arXiv subjects

Xuanhe Li

Publications and source records attributed to Xuanhe Li.

4 recordsLinked to original sources

arXiv2025

A Unified Thermo-Chemo-Mechanical Framework for Bulk and Frontal Polymerization: Coupled Kinetics and Front Stability

Polymerization is a fundamental chemical process enabling large-scale production of material components across modern industries. By transforming a monomer mixture into a cross-linked polymer network, polymerization induces changes in temperature and material properties such as density and stiffness, which can generate residual stress and warping through coupled mechanisms that remain incompletely understood. Depending on processing conditions, polymerization may occur either in the bulk, sustained by continuous external energy input, or as a self-sustaining exothermic reaction front, commonly referred to as frontal polymerization. While frontal polymerization offers rapid and energy-efficient curing, its localized reaction zone produces sharp spatial gradients that amplify thermo-chemo-mechanical coupling effects. In this work, we develop a thermodynamically consistent framework that captures both bulk and frontal polymerization, incorporating stress-dependent reaction kinetics and the evolution of the stress-free configuration during curing. Using a narrow reaction-zone approximation in a uniaxial setting, we derive analytical predictions for propagation velocity, residual stress development, and stability. A perturbation analysis yields a stability criterion that generalizes the classical Zeldovich number by accounting for heat loss and mechanical loading, and enables construction of a phase diagram distinguishing stable, unstable, and quenched propagation regimes.

cond-mat.soft↗

arXiv2025

Interfacial Cavitation with Surface Tension: New Insights into Failure of Particle Reinforced Polymers

Understanding and mitigating the failure of reinforced elastomers has been a long-standing challenge in many industrial applications. In an early attempt to shed light on the fundamental mechanisms of failure, Gent and Park presented a systematic experimental study examining the field that develops near rigid beads that are embedded in the material and describe two distinct failure phenomena: cavitation that occurs near the bead in the bulk of the material, and debonding at the bead--rubber interface [Gent, A.N. and Park, B., 1984. Journal of Materials Science, 19, pp.1947-1956]. Although the interpretation of their results has not been challenged, several questions stemming from their work remain unresolved. Specifically, the reported dependence of the cavitation stress on the diameter of the bead and the counterintuitive relationship between the delamination threshold and the material stiffness. In this work, we revisit the work of Gent and Park and consider an alternative explanation of their observations, interfacial cavitation. A numerically validated semi-analytical model shows that in {the} presence of surface tension, defects at the bead-rubber interface may be prone to cavitate at lower pressures compared to bulk cavitation, and that surface tension can explain the reported length-scale effects. A phase-map portrays the distinct regions of `cavitation dominated' and `delamination dominated' failure and confirms that for the expected range of material properties of the rubbers used by Gent and Park, interfacial cavitation is a likely explanation. Crucially, this result offers a new avenue to tune and optimize the performance of reinforced polymers and other multi-material systems.

cond-mat.soft↗

arXiv2023

Mechanical Forces Quench Frontal Polymerization: Experiments and Theory

Frontal polymerization is a promising energy-saving method for rapid fabrication of polymer components with good mechanical properties. In these systems, a small energy input is sufficient to convert monomers, from a liquid or soft solid state, into a stiff polymer component. Once the reaction is initiated, it propagates as a self-sustaining front that is driven by the heat released from the reaction itself. While several studies have been proposed to capture the coupling between thermodynamics and extreme chemical kinetics in these systems, and can explain experimentally observed thermo-chemical instabilities, only few have considered the potential influence of mechanical forces that develop in these systems during fabrication. Nonetheless, some experiments do indicate that local volume changes induced by the competing effects of thermal expansion and chemical shrinkage, can lead to significant deformation or even failure in the resulting component. In this work, we present a unique experimental approach to elucidate the effect of mechanics on the propagation. Our experiments reveal that residual stresses that arise in frontal polymerization are not only a potential cause of undesired deformations in polymer products, but can also quench the reaction front. This thermo-chemo-mechanically coupled effect is captured by our theoretical model, which explains the mechanical limitations on frontal polymerization and can guide future fabrication. Overall, the findings of this work suggest that mechanical coupling needs to be taken into consideration to enable industrial applications of frontal polymerization at large scales.

cond-mat.soft↗

arXiv2021

Nonlinear Inclusion Theory with Application to the Growth and Morphogenesis of a Confined Body

One of the most celebrated contributions to the study of the mechanical behavior of materials is due to J.D. Eshelby, who in the late 50s revolutionized our understanding of the elastic stress and strain fields due to an ellipsoidal inclusion/inhomogeneity that undergoes a transformation of shape and size. While Eshelby's work laid the foundation for significant advancements in various fields, including fracture mechanics, theory of phase transitions, and homogenization methods, its extension into the range of large deformations, and to situations in which the material can actively reorganize in response to the finite transformation strain, is in a nascent state. Beyond the theoretical difficulties imposed by highly nonlinear material response, a major hindrance has been the absence of experimental observations that can elucidate the intricacies that arise in this regime. To address this limitation, our experimental observations reveal the key morphogenesis steps of Vibrio cholerae biofilms embedded in hydrogels, as they grow by four orders of magnitude from their initial size. Using the biofilm growth as a case study, our theoretical model considers various growth scenarios and employs two different and complimentary methods -- a minimal analytical model and finite element computations -- to obtain approximate equilibrium solutions. A particular emphasis is put on determining the natural growth path of an inclusion that optimizes its shape in response to the confinement, and the onset of damage in the matrix, which together explain the observed behavior of biofilms. Beyond bacterial biofilms, this work sheds light on the role of mechanics in determining the morphogenesis pathways of confined growing bodies and thus applies to a broad range of phenomena that are ubiquitous in both natural and engineered material systems.

cond-mat.soft↗