arXiv subjects

Robert J. Wagner

Publications and source records attributed to Robert J. Wagner.

5 recordsLinked to original sources

arXiv2026

Physically-Motivated Primitive Path Analysis of Entangled Polymer Networks

Physical entanglements between polymer chains enhance the moduli, strength, and toughness of elastomers and gels, yet relating entanglement micromechanics to macroscopic mechanical benefits remains difficult. Experimentally investigating entanglements is challenging due to their nanoscale sizes, subsurface locations, and chemical indistinguishability from their surroundings. Computationally mapping structure-property relations is costly when using physics-based models that enable direct entanglement observation, such as coarse-grained molecular dynamics (CGMD). Entanglements are also transient, configuration-dependent features without clear quantitative definitions. To address this ambiguity, we introduce an approach that quantitatively defines local entanglements along simulated polymer backbones using the Gaussian Linking Number, and introduce a geometric center of entanglement verified to represent the position through which entropic chain forces are transmitted via Kremer-Grest CGMD simulations. Unlike existing approaches, which output a single linking number for chain pairs, our method identifies the multitude of load-transmitting inter- and intra-chain entanglements along a polymer's backbone. To bridge scales, we introduce a topological distillation algorithm that converts entangled CGMD networks into representative discrete network models (DNMs), representing entanglements as vertices and primitive paths as load-transmitting edges. Our DNMs reproduce small-strain virial stress predictions of the Kremer-Grest model with a 97% reduction in computational cost, verifying both physical accuracy and computational efficiency. This distillation procedure will facilitate physics-based, predictive modeling of entangled network mechanics, from polymers to architected metamaterials.

cond-mat.soft↗

arXiv2024

Bond exchange reactions as a paradigm for mitigating residual stress in polymer matrix fiber composites

Polymer matrix fiber composites often suffer from residual stresses due to differences in coefficients of thermal expansion between the fibers and resins, as well as contractile strain of the resins during curing. To address residual stress driven composite failure, we propose the use of vitrimers as composite resins, which can undergo thermally activated, stress alleviating, bond exchange reactions (BERs). We conduct fiber Bragg grating measurements for a single glass fiber within bulk vitrimer. These show that the fiber strain in vitrimers with 5% catalyst is significantly lower than in those with 0% catalyst (minimal BER expected) during both curing and post-curing phases. We developed a finite deformation, micromechanically-inspired model that incorporates curing, thermal processes, and BERs, and then implemented this model it into finite element software to simulate stress evolution within single fiber composite systems. The combination of experimental and computational results reveals that BERs can effectively mitigate, but not eliminate, the residual stress in polymer matrix fiber composites.

cond-mat.soft↗

arXiv2024

A foundational framework for the mesoscale modeling of dynamic elastomers and gels

Discrete mesoscale network models, in which explicitly modeled polymer chains are replaced by implicit pairwise potentials, are capable of predicting the macroscale mechanical response of polymeric materials such as elastomers and gels, while offering greater insight into microstructural phenomena than constitutive theory or macroscale experiments alone. However, whether such mesoscale models accurately represent the molecular structures of polymer networks requires investigation during their development, particularly in the case of dynamic polymers that restructure in time. We here introduce and compare the topological and mechanical predictions of an idealized, reduced-order mesoscale approach in which only tethered dynamic bonding sites and crosslinks in a polymer's backbone are explicitly modeled, to those of molecular theory and a Kremer-Grest, coarse-grained molecular dynamics approach. We find that for short chain networks at intermediate polymer packing fractions, undergoing relatively slow loading rates, the mesoscale approach reasonably reproduces the chain conformations, bond kinetic rates, and ensemble stress responses predicted by molecular theory and the bead-spring model. Further, it does so with a 90% reduction in computational cost. These savings grant the mesoscale model access to larger spatiotemporal domains than conventional molecular dynamics, enabling simulation of large deformations as well as durations approaching experimental timescales (e.g., those utilized in DMA). While the model investigated is for monodisperse polymer networks in theta-solvent, without entanglement, charge interactions, long-range dynamic bond interactions, or other confounding physical effects, this work highlights the utility of these models and lays a foundational groundwork for the incorporation of such phenomena moving forward.

cond-mat.soft↗

arXiv2023

The mechanical response of fire ant rafts

Fire ants (Solenopsis invicta) cohesively aggregate via the formation of voluntary ant-to-ant attachments when under confinement or exposed to water. Once formed, these aggregations act as viscoelastic solids due to dynamic bond exchange between neighboring ants as demonstrated by rate-dependent mechanical response of 3D aggregations, confined in rheometers. We here investigate the mechanical response of 2D, planar ant rafts roughly as they form in nature. Specifically, we load rafts under uniaxial tension to failure, as well as to 50% strain for two cycles with various recovery times between. We do so while measuring raft reaction force (to estimate network-scale stress), as well as the networks' instantaneous velocity fields and topological damage responses to elucidate the ant-scale origins of global mechanics. The rafts display brittle-like behavior even at slow strain rates (relative to the unloaded bond detachment rate) for which Transient Network Theory predicts steady-state creep. This provides evidence that loaded ant-to-ant bonds undergo mechanosensitive bond stabilization or act as \say{catch bonds}. This is further supported by the coalescence of voids that nucleate due to biaxial stress conditions and merge due to bond dissociation. The characteristic timescales of void coalescence due to chain dissociation provide evidence that the local detachment of stretched bonds is predominantly strain- (as opposed to bond lifetime-) dependent, even at slow strain rates, implying that bond detachment rates diminish significantly under stretch. Significantly, when the voids are closed by restoring the rafts to unstressed conditions, mechanical recovery occurs, confirming the presence of concentration-dependent bond association that - combined with force-diminished dissociation - could further bolster network cohesion under certain stress states.

cond-mat.soft↗

arXiv2020

Collective treadmilling in fire ant rafts permits sustained protrusion growth

Condensed active matter is exemplary for its capacity to morph and exhibit internal flows despite remaining cohered. To facilitate understanding of this ability, we investigate the cause of finger-like protrusions that emerge from super-organismal, aggregated rafts of fire ants (Solenopsis invicta). While these features are easily observed, what permits their recuring initiation, growth, and recession is not immediately clear. Ants rafts are comprised of a floating, structural network of interconnected ants on which a layer of freely active ants walks. We show here that sustained shape evolution is permitted by treadmilling defined by the competition between perpetual raft contraction due to displacement of bulk structural ants into the active layer, and outwards raft expansion due to deposition of free ants into the structural network at the edges. Furthermore, we see that protrusions emerge due to asymmetries in the edge deposition rate of surface ants, and we provide strong evidence that these asymmetries occur stochastically due to wall accumulation effects and local alignment interactions. Together these effects permit the cooperative, yet spontaneous formation of protrusions that fire ants utilize for functional exploration and to escape flooded environments. Ant raft dynamics mirror the treadmilling that facilitates morphogenesis and motility of cytoskeletons, thus providing another example in which reshaping of condensed active matter is explained by free constituent transport, yet whose continuance relies on perpetual phase transition between structural and free members.

cond-mat.soft↗