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DOE OSTI · 3025391

Intermediate time sub-diffusion and stress relaxation in ring polymer melts

Mei, Baicheng [Univ. of Illinois at Urbana-Champaign, IL (United States)] (ORCID:000000030289248X)·Grest, Gary S. [Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)] (ORCID:0000000252609788)·O’Connor, Thomas C. [Carnegie Mellon Univ., Pittsburgh, PA (United States)] (ORCID:0000000293930295)·Schweizer, Kenneth S. [Univ. of Illinois at Urbana-Champaign, IL (United States)] (ORCID:0000000312753790)
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Abstract

The slow dynamics of non-concatenated ring melts remains a frontier problem in polymer science with implications for many soft material environments including cellular biophysics. Here, in this work, we report large-scale simulations of model ring melts that analyze the monomer and center-of-mass (CM) mean square displacements (MSD) and stress relaxation function on intermediate time and length scales. The degree of dynamical slowing down is characterized by the maximally sub-diffusive fractional time scaling exponents. The data span an exceptionally wide range of ring degrees of polymerization and stiffnesses and are not successfully organized based on the classic measure linear chain entanglement, N/N e . Rather, we find that the crossover degree of polymerization, N D , based on ring macromolecular caging that successfully allows master curves to be constructed for the long-time CM self-diffusion constant also collapses these temporal dynamic scaling exponents. Different properties display different exponents and exhibit one or two regimes of linear variation with the logarithm of N D / N . A distinct crossover of the CM-MSD and stress relaxation exponents emerges at sufficiently large N or stiffness that is not found for the monomer MSD, indicating a novel form of dynamic decoupling. This crossover aligns with the predicted critical degree of polymerization for transitioning from a weak to strong caging regime, indicative of activated transport. The latter may reflect the emergence of an intermolecular collective contribution to stress in analogy with dense soft colloidal matter. Suggestions are made for future theoretical work to address the rich patterns of behavior discovered.

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Anomalous diffusionCage effectComputer simulationHopping transportMass diffusivityMechanical stressMolecular dynamicsPolymersStatistical mechanics theories and methods

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Mei, Baicheng [Univ. of Illinois at Urbana-Champaign, IL (United States)] (ORCID:000000030289248X), Grest, Gary S. [Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)] (ORCID:0000000252609788), O’Connor, Thomas C. [Carnegie Mellon Univ., Pittsburgh, PA (United States)] (ORCID:0000000293930295), Schweizer, Kenneth S. [Univ. of Illinois at Urbana-Champaign, IL (United States)] (ORCID:0000000312753790). 2026-03-04. Intermediate time sub-diffusion and stress relaxation in ring polymer melts. https://doi.org/10.1063/5.0315238

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DOE OSTI2025

Beyond fragmented dopant dynamics in quantum spin lattices: Robust localization and non-Gaussian diffusion

The motion of dopants in magnetic spin lattices has received tremendous attention for at least four decades due to its connection to high-temperature superconductivity. Despite these efforts, we lack a complete understanding of their behavior, especially out of the equilibrium and at nonzero temperatures. In this paper, we take a significant step towards a much deeper understanding based on state-of-the-art matrix-product-state calculations. In particular, we investigate the nonequilibrium dynamics of a dopant in two-leg 𝑡−𝐽 ladders with antiferromagnetic XXZ spin interactions. In the Ising limit, we find that the dopant is localized for all investigated nonzero temperatures due to an emergent disordered potential, with a localization length controlled by the underlying correlation length of the spin lattice, which increases exponentially with decreasing temperature. The dopant, hereby, only delocalizes asymptotically in the zero temperature limit. This greatly generalizes the localization effect discovered recently in Hilbert space fragmented models [Phys. Rev. Res. 6, 023325 (2024), SciPost Phys. Core 7, 054 (2024)]. In the presence of spin-exchange processes at rate 𝛼, the dopant diffuses with a diffusion coefficient, 𝐷 ℎ , depending nonmonotonically on 𝛼. It initially increases linearly as 𝐷 ℎ ∝ 𝛼 for 𝛼 ≪ 1 before dropping off as 𝛼 −1 for 𝛼 > 1. Moreover, we show that the underlying spin dynamics at infinite temperature behaves qualitatively the same, albeit with important quantitative differences. We substantiate these findings by showing that the dynamics features self-similar scaling behavior, which strongly deviates from the Gaussian behavior of regular diffusion, especially for weak spin exchange. Finally, we show that the diffusion coefficient 𝐷 ℎ follows an Arrhenius relation at high temperatures, whereby it is exponentially suppressed upon cooling.

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