanoconfinement” effects on the glass transition—has been an open question for a quarter century. We first analyze experimental and simulation results over the last decade to construct an overall phenomenological picture...

Venkatesh Meenakshisundaram, Jui-Hsiang Hung, David S. Simmons, Design rules for glass formation from model molecules designed by a neural-network-biased genetic algorithm, Soft Matter, 15 (2019) 7795-7808. 10.1039/C9SM01486A

ABSTRACT: The glass transition – an apparent amorphous solidification process – is a central feature of the physical properties of soft materials such as polymers and colloids. A key element of this phenomenon is the observation of a broad spectrum of deviations from an Arrhenius temperature of dynamics in glass-forming liquids, with the extent of deviation quantified by the “fragility” of glass formation. The underlying origin of “fragile” glass formation and its dependence on molecular structure remain major open questions in condensed matter physics and soft materials science. Here we employ molecular dynamics simulations, together with a neural-network-biased genetic algorithm, to design and study model rigid molecules spanning a broad range of fragilities of glass formation. Results indicate that fragility of glass formation can be controlled by tuning molecular asphericity...

Jui-Hsiang Hung, Tarak K Patra, Venkatesh Meenakshisundaram, Jayachandra Hari Mangalara, David S Simmons , Universal localization transition accompanying glass formation: insights from efficient molecular dynamics simulations of diverse supercooled liquids, Soft Matter, 15 (2019) 1223-1242. doi: 10.1039/C8SM02051E. (Inside cover article, In Special Issue on Soft Matter Emerging Investigators)

ABSTRACT: The origin of the precipitous dynamic arrest known as the glass transition is a grand open question of soft condensed matter physics. It has long been suspected that this transition is driven by an onset of particle localization and associated emergence of a glassy modulus. However, progress towards an accepted understanding of glass formation has been impeded by an inability to obtain data sufficient in chemical diversity, relaxation timescales, and spatial and temporal resolution to validate or falsify proposed theories for its physics. Here we first describe a strategy enabling facile high-throughput simulation of glass-forming liquids to nearly unprecedented relaxation times. We then perform simulations of 51 glass-forming liquids, spanning polymers, small organic molecules, inorganics, and metallic glass-formers, with longest relaxation times exceeding one microsecond. Results identify a universal particle-localization transition accompanying glass formation ...

Xiao Zhang, Jing Zhao, Changhuai Ye, Tzu-Yu Lai, Chad R Snyder, Alamgir Karim, Kevin A Cavicchi, David S Simmons, Dynamical Correlations for Statistical Copolymers from High-Throughput Broad-Band Dielectric Spectroscopy, ACS Combinatorial Science, 21, 4 (2019) 276-299. doi: 10.1021/acscombsci.8b00160.

ABSTRACT: Broad-band dielectric spectroscopy (BDS) provides a powerful method of characterizing relaxation dynamics in diverse materials. Here we describe and employ a novel instrument for high-throughput broad-band dielectric spectroscopy (HTBDS) that accelerates this capability...

Scott M Smith, David S Simmons , Poisson ratio mismatch drives low-strain reinforcement in elastomeric nanocomposites, Soft Matter, 15 (2019) 656-670. doi: 10.1039/C8SM02333F.

ABSTRACT: Introduction of nanoparticulate additives can dramatically impact elastomer mechanical response, with large enhancements in modulus, toughness, and strength. Despite the societal importance of these effects, their mechanistic origin remains unsettled. Here, using a combination of theory and molecular dynamics simulation, we show that low-strain extensional reinforcement of elastomers is driven by a nanoparticulate-jamming-induced suppression in the composite Poisson ratio. This suppression forces an increase in rubber volume with extensional deformation, effectively converting a portion of the rubber's bulk modulus into an extensional modulus. A theory describing this effect is shown to interrelate the Poisson ratio and modulus across a matrix of simulated elastomeric nanocomposites of varying loading and nanoparticle structure. This model provides a design rule for structured nanoparticulates that maximizes elastomer mechanical response...

2018

Daniel Diaz-Vela, Jui-Hsiang Hung, David S Simmons, Temperature-Independent Rescaling of the Local Activation Barrier Drives Free Surface Nanoconfinement Effects on Segmental-Scale Translational Dynamics near Tg, ACS Macro Letters, 7 (2018) 1295-1301. doi: 10.1021/acsmacrolett.8b00695.

ABSTRACT: Near-interface alterations in dynamics and glass formation behavior have been the subject of extensive study for the past two decades, both because of their practical importance and in the hope of revealing underlying correlation lengths underpinning glass transition more generally. Here we employ molecular dynamics simulations of thick films to demonstrate that these effects emerge, for segmental-scale translational dynamics at low temperature, from a temperature-independent rescaling of the local activation barrier. This rescaling manifests as a fractional power law decoupling relationship of local dynamics relative to the bulk...

Chao Wang, Clinton G Wiener, Pablo I Sepulveda-Medina, Changhuai Ye, David S Simmons, Ruipeng Li, Masafumi Fukuto, RA Weiss, Bryan D Vogt, Antifreeze Hydrogels from Amphiphilic Statistical Copolymers , Chemistry of Materials, 31 (2018) 135-145. doi: 10.1021/acs.chemmater.8b03650.

ABSTRACT: Prevention of ice formation is a critical issue for many applications, but routes to overcome the large thermodynamic driving force for crystallization of water at significant supercooling are limited. Here, we demonstrate that supramolecular hydrogels formed from statistical copolymers of 2-hydroxyethyl acrylate (HEA) and 2-(N-ethylperfluorooctane sulfonamido)ethyl methacrylate (FOSM) exhibit a degree of ice formation suppression unprecedented in a synthetic material. The mechanisms of ice prevention by these hydrogels mimic two methods used by nature: (1) hydrogen bonding of water to highly hydrophilic macromolecular chains and (2) nanoconfinement...

Yizi Cheng, Junhong Yang, Jui-Hsiang Hung, Tarak K Patra, David S Simmons, Design Rules for Highly Conductive Polymeric Ionic Liquids from Molecular Dynamics Simulations, Macromolecules, 51 (2018) 6630-6644. doi: 10.1021/acs.macromol.8b00572.

ABSTRACT: Polymeric ionic liquids (PILs) are of considerable interest as next-generation battery materials due to their potential to combine the solid-state stability of polymers with the high ion conductivities of ionic liquids. However, polymerization of ionic liquids to form a polymer generally leads to a suppression in ion transport rates that has proven to be a major barrier to the realization of commercially viable PIL solid electrolytes. Here we employ a combination of all atom and coarse-grained molecular dynamics simulations to identify strategies by which ion conductivity can be maximized by maximizing both PIL segmental relaxation rates and the extent of ion transport decoupling from chain...

Jui-Hsiang Hung, Jayachandra Hari Mangalara, David S Simmons, Heterogeneous Rouse Model Predicts Polymer Chain Translational Normal Mode Decoupling, Macromolecules, 51 (2018) 2887-2898. doi: 10.1021/acs.macromol.8b00135.

ABSTRACT: It has been known for 50 years that polymers exhibit chain normal mode decoupling upon approach to the glass transition, with chain dynamics exhibiting a weaker temperature dependence than segmental dynamics. Inspired by Sokolov and Schweizer’s suggestion that this thermorheological complexity is a consequence of dynamic heterogeneity in the supercooled state, we generalize the Rouse model to account for a distribution of segmental mobilities. The heterogeneous Rouse model (HRM) predicts chain translational normal mode decoupling as a manifestation of diffusion/relaxation decoupling (Stokes–Einstein breakdown)—a consequence of differences in how normal modes average over a distribution of segmental mobilities. Molecular dynamics simulations agree with theoretical predictions, with the HRM found to quantitatively predict deviations from Rouse scaling of the translational friction coefficient based on the observed degree of Stokes–Einstein breakdown.

Xiao Zhang, Jing Zhao, Chad R Snyder, Abdullah Al‐Enizi, Ahmed Eltazahry, David S Simmons, Alamgir Karim, Structure, nanomechanics, and dynamics of dispersed surfactant‐free clay nanocomposite films, Polymer Engineering & Science, 58 (2018) 1285-1295. doi: 10.1002/pen.24693.

ABSTRACT: The current work presents a new approach to achieve high quality dispersion of surfactant‐free nanoclay tactoid particles in sub‐micron thin films despite the absence of organic modifier...

2017

Venkatesh Meenakshisundaram, Jui-Hsiang Hung, Tarak K Patra, David S Simmons, Designing Sequence-Specific Copolymer Compatibilizers Using a Molecular-Dynamics-Simulation-Based Genetic Algorithm, Macromolecules 50 (2017) 1155-1166. doi: acs.macromol.6b01747 .

ABSTRACT: Compatibilizers—surfactant molecules designed to improve the stability of an interface—are employed to enhance material properties in settings ranging from emulsions to polymer blends. A major compatibilization strategy employs block or random copolymers composed of distinct repeat units with preferential affinity for each of the two phases forming the interface. Here we pose the question of whether improved compatibilization could be achieved by employing new synthetic strategies to realize copolymer compatibilizers with specific monomeric sequence. We employ a novel molecular-dynamics-simulation-based genetic algorithm to design model sequence-specific copolymers that minimize energy of a polymer/polymer interface. Results indicate that sequence-specific copolymers offer the potential to yield larger reductions in interfacial energy than either block or random copolymers, with the preferred sequence being compatibilizer concentration dependent. By employing a simple thermodynamic scaling model for copolymer compatibilization, we pinpoint the origins of this sequence specificity and concentration dependence...

Tarak K Patra, Venkatesh Meenakshisundaram, Jui-Hsiang Hung, David S Simmons, Neural-Network-Biased Genetic Algorithms for Materials Design: Evolutionary Algorithms That Learn, ACS Combinatorial Science, 19 (2017) 96-107. doi: 10.1021/acscombsci.6b00136 .

ABSTRACT: ... Here we describe a new strategy, the neural-network-biased genetic algorithm (NBGA), for combining genetic algorithms, machine learning, and high-throughput computation or experiment to discover materials with extremal properties in the absence of pre-existing data. Within this strategy, predictions from a progressively constructed artificial neural network are employed to bias the evolution of a genetic algorithm, with fitness evaluations performed via direct simulation or experiment. In effect, this strategy gives the evolutionary algorithm the ability to “learn” and draw inferences from its experience to accelerate the evolutionary process...

Mithun Chowdhury, Yunlong Guo, Yucheng Wang, Weston L Merling, Jayachandra H Mangalara, David S Simmons, Rodney D Priestley, Spatially Distributed Rheological Properties in Confined Polymers by Noncontact Shear, The Journal of Physical Chemistry Letters, 8 (2017)1229-1234. doi: 10.1021/acs.jpclett.7b00214 .

ABSTRACT: When geometrically confined to the nanometer length scale, a condition in which a large portion of the material is in the nanoscale vicinity of interfaces, polymers can show astonishing changes in physical properties. In this investigation, we employ a unique noncontact capillary nanoshearing method to directly probe nanoresolved gradients in the rheological response of ultrathin polymer films as a function of temperature and stress. Results show that ultrathin polymer films, in response to an applied shear stress, exhibit a gradient in molecular mobility and viscosity that originates at the interfaces. We demonstrate, via molecular dynamics simulations, that these gradients in molecular mobility reflect gradients in the average segmental relaxation time and the glass-transition temperature.

Jonathan Lee, Jayachandra Hari Mangalara, David S Simmons, Correspondence between the rigid amorphous fraction and nanoconfinement effects on glass formation, Journal of Polymer Science Part B: Polymer Physics, 55 (2017) 907-918. doi: 10.1002/polb.24324. (Cover Article)

ABSTRACT: Semicrystalline polymers' properties are strongly influenced by the presence of a “rigid amorphous fraction” (RAF) of material with enhanced glass transition temperature and suppressed mobility. Here, we review striking similarities between this domain and near‐interface regions of altered dynamics in other nanostructured polymers, including thin films, nanocomposites, layered films, and ionomers. Building on these observations, we present results of molecular simulations of model nanolayered polymers in which one layer is crystalline. The results of these simulations, which bear close similarities to semicrystalline homopolymers while providing a well‐defined equilibrium crystal size, suggest that the RAF shares the same phenomenology and mechanism as dynamic interphases in these other polymeric systems. Specifically, the fraction of rigid amorphous material is driven by the scale of cooperative rearrangements characteristic of dynamics in polymers and other supercooled liquids. These results situate the RAF as a specific instance of a general tendency towards broad dynamic interphases in polymers and other glass‐forming materials.

Scott M Smith, David S Simmons, Horizons for Design of Filled Rubber Informed by Molecular Dynamics Simulation, Rubber Chemistry and Technology, 90 (2017) 238-263. doi: 10.5254/rct.17.82668 .

ABSTRACT: Fillers such as carbon black provide a long-standing and essential strategy for the mechanical reinforcement of rubber in tires and other load-bearing applications. Despite their technological importance, however, the microscopic mechanism of this reinforcement remains a matter of considerable debate. A predictive understanding of filler-based reinforcement could catalyze the design of new rubber-filler composites with enhanced performance. Molecular dynamics simulations of rubber mechanical response in the presence of structured fillers offer a new strategy for resolving the origins of filler-based reinforcement and guiding filler design. Results of for ideal rubber-filler dispersions over a range of filler structures suggest that neither hydrodynamic effects nor non-deformable “bound rubber domains” are necessary to achieve high reinforcement...

Jayachandra Hari Mangalara, Mark E Mackura, Michael D Marvin, David S Simmons, The relationship between dynamic and pseudo-thermodynamic measures of the glass transition temperature in nanostructured materials, The Journal of Chemical Physics, 146 (2017) 203316. doi: 10.1063/1.4977520 .

ABSTRACT: Despite decades of research on the effects of nanoconfinement on the glass transitiontemperature Tg, apparent discrepancies between pseudothermodynamic and dynamic measurements of these effects have raised questions regarding the presence of long-ranged interfacial dynamic gradients in glass-forming liquids. Here we show that these differences can be accounted for based on disparities in these methods’ weightings over local Tg’s within an interfacial gradient. This finding suggests that a majority of experimental data are consistent with a broad interfacial dynamic interphase in glass-forming liquids.

Jayachandra Hari Mangalara, Michael D Marvin, Nicholas R Wiener, Mark E Mackura, David S Simmons, Does fragility of glass formation determine the strength of Tg-nanoconfinement effects? The Journal of Chemical Physics, 146 (2017) 104902. doi: 10.1063/1.4976521 .

ABSTRACT: Nanoscale confinement has been shown to alter the glass transition and associated mechanical and transport properties of glass-forming materials. Inspired by expected interrelations between nanoconfinement effects, cooperative dynamics in supercooled liquids, and the “fragility” (or temperature-abruptness) of the glass transition, it is commonly expected that nanoconfinement effects on Tg should be more pronounced for more fragile glass formers. Here we employ molecular dynamics simulations of glass formation in the bulk and under nanoconfinement of model polymers in which we systematically tune fragility by several routes. Results indicate that a correlation between fragility and the strength of nanoconfinement effects is weak to modest at best when considering all systems ...

2016

Weston L Merling, Johnathon B Mileski, Jack F Douglas, David S Simmons, The Glass Transition of a Single Macromolecule, Macromolecules, 49 (2016) 7597-7604 . doi: acs.macromol.6b01461.

ABSTRACT: We employ molecular simulations to show that a generic isolated macromolecule behaves as a glass-forming liquid in an extreme state of nanoconfinement. Specifically, its glass transition temperature Tg is strongly suppressed with respect to a corresponding bulk-state polymer, but adsorption onto an attractive substrate can reverse this effect. Results indicate that observations of bulklike Tg in individual nanoconfined systems can result from a “compensation point” between Tg enhancement and Tg suppression. In addition to their implications for the understanding of nanostructured synthetic materials, these results are also likely relevant to the dynamics of globular and adsorbed biological macromolecules.

Jayachandra Hari Mangalara, Michael D Marvin, David S Simmons, Three-Layer Model for the Emergence of Ultrastable Glasses from the Surfaces of Supercooled Liquids, The Journal of Physical Chemistry B, 120 (2016) 4861-4865. doi: 10.1021/acs.jpcb.6b04736 .

ABSTRACT: Ultrastable glasses produced by vapor deposition exhibit properties consistent with glasses that have been aged for thousands of years or more. These materials’ properties are believed to emerge from the presence of a mobile layer at the surface of supercooled liquids that allows access to lower-energy states. However, the precise mechanism by which this enhanced mobility is translated into ultrastable glass behavior remains incompletely understood. Here we show that enhanced densities and stabilities consistent with ultrastable glasses specifically can emerge as a result of a mismatch in the length scales of thermodynamic and dynamic gradients at the surfaces of equilibrium supercooled liquids. In particular, ultrastable glass properties can be understood within a three-layer model of the interface in which a “facilitated layer” intermediate between the surface and bulk exhibits bulk-like liquid-state density but suppressed Tg. This mismatch in length-scale has previously been correlated with the scale of cooperative rearrangements in the supercooled state, suggesting that ultrastable glasses may be a direct consequence of the cooperative nature of dynamics in equilibrium supercooled liquids.

David S Simmons, An Emerging Unified View of Dynamic Interphases in Polymers, Macromolecular Chemistry and Physics, 217 (2016) 137-148. doi: 10.1002/macp.201500284.

ABSTRACT: The properties of nanostructured polymeric materials reflect the presence of a dynamic interphase in which polymer segmental dynamics, glass formation, mechanics, and transport properties are altered from their bulk states. Examples include nanoconfinement effects on polymer nanofilms, reinforcement effects in polymer nanocomposites and filled rubbers, formation of a rigid amorphous fraction in semicrystalline polymers, and observation of suppressed‐mobility domains around ionic aggregates in ionomers. Whereas many of these phenomena have been viewed as emerging from system‐specific mechanisms, here recent work is highlighted that contributes to an emerging unified understanding of the dynamic interphase in all of these systems. This view suggests that the size scales of dynamic interphases in polymers are generally determined by the scale of cooperative rearrangements intrinsic to relaxation dynamics in supercooled glass‐forming liquids. It also implicates alterations in local segmental rattling as playing a central role in the dynamic interphase, and it suggests that near‐interface modifications in dynamics exhibit a general dependence on the interfacial work of adhesion and the relative high‐frequency moduli of the interface‐adjacent domains.

2015

Jayachandra Hari Mangalara, David S Simmons, Tuning Polymer Glass Formation Behavior and Mechanical Properties with Oligomeric Diluents of Varying Stiffness, ACS Macro Letters, 4 (2015) 1134-1138. doi: 10.1021/acsmacrolett.5b00635.

ABSTRACT: Small-molecule diluents are important tools in the control of polymers’ glass formation, transport, and mechanical properties. While recent work has indicated that these diluents can impose a more diverse range of effects than previously appreciated, use of these additives to rationally control polymer properties requires a predictive understanding of their effects. Here we employ molecular dynamics simulations to show that diluent-induced changes in a polymer’s glass transition temperature Tg can be predicted based on the diluent’s Debye–Waller factor ⟨u2⟩, a measure of picosecond time scale rattle-space, via a functional form previously found to predict nanoconfinement-induced shifts in polymer Tg. Moreover, we show that diluent-induced alterations in polymer segmental relaxation time are related to changes in modulus and ⟨u2⟩ via the Generalized Localization Model of relaxation. These results provide new design principles for the use of oligomeric diluents in achieving independent, targeted control of structural relaxation and glassy moduli.

Dihui Ruan, David S Simmons, Glass Formation near Covalently Grafted Interfaces: Ionomers as a Model Case, Macromolecules, 48 (2015) 2313-2323. doi: 10.1021/acs.macromol.5b00025 .

ABSTRACT: In materials ranging from ionomers to semicrystalline polymers, the glass transition—the central phenomenon determining the dynamic and mechanical behavior of amorphous materials—can reflect profound alterations by covalent grafting to surfaces or inclusions. The understanding of these alterations in ionomers has long been underpinned by the view that these grafts serve as “tethers”, suppressing dynamics of attached chains out to a distance related to their molecular stiffness. Here we describe simulations of model ionomers suggesting the need for a fundamental reconsideration of this understanding. Specifically, as in nanocomposites and nanoconfined glass-formers, we find that near-interface mobility suppression is mediated by cooperative rearrangements intrinsic to glass-forming liquids rather than by a unique covalent tethering effect. While remaining consistent with many earlier predictions, these results bring dynamics near ionomeric aggregates and other grafted interfaces into agreement with a body of literature suggesting the universal presence of a cooperative dynamic length scale in glass-forming liquids that dominates their relaxation behavior and establishes their fundamental length scale of local dynamic gradients.

Changhuai Ye, Clinton G Wiener, Madhusudan Tyagi, David Uhrig, Sara V Orski, Christopher L Soles, Bryan D Vogt, David S Simmons, Understanding the Decreased Segmental Dynamics of Supported Thin Polymer Films Reported by Incoherent Neutron Scattering, Macromolecules, 48 (2015) 801-808. doi: 10.1021/ma501780g .

ABSTRACT: Incoherent neutron scattering (INS) has commonly reported a suppression of segmental dynamics for supported thin polymer films as thickness is decreased, which is counter to expectations based on other measurement techniques such as ellipsometry and fluorescence. Here INS is utilized to measure the dynamics of thin films of comb polystyrene (PS) from 50 to 525 K. There is a significant suppression in dynamics as determined from the ∼5 ns Debye–Waller factor, ⟨u2⟩, as measured via INS for films as thick as 213 nm, while there is no change in the glass transition temperature (Tg) as determined by ellipsometry for films as thin as 20 nm. This poor correlation between Tg from ellipsometry and dynamics as measured by ⟨u2⟩ is attributed to contamination of nanosecond ⟨u2⟩ by incipient relaxation processes, differences in sensitivity to the postulated dynamically dead layer near the substrate due to the relative weighting of the distribution of dynamics between the two techniques, differences in the time scales probed, and possible decoupling between fast and slow dynamics under nanoconfinement. These results suggest that branching of PS significantly increases the interactions with the substrate to suppress the dynamics. Both technique-specific sensitivity to time scales and its weighing of the average over the gradient in dynamic properties present at the interfaces are important to consider when qualitatively different phenomena are inferred from different measurements.

Dihui Ruan, David S Simmons, Roles of chain stiffness and segmental rattling in ionomer glass formation, Journal of Polymer Science Part B: Polymer Physics, 53 (2015) 1458-1469. doi: 10.1002/polb.23788

ABSTRACT: Ionomers–polymers with bound ionic moieties—have received interest for their exceptional properties including high toughness and ion conductivity. One of the primary effects of introduction of covalently bound ions to a polymer is alteration of its glass transition and segmental dynamics. The standard model for these alterations attributes them to a covalent ‘tethering’ effect in which segments near ionic aggregates are immobilized within a range related to the chain persistence length. Here, results from molecular dynamics simulations of glass formation in model ionomers of varying chain stiffness indicate that chain persistence length does not play a central role in ionic‐aggregate‐induced suppression of local segmental dynamics. Instead, these alterations are found to accord with more universal interface‐induced alterations in glass‐forming liquids, consistent with a recent study in which the present authors found a correlation between near‐aggregate mobility suppression and the scale of segmental cooperative rearrangements. In this case, results indicate that shifts in the overall segmental relaxation times of these polymers can be quantitatively rationalized in terms of alterations in the Debye‐Waller factor, reflecting changes in the local picosecond‐timescale segmental rattling.

2014

Ryan J Lang, Weston L Merling, David S Simmons, Combined Dependence of Nanoconfined T g on Interfacial Energy and Softness of Confinement, ACS Macro Letters, 3 (2014) 758-762. doi: 10.1021/mz500361v.

ABSTRACT: We employ molecular dynamics simulations of nanolayered polymers to systematically quantify the dependence of Tg nanoconfinement effects on interfacial energy and the “softness” of confinement. Results indicate that nanoconfined Tg depends linearly on interfacial adhesion energy, with a slope that scales exponentially with the ratio of the bulk Debye–Waller factors ⟨u2⟩ of the confined and confining materials. These trends, together with a convergence at low interfacial adhesion energy to the Tg of an equivalent freestanding film, are captured in a single functional form, with only three parameters explicitly referring to the confined state....

MD Marvin, RJ Lang, DS Simmons, Nanoconfinement effects on the fragility of glass formation of a model freestanding polymer film, Soft Matter, 10 (2014) 3166-3170. doi: 10.1039/C3SM53160K.

ABSTRACT: Evidence suggests that the fragility (m) of glass formation both underpins and is sensitive to nanoconfinement effects on the glass transition. Here we present data indicating that nanoconfinement-induced changes in m of freestanding films emerge from a dominance of finite-size-driven fragility suppression over interfacial fragility enhancement.

Mark E Mackura, David S Simmons, Enhancing heterogenous crystallization resistance in a bead‐spring polymer model by modifying bond length, Journal of Polymer Science Part B: Polymer Physics, 52 (2014) 134-140. doi: 10.1002/polb.23398.

ABSTRACT: Linear bead‐spring chain models have been widely employed to study the glass‐formation of polymer melts due to their apparent resistance to homogenous crystallization. In this article, we present simulation data demonstrating that, in contrast to their bulk behavior, widely used bead‐spring models are subject to rapid heterogenous nucleation and crystallization when exposed to rigid crystalline walls. We propose a modified bead‐spring model, employing the finitely extensible nonlinear elastic bond, and show that it exhibits robust glass‐formation in the presence of crystalline walls. This new model will enable study of supercooled polymers in the presence of...

2013

Ryan J Lang, David S Simmons, Interfacial Dynamic Length Scales in the Glass Transition of a Model Freestanding Polymer Film and Their Connection to Cooperative Motion, Macromolecules, 46 (2013) 9818-9825. doi: 10.1021/ma401525q.

ABSTRACT: Nanoscale confinement alters the dynamics of glass-forming liquids in films of order 100 nm in thickness. A common hypothesis for the origin of this long range posits that interfacial dynamics propagate into the film via cooperatively rearranging regions (CRRs). However, the precise nature of the dynamic interface remains uncertain, and its identification with CRRs has yet to be firmly established. Based on results from coarse-grained molecular dynamics simulations of a freestanding polymer film, here we show that an apparent qualitative discrepancy between computational and several experimental measures of the interfacial dynamic length scale results from a difference in definition and not from a difference in underlying behavior of experimental and simulated systems...

David S Simmons, Isaac C Sanchez, Scaled Particle Theory for the Coil–Globule Transition of an Isolated Polymer Chain, Macromolecules, 46 (2013) 4691-4697. doi: 10.1021/ma400338d .

ABSTRACT: We have developed a scaled particle theory for the coil–globule transition (CGT) of a chain of attractive hard spheres in an attractive hard sphere solvent. The model predicts a lower critical solution temperature (LCST) related CGT that merges with a high temperature upper critical solution temperature (UCST) related CGT at a high pressure hypercritical point. The predicted hypercritical point is in good agreement with simulations of a bead–spring polymer chain in a symmetric solvent, without the use of any adjustable parameters. The model predicts that increasing bulkiness of monomers relative to solvent molecules, decreasing polymer cohesive energy relative to solvent cohesive energy, and reduced chain length stabilize the expanded coil state, in qualitative agreement with simulations. These trends in the UCST-CGT and LCST-CGT are found to emerge from incompressible-solvent effects and equation-of-state effects, respectively, consistent with the enthalpic origin of the UCST and the entropic origin of the LCST.

David S Simmons, Marcus T Cicerone, Jack F Douglas, Response to “Comment on ‘Generalized Localization Model of Relaxation in Glass-Forming Liquids’” by A. Ottochian et al., Soft Matter, 9 (2013) 7892-7899. doi: 10.1039/C3SM51316E .

Department of Chemical, Biological, and Materials Engineering

University of South Florida

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