arXiv subjects

Alexei P. Sokolov

Publications and source records attributed to Alexei P. Sokolov.

4 recordsLinked to original sources

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

Unexpectedly large entropic barrier controls bond rearrangements in vitrimers

Vitrimers are a relatively new class of polymeric materials containing associative covalent dynamic bonds that make them recyclable by design. However, the fundamental mechanisms controlling their viscoelastic properties remain poorly understood. Our detailed studies of relaxation dynamics and viscoelastic behavior of model vitrimers revealed that the density of dynamic covalent crosslinks has no influence on chain dynamics (beyond a weak change in the glass transition temperature), yet it strongly affects the linear viscoelasticity of vitrimers. Increasing the crosslink density induces a sol-gel transition consistent with predictions of classical gelation theory, demonstrating its applicability to vitrimers. Remarkably, the temperature-dependent analysis of the bond rearrangement time reveals an unexpectedly large negative activation entropy in the transition state that strongly slows down the bond exchange process despite its relatively low activation enthalpy. This insight explains the unusual long timescale for bond rearrangement in vitrimers and highlights the significance of entropy in controlling the viscoelasticity of dynamic covalent networks.

cond-mat.soft↗

arXiv2016

Identification of structural relaxation in the dielectric response of water

One century ago pioneering dielectric results obtained for water and n-alcohols triggered the advent of molecular rotation diffusion theory considered by Debye to describe the primary dielectric absorption in these liquids. Comparing dielectric, viscoelastic, and light scattering results we unambiguously demonstrate that the structural relaxation appears only as a high-frequency shoulder in the dielectric spectra of water. In contrast, the main dielectric peak is related to a supramolecular structure, analogous to the Debye-like peak observed in mono-alcohols.

cond-mat.soft↗

arXiv2015

Revealing spatially heterogeneous relaxation in a model nanocomposite

The detailed nature of spatially heterogeneous dynamics of glycerol-silica nanocomposites is unraveled by combining dielectric spectroscopy with atomistic simulation and statistical mechanical theory. Analysis of the spatial mobility gradient shows no 'glassy' layer, but the alpha relaxation time near the nanoparticle grows with cooling faster than the alpha relaxation time in the bulk, and is ~ 20 times longer at low temperatures. The interfacial layer thickness increases from ~ 1.8 nm at higher temperatures to ~ 3.5 nm upon cooling to near Tg. A real space microscopic description of the mobility gradient is constructed by synergistically combining high temperature atomistic simulation with theory. Our analysis suggests that the interfacial slowing down arises mainly due to an increase of the local cage scale barrier for activated hopping induced by enhanced packing and densification near the nanoparticle surface. The theory is employed to predict how local surface densification can be manipulated to control layer dynamics and shear rigidity over a wide temperature range.

cond-mat.mtrl-sci↗